Ion-Pair Conductivity Theory IV: SPAN Surfactants in Toluene and the Role of Viscosity

Abstract

© 2018 The Electrochemical Society. There have been number of papers published lately denying the role of ion-pairs role in the ionization of non-polar liquids containing surfactants. We have conducted several studies published in this Journal that unambiguously confirm the essential role of ion-pairs, and have suggested a theory that fits experimental conductivity data for a wide variety of non-polar liquid mixtures with amphiphilic substances. However, one contradiction remains between our very first paper on this subject and our recent experiments: specifically the linear dependence of conductivity on surfactant concentration found in the former, versus an exponential dependence found in the latter. We resolve this contradiction here by re-measuring mixtures of toluene with SPAN surfactants within the complete composition range. Conductivity is indeed a linear function of SPAN concentration below 50%. However, this linearity is result of viscosity being an exponential function of the SPAN concentration. Conductivity multiplied by viscosity is an exponential function of SPAN concentration as theory predicts for liquid mixtures with ion-pairs.