Electronic Supplementary Material

Physicochemical Investigation of Biocompatible Mixed Surfactant Reverse Micelles: III. Aqueous NaCl Solubilization, Thermodynamic Parameters of Desolubilization Process and Conductometric Studies

Kaushik Kundu and Bidyut K. Paul*

Surface and Colloid Science Laboratory, Geological Studies Unit

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

*author for correspondence <

Phone: (Off) (+91) (33)-2575-3164

Fax : (+91) (33)-2577-3026

Isopropyl palmitate Isopropyl myristate

Ethyl oleate (EO) Dioctyl sulfosuccinate sodium salt (DOSS)

SMO20EO (HLB = 15) STO20EO (HLB = 11)

Scheme. S1. Chemical structures of oils, ionic and nonionic surfactants

Fig. S1. Solubilization of aqueous NaCl in DOSS/STO20EO/EO mixed RMs at 303 K with surfactant concentration of 0.10 × 103 mol/m3 at different XSTO20EO; where, XSTO20EO = 0.025 (●), XSTO20EO = 0.05 (■), XSTO20EO = 0.1 (▲) and XSTO20EO = 0.2 (*).

1. Solubilization of water in mixed RMs in polar lipophilic oils

DOSS RMs with total surfactant concentration, [ST] of 0.10 × 103 mol/m3 in IPM, IPP and EO have been observed to solubilize a substantial amount of water, w0 (= [water]/[surfactant]) ~ 22, 14 and 17, respectively. It can be observed from Figs. 1A and 1B and Table 2 that the addition of nonionic surfactant(s) (STO20EO and SMO20EO) to DOSS/IPM or, IPP or, EO (at a fixed [ST] of 0.10 × 103 mol/m3) has been observed to enhance the solubilization capacity of water. The solubilization capacities of DOSS/ STO20EO and SMO20EO systems are observed to increase with increasing nonionic content (XSTO20EO or SMO20EO) in a mixture of DOSS/STO20EO or SMO20EO, and then decrease after passing through maxima (w0,max, indicates maximum amount of water solubilized in RMs). The results are presented in Figs. 1A and 1B. It has been observed that w0,max occurs at XSTO20EO or SMO20EO,max = 0.05 (where, XSTO20EO or SMO20EO,max indicates the mole fraction of STO20EO or SMO20EO, at which maximum solubilization of water occurs) for all three oils (Figs. 1A and 1B). It has also been observed that, w0,max values varies with the type of oils (herein studied). Values of w0,max follows the order, IPP < EO < IPM.

The rationale behind using these nonionic surfactants is to underline their role of hydrophobic configuration in water solubilization of mixed RMs. It can be observed from Table 2 that the DOSS/STO20EO exhibits higher maximum solubilization capacity (w0,max) (34.26, 32.41 and 27.78) compared to DOSS/SMO20EO (29.63, 25.93 and 22.22) at Xnonionic,max = 0.05 in oils (IPM, EO and IPP, respectively). From the scrutiny of the chemical structures and HLB values of the nonionic surfactants, it can be noted that STO20EO contains the same number EO units in chain (i.e., 20) as that of SMO20EO, but STO20EO (HLB = 11) is more hydrophobic than SMO20EO (HLB = 15) due to the presence of bulkier hydrophobic configuration. Hence, it can be inferred that w0,max of these systems is influenced by the difference in chemical structures and configurations of the nonionic surfactants and type of oils. Enhancement in water solubilization capacity in mixed surfactant RMs was also reported earlier [1-7].

2. Effect of nonionic surfactant on solubilization of water in mixed RMs (DOSS/STO20EO or SMO20EO) in polar lipophilic oils (EO, IPM and IPP)

In the present study, addition of nonionic surfactants with similar polar head group but differ in hydrophobic configurations to DOSS/oil(s)/water RMs produces synergism in water solubilization capacity. Solubilization of water in reverse micellar system depends upon many factors, for example, type of surfactant (and cosurfactant or a second surfactant), oil, temperature, and additives, etc. [8]. But the main driving force of such solubilization is the spontaneous curvature and the elasticity (or rigidity) of the interfacial film formed by the surfactant system [9]. If the interfacial curvature and the bending elasticity are fixed, solubilization can be maximized by minimizing the interfacial bending stress of the rigid interface and the attractive interdroplet interaction [10]. 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) of the surfactant plays an important role in water solubilization. For a mixed surfactant RM, the effective packing parameter (Peff) follows the additive relation [11]:

Peff = [(xv/al)A + (xv/al)B]/(xA+xB) (1)

where xA and xB are the mole fractions of surfactant A and B in the mixture, respectively. Nonionic surfactants have larger head group area than DOSS [8, 12], and thus on mixing STO20EO or SMO20EO with DOSS, Peff is decreased owing to the increase in ‘a’, which in turn increases the natural radius of curvature (R). Thus, DOSS/nonionic(s)/oil RMs can accommodate more water than the DOSS/oil RMs, and with increasing Xnonionic, solubilization capacity of the mixed system increases. But with increasing the droplet size, interdroplet interaction also increases, and at a certain droplet radius, Rc (the critical radius of curvature) interdroplet interaction starts governing the solubilization behavior and the fluid interfaces can coalesce in order to break the monophasic system into two distinct phases. Thus beyond Rc, addition of nonionic surfactant decreased the solubilization capacity. For all these mixed systems, the ascending curve in the solubilization capacity versus 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. [13]. They reported that the main driving force for water solubilization in mixed surfactant, DOSS/CiEj RMs (where ‘i’ is the number of carbon atoms in the hydrophobic part and ‘j’ is the number of ethylene oxide units in the hydrophilic part of the nonionic surfactant) is the presence of the hydrophilic group of the nonionic surfactant.

In the present study, we also performed dye solubilization experiment [8, 14] to estimate the phase separation phenomenon in single DOSS/oil(s)/water as well as mixed DOSS/nonionics/oil(s)/water with Xnonionic < Xnonionic,max and Xnonionic ≥ Xnonionic,max. The results have been depicted in our previous report [15].

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) as a result of curvature effect and the critical radius of droplets (Rc) due to the result of attractive interaction among the droplets. A maximum in solubilization occur when both of these effects are minimized [9].

3. Solubilization efficiency parameter (SP*water and SP*NaCl)

Maximization of solubilization capacity of water in mixed RMs (w0,max) have been found to depend on the content, hydrophobic moiety, polar head group of the added nonionic surfactant and oil type vis-à-vis its chemical structure. An attempt has been made to introduce a new parameter, “solubilization efficiency parameter (SP*)” to underline the efficacy of oils (of different lipophilicities) on synergistic behavior in solubilization capacity in mixed systems. Solubilization efficiency parameter (SP*water or SP*NaCl) can be defined as a relative increase in solubilization capacity of water or aqueous NaCl in presence of surfactant mixture as a unique number for a particular oil at constant composition in microemulsion systems. Mathematically, SP*water or SP*NaCl are represented as follows:

SP*water = (w0,max - w0)/w0 (2)

SP*NaCl = (wNaCl,max - w[NaCl]=0(M))/w[NaCl]=0(M) (3)

Mathematical symbols are used with their usual significance (mentioned earlier). The calculated values SP*water or SP*NaCl are presented in Tables 2-4. A representative diagram of solubilization efficiency parameter (SP*NaCl) as a function of XSTO20EO for all oils, has been presented (Fig. S2). It has been observed that SP*water and SP*NaCl follows the order: IPM < EO < IPP, which is in the reverse order of both w0,max and wNaCl,max. Order of SP*water has also been found to be of reverse order proposed by Shah et al. (which states that solubilization of water decreases with increase in molecular volume of oil) [9], with exchange in position of EO with respect to IPP. Further, it is evident from Fig. S2, SP*NaCl increases with increase in XSTO20EO for studied oils, which corroborate well with results presented in Sec. 3.2.1. The result might be explained in the following way: this type of fatty acid ester oils have a tendency to penetrate in the surfactant palisade layer, depending on the alkyl chain (short) associated with ester moiety and make oil/water interface rigid. It has been reported by Shah et al. [9], that the solubilization of a microemulsion with a rigid interface can be increased by increasing fluidity of the interface. So, variation in penetration ability of these oils due to their different chemical structures exhibit such type of order of SP*water and SP*NaCl (as mentioned above). Owing to the presence of cis-double bond in EO (i.e., adjacent hydrogen atoms are on the same side of the double bond), it has a natural tendency for bending [16]. Thus, the conformational freedom of the fatty acid chain of EO restricts due to the bending configuration and as a result, the oil/water interface becomes less flexible compared to saturated chain containing IPP system. Hence, SP*water and SP*NaCl values of EO is lower than IPP. All these phenomena are attributed to the fact that the solubilization depends not only on the geometric parameters but also on branching, saturation or unsaturation in the oil molecule etc. In the present case, it might be the mutual miscibility between the hydrophobic chains of the surfactant(s) and oil that affects the degree of oil penetration into the amphiphilic film as well as the spontaneous curvature [17, 18]. Hence, it can be inferred that the efficacy of the solubilization of the dispersed phase(s) in mixed RMs depends on both the fatty acid chain length (as indicated by increases with increase in molar volume or fatty acid chain length) and chemical structures of the oils.

Fig. S2. Solubilization efficiency parameter (SP*NaCl) as a function XSTO20EO in DOSS/STO20EO/oil/water mixed reverse micellar system.

References:

[1] Li Q, Li T, Wu J (2002) Water Solubilization Capacity and Conductance Behaviors of AOT and NaDEHP Systems in the Presence of Additives. Colloids Surf A 197: 101-109.

[2] Paul BK, Mitra RK (2005) Water Solubilization Capacity of Mixed Reverse Micelles: Effect of Surfactant Component, the Nature of the Oil, and Electrolyte Concentration. J Colloid Interface Sci 288: 261-279.

[3] Mitra RK, Paul BK (2005) Effect of NaCl and Temperature on the Water Solubilization Behavior of AOT/Nonionics Mixed Reverse Micellar Systems Stabilized in IPM Oil. Colloids Surf A 255: 165-180.

[4] Mitra RK, Paul BK (2005) Investigation on Percolation in Conductance of Mixed Reverse Micelles. Colloids Surf A 252: 243-259.

[5] Lundsten G, Backlund S, Kiwilsza G (1995) Solubility limits of water in systems of aromatic oils and nonionic surfactants. Prog Colloid Polym Sci 97: 194-198.

[6] Seedher N, Manik N (1993) Solubilization in mixed surfactant reverse micellar systems. J Surface Sci Technol 9: 81-86.

[7] Bumajdad A, Eastoe J, Griffiths P, Steytter DC, Heenan RK, Lu JR, Timmins P (1999) Interfacial Compositions and Phase Structures in Mixed Surfactant Microemulsions. Langmuir 15: 5271-5278.

[8] Liu DJ, Ma JM, Cheng HM, Zhao ZG (1998) Solubilization Behavior of Mixed Reverse Micelles: Effect of Surfactant Component, Electrolyte Concentration and Solvent. Colloids Surf A 143: 59-68.

[9] Leung R, Shah DO (1987) Solubilization and Phase Equilibria of Water-in-Oil Microemulsions: I. Effects of Spontaneous Curvature and Elasticity of Interfacial Films. J Colloid Interface Sci 120: 320-329, Solubilization and Phase Equilibria of Water-in-Oil Microemulsions: II. Effects of Alcohols, Oils, and Salinity on Single-chain Surfactant Systems. J Colloid Interface Sci 120: 330-344.

[10] Evans DF, Mitchell DJ, Ninham BW (1986) Oil, Water, and Surfactant: Properties and Conjectured Structure of Simple Microemulsions. J Phys Chem 90: 2817-2825.

[11] Mitchell DJ, Ninham BW (1981) Micelles, Vesicles and Microemulsions. J Chem Soc Faraday Trans II 77: 601-629.

[12] Liu D, Ma J, Cheng H, Zhao Z (1998) Investigation on the Conductivity and Microstructure of AOT/Non-ionic Surfactants/Water/n-Heptane Mixed Reverse Micelles. Colloids Surf A 135: 157-164; (1999) Conducting Properties of Mixed Reverse Micelles. Colloids Surf A 148: 291-298.

[13] Nazario LMM, Hatton TA, Crespo JPSG (1996) Nonionic Cosurfactants in AOT Reversed Micelles: Effect on Percolation, Size, and Solubilization Site. Langmuir 12: 6326-6335.

[14] Hou MJ, Shah DO (1987) Effects of the Molecular Structure of the Interface and Continuous Phase on Solubilization of Water in Water/Oil Microemulsions. Langmuir 3: 1086-1096.

[15] Kundu K, Paul BK (unpublished results) (2013) Physicochemical Investigation of Mixed Surfactant Reverse Micelles: I. Water Solubilization and Conductometric Studies (Communicated).

[16] Kaur G, Chiappisi L, Prevost S, Schweins R, Gradzielski M, Mehta SK (2012) Probing the Microstructure of Nonionic Microemulsions with Ethyl Oleate by Viscosity, ROESY, DLS, SANS, and Cyclic Voltammetry. Langmuir 28: 10640-10652.

[17] Fanun M (2008) Water Solubilization in Mixed Nonionic Surfactants Microemulsions. J Dispersion Sci Technol 29: 1043-1052.

[18] Mehta SK, Kaur G, Munteja R, Bhasin KK (2009) Solubilization, Microstructure, and Thermodynamics of Fully Dilutable U-type Brij Microemulsion. J Colloid Interface Sci 338: 542-549 and references therien.