Tween-based microemulsions: a percolation view

Results obtained on electrical conductivity, density, ultrasonic velocity and viscosity for the macroscopically homogeneous, stable, optically transparent and isotropic media — generally called microemulsions, are presented for a number of systems incorporating non-ionic surfactant: Tween 20, water, benzene and alkanol (C1–C8,C10) as a function of volume fraction of water (φ) at 30°C. The experimental path followed in each system has been characterized by a fixed molar ratio of chosen alkanol to the surfactant (na/nS)=2. Water is used as titrating component hence varying the value of ω from 0 to 180 within the system, where ω is the molar ratio of the aqueous phase to Tween 20+alkanol. In this paper, a phenomenological theory for the effects of interactions on the conductivity of water in oil microemulsions in the dilute limit of spherical droplets is analyzed. The density (ρm) and the isentropic compressibility (ks,m) of the micellar phase are derived from the experimental density (ρ) and ultrasonic velocity (u) data. The results indicate a trend towards an enhanced water like character of the dispersed phase at high volume fraction of water (φ). Viscosity varies in a non-monotonic way, giving two peaked plots. In the phase diagram, the realms-of-existence of single phase (i.e. microemulsion formation), double phase and mesophase have been delineated for the system containing propanol as co-surfactant. A simple structural model has been applied for the calculation of the various parameters i.e. aggregation number (n), core radius (rn) and surface number density of the surfactant molecules at the interface (αS).