Solution behavior of quenched or strongly charged polyampholytes in aqueous-salt solutions

Abstract

Quenched (or strongly charged) polyampholytes based on fully charged anionic monomer — sodium salt of 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) and cationic monomer — (3-acrylamidopropyl)-trimethylammonium chloride (APTAC) were synthesized by radical copolymerization and their behavior was studied in aqueous-salt solutions. A series of unbalanced (AMPS-25 and AMPS-75) and balanced (AMPS-50) quenched polyampholytes were characterized by 1H NMR, FTIR, GPC, DLS, viscosity, DSC, TGA. It was established that at the isoelectric point (IEP) the quenched polyampholytes in aqueous solutions are stabilized by cooperative intraionic contacts between strong charged anionic and cationic moieties. The conformational state of AMPS-25 and AMPS-75 in aqueous solution is represented as «core» and «shell» structure. The «core» part behaves polyampholyte character (PA region) while the «shell» part exhibits polyelectrolyte effect (PE region). Increasing of the ionic strength tends to shrink the polyelectrolyte «shell» and to swell the polyampholyte «core». In contrast to AMPS-25 and AMPS-75, the hydrodynamic size of balanced polyampholyte AMPS-50 at the IEP increases upon increasing of the ionic strength demonstrating the antipolyelectrolyte behavior. In aqueous solution the isoelectric points of quenched polyampholytes were found to be pH 6.10.1. In the presence of KCl the positions of the IEP shifted to pH 6.5–7.0 due to specific binding of chloride ions by quaternary ammonium groups of «quenched» polyampholytes.