Supplementary Material

The Schulman method of cosurfactant titration of the oil/water interface

(dilution method): A review on a well-known powerful technique in

interfacial science for characterization of water-in-oil microemulsions

Soumik Bardhan a, Kaushik Kundu b#, Gulmi Chakraborty a, Swapan K. Saha a*,

Bidyut K. Paul b*

a Department of Chemistry, University of North Bengal, Darjeeling-734 013, Indiab Surface and Colloid Science Laboratory, Geological Studies Unit

Indian Statistical Institute, 203, B.T. Road, Kolkata-700 108, India

#Current Address: Department of Biotechnology, Bhupat and Jyoti Mehta School of

Biosciences, Indian Institute of Technology Madras, Chennai 600036, India

Authors for correspondence:

Prof. Bidyut K. Paul Prof. Swapan K. Saha

Surface and Colloid Science Laboratory Department of Chemistry

Geological Studies Unit, Indian Statistical Institute University of North Bengal

203, B.T. Road, Kolkata-700 108, India Darjeeling-734 013, India

E-mail: bidyut.isical@gmail.com E-mail: ssahanbu@hotmail.com

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Background of the dilution method

Early in 1982 to 1987, reports on exploring the dilution method were due to Birdi et al. for

water/SDS/pentanol/benzene microemulsions [1], Singh et al. [2], followed by Kumar et al.

for water/CTAB/2-ethyl-1-hexanol/pentane or hexane or heptane microemulsions [3] and

Verhoecku et al. [4] for SDS/Pn stabilized system. Pithapurwala and Shah [5] acknowledged

modified Schulman-Bowcott model for the evaluation of interfacial composition of oil-

external microemulsion formed with sodium stearate, pentanol, brine, and linear

hydrocarbons (with > C10) as oils (viz. decane, dodecane and hexadecane). It was reported

that the molecular packing was highest at the optimal salinity which corresponded to

maximization of molar ratio of alcohol to surfactant at the oil/water interface. Later on, Singh

et al. [6] investigated the influence of chain length of alkanes (pentane to heptane and

benzene) as oil and amine (n-alkylamines and cyclohexylamine) as cosurfactant on water

solubilization behavior of cationic microemulsions (CTAB and CPC) from dilution

experiments. In 1998, Gu et al. [7] performed calorimetric investigation on the partition of n-

pentanol (Pn) between external oil phase (heptane, Hp) and the interface of w/o

microemulsion, sodium dodecyl benzenesulfonate (DDBS)/Pn/Hp/water. The results showed

fine changes in the structure of water-in-oil emulsion and the microemulsion droplets.

Further, the alcohol/surfactant mole ratio in the interface of the droplets, and also the standard

thermodynamic functions of alcohol transition from the external phase to the interface were

derived from calorimetric data. During analysis of these results, a criticism was put

forwarded by the authors on the basics of dilution method or the relationships used in the

dilution method by affirming that the calorimetric method provided better-quality of results

compared to the dilution method. However, in the following year, Moulik et al. [8]

investigated two w/o microemulsion systems; water/cetylpyridinium chloride (CPC)/alkanol

(1-butanol, 2-butanol, 2-methyl-1-butanol, 1-pentanol and 1-hexanol)/alkane (C6 and C7) and

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same microemulsion system (as adopted by Gu et al. [7]) at different temperatures by the

dilution method, in order to collect additional data for the sake of comparison and

interpretation. According to the authors, the dilution method for the understanding of the

interfacial cosurfactant/surfactant composition and the distribution of the cosurfactant

between the interface and bulk oil phase, was found to be sound and useful. Further, it was

demonstrated that the equations involved in both the methods were, in fact, identical and that

the criticism of Gu et al. [7] was incorrect. Later, a series of papers on w/o microemulsion

systems have been reported.by employing the dilution method. Of these, a significant

contribution in utilizing this elegant and powerful method to accomplish formation of

microemulsion through consistent motivation, has been made by the school led by Professor

S.P. Moulik, Centre for Surface Science, Jadavpur University, Kolkata, India. Subsequently,

Digout et al. evaluated thermodynamic and structural parameters of w/o microemulsions

comprising water/CPC/alkanols (C4-C6)/alkanes (C8 – C10) by the dilution method [9]. In this

vein, Hait and Moulik [10] introduced a polar amphiphilic (biocompatible) oil, isopropyl

myristate (IPM) for formation of w/o microemulsions based on surfactants of different charge

types, CPC, CTAB and SDS with 1-butanol as cosurfactant along with the evaluation of

structural parameters at different physicochemical environments. The results were analysed

using the dilution method. Further, Bayrak [11] reported stable microemulsion system using

two non-ionic surfactants, Triton X-100 and Triton X-405 with the variation in cosurfactant

chain length (C5OH and C7OH) and oil topology (hexane, heptane, octane, decane and

benzene) in 2004. As the dilution method (or a cosurfactant titration of the oil/water interface

proposed by Bowcott and Schulman [12]) is a simple and inexpensive method which is

accomplished by repetitively oil dilution with cosurfactant titration till attainment of stable

microemulsions, doubts are often expressed on the reliability of data obtained. In the same

year, Palazzo et al. [13] published a paper entitled, “Does the Schulman’s titration of

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microemulsions really provide meaningful parameters?” The interfacial compositions of

CTAB/n-hexane/1-pentanol/water w/o microemulsion system were measured using the

dilution method and found satisfactory agreement with high accuracy with those obtained by

using the pulsed gradient spin-echo NMR (PGSE-NMR) technique. They further pointed out

that this method finds applications in scattering and diffusion studies, because it provides

extrapolation to single-particle properties by reducing inter-particle interactions of the

microemulsions without changing its composition. Further, the same system with high

concentration of Pn was investigated to underline its influence on phase equilibria and

mesophase structure using the dilution method and PGSE-NMR technique [14]. It is worthy

to mention that Abuin et al. employed UV-Visible spectroscopy as a tool for titration

experiment involved in the dilution method for characterization of various microemulsion

systems in 2004 [15]. In the following years, Zheng et.al extended the field of the dilution

experiment through characterization of Gemini surfactant based microemulsion, C12-s-C12 or

C12-OEx-C12/n-hexanol/n-heptane for the first time [16, 17]. In 2006, the applicability of the

dilution method on mixed cationic-non-ionic surfactant microemulsions, CTAB/Brij-58/1-

butanol (1-pentanol)/heptane (decane) was first reported by Paul and co-workers [18].

Subsequently, Paul et al. investigated water (or, aqueous NaCl)/oil (decane and dodecane)

microemulsions stabilized by anionic (SDS), cationic (CPC) and non-ionic [polyoxyethylene

(23) lauryl ether, Brij-35] surfactants and 1-pentanol (Pn) as cosurfactant [19]. During the

period 2006-2007, a series of papers were published on the investigation of interfacial

composition, thermodynamic properties and structural parameters of anionic and cationic

surfactant (alkyltrimethylammonium halide or alkylammonium bromide and its mono-, di-,

and trihydroxyethylated head group analogues) stabilized w/o microemulsions using linear

chain alkanols (viz. C4OH to C9OH) and heptane, isooctane and isopropyl myristate (IPM) as

oils by the dilution method [20-22]. Wang et al. [23] investigated the effect of cosurfactant of

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different types with linear and branched chains (viz., n-butanol, n-pentanol, iso-pentanol, n-

hexanol, n-octanol) on the formation of diesel oil microemulsion,

water/cetyltrimethylammonium chloride (CTAC)/alkanol/diesel oil. The interfacial

composition and thermodynamic properties of these microemulsions were investigated by the

dilution method. It is interesting to note that under identical physicochemical conditions, n-

octanol is more suitable cosurfactant than the others in formation of w/o diesel oil

microemulsions. In 2009 and onwards, another group led by Panda and his coworkers [24,

25] investigated formation of w/o microemulsions stabilized by cationic (CnTAB, where n

stands for 10, 14, 16), nonionic (Tween-20, sorbitan polyalkanoates of different alkyl chain

lengths) surfactants, 1-butanol and 1-pentanol as cosurfactants and heptane as oil by the

dilution method. Since 2006 Paul and his coworkers published a series of papers on

interfacial surfactant/cosurfactant compositions, structural and thermodynamic parameters of

single and mixed cationic or anionic-nonionic surfactant microemulsions at different

physicochemical environments by employing the dilution method [26-29, 18, 19, 30-33].

During a decade, the formulation of microemulsions has been pursued with an ionic liquid

(IL) as a substitute of water or oil or as an additive (aqueous), because of their proposed

environmentally-benign nature with many unique and attractive properties. The objective of

these studies was to combine a nanostructure of a microemulsion with the unique properties

the ILs [34, 35] IL-based microemulsion systems are of current interest [36-38]. IL-in oil

(IL/O) nonaqueous microemulsions have attracted much attention from the viewpoints of

theoretical and application prospects in various fields [39, 40]. Chaoumont et al. [41] reported

the molecular dynamics study of the interface between water and room-temperature ILs. The

first report on the investigation on the formation of w/o microemulsions involving surfactant-

like IL, 1-alkyl-3-methylimidazolium bromides/alcohol/alkane/water (brine) by the dilution

method, is due to Chai et al. [42]. Subsequently, a series of papers have been published on the

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investigation of IL-based microemulsion systems by the dilution method by Chai et al. [43],

Moulik et al. [44], Panda et al. [45], Kar et al. [33], and Wang et al. [46].

Dilution study with single surfactants

The compositions of the cosurfactant and surfactant at the interface, the distribution of the

cosurfactant between the interface and the continuous oil phase, and the energetics of transfer

of the cosurfactant from oil to the interface were evaluated for cationic and anionic

surfactant-derived w/o microemulsions, water/cetylpyridinium chloride, CPC/ 1-butanol or 2-

butanol or 2-methyl-1-butanol or 1-pentanol or 1-hexanol/hexane or heptane and

water/sodium salt of dodecylbenzenesulfonate, DDBS/1-pentanol/heptane, respectively by

Moulik et al. [8] using the method of dilution The distribution of cosurfactant between the

interface and the oil phase was found to depend on number of factors, namely, their

hydrophobic/hydrophilic molecular nature, the nature of the oil, interfacial forces, and

interaction with the surfactant, system composition, and thermal conditions. The cosurfactant

transfer process was spontaneous for all the systems. Under identical environmental

conditions, DDBS was a more effective surfactant than CPC in transferring cosurfactant from

the oil to the interface. Such transfer process was found to be mostly exothermic, whereas

both positive and negative entropy values of the transfer process were observed. Further, the

enthalpy and entropy of the transfer process were fairly compensated each other. A

correlation between the free energy of transfer and carbon number of the alkanols was

examined and reported to be exponentially dependent [8]. Digout et al. [9] also evaluated

thermodynamic properties and structural parameters of w/o microemulsions comprising

water/CPC/alkanols (C4-C6)/alkanes (C8-C10) by the dilution method. The degree of

spontaneity depended on the chain length of both alkanol and alkane. The free energy of

alkanol transfer process (ΔG0t) varied in an opposite manner for even-numbered versus odd-

Page 6 of 25

numbered alkanes. The transfer process was least spontaneous for C4 alkanol and increased

with increase in chain length of alkanols. It was suggested that the formation of

microemulsion can be more ordered or disordered depending on their chemical and thermal

conditions. In the same year, Moulik and his coworkers [47] also estimated the spontaneity of

alkanol transfer process as well as different structural parameters for water/CPC/1-alkanol

(C4 or C5)/n-hexane microemulsion systems. ΔG0t values were found to be decreased with

increase in surfactant/cosurfactant mass ratio (σ) for CPC/1-pentanol (Pn) system whereas

mild dependence of ΔG0t on σ was evidenced for CPC/1-butanol (Bu) system. The transfer

process for Pn based system was slightly more spontaneous than the Bu based system.

However, the transfer process was entropy-controlled for both systems. The water pool

diameter of the droplets were lower for Pn based system than Bu based system, whereas

reverse trend was observed for the ratio of the interfacial aggregation number. However, the

size of the droplets increased mildly with temperature for Bu based systems.

In a subsequent study, Hait and Moulik [10] reported stabilization and destabilization of polar

lipophilic oil, isopropyl myristate (IPM) derived w/o microemulsions under varied amounts

of surfactant and water at different temperatures by the dilution method for the first time. To

understand the differential behavior of surfactants towards the distribution of 1-butanol (Bu)

between the interface and oil phase at the threshold level of stability, three surfactants of

different polar head groups, namely, sodium dodecylsulfate (SDS), CPC and

cetyltrimethylammonium bromide (CTAB) were used. The efficacy of the Bu association

with the surfactants followed the order, CPC > CTAB > SDS, which reflected the degree of

favorable/unfavorable interaction of the surfactant with Bu. At a constant level of surfactant

addition, increasing water content required increasing population of Bu per mole of

surfactant. The transfer of Bu from oil to the interface was an endothermic process with

positive entropy change and was energetically favored with increase in temperature. The

Page 7 of 25

associated ΔH0t and ΔS0

t values on the whole followed the sequence, (ΔH0t)CTAB < (ΔH0

t)SDS <

(ΔH0t)CPC. Further, effective radius of the water droplet (Rw) and radius of microemulsion

droplet (Re) were found to be increased with increase in content of water, so does the ratio of

average aggregation number of surfactant-cosurfactant on the droplet surface (Ns/Na). At a

low surfactant stoichiometry at interface, greater amount of cosurfactant was required to

stabilize the system. At higher proportions, the compositions of surfactants and cosurfactants

tended towards equilibrium. Also, Fu et al. [48] evaluated the thermodynamic function

ΔG0o→i and structural parameters of anionic surfactant based microemulsion systems to

understand the effects of type of oils or diluents, alcohol, temperature and water content.

They used for the first time organo-phosphoric extractants and their sodium salts as anionic

surfactants, namely, Cyanex272 [sodium salt of di (2,4,4-trimethylpentyl) phosphoric acid,

NaDTMPP], Cyanex301 [di (2,4,4-trimethylpentyl) dithiophosphinic sodium, NaDTMPDTP]

and Cyanex302 [di (2,4,4-trimethylpentyl) monothiophosphinic sodium, NaDTMPTP]. The

order of - ΔG0o→i values followed the sequence, Cyanex272 < Cyanex302 < Cyanex301 and

also, the order was found to be, alkane > CCl4 > aromatics, depending on the oil type. The

free energy values also decreased with increasing water content, which revealed that the

higher water content was unfavorable to the formation of monophasic microemulsion of these

extractants. This study was proposed to be valuable for the application of these extractants

[48]. Later on, the free energy of cosurfactant transfer (ΔG0s) and the adsorption free energy

per methylene group of the alkanol (ΔG0s, alkanol/CH2) for water/CTAB or SDS/n-alkanol

(C4OH-C8OH)/oil (C5H-C7H) microemulsions were estimated at various temperatures (25-

350C) by Kumar and Kabir-ud-Din [49]. ΔG0s values were more negative for CTAB

microemulsion than SDS microemulsion which indicated stronger association between

CTAB-cosurfactant than SDS-cosurfactant. This trend was explained from view point that n-

alkanols were slightly deprotonated which assisted the cosurfactant to associate strongly with

Page 8 of 25

cationic CTAB than with anionic SDS. The values of ΔG0s, alkanol/CH2 were found to be in the

range of -500 to -850 J mol-1 and decreased with the number of carbon atoms in the alkyl

chain of the cosurfactant. It was also evident that the ΔG0s, alkanol/CH2 values were much lower

for pentane to heptane as compared to benzene and increased with the number of carbon

atoms in the alkyl chain-length of the oil phase.

All the studies, discussed in preceding paragraphs, were related to the dilution method

study involving ionic surfactants (anionic and cationic) based microemulsions. In view of

this, Guo et al. [50] evaluated spontaneity (Δc-iG0) of the alkanol transfer process as well as

structural parameters of non-ionic, Triton X-100/n-CnH2n+1OH/H2O microemulsions. It was

observed that the effective radius (Re) and radius of the water pool (Rw), the thickness of the

interfacial layer (L) and the aggregation number (ň) of the systems increased with increase in

solubilized water content whereas the total droplet numbers (Nd), the total interfacial area

(Ad) and Δc-iG0 values decreased. On the other hand, Re, Rw, L and ň increased and Δc-iG0

value decreased drastically, respectively with increasing chain length of alcohols. In

continuation, Bayrak [11] also determined the partitioning of alcohol at the interface of

water/Triton X-100 or Triton X-405/n-alcohols (C5 and C7)/n-alkanes (C6, C7, C8 and C10 and

benzene) microemulsions. The number of moles of alcohols per mole of surfactant at the

interface was increased with an increase in the oil chain length. Further, -ΔG0t values

increased with an increase in the chain length of the alcohol whereas reverse trend was

observed with an increase in chain length of oil. Overall, ΔG0t values were less negative for

Triton X-405 systems than Triton X-100 systems, which indicated that the alcohols are

preferentially associated with Triton X-405 more than Triton X-100 surfactant. Abuin et al.

[15] investigated the effect of alcohols, n-hexanol, n-decanol, 2,4-dimethyl-3-pentanol and 3-

ethyl-3-pentanol on water-in-oil (for example, n-hexane, n-dodecane, cyclohexane and 2,2,4-

trimethylpentane) microemulsions stabilized by tetradecyltrimethylammonium bromide,

Page 9 of 25

TTAB through refinement of the dilution method by using UV-Vis spectroscopy as a tool to

monitor the titrimetric analysis. It was reported that the efficiency of alcohols in stabilizing

the microemulsion system followed the order, n-alcohols < cyclohexanol < branched

alcohols, and that of solvent (oil) topology had minimal role to play in the stabilization

process. In a concluding remark, it was accounted that the critical amount of alcohol at the

interface was mainly dependent on the alcohol topology and, to a lesser extent, on the alcohol

size, and the solvent topology. This methodology was adopted the same principal as

envisaged by Moulik et al. in 2000 [8].

Interfacial, thermodynamic properties and structural parameters of w/o

microemulsions comprising of water/CTAB or SDS/n-alkanol stabilized in IPM was further

studied by Mohareb et al. using the dilution method [20]. The distribution of n-alkanol (with

varying chain length from C5 to C9) between bulk oil to the oil/water interface vis-à-vis their

energetics as a function of chain length of the co-surfactants and temperature were evaluated.

The results indicated that, both the alkanol transfer process of CTAB (C16) based systems was

more spontaneous, and overall droplet size (Re) of CTAB stabilized microemulsions was

more enlarged than SDS (C12). The chain length of cosurfactant governed the degree of

spontaneity of microemulsion formation. The standard Gibbs free energy of the alkanol

transfer process turn out to be progressively negative up to C7 (heptanol), i.e., progressed

spontaneously, but fell off at C8 and C9. The alkanol transfer process that guided the

formation of microemulsions was entropically driven. The trend of ΔH0t values followed the

order: ΔH0t (CTAB/C4OH) < ΔH0

t (SDS/C4OH). Moreover, the ratio of average aggregation

numbers of surfactant-cosurfactant on the droplet surface (Na/Ns) was higher for SDS based

microemulsions than CTAB.

Investigation on the formation of quaternary ammonium Gemini surfactants in apolar solvent,

by the dilution method was reported by Zhao and his coworkers for the first time [16]. In this

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contribution, they illustrated the prominent influence of spacer length of alkanediyl-α,ω-

bis(dimethyldodecylammonium bromides), C12-s-C12.2Br (where, s = 2-6) on the interfacial

composition of n-hexanol in water/C12-s-C12.2Br/n-hexanol/n-heptane microemulsion. The

ratio of moles of cosurfactant, n-hexanol and Gemini surfactant (C12-s-C12.2Br) on droplet

surface initially increased with increasing water content up to a certain limit and thereafter, a

decreasing trend was observed. However, a regular increasing trend of same was reported

with hike of spacer length(s) from 2 to 6. A spontaneous formation of microemulsions was

evidenced for all systems. The dilution experiments showed that the effective radius of the

water droplet (Rw) and effective radius of droplet in the solution (Re) varied by 0.83 nm and

1.57 nm to 5.82 nm and 7.19 nm with increase water content (ω) from 10 to 50, respectively.

Whereas, Re values were decreased from 3.02 nm to 2.15 nm with increasing spacer length

(s) from 2 to 6. Conversely, with single surfactant analogue systems (water/CPC or CTAB/n-

butanol/IPM), the susceptibility towards water was comparatively low [10]. The individual

packing parameter (P) was higher in C12-s-C12.2Br compared to CPC, indicating formation of

larger aggregates. Further, P values decreased with increasing spacer length (s = 2→6), which

indicated that larger amount of cosurfactant (herein, n-hexanol) were present on the droplet

surface. Zheng et al. extended the dilution method study of Gemini surfactant w/o

microemulsions, water/C12-EOx-C12.2Br (x = 1-3)/n-hexanol/n-heptane by incorporating

oxyethylene group as spacer [17]. Compared to C12-2-C12.2Br based systems, C12-EOx-C12

based systems showed higher population of n-hexanol (nai/ns) on the droplet surface rather

than in oil, due to relatively larger size of the head group. The interfacial compositions of n-

hexanol in these microemulsions with different oxyethylene groups as spacer were evaluated.

With increase in spacer-length of C12-EOx-C12.2Br, (nai/ns) increased from 3.90 to 5.16, while

the effective radius of the water droplet (Rw) showed gradual decreasing tendency. Another

important feature of these microemulsions was evidenced its variation in droplet size with

Page 11 of 25

spacer chain length and provided a potential application for the synthesis of nanoparticles

with small size.

The characterization of cationic w/o microemulsions comprising cetylammonium

bromide (C0) and with mono-(C1), di-(C2) and tri-(C3) hydroxylated head group of surfactant

by employing the dilution method was reported by Mitra et al. [21]. The transfer of n-hexanol

from oil (isooctane) to the droplet interface was exothermic with negative entropy change.

Further, it was reported that droplet number varied inversely with temperature, and

microemulsion with C3 revealed larger droplet number than C2 analogue at comparable

conditions. The population of surfactant and cosurfactant on the droplet interface was

comparatively lower for C3 than C2 analogues. C3-derived microemulsion demonstrated to be

more convenient for enzyme accommodation at the interface to augment better catalytic

activity. Maiti et al. [22] also reported w/o microemulsions comprising of

water/octadecyltrimethylammonium bromide (C18TAB)/n-butanol/n-heptane. They inferred

that the enthalpy and entropy of butanol transfer process were independent of temperature at

water to surfactant molar ratio (ω), 10→25 i.e. with zero specific heat. Further, the transfer

process of butanol from bulk to the interface progressed isoenthapically above ω (= 10). They

also showed that at a given temperature and [surfactant], the droplet sizes increased with

increasing water content whereas, growth had a decreasing trend with increasing temperature

at fixed [surfactant] and water content. Paul et al. [19] explored evaluation of interfacial

composition and thermodynamics of formation of surfactant of different charge types, viz.

CPC, SDS and polyoxyethylene (23) lauryl ether (Brij-35) derived w/o microemulsion

systems in presence of 1-pentanol (Pn) as cosurfactant and decane (Dc) and dodecane (Dd) as

oil under varied conditions of molar ratio of water to surfactant (ω), salt (NaCl)

concentrations and temperature. The results revealed that cationic CPC was more efficient

than anionic SDS regarding formation of stable microemulsion. The head group size of CPC,

Page 12 of 25

which could modify the relative adjustment pattern of amphiphiles (or molecular interaction

between CPC and Pn) at the interface, was one of the factors that decide the packing of the

amphiphiles at the droplet interface. In addition, delocalization of the charge as well as less

charge shielding due to the presence of pyridine ring in CPC might contributed the factor that

influence the accommodation of Pn at the droplet surface compared to other surfactant

systems. The presence of NaCl favored the transfer of cosurfactant from oil to interface in

case of CPC and SDS systems whereas, the effect was opposite for Brij-35 system. With

increase in temperature, the transfer process was exothermic with more organized interface

for CPC systems whereas, endothermic with less organized interface was observed for Brij-

35 and SDS stabilized systems. The effect of temperature in presence of NaCl showed similar

trends. In this study, Re and Rw values were found to be 3.43 and 2.27 nm and 7.20 and 6.33

nm, respectively, for CPC/Dc system, and the corresponding values for CPC/Dd system were

3.43 and 2.23 nm and 6.31 and 5.32 nm at water to surfactant molar ratio (ω) 10 and 50,

respectively. Re and Rw values were found to be 4.65 and 3.20 nm and 7.72 and 6.48 nm,

respectively, for SDS/Dc system and the values were found to be 4.58 and 3.12 nm and 7.13

and 5.87 nm for SDS/Dd system at ω = 10 and 40, respectively. The Re and Rw values showed

a decreasing trend while decane was replaced by dodecane for CPC and SDS stabilized w/o

microemulsion systems. In contrary, the observed values were increased while Dc was

replaced by Dd for Brij-35 systems. Since dilutability of microemulsions is a major concern

for their use in the drug delivery systems, Mehta et.al [51] reported characterization of a U-

type microemulsion system, water/Brij-96 (polyoxyethylene 10 oleoyl ether)/butanol or

pentanol or hexanol/ethyl oleate (EO) with respect to variation in lipophilicity of different

cosurfactants, chemical structure of oil, water solubility, water solubilization capacity and

temperature from dilution measurements and instrumental techniques. The results revealed

from the thermodynamics of the transfer of alcohols from bulk to the interface that Gibbs free

Page 13 of 25

energy of the alkanol transfer process was spontaneous for all these systems at all

temperatures. The standard enthalpy and entropy changes of the alkanol transfer process were

mostly negative. The changes in the microstructure, state and dissolution behavior of the

added water of these microemulsions were also analyzed using conductivity, UV–visible

spectroscopy and FT-IR. Report on characterization of different charge types of w/o

microemulsions, for example, anionic SDS or cationic didodecyltrimethylammonium

bromide, DTAB or nonionic polyoxyethylene sorbitan monolaurate, Tween-20/n-butanol or

n-pentanol/n-heptane by the dilution method was reported by De et al. [24]. The spontaneity

of formation of these microemulsions was shown to be comparable for both DTAB and SDS,

whereas for Tween-20 based systems spontaneity was about 30% lower. The effective order

of microemulsion formation was DTAB > SDS > Tween-20, irrespective of type of alkanol

used. ΔH0t values were all negative (exothermic) for DTAB/butanol/ or pentanol system at

the studied temperature range. But SDS/butanol system showed exothermicity of the transfer

process at higher temperatures. All other systems were endothermic in nature. Tween-20

based microemulsions were more endothermic than SDS based microemulsions. The DTAB

derived systems were enthalpy driven, whereas the rest (with one or two exceptions) was

entropy driven. At a fixed water content, the difference between Re and Rw values were

continuously decreased with temperature for DTAB based systems, whereas both SDS and

Tween-20 derived systems upheld constancy. Such tuneable physicochemical parameters

were attributed to the variation in the head group of three surfactants. In a subsequent study,

they employed the dilution method to derive different physicochemical parameters for the

formation of water/surfactant + n-butanol/n-heptane w/o microemulsions where both cationic

and nonionic surfactants of two series were used [25]. The cationic surfactants used were

alkyltrimethylammonium bromides (CnTAB, n = 10, 14 and 16) while the non-ionic

surfactants were polyoxyethylene (20) sorbitan monoalkanoates (polysorbate), viz., palmitate

Page 14 of 25

(PS 40), stearate (PS 60) and oleate (PS 80). The spontaneity of formation of microemulsions

was increased with the surfactant chain length for both cationic and nonionic surfactants.

Nonionic surfactants with bulkier head groups required a higher amount of n-butanol

compared to cationic surfactants. The transfer of butanol from oil to the interface was

endothermic with positive entropy change. The effective size of the droplets (Re) increased

linearly with the surfactant chain length for both types of surfactants. For cationic surfactants

a 50% reduction in droplet size with temperature was noted while for nonionic surfactants, it

was only 17%. Surfactants with longer hydrocarbon tails were advocated to produce a better

surface coverage compared to the shorter analogues. Very recently, Mandal and his co-

workers evaluated physicochemical and thermodynamic parameters of both anionic (SDS)

and cationic (CTAB) microemulsion systems from dilution experiments, in view of their

suitability in oil recovery process [52]. They explored the effect of alkane carbon number

(ACN) on the maximum water solubilization capacity of microemulsion, and also,

corroborated the Bansal, Shah, O’Connell (BSO) equation and the effect of chain length

compatibility by dilution method. The influence of branching in cosurfactant chain on the

water solubilization limits of microemulsion systems composed of surfactants with different

polar head groups, SDS and CTAB was investigated. Dilution experiments were carried out

in these systems, using four alkanes (hexane, heptane, decane and dodecane) as oil phase and

pentan-1-ol or 3-methyl-1-butanol as cosurfactant, at different temperatures (303, 313 and

323K) to determine the interfacial compositions and thermodynamic parameters of the

microemulsions. The mole fractions of cosurfactant in the interfacial region (X ics) and the

bulk oil phase (Xocs) were higher for SDS microemulsions than CTAB microemulsions, and

was discussed on the basis of the hydrophobicity as well as exposure of the charge of the

head group of two surfactants. Gibbs free energy change of cosurfactant transfer obtained

from dilution experiments confirmed that the formation of these microemulsions was

Page 15 of 25

spontaneous. The degree of spontaneity of their formation depended on chain length of oil,

Higher the chain length of oil, less spontaneous was the formation process, i.e., the negative

values of the change in Gibbs free energy of cosurfactant transfer decreased with increase in

oil chain length. The partitioning of cosurfactant at the interface and the oil phase increased

with increase in oil chain length. CTAB showed higher maximum solubilization of water than

SDS system, irrespective of type of cosurfactant used. ΔH0t showed that the cosurfactant

transfer process occurs with absorption of heat (endothermic) or release of heat (exothermic),

depending on the temperature.

Recently, the dilution method was also carried out in water-in-diesel oil

microemulsion systems, using diesel oil as oil phase, cetyltrimethylammonium chloride

(CTAC) as surfactant and butanol, pentanol, iso-pentanol, n-hexanol and n-octanol as

cosurfactants by Wang et al. [23]. The solubility of all alkanols was found to be decreased in

diesel oil with increasing temperature, whereas an opposite trend was followed for n-octanol.

On the other hand, interfacial populations were higher and lower for n-butanol and n-octanol,

respectively. The diesel oil microemulsions were formed spontaneously no matter which

alkanol used for formation of these microemulsions. The transfer process was found to be

endothermic for all alkanols, except n-octanol caused the release of heat (exothermic). It was

reported that the interface and its surroundings constituted by n-octanol and CTAC showed

more orderly. Finally, it was concluded that n-octanol was more suitable cosurfactant than

others in forming water-in-diesel oil microemulsion under similar environmental conditions.

In continuation, they also measured interfacial composition and thermodynamic parameters

of CTAC and diesel oil based microemulsions stabilized by cosurfactants (as used in previous

report) in presence of aqueous HCl (mass fraction 20%) by the dilution method [53]. A large

amount of cosurfactant was required to form aqueous HCl containing microemulsions

compared with aquo-microemulsions. However, former set of microemulsions were found to

Page 16 of 25

be more spontaneous than latter set of microemulsions. Comparing the structural parameters

of the two microemulsion systems (i.e., aquo- and aqueous HCl based systems), the Re and

Rw values in aqueous HCl based microemulsions at high HCl content in water pool were

smaller than that of aquo-microemulsions with 2-methyl-1-butanol, 1-hexanol and 1-octanol

as cosurfactants. Similar to aquo-microemulsions, 1-octanol was proposed to be more

suitable cosurfactant than others for aqueous HCl based systems.

Page 17 of 25

Table S1. Variation of K, I, ∆Gc→i and D with surfactant, cosurfactant and temperature*

Variation of Surfactant for water/surfactant/1-Hexyl alcohol/heptane microemulsion at 308 K

K I ∆Gc→i

(kJ/mol)

D (nm)

NP-9 0.072 6.41 6.32 60.9

CTAB 0.063 5.21 6.56 45.2

AS 0.043 5.12 7.70 56.3

Variation of Cosurfactant for water/AS/alcohol/heptane microemulsion

1-Butyl alcohol 0.065 6.77 6.58 63.4

1-Amyl alcohol 0.051 3.41 8.76 51.5

1-Hexyl alcohol 0.023 6.16 7.12 53.8

1-Heptyl alcohol 0.017 5.29 9.70 47.1

Variation of Temperature (K) for water/AS/1-Hexyl alcohol/heptane microemulsion

298 0.023 6.16 7.12 53.8

308 0.043 5.12 7.70 56.3

318 0.037 5.01 8.32 61.3

328 0.021 4.67 10.04 65.6

*Taken from He et al. [54] with permission from Elsevier.

Page 18 of 25

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