Volumetric and transport properties in microemulsions and the point of view of percolation theory.

Abstract

Microemulsions have been prepared from either benzene, toluene, ethylbenzene, or o-xylene and water using a mixture of surfactants of Span 80 and Tween 20 in the volume ratio of 1:4. These formulated microemulsions having propanol as the cosurfactant have been characterized by measuring their dynamic properties, i.e., conductance \ensuremath{\sigma}, density \ensuremath{\rho}, viscosity \ensuremath{\eta}, and ultrasonic velocity u, as a function of the volume fraction of water \ensuremath{\varphi} at 30 \ifmmode^\circ\else\textdegree\fi{}C. The conductance values change exponentially as the volume fraction of water is increased. The use of a percolation model gives reasonable agreement between the experimental and calculated values. The viscosities vary in a nonmonotonic way, giving two peaked plots when the water content increases, indicating a lack of correlation with conductivity. The ultrasonic velocity and isentropic compressibility ${\mathit{K}}_{\mathit{S}}$ data throw some light on structural changes that occur in the microemulsion. An attempt has also been made to calculate the density ${\mathrm{\ensuremath{\rho}}}_{\mathit{m}}$ and isentropic compressibility ${\mathit{K}}_{\mathit{S},}$m of the micellar phase from the experimental data. The results indicate a trend toward an enhanced waterlike character of the dispersed phase at a high volume fraction of water \ensuremath{\varphi}.