Physicochemical investigation of mixed surfactant reverse micelles: Water solubilization and conductometric studies

Colloids and Surfaces A: Physicochem. Eng. Aspects 433 (2013) 154–165

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Physicochemical investigation of mixed surfactant reverse micelles: Water solubilization and conductometric studies Kaushik Kundu, Bidyut K. Paul ∗ Surface and Colloid Science Laboratory, Geological Studies Unit, Indian Statistical Institute, 203, B.T. Road, Kolkata 700108, India

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Mixed • • • •

ionic/nonionic reverse micelles exhibit synergism in solubilization capacity. Hydrophobic moiety and head group of nonionic’s affect the solubilization phenomena. Solubilization of water in oil(s) is more stabilized in isopropyl palmitate. Solubilization efficiency parameter has been evaluated to underline efficacy of the oils. Structure of polar lipophilic oils influences water solubilization and related phenomena.

a r t i c l e

i n f o

Article history: Received 17 January 2013 Received in revised form 29 April 2013 Accepted 3 May 2013 Available online xxx Keywords: Mixed reverse micelle Polar lipophilic oil Solubilization efficiency parameter Thermodynamics of dissolution Percolation phenomena Additive

a b s t r a c t Solubilization of water in mixed reverse micelles (RMs) comprising sodium bis(2-ethylhexyl) sulfosuccinate (AOT), and polyoxyethylene (20) sorbitan trioleate (Tween-85) or polyoxyethylene (20) sorbitan monooleate (Tween-80) or sorbitan trioleate (Span-85) has been studied at different compositions (Xnonionic = 0–1.0) at a total surfactant concentration, ST = 0.1 mol dm−3 in polar lipophilic 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 AOT/oil(s)/water systems. The maximum water solubilization capacity (ω0,max ) and Xnonionic,max (mole fraction at which synergism occurs) have been influenced by polar head group and hydrophobic moiety of nonionic surfactant. The standard free energy change of dissolution of water (Gs0 ) of these systems depends on water content, XTween-85 and oil. Solubilization efficiency parameter (SP∗water ) has been evaluated to underline the efficacy of oils in obtaining maximum water solubilization capacity in mixed RMs. Conductance behavior of these systems in absence and presence of additives (bile salts and hydrotrope) has also been investigated under varied water content (ω) at 303 K. Volumeinduced percolation threshold (ωp ) depends on XTween-85 , oil type, and additives. An attempt has been made to give an insight to the mechanism of solubilization phenomena, standard free energy change of dissolution of water, percolation in conductance and microstructures of these systems by dynamic light scattering (DLS) measurements, wherein the chemical structures of both nonionic surfactants and polar lipophilic oils played significant role. © 2013 Elsevier B.V. All rights reserved.

1. Introduction ∗ Corresponding author. Tel.: +91 33 2575 3164; fax: +91 33 2577 3026. E-mail addresses: [email protected] (K. Kundu), [email protected], [email protected], [email protected] (B.K. Paul). 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.05.009

Reverse micelles (RMs) are macroscopically homogeneous mixtures of oil, water (or, sodium chloride) and surfactant(s), which in the microscopic level individual domains of oil and water separated

K. Kundu, B.K. Paul / Colloids and Surfaces A: Physicochem. Eng. Aspects 433 (2013) 154–165

by a monolayer of surfactant(s). They possess some unique characteristics such as thermodynamic stability (imparting long shell life), compartmentalized polar and non-polar dispersed nano-domains, ease of formation, ultralow surface tension, low viscosity, large surface area and optical transparency. One of the most important features of RMs is the presence of highly structured yet heterogeneous water molecules present in biological systems such as membranes. These properties qualify them to be prospective drug delivery systems provided they are composed of nontoxic excipients [1,2]. 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, and salt concentration [3,4]. An important aspect of RM/microemulsion design is its ability to solubilize a maximum amount of dispersed phase (i.e. water) into the continuous phase. Single surfactant does not necessarily produce the best RMs. Method which has been suggested to enhance water solubilization capacity in RMs is using surfactant mixtures. Stabilizing the RM with a mixture of surfactants is very common. In commercial applications, blending of surfactants is the rule rather than the exception. This is for many reasons, but mainly because the surfactant mixtures may improve the product performance. Minimizing the amount of surfactant added to any product or processes gives obvious economic and environmental benefits, and an effective means of doing this is with synergistic mixtures [5]. The investigation on the solubilization capacity of anionic or cationic surfactant with nonionic surfactant mixtures in non polar solvents showed that the solubilization of water (or, sodium chloride) increased significantly with the incorporation of nonionic surfactant [6–8]. The structure and properties of RMs have been investigated extensively by employing a variety of physicochemical techniques, for example, Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR), fluorescence spectroscopy, scattering techniques, calorimetry, dynamic light scattering (DLS), and conductivity [9–19]. Among these, electrical conductivity provides a convenient, useful, and accessible tool for probing the microstructure of RMs. Of the different physical properties of RMs, percolation of conductance is striking; where many fold (100–1000 times) increase in conductance can take place after a threshold volume fraction of the dispersant (water) at a constant temperature or after a threshold temperature at a constant composition [20–22]. The basic understanding of percolation process and related aspects in water-in-oil (w/o) microemulsions/RMs has been reported by a number of research workers [22–25]. Understanding the percolation process is also important for performing enzymatic reactions in w/o microemulsion or RMs [26]. Other technique which can contribute significantly in understanding the inter-micellar interaction in RMs is dynamic light scattering (DLS) [27]. Coupling the conductance study with DLS technique can give better insight into the percolation mechanism vis-à-vis droplet dimension and the polydispersity index (PDI) of mixed RMs [3]. Till now, most of the studies regarding the solubilization phenomenon and conducting properties coupling with droplet dimensions in RMs stabilized by both single and mixed surfactant systems have been carried out using linear hydrocarbons as solvent. However, polar lipophilic oils, which possess different chemical structures and physicochemical properties compared to the hydrocarbon oils, are widely used in biologically resembling systems, pharmaceuticals and drug delivery. Such studies using these oils are seldom reported in literature [6,7]. Very recently, Mehta et al. [28] reported the fully characterization of polyoxyethylene (10) oleyl ether (Brij-96)/cosurfactant (C2 OH C6 OH)/ethyl oleate (EO) or isopropyl palmitate (IPP) or isopropyl myristate (IPM)/water microemulsion systems using conductivity, optical

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microscopy, dilution method, absorption, and FTIR spectroscopy. Also, Wang et al. [29] formulated self-nanoemulsifying drug delivery systems (SNEDDS) using polyoxyethylene sorbitan fatty acid esters (Tweens) and sorbitan fatty acid esters (Spans) as surfactants and IPM or EO or methyl oleate (MO) or methyl decanoate (MD) as oils to improve the dissolution rate of ibuprofen (a model poorly water soluble drug). Their studies also suggested possibility for controlling the droplet size yielded by SNEDDS. In the present report, we contemplate to undertake studies on the solubilization and thermodynamics of dissolution of water (Gs0 ), and conductance behavior of AOT RMs in the presence of non-ionic surfactants with different chemical structures that is, with similar or dissimilar polar head group and hydrophobic moieties [viz., polyoxyethylene (20) sorbitan trioleate (Tween-85) or polyoxyethylene (20) sorbitan monooleate (Tween-80) or sorbitan trioleate (Span-85)] in polar lipophilic oils of different chemical structures [consisting of long fatty acid chain and short alkyl chain (linear or branch) on either side of hydrophilic ester moiety, viz. EO, IPM, IPP] at different physicochemical conditions. Further, droplet size has been measured of these systems by dynamic light scattering (DLS) technique by changing compositions and oil types. Also, the effect of additives [viz. sodium cholate (NaC), sodium deoxycholate (NaDC), sodium taurodeoxycholate (NaTDC) and sodium salicylate (NaSl)] on the percolation of conductance in mixed RMs has also been taken up, as such studies in mixed RMs are rarely reported [19]. These studies would be of much importance and significance to underline the solubilization of water, microstructure and droplet dimensions of these novel systems in absence and presence of additives. Solubilization efficiency parameter (SP∗water ) has been evaluated to underline the efficacy of a particular oil in obtaining maximum water solubilization capacity at the corresponding composition in mixed RMs on the basis of a relative increase in solubilization of the dispersed phase (herein, water) in the oil compared to single AOT-based RM. Further, an attempt has been made to decipher the physicochemical concept of SP∗water in the light of standard free energy change of dissolution of water in these systems. Polar lipophilic oils are noteworthy to investigate because of their structural resemblance to the lipids in living systems and they are expected to be environment friendly [30]. AOT or nonionic surfactants (Brijs, Tweens, and Spans) stabilized in IPM, IPP, and EO find applications in biologically relevant microemulsion systems [30–34]. Further, bile salts play vital roles in a number of physiological processes such as lipid digestion, drug adsorption, and cholesterol solubilization [35]. Finally, an attempt has been made to improve the understanding of the synergism in solubilization of water due to the addition of nonionic surfactant(s) to ionic surfactant in these oils, wherein variation in their chemical structures has been taken into account. And, also appearance of maximum in solubilization capacity has been examined to correlate microstructural variation due to the curvature effect and attractive interaction of the surfactant aggregates from conductance, DLS measurements and thermodynamic approach. Such a comprehensive study in mixed RMs is not reported in literature.

2. Experimental 2.1. Materials Sodium bis(2-ethylhexyl) sulfosuccinate (AOT, 99%), polyoxyethylene (20) sorbitan trioleate (Tween-85), polyoxyethylene (20) sorbitan monooleate (Tween-80) are purchased from Sigma, USA. Sorbitan trioleate (Span-85) is a product of Fluka (Switzerland). The surfactants were used without further purification. Ethyl oleate (EO, ≥98%), isopropyl myristate (IPM, ≥98%), isopropyl palmitate (IPP, 99%), sodium cholate (NaC, ≥99%), sodium

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Scheme 1. Chemical structure of oils and additives.

deoxycholate (NaDC, ≥97%), sodium taurodeoxycholate (NaTDC, ≥97%) and sodium salicylate (NaSl, ≥99.5%) are products of Fluka (Switzerland). Sudan IV and Eosin Blue are AR grade products of SRL, India. The chemical structures of oils and additives, and surfactants are represented in Scheme 1 and Scheme S1 (supplementary materials), respectively. Double distilled water was used with conductance less that 3 ␮S cm−1 .

2.2. Methods Water (or, aqueous solution of additives of different concentrations) was gradually injected using microsyringe of varying capacity into 3 ml of 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 0.1 mol dm−3 . The onset of permanent turbidity at each composition of surfactant mixture in oil denotes maximum solubilization of water at end point of titration. For experiments with the additives, their aqueous solutions were used instead of pure water in the preparation of RMs at a fixed concentration. All the experiments were repeated 2–3 times and mean results were taken.

The results of these experiments have been dealt in the subsequent section. Conductivity measurements were made using an automatic temperature-compensated conductivity meter, Thermo Orion, USA (Model 145A plus), at 303 K, with cell constant of 1.0 cm−1 . The reproducibility of the conductance measurement was found to be within ±1%. The average hydrodynamic radius (dh ) of the droplets was determined by dynamic light scattering (Nano ZS90, Malvern Instruments, UK) equipped with a He–Ne laser operating at 633 nm and at a scattering angle of 90◦ . Measurements were performed at 298 K. The mechanism of the instrument is as described elsewhere [36]. Before addition of water, the surfactant(s) and the oil phase mixture was filtrated using a 0.22 ␮m pore size membrane (Millex® -GP) to remove possible dust particles. For a certain system, the hydrodynamic radius of droplets (dh ) is correlated with diffusion coefficient (D), viscosity of solvents (), and temperature (T), which is expressed by the Stokes–Einstein equation: dh =

kB T 3D

where kB denotes the Boltzmann constant.

(1)

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35 IPM IPP EO

(A)

30

30

Solubilization capacity (ω0)

Solubilization capacity (ω0)

35

157

25 20 15

(B)

25 20 15

IPM

10

IPP EO

5 0

10 0

0.2

0.4

0.6

0.8

0

1

XTween-85

0.05

0.1

0.15

0.2

XTween-80

Fig. 1. Solubilization of water in mixed AOT/nonionic/oil reverse micellar systems as a function of concentration of (A) Tween-85 (XTween-85 ) and (B) Tween-80 (XTween-80 ) at fixed total surfactant concentration of 0.1 mol dm−3 and 303 K.

3. Results and discussion 3.1. Solubilization of water in mixed RMs stabilized in polar lipophilic oils Water solubilization capacity of mixed anionic (AOT)/nonionics (Tween-80 or Tween-85 or Span-85) RMs (at Xnonionic = 0 → 1.0) in polar lipophilic oils (viz. EO, IPM and IPP) at 303 K are presented in Fig. 1A and B and Table 1. Apart from difference in molar volume (Table S1), these oils contain short alkyl chain [linear (ethyl) or branch (isopropoyl)] on either side of hydrophilic ester moiety and long fatty acid chain of different lengths, viz., myristate, palmitate and oleate. Of these oils, EO contains a cis-double bond in the hydrocarbon chain of fatty acid (Scheme 1). Single AOT RMs (Xnonionic = 0) with total surfactant concentration, [ST ] of 0.1 mol dm−3 in IPM, IPP and EO have been observed to solubilize a substantial amount of water, ω0 (=[water]/[surfactant]) ∼ 22, 14 and 17, respectively. The solubilization capacities of mixed AOT/nonionic surfactant(s) RMs are observed to increase with increasing nonionic content (Xnonionic ), and then decrease after passing through maxima (ω0,max , indicates maximum amount of water solubilized in RMs). The results are depicted in Fig. 1A and B for AOT/Tween-85 or Tween-80 system, respectively in three oils. Similar plots for AOT/Span-85 systems have not been exemplified here. These nonionic surfactants possess different chemical structures with respect to hydrophobic configurations, similar or dissimilar polar head groups and HLB values

(Scheme S1) and their solubilization behaviors in the presence of AOT, are expected to depend on physicochemical conditions of the studies undertaken. The rationale behind using these nonionic surfactants is to underline the role of their polar head groups and/or hydrophobic configurations in water solubilization capacity of mixed RMs. Similar enhancement in water solubilization capacity in mixed surfactant RMs was reported earlier [3,4,6,7,13]. The influence of chemical structure and configuration of the nonionic surfactants and type of oil on both ω0,max and Xnonionic,max (mole fraction at which synergism occurs) has been dealt in subsequent sections. 3.2. Comprehensive analysis of solubilization of water in mixed RMs stabilized in polar lipophilic oils 3.2.1. Effect of nonionic surfactant In the present study, addition of nonionic surfactant(s) of different types and chemical structures/configurations (Tweens and Span) (Scheme S1) to AOT/oil(s)/water RMs shows synergism in their solubilization capacities and the results are presented in Table 1. Solubilization of water in RM depends upon many factors, for example, type of surfactant (and cosurfactant or a second surfactant), oil, temperature, and additives [3]. But the main driving force of such solubilization is the spontaneous curvature and the elasticity (or rigidity) of the interfacial film formed by the surfactants [37]. If the interfacial curvature and the bending elasticity are fixed, solubilization can be maximized by minimizing the interfacial bending

Table 1 Solubilization of water (ω0,max ) in mixed AOT/nonionic reverse micellar systems in different oils with total surfactant concentration, ST = 0.1 mol dm−3 at 303 K. Surfactant

Oil

ω0 a

ω0,max b

SP∗water c

AOT/Tween-85

IPM IPP EO

22.22 13.89 16.67

34.26 (0.05) 27.78 (0.05) 32.41 (0.05)

0.54 0.99 0.94

AOT/Span-85

IPM IPP EO

22.22 13.89 16.67

31.48 (0.4) 24.07 (0.4) 27.78 (0.4)

0.42 0.74 0.67

AOT/Tween-80

IPM IPP EO

22.22 13.89 16.67

29.63 (0.05) 22.22 (0.05) 25.93 (0.05)

0.33 0.60 0.56

a b c

ω0 = [water]/[AOT] in oils at [ST ] = 0.1 mol dm−3 at 303 K. The maximum solubilization of water at a particular XTween-85 given in the parenthesis. Solubilization efficiency parameter with respect to oil.

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stress of the rigid interface and the attractive interdroplet interaction [38]. The critical packing parameter (CPP, given by P = v/al where ‘v’ and ‘l’ are the volume and the length of hydrophobic chain, respectively, and ‘a’ the area of polar head group of the surfactant) plays an important role in water solubilization. For a mixed surfactant RM, the effective packing parameter (Peff ) follows the additive relation [39]: Peff =

[(xv/al)A + (xv/al)B ] xA + xB

(2)

where xA and xB are the mole fractions of surfactant A and B in the mixture, respectively. Any factor that increases the rigidity of the interface and hence the natural radius of curvature (R), increases the solubilization capacity significantly. Nonionic surfactants have larger head group area than AOT [40], and thus on mixing Tween-85 or Tween-80 or Span-85 with AOT, Peff is decreased owing to the increase in ‘a’, which in turn increases the natural radius of curvature (R). Thus, AOT/nonionic(s)/oil RMs can accommodate more water than the AOT/oil RMs, and with increasing Xnonionic , solubilization capacity of the mixed system is increased. But such an increase in R increases the interdroplet interaction, and at a certain value of R ∼ Rc (critical radius of curvature), the interdroplet interaction starts to govern the solubilization process and limits the solubilization capacity. Thus beyond Rc , addition of nonionic surfactant decreased the solubilization capacity. For all these mixed systems, the ascending curve in the solubilization capacity–Xnonionic profile is governed by the curvature effect due to the rigidity of the interface and the descending curve is governed by the interdroplet interaction effect. Our observations are also supported by the argument of Nazario et al. [41]. They reported that the main driving force for water solubilization in mixed surfactant, AOT/Ci Ej RMs (where ‘i’ is the number of carbon atoms in the hydrophobic part and ‘j’ is the number of EO chains in the hydrophilic part of the nonionic surfactant) is the presence of the hydrophilic group of the nonionic surfactant. In the present study, addition of these nonionic surfactants to AOT/oil(s)/water RMs produces synergism in their solubilization capacities (ω0,max ) at different concentrations (Xnonionic ), depending upon the nonionic surfactant and oil type. ω0,max and Xnonionic,max values are presented in Table 1. However, overall ω0,max for these systems follows the order: AOT/Tween85 > AOT/Span-85 > AOT/Tween-80, whereas Xnonionic,max follows the order: AOT/Span-85 > AOT/Tween-85 ∼ AOT/Tween-80 in three oils. A plausible explanation for the trend observed for ω0,max can be suggested from the view point of difference in chemical structures/configurations of the added nonionic surfactants. Both Tween-85 and Tween-80 possess same number (20) of POE chains as polar head group, but differ in hydrophobic moieties in respect of the presence of number of double bonds (Scheme S1). Such double bonds are positioned in the middle of the long hydrocarbon chain of ester linkage of these surfactants, which impose stereo-chemical constraints on the system. Consequently, the hydrocarbon chain bends and its volume increase considerably, but its length is slightly decreased. All these factors reduce the attractive interaction between the hydrophobic chains of the surfactant molecules and favor the formation of w/o microemulsion/RM as packing ratio (P = v/al) is raised due to decrease of hydrocarbon chain length (l). As Tween-85 contains more double bonds (three) in hydrophobic chains than Tween-80 (one double bond) (Scheme S1), the attractive interaction between hydrophobic chains of Tween-85 is less than Tween-80 [42]. It was reported earlier that, the enhanced attractive interaction is related to the penetration of oil molecules into the interfacial film, and subsequently, leads to change in interfacial curvature and fluidity. The penetration of oil molecules into the palisade layer of the interface (solute–solvent interaction) results in more

rigid surfactant chains (i.e. less attractive interaction between droplets) and promotes higher water solubilization [37]. Hence, AOT/Tween-85-based systems favor oil penetration compared to AOT/Tween-80-based systems, and promote higher water solubilization (i.e., higher ω0,max value) in former system than the latter. Again, Span-85 and Tween-80 differ in their chemical structures in respect of both hydrophobic and hydrophilic moieties (Scheme S1). So far as hydrophobic moiety of Span-85 is concerned, it contains more double bonds (three) in their hydrophobic chains than Tween-80 (one double bond) and subsequently, the attractive interaction between hydrophobic chains of Span-85 is less than Tween-80. Hence, AOT/Span-85 systems produce higher ω0,max (31.48, 24.07 and 27.78) than AOT/Tween-80 systems (29.63, 22.22 and 25.93) in IPM, IPP and EO, respectively [37,42]. On the other hand, AOT/Tween-85 systems (34.26, 27.78 and 32.41) produce higher ω0,max than AOT/Span-85 systems (31.48, 24.07 and 27.78) in IPM, IPP and EO, respectively, though both Tween-85 and Span85 possess similar hydrophobic moiety (Scheme S1) (vide Table 1). The result can be explained in the following way; ester moiety with long hydrocarbon chains (C18 ) is attached with tetrahydrofuran ring via polyoxyethylene chains in Tween-85, whereas ester moiety is directly attached with tetrahydrofuran ring in Span-85 (Scheme S1). It is quite likely that the difference in attachment or positioning of the hydrophobic chain with polar moiety may affects the solubilization behavior (i.e., ω0,max values) in AOT/Tween-85 and AOT/Span-85 systems. Unlike Tween-85 and Tween-80, Span85 does not contain any POE chain; rather a sorbitan ester moiety represents as a polar constituent (Scheme S1). Furthermore, the interfacial behavior of Span-85 varies as a result of differences in molecular structure and hydrophobicity [43]. It has been mentioned earlier that ω0,max values obtained at higher Xnonionic,max (=0.4) for AOT/Span-85 systems compared to AOT/Tween-85 or Tween-80 systems (Xnonionic,max = 0.05) in IPM, IPP and EO oils. This phenomenon can be explained in the following manner; growth of microemulsion droplets during the solubilization process and consequently the phase separation in such systems is limited by two opposing factors, namely, the radius of spontaneous curvature (Ro ) of interface as a result of curvature effect and the critical radius of droplets (Rc ) due to the attractive interaction between microemulsion droplets. Therefore, a maximum solubilization occurs as a result of the compromise between these two opposing factors [37]. Incorporation of Tween surfactants with large head group (20 POE) into the interface decreases the effective packing parameter (Peff ) by increasing area of polar head group, thereby increasing the natural radius of curvature (R). Whereas, Span-85 having no POE chain, increases the interfacial rigidity. So, incorporation of Span-85 into the interface cannot increase the natural radius (R) as much as earlier than Tweens. Due to rapid increase in R in Tween-based systems, the interdroplet interaction starts to govern the solubilization phenomena at lower molar ratio of Tweens in mixed AOT/Tween systems, whereas it starts at higher molar ratio of Span for AOT/Span systems. Hence, AOT/Span-85 systems exhibit ω0,max at higher Xnonionic,max (=0.4) compared to AOT/Tween-85 or Tween-80 (Xnonionic,max = 0.05) in IPM, IPP and EO oils. 3.2.2. Effect of oil In order to underline the role of polar lipophilic oils on the solubilization phenomenon in AOT or AOT/Tween-85 or Tween-80 or Span-85/oil(s)/water systems, an attempt has been made to analyze our results on the basis of the model developed by Shah et al. [37,44] using the concept of the molar volume or alkyl chain length of the hydrocarbon oils. This model predicts that at a given surfactant concentration, the maximum solubilization capacity of a microemulsion system can be obtained by adjusting the interfacial curvature and elasticity to an optimum value at which the bending

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stress and the attractive forces of the interface are both minimized. Hence, one can increase the solubilization of a microemulsion with a rigid interface by increasing its natural radius and fluidity of the interface. On the other hand, the solubilization of a microemulsion of a fluid interface can be increased by increasing the interfacial rigidity and decreasing the natural radius. In the present investigation, we performed the dye solubilization experiment [3,6,44] to underline the interplay between the curvature effect and the interdroplet interaction on the solubilization phenomenon in single AOT/oil/water as well as mixed AOT/nonionic(s)/oil(s)/water (where oil represents EO, IPM and IPP) at compositions Xnonionic < Xnonionic,max and Xnonionic ≥ Xnonionic,max from appearance of phase separation visà-vis intensity of colors of the upper and lower phases upon the addition of water at 303 K. The results have been depicted through solubilization capacity versus XTween-85 or Tween-80 profile in Fig. 1A and B and Table 1 and dealt in supplementary materials (Section 1). From the dye solubilization experiment for these systems, it can be inferred that the ascending curve of the solubilization capacity is the curvature branch (R0 ), whereas the descending curve is the interdroplet interaction branch (RC ). Similar observation for AOT/nonionic(s)/IPM/water RMs was reported earlier [6,7]. It can be observed from Table 1 that maximum solubilization capacity of water in single AOT (ω0 ) or mixed AOT/Tween-85 or Tween-80 or Span-85 systems (ω0,max ) in studied oils follows the trend: IPP < EO < IPM, which is not consistent with the model proposed by Shah et al. [37], which states that smaller molar volume or shorter linear hydrocarbon chain of the oil, larger is the effect of increasing radius of curvature, and hence higher is the solubilization capacity [45]. Herein, EO and IPP have interchanged their positions (Table S1), which indicates a deviation from Shah et al. model [37,44], as mentioned earlier. Such deviation may be due to difference in chemical structures/configurations of these oils compared to linear hydrocarbons. This aspect was not taken into account in earlier model, as most of the oils were used as linear hydrocarbons. Such an unusual behavior of polar lipophilic oils 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 cross-sectional 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 [46]. Further, the effect of oil on the solubilization as well as geometry of the aggregates can be explained in terms of the packing of the oil into the micelles, which is determined by its chemical structure. Long chain fatty acid esters are oils of a fair degree of lipophilicity along with polar ester moiety. The differences in behavior of these oils toward water solubilization in these systems on account of their different chemical structures, can be rationalized from the following view points: (1) long carbon chain length of saturated (myristate and palmitate) and cis-unsaturated (oleate) fatty acids, (2) straight (ethyl) and branching (isopropyl) in short alkyl chain which constitutes the ester group, and (3) relative contribution of long fatty acid chain and short alkyl chain. 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 arrangement of surfactant molecules at the interface [42]. It is probable that the alkyl chain (short) of the oil is located in the palisade layer of the micelles along with the ester group, whereas 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 [47]. Further, it can be noted that degree of penetration of oil with branching in alkyl chain, for example IPM or IPP [29] or cis-unsaturation in the long fatty acid chain, for example EO [29,48] may contributes toward modulation

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of the mixed surfactant interfacial curvature (compared to linear hydrocarbons, wherein all these factors are absent), resulting in its more flexibility or rigidity, and thus influences the preferred surface curvature, which further affects the solubilization capacity. That is why, water solubilization capacity of these systems is not following the order in respect of molar volume or alkyl chain length of the oils unlike linear hydrocarbons. On the contrary, it depends on chemical structure, solubilization site, degree of penetration of the oils vis-à-vis interactions between (i) fatty acid chain of oil and tails of surfactants at the interface or (ii) short alkyl chain of oil connected to ester moiety ( O CH2 ) and tails of surfactants or (iii) polar head groups of surfactants (POE chain of nonionic surfactant and carbonyl ester group of AOT) and fatty acid chain of oil apart from the interaction between AOT/nonionic surfactants, in tune with the report of Afifi et al. [47] for wormlike micelles in alkyl ester oils and Kaur et al. [48] for Brij-96/butanol/ethyl oleate/water RMs. However, the molecular arrangement of the constituents or microstructure of these mixed RMs, can be deciphered more accurately from SANS or 1 H NMR rotating frame nuclear Overhauser effect spectroscopy (ROSEY) studies [47,48], which is beyond our scope for the present investigation. Fanun [42] reported the difference in water solubilization capacity in mixed ethoxylated mono-di-glyceride/sucrose laurate microemulsion systems stabilized in three oils (IPM, caprylic-capric triglyceride and R (+)limonene) and explained in terms of degree of oil penetration in the surfactant palisade layer due to difference in chemical structures of the oils. 3.3. Solubilization efficiency parameter (SP*) An attempt has been made to evaluate “solubilization efficiency parameter (SP∗water ) @@” to underline the efficacy of a particular oil in obtaining maximum water solubilization capacity (ω0,max ) at the corresponding composition (XTween-85,max ) and temperature (303 K) in mixed RMs on the basis of a relative increase in solubilization of the dispersed phase (herein, water) in the oil compared to single AOT-based RM (ω0 ). This parameter can be considered as an intrinsic oil property. Consequently, variation in solubilization of water in oils of different lipophilities in mixed RMs can be rationalized in respect of their chemical structures and configurations [49]. Mathematically, SP∗water is represented as follows [49]: SP∗water =

ω0,max − ω0 ω0

(3)

Mathematical symbols are used with their usual significance (mentioned earlier). The calculated values are presented in Table 1. A representative diagram of solubilization efficiency parameter (SP∗water ) as a function of XTween-85 for studied oils, has been presented (Fig. 2). It can be observed from Fig. 2 that SP∗water follows the order: IPM < EO < IPP, which clearly indicates that IPP and IPM can be considered as most efficient and least efficient oils, respectively in solubilization of water in mixed RMs. In other words, this trend is not consistent with the molar volume or alkyl chain length of oils. Herein, the positions of EO and IPP are also interchanged. Further, it is evident from Fig. 2 that, SP∗water shown maximum at XTween-85 = 0.05 for studied oils, indicating the efficacy of oils corroborates well with the composition at which maximum water solubilization occurs. Very recently, similar trend has been reported for SP∗NaCl (solubilization of aqueous NaCl in these systems) by considering this parameter as an intrinsic oil property [49]. Hence, it can be inferred that the studied oils do not behave as linear hydrocarbons due to the presence of ester moiety as well as inherent structural characteristics (mentioned earlier). In the subsequent section, dissolution of water in mixed RMs has been analyzed from the view point of the thermodynamic concept.

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3 IPP

∆G 0s (kJ/mol)

2.5

EO IPM

2

1.5

1

0

0.1

0.2

0.3

0.4

XTween-85

Fig. 2. Solubilization efficiency parameter [SP∗water ], as a function of concentration of Tween-85 (XTween-85 ) in mixed AOT/Tween-85/oil/water reverse micellar systems at 303 K.

3.4. Thermodynamics of dissolution of water in mixed RMs Solubilization of water in mixed surfactant(s)/oil medium up to the maximum solubilization limit or phase separation point leading to the formation of RMs which can be considered as maximum solubilization capacity of water. The corresponding standard free energy change of dissolution of water (Gs0 ) at a constant temperature can be obtained from the relation [49–53]: Gs0 = −RT ln Xd

(4)

where Xd is the mole fraction of dispersed phase (herein, water), R is the gas constant, and T is the experimental temperature (303 K). Gs0 has been estimated on the basis of mole fraction of the dispersed water. The apparent Gs0 values are good enough to exhibit a comparative trend of results obtained under different experimental conditions. The calculated values of Gs0 according to Eq. (4) for mixed AOT/Tween-85/oil(s)/water systems at 303 K have been presented in Fig. 3 (as representative plot). From Fig. 3, it has been observed that Gs0 values are positive at all compositions. Gs0 values in the present report are comparable with those reported earlier for single and mixed surfactant microemulsions using biocompatible oils [52,54,55]. Gs0 values have been found to depend on water solubilization capacity at different mixing ratios of the surfactant (i.e., XTween-85 ) for all studied oils, and decrease with increase in water content (number moles of water, nw ) indicating that the addition of water to these systems disrupts their organization. From the plot of Gs0 versus XTween-85 (Fig. 3), it has been observed that Gs0 values decrease with addition of Tween-85 and shown minima at 0 , standard free energy of disXTween-85 = 0.05 (designated as Gs∗ solution of water at maximum water solubilization, ω0,max ), where maxima in water solubilization capacity were observed for all these systems and beyond which, Gs0 values have been found to increase with further addition of Tween-85 in AOT/oil(s)/water systems. This indicates that interdroplet interaction between the droplets starts to govern the solubilization process, and further increasing the concentration of Tween-85 in the mixed system, makes the system more organized. Hence, it can be inferred that there might be a correlation between the standard free energy change of water

Fig. 3. Free energy of dissolution of water (Gs0 ) as a function of concentration of Tween-85 (XTween-85 ) in mixed AOT/Tween-85/oil/water reverse micellar systems at 303 K.

dissolution (Gs0 ) and the two opposite effects that determine the maximum solubilization of water in RMs that is, compromise between the curvature of the surfactant film separating the oil and water, and interdroplet interaction between droplets [37], as mentioned in Section 3.2.1. 0 ) values Further, from Fig. 3 it can be observed that Gs0 (or, Gs∗ depend on the type of oil and follow the order: IPM < EO < IPP, which is in consistence with the order of SP∗water . Hence, it can be inferred that Gs0 depends on the chemical structure and configuration (viz., branching, unsaturation, etc.) of oils, not on molar volume or fatty acid chain length of oil, as discussed in Section 3.2.2. Gs0 0 ) values are higher for IPP-based system, whereas IPM(or, Gs∗ based system exhibits the least values. These results indicate that the dissolution of water in oil is more organized in IPP-based system compared to IPM- and EO-based systems. Very recently, similar trend of oils (as used in the present study), i.e., IPM < EO < IPP, has 0 0 been reported for Gs,NaCl (standard free energy change), Hs,NaCl 0 (standard enthalpy change) and Ss,NaCl (standard entropy change) of dissolution of aqueous NaCl in mixed AOT/Tween-85 RMs [49]. Further, it was reported that these oils, because of their dual characteristics (viz. polar and lipophilic) can undergo both hydrophilic and hydrophobic interactions with the surfactants during the solubilization process and thereby contributing a fair share to the energetic process. Earlier, Fanun [42] reported that the microemulsification of water in oil is more organized in R (+)-limonene based system compared to the caprylic-capric triglyceride and IPM for mixed sucrose laurate/ethoxylated mono-di-glyceride-based nonionic-nonionic w/o microemulsion systems. The overall thermodynamic properties are a complex combination of a number of processes viz., disruption of solvent structure and penetration of dispersant, transfer of amphiphiles from bulk to the interface, stabilization of the droplets by the amphiphiles and their orientation/organization at the interface between oil and water [54]. 0 3.5. Correlation between SP∗water and Gs0 or Gs∗

In this section, an attempt has been made to decipher the physicochemical concept of solubilization efficiency parameter (SP∗water ) in the light of standard free energy change of dissolution of water 0 ) in mixed RMs stabilized in polar lipophilic oils at (Gs0 or Gs∗

K. Kundu, B.K. Paul / Colloids and Surfaces A: Physicochem. Eng. Aspects 433 (2013) 154–165

25

161

35 30

20

10

σ (μS cm-1)

σ (µS cm-1)

15

X=0.1

25

X=0.1 X=0.2 X=0.3 X=0.4 X=0.5

X=0.2 20

X=0.3

15 10 5

5

0

0

0

0

5

10

15

20

25

5

10

15

30

ω Fig. 4. Conductance study of mixed AOT/Tween-85/IPP/water reverse micellar systems as a function of ω with varying concentration of Tween-85 (XTween-85 ) at fixed total surfactant concentration of 0.1 mol dm−3 and 303 K.

303 K, which has been dealt in supplementary materials (Section 3). 3.6. Conductometric studies In this section an attempt has been made to correlate the maximum solubilization of water with percolation of conductance in mixed RMs. It is also known that the introduction of additives into RMs can significantly affects the process of percolation by altering the texture of the interface. The electrical conductivity is especially sensitive to the aggregation of the droplets. More precisely, it is a structure sensitive property [1]. Percolation of conductance study extracts information about the nature of interaction among the droplets in microemulsions or RMs. The paper of 1978 by Lagues [56] is first to interpret the dramatic increase of the conductivity with droplet volume fraction for a w/o microemulsion in terms of a percolation model and termed this physical situation as stirred percolation, referring to the Brownian motion of the medium. In the present investigation, conductance studies have been carried out for mixed surfactant RMs wherein solubilization maxima (ω0,max ) are observed. In addition, samples from both sides of the corresponding maxima, i.e., ascending or descending branches of solubilization capacity versus XTween-85 profile are also chosen. 3.6.1. Effect of nonionic content (XTween-85 ) on the percolation of conductance with varying ω in mixed AOT/Tween-85 RMs Conductance of AOT/EO or IPM or IPP/water RMs were investigated as a function of ω with total surfactant concentration, [ST ] of 0.1 mol dm−3 at 303 K. The conductivity remains low and almost unchanged with increasing water content (ω) up to phase separation in all these systems, which corresponds to that for non-conducting oil phase (figures are not depicted). But addition of Tween-85 at different proportions into above systems was observed to increase conductivity of such mixed systems. Figs. 4 and 5 depict the conductivity of mixed AOT/Tween-85/IPP or EO/water RMs, respectively as a function of ω at different compositions (XTween-85 = 0.1–0.5) and a sudden increase in conductance (i.e., percolation in conductance) has been observed.

ω

20

25

30

35

40

Fig. 5. Conductance study of mixed AOT/Tween-85/EO/water reverse micellar systems as a function of ω with varying concentration of Tween-85 (XTween-85 ) at fixed total surfactant concentration of 0.1 mol dm−3 and 303 K.

Derived data (i.e., ωp at corresponding XTween-85 ) in oils are presented in Table 2. It is evident from Table 2 and Figs. 4 and 5 that volume-induced percolation threshold (ωp ) is shifted toward lower ω values with increase in XTween-85 . Addition of Tween85 with large polar head group size increases the droplet size by decreasing the packing parameter, which in turn facilitates the droplet coalescence that limits the solubilization capacity. All these systems (AOT/Tween-85/oil(s)/water) showed solubilization maxima at XTween-85,max = 0.05 (Table 1). The shaded region in the solubilization capacity versus XTween-85 profile represents the percolation range for AOT/Tween-85/IPP/water, respectively (Fig. S1, as representative plot in supplementary materials). It has been observed that conductivity remains low and practically unchanged for systems chosen from ascending branch, which confirms their hard-sphere type droplet nature. Percolation of conductance has been observed for all mixed RMs, which lie along the descending branch and confirms interdroplet interaction between the droplets. A similar trend was also observed for the blends, AOT/Tween-85 stabilized in IPM (not exemplified here). Single AOT/oil(s)/water RM can be assumed to be so rigid that it cannot form any such fused droplet network to facilitate percolation. These oils possess both high molar volume (Table S1) and high viscosity (>4.0 mPa s at 30 ◦ C [57]) than conventional linear hydrocarbon oils. According to Mitra et al. [6,7] and Zhang et al. [57], oil with high molar volume as well as high viscosity (IPM), it is difficult for droplets to collide and aggregate into clusters resulting in rigidity of the interfacial film, which make these systems

Table 2 Volume induced percolation threshold (ωp ) of mixed AOT/Tween-85/oil(s)/water reverse micellar systems with ST = 0.1 mol dm−3 with varying compositions (XTween-85 ) at 303 K. XTween-85

0.1 0.2 0.3 0.4 0.5

ωp EO

IPM

33.06 21.67 19.44 18.06 –

34.72 26.39 20.83 19.44 –

IPP 25.00 18.06 16.67 16.11 15.28

K. Kundu, B.K. Paul / Colloids and Surfaces A: Physicochem. Eng. Aspects 433 (2013) 154–165

non-percolating. But incorporation of Tween-85 into AOT/oil(s)/water systems induces percolation in conductance at a certain XTween-85 (in the present study, greater than XTween-85,max ). The fluidity of the interface and the attractive interaction among the aggregates are the most important factors that determine the exchange rate of the ions and water molecules during the fusion processes. Any factor that increases the fluidity of the interface and/or the interdroplet interaction would also increase the ease of conductance percolation. The droplet size of RMs depends upon the flexibility of the interface, which in turn is related to changes in the critical packing parameter (CPP, given by P = v/al). Lower value of P leads to higher interfacial flexibility and consequently, facilitates percolation. For mixed AOT/Tween-85 systems, the packing parameter gets modified due to the presence of Tween-85 at the interface and Peff can be expressed according to Eq. (2). It follows from Eq. (2) that Peff takes up a lower value than that of P due to the presence of Tween-85, and consequently the mixed systems acquires larger droplet size than the single AOT/oil(s) system. This observation has also been supported by DLS measurements, which has been dealt in Section 3.7. 3.6.2. Effect of oil on the percolation of conductance with varying ω in mixed RMs Table 2 represents the effect of oil type on the percolation of conductance in AOT/Tween-85 blend systems (with ST equals to 0.1 mol dm−3 ) with varying compositions (XTween-85 = 0.1–0.4) at 303 K. It is evident from Table 2 that ωp follows the order: IPP < EO < IPM, which is not consistent with the order of molar volume or hydrocarbon chain length of oils as proposed by Shah et al. [37] for linear hydrocarbons. Larger alkyl chain length of the oil, greater is the effect on ωp , i.e., smaller is the value of ωp . Herein, the positions of EO and IPP are interchanged. It can be mentioned that the role of these polar lipophilic oils in solubilization of water in this mixed RMs, has been dealt in Section 3.2.2. However, this trend can be summarized from the view point of variation in penetrating ability of these oils because of the presence of branching in isopropyl group (in IPM and IPP) and linear ethyl group with cis-unsaturation (in EO) with the varying fatty acid chain length in addition to common ester moiety. All these factors might play an important role in tuning interfacial flexibility or rigidity, attractive interdroplet interaction as well as droplet dimensions and subsequently ωp . Liu et al. [40] reported that that not only the molecular volume but the configuration of solvent molecules has significant effect on the conducting properties of AOT/Brijs/linear or cyclic hydrocarbon oil(s) and substantiated from DLS measurements. However, the order of ωp with varying oil type in mixed AOT/Tween-85 RMs has been further supported from measurement of droplet radius (by DLS), which has been dealt in subsequent section. Further, the role of oils in influencing ωp of these mixed RMs has been discussed in supplementary materials (Section 4). 3.6.3. Influence of third component (additives) on the percolation of conductance in mixed RMs The results of the effect of additives [bile salts, sodium cholate (NaC), sodium deoxycholate (NaDC), sodium taurodeoxycholate (NaTDC) and hydrotrope, sodium salicylate (NaSl)] on the percolation of conductance in mixed AOT/Tween-85 RMs in IPM and IPP as a function of water content (ω) with [ST ] equals to 0.1 mol dm−3 at XTween-85 = 0.2 and 303 K are presented in Table 3 and Fig. 6 (a representative plot for IPM-based system). It can be observed from Table 3 that, NaC, NaDC, NaTDC and NaSl assist percolation in AOT/Tween-85/IPM (or IPP)/water RMs, and with increasing concentration of additives, percolation threshold (ωp ) shifted toward lower ω. Further, it is evident that the efficiency order of the additives in reducing ωp is NaC > NaDC > NaTDC > NaSl at comparable concentration of additives (0.045 mol dm−3 ). The observed effect

Table 3 Volume induced percolation threshold (ωp ) of mixed AOT/Tween-85/oil(s)/water reverse micellar systems with [ST ] = 0.1 mol dm−3 by varying concentration of additives at XTween-85 = 0.2 and 303 K. Additive

Concentration (mol dm−3 )

ωp IPM (IPP)

Sodium salicylate (NaSl)

0 0.015 0.030 0.045

26.39 (18.07) 24.17 23.34 22.51 (17.22)

Sodium cholate (NaC)

0.015 0.030 0.045

20.84 20.56 20.28 (16.67)

Sodium taurodeoxycholate (NaTDC)

0.045

21.39 (17.08)

Sodium deoxycholate (NaDC)

0.045

20.83 (16.94)

of additives on the percolation phenomenon in mixed surfactant RMs can be explained by extending the “channel-formation followed by material transfer” mechanism [19,41]. The presence of additives affects the phenomenological process of percolation in conductance as a whole in these systems, because of the difference in their chemical structures and physicochemical properties which may influence the curvature of the interface [1]. Hence, these additives in the mixed RMs may further modify the effective packing parameter (Peff ) (i.e., overall modification of Peff due to addition of Tween-85 and additives to AOT/oil/water RMs) and thereby assist the percolation process. However, the degree of variation in assistance of percolation can be explained from their chemical structures given in Scheme 1. NaC contains three hydroxyl groups in steroid ring, whereas both NaDC and NaTDC possess two hydroxyl groups. So, ability of NaC to form H-bond with POE chains of Tween-85 and water is comparatively better than other two additives, and in consequence channel-formation followed by material transfer becomes easier for NaC. However, NaTDC also contains steroid ring with two hydroxyl groups, but it has a long chain connected with cyclopentane ring of steroid part compared to NaC and NaDC, which may affects the easier motion of ions through channel and rate of

35 30 25

σ (μS cm-1)

162

NaTDC NaDC

20

NaC NaSl

15

Water

10 5 0

0

10

20

30

40

ω Fig. 6. Conductance study of mixed AOT/Tween-85/IPM/water reverse micellar systems as a function of ω at fixed concentration of Tween-85 (XTween-85 = 0.2) and total surfactant concentration of 0.1 mol dm−3 in the presence of additives at 303 K, where [additive] = 0.045 mol dm−3 .

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163

Table 4 Hydrodynamic radius (dh ) and polydispersity index (PDI) of mixed AOT/Tween-85/oil(s)/water reverse micellar systems under various physicochemical environments at 303 K. System

Composition

dh (nm)

PDI

AOT/Tween-85/IPM/water

XTween-85 = 0, ST = 0.10 mol dm−3 , ω = 10 XTween-85 = 0.1, ST = 0.10 mol dm−3 , ω = 10

9.923 13.231

0.195 0.298

AOT/Tween-85/EO/water

XTween-85 = 0, ST = 0.10 mol dm−3 , ω = 10 XTween-85 = 0.1, ST = 0.10 mol dm−3 , ω = 10

11.217 15.904

0.198 0.199

AOT/Tween-85/IPP/water

XTween-85 = 0, ST = 0.10 mol dm−3 , ω = 10 XTween-85 = 0.1, ST = 0.10 mol dm−3 , ω = 10

15.839 17.216

0.188 0.281

assistance is somewhat lower than that of NaC and NaDC. However, NaSl possesses different chemical structure and physicochemical properties than other additives (i.e., bile salts). NaSl contains both hydroxyl and carbonyl groups (as polar functional groups), which enable them to form channels between adjacent droplets by adhering to the mixed surfactant layer of the droplets [58]. Attractive as well as H-bonding interactions between polar functional groups present in NaSl and Tween-85 (POE as polar head group) might facilitate channel formation and in consequence assist percolation process. However, the presence of aromatic ring in NaSl makes channel formation difficult [22] compared to other three additives and consequently, degree of assistance shows lower among all the additives used in this investigation. Li et al. [58] reported increase of droplet size in the presence of NaSl, which also facilitates percolation of conductance phenomenon. The type of oil also affects ωp in the presence of additives at comparable environments. Additives exert greater assisting effect on ωp in IPP (palmitate) than IPM (myristate). The results can be explained by the increased droplet size results in increased interdroplet interaction with increasing the fatty acid chain length of oil (with same isopropyl group as short alkyl chain), which has been dealt by employing DLS measurements in subsequent section. 3.7. Dynamic light scattering (DLS) study in mixed RMs In order to test the validity of the percolation phenomenon estimated from conductance measurement or to illustrate the mechanism of percolation in mixed RMs, hydrodynamic radius (dh ) and the size distribution of reverse micellar droplets have been measured by dynamic light scattering (DLS) technique. In this section, we employ the DLS technique to investigate the hydrodynamic radius (dh ) of mixed AOT/Tween-85/IPM or IPP or EO/water RMs, as a function of XTween-85 (=0 and 0.1) at a fixed ω (=10) and ST (=0.1 mol dm−3 ). The values of the hydrodynamic radius (dh ) with polydispersity index (PDI) of selected samples with compositions are presented in Table 4. It has been observed that dh increases with increase in XTween-85 for all these systems at comparable conditions. The result further indicates that flexibility of the oil/water interface increases with increase in droplet size, which in turn increase the interdroplet interaction and facilitates percolation process. Similar results were reported by Liu et al. [40] and Nazario et al. [41] for AOT/Brij(s)/heptane/water and AOT/Ci Ej /isooctane/water RMs, respectively from DLS measurements. Further, it has been observed that dh depends on oil type and follows the order: IPM < EO < IPP at comparable composition for both single and mixed surfactant RMs. This trend reflects reverse order of ωp (i.e., IPP < EO < IPM, presented in Section 3.6.2), as expected. Larger the droplet size, lower is ωp . However, the droplet sizes are not in consistent with fatty acid chain length of the oils, and not confirming to the previous reports, which deal with percolation phenomenon in water-in-oil AOT/linear hydrocarbon(s)/water systems [50,59]. According to these reports, attractive interaction between droplets increase significantly with increasing chain

length or molar volume of linear hydrocarbon(s) (because of reduction in solvent penetration into the interface) and lowers percolation threshold. Further, the trend of dh (from DLS measurements) corroborates well with SP∗water and Gs0 (theoretically derived), which justifies inherent structural characteristics of these oils in water solubilization of mixed RMs from the viewpoint of efficacy of oils and thermodynamic aspects. Hence, the droplet radius (dh ) contributes to understand well the mechanism of percolation in conductance and microstructures of mixed AOT/Tween-85 RMs stabilized in polar lipophilic oils (viz., IPM, IPP and EO). 4. Conclusions The present study represents a systematic investigation on the effects of composition of surfactant (Xnonionic ), polar head group and hydrophobic moiety of nonionic surfactant, and type of oils on the water solubilization capacity of the mixed AOT/Tween-85 or Tween-80 or Span-85/water RMs stabilized in three polar lipophilic oils (EO, IPM, IPP) at 303 K. The addition of nonionic surfactants of different chemical structures (Scheme S1) (Tween-85 or Tween-80 or Span-85) induces synergism in solubilization capacity of water in AOT/EO or IPM or IPP/water RMs. Both ω0,max (maximum water solubilization capacity) and Xnonionic,max (mole fraction at which synergism occurs) depend on chemical structure of both nonionic surfactants and oils. ω0,max follows the order: AOT/Tween85 > AOT/Span-85 > AOT/Tween-80, whereas Xnonionic,max follows the order: AOT/Span-85 > AOT/Tween-85 ∼ AOT/Tween-80 in studied oils. Variation of these two parameters has been explained in view of the difference in chemical structures with respect to both hydrophobic and hydrophilic moieties of the nonionic surfactants (Scheme S1). Further, the polar lipophilic oils have been found to influence both ω0 (single AOT) and ω0,max (mixed AOT/Tween85 or Tween-80) and follow the order: IPP < EO < IPM, which is not consistent with the report of Shah et al. [37] that higher the molar volume or long hydrocarbon chain of linear hydrocarbons, lower is the solubilization capacity. In this report, the positions of EO and IPP are interchanged. Solubilization efficiency parameter (SP∗water ) has been evaluated to underline the efficacy of the oils in obtaining maximum water solubilization capacity of mixed AOT/Tween-85 RMs on the basis of relative increase in solubilization of the dispersed phase (herein, water) in oils of different lipophilicities (ω0,max ), compared to single AOT-based RM in corresponding oil (ω0 ). The order of SP∗water follows: IPM < EO < IPP, which is not consistent with Shah et al. model [37], as the positions of EO and IPP are interchanged. The inconsistency in both cases may be attributed to inherent differences in chemical structures of the oils, which was beyond scope to take into consideration by Shah et al. [37] model for linear hydrocarbons. Standard free energy of dissolution of water (Gs0 ) of these systems at 303 K has been found to be positive and depends on the mixing ratio of the surfactants (XTween-85 ) and oil type. Minima in Gs0 values have been observed 0 ), which is a result of the comat XTween-85 = 0.05 (designated as Gs∗ promise between the two opposite effects that is, the spontaneous

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curvature of the interface and the attractive interactions among the droplets that limits the growth of droplets during the solubilization 0 values process (Fig. 3, Gs0 – XTween-85 profile). Both Gs0 and Gs∗ follow the order: IPM < EO < IPP, which indicates that the solubilization of water in oil(s) is more stabilized in IPP and less stabilized in IPM at comparable conditions. Further, the order of these thermodynamic parameters support the overall order of SP∗water (i.e., IPM < EO < IPP), which demonstrates that the variation in penetrating ability of these oils because of the presence of branching in isopropyl group (in IPM and IPP) and linear ethyl group with cisunsaturation (in EO) with the varying fatty acid chain length in addition to common ester moiety, plays a decisive role in water solubilization phenomenon in mixed RMs. Addition of Tween-85 to AOT/EO or IPM or IPP/water RMs, induces percolation in conductance with increase in ω at 303 K. ωp (volume-induced threshold percolation) depends on both XTween-85 and oil type. Additives (bile salts; NaC, NaDC, NaTDC and hydrotrope; NaSl) assist percolation threshold (ωp ) in AOT/Tween-85/IPM (or, IPP)/water RMs at a fixed composition (XTween-85 = 0.2) and temperature (303 K) by altering the texture of the interface. The efficiency order of the additives in reducing ωp is NaC > NaDC > NaTDC > NaSl at comparable concentration (0.045 mol dm−3 ). The mechanism of action of these additives has been explained on the basis of their structural and physicochemical properties. Along with conductivity, DLS measurements have been utilized to characterize the influence of various physicochemical parameters (comparable to conductivity measurements) on hydrodynamic radius (dh ). dh depends on oil type and follows the order: IPM < EO < IPP at comparable compositions, which corroborates well with the trends observed for SP∗water and Gs0 . Further, dh contributes to understand well the mechanism of percolation in conductance and microstructures of the mixed RMs in these oils. Understanding of the relationship between microstructure and composition of RMs is important to prepare and optimize RMs for protein purification process [60] and efficient use in drug delivery [61]. Such a comprehensive study consisting of water solubilization capacity, standard free energy of water dissolution, conductance and DLS measurements of mixed AOT/nonionic(s) RMs in polar lipophilic oils of different chemical structures and configurations has not been reported earlier and could form the basis of a formulation rationale while developing these systems for applications in encapsulation of bioactive molecules, etc. [47]. Acknowledgements The financial support in the form of an operating research grant to Professor B.K. Paul and Senior Research Fellowship to Mr. Kaushik Kundu from the authority of Indian Statistical Institute, Kolkata, India are thankfully acknowledged. We thank Mr. Shuvabrata De (GSU, ISI) for his help in preparation of Schemes. Professor K.P. Das and Mr. Victor Banerjee of Bose Institute, Kolkata are also acknowledged for extending us the facility of using DLS. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa. 2013.05.009. References [1] (a) S.P. Moulik, B.K. Paul, Structure, dynamics and transport properties of microemulsions, Adv. Colloid Interface Sci. 78 (1998) 99; (b) B.K. Paul, S.P. Moulik, Microemulsions: an overview, J. Dispersion Sci. Technol. 18 (1997) 301.

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