Viscosity, conductivity, and dielectric relaxation of waterless glycerol–sodium bis(2-ethylhexyl) sulfosuccinate–isooctane microemulsions: The percolation effect

The waterless microemulsion glycerol-AOT-isooctane [AOT, sodium bis(2-ethylhexyl) sulfosuccinate] was systematically studied as a function of temperature T, volume fraction (glycerol plus AOT) φ, molar ratio n=[glycerol]/[AOT], and the salt content ${\mathit{p}}_{\mathit{s}}$ in the glycerol. The properties studied are dynamic viscosity η, electric conductivity σ, and dielectric relaxation ${\mathrm{\epsilon }}_{\mathit{R}}^{\mathrm{*}}$. At fixed T, n, and ${\mathit{p}}_{\mathit{s}}$ an increase in the conductivity and dynamic viscosity is observed when the volume fraction increases. Dielectric relaxation may be represented as a generalized Davidson-Cole distribution of the relaxation time. The quantities (1/σ)(dσ/dφ) and (1/η)(dη/dφ) pass through a maximum, as does the static permittivity ${\mathrm{\epsilon }}_{\mathit{s}}$ (in the latter case the maximum is often followed by a minimum). At the same time the inverse of the characteristic frequency of dielectric relaxation 1/${\nu }_{\mathit{R}}$ passes through a maximum. The results are discussed in the framework of percolation theory. The application of the asymptotic laws of percolation is discussed. For viscosity, the analysis of the results provides a good comparison between theoretical and experimental values taking as critical percolation exponents μ’≃2 (for φ>${\mathrm{\phi }}_{\mathit{c}}$), where ${\mathrm{\phi }}_{\mathit{c}}$ is the percolation threshold and s’≃1.2 (φ${\mathrm{\phi }}_{\mathit{c}}$), which are the values predicted by the dynamic theory of percolation. By determining the η(φ) curves for various conditions, we were able to establish the variations of ${\mathrm{\phi }}_{\mathit{c}}$(T), ${\mathrm{\phi }}_{\mathit{c}}$(n), and ${\mathrm{\phi }}_{\mathit{c}}$(${\mathit{p}}_{\mathit{s}}$). It was observed that ${\mathrm{\phi }}_{\mathit{c}}$ decreases when T and n increase or when ${\mathit{p}}_{\mathit{s}}$ decreases. This corresponds to an increase in the interactions between droplets within the system. Finally, according to the value of φ, the viscosity may increase or decrease with increasing temperature. This curious effect can be explained by appropriate application of percolation theory.