AMPHIPATHIC POLYELECTROLYTES AT THE N-OCTANE WATER INTERFACE

The surface adsorption at the n-octane/water interface of different polysoaps made from poly(maleic anhydride-co-styrene) has been studied. Interfacial tension measurements were carried out according to the Du Nouy method. The interfacial tension decreases more rapidly as the chain length of the side chain increases. The excess surface concentrations, GAMMA, were determined according to the Gibbs-Szyszkowski equation. The observed behavior of GAMMA with polymer concentration is entirely different from that found for the same system at the air/water interface. Results are interpreted in terms of the low cohesion of gaseous air compared with that of liquid n-octane. The standard free energy of adsorption, DELTAG(ads)degrees, was found to be more negative as the length of the side chain increased. A plot of DELTAG(ads)degrees versus the number of carbon atoms shows a linear behavior Which is consistent with a contribution of -610 calories per methylene group to DELTAG(ads)degrees This result is similar to that found for the air/water interface for the same polymer. In addition, an intercept of -590 calories was obtained for the contribution of the functional groups belonging to the main chain. This value is lower than that reported for the air/water interface, which suggests that these functional groups behave as desorbing agents. (C) 1994 Academic Press, Inc.