Study on the Effects of Nonelectrolyte Additives on the Phase, Thermodynamics, and Structural Changes in Micelles of Silicone Surfactants in Aqueous Solutions from Surface Activity, Small Angle Neutron Scattering, and Viscosity Measurements

Abstract

Association properties of two silicone surfactants based on poly(dimethylsiloxane)-graft-polyethers in aqueous solutions of 2-butoxyethanol, poly(ethylene glycol), and glucose were determined using various techniques such as surface tension, small-angle neutron scattering, and viscosity. Dilute solution phase diagrams were also constructed, and cloud points were measured for different concentrations of both the surfactants in additive aqueous solutions. The thermodynamic parameters for the micellization were obtained from the temperature-dependent data on critical micelle concentration. The influence of given additive on the micellization of silicone surfactants was monitored from the changes in the free energy of micellization values for the surfactant solutions in water and water + additives. The cloud points and the critical micelle concentration values for the surfactant solutions were found to be decreased in the presence of the selected three additives. The analysis of the changes in the relative permittivities and partial molar volumes for the surfactant solutions in the presence of water and water + additives suggests that the solvent environment around the surfactant solute molecules is different in mixed solvent systems vis-a-vis water and the micellization and surface activity of silicone surfactants were dictated predominantly by the preferential hydration of cosolute additives. The analysis of small angle neutron scattering curves for the surfactant aqueous solutions in the presence of additives showed that the micelles formed have oblate ellipsoidal shape at 30 °C with, however, increased characteristic axial ratios than those in pure water. The increase in the concentration of additives has also increased the size of the micelles. The increase in temperatures corresponding to the values close to the turbid boundaries in the phase diagrams caused a transition from the oblate to the disklike micellar shape. The changes in the hydration values of the micellar associates were monitored from the dilute solution viscosity measurements. The dehydration of the micelles in the presence of additives at 30 °C and also at elevated temperatures occur from the interior to the fringe of the core−outer shell parts and to the outer shell successively.