Journal of Colloid and Interface Science 402 (2013) 19–26

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Journal of Colloid and Interface Science

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Interaction forces between silica surfaces in cationic surfactant solutions: An atomicforce microscopy study

Liset A.C. Lüderitz, Regine v. Klitzing ⇑Stranski-Laboratorium für Physikalische und Theoretische Chemie, Institut für Chemie, Technische Universität Berlin, Strasse des 17. Juni 124, D-10623 Berlin, Germany

a r t i c l e i n f o

Article history:Received 13 July 2012Accepted 5 November 2012Available online 28 November 2012

Keywords:AggregatesForcesMorphologyNanobubblesSilicaSurfactants

0021-9797/$ - see front matter � 2012 Published byhttp://dx.doi.org/10.1016/j.jcis.2012.11.007

⇑ Corresponding author. Fax: +49 30 31426602.E-mail address: klitzing@mailbox.tu-berlin.de (R. v

a b s t r a c t

The interaction forces between silicon oxide surfaces in the presence of surfactant solutions were studied.Based on the qualitative and quantitative analysis of these interaction forces the correlation with thestructure of the aggregates on the surfaces is analyzed. A colloidal probe atomic force microscope(AFM) was used to measure the forces between two colloidal silica particles and between a colloidal par-ticle and a silicon wafer in the presence of hexadecyltrimethylammonium bromide (CTAB) at concentra-tions between 0.005 mM and 1.2 mM. Different interaction forces were obtained for the silica particle–silica particle system when compared to those for the silica particle–silicon wafer system for the samestudied concentration. This indicates that the silica particles and the silicon wafer have different aggre-gate morphologies on their surfaces. The point of zero charge (pzc) was obtained at 0.05 mM CTAB con-centration for the silica particles and at 0.3 mM for the silica particle–silicon wafer system. This indicatesa higher charge at the silicon wafer than at the silica particles. The observed long range attractions areexplained by nanobubbles present at the silicon oxide surfaces and/or by attractive electrostatic interac-tions between the surfaces, induced by oppositely charged patches at the opposing Si oxide surfaces.

� 2012 Published by Elsevier Inc.

1. Introduction

The morphologies, functions, and applications of surfactants arediverse. There are natural surfactants such as the phospholipidprotein, a pulmonary surfactant [1] used to reduce the surface ten-sion in the lung. In addition synthetic surfactants are used inindustry, including textile, cleaning products, cosmetics, food,and others. Depending on the concentration the surfactant canact as a destabilizing or stabilizing agent. Therefore, it is importantto understand the interaction forces between two surfaces in pres-ence of the surfactant. Related to that, the morphology of the sur-factant aggregates on the surfaces has a dominant effect. Themorphology of the surfactant aggregates in the bulk depends onthe critical packing parameter [2,3]. Therefore, different aggregatemorphologies like spherical, cylindrical, globular, oblate micelles,single and multiwalled vesicles, microtubules, bilayers, lamellarphases, and inverted structures can be found depending on the sizeratio between head group and hydrophobic tail of the surfactant[3]. It is known that hexadecyltrimethylammonium bromide(CTAB) forms spherical micelles in the bulk at the critical micelleconcentration (CMC) [4–6]. Additional parameters, like surfacecharge, head groups and interaction between the hydrophobic tail,define the structure of the adsorbate on surfaces [4,7]. Several

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. Klitzing).

studies of the adsorption of surfactant to a hydrophilic surface, likemica and silica, have been performed [6–9]. Tyrode et al. [8] men-tion the two limit cases for adsorbing charged ionic surfactant at anoppositely charged hydrophilic surface below the CMC. For lowsurface charge systems, where the electrostatic interaction be-tween head groups and surface is weak, no monomer adsorptionat low concentration takes place. At a certain concentration, thecritical surfactant aggregation concentration (csac), aggregatesstart to adsorbe to the surface. No monolayer formation is presentin this system, since a monolayer with aliphatic chains facing to-wards the aqueous solution would be entropically unfavored.Due to hydrophobic interactions additional surfactant molecules(with the hydrophilic groups facing out to the water) will start toadsorbe before monolayer coverage is reached. In case of high sur-face charge, monolayer formation is favoured due to strong elec-trostatic attraction between the surfactant head groups and theoppositely charged surface. Subramanian et al. [4] studied the ef-fect of the counterion on the shape of the adsorbed aggregateson the silica surface with AFM and demonstrated that it is possibleto change the morphology of the CTAB from spherical to cylindricalby changing the counterion. With Cl� ions for example sphericalmicelles are obtained whereas with Br� ions cylinder aggregateson the silica surface are reported. Velegol et al. [9] obtained similarresults studying the adsorbed layer at concentrations from 0.9 mMto 10 mM. Other authors [10,11] reported the presence of micelleson the silica surface above the CMC studied with optical reflectom-

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etry and surface force apparatus respectively. Stiernstedt et al. [2]measured the surface force between silica particles across tetrade-cyltrimethylammonium bromide (TTAB) with a bimorph surfaceforce apparatus. The obtained adsorbed layer thickness is 4 nm.Rutland and Parker [11] could not conclude if a patchy bilayer orflattened micelles were present on the silica surface close to theCMC. They observed a strong dependency of the adsorption ofCTAB molecules on the surface charge of the particles. Raman scat-tering and sum frequency spectroscopy showed that the thicknessof the CTAB layer on silica particles is about 3 nm. Surface neutral-ization is obtained around 0.1 mM surfactant concentration. Nei-ther of the two principal models mentioned in the literature(‘‘The two step model’’ and ‘‘The four region model’’) can explainthe adsorption to the silica surface [8].

The surface forces in the presence of surfactants have beeninvestigated by several authors. Parker et al. [12] studied the inter-actions between glass particles with a surface force apparatusacross CTAB solutions of different concentrations. They observedattractive interactions between two silica surfaces for distancesat about 20 nm. The attractive forces could not be explained byvan der Waals forces and they were present after the charge rever-sal and at higher concentrations. Several researchers try to explainthe long range attraction observed between hydrophobic surfaces.Craig et al. [13] reported long range hydrophobic attraction ofabout 40 nm for cetylpyridimium chloride (CPC) in 100 mM NaCladsorbed to silica surfaces. Carambassis et al. [14] associate thelong range attraction between hydrophobic surfaces with the pres-ence of bubbles. In a further work Craig et al. [15] obtained aslightly less attractive hydrophobic force for adsorbed CPC layerson the silica surface in 100 mM NaCl when the surfactant solutionwas degassed. Although the influence of dissolved gas on thehydrophobic interaction was suspected, they could not prove thatnanobubbles may be responsible for the observed attractions. Ke-kicheff and Spalla [16] reported the correlation between the pre-factor of the long range hydrophobic interaction for silicaparticles in CTAB solutions and the ionic strength of the solutions,supporting the hypothesis that the long range attraction betweenhydrophobic surfaces may have electrostatic origin. In the workof Yaminski et al. [17] microcavitation between the adsorbedpatches of CTAB on the surface is considered to explain the largerange hydrophobic interactions. Pashley et al. [18] studied the phe-nomenon of cavitation in CTAB monolayer. No evidence of cavita-tion was observed for this system since the contact angle was lessthan 90�.

So far, no systematic study of the interaction forces in the pres-ence of cetyltrimethylammonium bromide (CTAB) has been carriedout using AFM and still the mechanism for the long range hydro-phobic attraction present in those systems is debated. In this workthe interaction forces between a pair of silica particles (system I)and between a silica particle and a silicon wafer (system II) in CTABsolutions were measured over a large range, from well below theCMC to concentrations above the CMC. A prediction of the aggre-gates structure on the silicon oxide surfaces in dependence on sur-factant concentration is established based on the qualitative andquantitative analysis of the interaction curves.

2. Experimental section

2.1. Materials

A suspension of silica particles of 4.63 lm in mean diameter(10% solid content) was purchased from Bangs Laboratories. Solu-tions of cetyltrimethylammonium bromide (CTAB, analytic grade,Aldrich) were prepared in a concentration range from 0.005 mMto 1.2 mM in pure water (Milli-Q). CTAB was water soluble up to

1.2 mM at room temperature. Clear solutions were obtained atall concentrations, which shows that the experiments were per-formed above the Krafft temperature.

2.2. Methods

A colloidal silica particle was glued to the end of a tip-less AFMcantilever (CSC12, l-mach, Lithonia) with a nominal spring con-stant of 0.03 N/m. Another particle was glued to a glass slide (Men-zel-Gläser, Germany) using an optical microscope and amicromanipulator. The colloidal probes and the glass slides withattached particles were cleaned with ethanol and water and placedin an air plasma cleaner for 20 min (Diener electronic. Femtotimer). The silicon wafers (type-P) Wacker Siltronic Burghausenwere cut and cleaned in piranha solution H2O2/H2SO4:50/50 for30 min. Thereafter, they were washed with milli-Q water and thenimmediately used for the experiments. This method allows the cre-ation of an oxide layer on the silicon wafer surface and renders thesurface highly hydrophilic.

The force measurements between the two silica particles [19]were performed using a MFP-3D Asylum Research atomic forcemicroscope mounted on an inverted optical microscope (OlympusIX71). This technique is well described elsewhere [20–22]. In brief,a cantilever with the colloidal probe is fixed at the AFM head andthe glass slide with the attached particle is placed on the scanner.The two opposing particles are optically aligned. A laser is pointedat the end of the cantilever. The cantilever moves in the z-directionand the deflection of the cantilever while approaching the surfacesis registered by a photosensitive detector. The spring constant isdetermined using the thermal noise method. In the following onlyapproach curves are shown. At least 50 repetitions were done.After each measurement an optical microscope was used to check,if the particle was still attached to the cantilever. For the particle–particle system this check was performed during the measure-ments using an inverse microscope placed in the AFM. The mea-surements performed with one pair of spheres are reported. Thevelocity of the approach was 600 nm/s. The measurements withMPF-3D were obtained at room temperature at 1 atm.

The zeta potentials were measured using a Malvern ZetasizerNano ZS with a HeNe laser. Different solutions of CTAB from0.005 to 1.2 mM containing 0.1% silica particles were prepared.These suspensions were placed in a clear disposable zeta cell(DTS1060C) and equilibrated for 2 min at 25 �C in the equipmentbefore starting measurements [23]. The average of 10 zeta poten-tial measurements of the same sample was taken as the zeta po-tential value.

2.3. Simulations

The simulations are based on the DLVO theory. The DLVO the-ory only takes two kinds of forces into account to explain the sta-bility of colloids in a suspension: electrostatic double layer and vander Waals forces. The electrostatic interactions are calculated solv-ing numerically the non-linear Poisson–Boltzmann equation fortwo parallel plates using two boundary conditions, constant chargeand constant potential. The van der Waals forces are the sum of theinteractions between atomic and molecular dipoles in the particlesor between different particles [24]. According to Borkovec et al.[25] the Derjaguin approximation is still valid for a particle sizearound of 4.63 lm. The algorithm used was proposed by Chanet al. [26]. The software used was written by McKee [27] basedon the algorithm of Chan [26]. In the algorithm the electrostaticinteractions are computed across symmetrical electrolytes. Theplanar Poisson Boltzmann equation is integrated to obtain a differ-ential equation, and using the two boundary conditions, the elec-trostatic free energy of interaction is calculated for each distance

L.A.C. Lüderitz, R.v. Klitzing / Journal of Colloid and Interface Science 402 (2013) 19–26 21

from x = xmidplane to x = 0 (at the surface) by a numerical technique(e.g. a fourth order Runge–Kutta method). A water dielectric con-stant � = 78 and a Hamaker (Si/water/Si) constant = 8.5 � 10�21 J[28] is assumed.

3. Results

3.1. Interaction forces between two silica particles (system I)

Fig. 1 shows the interaction forces between two silica particlesat a surfactant concentration from 0 to 0.1 mM. The pH of the solu-tions was around 5.8 where silica is negatively charged. The diam-eter of the particles was 4.63 lm. The fitting of the experimentalcurves to the DLVO theory provides the potential, u and ionicstrength, I. Each experimental curve was fitted with both boundaryconditions, constant charge and constant potential. For reasons ofclarity, in Fig. 1 only the fit for the experimental curve at 0.1 mMionic strength is shown. The constant charge boundary conditionfits very well at larger distances but for distances smaller than20 nm, the experimental curve lies between the two boundary con-ditions. Neither the charge nor the potential remains constant. Therespective potentials, u and decay length, k�1 are shown in Table 1.The repulsion decreases with increasing surfactant concentrationfrom 0 to 0.05 mM. The interaction in water was repulsive for alldistances. Below 5 nm a small attraction was observed which canbe interpreted as a van der Waals force. At 0.005 mM repulsionwas observed for the whole range of studied forces. The diffuselayer potential u was decreased from �30 mV (for water) to�14 mV and the Debye length k�1 of 42.87 nm was similar to theDebye length of water and corresponds to an ionic strength I of0.005 mM. In force measurements, only the value of the potentialcan be determined. Based on zeta potential measurements underthe same conditions, the sign of the potential is inferred. A furtherincrease of the CTAB concentration gave a weaker repulsion at sep-aration >10 nm, and for smaller distances attraction was observed.At 0.03 mM surfactant concentration only weak repulsion was de-tected, and at 0.05 mM CTAB concentration no repulsion was ob-served at all, only attractive interaction with a jump-in distancearound 20 nm. When the concentration was increased further to0.1 mM repulsion occurred again. This leads to the conclusion that

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water, pH=5.8,

Fig. 1. Forces between a pair of colloidal silica particles (system I) across CTABsurfactant solution, from 0 to 0.1 mM surfactant concentration. Hamaker constantA = 8.5 � 10�21 J. DLVO_CC (constant charge) and DLVO_CP (constant potential) fitsare shown for 0.1 mM surfactant concentration.

the point of zero charge (pzc) is between 0.05 and 0.1 mM CTABconcentration.

In Fig. 2 the respective interaction forces are shown for a con-centration regime from 0.1 to 0.5 mM. For a better understandingonly the best fits to the experimental curves are shown. The con-stant charge boundary condition is fitting the experimental curvereally well at 0.2 and 0.3 mM for distances larger than 10 nmwhere repulsion was observed. At smaller distances short rangeattraction dominates the interactions. At 0.4 mM, the constant po-tential fits better almost until contact. Repulsive interactions areseen for larger distances and at smaller distances only a smallattraction is observed. At 0.5 mM the experimental curve lies be-tween the constant charge and the constant potential boundaryconditions for distances smaller than 20 nm. No short range attrac-tion is seen anymore, the interactions are monotonic repulsive atthis concentration. The diffuse layer potential u increased from0.1 mM to 0.4 mM surfactant concentration (see Table 1). The De-bye length k�1 correlates well with the theoretical one and wasdecreasing with increasing surfactant concentration, as expected.

In the concentration regime from 0.5 to 1.2 mM CTAB (seeFig. 3) repulsion dominated the interaction over the whole range.The fits shown for 0.5 mM (Fig. 2), 0.8 mM and 1.0 mM (Fig. 3)were calculated assuming that the surfaces were in contact. Theconstant charge and constant potential boundary conditions fitwell at larger distances. For distances smaller than 20 nm, theexperimental curve lies between the two boundary conditions.No adhesion in the retraction curves (data not shown) was seenfor concentrations larger than 0.5 mM. An unexpected decreasein the repulsion with increasing surfactant concentration was ob-served in this concentration regime. Therefore the DLVO fits werealso calculated assuming that the surfaces were not in contact tak-ing into account the adsorption of aggregates that cannot be re-moved from the surface by the applied force. Under thisassumption, the constant charge boundary condition fits the exper-imental curves well until contact (data not shown). The fitted dif-fuse layer potentials are reported in Table 1).

3.2. Interaction forces between a silica particle and a silicon wafer(system II)

In order to get information about the adsorption of surfactant atthe silicon wafer, force curves were recorded for interactions be-tween a silica particle and a silicon wafer (system II) for the sameCTAB concentration range (0.005–1 mM). The interaction curveswere fitted with the DLVO theory. The Derjaguin approximationis still valid for this system and since it is difficult to simulatetwo different surfaces, a symmetric system (two planar surfaces)was assumed. The obtained diffuse layer potential serves to showthe changes in the system. The fitted diffuse layer potentials are re-ported in Table 1. The force curves for two concentrations below0.3 mM are shown in Fig. 4. Only attraction was observed for theseconcentrations and the jump-in distance was around 20 nm for0.005 mM and 50 nm for 0.05 mM. The interactions at 0.1 mMand 0.2 mM (data not shown) remain attractive with a jump-in dis-tance around 50 nm. The interactions between two silica particles(system I) at 0.05 mM (jump-in distance around 15 nm) are alsorepresented in this figure. It can be seen that the attraction is largerin magnitude and range for system II.

Fig. 5 shows the force curves for concentrations ranging from0.3 mM to 0.8 mM. At 0.3 mM CTAB concentration the observedattraction has a jump-in distance of about 43 nm and no repulsionoccurs at long range. From 0.4 mM up to 0.8 mM repulsion was ob-served for distances larger than 20 nm. The better DLVO fits areshown for 0.4, 0.5 and 0.8 mM surfactant concentration. At0.4 mM the constant charge as well as the constant potentialboundary condition fits the experimental curve very well at dis-

Table 1Results for simulations of direct force measurements by the Poisson–Boltzmann theory.

CTAB concentration(mM)

Diffuse layer potential system (I) u(mV)

Diffuse layer potential system (II) u(mV)

Experimental decay length k�1

(nm)Zeta potential f(mV)

0.00 �30.0 ± 0.0 �45.0 ± 0.0 42.2 �55.00.005 �14.0 ± 1.6 0.0 42.9 0.00.008 �15.0 ± 0.0 0.0 42.9 0.00.01 �15.0 ± 0.0 0.0 42.9 0.00.03 0.0 0.0 n.d. +21.3 ± 4.20.05 0.0 0.0 n.d. +26.9 ± 5.30.10 +36.0 ± 0.0 0.0 30.0 +20.5 ± 2.00.20 +48.0 ± 0.0 0.0 21.4 +35.0 ± 6.80.30 +48.6 ± 2.8 0.0 17.5 +62.7 ± 3.60.40 +72.2 ± 6.2 +46.0 ± 2.6 15.1 +58.8 ± 12.40.50 +57.2 j +75.0⁄⁄ ± 7.5 +75.0 j +50.0⁄ ± 5.0 13.6 +95.8 ± 1.30.80 +38.2 j +60.0⁄⁄ ± 0.6 +158.0 j +80.0⁄ ± 2.7 10.7 +108.0 ± 7.11.00 +30.0 j +45.0⁄⁄ ± 0.0 +44.0 j +85.0⁄⁄ ± 5.3 8.8 +127.0 ± 2.71.20 +30.0 j +45.0⁄⁄ ± 0.0 Not measured 8.78 +111.0 ± 3.9

⁄ From fitting with the plane of origin of charge taken at 4 nm from each surface.⁄⁄ Assuming that the surfaces were not in contact (micelles/bilayer adsorption).

22 L.A.C. Lüderitz, R.v. Klitzing / Journal of Colloid and Interface Science 402 (2013) 19–26

tances larger than 20 nm. The interactions for distances smallerthan 20 nm were attractive. At 0.5 mM the better fit is obtainedby the constant charge boundary condition. A non-DLVO repulsionwas observed from 20 nm down to 10 nm followed by a plateaudown to contact. At 0.8 mM also the constant charge boundarycondition fits better and a non-DLVO repulsion from 20 nm downto 13 nm was observed followed by a plateau down to contact. Itis important to note that the plateau did not occur between silicaparticles (see Fig. 3). Since a plateau around 10 nm was observedat 0.5 and 0.8 mM corresponding to the adsorption of aggregateson the surface, the DLVO fits at these concentrations were calcu-lated with the plane of charge displaced by 4 nm from each surface(see Table 1). The obtained fitted diffuse layer potentials for theunshifted plane of charge are shown in Table 1. At 1 mM the inter-action was monotonically repulsive (Fig. 6). The experimentalcurve was shifted by 8 nm, considering the presence of aggregateson the surface, which could not be removed by the applied force atthis concentration. The original and the shifted curves are bothshown with the corresponding fits, the obtained diffuse layerpotentials are given in Table 1.

3.3. Point of zero charge

From Fig. 1 one can conclude that the point of zero charge (pzc)for silica particles (system I) occurred at about 0.05 mM surfactantconcentration. Since a symmetric system is measured, the sign ofthe potential can also be inferred from the trend of the forcecurves. The interactions are changing with concentration, fromrepulsion at low surfactant concentration to attraction at0.05 mM (pzc) to repulsion again at higher concentrations. Silicaparticles are negatively charged in water at normal pH [30]. There-fore, a negative sign is assumed for surfactant concentrations be-low 0.05 mM and a positive charge for concentrations above0.05 mM. The dependence of the charge of the silica on surfactantconcentration and the point of zero charge was confirmed by zetapotential measurements. The zeta potentials and the fitted diffuselayer potentials are shown in Table 1. For surfactant concentrationsbelow 0.03 mM the zeta potentials values were low and no stablevalues were obtained. From 0.03 mM onwards, positive values ofzeta potentials were obtained, meaning that the pzc in zeta poten-tial measurements was around this concentration. The zeta poten-tial increased with increasing surfactant concentration indicatingan increase in adsorbed amount of surfactant. For system I the fit-ted diffuse layer and the zeta potentials show the same tendency atconcentrations below 0.4 mM. Both are increasing with surfactant

concentration. At concentrations above 0.4 mM the zeta potentialcontinuously increased with concentration, as expected, whereasthe fitted diffuse layer potential shows an ‘‘apparent’’ decrease.From Fig. 5 the point of zero charge for system II can be deduced.It occurred at about 0.3 mM surfactant concentration.

4. Discussion

4.1. Interaction between two silica particles (system I)

The fitted potentials are called ‘‘effective potentials’’ becausealthough the DLVO theory is applied for the analysis, there areother non-DLVO forces which influence the interactions, likehydrophobic, hydration and steric forces. From zeta potential mea-surements it is known that the silica particles have a negative sur-face potential in water. The addition of 0.005 mM CTAB to watercauses a decrease in the diffuse layer potential from u = �30 mVto u = �14 mV (see Fig. 1). The interaction curve is still repulsive.The potential is decreased due to the adsorption of positivelycharged surfactant monomers to the silica particles. The attractionobserved at short range for the interaction in water was overcomeat this surfactant concentration and short range repulsion wasobserved.

A correlation between the measured interaction curves and thepossible surfactant structures on the silica surface can be estab-lished. From the quantitative analysis of the interaction curve(the fitting to the DLVO theory) it is known that adsorption oc-curred at 0.005 mM because the diffuse layer potential is decreasedwith respect to the diffuse layer potential for the interactions inwater. Based on the qualitative analysis of the force curve at thisconcentration a parallel arrangement is the most probable (seeFig. 7). If the surfactant molecules were arranged perpendicularto the surface, attraction would be observed at short range dueto hydrophobic interactions. Since short range repulsion was ob-served, it is more probable that the surfactant monomers were ar-ranged in parallel to the surface and that the observed short rangerepulsion was due to the dehydration of the ammonium headgroups. At this concentration the hydration forces overcome thehydrophobic forces and/or van der Waals forces (see Fig. 7). Otherauthors [8] reported a perpendicular arrangement of the surfactantmolecules with the hydrophobic tails facing towards water at lowsurfactant concentration.

Increasing the concentration further from 0.008 mM to0.05 mM (see Fig. 1) turned the interactions from repulsive toattractive. More and more surfactant molecules interact electro-

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water, pH=5.8, 0.1 mM 0.2 mM 0.3 mM 0.4 mM 0.5 mM DLVO_CC DLVO_CP

Fig. 2. Forces between a pair of colloidal silica particles (system I) across CTABsurfactant solutions from 0.1 to 0.5 mM surfactant concentration. Hamakerconstant A = 8.5 � 10�21 J. DLVO_CC (constant charge) and DLVO_CP (constantpotential) fits are shown for 0.2 mM (DLVO_CC), 0.3 mM (DLVO_CC – overlaps fit at0.2 mM), 0.4 (DLVO_CP) and 0.5 mM (DLVO_CC and DLVO_CP) surfactantconcentration.

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water, pH=5.8 0.5 mM 0.8 mM1 mM 1.2 mM DLVO_CC DLVO_CP

Fig. 3. Forces between a pair of colloidal silica particles (system I) across CTABsurfactant solutions from 0.5 to 1.2 mM surfactant concentration. Hamakerconstant A = 8.5 � 10�21 J. DLVO_CC (constant charge) and DLVO_CP (constantpotential) fits (assuming that surfaces are in contact) are shown for 0.8 and 1 mMsurfactant concentration.

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0.005 mM (silica particle-silicon wafer) 0.05 mM (silica particle-silicon wafer) 0.05 mM (silica particle-silica particle)

Fig. 4. Forces between a colloidal silica particle and a silicon wafer (system II)across CTAB surfactant solutions at 0.005 and 0.05 mM surfactant concentration.Forces between two colloidal silica particles (system I) at 0.05 mM surfactantconcentration. Hamaker constant A = 8.5 � 10�21 J.

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0.3 mM 0.4 mM 0.5 mM 0.8 mM DLVO_CC DLVO_CP

Fig. 5. Forces between a colloidal silica particle and a silicon wafer (system II)across CTAB surfactant solutions from 0.3 to 0.8 mM surfactant concentration.Hamaker constant A = 8.5 � 10�21 J. DLVO_CC (constant charge) and DLVO_CP(constant potential) fits are shown for 0.4 mM surfactant concentration. For theDLVO_CC fits shown at 0.5 mM and 0.8 mM the plane of charge was set 4 nm awayfrom each surface. The data at 0.4 mM and 0.5 mM surfactant concentration wastaken from [29].

L.A.C. Lüderitz, R.v. Klitzing / Journal of Colloid and Interface Science 402 (2013) 19–26 23

statically with the silica surface. That is the most probable interac-tion because the silica particles are hydrophilic and negativelycharged and the CTAB head group is positively charged. At0.05 mM only attraction was observed with a jump-in distance ataround 20 nm. The range is larger than for pure van der Waalsforces, which indicates that additional interactions (like hydropho-bic ones) come into play. The hydrophobic interactions observed inthe force curves suggest that the most probable arrangement ofsurfactant aggregates is perpendicular to the surface (see Fig. 7).The point of zero charge (pzc) was obtained at this concentration.Parker et al. [12] also observed neutralization of the silica surfaceat this concentration. Although the range of the interaction wassimilar, the magnitude was larger in Parker’s work. Yaminskiet al. [17] reported the point of zero charge for silica at the sameconcentration and argued that below this concentration the forma-tion of patches on the surface is possible, since the energy gain is

larger for patches than for isolated molecules. A further increasein the surfactant concentration to 0.1 mM resulted in a repulsivelong range interaction meaning that a charge reversal from nega-tive to positive occurred. The obtained potential was +36 mV.The sign of the potential can be inferred from the trend of the forcecurves and was also confirmed by zeta potential measurements.The interaction cannot be fitted by the constant charge/potentialmodel for distances smaller than 20 nm indicating that at smallerdistances there is a variation of both, the surface charge and thesurface potential. At distances <10 nm attraction was observed,which may be due to the expulsion of aggregates present on bothsurfaces. Near to contact, for distances <2 nm, repulsion was ob-served again which may be steric in origin. The charge reversalof the surface is associated to the adsorption of surfactant mole-cules with the hydrophilic head facing out to the water. The aggre-gates can easily be removed during the force measurements (seeFig. 7). The repulsion continued increasing with increasing surfac-

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1 mM 1 mM (offset 8 nm) DLVO_CC1

DLVO_CP1 DLVO_CC2DLVO_CP2

Fig. 6. Forces between a colloidal silica particle and a silicon wafer (system II)across 1 mM CTAB surfactant solution. Hamaker constant A = 8.5 � 10�21 J. Theexperimental curve was offset 8 nm under the assumption that micelles/patchybilayers were adsorbed to the surface. DLVO_CC (constant charge) and DLVO_CP(constant potential) fits are shown for the experimental curve (DLVO_CC1,DLVO_CP1) and the shifted curve (DLVO_CC2, DLVO_CP2). Data taken from [29].

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Fig. 8. Interaction forces between two silica particles (system I) and between aparticle and a silicon wafer (system II) at 0.4 mM surfactant concentration. TheDebye length was 15.2 nm for both cases.

24 L.A.C. Lüderitz, R.v. Klitzing / Journal of Colloid and Interface Science 402 (2013) 19–26

tant concentration (Fig. 2). The effective potentials are shown inTable 1. At 0.2 and 0.3 mM the constant charge boundary conditionmatches the experimental curves really well at larger distances.For distances smaller than 10 nm no fit is seen at all because theinteractions are not DLVO like and correspond to the expulsionof aggregates from the surface. At 0.3 mM the attraction is weakand starts to be replaced by a soft short range repulsion. At0.4 mM surfactant concentration, the experimental curve is bestfitted with the constant potential boundary condition until smallerdistances. A potential of 72 mV was obtained. The change in shortrange interaction from attraction at 0.2 mM to soft repulsion at0.3 mM finishing with a strong repulsion at 0.4 mM can be corre-lated with the stiffness of the aggregates adsorbed to the surface.The aggregates are closer and stiffer with increasing concentration(see Fig. 7).

A further increase in concentration to 0.5 mM led to a decreasein the diffuse layer potential to +57 mV (see Table 1). The diffuselayer potential decreased until the critical micelle concentrationwas reached where it stayed constant (see Fig. 3). This decreaseis unexpected. The experiments were repeated with another pairof silica particles showing the same tendency (data not shown).

Fig. 7. Possible surfactant morphologies depen

Interestingly, the interaction curves cannot be fitted by the con-stant charge/potential model at distances smaller than 20 nm.The fits were corrected for concentrations from 0.5 mM onwards,under the assumption that the surfaces were not in contact dueto the adsorption of micelles/patchy bilayers on the surfaces. Stilla decrease in diffuse layer potential is observed from 0.5 mM on-wards (see Table 1). That is explained by condensation of Br� coun-terions to the highly charged surfaces (surfaces with a denser outerlayer of surfactant) during the force measurements. Hence lowerdiffuse layer potentials are obtained. In contrast the zeta potentialincreases with increasing surfactant concentration (see Table 1), asexpected. The interactions are monotonic repulsive in this concen-tration regime (from 0.5 mM to 1 mM) indicating that the aggre-gates are stiffer with increasing concentration.

4.2. Interaction between a silica particle and a silicon wafer (system II)

The interaction of a silica particle with a silicon wafer is repre-sented in Figs. 4–6. At concentrations as low as 0.005 mM a largeattraction was observed. This could be caused by aggregate patchesof CTAB with the hydrophobic tails facing out to the water presenton the surface of the silicon wafer (see Fig. 7). At 0.05 mM the

ding on the concentration (not to scale).

300

250

200

150

100

50

0

nm

nm

0 50 100 150 200 250 300nm

4

3

2

1

0

-2nm-1012

300250200150100500nm

nanobubble

micropancake

Hei

ght

Fig. 9. AFM tapping mode of a silicon wafer at 0.3 mM surfactant concentration.

L.A.C. Lüderitz, R.v. Klitzing / Journal of Colloid and Interface Science 402 (2013) 19–26 25

interaction between a silica particle and an oxidized silicon waferstill remained attractive. At this concentration the silica particlewas neutralized, that means that the observed attraction has tobe produced by hydrophobic interactions between the two sur-faces. Therefore, it is probable that the surfactant was adsorbedat the silicon wafer surface with the hydrophobic tails facing outto the water, as shown in Fig. 7. The long range attraction occurredup to 0.3 mM (see Fig. 5). It is known from force and zeta potentialmeasurements that at this concentration the silica particle waspositively charged. Hence the observed attraction may be due toelectrostatic interaction between the already positive silica particle(charge reversal at 0.05 mM) and the bare silicon wafer (the areaswithout surfactant adsorption). Therefore, it is proposed that thesurfactant adsorbed patchwise to the silicon wafer surface (seeFig. 7). The validity of this statement is confirmed later in this pa-per (see Fig. 9).

Fig. 5 shows the interactions between a silica particle and a sil-icon wafer from 0.3 to 0.8 mM. The point of zero charge for thissystem was 0.3 mM. At 0.4 mM a long range double layer repulsionwas present. The short range attraction started at around 17 nmand is non-DLVO. The surfactant starts to be adsorbed with thehydrophilic tails facing out to the water (see Fig. 7). At 0.5 mMthe short range interaction occurred at around 8 nm which corre-sponds to the expulsion of micelles/patchy bilayers from the twoopposing surfaces. At 1 mM (CMC) complete micelles/patchy bilay-ers were present on the silica/silicon wafer surfaces, and they couldnot be removed under the applied force. Two DLVO fits were done.The results are shown in Table 1. The experimental curve can befitted well with a diffuse layer potential of 44 mV, assuming thatthe surfaces (silica/silicon wafer) are in contact (D = 0) or with adiffuse layer potential of 85 mV assuming that the surfaces (sil-ica/silicon wafer) are separated by 8 nm due to the adsorption ofmicelles/patchy bilayers. The non-DLVO repulsion observed inthe shifted curve can be explained by steric forces between the mi-celles/patchy bilayers (see Fig. 6).

4.3. Comparison between the silica particle–silica particle system (I)and the silica particle–silicon wafer system (II)

A comparison of the interactions in the two systems gives thefollowing results. At low CTAB concentration (e.g. 0.05 mM) theattraction is larger for system II. Since the attractions were dueto hydrophobic interactions, the silicon wafer was more hydropho-bic than the silica particles. Consequently the hydrophobic patcheswere larger at the silicon wafer (see Fig. 4). At 0.4 mM surfactantconcentration (Fig. 8) the Debye length correlates well with thetheoretical one for both systems but the interaction looks different.A greater repulsion was obtained for system I compared to systemII. That indicates that the outer layer of surfactant was more denseat the silica particle surface. The qualitative analysis of both sys-tems from 0.4 mM onwards led to the conclusion that it was notpossible to remove the aggregates from the silica particles underthe applied force in system I whereas aggregate expulsion wasseen for system II up to 0.8 mM. The aggregates at the silica parti-cles must be stiffer and more closely packed than on the siliconwafer surface from 0.4 mM onwards. In general, the aggregatemorphology at the silica particles was different than at the siliconwafer, which was verified with the different force curves obtainedfor the same concentration. The silica particles and the silicon wa-fer have the same surface chemistry, but the surface treatment hasa great influence on the surface charge and hence the type ofaggregates on the surface. Other authors confirm that differentpreparation methods cause differences in surface charge [31].The point of zero charge (pzc) for system I was obtained at0.05 mM surfactant concentration and for system II at 0.3 mM.That indicates that the silicon wafer carries a larger surface charge.

4.4. Non-DLVO forces

Non-DLVO forces were observed in system I and II at 0.05 mM.The jump-in distance for system II was about 43 nm which is larger

26 L.A.C. Lüderitz, R.v. Klitzing / Journal of Colloid and Interface Science 402 (2013) 19–26

than that for system I. The bridging mechanisms through nanobub-bles is an explanation for the hydrophobic forces observed at thisconcentration. In previous experiments performed in our groupnanobubbles were found on the silicon oxide surface at 0.05 mMsurfactant concentration [29]. When two surfaces with nanobub-bles are approaching, a thin free–standing lamella is formed be-tween the bubbles. When it breaks the bubbles will bridge and ajump-in contact will be observed in the force curves. At 0.3 mMa large attraction was observed for system II. It was proposed thatthe surfactant is adsorbed patchwise and that electrostatic interac-tions between the already positive silica particle and the still neg-ative bare areas at the silicon wafer also play a role in the observedattraction. An AFM image of the silicon wafer at this concentration(see Fig. 9) shows that two different morphologies were present onthe surface, so-called ’’micropancake’’ (thin layers of air) with somenanobubbles on the top and some areas without surfactant adsorp-tion. The observed morphology is similar to that in reference [32].It indicates that the adsorption at the silicon wafer occurs patch-wise at low surfactant concentration. Patchwise adsorption is alsoreported at mica surfaces in the presence of low concentration ofC16TAB [33] and at silica surfaces in the presence of low concentra-tion of C18TACl [34]. Micropancakes as well as nanobubbles may becovered with surfactant. Attractive electrostatic interaction be-tween the silica particle (positive) and the silicon wafer (negativeareas without adsorption) as well as the bridging through nano-bubbles can explain the attractive interaction observed at this con-centration. Non-DLVO forces due to expulsion of aggregates orsteric forces between micelles/patchy bilayers were also observedat higher concentrations.

5. Conclusions

The interaction forces between two silica particles (system I)and between a silica particle and a silicon wafer (system II) inthe presence of aqueous CTAB solutions with concentrations be-tween 0.005 and 1 mM were measured using AFM. The forcecurves were correlated to the surfactant morphologies (Fig. 7).Both systems show a charge reversal from negative to positivecaused by the adsorption of cationic surfactant (CTAB) at the for-mer negatively charged silicon oxide surface. The interactions ofthe two systems were different for the same studied surfactantconcentration. The point of zero charge was obtained at 0.05 mMfor the silica particle–silica particle system (system I) as in Parker’swork [12] and at 0.3 mM for the silica particle–silicon wafer sys-tem (system II). This leads to different aggregate morphologies atthe silica particle surface and at the surface of the silicon wafer.An explanation for the difference might be the surface treatment:the silica particles were plasma cleaned, whereas the silicon waferswere treated with a piranha solution. In another study the samesurface treatment was applied to the silica particles and the siliconwafers and no differences in the interaction were observed [35]. Ata low CTAB concentration (for Example 0.05 mM) long-rangeattraction was observed. The attraction was larger in range andmagnitude for the silica particle–silicon wafer system and startsat distances larger than 40 nm. They cannot be caused by van derWaals attraction, but they are explained by the presence of nano-bubbles probing hydrophobic patches on the surfaces. The attrac-tion occurs when the nanobubbles bridge. Obviously, on a siliconwafer surface larger hydrophobic patches are present than on thesurface of the silica particles. Another explanation for long-range

attraction is the electrostatic attraction between oppositelycharged patches. At higher surfactant concentration (0.4 mM on-wards), monotonic repulsion between the two silica particles wasobserved. In contrast to this, aggregate expulsion could be ob-served in the interaction curves for the silica particle–silicon wafersystem up to a CTAB concentration of 0.8 mM. The difference is ex-plained by the different stiffness of the surfactant aggregates at thetwo surfaces (silica particle and silicon wafer). For the same stud-ied concentrations, the outer layer of surfactant was denser for thesilica particles and the aggregates on the silica particles were stifferand more closely packed. The stiffer aggregates are more difficultto remove.

Acknowledgments

The authors thank the DFG for financial support via the SPP1273 ‘‘Kolloidverfahrenstechnik (KL-1165/10)’’. Gerhard Findeneggis acknowledged for the helpful discussions and Gabi Hedicke isacknowledged for surface tension measurements of CTAB.

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