Physicochemical Investigation of Biocompatible Mixed Surfactant ...

J Surfact Deterg (2013) 16:865–879 DOI 10.1007/s11743-013-1473-1

ORIGINAL ARTICLE

Physicochemical Investigation of Biocompatible Mixed Surfactant Reverse Micelles: III. Aqueous NaCl Solubilization, Thermodynamic Parameters of Desolubilization Process and Conductometric Studies Kaushik Kundu • Bidyut K. Paul

Received: 23 December 2012 / Accepted: 26 March 2013 / Published online: 23 April 2013 Ó AOCS 2013

Abstract Solubilization of water and aqueous NaCl in mixed reverse micelles (RMs) comprising sodium bis(2ethylhexyl) sulfosuccinate (AOT), and polyoxyethylene (20) sorbitan trioleate or polyoxyethylene (20) sorbitan monooleate has been studied at different compositions (Xnonionic = 0–1.0) at a total surfactant concentration, ST = 0.10 9 103 mol m-3 in biocompatible oils of different chemical structures; viz., ethyl oleate (EO), isopropyl myristate (IPM) and isopropyl palmitate (IPP) at 303 K. The enhancement in water solubilization (i.e., synergism) has been evidenced by the addition of nonionic surfactant to dioctyl sulfosuccinate/oil(s)/water systems. Addition of NaCl in these systems at different Xnonionic enhances their solubilization capacities further until a maximum, xNaCl,max is reached. xNaCl,max and [NaCl]max (concentration at which maximization of NaCl solubilization occurs) depend on type of nonionic surfactant, its content (Xnonionic) and oil. A new solubilization efficiency parameter (SP*water or SP*NaCl) has been proposed to compare solubilization phenomena in these oils. The energetic parameters of the desolubilization process of water or aqueous NaCl in single and mixed RMs have been estimated. Energetically, the water dissolution process in oil has been found to be more exothermic as well as more organized in IPP. Overall, the dissolution of water and aqueous NaCl in mixed RMs is entropically driven process.

Electronic supplementary material The online version of this article (doi:10.1007/s11743-013-1473-1) contains supplementary material, which is available to authorized users. K. Kundu B. K. Paul (&) Surface and Colloid Science Laboratory, Geological Studies Unit, Indian Statistical Institute, 203, B.T. Road, Kolkata 700 108, India e-mail: [email protected]

Conductance behavior of these systems in the presence of NaCl has been investigated under different [NaCl] at 303 K. An attempt has been made to give an insight to the mechanism of solubilization phenomena, percolation in conductance and microstructures vis-a`-vis role of biocompatible oils in these systems. Keywords Reverse micelle Mixed surfactant Biocompatible oil NaCl Solubilization efficiency parameter Synergism Desolubilization process Energetic parameter Percolation phenomena

Introduction Microemulsions and reverse micelles (RMs) can be defined as thermodynamically stable, isotropically clear dispersions of two immiscible liquids, consisting of microdomains of one or both liquids stabilized by an interfacial film of surface active molecules that form spontaneously without the need for high shear equipment. Typical waterin-oil (w/o) microemulsions or RMs consist of nanoscopic pools of water dispersed in non-polar solvent [1]. Because of their improved drug solubilization (including watersoluble, oil-soluble and amphiphilic drugs) and modified drug release characteristics, they have received enormous attention as drug carrier systems during the past few years [2, 3]. For potential uses in biology and medicine, microemulsion/RM systems are required to be biocompatible and hence the choice of oil and amphiphile(s) used in their formulation is restricted [4]. Development, characterization and biotechnological studies on ‘‘biocompatible microemulsions’’ as potential vehicles for drug delivery, pharmaceutical preparations, enzymatic studies, protein purification, and extraction have become a thrust area of

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research [5]. These applications depend crucially on the water solubilization capacity, which changes in response to the environmental variables such as surfactant properties, composition, type of oil, temperature, valence of counter ions, salt concentration, etc. [6, 7]. As salinity in w/o microemulsions is reported to affect the attractive interaction among the droplets [6, 8–10], a knowledge of the effect on the solubilization behavior of NaCl at different concentrations in single as well as mixed surfactant based RMs would be worthwhile. Further, variation of ionic strength has been reported to lead to salting out effects, resulting in the transfer of proteins from the organic to the aqueous phase, which facilitates protein purification processes [11]. Studies on the extent of water solubilization in RM was initiated by Kon-No and Kitahara [12–14]. Further, Liu et al. [6] reported on the solubilization of water and aqueous NaCl solution in mixed RMs. Li et al. [7] investigated the influence of two typical additives, long chain organic molecule, bis (2-ethylhexyl) phosphoric acid (HDEHP) and inorganic electrolyte (NaCl) on the water solubilization capacity of dioctyl sulfosuccinate and NaDEHP in n-heptane solutions. The effects of the variable (additives, water content and temperature) on the water solubilization capacity and the structure of the oil/water interface have also been examined by measuring the electrical conductivity of these systems. The dissolution of water in surfactant/oil medium leading to microemulsion formation is an important aspect of the study of microheterogeneous systems. Thermodynamic studies on the formation behavior of RMs and the energetics of the interaction of the components based on mixed surfactant systems in biocompatible oils are comparatively rare [15]. The single and multiphase formation of RMs and their internal structures and particle aggregations are vital pieces of information for their preparation, application, and use. But knowledge of their internal arrangement, structure, and interaction need augmentation through thermodynamic studies. Although the complex nature of RMs has restricted the rapid growth of their thermodynamic status, several direct calorimetric studies have appeared in the literature [16–21]. An important aspect of RM/microemulsion design is the ability to solubilize a maximum amount of dispersed phase (i.e., water or aqueous NaCl) into the continuous phase. A single surfactant does not necessarily produce the best microemulsions. A method that has been suggested to enhance the solubilization capacity of RMs is the use of surfactant mixtures. Mixed surfactant systems play an important role in a number of applications in areas related to medicine, enhanced oil recovery, etc., in view of the fact that mixtures of surfactants often exhibit interfacial properties that are more pronounced than those of the individual surface-active components of the mixture [22–24]. Studies

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on mixed surfactant systems, therefore, are of considerable interest froma fundamental as well as an applied viewpoint. In a previous report [25], we investigated the water solubilization, conductometric studies and thermodynamics of water dissolution of biocompatible sodium bis(2-ethylhexyl) sulfosuccinate (abbreviated DOSS) RMs in the presence of non-ionic surfactant [polyoxyethylene (20) sorbitan trioleate (abbreviated STO20EO)] in polar lipophilic oils [viz. ethyl oleate (EO), isopropyl myristate (IPM) and isopropyl palmitate (IPP)] under various physicochemical environments. As part of our continuing research into understanding the mechanism of solubilization phenomena, percolation in conductance and microstructures of microemulsion/RM systems, wherein the chemical structures of polar lipophilic oils play a significant role, we undertook a detailed study of the solubilization behavior of mixed DOSS/non-ionic surfactant [STO20EO or polyoxyethylene (20) sorbitan monooleate (SMO20EO) with same number (20) of polyethoxylated chain, but different in hydrophobic moiety]/IPM or IPP or EO systems in the absence and presence of electrolyte (sodium chloride, NaCl), as NaCl can also strikingly affect the percolation of conductance of w/o microemulsion [26– 28]. This report also presents the temperature dependent solubilization behavior of DOSS or DOSS/nonionic(s)/IPM or IPP or EO systems, both in the absence and presence of the salt NaCl. The standard free energy, standard enthalpy and standard entropy changes of dissolution of water or aqueous NaCl have been evaluated with a view to understanding the mechanism of microemulsion formation and spontaneity of the process. Also, a thermodynamic study of the dissolution process undertaken herein is expected to offer information from the standpoint of catalytic activities and reactivity of enzymes in non-aqueous solution environments [29, 30]. The selection of the components was not arbitrary, as justification for using these kinds of ingredients have been reported in our earlier work [25]. Above all, the basic objective of this study was to understand the physicochemical nature of microemulsion/RMs composed of pharmaceutically accepted ingredients from the viewpoint of the future employment of these model systems in drug delivery, enzyme kinetics, protein purification processes, etc., as such studies are rarely reported in the literature.

Materials and Methods Materials Sodium bis(2-ethylhexyl) sulfosuccinate (commercial brand AOT, 99 %, abbreviated as dioctyl sulfosuccinate DOSS in what follows), polyoxyethylene (20) sorbitan


trioleate (brand Tween-85, abbreviated STO20EO), and polyoxyethylene (20) sorbitan monooleate (brand Tween80 abbreviated SMO20EO) were purchased from Sigma (St. Louis, MO) and used without further purification. Isopropyl myristate (IPM), isopropyl palmitate (IPP) and ethyl oleate (EO) are products of Fluka (Buchs, Switzerland). Sudan IV and Eosin Blue are AR grade products of SRL (Mumbai, India). Sodium chloride (NaCl) is a product of SRL. The chemical structures of surfactants and oils are represented in Scheme S1 (Electronic Supplementary Material). Double distilled water was used with conductance less that 3 9 102 lS m-1. Methods Water (or aqueous solution of NaCl at different concentrations) was gradually injected using a microsyringe of varying capacity into 3 ml surfactant(s) solution in organic solvent (oil) maintained at constant temperature (303 K) with constant stirring in a vortex shaker. The surfactant solution was fixed at a concentration of 0.10 9 103 mol m-3. The onset of permanent turbidity at each composition of surfactant blend in oil denotes maximum solubilization of water or NaCl at the end point of titration. All the experiments were repeated 2–3 times and mean results were taken. The results of these experiments are presented in the subsequent section. Conductivity measurements were made using an automatic temperature-compensated conductivity meter, Thermo Orion (Model 145A plus; http://www.thermo scientific.com), at 303 K, with cell constant of 1.0 9 102 m-1. The uncertainty in measurement was not more than ± 1 %.

Results and Discussion Solubilization of Water in Mixed RMs in Polar Lipophilic Oils Water solubilization capacity of mixed anionic (DOSS) and nonionic (SMO20EO or STO20EO) RMs (at XSMO20EO or STO20EO = 0 ? 1.0) in three different oils (viz. EO, IPP and IPM) with their chemical structures (Scheme 1) and molar volumes (Table 1) at 303 K are presented in Fig. 1 and Table 2. Further details of the investigation are provided in Section 1 of the Electronic Supplementary Material. Solubilization of NaCl (Aqueous) in Mixed RMs in Polar Lipophilic Oils The effect of aqueous NaCl on the solubilization capacity of mixed DOSS/STO20EO or SMO20EO/oil(s) RMs was

867 Table 1 Molar volume of solvents Solvent

Molar volume (m3 mol-1) 9 10-6

EO

356.92

IPP

349.95

IPM

316.69

EO ethyl oleate, IPP isopropyl palmitate, IPM isopropyl myristate

investigated at different contents of STO20EO [XSTO20EO = 0.025 (\XSTO20EO,max), 0.05 (=XSTO20EO,max) and 0.1 and 0.2 ([XSTO20EO,max), where XSTO20EO,max indicates the composition at which maximum solubilization capacity of water occurs] and SMO20EO (XSMO20EO,max = 0.05) with total surfactant concentration of 0.10 9 103 mol m-3 at 303 K. Addition of aqueous NaCl solution in these systems at a particular XSTO20EO or SMO20EO further enhances the solubilization capacity up to a maximum (indicated as, xNaCl,max) at an optimal [NaCl] (or, threshold concentration), and thereafter solubilization capacity decreases for all oils studied. The threshold concentration at which solubilization capacity attains maximum value has been denoted as [NaCl]max. Both xNaCl,max and [NaCl]max for DOSS/STO20EO or SMO20EO/oil(s)/ aqueous NaCl RMs at 303 K are presented in Tables 3 and 4. Solubilization capacity versus [NaCl] profile as a representative plot at XSTO20EO = 0.025, 0.05 and XSMO20EO = 0.05 have been presented in Figs. 2 and 3, respectively. Both xNaCl,max and [NaCl]max depend on the nonionic surfactant and oil type. Similar observations were also reported earlier [6, 8, 15, 25, 27]. Further, both of these parameters depend on the composition (i.e., XSTO20EO or SMO20EO) of the mixed RMs. The dependence of both xNaCl,max and [NaCl]max on composition (XSTO20EO) and oil type can be understood from the plots of xNaCl,max or [NaCl]max versus XSTO20EO for all studied oils (Fig. 4) and is disussed in the next section. Effect of XSTO20EO on Solubilization of NaCl (Aqueous) From the solubilization capacity of NaCl (xNaCl,max)– XSTO20EO profile (Fig. 4a), it is evident that xNaCl,max passes through a maximum at XSTO20EO = 0.05 (where maximum water solubilization capacity was evidenced) for a DOSS/TSTO20EO-based system in EO, whereas IPM- and IPP-derived systems show a maximum at XSTO20EO = 0.2 with a sharp inflection (or hump) at XSTO20EO = 0.05. Further, it was observed from the plot of [NaCl]max versus XTSTO20EO that, with increasing content of STO20EO (i.e., XSTO20EO) in these systems, [NaCl]max increases at constant temperature of 303 K (Fig. 4b). Figure S1 (Electronic Supplementary Material) also represents

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Fig. 1 Solubilization of water in sodium bis(2-ethylhexyl) sulfosuccinate (DOSS)/ nonionic/oil mixed reverse micelles (RMs) at 303 K with total surfactant concentration of 0.10 9 103 mol m-3. a Sorbitan trioleate (STO20EO), b sorbitan monooleate (SMO20EO)

Table 2 Solubilization of water (x0,max) in sodium bis(2-ethylhexyl) sulfosuccinate (DOSS)/nonionic mixed reverse micelles (RMs) in different oils at 303 K with total surfactant concentration, ST = 0.10 9 103 mol m-3 Surfactant DOSS/STO20EO

DOSS/SMO20EO

Oil

xa0

xb0,max

SP*cwater

EO

16.67

32.41 (0.05)

0.94

IPM

22.22

34.26 (0.05)

0.54

IPP

13.89

27.78 (0.05)

0.99

EO

16.67

25.93 (0.05)

0.56

IPM

22.22

29.63 (0.05)

0.33

IPP

13.89

22.22 (0.05)

0.60

STO20EO Sorbitan trioleate, SMO20EO sorbitan monooleate a

x0 = [water]/[DOSS] in different oil at [ST] = 0.10 9 103 mol/m3 at 303 K b

x0,max represents the maximum solubilization of water at a particular XSTO20EO given in the parenthesis c

SP*water represents solubilization efficiency parameter of water

the solubilization of aqueous NaCl in a DOSS/STO20EO/ EO RM system at different XSTO20EO (=0.025–0.2). Incorporation of STO20EO with large polar head group (20 ethylene oxide units group) decreases the effective packing parameter (Peff) by increasing a (area of polar head group), thereby increasing the natural radius of curvature (R). Such an increase in R increases the interdroplet interaction among the droplets, making the interfacial layer more flexible. According to Liu et al. [6], any factor that increases the rigidity of the interfacial film of the reverse micellar microdroplet will decrease [NaCl]max and vice versa. That is why, with increasing XSTO20EO, maximum solubilization of NaCl is evidenced at higher [NaCl]. A similar observation was also reported earlier [6, 8, 15, 27].

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Effect of Nonionic Surfactants on Solubilization of NaCl The solubilization capacity of NaCl in the mixed systems, DOSS/nonionic surfactant (STO20EO or SMO20EO) was investigated in three oils at a fixed composition (XSTO20EO or SMO20EO,max = 0.05) at 303 K. The result of solubilization behavior of these mixed systems is presented in Tables 3 and 4, where it can be observed that the DOSS/ STO20EO exhibits higher xNaCl,max compared to DOSS/ SMO20EO, whereas DOSS/SMO20EO requires higher [NaCl]max compared to DOSS/STO20EO at Xnonionic,max = 0.05 to attain maximum solubilization in these oils. The dependence of both xNaCl,max and [NaCl]max on the chemical structures and configurations of the nonionic surfactants is dealt with below. Effect of Oil on Solubilization of NaCl The solvents (i.e., oils) also have significant influence on both xNaCl,max and [NaCl]max of these systems, as evidenced from Tables 3, 4 and Fig. 4. It was observed that [NaCl]max follows the order: IPM B EO \ IPP. The results indicate, from the overall order of [NaCl]max, that the molar volume of the oil is not only the deciding factor for determining [NaCl]max, but also that their chemical structures and configurations (i.e., overall chemical architecture) contribute to the process, as considered further below. Liu et al. [6] and Mitra et al. [8] reported that not only the molar volume of oils but also their configuration has an influence on [NaCl]max for DOSS/nonionic mixed RMs in linear hydrocarbons and IPM (along with IBB and Cy) oils, respectively. Hence, it can be inferred from these results that both nonionic content (XSTO20EO) and the type of oil (viz.


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Table 3 Solubilization of aqueous NaCl in DOSS/STO20EO mixed RMs in different oils with ST = 0.10 9 103 mol m-3 at different X and at 303 K Oil

XSTO20EO

x0,max or, x[NaCl]=0a

xNaCl,maxb

10-3 [NaCl]max (mol/m3)c

SP*NaCld

EO

0.025

25.93

33.33

0.0625

0.29

IPM

27.78

35.19

0.05

0.26

IPP

21.30

27.78

0.1

0.30

32.41

41.11

0.0625

0.27

IPM

34.26

42.59

0.05

0.24

IPP

27.78

36.11

0.1

0.30

28.70 31.48

39.81 40.74

0.08 0.08

0.39 0.29

24.07

37.04

0.125

0.54

24.07

37.81

0.15

0.65

IPM

26.85

43.52

0.15

0.62

IPP

22.22

42.59

0.22

0.92

EO

EO IPM

0.05

0.1

IPP EO

a

0.2

x0,max or x[NaCl]=0 represents solubilization of water in absence of NaCl at corresponding XSTO20EO

b

xNaCl,max represents maximum solubilization of aqueous NaCl at [NaCl]max

c

[NaCl]max = Threshold NaCl concentration at which solubilization capacity of NaCl reaches maximum

d

SP*NaCl represents solubilization efficiency parameter of aqueous NaCl

Table 4 Solubilization of aqueous NaCl in DOSS/SMO20EO mixed RMs in different oils with ST = 0.10 9 103 mol m-3 at fixed XSMO20EO and at 303 K. For definitions, see Table 3 Oil

XSMO20EO

EO IPM

0.05

IPP

x0,max or, x[NaCl]=0

xNaCl,max

10-3 [NaCl]max (mol m-3)

SP*NaCl

25.93

38.89

0.12

0.50

29.63

44.44

0.10

0.50

22.22

34.26

0.15

0.54

Fig. 2 Solubilization of aqueous NaCl in DOSS/ STO20EO/oil mixed RMs at 303 K with total surfactant concentration of 0.10 9 103 mol m-3. a XSTO20EO = 0.025, b XSTO20EO = 0.05

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Fig. 3 Solubilization of aqueous NaCl in DOSS/SMO20EO/oil mixed RMs at fixed XSMO20EO = 0.05 and at 303 K with total surfactant concentration of 0.10 9 103 mol m-3

overall chemical architecture) play significant roles in the solubilization of NaCl in mixed RMs. Comprehensive Analysis of Solubilization of Water or Aqueous NaCl in Mixed RMs (DOSS/STO20EO or SMO20EO) in Three Different Oils (EO, IPM and IPP) Effect of Nonionic Surfactant In the present study, addition of nonionic surfactants with asimilar polar head group (20 ethylene oxide units) but differing in chemical structures/configurations and HLBs (Scheme S1) to DOSS/IPM (or IPP or EO)/water RMs produces synergism in water solubilization capacity, as discussed in detail in section 2 of the Electronic Supplementary Material. Further, the addition of aqueous NaCl solution to these systems at a particular XSTO20EO or SMO20EO further Fig. 4 Maximum solubilization capacity of NaCl (aqueous), xNaCl,max (a) and threshold concentration of NaCl at which maximum solubilization occurs, [NaCl]max (b) as a function of STO20EO content (XSTO20EO) in DOSS/STO20EO/oils/NaCl (aqueous) mixed RMs at 303 K

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enhances the solubilization capacity up to a maximum (indicated as, xNaCl,max) at an optimal [NaCl] (say, [NaCl]max), and thereafter solubilization capacity decreases for studied oils. xNaCl,max and [NaCl]max for both DOSS/ STO20EO or SMO20EO/oil(s)/aqueous NaCl RMs at 303 K are presented in Tables 3 and 4. The overall x0,max and xNaCl,max for these systems follows the order: DOSS/ STO20EO [ DOSS/SMO20EO, whereas [NaCl]max follows the order; DOSS/SMO20EO [ DOSS/STO20EO in all oils (EO, IPM and IPP). The above features reflect the fact that solubilization phenomena in these mixed RMs is not straightforward. A plausible explanation can be suggested from the viewpoint of differences in chemical structures/configurations of the added nonionic surfactants. Both nonionics possess the same number (20) of EO units as polar head groups, but differ in hydrophobic configurations/moieties with respect to the number of double bonds (Scheme S1). As such, a double bond positioned in the middle of the long hydrophobic chain of the ester linkage of these surfactants imposes stereo-chemical constraints on the system. Consequently, the chain bends and its volume increases considerably and its length is slightly decreased. This reduces the (interdroplet) attractive force between the hydrophobic chains of the surfactant molecules and favors the formation of w/o microemulsions as a result of the increase in packing ratio. As STO20EO contains more double bonds (three) in their hydrophobic chain than SMO20EO (one) (Scheme S1), the attractive force between hydrophobic chains of STO20EO is less than that of SMO20EO. On the other hand, this distorted configuration of the surfactant chains prevents close packing in the interfacial layer and may facilitate oil penetration. Penetration of the oil into the interface is related to an additional property of double bonds. The hydrophobic chain of unsaturated surfactants is liquid at ambient temperature; thus, the chain becomes flexible, thereby facilitating oil penetration. So, for a DOSS/STO20EO-based system, oil penetration is favored compared to a DOSS/SMO20EO-based system.


All these considerations promote higher water or NaCl solubilization (i.e., a higher x0,max or xNaCl,max value) in a DOSS/STO20EO blend system than in a DOSS/SMO20EO blend system [31]. On the other hand, as STO20EO contains more double bonds (three) in the hydrophobic chain than SMO20EO (one) (Scheme S1), the attractive force between hydrophobic chains of STO20EO is less than that of SMO20EO. Thereby, the oil/water interface comprising DOSS/ STO20EO blends is less flexible compared to that of DOSS/SMO20EO blends. According to Liu et al. [8], any factor that increases the rigidity of the interfacial film of the reverse micellar microdroplet will decrease [NaCl]max and vice versa. That is why maximum solubilization of NaCl is evidenced at higher [NaCl] for DOSS/SMO20EObased systems. A similar observation was also reported earlier by Mitra et al. on DOSS/various nonionics/IPM or IBB or Cy systems [8, 15, 27]. Effect of NaCl (Aqueous) The maximum solubilization obtained in the solubilization capacity versus [NaCl] profile in mixed RMs can be explained on the basis of a salting in and salting out process [7, 8, 15, 27, 32–34]. Addition of electrolyte (NaCl) can expel part of the AOT molecules from the aqueous phase to the organic phase to form RM and hence increases solubilization. In the mixed RM, ethoxylated chains of nonionics coil around the charged head groups of DOSS screening the electrostatic repulsions and high charge density of sulfonate head group of DOSS causes a strong ion–dipole interaction with the polyethyleneoxide chains of nonionics [22]. It is known that the addition of electrolyte diminishes the effective polar head area of the surfactants by screening the electrostatic repulsion, which makes surfactant membranes more rigid and also decreases the attractive interactions among the droplets [7, 8, 15, 27, 32– 34]. This in turn increases solubilization. At a higher concentration of NaCl a different phenomenon overcomes this effect. Addition of NaCl decreases the thickness of the electrical double layer of the charged interfacial film and the effective polar area of the surfactant also decreases. This in turn increases the tendency of the surfactant to form a natural negative curvature, which decreases the solubilization. At XSTO20EO or SMO20EO,max, the systems can be assumed to be governed by the interdroplet interaction mechanism, and the addition of NaCl into such systems can decrease such interdroplet interaction, thereby increasing solubilization. So, the ascending curve of AOT/STO20EO or SMO20EO/oil/NaCl (aq.) solubilization capacity versus [NaCl] profile has been found to be the Rc branch, with the descending curve as the R0 branch. Similar results were reported by Zana and co-workers [35, 36] for cationic

871

surfactants/aromatic oils/NaCl (aq.) systems. Our results are also consistent with the findings of Derouiche and Tondre [37] for DOSS/decane or dodecane/NaCl (aq.) systems and Mitra and Paul [8, 15, 27] for DOSS/nonionics/ IPM/NaCl systems. Effect of Oil In order to ascertain the role of polar lipophilic oils (herein reported, consisting of a long fatty acid chain and hydrophilic ester moiety with short alkyl chain and possess high molar volume in the range of (317 9 10-6–357 9 10-6 m3 mol-1) on the solubilization phenomenon in DOSS or DOSS/STO20EO or SMO20EO/oil/water systems, an attempt was made to analyze our results on the basis of the model developed by Shah et al. [32–34]. As can be observed from Tables 2, 3, 4, the overall solubilization capacity of water (x0 and x0,max) and NaCl (xNaCl,max) for all studied oils follow the order: IPP \ EO \ IPM. Herein, EO has exchanged positions with respect to IPP, and hence the order is anomalous with respect to their molar volumes (Table 1). This may be due to the differences in chemical structures/architectures of these oils compared to linear hydrocarbon oils. Hence, it is suggested that not only the molar volume of oils but also their chemical structures and configurations (overall chemical architecture) have an influence on x0, x0,max and xNaCl,max. Such unusual behavior can be explained in the following way: the structural changes induced by addition of oil usually fall under two main scenarios: one is the ‘‘penetration effect’’, in which oil molecules penetrate into the surfactant palisade layer and expand the effective crosssectional area, as. The other is ‘‘swelling effect’’, in which oil molecules are solubilized in the core of aggregates and expand the volume of aggregates. In this case, ‘as’ is almost constant [38]. Further, the effect of oil on solubilization as well as on the geometry of the aggregates can be explained in terms of packing of the oil into micelles, which is determined by its chemical structure. Long chain fatty acid esters are oils with a fair degree of lipophilicity along with a polar ester moiety. The differences in the behavior of these oils towards water solubilization in these systems on account of their different chemical structures can be rationalized from the following viewpoints; (1) long carbon chain length of saturated (myristate and palmitate) and cisunsaturated (oleate) fatty acids; (2) straight (ethyl) and branching (isopropyl) in alkyl chain (short), which constitutes the ester group; and (3) relative contribution of fatty acid chain and alkyl chain (short). As a result, the degree of oil penetration into the surfactant aggregate varies, and depends on its location (or site), which probably affects intermolecular spacing as well as the arrangement of surfactant molecules at the interface [39]. It is probable that

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the alkyl chain (short) of the oil is located in the palisade layer of the micelles along with the ester group, whereas the fatty acid chain (longer) is more likely to be associated with the hydrophobic tails of the surfactants at the interface, rather than forming an oil pool in the core of the aggregates [40]. Further, it can be noted that the degree of penetration of an oil with branching in the alkyl chain, for example IPM [41], or cis-unsaturation in the long fatty acid chain, for example EO [41, 42], may contribute towards modulation of the mixed surfactant interfacial curvature, resulting in its greater flexibility or rigidity, and thus influence the preferred surface curvature, which further affects the solubilization capacity. This explains why water or NaCl solubilization capacities in these systems do not follow the same order with respect to molar volume of oils unlike linear hydrocarbons, and depend rather on chemical structure, solubilization site, and penetration of the oil into the interface. However, the molecular arrangement of the constituents or microstructure of these mixed RMs can be deciphered more accurately from SANS or 1H NMR rotating frame nuclear Overhauser effect spectroscopy (ROSEY) studies [40, 42], which is beyond the scope of the present investigation. Fanun [39] reported the difference in water solubilization capacity in a mixed ethoxylated mono-di-glyceride/ sucrose laurate system stabilized in three studied oils (IPM, caprylic-capric triglyceride and R(?)-limonene) and explained differences in the degree of oil penetration in terms of differences in the chemical structures of the surfactant palisade layer. Solubilization Efficiency Parameter (SP*water and SP*NaCl) An attempt was made to introduce a new parameter, ‘‘solubilization efficiency parameter (SP*)’’ to underline the efficacy of polar lipophilic oils on synergism in the solubilization capacity of water or NaCl in mixed RMs. This aspect is dealt with in section 3 of the Electronic Supplementary Material. Thermodynamics of Desolubilization of Water or Aqueous NaCl in Mixed Reverse Micellar Systems Water can be solubilization in a mixed surfactant(s)/oil medium up to the maximum solubilization limit or phase separation point leading to the formation of water-in-oil microemulsions, which can be considered as maximum solubilization capacity of water. The corresponding standard free energy change of dissolution of water, (DG0s ) at constant temperature can be obtained from the relation [17, 19, 43, 44]:

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DG0s ¼ RT ln Xd

ð1Þ

where, Xd is the mole fraction of dispersed phase (here, water), R is the gas constant, T is the experimental temperature (303 K). The estimation of DG0s was estimated based on mole fraction of the dispersed water. The apparent DG0s values (reported herein) are good enough to exhibit a comparative trend of results obtained under different experimental conditions. The Gibbs-Helmoltz equation has been used to obtain the standard enthalpy change for dissolution of water, DHs0 . o DG0s =T 0 ð2Þ DHs ¼ oð1=T Þ And finally the standard entropy change of dissolution of water, DS0s is given by the relation: DHs0 DG0s 0 DSs ¼ ð3Þ T Considering the phase separation point of maximum solubility of electrolyte (NaCl), the free energy of dissolution of aqueous NaCl, (DG0s;NaCl ), the enthalpy of 0 dissolution of aqueous NaCl, (DHs;NaCl ) and the entropy of dissolution of aqueous NaCl, (DS0s;NaCl ) at the microdispersed state were been estimated according to Eqs. (1)–(3). For aqueous NaCl-based systems, Xd represents the mole fraction of dispersed NaCl at 303 K. A representative plot of DG0s;NaCl against concentration of NaCl (mol m-3) (Fig. 5) is depicted for DOSS/STO20EO/ IPM (or IPP, or EO) systems at a fixed composition (XSTO20EO = 0.05, where x0,max was observed) in the presence of NaCl. A similar plot can be drawn for DOSS/ SMO20EO based systems (not shown). The values of all thermodynamic parameters calculated according to Eqs. (1)–(3) for DOSS/STO20EO or SMO20EO/oil(s) systems in the absence and presence of NaCl are presented in Tables 5 and 6. DG0s and DG0s;NaCl values have been found to be positive for all microemulsion-forming solutions, since a dispersion process is non-spontaneous and corroborates well with previous reports [20, 21, 45–49]. For titration with water or aqueous NaCl, a moderate decrease of DG0s and DG0s;NaCl were observed upon increasing the amount of water or aqueous NaCl (i.e., x0 or x0,max or xNaCl,max), leading to formation of microheterogeneous solutions. DG0s values for single DOSS as well as mixed DOSS/STO20EO or SMO20EO systems follow the order: IPM \ EO \ IPP, which corroborates well with corresponding water solubilization limit (x0 or x0,max) as discussed earlier (see section on ‘‘Solubilization of Water in Mixed RMs in Polar Lipophilic Oils’’). A similar correlation between


Fig. 5 Free energy of dissolution of NaCl (DG0s;NaCl ) as a function of concentration of NaCl at 303 K

‘solubilization efficiency parameter’ (SP*) and DG0s was shown in our previous report [25]. Thus, several factors (for example, the content and configuration of the polar head group of nonionic surfactants, their hydrophobic moiety, etc., for a particular oil) determine the value of x0,max for these mixed systems. It can be inferred that, at the solubilization maximum, contributions of all these factors are optimized in order to attain x0,max as revealed from the thermodynamic point of view. The solubilization process yielding w/o microemulsion continues up to the phase transition point. The calculated enthalpy thus represents the overall associated heat up to the phase separation point per mole of water added and stands for the enthalpy of solution of water in oil ? 0 amphiphile medium. Both DHs0 and DHs;NaCl are found to be negative, which indicates that the solubilization process ends up with release of heat (i.e., is an exothermic process). The exothermicity of solubilization of water in a alkylphenol ethoxylate/butanol/heptane ternary mixture was reported in the literature [20]. Also, the DS0s and DS0s;NaCl values are all negative, with reasonable magnitude. The dispersed droplets of water or aqueous NaCl are surrounded by a layer of amphiphile in the dispersion medium in the organized state so that the entropy change is negative. Further, during dispersion, the normal molecular arrangement of the dispersion medium is initially broken, which subsequently rearranged [45]. The DS0s and DS0s;NaCl values are negative for water dispersion, which has a similarity with the solubilization of non-polar compounds in water called the hydrophobic effect [50], where the entropy of solution is negative by the formation of icebergs

873

surrounding the non-polar body. In a calorimetric study, negative DS0s was observed for water dissolution in DOSS/ alkanol medium [17]. Negative DS0s was also observed for water dissolution in isopropyl myristate/medium-chain glyceride/SMO20EO medium [45]. Both negative enthalpy and entropy of dispersion were reported by Acharya et al. for a (Ricebran, RB ? IPM)/(ethoxylated oleyl alcohol ? i-PrOH)/water system [49] and for a coconut oil/ (ethosxylated cetyl alcohol ? 2-PrOH) (1:1)/water system [51]. Dissolution of water into amphiphile/oil medium can be modeled as four major processes: (1) endothermic dispersion of water, (2) endothermic penetration of water in the interior of RMs (aggregates), (3) exothermic reorganization of the amphiphile at the oil/water interface, and (4) exothermic organization of the penetrated water. The sum total heat shows exothermicity: the sum of the contribution of the processes one and two are, therefore, less than those of the three and four [19]. From Table 5, it can be observed that DHs0 more exothermic in mixed DOSS/nonionics (XSTO20EO or SMO20EO = 0.05) systems compared to single DOSS system in these oils. The greater solvation of polyoxyethylene head group of STO20EO or SMO20EO (compared to DOSS) [52] may contribute an exothermic share to the overall thermal events, making the resultant enthalpy of water solution more exothermic with mixed DOSS/STO20EO or SMO20EO systems. On the other hand, DOSS/SMO20EO system makes a larger exothermic contribution towards the water dissolution process compared to DOSS/STO20EO systems at comparable conditions. Also, the water dissolution process in oil is more stabilized or organized (higher negative DS0s value) in the DOSS/SMO20EO system compared to the DOSS/ STO20EO system. This indicates that not only the polar head group but also the hydrophobic moiety of the amphiphile (nonionic surfactant) influences overall thermal events. Oil also plays an important role in water dissolution processes. The values of DHs0 and DS0s for all these systems follow the order; IPM \ EO \ IPP, which is consistent with the order of DG0s or DG0s;NaCl and SP*. Energetically, the water dissolution process in oil is more exothermic (higher negative DHs0 value), and is more stabilized/organized (higher negative DS0s value) in IPP. Thus, it can be concluded that this type of polar lipophilic oil (IPM or EO or IPP) can undergo both polar (arising from the carbonyl group present in the oil and surfactant) and hydrophobic interactions with surfactants (DOSS or STO20EO or SMO20EO) during the solubilization process, thereby contributing a fair share to the energetic process. It has also been observed that systems with larger - DHs0 values possess larger -DS0s values,

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874 Table 5 Thermodynamic parameters of desolubilization processa (DG0s , DHs0 and DS0s ) for different mixed RMs with total surfactant concentration of 0.10 9 103 mol/m3


System with composition

Oil

Temperature (K)

DG0s (kJ mol-1)

DOSS/oil/water XSTO20EO or STO20EO = 0

IPM

303

2.28

313

2.42

323

2.57

EO

IPP

DOSS/STO20EO/ oil/water XSTO20EO = 0.05

IPM

EO

IPP

DOSS/SMO20EO/ oil/water XSMO20EO = 0.05

IPM

EO

IPP a

The error limits in DG0s , DHs0 and DS0s are ±4, ±6 and ±10 % respectively

303

2.55

313

2.72

323

2.94

303

2.86

313

3.09

323

3.34

303

1.68

313

1.79

323 303

1.92 1.69

313

1.85

323

2.04

303

1.73

313

1.97

323

2.17

303

1.87

313

2.11

323

2.45

303

1.89

313

2.18

323

2.50

303

2.12

313

2.48

323

3.05

DHs0 (kJ mol-1)

DS0s (J K-1 mol-1)

-2.24

-14.0

-3.47

-19.0

-4.24

-23.0

-2.26

-11.0

-3.57

-17.0

-5.11

-22.0

-6.93

-28.0

-7.37

-30.0

-12.05

-46.0

0 Table 6 Thermodynamic parameters of desolubilization processa (DG0s;NaCl , DHs;NaCl and DS0s;NaCl ) for different mixed RMs with total surfactant concentration of 0.10 9 103 mol m-3

DG0s;NaCl (kJ mol-1)

0 DHs;NaCl (kJ mol-1)

DS0s;NaCl (J K-1 mol-1)

303

1.51

-1.99

-11.0

313

1.62

323

1.74 -5.83 (-4.28)

-24.0 (-20.0)

Temperature (K) [DOSS/STO20EO/IPP/NaCl (aqueous)]b

[DOSS/SMO20EO/IPP/NaCl (aqueous)]c 303

1.87 (1.56)d

313

2.03 (1.80)

323

2.27 (2.04)

a

The error limit in DG0s , DHs0 and DS0s are ±4, ±6 and ±10 % respectively

b

XSTO20EO = 0.05, [NaCl] = 0.10 9 103 mol m-3

c

XSMO20EO = 0.05, [NaCl] = 0.10 9 103 mol m-3

d

0 Values in parenthesis indicate DG0s;NaCl , DHs;NaCl and DS0s;NaCl for DOSS/SMO20EO/IPP/NaCl (aqueous) system at XSMO20EO = 0.05, and 3 -3 [NaCl] = 0.15 9 10 mol m

123


i.e., the greater the interaction strength among the constituents, the greater the ordering of water [53] (Table 5). It can be observed from Table 6 that values of both the 0 energetic parameters (DHs;NaCl and DS0s;NaCl ) decrease with increase in [NaCl] at fixed compositions (i.e., with total surfactant concentration of 0.10 9 103 mol/m3 and XSMO20EO,max = 0.05) for a DOSS/SMO20EO blended 0 system in IPP. Also, both DHs;NaCl and DS0s;NaCl -values are lower than the corresponding values of DHs0 and DS0s at comparable conditions, indicating that solubilization of NaCl in mixed RMs is less exothermic and less organized than in water. From these thermodynamic results, it can be inferred that the presence of NaCl influences the interfacial rigidity vis-a`-vis the interaction among the droplets in these systems. On the other hand, the content, size and configuration of polar head groups, and the hydrophobic moiety of nonionic surfactants play a significant role under this changed environment (aqueous NaCl instead of water). As mentioned earlier, incorporation of NaCl makes the interfacial film less flexible by shielding the charge of DOSS head groups, thereby reducing head group repulsion as well as attractive interactions (ion-dipolar interaction between DOSS and any nonionic) among the droplets. The possibility of formation of hydrogen bonds between water inside the droplets and the polar interface may become more feasible, which in turn makes the desolubilization process less exothermic [15]. These views corroborate the further maximization of the solubilization capacity of mixed systems in the presence of NaCl at their corresponding Xnonionic,max as discussed earlier (see section on ‘‘Solubilization of NaCl (Aqueous) in Mixed RMs in Polar Lipophilic Oils’’). The overall thermodynamic properties are a complex combination of a number of processes, viz., disruption of solvent structure and penetration of the dispersant (oil or water), fragmentation of the dispersant, transfer of amphiphile from the bulk to the interface, stabilization of the droplets by the amphiphile, and their orientation/organization at the interface between oil and water [49]. More intimate knowledge of the mixing and related phenomena is required for quantification of the process. Deciphering the individual contributions toward the overall process would be a worthwhile task, and may be taken up in a future study. Conductometric Studies In this section, an attempt was made to correlate the maximum solubilization of electrolyte (NaCl) with percolation of conductance in mixed RMs.

875 Table 7 Percolation threshold (xp) in DOSS/STO20EO/oil(s)/NaCl (aqueous) mixed RMs in different oils with ST = 0.10 9 103 mol m-3 at XSTO20EO = 0.2 (fixed) and 303 K with varying [NaCl] (mol m23) 10-3[NaCl] (mol m-3)

xp EO

IPM

IPP

0

21.67

26.39

18.06

0.03

25.00

27.78

0.05 0.0625

– –

– 29.17

0.08

34.72

–

0.1

35.27

36.11

0.125

44.44

45.00

– 20.83 – – 26.67 –

0.15

–

–

29.17

0.175

–

–

36.11

Effect of [NaCl] on the Percolation of Conductance in Mixed RMs The effect of NaCl at different concentrations (viz. 0.03 9 103 to 0.175 9 103 mol/m3, covering below and above [NaCl]max, at which xNaCl,max is obtained) on conductivity of the mixed surfactant RMs DOSS/STO20EO/ oil(s)/water at a fixed XSTO20EO = 0.2 with [ST] equals to 0.10 9 103 mol/m3, was studied at 303 K and the results are presented in Table 7. Representative plots of conductance versus x as a function of [NaCl] in DOSS/STO20EO/ IPP or IPM or EO systems are depicted in Figs. 6 and 7, respectively, and the percolation zone in the solubilization capacity–[NaCl] profile of DOSS/STO20EO/IPP system is depicted in Fig. 8. Figures for the other systems are shown. It is evident from Table 7 that the presence of NaCl shifts the percolation threshold (xp) towards higher x with respect to water at comparable composition, i.e., XSTO20EO = 0.2 for all studied systems. At [NaCl] = 0.20 9 103 mol/m3, no percolation was observed for the above system. Furthermore, examination of Fig. 8 reveals that percolation occurs at [NaCl] \ [NaCl]max. As [NaCl] increases, the xp increases. In other words, the effect is more pronounced at higher [NaCl]. However, the effect of NaCl at comparable concentration, viz. 0.10 9 103 mol m-3 (\[NaCl]max) on percolation threshold (xp) was seen to depend on oil type. xp (NaCl) follows the same trend as that of xp (water); IPM [ EO [ IPP, as discussed below. It can be inferred from generalization of Fig. 8 to all these systems that percolation of conductance is exhibited by the samples chosen from the ascending curve (indicating droplets with fluid interface vis-a`-vis the interdroplet interaction branch, Rc); on the other hand, the samples chosen from the descending branch (i.e., curvature branch, Ro) have been found to be low conducting and a

123

876


Fig. 6 Conductance study of DOSS/STO20EO/IPP (a) or IPM (b)/NaCl (aqueous) mixed RMs at fixed XSTO20EO = 0.2, ST = 0.10 9 103 mol m-3 and 303 K

Fig. 7 Conductance study of DOSS/STO20EO/EO/NaCl (aqueous) mixed RMs at fixed XSTO20EO = 0.2, ST = 0.10 9 103 mol m-3 and 303 K

Fig. 9 Conductance study of DOSS/STO20EO/oil(s)/NaCl (aqueous) mixed RMs at fixed XSTO20EO = 0.2, [NaCl] = 0.10 9 103 mol m-3, ST = 0.10 9 103 mol m-3 and 303 K

non-interacting hard sphere type droplet structure can be assumed. It is known that addition of electrolyte diminishes the effective polar head area of the surfactants by screening the electrostatic repulsion, which makes the surfactant membranes more rigid and decreases the attractive interactions among the droplets. In the presence of NaCl, large activation energies are needed to create the positive curvature of local regions and thus percolation is hindered, which in turn increases solubilization [7, 15, 27, 34]. Effect of Oil on the Percolation of Conductance with Varying x in Mixed RMs in Presence of NaCl

Fig. 8 Percolation zone (shaded) of DOSS/STO20EO/IPP/NaCl (aqueous) mixed RMs at 303 K with total surfactant concentration of 0.10 9 103 mol m-3 in solubilization capacity-[NaCl] profile

123

Figure 9 illustrates the effect of oil type on the percolation of conductance in DOSS/STO20EO blend systems (with ST = 0.10 9 103 mol m-3) in the presence of NaCl at fixed XSTO20EO = 0.2, [NaCl] = 0.10 9 103 mol m-3 and at 303 K. From Table 7 and Fig. 9, it is clear that xp (NaCl)


follows the same trend as that of xp (water): IPM [ EO [ IPP, indicating that longer fatty acid chain length (oleate or palmitate) tends to reduce xp much more than do smaller chains (myristate). In other words, the trend in reducing x-induced percolation (xp) is as follows; IPP [ EO [ IPM, which does not follow clearly from the order of molar volume of oils, as proposed by Shah et al. [32, 33] for linear hydrocarbons. The positions of IPP and EO are interchanged, and the order is anomalous with respect to their molar volumes (Table 1). Apart from variation in fatty acid hydrocarbon chain length, variation in penetrating ability due to presence of an ethyl (EO) or isopropyl (IPM and IPP) group might also play important role in tuning interfacial flexibility or rigidity, attractive interdroplet interaction as well as droplet dimensions and subsequently xp. According to Afifi et al. [40], the shorter alkyl chain (herein ethyl or isopropyl) associated with the ester moiety of these oils is buried inside the interfacial film. Therefore, EO adsorbs more inside the film than IPP due to the presence of shorter ethyl chain compared to the isopropyl chain connected to the ester moiety. Hence, the penetration ability of EO is greater than that of IPP and, consequently, EO makes the oil/water interface more rigid than does IPP. Thus, the efficiency order of EO in reducing xp is lower than IPP. Additionally, due to the presence of a cis-double bond in EO (i.e., adjacent hydrogen atoms are on the same side of the double bond, Scheme S1), it has a natural tendency to bend [42]. Thus, the conformational freedom of the fatty acid chain of EO is restricted due to the bending configuration and as a result, the oil/water interface becomes less flexible compared to the saturated chain containing IPP system. Hence, the efficiency order of EO in reducing xp is lower than that of IPP. Liu et al. [54, 55] and Mitra and Paul [8] reported that not only the molecular volume but also the configuration of solvent molecules has significant effects on the conducting properties of DOSS/alcohol ethoxylates/hydrocarbon oil(s) and DOSS/nonionic(s)/IPM RMs, respectively.

Conclusions The present study represents a systematic investigation into the effects of composition of the surfactant (Xnonionic), hydrophobic moiety of nonionic surfactants, [NaCl], and the type of oil on the solubilization phenomena and microstructure of mixed DOSS/STO20EO or SMO20EO/ water (aqueous NaCl) RMs stabilized in three polar lipophilic oils (EO, IPM and IPP) at 303 K. In addition, the thermodynamic parameters of desolubilization process 0 [DG0s or DG0s;NaCl , DHs0 or DHs;NaCl and DS0s or DS0s;NaCl ] of these systems were evaluated under varied physicochemical environments.

877

The addition of STO20EO or SMO20EO induces synergism in the solubilization capacity of water in DOSS/EO or IPM or IPP/water RMs. x0,max depends on the hydrophobic moiety of nonionic surfactants and type of oils. The addition of NaCl further enhances the solubilization capacity (xNaCl,max) of mixed RMs in these oils at threshold concentration, [NaCl]max at 303 K and depends on Xnonionic, hydrophobic moiety of nonionic surfactants and oil. The magnitude of [NaCl]max increases with increase in XSTO20EO. Further, x0, x0,max, and xNaCl,max (at XSTO20EO,max), respectively, for all studied oils follows the order: IPP \ EO \ IPM, which is not consistent with the report of Shah et al. [32, 33]. Herein, EO has exchanged its position with IPP, which possesses certain dissimilarity in structure compared to other oils. The presence of both short alkyl chain [ethyl or isopropyl (branching)] and fatty acid chains of different lengths (myristate or palmitate or oleate with cis-unsaturation) in these oils contributes towards modulation of the mixed surfactant interfacial curvature, resulting in greater flexibility or rigidity, thus influencing the preferred surface curvature, which further affects the solubilization capacity. As a result, these parameters do not follow the expected order with respect to molar volume; instead the order depends on chemical structure, solubilization site, and penetration of these oils into the interface. SP*water or SP*NaCl were evaluated to underline the efficacy of oils on the synergistic behavior in solubilization capacity in mixed RMs. The order of both these parameters follows: IPM \ EO \ IPP, which is not consistent with the molar volume of oils. Overall, x0,max and xNaCl,max for these systems follows the order: DOSS/STO20EO [ DOSS/SMO20EO, whereas [NaCl]max follows the order; DOSS/SMO20EO [ DOSS/STO20EO in all oils (EO, IPM and IPP). DG0s and DG0s;NaCl of these systems was found to be positive for all systems, and to depend on Xnonionic, the hydrophobic moiety of the nonionics and [NaCl]max, respectively. DG0s values for AOT as well as DOSS/ STO20EO or SMO20EO systems follow the order: IPM \ EO \ IPP, which corroborates well with the corresponding water solubilization limit (x0 or x0,max). Both 0 DHs0 and DHs;NaCl were found to be negative, indicating that the solubilization process ends in the release of heat (i.e., exothermic process). Also, the DS0s and DS0s;NaCl values are all negative of comparable magnitude. DHs0 were found to be more exothermic in DOSS/STO20EO or SMO20EO (XSTO20EO or SMO20EO = 0.05) systems compared to DOSS system in these oils. On the other hand, the ADOSS/SMO20EO system shares a larger exothermic contribution towards the water dissolution process compared to the DOSS/STO20EO system under comparable conditions. Also, the water dissolution process in oil was found to be more organized (with larger negative value of

123

878

DS0s ) in the DOSS/SMO20EO system compared to the DOSS/STO20EO system. The values of DHs0 and DS0s for all these systems follow the order; IPM \ EO \ IPP, which is consistent with the order obtained for DG0s or DG0s;NaCl and SP*. Energetically, the water dissolution process in oil was found to be more exothermic as well as 0 and DS0s;NaCl decrease more organized in IPP. Both DHs;NaCl with increasing [NaCl] at a fixed compositions for a DOSS/ SMO20EO blended system in IPP. Also, values of both 0 and DS0s;NaCl are lower than DHs0 and DS0s , indiDHs;NaCl cating that solubilization of NaCl in mixed RMs is less exothermic and less organized than in water. DOSS/oil(s)/ water RMs are non-percolating, but the addition of STO20EO to these systems induces percolation in conductance with increase in x at 303 K. xp depends on [NaCl] and oil type. xp (NaCl) follows the same trend as that of xp (water): IPM [ EO [ IPP. The appearance of solubilization maxima in these mixed RMs (in the absence and presence of NaCl) were correlated with the occurrence of percolation of conductance in such systems. The understanding of the solubilization capacity and dissolution behavior of added water or aqueous NaCl in these systems might be expected to play a key role in making these microemulsion/RM vehicles a good carrier for pharmaceutical products. Also, characterization of the internal structure of these microemulsion/RM vehicles by means of conductance in percolation is very important for the diffusivity of the phases and thereby also for the diffusion of a drug in the respective phases. Understanding the relationship between microstructure and composition of RMs is important in order to prepare and optimize RMs for protein purification processes [56] and for efficient use in drug delivery [57]. Acknowledgments Financial support in the form of an operating research grant to B.K.P. and Senior Research Fellowship to K.K. from the authority of Indian Statistical Institute, Kolkata, India is thankfully acknowledged.

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Author Biographies Kaushik Kundu studied chemistry at the Jadavpur University, Kolkata and received his MSc degree (specialization in physical chemistry) in 2008. He is a PhD student under the supervision of Professor Bidyut K. Paul in the Indian Statistical Institute, Kolkata. His research interest is in the field of physico-chemical studies of mixed surfactant microemulsions/reverse micelles involving biocompatible ingredients. Bidyut K. Paul is a professor at the Surface and Colloid Science Laboratory, Geological Studies Unit, Indian Statistical Institute, Kolkata, India. His research interests are in colloid and interface science with special reference to mixed surfactant micelles and microemulsions/reverse micelles, adsorption of surfactants at the solid-solution interface. He also interested in the synthesis and characterization of mesoporous oxide materials in soft templates.

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