TEMPERATURE-DEPENDENT ADSORPTION AND SURFACE FORCES IN AQUEOUS ETHYL(HYDROXYETHYL)CELLULOSE SOLUTIONS

Forces between hydrophobized mica surfaces across an aqueous solution of ethyl(hydroxyethyl)cellulose (EHEC) have been studied, inter alia, as a function of surface separation and temperature. EHEC adsorbs strongly to hydrophobic surfaces in a rather flat conformation. The force is reversible on approach and separation up to a temperature of about 40-degrees-C. As the temperature was increased from 20 to 37-degrees-C, which is slightly above the cloud point (CP = 35-degrees-C), the adsorbed amount increased by a factor of 3. The force remained monotonically repulsive, despite the fact that the temperature was higher than CP. When the temperature was increased further, the adsorbed amount, as well as the thickness of the adsorbed layer (at a high compressional force), remained essentially unchanged. However, due to the decreased solvency, there is a contraction of the outer part of the adsorbed layer. The force was essentially repulsive at 41-degrees-C, with only a very weak attraction observed on separating the surfaces. At 52-degrees-C, a weak attraction was observed on both approach and separation, as would be expected in a significantly worse than theta solvent. As the temperature was decreased again, the adsorbed amount as well as the compressed adsorbed layer thickness decreased. The adsorbed amount was the same (within experimental uncertainty) after heating as before. However, the force curve was shifted outward after heating as compared to the situation before heating. Hence, the temperature dependence of the adsorption, in the absence of constraints on the adsorbed amount, is not completely reversible, at least over the incubation time used (12 h).