The chemistry of tetrameric acids in petroleum

Advances in Colloid and Interface Science 205 (2014) 319–338

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Advances in Colloid and Interface Science

j ourna l homepage: www.e lsev ie r .com/ locate /c i s

The chemistry of tetrameric acids in petroleum

Johan Sjöblom a, Sébastien Simon a,⁎, Zhenghe Xu b

a Ugelstad Laboratory, Norwegian University of Science and Technology, 7491 Trondheim, Norwayb Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB T6G 2V4, Canada

⁎ Corresponding author at: Ugelstad Laboratory, NorweE-mail address: [email protected] (S

0001-8686/$ – see front matter © 2014 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.cis.2013.12.007

a b s t r a c t
a r t i c l e i n f o
Available online 27 December 2013

Keywords:ARNTetrameric acidsCalcium naphthenate depositsFlow assurance

This article reviews the properties of a novel class of molecules: the tetrameric acids. These molecules havebrought a large interest in petroleum science since the discovery of the family of molecules named ARN in2004. ARN, which is naturally present in oil, is responsible, by reaction with calcium ion, of the formation ofcalcium naphthenate deposits; organic deposits that cause irregularities in crude oil production and processing.In order to study the properties of ARN, a model tetrameric acid molecule mimicking some of its propertiesnamed BP-10 has been developed in 2008 by Nordgård and Sjöblom and has been extensively used since then.After presenting the experimental techniques used to study the tetrameric acids, this review describes in detail thestructure, preparation, detection and the bulk and interfacial properties of tetrameric acids ARN and BP-10. Finallythe prediction of the operational problems with calcium naphthenate precipitation in new fields is discussed.

© 2014 Elsevier B.V. All rights reserved.

Contents

1. Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3202. Experimental techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320

2.1. Preparation of tetrameric acid samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3202.1.1. Preparation of ARN samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3202.1.2. Preparation of model compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320

2.2. Analytical techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3202.2.1. Determination of ARN structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3202.2.2. Quantification of ARN content in deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3202.2.3. Determination of glass transition temperature Tg of calcium naphthenate deposits . . . . . . . . . . . . . . . . . . . . . . 320

2.3. Experimental techniques for characterization of tetrameric acids in solution . . . . . . . . . . . . . . . . . . . . . . . . . . . 3202.3.1. Determination of phase diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3202.3.2. Cryogenic transmission electron microscopy (cryo-TEM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3202.3.3. Scattering techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3212.3.4. Titration techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3212.3.5. Reactions between the model molecule BP-10 and M2+ followed by isothermal titration calorimetry (ITC) . . . . . . . . . . . 322

2.4. Interfacial analysis technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3222.4.1. Surface and interfacial tension measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3222.4.2. The Langmuir balance technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3222.4.3. Shear interfacial rheology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323

3. The structure of tetrameric acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3234. Detection of tetrameric acids in oils and deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3245. Model compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3246. Bulk properties of tetraacids in aqueous phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324

6.1. Acidity of tetrameric acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3256.2. Self-association properties of tetrameric acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326

6.2.1. Phase diagrams of Na4BP-10 systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3266.2.2. Vesicles and micelles formed by model tetrameric acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327

7. Properties of tetraacids at interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3277.1. Surface activity of C80-TA and BP-10 at the liquid/air and liquid/liquid interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3287.2. Interaction between mono-acid and C80-TA at liquid/air interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3297.3. Emulsion-forming properties of C80-TA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330

gian University of Science and Technology, N-7491 Trondheim, Norway. Tel.: +47 73 59 16 57, +47 48 04 90 31 (mobile).. Simon).

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8. Bulk and interfacial reactions with M2+ and selectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3308.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3308.2. Thermodynamic approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3308.3. Interfacial reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3328.4. Cation selectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333

9. Risk assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336

1. Background

Successful and economical flow (flow assurance) is highly desirablein crude oil production and processing. Obstacles to challenge the flowcan be emulsions [1], foam, asphaltene deposition [2–4] and recentlydiscovered calcium naphthenate deposition [5–8]. The consequenceson oil production — shut down of a field to clean out separators — areimportant: in today's dollars, a field producing 200,000 bpd at 100$per barrel is a gross loss of 20 million dollars a day in lost or deferredproduction. At first the calcium naphthenate deposition in separatorsand equipment was believed to originate from a simple reactionbetween calcium in the water phase and monomeric fatty or naphthenicacids present in the oil phase. Such reaction is known to form complexeswith a given stoichiometry of 1:2 between the constituents [9–11]. How-ever, it was soon found that the problem could not be oversimplified inthis way. An extensive characterization campaign was initiated byConocoPhillips and Statoil and resulting in completely newand surprisingfindings [12,13]. The characterization of the deposits showed that a dom-inating constituent in these deposits was a family of 4-protic carboxylicacids with 4–8 unsaturated rings in the hydrocarbon skeleton and amolar mass in the range of 1227–1235 g·mol−1 [12,14]. In the earlystage an acid with the mass 1231 was identified to have the structureC80H142O8.

After the discovery of the tetrameric acid it was found that this fam-ily of molecules was the main constituent responsible for naphthenatedepositions in large operational offshore fields in Norway, UK, Chinaand West Africa [12,15]. These molecules were named ARN by theirdiscoverer (word meaning “eagle” in Norwegian) [12]. The name C80-tetraacids (abbreviated C80-TA) is also found in the literature [16].

2. Experimental techniques

2.1. Preparation of tetrameric acid samples

2.1.1. Preparation of ARN samplesARN samples are obtained by extraction from calcium naphthenate

deposits obtained from production sites using acid treatment with sol-vent extraction followed by clean up with the Acid-IER (Ion ExchangeResin) method [17]. The first step in that process is to wash the samplethoroughly in toluene to remove residual crude oil and oil-solublefractions. The remaining solid was then placed in a 2:1 volume mixtureof toluene and 1 MHCl solution to dissolve the naphthenate. The naph-thenic acids were selectively isolated from other potentially occurringpolar compounds by the use of the ion-exchange resin QAE SephadexA-25 and elution with formic acid. The solvent was removed from thenaphthenic acids by evaporation on a rotary evaporator at 60 °C andfurther by drying in an oven at the same temperature. Some aspects ofthis method are discussed in Section 5.

2.1.2. Preparation of model compoundsThemodel compounds BP-10 and Pe-10were prepared according to a

method developed by Nordgård and Sjöblom [18]. Briefly Pe-10 has beenprepared via esterification of 11-bromoundecanoic acid, alkylation ofperylene-3,4,9,10-tetracarboxylic dianhydride with the x-bromo ester ina phase transfer reaction in alkali aqueous solution and subsequent hy-drolysis of themethyl esterswith KOH in EtOH to yield the final tetraacid.BP-10was prepared by alkylation of 2,2′,4,4′-tetrahydroxybenzophenone

with the ω-bromo ester using K2CO3 in DMF at 80 °C and subsequentester hydrolysis using KOH in MeOH.

2.2. Analytical techniques

2.2.1. Determination of ARN structureTheARN structure has been determined by a combination ofmodern

NMR techniques by Lutnaes et al. [19]. The techniques they have usedare a series of edited 1D and correlation NMR spectra, including COSY,ROESY, HSQC, HMSC, 1,1-ADEQUATE, and 2D and 3D HSQC-TOCSY.

2.2.2. Quantification of ARN content in depositsThe ARN content in deposits can be determined according to the

method developed by Simon et al. [20]. This method consists of foursteps. Molecules present in the deposit are first dissolved in a mixtureof toluene and 2-butanol after an acidic treatment. All acid moleculesare then selectively extracted and concentrated by a solid-phase extrac-tion (SPE) method. After derivatization of acids into their naphthacylesters to increase the sensitivity of detection, ARN is separated anddetected by reversed-phase HPLC with UV detection.

It must be noticed that the second part of this method is designed toextract naphthenic acid at analytical scale (120 μmol of acid functions).The method used to obtain ARN in large amounts (a few grams) isdescribed in Section 2.1.1.

2.2.3. Determination of glass transition temperature Tg of calciumnaphthenate deposits

Differential scanning calorimetry (DSC)was used to determine ther-mal phenomena of calcium naphthenate deposits [21]. Experimentswere blanketed with a nitrogen atmosphere to exclude oxygen effects.All samples were first heated to the maximum temperature of thecycle, 300 °C for the tetraacid and 350 °C for naphthenate, in order toobtain a representative thermal history and to evaporate lower-boiling impurities. The sample was heated at 58 °C/min to the maxi-mum temperature, kept isothermal for 5 min, cooled at the same rateto −50 °C and kept isothermal for another 5 min. This cycle was re-peated twice and the thermal transitions were recorded on the secondcycle.

2.3. Experimental techniques for characterization of tetrameric acidsin solution

2.3.1. Determination of phase diagramThe isotropic L1 and L2 regions of Na4BP-10/alcohol/H2O system

were determined by determining the transition point from clear solu-tion to cloudy suspensionwhen alcoholwas added to the samples of dif-ferent Na4BP10/H2O weight ratios. The transition points were checkedby addingNa4BP-10 into the sampleswith different alcohol/H2Oweightratios. Water is titrated into samples of different Na4BP10/alcoholweight ratios and centrifuged after mixing. The appearance of liquid-like phase observed is the end point of S region. The phase boundariesof liquid crystals were determined with polarizing microscope [22].

2.3.2. Cryogenic transmission electron microscopy (cryo-TEM)Cryo-TEM sampleswere prepared as follows: A small droplet of each

samplewas placed on a carbon-coated, lacey film-supported andmicro-perforated TEM copper grid. The excess solution of each sample was

321J. Sjöblom et al. / Advances in Colloid and Interface Science 205 (2014) 319–338

blottedwith a piece of filter paper. Careful blotting produced thin liquidfilms with a thickness of 100–400 nm, which freely spanned themicro-pores of the TEM grids. The sample grids were then quickly immersedinto liquid ethane and kept there before imaging with the transmissionelectron microscope [22].

2.3.3. Scattering techniques

2.3.3.1. Introduction. The scattering techniques allow quantitativedetermination of size, shape and structure of colloidal particles. Thesetechniques are based on interactions between incident radiationand particles. As a result the radiations are scattered into differentdirections.

These techniques are ranked based on:

– The nature of the radiation (light, X-rays or neutrons)– Themeasured signal: whether the average scattered intensity or the

time fluctuation of the scattered signal is measured. In the case oflight these techniques are named respectively static light scattering(SLS) and dynamic light scattering (DLS).

2.3.3.2. Static light scattering (SLS). SLS was used to determine theweight-average molar mass Mw and the second virial coefficient A2 ofthe Na4BP-10 micelles. SLS is based on the measurement of lightscattering at different Na4BP-10 concentrations, C. By fitting the exper-imental results to thewell known Rayleigh equation, theMw and A2 canbe obtained.

K � CRθ

¼ 1Mw

þ 2 � A2 � C ð1Þ

where Rθ is the Rayleigh ratio and K is the optical constant:

K ¼ 2π2

λ40NA

n0dndC

� �2ð2Þ

where NA is the Avogadro number, n0 is the refractive index of waterand dn/dC is the concentration dependence of refractive index. Eq. (1)is valid only if the size of the micelle is negligible compared to thewavelength of the incident light. This means that the diameter of themicelles must be smaller than λ0/20. Since the maximum micellediameter measured for Na4BP-10 is 5.2 nm, this assumption is valid.

2.3.3.3. Dynamic light scattering (DLS). DLS was used to determine thehydrodynamic diameter of micelles and vesicles formed by Na4BP-10in aqueous solution. The time correlation function G(2)(t,q) wasmeasured on filtered Na4BP-10, where t is the decay time and q isscattering vector (q = (4πn / λ0)sin(θ/2)). G(2)(t,q) can be related tothe normalized first-order electric field time correlation function|g(1)(t,q)| via the Siegert relation [23] as

G 2ð Þ t;qð Þ ¼ A 1þ β g 1ð Þ t;qð Þ2�� h i

ð3Þ

where A (≡bI(0)N2) is the measured baseline. For the broadly distributedrelaxation spectrum, |g(1)(t,q)| is related to a characteristic relaxationtime distribution G(τ) by

g 1ð Þ t;qð Þ�� ≡ E 0;qð ÞE� t;qð Þ� �

= E 0;qð ÞE � 0;qð Þh i ¼Z∞

0

G τð Þe−t=τdτ ð4Þ

whereG(τ) can be calculated from the Laplace inversion of themeasuredG(2)(t,q) on the basis of Eqs. (1) and (2); a translational diffusivecoefficient distribution G(D) by Г = Dq2, where Г = 1 / τ, andthe hydrodynamic radius distribution by the Stokes–Einstein equation:Rh = kBT / 6πηD, where η, kB and T are the solvent viscosity, theBoltzmann constant, and the absolute temperature, respectively.

2.3.3.4. Small-angle neutron scattering (SANS). SANS is a highly usefulmethod for studying in detail structures at the nanoscale [24], since itcan distinguish the contribution of the particle shape and size fromthat of the interaction due to e.g. surface charge. Theoretically thistechnique is similar to SLS.

SANS measures the intensity I(q) scattered by the sample as a func-tion of the wave scattering vector q depending on the scattering anglewhen a sample is bombarded by neutrons. The scattered intensity I(q)probes the correlations between colloid-rich regions at a scale of theorder of q−1. For a particle (like micelle) in a solvent system, a generalexpression of I(q) can be derived [24]:

I qð Þ ¼ φ � v � ΔρSLD2 � F qð Þ � S qð Þ ð5Þ

with

• ϕ the particle volume fraction,• v the volume of a particle,• ΔρSLD2 the contrast term; Δρ is the scattering length density differencebetween particles and solvent, determined from density and chemicalcomposition of solvent and particles,

• F(q), the form factorwhich is a function of shape, size and polydisper-sity of particles,

• S(q), the structure factor which depends on the inter-particleinteractions.

2.3.4. Titration techniquesThis method is used to determine the pKa of tetrameric acids

(Section 7).The titrations were performed with a high-precision potentiometric

titration instrument, developed at Umeå University [25–29]. All titrationswere performed in an oil bath thermostated at 25 ± 0.1 °C. During thetitrations the solutions were protected from CO2 contamination usinginert N2-gas. The cell used for the determination of [H+] was:

−Ag/AgCl | ionic media (NaCl) || solution/suspension in NaCl(aq) | -

Glass electrode+.

The potential (E, in mV) of this cell is given by Eq. (6):

E ¼ E0 þR � T � ln10

F� log Hþh i

þ E j ð6Þ

where E0 is an apparatus constant. R in Eq. (6) is the gas constant, T is theabsolute temperature and F is the Faraday constant. Ej is a function of[H+], two junction potentials jac and jalk, and the ionic product ofwater, Kw, according to Eq. (7):

E j ¼ jac � Hþh iþ jalk � Kw � Hþh i−1

: ð7Þ

In Eq. (7), relevant values for Kw, jac and jalk were adopted fromSjöberg et al. [29]. The equilibrium condition was set to a drift in E lessthan 0.2 mV/h.

After calibration, titration was performed using dilute HCl(aq) andNaOH(aq) solutions with corrected ionic strength (NaCl) in the acidicdirection and the alkaline direction, respectively.

The data from the potentiometric titrations are illustrated in theform of Z-plots, where Z is defined as the average number of protonsabsorbed by each molecule of BP-10. This definition is shown in Eq. (8):

Z ¼ − ≡ COO−½ �BP10½ � ¼ H−hþ Kw � h−1

Bð8Þ

where H and B are the total concentrations (mol/dm3) of protons andtetraacid, respectively. h is the concentration (mol/dm3) of free protons,[H+], and Kw is the auto-ionization product of water.

mN.m-1

c 25-50

v 0.1 GLE/G

LE

LC

I

S

Molecular area (nm2.molecule-1)0.2 0.25 0.50 10

π

π

π

≈

≈

Fig. 1. Composite two-dimensional pressureπ versus areaσ isotherm,which includes awideassortment of monolayer phenomena. Note that the scale of the figure is not uniform so thatall features may be included on one set of coordinates. The sketches of the surfactants showthe orientations of the molecules in each phase at various phases of compression.Redrawn from reference [34].


2.3.5. Reactions between the model molecule BP-10 and M2+ followed byisothermal titration calorimetry (ITC)

ITCmeasurements [30] were performed at 50 °C using a Nano-ITC 2Gtitration microcalorimeter (TA instruments, USA). The integratedinjection-stirrer assembly (250 μL) was filled with metal nitrate(M(NO3)2) aqueous solutions, which was titrated in portions of 10 μL or5 μL into the calorimeter vessel that initially contained 1 mL Na4BP-10aqueous solution. The pH values of bothM(NO3)2 and Na4BP-10 aqueoussolutions were adjusted to 10. The reference cell was filled withMilliporeMilli-Q water. The titration was carried out at the constant stirring speedof 250 rpm to ensure thorough mixing. Each injection was completed in10 s. To allowequilibration, therewas a 420-s timedelay between succes-sive injections. Raw datawere obtained as a plot of heating rate (μJ/s) as afunction of titration time (s). These data were then integrated to give aplot of enthalpy change per mole of injectant (M2+), ΔHobs (kJ/mol)(molar enthalpy of reaction) as a function of M2+ to Na4BP-10 molarratio.

2.4. Interfacial analysis technique

2.4.1. Surface and interfacial tension measurementsIn a bulk phase, molecules will be surrounded by other molecules

such that the molecular interactions are equal in all directions. At aninterface however, the molecules in one phase will come in contactwith molecules in the other phase with, for two immiscible liquids,very different properties regarding polarity and intermolecular forces.The molecules at the interface will no longer be symmetricallysurrounded by equal molecules causing a net drag force towards thebulk phase. The increase in energy by surface area due to the loss ofbulk interactions is called surface (water/air) or interfacial (water/oil)tension [1].

Severalmethods exist tomeasure interfacial tension [31,32] but onlythe two most common ones are presented.

A tensiometer equipped with either a Wilhelmy's plate or a DuNoüy's ring can determine surface and interfacial tensions. The forceat the surface is measured by the tensiometer, when the ring is pulledup from the surface.

The pendant drop technique allows measurement of surface andinterfacial tension [33]. This technique is particularly adapted to studythe dynamic interfacial tension i.e. the variations of γ with time. Thistechnique can be favorable compared with the Wilhelmy's plate andDu Noüy's ring methods since the interface is not disturbed during themeasurement.

The pendant dropmeasures the shape of a drop placed at the tip of aneedle or a hook. The shape depends on two different forces:

– First the Laplace pressure:

ΔP ¼ γ � 1R1

þ 1R2

� �ð9Þ

where the pressure difference across the drop interface is ΔP and R1

and R2 are the radii of curvature of the drop, and then– The gravity and the buoyancy forces:

ΔP ¼ ΔP0 þ Δρð Þgz ð10Þ

with ΔP0 being the pressure difference at the vertical distance z, Δρ,the difference in the densities of the two bulk phases (inside andoutside) the drop), g, the gravity acceleration and z, the verticaldistance.

By combining those two equations to fit the shape of the dropdetermined by a camera, it is possible to determine the interfacial tensionof the system provided that the densities of the bulk phases are known.

2.4.2. The Langmuir balance techniqueThe Langmuir balance technique is used to study film formation and

monolayer properties of surface active materials not soluble in water.The technique can give information on how much area each moleculetakes at interface/surface, how the molecules interact with each otherand how the molecules pack at interfaces.

The instrument consists of a shallow rectangular trough and twomoving barriers. The surface pressure is measured with a Wilhelmypaper probe [31]. This probe determines the surface pressure bymeasuring the force due to surface tension on the probe that is partiallyimmersed in the sub-phase. The dimension of the plate then allows oneto convert the force into surface tension (mN/m). A typical measure-ment is carried out by filling up the trough with the water phase. Thepaper probe is hanged on the balance so that it is partially immersedin the liquid. Then the sample containing the amphiphiles is spreadonto the surface. After the solvent has completely evaporated thelayer of surfactants is compressed with moveable barrier blocks. Asthe compression proceeds the amphiphiles will start to interact witheach other, causing an increase in the surface pressure (π) accordingto Eq. (11),

π ¼ γ0−γ ð11Þ

where γ0 is the surface tension at a clean surface and γ is the surfacetension in the presence of the sample.

The classical approach to explain the behavior of surface activemonolayers was given by Langmuir [34,35]. He conducted experimentswith a troughmonitoring the surface pressure (π = γ0 − γ) as a func-tion of molecular area on the surface, a so-called π–A isotherm. Whenthe total area available for the molecules is reduced due to the positionof the barriers, one observes a transition from a gaseous state (G) to twosubsequent liquid-like states named liquid expanded (LE) and liquidcondensed (LC) to a solid–liquid state (S). The change between thesestates lies within the interaction between themolecules. In the gaseousstate, the molecules have large available surfaces and practically nointeraction between them. The molecules have an ideal behavior withno interaction profiles. When the distance between themolecules is re-duced, a condensation will take place indicating a stronger intermolec-ular interaction. Finally, when the packing of the molecules is optimal,the hydrocarbon chains are in hexagonal array characteristic for asolid state. Between the gaseous, liquid and solid states, there areequilibrium states for instance G + L, L + S and so on where thesurface pressure is constant (Fig. 1).

Aqueous phase

Oil phase

Bi-cone geometry

Fig. 2. Schematic of the shear interfacial rheometer geometry. For the study of the films formed by BP-10 and calcium ions, the aqueous phase comprised calcium and a buffer while the oilphase contains BP-10 [41].


Langmuir balance can be combined with a Brewster angle micro-scope (BAM) for morphological characterization of the monolayer.Langmuir balance equipped with BAM has previously proved to be apowerful combination to visualize domain structures of monolayers ofcarboxylic acids [36], surface morphology of polymer films [37], andmiscibility of mixed monolayers of two-component systems [38,39].

2.4.3. Shear interfacial rheologyShear interfacial rheology was used to determine the rheological

properties of interfacial films formed by BP-10 and calcium ions.Interfacial shear rheology experiments were realized by utilizing a

shear- and stress controlled rheometer with an oscillating biconicalbob geometry [40]. A schematic of the measurement cell of thisapparatus is given in Fig. 2.

In interfacial shear rheology, a shear deformation at the interface isapplied while the area is kept constant. In oscillation test, a sinusoidalshear deformation γ

γ ¼ γ0 sin ωtð Þ ð12Þ

is performed at an angular frequencyω and strain amplitude γ0, and anout-of-phase response is obtained for the stress tensor σ with phaseangle δ

σ ¼ σ0 sin ωtþ δð Þ: ð13Þ

The storage modulus G′ and loss modulus G″ are derived from

G′ ¼ τ0γ0

cos δð Þ ð14Þ

G″ ¼ τ0γ0

sin δð Þ: ð15Þ

They describe the elasticity and viscosity of the film, respectively.

Fig. 3. Formula of the most abundant C80 6-r

3. The structure of tetrameric acids

In order to start-up remediation/mitigation operations, informationon the structure of the compounds responsible for the deposition be-came important. Statoil and ConocoPhillipswere in leadof the analyticalwork resulting in an almost complete clarification of the molecularstructure of the C80 molecule [12,13]. The analytical work comprisedcollection of deposits, processing of the deposits and finally isolationusing the Acid-IER (Ion Exchange Resin) method [17,42]. Structuralelements were determined by means of 13C NMR, Liquid Chromatogra-phy–Mass Spectrometry (LC–MS). Also VPO (Vapor Phase Osmometry)and Gas Chromatography were used in the characterization work. Thefinal molecular weight was determined by a homebuilt 9.4 T FT-ICRMass Spectrometer at National High Magnetic Field Laboratory ofFlorida State University (USA).

The extensive work by Statoil and ConocoPhillips was almost com-plete with regard to molecular structure. Utilizing updated and moremodern NMR technology 1D and 2D (COSY, ROESY…) and 3D HSQC-TOCSY it was possible to resolve a final structure for the ARN family[19,43–45]. Fig. 3 presents the structure of the most abundant memberof the ARN family. This molecule C80H142O8 has 80 carbon atoms, 4 car-boxylic acid functions and 6 cyclopentyl rings. The other isomers of theARN family differ by the number of cyclopentyl rings: 4 (C80H146O8), 5(C80H144O8), 7 (C80H140O8) and 8 (C80H138O8). Derivatives containing81 and 82 carbon atomswere also detected [44]. The relative abundanceof the isomers depends on the origin of the oil in which ARN sampleswere extracted [44]. More anecdotic, an ester of C80-TA was recentlydetected in one North Sea oilfield pipeline, but it could be the result ofesterification of C80-TA with production chemicals [46].

The structure presented in Fig. 3 has led Lutnaes et al. to sug-gest that ARN was synthesized by Archaea micro-organisms [19].The difference of relative abundance of the isomers with the originof the oil could therefore be related to growth of the Archaeafrom which the acids are thought to originate at different averagetemperature [44].

ing tetraacid after Lutnaes et al. [19,43].


4. Detection of tetrameric acids in oils and deposits

The detection and determination of the ARN concentration indeposits and crude oils is of paramount importance:

– To identify the nature of the deposit i.e., calcium naphthenate,waxes, asphaltenes….

– To estimate at an early stage potential for fouling and need for treat-ment. ARN can be present in a field but this does not always lead tothe formation of deposit as shown by Brocart et al. [47].

The detection of ARN in deposits ismostly done byMass Spectrometry(MS) generally after an extraction and concentration step. Two mainextraction steps are involved:

– After removing residual oil, the deposit is treated with an acidfollowed by extraction with an organic solvent. The extract is thencharacterized by MS [48–50].

– A more specific method exists to specifically extract naphthenicacids from deposits. After removing residual oil, acid treatmentand extraction, the naphthenic acids are selectively isolated fromother potentially occurring polar compounds present in the extractby use of ion-exchange resin. The final extract is here also character-ized by MS [14]. However due to its selectivity and time consump-tion, this method is generally used to purify ARN samples for extraanalysis [14].

Different MS techniques can be implemented to detect ARN in theextracts recovered with the methods presented above. Negative-ionelectrospray ionization Fourier transform ion cyclotron resonancemass spectrometry (ESI FT-ICR MS) has been used by Mapolelo et al.[48,49] and Brandal et al. [14]. The advantage of this method is thehigh resolving power of FT-ICR MS combined with the fact that thenegative-ion ESI ion source generates [M − H]− without extensiveion fragmentation or matrix interference. More affordable ES/MS hasalso been implemented [50,51]. Atmospheric pressure chemical ioniza-tion (APCI) has been used as ionization source as well [50].

In order to quantify the content of ARN in deposits, only onemethoddescribed in the literature exists [20]. This method involves the prepar-ative steps described above. After this the acids initially present in thedeposit are selectively isolated by a solid-phase extraction method.Then ARN and the naphthenic are derivatized to make them UV active[52–54] and finally ARN is separated and detected by reversed-phaseHPLC with UV detection. Three different deposits were characterizedby this method (Table 1) [20]. The TBN values represent the totalamount of bases (carboxylate functions) present in the deposits. Theresults show that the ARN content in these deposits is quite similarvarying between 28 and 41% w/w. ARN is the major component in theacid fraction (between 55 and 75% mol/mol).

Contrary to deposits, detecting ARN in petroleum crude oils is a chal-lenge owing to its very low concentration [50] and the complexity of thecrude oil matrix. Total has developed a method to detect ARN in crudeoils [47]. After an extraction step, the sample is analyzed by HPLC/MSwith electrospray ionization. The presence of ARN is guaranteed whenthe monocharged [M − H]− and the corresponding bicharged anion[M − 2H]2− are detected simultaneously. However this method is notquantitative.

Table 1Characteristics of 3 calcium naphthenate deposits A, B and C: Total Base Number (TBN),mass percent of ARN in deposits, and mol percent in acid functions from ARN in deposits[20].

Deposit Aspect TBN mg KOH/g % Ca2ARN w/win deposit

nCOOHfromARN=nHþtotal

% mol/mol

A “Solid” 83 38 78B “Thick liquid” 87 28 55C “Solid” 95 41 75

To our knowledge there exist two quantitative methods to deter-mine the ARN concentration in crude oil; one developed by Statoil andthe other by the Ugelstad Laboratory. The Statoil method published asa patent [55] consists of selective absorption/adsorption of ARN acidsby a solid medium (calcium hydroxide) then isolation of the solidmedium and transfer of the ARN acids to an organic solvent. The finalquantification method is not disclosed: the authors just claimed thatMS, GC and UV can be used with no other information. The methoddeveloped at Ugelstad Laboratory [56] is based on the one developedto analyze calcium naphthenate deposits with extra purification stepto obtain a less complex matrix to analyze by HPLC/UV. This method isfor instance used by Total in their process to assess risk of calciumnaphthenate deposition [6].

5. Model compounds

Formechanistic studies of calcium naphthenate deposition, relative-ly large amounts of C80-TA are required. As C80-TA is found in large pro-portion in organic deposits, the easiest (and for nowonly)way to obtainthis molecule is to isolate it from calcium deposits by the ion-exchangeresin (IER) method developed by Statoil [17]. However, this methoddoes not always give satisfactory purity of the tetraacids since it extractsall the carboxylic acid types of molecules present in deposits and notonly C80-TA. Moreover C80-TA is difficult to detect since it has no chro-mophore. Amethod has been developed to detect it but it requires to es-terify C80-TA into an aromatic ester; a procedure which can be timeconsuming [20]. Considering the lack of chromophore in the tetraacidsand the purity issue brought forward, a need for UV active and fluoresc-ing model compounds was clearly defined. Comparative studies of thefundamental properties between natural tetraacids and synthesizedmodel compounds could therefore be conducted both in bulk and at in-terfaces. In this regard, Nordgård and Sjöblom have developed modelmolecules displaying a chemical structure close to the C80-TA's tomimic the properties of C80-TA [18]. These molecules named BP-10and Pe-10 (Fig. 4) are built of an aromatic core with covalently bondfour decyl chains with four carboxylic acid groups in terminal position.Their molecular weight (between 983 and 1165 g·mol−1) is similar tothe C80-TA's (1231 g·mol−1). These molecules are pure and easy todetect due to the presence of chromophore (Fig. 4). The names BP10and Pe-10 come from their structure in which either a benzophenone(BP) or a perylene (Pe) core is linked to decyl chains (10) via an etheror an ester bond.

In order to validate the representativeness of thesemodelmolecules,an extensive comparison has been done (Table 2), which shows thatthese molecules and in particular BP-10 is able to mimic some of theproperties of C80-TA [57]. In particular these three molecules displaysimilar surface active properties (surface tension and interfacial ten-sion), exhibit similar critical micellar concentrations (CMC) and reactiv-ity with calcium. However, some differences exist. The main ones arethe solubility and thermal properties of model compounds comparedwith C80-TA. Since C80-TA is less polar than the two model compounds,it is more soluble in aromatic solvents like toluene and xylene. Whenfully ionized, it is less soluble inwater than BP-10 [16]. The observed dif-ferences are likely linked with presence of aromatic rings contained inBP-10 that have π–π interactions (possibility of aromatic ring stacking).

6. Bulk properties of tetraacids in aqueous phase

C80-TA and BP-10 have two different features that dictate their prop-erties in aqueous solution:

– They are weak acid, a property which is characterized by an acidconstant Ka and its pKa = −Log Ka.

– They are amphiphilic and therefore display properties of surfactants,including self-association properties.

Fig. 4. Structure of Pe-10 and BP-10, molecules developed as model for C80-TA.Preparation described in Nordgård and Sjöblom [18].


6.1. Acidity of tetrameric acids

The dissociation constant of BP-10 has been determined directly bythe use of high precision potentiometric titration [27] and indirectlyby measuring the apparent partition coefficient of BP-10 between anaqueous and oil phase [58]. It must be noticed that at the interface,the pKa could be, a priori, different.

Sundman et al. [27] used the first technique to investigate the pKa oftetrameric acids. To reach this goal a series of tetraacids, derivatives ofBP-10, with one to ten methylene groups in the alkyl chains givingBP-1–BP-10 was synthesized and studied (Fig. 5) at different salinities,20 to 600 mM NaCl. The measurements were done at a single concen-tration of BP-n of 1 mM. At this concentration BP-1, BP-3 and BP-7 atall the salinities and BP-7 in 20 mM NaCl form monomers in solutionwhile BP-7 in 600 mMNaCl and BP-10 at all the salinities formmicelles.

Results have shown that the pKa of different BP-n depends stronglyof their self-association properties in water and the chain lengths n.

The most hydrophilic tetraacids BP-1 and BP-3 (titration curves forBP-3 given in Fig. 6) were analyzed below their CMC (values notmeasured but assumed to be higher than for BP-5). They displayedaqueous dissociation behavior with separated dissociation step due tothe close proximity between the carboxylic acid functions. Their behav-ior was successfully modeled by common aquatic acid–base equilibria.

For intermediate chain length (BP-5, CMC varies from 20 to 8.4 mMin respectively 100 and 600 mM of NaCl), only one protonation step ismeasured due to the greater distance between the carboxylic acidfunctions.

The titration curves for BP-10 (titration curves given in Fig. 7) andBP-7 above the CMC (CMC of 0.14 mM and 0.6 μM for respectivelyBP-7 and BP-10 in NaCl 600 mM) showed a completely different behav-ior with a hysteresis effect, depending on the direction of titration, andmore influence of the ionic strength due to their self-association proper-ties. Due to this complexity, a simple equilibriummodel will not sufficeto model the raw data.

An apparent pKa defined as the −Log[H+] at which 50% of thecarboxylic groups are deprotonated can be determined for each systemunder investigation (Fig. 8). An obvious observation is that the apparent

Table 2Comparison between the properties of model compounds Pe-10 and BP-10 and indigenous C8Comparison partially presented in Nordgård et al. [57].

Property Similarities Comments

Critical micellar concentration (CMC) Yes Same order of magnitudSolubility in oil Partially Models not well solubleSolubility in water Different BP-10 more soluble thanPartitioning vs pH Yes Partitioning data given iLangmuir films Some BP-10 forms the Ca-filmReactions with Ca2+ YesEmulsions Yes Generally oil-in-water [5Interfacial activity Yes Similar interfacial tensioTg No No clear Tg for models [5

pKa values of the carboxylic acid groups increase with the alkyl chainlength, reaching unexpected high values for carboxylic acid function inthe case of BP-10 (≈7–8). This increase is due to the increased self-association with increasing the chain length. This is in good agreementwith previous observations on long-chain fatty acids [60].

These resultswere complemented by a recent study dealingwith thedetermination of the apparent partition coefficient of BP-10 between anaqueous and oil phase [58]. Themethodology is the following: Solutionsof BP-10 dissolved in oil (here chloroform) are put in contactwithwaterphases at different pH, with the BP-10 concentration being keptconstant. After reaching equilibrium, the concentration of BP-10 isdetermined in both phases.

Fig. 9 presents an example of such a determination depicting thevariation of the concentration of BP-10 in the water phase as a functionof its pH for an initial concentration in oil of 0.1 mM [58]. At low pH, theconcentration in water is low and constant; from pH 7 and above, theconcentration inwater increases due to the ionization of the naphthenicacids. A model based on the oil/water partition coefficient Kwo

[58,61,62] and acid dissociation constant Ka [27] allows to obtain agood fit of experimental data. The results of the fit are summarized inTable 3. In good agreement with the potentiometric titration, the pKa

of BP-10 is quite high (≈7–8) compared with classical values forcarboxylic acids (≈5). These high values can be due to the self-association of BP-10, especially considering that the concentration ofBP-10 is higher than its CMC in the experiment performed. At someionic strengths two pKa values must be considered to obtain a good fitof experimental data (Fig. 9). This apparent inconsistency noted inFig. 8 can be resolved by understanding that it is the pH measured atthe half dissociation point that is reported in this figure, which doesnot necessarilymean that there is only one pKa value for BP-10. Second-ly the partition coefficients were measured at a BP-10 concentrationhigher than its CMC,which favors the self-association of this compound.

The aqueous phase ionic strength has some influence on the parti-tion coefficient Kwo of BP-10 (Table 3). It was shown that this coefficientdecreases with increasing salinity of aqueous phase. This decrease canbe explained by a salting out effect of the added cations, reducing thesolubility of BP10 in the water phase [58].

0-TA extracted from calcium naphthenate deposits.

e: CMC order: BP-10 b C80-TA b Pe-10. CMC values in [57] and in [27]in toluene at room temperature, CHCl3 better for models.C80-TA. Solubility for C80-TA in Table 4 in [16]. Form different structures in water [16].

n [58]at higher pH [57].

9]n values for instance [14,18]7] contrary to C80-TA [21]

image of Fig.�4

Fig. 5. Structure of BP-1, BP-3, BP-5, BP-7 and BP-10 used in the apparent pKa determina-tion study by Sundman et al. [27]. n denotes the number of methylene groups. Molecularweights: BP-1: 478 g/mol, BP-3: 591 g/mol, BP-5: 703 g/mol, BP-7: 815 g/mol, BP-10:983 g/mol.

Fig. 7. Z-plot for the precipitation of BP10 (titration from basic to acidmedium), above theCMC (approx 1 mM). The symbols represent data recorded at (♦) 20 mM, (▲) 50 mM,(★) 100 mM, and (■) 600 mM ionic strengths, respectively.


6.2. Self-association properties of tetrameric acids

6.2.1. Phase diagrams of Na4BP-10 systemsCrucial for the self-association in aqueous media is the solubility of

monomeric TA. It has been found that there is a substantial differencebetween indigenous ARN acids and BP-10-based synthetic molecules[16]. For the latter ones phase diagrams with Na4BP-10/water/butanolor pentanol have been determined [22] revealing not only an extensivemicellization but also the occurrence of a lamellar crystalline D phase. Atlow concentrations there is a co-existence of micelles and vesicles forNa4BP-10 solutions.

The BP-10 self-association can be rationalized through the CriticalPacking Parameter (CPP) valid to distinguish self-assembly structuresfor surfactant systems:

CPP ¼ v�l�a ð16Þ

where v is the volume occupied by the hydrocarbon chain, l is the lengthof the fully extendedhydrocarbon chain and a is a topological area occu-pied by surfactant head groups. For small values of the CPP, i.e. b1/3spherical geometries are predominating. Phenomenologicallywe detectmicelles in water with strong electrostatic repulsion between chargedhead groups which could alter the CCP derived purely from geometryof molecules. Therefore, the actual CPP can be altered by addition ofelectrolytes (normally multivalent ones). By so increasingly addingthe electrolyte one reduces the CPP value and alters the aggregate ge-ometries. Micellar solutions usually experience a phase transition intoliquid crystals upon increasing surfactant concentrations or addition of

Fig. 6. Potentiometric titration curves of BP-3: Z defined as the average number of protonsadsorbed to eachmolecule of acid as a function of pH. The symbols represent data record-ed at (■) 20 mM, (★) 100 mM, and (♦) 600 mM ionic strengths, respectively. The dotted,dashed and solid lines represent fitting with acid/base models [27].

polar additives (like alcohols and acids) [63]. In liquid crystals onedimension of the structure is “infinite”with regard tomolecular dimen-sions. In a hexagonal phase it is the dimension (length) of the hexago-nally oriented cylinders, while in a lamellar phase it is the dimensionof the surfactant sheet. In a lamellar liquid crystal thepackingparameterhas a value close to 1. Themicrostructure here is a molecular packing inbilayers. When the packing parameter exceeds unity, reversedstructures, mainly reversed micelles, will form.

Ordinary monoacids (fatty acids or naphthenic acids) are micelle-forming compounds in solutions above their pKa values. Due to ioniza-tion the criteria for achieving a CPP value of 1/3 are fulfilled. Higher con-centrations of the surfactant will normally render a transition from themicellar L1 phase into a hexagonal liquid crystalline phase E. In order toachieve a packing parameter close to or above 1 the fatty acid must becombined with a polar additive such as alcohol or fatty acid. In systemsof sodium carboxylates/water/free carboxylic acid or a long-chainalcohol it is not common to find both lamellar D phases, in some casesreversed hexagonal F phases and reversed micellar L2 solution phases.

The partial phase equilibrium of two model systems, Na4BP-10/n-C4H9OH/water and Na4BP-10/n-C5H11/OH/water at 25 °C, has been de-termined. As shown in Fig. 10, both systems are dominated by a micellarsolution phase L1 at lower Na4BP-10 concentrations, followed by a smalllamellar liquid crystalline phase (D phase) at higher Na4BP-10 concentra-tions. The polarized micrograph in the inset shows the typical texture oftheDphase. From the phase equilibrium it is evident that the tetrasodium

Fig. 8.Apparent pKa of BP-3 to -10 at 0.5–1 mMconcentration, defined as the−log[H+] atwhich 50% of the carboxylic groups are deprotonated, in 20–600 mMNaCl as a function ofthe number of carbon atoms in the alkyl chain [27].

image of Fig.�5

image of Fig.�7

image of Fig.�8

Fig. 9. Partitioning of BP10 between CHCl3 and buffer solutions at 184 mM K+ as a func-tion of final pH with an initial concentration of BP-10 in the oil phase of 0.1 mM. Thesolid, dotted and longdashed lines represent the bestfit by the sumof least squaremethodwith Kwo = 4.7 · 10−4, pKa = 7.3 forModel 1 (assuming equal acid constant for all the 4BP-10's acid functions), Kwo = 4.5 · 10−4 and pKa = 6.7 and 8.3 for Model 2 (model as-suming 2 acid dissociation constants), Kwo = 4.3 · 10−4 and pKa = 6.7, 6.8, 8.0 and 8.3for Model 3 (assuming 4 acid dissociation constants).Reproduced with permission from [58].


salt of BP10 can be dissolved into an aqueous phase up to 40%. The mi-celles are capable of solubilizing the medium-chain alcohol, 1-butanolup to 31% and 1-pentanol up to 28%. The lamellar phase on the otherhand can hold approximately 5–30% of the alcohol. Contrary to systemscontaining a carboxylate salt of a monoacid, in which the L2 phase ex-tends into the alcohol corner of the phase diagram, no reverse structureswere observed for the model tetrameric acid BP-10 systems. It appearsthat the BP-10 in its completely ionized form is so polar that the additionof the medium chain alcohol cannot increase the CPP values above unity.The results in Fig. 10 clearly show that BP-10 is preferentially a micelleforming compound under these pH and temperature conditions. Further-more, the hexagonal liquid crystalline phase E is also lacking. The absenceof hexagonal liquid crystal phase E suggests that the BP-10molecule doesnot form cylindricalmicelles inside the L1 phase. In order to obtain a bilay-er structure (as in the D phase) the BP10 molecule must be combinedwith the alcohol molecule in order to reduce its surface charge densityand to reach a CPP value around 1.

6.2.2. Vesicles and micelles formed by model tetrameric acidsFig. 11 shows the results of the characterization of Na4BP-10

solutions byDLS at differentNa4BP-10 concentrations. At BP-10 concen-trations much lower than its CMC (≈0.1 mM in pure water [57]), theautocorrelation function decays sharply, leading to a single modelradius distribution of dimensions close to 100 nm. This distributioncorresponds to BP-10 vesicles. At higher BP-10 concentrations, thecorrelation function decays in two steps, leading to a bimodal radiusdistribution. The smaller aggregates with dimensions of 1–2 nm aremicelles, which will be discussed in the following section. The presenceof vesicles is responsible for the larger size peak around 100 nm. TheBP-10 molecule will probably form a bilayer of 5 nm thickness. For a

Table 3Apparent acidity constants assuming that BP-10 has two pKa (called model 2 in Fig. 9) and apartition coefficient determined from the experimental data at different ionic strengths [58].

Ionic strength [mM] Cation pKa1 pKa

2

50 K+ 7.7 7.8184 (‘ + 1 wt.% KCl’) K+ 6.7 8.3184 Na+ 6.8 8.0452 (‘ + 3 wt.% KCl’) K+ 6.6 7.8

unilamellar/multilamellar vesicles the total aggregate can have dimen-sions close to 100 nm as found in our study. There is no surprise thatthe BP-10 molecule forms vesicles as it can be considered essentially adouble layer surfactantwithwater boundaries at both sides of carboxyl-ic end groups.

The uptake of 1-butanol into the vesicles is by far more complicated.When butanol is incorporated into the bilayer of BP10 the molecularpacking conditions are altered, changing bilayer formation conditions.The maximum radius ratio in the absence and presence of butanol isabout 2.3. Depending on the surface forces between the bilayers theswelling can vary. If an alcohol is incorporated into a lamellar structureit has been shown that the swelling between the bilayers can exceed10 nm. The incorporation of butanol can also promote the formationof multilamellar structures. Well-defined spherical vesicles are verifiedby cryo-TEM images (Fig. 12). Due to the shield of electrostatic repul-sion, the addition of salt increases the size of vesicles, which is in linewith the DLS results. In addition, the fusion of vesicles as indicated inFig. 12b is also observed.

The micelles formed by fully ionized BP-10 were analyzed by staticand dynamic light scattering (SLS and DLS) [22] as well as by Small-Angle Neutron Scattering (SANS) [16]. These studies (Fig. 11) haveshown that, in addition to vesicles, Na4BP-10 forms micelles at concen-trations higher than its critical micellar concentration (22 μM in 20 mMNaCl and 0.6 μM in 600 mMNaCl [27]), which is the typical behavior ofa surfactant. The Na4BP-10 micelle diameters increase from 2.6 to5.2 nm when the ionic strength varies from 20 to 600 mM due to thescreening of electrostatic repulsion by NaCl. The aggregation numberwas determined by static light scattering and values were found to beequal to 4 (SANS), 13 (SLS) and 21 (SLS) in respectively 20, 100 and600 mM NaCl solutions. The Na4BP-10 micelle structure does notseem to vary up to concentrations as high as 50 g·L−1 nor by addingbutanol at a concentration of 0.9 wt.%.

To conclude the size and properties of micelles formed by Na4BP-10do not seem to significantly differ from the micelles formed by “classi-cal” surfactants like sodium dodecylsulphate (SDS) even if their chemi-cal structures are significantly different. For instance, the aggregationnumber of the micelles for this surfactant increases from 27 to 95upon increasing the NaCl concentration from 0 to 0.05 M [64] and thehydrodynamic radius of the micelles is 2.4 nm in pure water [65].These features are similar to those measured for the anionic Na4BP-10concerning both the micelle radius and the aggregation number if weconsider that one molecule of BP-10 is equivalent to 4 SDS molecules(since BP-10 has four “arms”).

The structures formed by the fully ionized C80-TA in aqueous solutionare more peculiar than the ones formed by Na4BP-10. C80-TA is less solu-ble in aqueous solution than Na4BP-10 owing to its higher apolar charac-ter. Moreover Na4BP-10 can formπ–π interactions contrary to C80-TA dueto its aromatic core. C80-TA forms very large structure in pure D2O and20 mM NaCl solutions, so large that their exact dimensions could not bedetermined by SANS. The shape of the scattering curves, in particularthe fact that the slope is found to be near −2 in the intermediate wavevector-range, can be an indication that these particles are elongated andpossibly of oblate ellipsoidal shape, although other models may also bepossible. The reason for the difference in behavior between Na4BP-10and C80-TA is not fully understood even if the lower solubility of the lattermust play a role. The results obtainedwith this compoundneed to be con-firmed or refined using a C80-TA sample of higher purity, when available,to eliminate any doubt about the possible influence of impurities on themeasurements.

7. Properties of tetraacids at interfaces

In this section the surface properties of C80-TA and BP-10 at liquid/air and liquid/liquid interfaces are presented. It must be pointed outthat no divalent cation is present in the systems considered in thissection.

image of Fig.�9

n-C4H9OH

BP10Na4

L1 SD

L2

a n-C5H11OH

H2O H2O BP10Na4

L1S

L1

D

b

Fig. 10. Phase diagrams of Na4BP-10/n-C4H9OH/H2O (a) and Na4BP*10/n-C5H11OH/H2O (b) systems at 25 ± 0.5 °C. The inset is the polarizedmicrograph of samplewithin LLC region [22].


7.1. Surface activity of C80-TA and BP-10 at the liquid/air and liquid/liquidinterface

One of themain properties of the tetrameric acids is their surface ac-tivity. Indeed thesemolecules are strongly surface active at liquid/liquidinterface as shown in Fig. 13. This figure shows that the IFT decreaseswhen the C80-TA concentration increases. Interestingly low IFT valuesare reached at very low concentrations. For instance a decline of theIFT of about 40 mN/m is observed for a concentration of only 5 μMC80-TA. Under the same conditions naphthenic acids extracted fromcrude oil have shown to cause a lowering of the IFT by less than

1E-4 0.01 1 100 10000 1000000-0.05

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Rh/ nm

f(R

h)

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Rh/

f(R

h)

Fig. 11. Plot of intensity–intensity time correlation function (top) andnormalized hydrodynamic ran-C4H9OH contents (from left to right: 0, 1 and 5 wt.%) [22].

30 mN/m at 2000 times as high concentrations as reported by Brandalet al. [14]. This surface activity increases with the pH due to ionizationof carboxylic acid groups, which is classical in the case of naphthenicacids [62,66,67]. A comparison study with BP-10 have shown that thismodel compound capture the interfacial properties of C80-TA [18].

The Langmuir film properties of C80-TA have also been studied.Isotherms obtained at different pH values are presented in Fig. 14 [14].As films are compressed, a transition from gaseous-like to liquid–liquidfilm is apparent. It corresponds to the point where the C80-TA starts tointeract and the position of the hydrophobic tails of the molecules willchange from horizontal to vertical to the surface. The film goes then

100 10000 1000000 1E-4 0.01 1 100 10000 1000000-0.05

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Rh/ nm

f(R

h)

dius (Rh) distributions (bottom) inNa4BP-10/n-C4H9OH/H2O systemat variousNa4BP-10 and

image of Fig.�10

Fig. 12. Cryo-TEM images of vesicles in the systemof Na4BP-10/n-C4H9OH/H2O (a) andNa4BP-10/NaCl/H2O (b). The concentrations of Na4BP-10 are 10 mM. The content of n-C4H9OH andNaCl are 1.0 wt.% and 20 mM, respectively. The scale bars are 100 nm [22].


through a liquid like state as the slope of the isotherms increases. At aspecific point, however, the resistance of the film is reaching its maxi-mum after which film collapses. The initial monolayer is then brokeninto multilayers and/or forced into the aqueous solution, and a furthercompression leads to just a small influence on the surface pressure [14].

As observed from Fig. 14, the surface pressure reaches a higher valueat elevated pH. This is a result of electrostatic repulsion betweendissociated carboxylic groups. The pressure-area isotherms at pH 2.3and 5.6 show quite similar shapes. This finding indicates that the degreeof dissociation is low. At these pHs the surface pressure starts to increaseat a molecular area of about 160 Å2/molecule. The higher area permolecule reflects the extended molecular structure of the C80-TA, com-prising four surface active head groups, suggesting a folded conforma-tion of the TA molecules with all the carboxylic groups in contact withthe water surface. At higher surface pressures, the isotherms graduallyflatten out and reach a non-horizontal plateau before the pressurecontinues to increase until the collapse. This plateau is attributed to atransition of conformation. At low surface pressure the C80-TA adoptsa flat conformation with all the carboxylic groups in contact withthe water surface. When the plateau-like region is reached, two ofthe carboxylic groups are expelled from the surface. A scheme of thismechanism is presented in Fig. 15.

7.2. Interaction between mono-acid and C80-TA at liquid/air interface

The determination of Langmuir isotherms can successfully be usedto studymixed films formed from several components. The interactionsand miscibility between components can therefore be studied. This hasbeen done for the system C80-TA/4-n-dodecyl-cyclohexane carboxylicacid at the liquid/air interface [68]. As mentioned above no divalentcation is present in the system.

Fig. 13. Dynamic interfacial tension at different C80-TA concentrations. System: Toluene–hexadecane (10/90 v/v)/pH = 9 [14].

Using the Langmuir trough and BAMmicroscopy, Brandal et al. [68]studied the interactions between C80-TA and a C18 monoacid (4-n-do-decyl-cyclohexane carboxylic acid) hereafter noted MA. Fig. 16 showscompression of isotherms for mixed monolayers of C80-TA and MA onpure water at different C80-TA molar ratio XTA. The isotherms to thevery left and right are those of pure MA and C80-TA, respectively. Forpure MA, the rise in pressure starts at around 40 Å2/molecule and nophase transition area is visible before the collapse. For C80-TA, thepressure starts to rise at around 160 Å2/molecule then the isothermgradually flattens out and reaches a non-horizontal plateau before thepressure continues to increase until the collapse. The interpretationhas been presented in the previous section.

The isotherms of the mixed monolayers are located between thoseof the pure components. The miscibility of two components in a mono-layer can be examined by calculation of the excess area (Aex) of themixed monolayer at the water surface. The excess area is determinedby comparing the average area per molecule at a given value of surfacepressure of a mixed monolayer consisting of components 1 and 2 (A12)with that of an ideally mixed monolayer (Aid):

Aex ¼ A12−Aid ¼ A12− X1A1 þ X2A2ð Þ ð17Þ

where A12 is themeasured area and Aid is the ideal area (when no inter-actions occur). Ai and Xi are the area and fraction of pure component i.For an ideally mixed monolayer or when the components are immisci-ble, the excess area will be zero and A12 will increase in proportion toX1. In Fig. 17, the ratio excess area/ideal area is plotted as a function ofthe composition. Although all the mixtures of TA and MA exhibit anegative deviation, which implies interactions between the compo-nents in the entire concentration range, the most negative deviationappears at 60% TA, which suggests stronger interactions for that compo-sition which leads to best fitting or packing of the molecules.

Fig. 14. Surface pressure (SP) vs. molecular area (Å2/molecule) at different pH levels forC80-TA [14].

image of Fig.�13

image of Fig.�14

Fig. 15.Transition between different conformations of TAmolecules at air–water and oil–water interfaces uponmonolayer compression, obtained fromLangmuir trough experiments [68].


7.3. Emulsion-forming properties of C80-TA

The emulsion forming properties of C80-TA have been studied byDahl Hanneseth et al. [59]. C80-TA preferentially forms an oil-in-water(o/w) emulsion atwater cuts ranging from10 to 90% v/v and pHvaryingfrom 4 to 10. A similar behavior was noticed if C80-TA is replaced by amodel mono-acid: stearic acid and when emulsions are prepared froma mixture of C80-TA and stearic acid. The emulsion stability maximizedat intermediate water cuts and increased with increasing pH when itvaries from 4 to 10. This increase is probably due to an increase in elec-trostatic repulsions between droplets due to the ionization of carboxylicacid functions at pH higher than their pKa.

The authors have also studied the influence of presence of NaCl andCaCl2 on the type of emulsions formed [59]. Addition of NaCl to thesystem let to a phase transition from o/w to w/o. This trend is attributedto a “salting out” effect whichmakes C80-TAmore oil soluble. An increaseof NaCl concentration also led to a decrease of the emulsion stability. Thisvariation is attributed to a decrease of electrostatic repulsions betweendroplets due to the decrease of the Debye length when the NaCl concen-tration is increased. This trend is classical andwell-explained by theDLVOtheory, theory that describes the forces between charged surfacesinteracting through a liquid medium [69,70]. For Ca2+-containing sys-tems no phase inversion occurred. However at high water cut theaddition of CaCl2 led to emulsion destabilization and formation of solidcalcium naphthenate.

8. Bulk and interfacial reactions with M2+ and selectivity

8.1. Introduction

When a tetrameric-containing oil phase is put in contact with acalcium-containing water phase, reactions occur if the pH is highenough; ionization of carboxylic acid groups and reaction betweenformed carboxylate and divalent cation, typically calcium. The outcome

Fig. 16. Surface pressure area isotherms formixedmonolayers of C80-TA andMAat pH 5.6.Reproduced with permission from [68].

of these reactions is very easily put into evidence as illustrated in Fig. 18.The calcium salt of C80-tetraacid tends to form a film at interface whilethe calcium naphthenate formed by a model monoprotic naphthenicacid tends to be dispersed in the oil phase. In this section, the resultsobtained when studying these reactions are presented. Two differentkinds of approaches have been implemented:

– A thermodynamic approach inwhich the compositions of the oil andaqueous phases are determined at the thermodynamic equilibrium.

– An approach in which the properties of the interfacial film of tetra-meric acids and calcium ions are determined.

8.2. Thermodynamic approach

Simon et al. [72] studied the effect of different parameters (calciumconcentration, temperature, NaCl concentration and initial BP-10concentration) on solubility of tetrameric acids in oil/water systems.In order to fulfill this goal, they put in contact of an oil phase with anaqueous phase containing NaCl and CaCl2 at different pHs. The remain-ing concentration of BP-10 after reaction with calcium was determinedas a function of pH. Examples of curves presenting the remainingconcentration of BP-10 in model oil (chloroform) as a function of pHfor an initial concentration of 200 μM in the oil phase are presented inFig. 19.

Every curve presents the same shape: At low pH, the BP-10 concen-tration is constant and equal to the initial BP-10 concentration. At agivenpH, dependingon the calciumconcentration, there is a sudden de-crease of theBP-10 concentration due to precipitation of Ca2BP-10. Thenthe concentration of BP-10 decreases with pH until being non-detectable by HPLC. These curves have been fitted to a simple thermo-dynamic model taking into account the oil/water partition coefficientKwo [58,61,62], acid dissociation constant Ka [27], solubility productconstant Ks [73,74], and bulk mass balances. According to this modelthe equilibrium BP-10 concentration in oil when a precipitate is present

Fig. 17. Excess area/ideal area versus mole fraction of C80-TA of mixed monolayers ofC80-TA and MA.Reproduced with permission from [68].

image of Fig.�15

image of Fig.�16

image of Fig.�17

Fig. 18. Picture of oil (toluene)–water (pH = 9 with Ca2+) systems with calciumnaphthenate of the C80-tetraacid (sample to the left) and of a mono-acid (p-(n-dodecyl)benzoic acid) (sample to the right) after stirring. The calcium salt of C80-tetraacid tendsto form a structure at the interface while the calcium naphthenate formed by themonoprotic synthetic monoacid is dispersed in the oil phase [71].


i.e. when the solubility product constant of Ca2BP-10 is reached, mustfollow the following equation:

H4TA½ �o ¼ Ks � 10−4�pH

Ca2þ� 2 � Kwo � K4

a

¼ A � 10−4�pH ð18Þ

with : A ¼ Ks

Ca2þ� 2 � Kwo � K4

a

: ð19Þ

The downwards parts of the curves can be successfully fitted by thesimple model with A × [Ca2+]2 = 2.6 · 1018 mol−1·L+1 for calciumconcentrations of 100, 30 and 10 mM. The equation well-representsthe sharp decrease of the BP-10 concentration in the oil phase. Forlower pH, the Ks equation is not valid since there is no precipitate.For lower concentrations of 4 and 2 mM, the simple model slightlyunderestimates the precipitation pH by approximately 0.2 pH units.

pH4 5 6 7 8 9

[BP

-10]

o M

0.00000

0.00005

0.00010

0.00015

0.00020

0.00025

2 mM CaCl24 mM CaCl210 mM CaCl230 mM CaCl2100 mM CaCl2

Fig. 19. Concentration of BP-10 in oil phase, [BP-10]o, after 2 days of shaking as a functionof pH for different Ca2+ concentrations [72]. Conditions: [BP-10]o,i = 200 μM,[NaCl] = 100 mM. The lines represents the equation [BP-10]o = A.10−4 pH withA × [Ca2+]2 = 2.6 · 1018 mol−1·L+1.

It has been found that Eq. (18) has been able to fit the curves for allthe conditions tested. However, the determined value of the parameterA is not fully consistent with its definition presented in Eq. (19).

With this approach the influence of different parameters on the sol-ubility of tetraacid in the oil/water has been studied and is summarizedin Table 4.

The same approach has been used for ARN-containing systems. Ithas been found that the variations of the ARN concentration in the oilphase with the pH of the aqueous phase follow a different relationshipfrom the one found for BP-10:

C80TA½ �o∝10−2 pH: ð20Þ

The reasons that the behavior of the model compound, BP-10, doesnot accurately predict the behavior of the ARN acids is not clear at thistime. First, as mentioned above, no data on the possible dimerizationof BP-10 and C80-TA are available but some clues can be put forward.First the dimerization of tetrameric acids in oil phase has not beentaken into account when Eq. (18) was established, a phenomenon thathas been shown for mono-acids [75,76]. Secondly BP-10 containsaromatic rings which can, in principle, form π–π interactions with thepossibility of aromatic ring stacking. C80-TA backbone is purely aliphaticand has no possibility to form such interactions.

Another thermodynamic approach has been implemented byMohammed et al. [77,78]. In their first article [78] they measured thepH change in an oil/water systemwith no calcium due to the solubiliza-tion of naphthenic acid extracted from a naphthenate deposit. Theycompared the obtained pH values with a model assuming a single“pseudonaphthenic acid” taking into account its oil/water partitioncoefficient Kwo and acid dissociation constant Ka. It was found that themodel agrees qualitatively well with the experimental data but notquantitatively. Such finding means that a more accurate or completemodel is required. In a subsequent article [77], they extended theirapproach by considering the precipitation of calcium naphthenate char-acterized by a solubility product constant Ks. In addition to pHmeasure-ment they have also measured the mass of precipitates formed atdifferent conditions. The results show that the proposed model is ingood qualitative agreement with the batch experimental results. Inparticular the model predicts that there is a maximum in the mass ofcalcium naphthenate precipitates with thewater cut at around 60–70%.

Another approach aiming to better understand the formationmech-anism of calcium naphthenate deposits was proposed by Ge et al. [30].They investigated the interactions betweenNa4BP-10 and Ca2+ in aque-ous solutions mainly by isothermal titration calorimetry (ITC), which isa popular and high precision technique to study chemical and physicalinteractions. Fig. 20 presents enthalpy change as a function of divalentcation to Na4BP-10 molar ratio when 2.0 mM Ca(NO3)2 was injectedinto 0.1 mM Na4BP-10. At this concentration there is only presence ofvesicles (C b CMC). The figure shows that the interaction betweenCa2+ and carboxylate group of Na4BP-10 is endothermic (ΔHmix N 0).This positive (endothermic) apparent enthalpy value suggests apredominant contribution of dehydrating calcium ions to the overall en-thalpy of the reaction. In this figure two transitions of enthalpy changeare visible. At the first addition of Ca2+, a relatively high enthalpyvalue is determined, which decreases sharplywith increasing Ca2+ con-centration. This reduction in themolar enthalpywith increasing calciumion concentration is attributed to the reduced amount of unbound car-boxylic groups and decreased charge density of vesicle. The enthalpycurve shows a slope that can be extrapolated to a [Ca2+]/[Na4BP-10]value of 1, indicating a complete reaction of carboxylate groups on theexternal surface of vesicles. Within this concentration range, theadded Ca2+ appears to react only with the external carboxylate groupswith minimum transport through the membrane. After complete reac-tion of external carboxylate groups with calcium ions at [Ca2+]/[Na4BP-10] ratio of 1, the additional calcium ions added are able topenetrate to the inner side of the vesicle and react with unbound

image of Fig.�18

image of Fig.�19

Table 4Summary of experiments realized using the model system: BP-10 in a chloroform/water system with calcium, sodium chloride and buffer in the aqueous solution [72].

Parameter Range investigated Effect of parameter onto precipitation

NaCl concentration 20–600 mM Negligible: Less than 0.15 pH unitTemperature 5–50 °C Negligible: Less than 0.15 pH unitCaCl2 concentration 4–100 mM 0.9 pH unit variation, follows [BP-10]o = A.10−4 pH with A × [Ca2+]2 = 2.6 · 1018 mol−1·L+1

with a slight overestimation of BP-10 solubility at low calcium concentration (b10 mM)Initial BP-10 concentration 20–200 μM No effect: superimposes onto a “master curve”Comparison Ca/Mg – 0.6 pH unit difference

12

14 a BP10Na4/mM

0.1


carboxylate groups on the inner surface of membranes, shown by theadditional enthalpy. This mechanism is summarized in Fig. 22.

The variations of the enthalpy as a function of divalent ion to Na4BP-10 molar ratio measured at higher concentrations of Na4BP-10, wherethere is coexistence of micelles and vesicles (C N CMC) is presented inFig. 21. This figure shows that the molar enthalpy is much lower atconcentrations above the CMC than that below CMC. In the micelledominated system (10.0 mM Na4BP-10), the transport behavior ofcalcium ions through the vesicle membrane becomes subordinate.Since divalent calcium ions unlikely penetrate into the hydrophobiccore of the micelle, only reactions of calcium ions with external carbox-ylate groups of Na4BP-10 on micelle are seen in 10 mM Na4BP-10 solu-tions, leading to not only a lower molar enthalpy but also a gradualdecrease in molar enthalpy of reaction with increasing calcium toNa4BP-10 molar ratio up to 2.

In this section only the thermodynamic aspect of the precipitation oftetrameric acid has been considered. However some important aspectsof avoiding the formation of calcium naphthenate deposits in oil indus-try are thought to be kinetics in nature, such as the action of inhibitors[79]. That is why it is of paramount importance to study the reactionoccurring between oilfield tetrameric acids and calcium at interfaces.

8.3. Interfacial reaction

The reaction between tetrameric acids and calcium ions at liquid/liquid interface has been studied by means of the Langmuir troughtechnique [57], pendant drop technique [57,80] and interfacial shearrheology [41].

The study of films with the Langmuir trough is first considered. Theresults obtained with no calcium have already been presented inSection 8. Brandal et al. [14] studied the stability of C80-TA films formedby this technique at basic pH when the carboxylic groups are ionized.They noticed that in the absence of calcium, the films are unstablemost likely because of dissolution of ionized C80-TA in the watersub-phase. The film stability is increased by adding calcium ions to thesubphase. The dissociated molecules most likely react with Ca2+ atthe interface to form less soluble calcium naphthenates.

0 1 2 3 4 5 6 7

0

2

4

6

8

10

12b

[Ca2+]/[BP10Na4]

ΔHm

ix/(

kJ/ m

ol o

f inj

ecta

nt)

Fig. 20. Enthalpy change as a function of divalent ion to Na4BP-10 molar ratio when2.0 mM Ca(NO3)2 was injected into 0.1 mM Na4BP-10 at 25 °C and pH 10.2 [30].

The study of the reaction between BP-10 and divalent cations by thependant drop technique (Section 2.4.1) has been carried out byNordgård et al. [57]. They noticed that the addition of a divalent cationin BP-10-containing system induces a variation of interfacial tensioneither increase or decrease. This variation depends on the nature ofdivalent cations (Mg2+, Ca2+, Sr2+ or Ba2+) and the cation to BP-10molar ratio [57].

This technique can give qualitative information on the rheologyproperties of the interface in the presence of divalent cations: After for-mation of a droplet and subsequent interfacial reaction, the droplet ismanually compressed and decompressed (Fig. 23) [57]. The pictures ob-tained clearly show the presence of an interfacial film. The film appearsto be solid-like, and is crumpling like a paper. Similar “crumplings” havealready been observed with asphaltenic systems for instance [81].

In order to characterize this solid “skin”, interfacial rheology is atechnique of choice. Interfacial shear rheology is a technique that allowsus to determine the rheological properties of liquid/air and liquid/liquidinterfaces. This technique has been used for instance to studyasphaltenes [82–85], surfactants [40,86], proteins [87–89], polymers[86,90], microgels [91] .

The interfacial reaction between Ca2+ and BP-10has been studied asa function of pH, calcium concentration (not show here) and mono-acids. The first parameter investigated is the pH.

In Fig. 24a is both the interfacial elastic (storage) and viscous (loss)modulus shown at four pH values, 4.96, 6.00, 6.35 and 7.97. Fig. 24bshows the elastic modulus and G′/G″ ratio after 300 min. The ratiosshown in Fig. 24b correspond to phase angles in the range from 0 to10°, thus all films can be considered as near-elastic films. At pH 4.96and 6.00 any elastic modulus was not observed up to ca 120 min ofaging, after which G′ increases well above G″ and the film becomesviscoelastic. Increasing the pH however only slightly to 6.35, the filmis largely of an elastic nature after only a short period of time as can

0 1 2 3 4 5 6 7-2

0

2

4

6

8

10 10.0

[Ca2+]/[BP10Na4]

ΔHm

ix/(

kJ/m

ol o

f inj

ecta

nt)

Fig. 21. Enthalpy change as a function of divalent ion to Na4BP-10molar ratiowhen titrat-ing different concentrations of BP10 with Ca(NO3)2 at 25 °C and pH 10.2 [30].

Fig. 22. Scheme of interaction between calcium ions and Na4BP-10 vesicle and concentration dependence of Ca2+ [30].


be seen in Fig. 24a. The increase inG′ after 300 min is over two orders ofmagnitude from pH 6.00 to pH 6.35. These results suggest a drastic in-crease in the mechanical properties when the pH is exceeding 6–6.2.This value corresponds to the beginning of formation of a network atthe liquid–liquid interface formed by crosslinking of ionized BP-10 byCa2+.

Extra information on the structure formed at interface can be obtainedby performing frequency sweep. The results of such tests performed atthe end of the aging experiments presented in Fig. 24a are representedin Fig. 25.

The frequency sweeps in Fig. 25 show that both G′ and G″ are almostindependent of frequency at pH 6.50 with G′ N G″ (typical for a gel[92]). At lower pH values, the elastic modulus's are similar for pH 6.17and 6.00, but the viscous modulus at pH 6.17 is lower than that atpH 6.00. Compared to the maximum of G′/G″ in Fig. 24b. The observedmaximum is attributed to a decreased fluidity (viscosity) as progressivedeprotonation takes place and carboxylate groups start to cross-link.Increasing the pH to pH 6.5 will, as previously stated, enhance filmgrowth. However the decrease ofG′/G″ after themaximum is not clearlyunderstood.

All these results show that, at pHhigher than 6.0–6.2, three is forma-tion of a gel at the interface by reactions between COO− functions of BP-10 (4 arms) and Ca2+. These bonds or interactions crosslink BP-10mol-ecules through calcium to create an interfacial gel at water/oil interface.

Finally the study on the interfacial rheology properties of BP-10 inthe presence of a model mono-acid acid showed that the total amountof acids present in crude oils is largely dominated by mono-acids andnot C80-TA.

Fig. 26 shows the results of interfacial shear rheology experimentscarried out at pH 6.5 and a constant concentration of BP10 = 0.02 mMwith varying monoacid concentration.

The results by adding a model monoacid to the system show, from arheological point of view, that the properties of the calcium naphthenatefilm are in fact changed dramatically with monoacid concentration. It is

believed that the monoacid inhibits gelation by acting as a terminationagent, whereby the presence of one or more monoacid molecules inbetween two neighboring tetraacid molecules prevents calcium fromconnecting these tetraacid molecules to give the cross-linked network,crucial for gelation of such interfacial films. Hence, the presence of themonoacid terminates the cross-linking between neighboring tetraacidmolecules and calcium.

8.4. Cation selectivity

This part aimed to obtain a better understanding of the relativestrength of the bulk interactions between BP-10 and several cationsand evaluating whether BP-10 shows selectivity toward Ca2+. Twoapproaches were considered.

As a first approach to study the binding of divalent cations toa tetracarboxylic acid in an aqueous solution, potentiometricpH-titrationswere carried out at conditions that correspond to previousstudies without the presence of any divalent cation [27] as presented inSection 8.1. Since theM2+-ions compete with H+ for affinity to BP-10, ashift in the potentiometric titration curve in the acidic direction isexpected as a result of the presence of such ions.

In Fig. 27 the data from titrations in the alkaline direction is shown[28]. It is noticeable that the presence of an excess of all the four M2+-ions gives some shift of the titration curve in the acidic direction, showingthat H+ are released from BP-10 at lower−log [H+] when theM2+-ionsare present. For three of the M2+-ions; Ba2+, Ca2+ and Sr2+, this shift ofthe titration results is seen throughout most of the curve, while for Mg2+

this phenomenon is only seen at Z values between 0 and−1.5. Actually,the data at Z b −1.5 with Mg2+ is overlapping with the data of noM2+. This is the evidence of a quite weak bonding behavior betweenBP-10 and Mg2+.

As the sensitivity of the potentiometric technique is not sufficient tocompare Ca2+, Sr2+ and Ba2+, a second method was used. Precipitateswere prepared by adding dropwise a Na4BP10 solution into a second

Fig. 23. Images taken of droplets after reactions between BP-10 and divalent cations and manually contracting/expanding the droplets. Conditions: Droplets composed of BP-10 (1 mM)dissolved in 1-octanol in an aqueous phase (pH ≈ 7.4, with divalent cation). Molar ratio M2+/BP-10 = 3 [57].


solution containing 2 different divalent cations at the same concentra-tion. The pH of both solutions was previously adjusted to pH 9.5 sothat BP-10 is fully ionized and contained either 20 or 600 mM NaCl.After 1 day of stirring, the solutionswere filtered and the recovered pre-cipitates were washed and dried at 60 °C. By determining the composi-tion of the formed precipitates, a difference of selectivity between

Fig. 24. a) Interfacial shear rheology at 1 Hz and 0.01% strain of 0.02 mMBP-10 in 9:1 xylene:CHfunction of pH after 300 min of aging (solid line with circles, left y-axis) and corresponding G′

cations can be put in evidence. Fig. 28 shows the composition of precip-itateswith exposure to the same concentration of two different divalentcations. It can be seen that Ca2+ shows higher affinity towards BP-10than the other divalent cations. It can further be seen that Mg2+

(Fig. 28) shows the least reactivity while Ba2+ and Sr2+ showmediumand roughly equal reactivity (Fig. 28b and c) towards BP-10. The last

Cl3 and 20 mMCa2+ in 10 mM buffer solution, and b) interfacial elasticmodulus (G′) as a/G″ ratios (dotted line with triangles, right axis) [41].

image of Fig.�23

image of Fig.�24

Fig. 25. Interfacial shear rheology of BP10 and Ca2+ as a function of frequency. Experi-ments are performed subsequent to the aging experiments shown in Fig. 24a under thesame conditions. Same compositions as indicated in the legend of Fig. 24 [41].

Fig. 27. Titration data from potentiometric titrations of BP10 in 600 mMNaCl solutions inthe alkaline direction. The titration curves illustrate data from titrations with 3:1 M2+:BP10 with Ca2+, Ba2+, Mg2+ and Sr2+. Data from titrations performed without anyM2+ are shown for comparison. [BP10]0 = 0.3–0.4 mM in all titrations [28].


observation is further evident when a mixture of Ba2+ and Sr2+, with-out Ca2+ (Fig. 28d) was allowed to react with BP10. Another observa-tion is that the sum of ndivalent cations per BP-10 molecule is in all casesclose to 2, supporting that neutral 2:1 precipitates have been formed,as expected.

By combining the results, the following order of affinity between BP10and different cations is proposed; H+ ≫ Ca2+ N Ba2+ ≈ Sr2+ N

Mg2+ ≫ Na+. This order between the divalent cations is in good agree-ment with the number of hydrated water complexed to the cation in so-lution, in combination with the cation size, i.e. the shielding density fromwater molecules. Thus, it is considered very likely that Ca2+ probablydominates in naphthenate deposits due to both a higher concentrationin the saline water [93] as well as a higher selectivity.

9. Risk assessment

Predicting the operational problems with calcium naphthenate pre-cipitation involves many steps of both physicochemical and analyticalchemical analysis. It requires a fundamental knowledge of the tetraacidin oil phase, at w/o interface and in water phase.

With access to a sample of the potential production site, the firstconcern is to have a reliablemethod to determine the content of the tet-rameric acid. The approach here can involve an extraction procedure ofeither the deposit or alternatively the crude oil, as is the case for riskassessment. Literature reports several attempts to quantitatively

Fig. 26. Elastic modulus (G′) and G′/G″ ratio after 30 min aging at pH 6.5 as a function ofmyristic acid concentration in the presence of a fixed concentration of BP-10 (0.02 mM)at 20 mM Ca2+ in 20 mMMES buffer (pH 6.50) [41].

determine the concentration of calcium naphthenate down to levelslike 1 ppm or even lower. The researchers at TOTAL in Pau have devel-oped a LC–MS analysis [47], which was found to be sensitive althoughnot sufficiently quantitative. The Ugelstad Laboratory developed aquantitative method that works nicely for calcium naphthenate detec-tion [20]. The method can be applied on both deposits and acidiccrude oils [56]. The Ugelstad Laboratory approach is based on an extrac-tion and followed by a chemical grafting of a chromophore allowing fora subsequent LC-UV quantification. The procedure is described in detailin Section 5. The method has been validated by comparison with other“known” data provided by Total and Statoil. If the content of ARN is di-rectly related to the forthcoming fouling/deposition one should treatanalytical data with precaution. The results are of course very depen-dent on what kind of sampling techniques have been used. Thismeans that a concentration of calcium naphthenate needs a larger ap-proach/model to be applicable for operational purposes. What is not re-vealed in the analytical approach is the availability of the tetrameric acidto react with multivalent ions in an aqueous phase. The availability ofthe TAmolecule depends on conditions in the oil phase, at the interfaceand in thewater phase. The composition of the oil phase is critical to thecomplexation between tetrameric acids and constituents in the oilphase. If the interaction is of acid-base type, this will also neutralizeone or several of the carboxylic groups and prevent further reactions.When water (together with M2+) is added to the oil phase (containingTA) the situation is radically changed.With the formation of an aqueousphase and corresponding w/o interface there will be a redistribution ofthe components in the oil phase. With appropriate pH conditions in thewater phase the TA molecules are generally very interfacially active[14]. With pHs above the pKa the deprotonation is complete and theTA molecules will possess maximal interfacial activity. If we considerthe conditions in onew/o emulsion dropletwhere the interface containsdissociated TA molecules and the aqueous phase Ca2+ molecules, criti-cal parameters will be interfacial packing and final distance between TAmolecules (concentration). Critical for a cross-link between TA mole-cules and the propagation of the reaction is the area occupied byindividual TAs. Such data can be obtained from area determinations inthin films of TAs, i.e. for instance in Langmuir films. Another aspect,but related, is the virial coefficient between molecules in self-assembled states. The packing of TAs is influenced by incomplete disso-ciation or incorporation of competing interfacially active molecules,such as naphthenate inhibitors or simple monoacids present in thegross composition.

The selectivity of what kind of deposit will occur is very much deter-mined from reaction conditions at the emulsion interface, explicitly sowith regard to the size of multivalent counterions. The final size is

image of Fig.�25

image of Fig.�26

image of Fig.�27

Fig. 28. Cation composition of precipitates prepared by putting in contact of tetra-acid and cation solutionswith final concentrations of 2 mMof BP-10 and amixture of two salts of 4 mMeach, with the following mixtures: (a) Ca2+ and Mg2+, (b) Ca2+ and Sr2+, (c) Ca2+ and Ba2+, and (d) Ba2+ and Sr2 + 28.


dependent of the number of hydration molecules that are attracted byone molecule: For barium the number ≈ 0, for calcium the number ≈ 2and for magnesium ≈ 6. Under equal interfacial reaction conditions onewould predict a much higher propensity for Ca-naphthenate depositsthan for Mg-deposit, which has been observed from field tests andlaboratory experiments [28].

The interfacial reactivity of TA:s is loosely related to the formation ofw/o or o/w emulsions. The formation of w/o emulsions will immenselyincrease the reaction area betweenM2+ and TA. Under operational con-ditions the pH is approximately 6–8, where the interface constitutionwould be a combination of dissociated/undissociated TA. The TAmolecules alone will not stabilize the emulsion droplet but form amixed surface layer with other constituents in the crude oil. These are,among others, asphaltenes, resins, waxes and fatty acids. The packingof all the components including the TA molecules will dictate thereaction condition inside each droplet. The final formation of a depositwill depend on the stability of the w/o emulsion. The central questionis whether a coalescence between droplets will bring a more favorablegrowth situation for nuclei originating from individual droplets.Obviously we have a first step of interfacial nucleation which propa-gates through a coalescence step to a final growth and deposition/precipitation.

Acknowledgment

The authors thank the JIP-2 consortium “Prediction of Ca-naphthenatedeposition in water–oil systems”, consisting of AkzoNobel, BakerPetrolite, BP, Champion Technologies, Chevron, Clariant Oil Services,ConocoPhillips, ENI, Shell Global Solutions, Statoil ASA, Talisman Energyand Total, for financial support of the present work. The authors also

thank Dr. Thomas D. Baugh (ConocoPhillips) for critical reviewing of themanuscript.

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The chemistry of tetrameric acids in petroleum