Adsorption of Triblock Copolymers of Ethylene Oxide and Propylene Oxide at the Air/Water Interface: The Surface Excess

Abstract

Neutron reflection has been used to measure the surface excesses of two poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) triblock copolymers (EPE) at the air/water interface over a range of bulk solution concentrations. The copolymers, of approximate formulas E23P52E23 and E9P22E9 and have similiar adsorption isotherms in terms of their molar adsorption, though quite different in terms of segmental adsorption. The isotherms have two steps, one at low concentrations leading to a plateau over about two decades of concentration followed by a substantial rise over the decade of concentration leading to the critical micelle concentration. In the plateau region, the area per ethylene oxide segment was found to be about 9 and 22 Å2 respectively for the two copolymers decreasing to 6.5 and 12 Å2 at the CMC at 25 °C. The thickness of the adsorbed layer showed little change between these two coverages. An increase in temperature to 35 °C shifted the adsorption isotherm pattern to concentrations about 1/30 of those at 25 °C, in line with the change in CMC. The neutron surface excesses were incorporated into the integrated Gibbs equation and fitted the surface tension curves well over most of the range of adsorption. However, a sharp change in surface tension at low polymer concentrations (about 10-3 wt/vol %) was found not to be consistent with the neutron measurements, and a model of the surface tension behavior in this region, based on adsorption from a polydisperse sample, has been proposed to explain this behavior. Earlier explanations of this surface tension behavior are shown to be inconsistent with the neutron reflection and surface tension results.