Physico-Chemical Studies on Microemulsion: Effect of Cosurfactant Chain Length on the Phase Behavior, Formation Dynamics, Structural Parameters and Viscosity of Water/(Polysorbate-20+n-Alkanol)/n-Heptane Water-in-Oil Microemulsion

The pseudo-ternary water-in-oil microemulsion system, comprising water/polyoxyethylene sorbitan monolaurate (Polysorbate-20) + n-alkanol/n-heptane, have been studied by phase manifestation, method of dilution, viscosity and dynamic light scattering measurements. Polysorbate-20, in combination with equal mass of cosurfactants (of varying chain length, from n-butanol to n-octanol) were used in studying the systems in the temperature range 303-323 K. Appearance of turbidity was noted visually, which indicated the attainment of immiscibility or phase separation; a clear dependence of the different phases on cosurfactant chain length was noted. By employing the method of dilution, associated thermodynamic parameters for the formation of water-in-oil microemulsion droplets were derived. Different associated structural parameters were derived through further computation of the data derived from the method of dilution. Unusual behavior of Polysorbate-20, compared to the conventional ionic surfactants, was noted. Viscosity measurements, as carried out with different compositions and temperatures, revealed the temperature and water pool size dependency of the microemulsion systems. Viscosity data did not follow the same trend during heating and the cooling process, due to the condensation effect. This phenomenon was further confirmed by dynamic light scattering measurements.