Characterizing colloidal behavior of non-ionic emulsifiers in non-polar solvents using electrical impedance spectroscopy

Abstract

Colloidal behavior of a widely used non-ionic emulsifier, sorbitan monooleate (Span80), was investigated in non-polar solvents (cyclohexane and xylene) using electrical impedance spectroscopy (EIS). The electrical characteristics of the colloidal mixtures were measured with frequency scans ranging from 1 Hz to 200 kHz. The conductances at low frequencies were found to increase with an increase in Span80 concentration. The source of conductivity for non-polar solvents using non-ionic emulsifiers is usually attributed to ionic impurities either in the Span80 or in the non-polar solvents. The measured electrical characteristics for pure Span80 and pure non-polar solvents revealed that the source of ionic conduction is impurities in Span80. It was confirmed that the ionic impurities in the non-polar solvents are in form of aggregate of ions, ion-pairs, and triple ions which is unaffected with the emulsifier concentration. Analyses using equivalent electrical circuits confirmed that the critical Maxwell-Wagner frequency is 0.6–1.8 Hz for the mixtures. The conductance-concentration profiles for the mixtures at 1 Hz showed transitions from a square root to a linear concentration dependence at the CMC. This indicated that the dissociation model holds below the CMC, while the fluctuation model applies above the CMC. The conductance profiles enabled estimates of the relative hydrophilic core radius and the fraction of charged micelles in both non-polar solvents.