Calorimetric study of non-ionic surfactants. Enthalpies and heat-capacity changes for micelle formation in water of C8E4 and Triton X-100 and micelle size of C8E4

Abstract

Calorimetric measurements have been made of the differential enthalpies of solution in water of liquid n-octyl tetraoxyethylene glycol monoether C8E4 and Triton X-100, as function of concentration, at three different temperatures. Experiments have also been carried out with C8E4 dissolved in 0.1 mol kg–1 KCl at 25 °C. Enthalpies of solution of monomers at infinite dilution and micelles at the c.m.c., respectively, were calculated and enthalpies of micelle formation have been derived. Heat-capacity changes for dissolution and for micelle formation were calculated from the temperature variation of the solution enthalpies. The aggregation number n of C8E4 micelles has been derived from the calorimetric titration experiments by applying the mass-action model to micelle formation. At 25 °C a value of n= 23 is found which is compatible with the formation of small spherical micelles with a radius equal to the length of the extended n-octyl chain. There are no indications of micellar growth from the calorimetric results as the liquid–liquid phase boundary is approached in the C8E4–water system. The presence of 0.1 mol kg–1 KCl had no significant effect on the enthalpies of solution and micelle formation of C8E4 and had no detectable influence on the aggregation number.