Physicochemical studies of mixed surfactant microemulsions with isopropyl myristate as oil

The present study is focused on evaluation of interfacial compositions and thermodynamic properties of w/o mixed surfactant [(sodium dodecylsulfate, SDS/polyoxyethylene (23) lauryl ether, Brij-35)/1-pentanol (Pn)/isopropyl myristate (IPM)] microemulsions under various physicochemical conditions by the dilution method. The number of moles of Pn at the interface (nₐ ⁱ) and bulk oil (nₐ ᵒ), and various thermodynamic parameters [viz. standard Gibbs free energy (ΔGₒ→ᵢ ⁰), standard enthalpy (ΔHₒ→ᵢ ⁰), and standard entropy (ΔSₒ→ᵢ ⁰) of the transfer of Pn from bulk oil to the interface] have been found to be dependent on the molar ratio of water to surfactant (ω), concentration of Brij-35 (XBᵣᵢⱼ₋₃₅), and temperature. Temperature-insensitive microemulsions with zero specific heat capacity (ΔCₚ ⁰)ₒ→ᵢ have been formed at specific compositions. The intrinsic enthalpy change of the transfer process (ΔH⁰)ₒ→ᵢ ∗ has been evaluated from linear correlation between ΔHₒ→ᵢ ⁰ and ΔSₒ→ᵢ ⁰ at different experimental temperatures. The present report also aims at a precise characterization on the basis of molecular interactions between the constituents and provides insight into the nature of the oil/water interfaces of these systems by conductivity and dynamic light scattering studies as a function of ω and XBᵣᵢⱼ₋₃₅. Conductivity studies reveal that incorporation of Brij-35 in non-percolating water/SDS/Pn/IPM systems makes them favorable for ω-induced percolation behavior up to XBᵣᵢⱼ₋₃₅⩽0.5. But further addition of Brij-35 causes a decrease in conductivity with increasing ω. Furthermore, the hydrodynamic diameters of the microemulsion droplets increase with increase in both XBᵣᵢⱼ₋₃₅ and ω. Correlations of the results in terms of the evaluated physicochemical parameters have been attempted.