Effects of water content and oil on physicochemical and microenvironmental properties of mixed surfactant microemulsions

In this report, phase behavior, conductivity, viscosity, dynamic light scattering, fluorescence lifetime, steady state fluorescence anisotropy and Fourier transform infrared spectroscopy (FTIR) techniques were employed for understanding of the physicochemical properties and microenvironment of water-in-oil microemulsion comprising equimolar (1:1) cetyltrimethylammonium bromide (C16TAB) and polyoxyethylene (20) cetyl ether (C16E20)/1-butanol/heptane or decane, with varying water content (ω) at 303 K. Both conductivity and viscosity of these systems were increased with increase in ω in both oils. Droplet size was also increased with increase in ω and corroborated well with the conductance and viscosity measurements, and depends on oil chain length. The physicochemical changes in the microenvironment with increase in ω were presented by measuring the changes in decay time using a fluoroprobe (7-hydroxycoumarin). The effect of hydration on the microstructure of these systems was studied by polarized fluorescence measurements. FTIR measurements reveal three states of water molecules, viz. trapped, bound and bulk water, in water pool of these systems. Further, the interfacial composition and free energy of transfer of 1-butanol from oil to the interface were evaluated by the dilution method. Changes in interfacial composition as a function of ω corroborate well with FTIR results indicating bound and bulk water.