Direct measurement of temperature-dependent interactions between non-ionic surfactant layers

Abstract

The force between two surfaces coated with pentaoxyethylene dodecyl ether, C₁₂E₅, and immersed in aqueous solution has been measured as a function of surface separation in the temperature range 15–37 °C. The surfaces were prepared by allowing the non-ionic surfactant to adsorb on to hydrophobised mica from the solution. At 15 °C the interaction is repulsive at all separations, but above 20 °C an attractive minimum appears at separations below ca. 4 nm. The attraction increases rapidly with temperature and is identified with the interaction that gives rise to the phase separation (‘clouding’) in micellar solutions of many non-ionic surfactants as well as in the poly-(ethylene oxide)–water system. The net interaction is composed of two large parts, one entropic that is attractive and one enthalpic that is repulsive. The two parts nearly cancel, making the net interaction repulsive at low temperatures but attractive at high. They apparently originate mainly from the hydration interactions between the oxyethylene head groups on one surface with those on the other, and are of the same order of magnitude as in pure poly(ethylene oxide)–water solutions. The thickness of the non-ionic surfactant layers increases with temperature, implying a decrease in surface area per head group. This indicates that the intralayer head-group interaction also becomes more attractive (or less repulsive) with temperature.