Ion Distribution and Polyion Conformation Displayed by Amphiphilic Polyacids in Aqueous and Organic Media

Abstract

The properties of aqueous and n-octanol solutions of amphiphilic polyelectrolytes — the polysoaps of maleic acid and cetyl (or decyl) vinyl ether — are described. Some important differences between the conformation and the counter-ion distribution in the two media are discussed. In water, the occurrence of intra-molecular hydrophobic interactions results in a compact conformation, which is however not that of a spherical mono-molecular micelle with no water incorporated. From conductivity and tracer self-diffusion measurements, it is shown that more than 20% of the alkali metal carboxylate groups are ionized. The polysoaps in n-octanol behave quite differently: although metal carboxylate groups can only be accommodated in organic media with a considerable free energy expense, an “inverted” compact intra-molecular micelle structure is not observed. It is assumed that a redistribution of metal carboxylate groups along the chain enables the partially neutralized form of the polymer to be solubilized in a low dielectric constant medium without undergoing a conformational collapse. The idea that carboxylate (R-COOA) and carboxylic (R-COOH) monomeric groups are not randomly mixed along the chain is inferred from the determination of the excess free energies of these groups and from conductivity measurements. A theoretical model in which the polymer is treated as an uncharged coil with all small metal ions “site bound” and non-homogeneously distributed, yields realistic pair contact energies and overall qualitative agreement with the experimental findings.