Temperature Effects on Micelle Formation and Particle Charging with Span Surfactants in Apolar Media

This paper examines the effects of temperature on the micellization and particle charging behavior of the Span surfactant series in an apolar environment. The critical micelle concentrations of each of six surfactants at five temperatures were measured by conductometric techniques. The thermodynamic properties of micellization were calculated using Gibbs-Helmholtz analysis. Magnesia particles were then dispersed in solutions of these surfactants, and their electrophoretic mobilities were measured at three temperatures. Preliminary small-angle neutron scattering (SANS) experiments were conducted to measure the size of aggregates (referred to as reverse micelles) of three of the surfactants. It was found that for all but one of the surfactants the critical micelle concentration (CMC) increased by as much as an order of magnitude across a 40 °C range of temperature. One of the surfactants exhibited a decrease in CMC upon increasing temperature, likely due to a decrystallization of the tails upon reverse micelle formation. The maximum particle mobilities decreased upon increasing temperature due to the increased electrostatic screening by charged reverse micelles at higher temperatures.