TEMPERATURE-DEPENDENT FORCES BETWEEN HYDROPHOBIC SURFACES COATED WITH ETHYL(HYDROXYETHYL)CELLULOSE

The forces acting between hydrophobic surfaces coated with a layer of a nonionic polymer, ethyl-(hydroxyethyl)cellulose (EHEC), were studied inter alia as a function of surface separation and temperature. EHEC adsorbs strongly at hydrophobic surfaces, and the force measurements were performed with a constant adsorbed amount. The force vs distance curves were reversible; i.e., the same forces were measured on approach and separation. The surface interaction is very sensitive to temperature. At room temperature, the forces are purely repulsive. At 44 °C (5 °C above the cloud point), the force curve remains monotonically repulsive but less long-range. Hence, the EHEC layer contracts due to the poor solvency at this temperature. The fact that no attraction is observed despite that the temperature is above the cloud point is rationalized by considering that the most hydrophilic segments are oriented toward the bulk solution. Consequently, the local x-parameter for the stabilizing moieties is, for this chemically heterogeneous polymer, different from the average x-parameter. However, an elastic repulsion, due to the loss of conformational entropy, also contributes significantly to the repulsive force at this temperature. At an even higher temperature (55 °C), a strong, but rather short-range, attractive force was observed. The temperature dependence of the force curves was found to be completely reversible at the incubation times used (approximately 15 h). © 1990, American Chemical Society. All rights reserved.