Ion-Pair Conductivity Theory: Mixtures of Non-Polar Media with Non-Ionic Surfactant

Abstract

In previously published papers [Dukhin, A., Parlia. S., J. Electrochem. Soc., 162, H1–H8 (2015); Parlia, S., Dukhin, A., Somasundaran, P., J. Electrochem. Soc., 163, H1–H6 (2016); Bombard, A., Dukhin, A., Langmuir, 30, 4517–4521 (2014)] we presented data on the conductivity of various alcohols mixed with several non-polar liquids. A peculiar range of the conductivity, in which it decayed exponentially with alcohol content, was discovered for all mixtures. We suggested a theory based on the “ion-pair concept” following fundamental principles formulated by Bjerrum, Onsager, and Fuoss. This theory fits the experimental data for all of these mixtures within the full concentration range, from pure alcohol to pure non-polar liquid. In this work we investigated the behavior of a new type of amphiphilic liquid, using non-ionic surfactants instead of alcohols. Measurements were conducted again across the full concentration range, which has never been done before for non-ionic surfactants. It was found that the exponential decay range exists for these mixtures as well. The earlier proposed ion-pairs conductivity theory was found to fit the experimental data well for these surfactant mixtures with toluene. This fitting procedure yields values for two adjustable parameters: size of the ions and ionic strength of the pure surfactant.