Effect of Polyoxyethylene Type Nonionic Surfactant and Polar

Article pubs.acs.org/jced

Effect of Polyoxyethylene Type Nonionic Surfactant and Polar Lipophilic Oil on Solubilization of Mixed Surfactant Microemulsion Systems Kaushik Kundu* and Bidyut K. Paul Surface and Colloid Science Laboratory, Geological Studies Unit, Indian Statistical Institute, 203, B.T. Road, Kolkata-700108, India S Supporting Information *

ABSTRACT: In the present report, the water solubilization capacity of water-in-oil microemulsion systems comprising mixed surfactants, that is, anionic sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and nonionic(s) [viz., polyoxyethylene (2) stearyl ether (Brij-72), polyoxyethylene (10) stearyl ether (Brij-76), polyoxyethylene (20) stearyl ether (Brij-78), polyoxyethylene (2) oleyl ether (Brij-92), polyoxyethylene (10) oleyl ether (Brij-97), and polyoxyethylene (20) oleyl ether (Brij-98)] was investigated in polar lipophilic oils [viz., ethyl myristate (EM), isopropyl myristate (IPM), and isopropyl palmitate (IPP)] with varying mole fractions of nonionic surfactant (Xnonionic) at a fixed surfactant concentration and 303 K. A maximum (i.e., synergism) in water solubilization (ω0,max) was observed at an optimal value of mole fraction of nonionic surfactant (Xnonionic,max). The solubilization efficiency parameter (SP*water) and the free energy of dissolution (ΔG0s) were evaluated for these mixed systems. All of the physicochemical parameters for solubilization (viz., ω0,max, Xnonionic,max, and SP*water) were found to be dependent on both nonionic surfactants and oils. A minimum in ΔG0s was observed at corresponding Xnonionic,max, where a maximum in SP*water was evidenced. The IPP-based system was found to be most efficient, whereas the EMbased system was least efficient. An attempt was made to correlate the interdependence of physicochemical (ω0,max or SP*water) and thermodynamic (ΔG0s) parameters.

1. INTRODUCTION Microemulsions or reverse micelles (RMs) are transparent, isotropic, and thermodynamically stable dispersions of otherwise immiscible water and oil stabilized by surfactant and/or cosurfactant.1 Solubilization of water in microemulsion systems has been attracted considerable attention, since it has significant applications in pharmaceutics, cosmetics, dry cleaning, micellar catalysis, and enzymatic catalysis, in addition to enhanced oil recovery (EOR) and chemical engineering.1,2 Further, Ahmed et al.3 reported that flotation of minerals (e.g., francolite, calcite, and dolomite) can be modified, using flotation reagents in the form of microemulsions by dilution with water. Solubilization is the process of incorporation of the solubilizate (i.e., the component that undergoes solubilization) into a system consisting of a solvent and colloidal aggregates (i.e., micelles or RMs).4 The topic of solubilization as a whole certainly deserves to be treated in detail due to ever-increasing role of microemulsions as “microreactors” for numerous organic and inorganic reactions.5,6 The water solubilization capacities of microemulsions are determined by two phenomenological parameters such as the spontaneous curvature and the elasticity of the interfacial film, which have been influenced by the constituents of the systems and experimental conditions.7 © 2013 American Chemical Society

Surfactant mixtures often give rise to enhanced performance over the individual components or exhibit synergism in their physicochemical properties. Because of this property, such mixtures are of theoretical interest and could potentially be employed in a wide range of applications.8 Thus it is expected that enhanced solubilization of water in water-in-oil (w/o) microemulsions could be achieved with certain surfactant mixtures. To achieve this goal, mixtures of surfactants can be used. An increase in water or brine solubilization capacity by the addition of nonionic surfactant for mixed anionic or cationic/nonionic microemulsion systems was reported earlier.9−11 The dissolution of water in surfactant/oil medium leading to microemulsion formation is an important aspect of the study of microheterogeneous systems. The thermodynamic studies on the formation of microemulsions as well as the energetics of the interaction of the components at the interface stabilized by mixed surfactant systems in biocompatible oils are scarcely reported.10,12 Earlier, most of the research were carried out on water solubilization and conductivity studies in single and mixed surfactant RMs by using linear hydrocarbons as continuous Received: June 24, 2013 Accepted: August 22, 2013 Published: September 3, 2013 2668

dx.doi.org/10.1021/je400597f | J. Chem. Eng. Data 2013, 58, 2668−2676

Journal of Chemical & Engineering Data

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medium.7,11,21 However, similar studies using polar lipophilic or alkyl ester oils are seldom reported in literature.9,10,12 Recently, these type of oils are an attractive research subject because they are different from the hydrocarbon oils with respect to physicochemical properties as well as chemical structures and find applications in biologically relevant systems.9 Recently, Mehta et al. 13 used different techniques (e.g., water solubilization, conductivity, FT-IR spectroscopy, etc.) to characterize U-type microemulsion systems comprising of polyoxyethylene (10) oleyl ether (Brij-96) as surfactant stabilized by cosurfactant (C2OH−C6OH) and oil [ethyl oleate (EO), isopropyl myristate (IPM), or isopropyl palmitate (IPP)]. In our previous report,14 we have investigated the water solubilization, conductometric studies, and thermodynamics of water dissolution of biocompatible RMs comprising of mixed surfactant(s) [anionic sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and polyoxyethylene (20) sorbitan trioleate (Tween85)] and polar lipophilic oils (viz. EO, IPM, and IPP) under various physicochemical conditions. As a part of our continuing research, the solubilization and thermodynamics of dissolution of water (ΔG0s) of mixed AOT/nonionic surfactant(s) (viz., Brij-72, Brij-76, Brij-78, Brij-92, Brij-97, and Brij-98) microemulsions stabilized in polar lipophilic oils [viz. ethyl myristate (EM), IPM, and IPP] have been taken at different physicochemical conditions. The nonionic surfactants used herein are of similar or dissimilar polar head groups and hydrophobic moieties. Polar lipophilic or alkyl ester oils consist of a hydrophilic ester moiety connected with long fatty acids and a short linear or branch alkyl or alcohol chain. Due to the structural resemblance of these type of oils to the lipids in living systems, they are being increasingly studied as environmentally benign media.15 Earlier, efforts were made to formulate and characterize biocompatible microemulsion systems using AOT or Brijs, Tweens, and Spans as surfactants and IPM, IPP, and EO as oils.15−19 Here, we intend to address different surfactant mixtures that exhibit synergism in the solubilization of water in w/o microemulsions and also to improve the basic understanding of the role of nonionic surfactant(s) and oil(s) in water solubilization process. This report might be helpful in understanding of using these novel compartmentalized systems as prospective media for various applications in a better way.

Table 1. Sample Materials and Information final mole fraction purity

source

initial mole fraction purity

AOT

Sigma

0.98

0.99

Brij-72, Brij-76, Brij-78, Brij-92, Brij-97, and Brij-98 EM, IPM, and IPP water

Sigma

vacuum desiccator none

0.98

distillation none

> 0.99

chemical name

Fluka Milli-Q

purification method

solution and the critical volume of water was noted. The experiments were done several times, and the mean result was taken. It was estimated that the error limits in the measurements for the transparency-to-turbidity transition were ± 0.02.

3. RESULTS AND DISCUSSION 3.1. Water Solubilization in AOT/Nonionic(s) Microemulsions Stabilized in EM, IPM, or IPP. Water solubilization capacity (i.e., molar ratio of water to surfactant, ω) of mixed AOT/Brij-72, Brij-76, Brij-78, Brij-92, Brij-97, or Brij-98/EM, IPM, or IPP microemulsions with varying mole fraction of nonionic surfactant in mixed systems (i.e., Xnonionic = 0→1.0) at 303 K are presented in Figures 1 to 3 and Table 3. Values of ω0, which can be defined as molar ratio of water to surfactant in single AOT microemulsion system, are ∼30, 22, and 14 in EM, IPM, and IPP, respectively at [ST] of 0.10 mol· kg−1. The water solubilization capacity of AOT-based microemulsions increases up to a maximum value at a certain composition of the nonionic surfactant(s) in mixed systems (Xnonionic,max, i.e., mole fraction at which synergism in water solubilization occurs), beyond which (Xnonionic,max) the solubilization capacity further decreases with increasing Xnonionic. Earlier, it was observed that the solubilization capability of the sodium bis(2-ethylhexyl) phosphate (NaDEHP)/Hp/water microemulsion significantly increases in presence of bis(2ethylhexyl) phosphoric acid (HDEHP).21 Also, the increased solubilization effect of mixed pentaethylene glycol monododecyl ether (C12E5)/didodecyldimethylammonium bromide (DDAB) microemulsion systems with water and Hp was reported by Bumajdad et al.22 Synergism in solubilization capability of water of microemulsion systems was reported also by our group.9,10,14 The effect of the nonionic surfactants and oils on both ω0,max (maximum water solubilization capacity of AOT/Brij microemulsion) and Xnonionic,max has been dealt in subsequent paragraphs. 3.1.1. Effect of Content of Nonionic Surfactant (Xnonionic) on the Solubilization of Water in Mixed Microemulsions. In the present report, synergism in water solubilization capacity in mixed AOT/nonionic surfactant(s) (Brij)/EM, IPM, or IPP microemulsions has been evidenced, and the findings are represented in Table 3. Two interfacial parameters, namely, spontaneous film curvature (the curvature that the amphiphilic film would like to attain) and the elasticity (or rigidity) of the interfacial film formed by the surfactants, influence the water solubilization behavior, and the synergism in solubilization capacity can be achieved by tuning these two parameters.23,24 Mainly, water solubilization in surfactant aggregates governs by the packing parameter of individual surfactant (i.e., P = v/al, where v and l are the volume and the chain length of hydrophobic moiety, respectively, and a is the area of polar headgroup of the surfactant). Mitchell and Ninham 25

2. EXPERIMENTAL SECTION 2.1. Materials. AOT was purchased from Sigma, USA and purified as reported earlier.20 Brij-72, Brij-76, Brij-78, Brij-92, Brij-97, and Brij-98 were purchased from Sigma, USA. Nonionic surfactants were used without further purification. The oils, isopropyl myristate (IPM), isopropyl palmitate (IPP), and ethyl myristate (EM) were products of Fluka (Switzerland) and purified as reported earlier.20 Water was double-distilled and deionized before use with a conductance less than 3 × 102 μS·m−1. Information about chemical samples is shown in Table 1. The chemical structures along with molar volumes of these oils are presented in Table 2. 2.2. Methods. Fixed amounts of surfactants (AOT or blend of AOT/Brij) at different proportions and oil (IPM, EM, or IPP) were mixed in a dry test tube and kept in a thermostatted water bath (accuracy, ± 0.1 K) at fixed temperature of 303 K. The total surfactant concentration, [ST] was fixed at 0.10 mol· kg−1. Water was gradually added in small intervals into it from a Hamilton microsyringe (Hamilton, USA) with constant stirring until a clear single-phase solution turned into biphasic or turbid 2669



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Table 2. Chemical Structure and Molar Volume of Oils

Figure 1. Solubilization capacity of water, ω, in mixed AOT/ nonionic(s)/ethyl myristate (EM)/water microemulsion systems as a function of mole fraction of nonionic surfactant in mixed systems, Xnonionic, at a fixed total surfactant concentration of 0.10 mol·kg−1 and 303 K. ⧫, Brij-72; ■, Brij-92; ▲, Brij-76; ×, Brij-97; ∗, Brij-78; and ●, Brij-98.

Figure 3. Solubilization capacity of water, ω, in mixed AOT/ nonionic(s)/isopropyl palmitate (IPP)/water microemulsion systems as a function of mole fraction of nonionic surfactant in mixed systems, Xnonionic, at a fixed total surfactant concentration of 0.10 mol·kg−1 and 303 K. ⧫, Brij-72; ■, Brij-92; ▲, Brij-76; ×, Brij-97; ∗, Brij-78; and ●, Brij-98.

established a relation for evaluating packing parameter of mixed surfactant microemulsion, that is, the effective packing parameter (Peff), which is shown below: Peff = [(xv /al)A + (xv /al)B ]/(xA + x B)

(1)

where, xA and xB are the mole fractions of surfactant A and B in the mixture, respectively. Factors that increase the natural radius of curvature (R) (e.g., more oil penetration, longer chain length of surfactant, higher concentration of electrolytes, higher temperature, etc.) increase the solubilization capacity significantly.23 AOT has a smaller headgroup area than polyoxyethelene types of surfactants (herein, Brij), and this is because of the presence of POE chain in polar headgroup of Brij.26 Earlier, it was noticed that the polar headgroup of the nonionic surfactant influences the water solubilization behavior of mixed AOT/CiEj RMs.27 Thus, on addition of nonionic surfactant(s) (Brij) with AOT, Peff is decreased due to increase in a. As a result, R increases for a mixed surfactant system, and subsequently, mixed AOT/Brij/oil microemulsions accommodate more water than the single AOT/oil microemulsions. Also,

Figure 2. Solubilization capacity of water, ω, in mixed AOT/ nonionic(s)/isopropyl myristate (IPM)/water microemulsion systems as a function of mole fraction of nonionic surfactant in mixed systems, Xnonionic, at a fixed total surfactant concentration of 0.10 mol·kg−1 and 303 K. ⧫, Brij-72; ■, Brij-92; ▲, Brij-76; ×, Brij-97; ∗, Brij-78; and ●, Brij-98. 2670



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Table 3. Maximum Solubilization Capacity of Water, ω0,max, in Mixed AOT/Nonionic(s) Microemulsion Systems in Different Oils with the Total Surfactant Concentration, ST, of 0.1 mol·kg−1 at 303 K ω0,max AOT/nonionic(s)/oil/water Brij-72 C18H37(OCH2CH2)2OH Brij-76 C18H37(OCH2CH2)10OH Brij-78 C18H37(OCH2CH2)20OH Brij-92 C18H35(OCH2CH2)2OH Brij-97 C18H35(OCH2CH2)10OH Brij-98 C18H35(OCH2CH2)20OH a

EM

IPM

IPP

37.04 (0.30)a

30.56 (0.30)

25.93 (0.30)

35.18 (0.05)

29.26 (0.05)

22.78 (0.05)

31.85 (0.05)

26.85 (0.05)

21.30 (0.05)

38.89 (0.30)

33.33 (0.30)

28.70 (0.30)

36.11 (0.05)

30.00 (0.05)

24.07 (0.05)

34.26 (0.05)

27.78 (0.05)

23.15 (0.05)

The values in parentheses indicate the mole fraction of nonionic surfactant at which maximum solubilization of water occurs, i.e., Xnonionic,max.

the solubilization capacity of the mixed system is increased with an increase in Xnonionic up to a certain value of R. When R reaches the critical value, that is, Rc (known as critical radius of curvature), the solubilization process has been governed by the interdroplet interaction. According to Leung and Shah,23 the interdroplet interaction among the droplets makes the oil/ water interface flexible, and as a result, solubilization capacity decreases. Thus beyond Rc, the addition of nonionic surfactant decreases the solubilization capacity of mixed systems. The ascending and descending curve of solubilization capacity− Xnonionic profile is governed by the curvature effect due to the rigidity of the interface and the interdroplet interaction effect, respectively, for all of these systems. Further, attainment of maximum water solubilization using a mixture of ionic/nonionic surfactants can be explained in the light of hydrophilic−lipophilic balance (HLB) of individual surfactants. The HLB is one of the most common methods to correlate surfactant structure with their effectiveness as emulsifiers. The HLB value indicates how the surfactant will behave in a solution, and it is closely related to its capability to solubilize substances.28 Huibers and Shah29 reported earlier that the best solubilization for the mixture occurs when HLB values of its components lie in 9−13 region.29 Further, they mentioned that good solubilization occurs when HLB of the surfactant mixture falls in a certain HLB region, which should be neither very water-soluble nor very oil soluble. As a result of attainment of these conditions, the surfactants are more favorable to partition at the interface. In such a case, solubilization for the mixtures is better than that of either component. Hence, HLB of the present systems at the mixed compositions (i.e., HLBmix) has been evaluated using the concept of Acosta et al.30 HLBmix = (X1HLB1 + X 2 HLB2)

Figure 4. Hydrophilic−lipophilic balance of mixed surfactant, HLBmix, for AOT/nonionic(s)/ethyl myristate (EM) or isopropyl myristate (IPM) or isopropyl palmitate (IPP)/water microemulsions as a function of mole fraction of nonionic surfactant in mixed systems, Xnonionic at fixed total surfactant concentration of 0.10 mol·kg−1 and 303 K. ▲, Brij-72 or Brij-92; ●, Brij-76 or Brij-97; and ■, Brij-78 or Brij-98.

occurs at HLBmix = 10.3−10.5 regions. According to Huibers and Shah,29 synergism in maximum water solubilization in w/o microemulsion was observed at HLBmix = 9 for a mixture of nonylphenyl ethoxylate surfactants (C9PhE1.5 and C9PhE12) in cyclohexane. 3.1.2. Effect of Polar Headgroup of Nonionic Surfactant on Solubilization of Water in Mixed Microemulsions. It has been observed from Table 3 that both the values of ω0,max and Xnonionic,max depend upon the polar headgroup and hydrophobic moiety of nonionic surfactant. However, the overall ω0,max for these systems follows the order: AOT/Brij-72 > AOT/Brij-76 > AOT/Brij-78 and AOT/Brij-92 > AOT/Brij-97 > AOT/Brij98, whereas Xnonionic,max follows the order: AOT/Brij-72 > AOT/Brij-76 ∼ AOT/Brij-78 and AOT/Brij-92 > AOT/Brij97 ∼ AOT/Brij-98 in all investigated oils. A plausible explanation for these types of trends has been presented below. Brij-72, Brij-76, and Brij-78 or Brij-92, Brij-97, and Brij-98 possess the same carbon number (C18) in hydrophobic moieties but differ in respect of the number of POE chains as polar headgroup. The above trend of ω0,max may be argued as

(2)

where X1 and X2 are the mole fractions of surfactant 1 and surfactant 2 in the mixture, respectively. HLB1 and HLB2 are the hydrophilic−lipophilic balances of surfactant 1 and surfactant 2, respectively. In these calculations, HLB of AOT (equals to 10.2) and HLBs of nonionic surfactants (vide. Figure 4) have been taken from Ghosh and Naskar31 and Griffin,32 respectively. A graphical illustration of HLBmix versus Xnonionic has been represented in Figure 4. It is evident from Figure 4 that maximum water solubilization for AOT/Brij-72 or Brij-92 systems occur at HLBmix = 8, whereas for AOT/Brij-76 or Brij78 or Brij-97 or Brij-98 systems, maximum water solubilization 2671



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follows: eq 1 predicts that the effective packing parameter (Peff) of the mixed microemulsion system decreases with the increase in EO chain lengths of the nonionic surfactant, and as a result the droplet radius increases. This can affect the natural tendency of mixed surfactants to form an microemulsion and hence also the solubilization behavior. The observed difference in solubilization capacity comes from the change in the rigidity of the oil/water interface, which generates due to the addition of nonionic surfactant of varying polar head groups or POE chains. The incorporation of Brij-78 or Brij-98 with large headgroup (20 POE) into the interface decreases the effective packing parameter (Peff) by increasing the area of the polar headgroup, thereby increasing the value of R compared to Brij76 or Brij-97 (10 POE) and Brij-72 or Brij-92 (2 POE). As a result of that the oil/water interface composed of AOT/Brij-76 or Brij-97 or Brij-72 or Brij-92 surfactants becomes more rigid compared to AOT/Brij-78 or Brij-98 systems, and consequently, former systems attain larger water solubilization capacity than latter.9,23 On the other hand, the decrease in the number of POE chains would gradually reduces the cohesive interaction, which is related to the force between the hydrocarbon chains at the interface as well as to the rigidity of the interface. The solubilization for such systems thus increases due to a decrease in the critical packing parameter.9 A similar argument is also valid when comparing Brij-76 or Brij-97 (10 POE) with Brij-72 or Brij-92 (2 POE). In our previous work,9 it was reported that the incorporation of nonionic surfactant with a large polar headgroup (Brij-56 or Brij-58 with 10 or 20 POE chains, respectively) into the oil/water interface increases the fluidity of the interface of AOT/cyclohexane (Cy) or isobutyl benzene (IBB) or IPM/water microemulsions and, thereby, decreases the solubilization. On the other hand, sorbitan monostearate (Span-60), due to the absence of POE groups, induces interfacial rigidity in the same systems, and thereby, the solubilization capacity of mixed AOT/Span-60 is increased. Further, Shah et al.33 also reported a similar observation wherein the addition of sorbitan esters (Spans) enhanced the water solubilization of AOT/hexadecane/water microemulsion. The maximum water solubilization has been obtained at higher Xnonionic,max (= 0.3) for AOT/Brij-72 or Brij-92 systems compared to AOT/Brij-76 or Brij-78 or Brij-97 or Brij-98 systems (Xnonionic,max = 0.05) in three oils, as mentioned above. The results indicate that nonionic surfactants with larger polar head groups (say, POE = 10 or 20) produce solubilization maxima at a lower Xnonionic,max value. A similar observation was also reported earlier for mixed AOT/Brij or Tween or Span systems stabilized in IPM, IPP, EO, Cy, or IBB oils.9,14 3.1.3. Effect of Hydrophobic Moiety of Nonionic Surfactant on the Solubilization of Water in Mixed Microemulsions. It has been observed from Table 3 that AOT/Brij-92, AOT/Brij-97, and AOT/Brij-98 systems produce a higher ω0,max than corresponding AOT/Brij-72, AOT/Brij-76, and AOT/Brij-78 systems, respectively. Brij-72 and Brij-92, Brij-76 and Brij-97, and Brij-78 and Brij-98 possess the same number of POE chains in their polar headgroup (i.e., 2, 10, or 20, respectively). But, Brij-92, Brij-97, or Brij-98 differs in hydrophobic moieties with respect to the presence of a double bond (which is present in the middle of the hydrocarbon chain of the surfactant) from Brij-72, Brij-76, or Brij-78, respectively. A similar explanation in this direction for the observed difference in water solubilization behavior was found in our earlier report for AOT/Tween-85 or Tween-80/EO, IPM, or IPP/water reverse micellar systems.14

Further, oil molecules may be penetrated into the interfacial film of Brij-92, Brij-97, or Brij-98 containing systems as a result of decrease in chain length of hydrophobic moieties, l (due to the presence of a double bond) of Brij-92, Brij-97, or Brij-98.34 According to Leung and Shah,23 oil penetration makes the interfacial film more rigid, and thus, solubilization increases. Hence, AOT/Brij-92, Brij-97, or Brij-98-based systems favor oil penetration compared to AOT/Brij-72, Brij-76, or Brij-78based systems and exhibit higher water solubilization (i.e., higher ω0,max values) in the former system than the latter. 3.1.4. Effect of Oil on Solubilization of Water in Mixed Microemulsions. It has been observed from Table 3 that water solubilization capacity (ω0 or ω0,max) in these systems in all investigated oils follows the order: EM > IPM > IPP, which is similar to the report of Shah et al.23,33 They reported that the solubilization capacity is higher for the smaller molar volume or the shorter carbon chain length of alkane due to more oil penetration at the interface. The carbon numbers in the fatty acid chain of EM and IPM (C14) are lower than that of IPP (C16), whereas the carbon numbers in the alcohol chain of IPP and IPM (C3) are higher than that of EM (C2). However, the order of the molar volume of these oils follows: IPP > IPM > EM (vide. Table 2). Hence, it is apparent that the overall water solubilization capacity in these mixed surfactant microemulsions (ω0,max) supports the model proposed by Shah et al.23 It can be mentioned that, during the interpretation of our results using Shah et al.23 model, we only consider the carbon number of either fatty acid chain or alcohol chain of these oils. However, the functional group, that is, the ester moiety (−COOR) present in these oils which imparts polarity, has not been taken into consideration. Alkyl ester oils with a high polarity behave in a different manner from hydrocarbons.23,35,36 It was reported earlier that polar oils retarded the interdroplet interaction and, hence, influenced the solubilization phenomena in microemulsion systems.35,36 In the present report, EM, IPP, or IPM possesses a higher dielectric constant compared to linear hydrocarbons.9 In our previous reports,12,14 it was found that water or aqueous NaCl solubilization parameters in mixed AOT/Tween85 RM stabilized in IPM, IPP, and EO do not follow the order with respect to molar volume; instead they depend on the chemical structure and variation in penetrating ability of these oils. We also explained the mechanism of solubilization in these types of oils in tune with the report of Afifi et al.37 for cholesterol-based wormlike micelles in alkyl ester oils and Kaur et al.38 for nonionic surfactant based microemulsions in EO. Earlier, Fanun34 explained the difference in water solubilization due to the difference in the chemical structures of IPM, caprylic-capric triglyceride, and R-(+)-limonene in mixed nonionic/nonionic microemulsion systems. Kuneida et al.39 earlier reported the structural characteristic of polar lipophilic or alkyl ester oils by considering either “penetration effect” or “swelling effect” of oils. In view of these, it can be concluded that solubilization capability of water might follow the order of molar volume or carbon number of the oils in the fatty acid chains or alcohol chains like linear hydrocarbons but the mechanism of solubilization may be different in these systems. 3.2. Solubilization Efficiency Parameter (SP*) of Mixed Microemulsions. To underline the efficiency of a particular oil to synergies the water solubilization capacity in mixed microemulsions, a new “solubilization efficiency parameter (SP*water)” has been introduced. It has been estimated on the basis of a relative increase in solubilization 2672



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3.3. Energetic Parameter of Solubilization of Water of Mixed Microemulsions. Considering the phase separation point to be the point of maximum solubilization, the energetics of solubilization (i.e., dissolution) in terms of free energy change of the microemulsification process have been estimated. The standard free energy change of dissolution of water (ΔG0s) at a fixed temperature can be evaluated from the relation:12,14,40−43

in mixed AOT/nonionic system (ω0,max) of water compared to single AOT-based system (ω0).12,14 Mathematically, SP*water is represented as follows:12,14 SP*water = (ω0,max − ω0)/ω0

(3)

Representative plots of SP*water versus XBrij‑72/Brij‑92 for studied oils have been illustrated as Figure 5. It can be observed from

ΔG 0 s = −RT ln Xd

(4)

where Xd is the mole fraction of dispersed phase (herein, water), R is the universal gas constant, and T is the experimental temperature (herein, 303 K). Since the water in microemulsion system exists in the form of nano water pools surrounded by interfacial film, it would be more appropriate to consider Xd as the mole fraction of the water droplets. Estimation of Xd requires a detailed knowledge about the droplet size. This could not be achieved at present for the lack of facility. In this report, the estimation of ΔG0s has been done on the basis of mole fraction of water. The apparent ΔG0s values are good enough to exhibit a comparative trend of results obtained under different experimental conditions. The calculated values of ΔG0s according to eq 4 for these systems have been presented in Table 4 and representative plot(s) for mixed AOT/Brij-72 or Brij-92/EM, IPM, or IPP/water systems have been illustrated in Figure 6 at 303 K. The ΔG0s values at all compositions are comparable with earlier reports.14,41,44,45 It Table 4. Standard Free Energy Change of Dissolution of Water, ΔG0s/kJ·mol−1 of Mixed AOT/Nonionic(s)/Oil(s) Microemulsions with a Varying Mole Fraction of Nonionic Surfactant in Mixed Systems, Xnonionic, at the Total Surfactant Concentration, ST, of 0.1 mol·kg−1 and 303 Ka,b AOT/nonionic(s)/oil(s)

ΔG0s/kJ·mol−1 in EM

ΔG0s/kJ·mol−1 in IPM

ΔG0s/kJ·mol−1 in IPP

Brij-72(0.10)c Brij-72(0.20) Brij-72(0.30) Brij-72(0.40) Brij-72(0.70) Brij-76(0.05) Brij-76(0.10) Brij-76(0.20) Brij-78(0.05) Brij-78(0.10) Brij-78(0.20) Brij-92(0.10) Brij-92(0.20) Brij-92(0.30) Brij-92(0.40) Brij-92(0.70) Brij-97(0.05) Brij-97(0.10) Brij-97(0.20) Brij-98(0.05) Brij-98(0.10) Brij-98(0.20)

1.867 1.758 1.660 1.991 3.921 1.724 1.991 2.379 1.852 2.085 2.499 1.805 1.724 1.602 1.867 2.829 1.692 1.805 2.042 1.758 2.149 2.451

2.101 1.959 1.828 2.156 4.089 1.884 1.976 2.409 2.002 2.409 2.946 2.064 1.881 1.718 2.002 3.169 1.852 1.960 2.288 1.954 2.288 2.907

2.570 2.214 1.918 2.274 5.553 2.096 2.214 2.670 2.192 2.570 3.660 2.419 2.286 1.783 2.122 3.465 2.019 2.286 2.943 2.174 2.438 5.182

Figure 5. Solubilization efficiency parameter, SP*water, as a function of mole fraction of Brij-72 or Brij-92, XBrij‑72/Brij‑92, of mixed AOT/Brij-72 or Brij-92/ethyl myristate (EM), isopropyl myristate (IPM), or isopropyl palmitate (IPP)/water microemulsion systems at a fixed total surfactant concentration of 0.10 mol·kg−1 and 303 K. ●, EM; ■, IPM; and ▲, IPP.

Figure 5 (as a representative figure) that SP*water follows the order: EM < IPM < IPP, which clearly indicates that IPP and EM can be considered as most efficient and least efficient oils, respectively, in the solubilization of water in mixed AOT/Brij72 or Brij-92 microemulsions. In other words, this trend is consistent with the molar volume or total carbon number in hydrophobic moiety of these oils. But it is in the reverse order of that expected from the Shah et al.23 model (i.e., solubility decreases with increased molecular volume of oil). However, such an order was also reported earlier, where the values of surfactant efficiency (Qm) and water solubility (Ws) for the Brij 96 microemulsion system vary in the order EO > IPP > IPM.13 Further, it is evident from Figure 5 that the SP*water of the studied oils (i.e., efficacy of oils) shows a maximum at XBrij‑72/Brij‑92 = 0.30, where the synergism in water solubilization occurs. Recently, we have reported a similar result for SP*water and SP*NaCl (solubilization efficiency parameter of aqueous NaCl) for AOT/Tween-85/IPM, IPP, or EO systems by considering these parameters as an intrinsic oil property.12,14

ΔG0s/kJ·mol−1 values are 1.951, 2.277, and 2.863 kJ·mol−1 in EM, IPM, and IPP, respectively, in single AOT microemulsions (i.e., at Xnonionic = 0). bThe uncertainty is ± 0.02 kJ·mol−1 for ΔG0s. cThe values in parentheses indicate the mole fraction of nonionic surfactant, i.e., Xnonionic. a

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Figure 6. Standard free energy change of dissolution of water, ΔG0s/kJ·mol−1, of mixed AOT/Brij-72 or Brij-92/EM, IPM, or IPP/water microemulsion systems as a function of mole fraction of Brij-72 or Brij-92, XBrij‑72/Brij‑92 at fixed total surfactant concentration of 0.10 mol·kg−1 and 303 K. ●, EM; ■, IPM; and ▲, IPP.

has been observed from Table 4 that ΔG0s values depend on the solubilization capacity of water (ω0 or ω0,max). With increase in water content, ΔG0s values decrease, which indicate that the addition of water disrupts the organization of constituents in microemulsion systems. From the plot of ΔG0s vs XBrij‑72/Brij‑92 (Figure 6), it has been observed that ΔG0s values decrease with the addition of Brij-72 or Brij-92 and show minima at XBrij‑72/Brij‑92 = 0.30 (designated as ΔG0s*, standard free energy of dissolution of water at ω0,max), and beyond which, ΔG0s values have been found to increase with further addition of Brij72 or Brij-92 to AOT/oil(s)/water systems. Hence, it can be inferred that there might be a correlation between the energetics of solubilization process (ΔG0s or ΔG0s*) and maximum solubilization of water (ω0,max) which can be obtained by tuning two interfacial parameters23 as mentioned in Section 3.1.1. Similar results was found in our previous report for AOT/Tween-85/EO or IPM or IPP/water RMs.14 Plots of ΔG0s vs Xnonionic for other systems have been presented in Supporting Information as Figures S1 and S2, where a similar trend has been observed. Further, from Figure 6, it can be observed that ΔG0s (or ΔG0s*) values depend on the type of oil and follow the order: EM < IPM < IPP, which is in consistence with the order of SP*water. Hence, it can be inferred that ΔG0s depends on the molar volume or fatty acid chain length of oils in conjugation with chemical structure of the oils, as discussed in earlier section (Section 3.1.4). These results signify that the water dissolution is more organized or stabilized in an IPP-based system compared to IPM and EM-based systems. Very recently, we reported that solubilization of water or aqueous NaCl is more organized in IPP and less organized in IPM for AOT/ Tween-85/EO or IPM or IPP/water aqueous NaCl RMs by evaluating standard free energy change, standard enthalpy change, and standard entropy change of dissolution of water or aqueous NaCl.12,14 According to the report of Fanun,29 microemulsification of water in R-(+)-limonene based mixed nonionic/nonionic microemulsion system found to be more organized than caprylic-capric triglyceride and IPM based systems. Earlier, Acharya et al.44 reported that the thermodynamic aspect of microemulsion formation are governed by rupture of H-bonding of water around surfactant’s headgroup and penetration of oil molecules into the oil/water interface,

transfer of surfactant(s) from bulk oil to the interface, and orientation of surfactant(s) at the oil/water interface, and so forth. 3.4. Interdependence of Physicochemical Parameter (SP*water) and Thermodynamic Parameter (ΔG0s or ΔG0s*) of Mixed Microemulsions. In this section, a theoretical model has been proposed to correlate the physicochemical concept of SP*water with the thermodynamic concept concerning ΔG0s or ΔG0s* in mixed surfactant microemulsions stabilized in polar lipophilic or alkyl ester oils at 303 K. SP*water can be written as: SP*water =

(ω0,max − ω0) ω0

⎞ ⎛ n w,max =⎜ − 1⎟ ⎠ ⎝ nw

(5)

where nw represents the number of moles of water (at Xnonionic = 0→1.0) and nw,max represents the number of moles of water at maximum solubilization point (at Xnonionic,max = 0.05 or 0.30); other terms are described earlier. Equation 5 can be rewritten as: n w,max * r + 1) = (SPwate nw (6) Now, from eq 4 we have, 0

Xd = e−ΔGs / RT

(7)

So, Xd can be represented as: nw Xd = (n w + no + ns)

(8)

where no and ns represent number of moles of oil and surfactant, respectively. Now, rearranging eqs 7 and 8, we get, 0

nw =

(no + ns)e−ΔGs / RT 0

(1 − e−ΔGs / RT )

(9)

Similarly, nw,max can be represented as; 0

n w,max = 2674

(no + ns)e−ΔGs*/ RT 0

(1 − e−ΔGs*/ RT )

(10)



Article

By using eqs 5, 9, and 10, one can find the relationship between SP*water and ΔG0s, n w,max

(SP*water + 1) =

nw

=

0

0

0

0

and less organized in EM. Thus, it can be concluded that the variation in chemical structures of alkyl ester oils (because of their dual characteristics) tunes the flexibility or rigidity of the oil/water interface vis-à-vis the solubilization phenomenon of mixed microemulsions. A theoretical model has been proposed to correlate the physicochemical concept of SP*water with the thermodynamic concept in the light of ΔG0s or ΔG0s*. This is one of the new insights vis-à-vis contributions to the field of colloid and interface science. The results of this work would help in better understanding of colloidal processes, for example, processes like encapsulation of bioactive molecules, drug delivery, and catalysis of chemical reaction in the microemulsion, which are related to the water solubilization in the droplet core.37

e−ΔGs*/ RT (1 − e−ΔGs / RT ) e−ΔGs / RT (1 − e−ΔGs*/ RT ) (11)

or, 0

SP*water =

0

(e−ΔGs*/ RT − e−ΔGs / RT ) 0

0

0

(e−ΔGs / RT − e−(ΔGs +ΔGs*)/ RT )

(12)

It is evident from eq 12 that composition-dependent SP*water is a function of both ΔG0s and ΔG0s*, which correlates maxima in SP*water versus XBrij‑92 (Figure 5) and minima in ΔG0s versus XBrij‑92 (Figure 6) at the same composition, i.e., XBrij‑92 = 0.30 from physicochemical point of view. This is an important advancement to validate the titrimetric method for evaluation of solubilization efficiency of polar lipophilic oils from thermodynamic point of view.

■

ASSOCIATED CONTENT

■

AUTHOR INFORMATION

S Supporting Information *

Free energy of dissolution of water, ΔG0s/kJ·mol−1 of mixed AOT/Brij-76 or Brij-97/EM, IPM, or IPP/water microemulsion systems as a function of Brij-76 or Brij-97 content, XBrij‑76/Brij‑97 (Figure S1) and mixed AOT/Brij-78 or Brij-98/ EM, IPM, or IPP/water microemulsion systems as a function of Brij-78 or Brij-98 content, XBrij‑78/Brij‑98 (Figure S2), at fixed total surfactant concentration of 0.10 mol·kg−1 and at 303 K. This material is available free of charge via the Internet at http://pubs.acs.org.

4. CONCLUSIONS The following conclusions can be drawn from the study. The addition of Brij (herein, Brij-72 or Brij-92, Brij-76 or Brij-97, and Brij-78 or Brij-98) improves significantly the water solubilization capacity of AOT/EM or IPM or IPP/water microemulsions that is synergism in water solubilization occurs at certain mole fraction of mixed surfactant. The solubilization parameter, that is, ω0,max and Xnonionic,max, depend on the nonionic surfactants and oils. The ω0,max values for these systems in three oils follows the trend: AOT/Brij-72 > AOT/ Brij-76 > AOT/Brij-78 and AOT/Brij-92 > AOT/Brij-97 > AOT/Brij-98, whereas Xnonionic,max follows the order AOT/Brij72 > AOT/Brij-76 ∼ AOT/Brij-78 and AOT/Brij-92 > AOT/ Brij-97 ∼ AOT/Brij-98. The result indicates nonionic surfactants with larger polar headgroup produce solubilization maximum at lower Xnonionic,max values. On the other hand, AOT/Brij-92, AOT/Brij-97, and AOT/Brij-98 systems produce higher ω0,max than corresponding AOT/Brij-72, AOT/Brij-76, and AOT/Brij-78 systems, respectively. This has been explained in view of the presence of double bond in hydrophobic moiety of Brij-92 or Brij-97 or Brij-98. Further, both ω0 (single AOT) and ω0,max (mixed AOT/Brij’s) for three investigated oils follow the order: EM > IPM > IPP. The order is found to be apparently consistent with Shah et al.23 model (which was dealt with linear hydrocarbons). To underline the efficiency of a particular oil to synergies the water solubilization capacity in mixed microemulsions, SP*water has been evaluated, which followed the trend: EM < IPM < IPP. The trend is reversed with respect to the order of ω0,max. It can be inferred that, role of chemical structure of the oils predominates over the molar volume of the oils. The addition of nonionic surfactant that is its content (Xnonionic) influences the ΔG0s values of these systems. ΔG0s values exhibit a minima at XBrij‑76/Brij‑78/Brij‑97/Brij‑98 = 0.05 and XBrij‑72/Brij‑92 = 0.30 (designated as ΔG0s*), where maxima in SP*water perceives. This phenomena has been explained in the light of two interfacial parameters (i.e., the spontaneous curvature and the interdroplet interactions), which governs solubilization process.23 Both ΔG0s and ΔG0s* values depend on type of oil at comparable conditions and follow the order: EM < IPM < IPP, which is consistent with the order of SP*water. This result indicates that the water solubilization is more organized in IPP

Corresponding Author

*Phone: +91-9433339042. E-mail: [email protected] (K.K.). Funding

B.K.P. and K.K. thankfully acknowledge the authority of Indian Statistical Institute, Kolkata for the financial support in the form of an operating research grant and Senior Research Fellowship, respectively. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Ms. Ankita Aich (Bankura Christian College, Bankura) for her help in the calculation of theoretical parameters.

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REFERENCES

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