Effect of NaCl and temperature on the water solubilization behavior of AOT/nonionics mixed reverse micellar systems stabilized in IPM oil

Solubilization of water in mixed reverse micellar systems formed with anionic surfactant (AOT) and nonionic surfactants (Brij-30, Brij-35, Brij-52, Brij-56, Brij-58, Brij-76, Tween-20, Tween-40, Span-20, Span-40, Span-60, Span-80) in isopropyl myristate (IPM) oils has been studied. The enhancement in water solubilization (i.e. synergism) has been evidenced by the addition of nonionic surfactants to AOT/IPM system. The maximum water solubilization capacity (ω0,max) and the mole fraction of nonionic surfactant at which maximization occurs (Xnonionic,max) in these mixed reverse micellar systems has been found to depend on surfactant component (size and nature of polar group and hydrocarbon moiety of the surfactant). The addition of electrolyte (NaCl) in these systems has been found to enhance the solubilization capacity (ωmax) depending upon the content and their EO chains and type of polar head group of nonionic surfactant used. The maximum solubilization of electrolyte, ωmax was obtained at an optimal concentration of it ([NaCl]max), which depends on the content and their EO chains and type of polar heads of the nonionic surfactants. The temperature induced solubilization of water (in absence and presence of additives) for AOT blended with nonionic surfactants (Brijs, Tweens, Spans) at different compositions has been investigated. The stability of these systems with respect to temperature has been found to be dependent on the content and nature of the polar head group as well as the hydrophobic moiety of the nonionics. The energetic parameters (ΔG¯°, ΔH¯° and ΔS¯°) of the desolubilization process of water in the mixed systems have been estimated from the solubilization capacity-temperature profile. An attempt has been made to shed more insight into the process of solubilization-desolubilization of water (or aqueous NaCl) in mixed reverse micelles stabilized in IPM oil on the basis of the model proposed by Shah et al. [M.J. Hou, D.O. Shah, Langmuir 3 (1987) 1086; R. Leung, D.O. Shah, J. Colloid Interf. Sci. 120 (1987) 320, 330] and thermodynamic approach.