The adiabatic compressibility of polyelectrolytes: Sodium and hydrochloride salts of acrylic acid–N-dimethylaminoethyl methacrylate copolymer



The results of adiabatic compressibility measurement for the 100% neutralized sodium and hydrochloride salts of three copolymers of acrylic acid and N-dimethylaminoethyl methacrylate, namely, AA-DAM 58, AA-DAM 43, and AA-DAM 33, are discussed. Similar to three unneutralized amphoteric polyelectrolytes, the ϕK2 and ϕV2's for these salts are found to be concentration independent. Since in the former both anionic and cationic groups are present, fully neutralized sodium salts and hydrochloride salts of these polyelectrolytes are expected to show a decrease in ϕK2i0 and ϕV2i0 values due to maximum electrostriction. In fact, in case of AA-DAM 58 with excess amino group in the chain (58%), the experimentally obtained values are found to be −9.6 × 10−4 cc/bar/mole and 159.6 cc/mole, and −3.3 × 10−4 cc/bar/mole and 164.4 cc/mole for sodium and hydrochloride salts, respectively, which corresponds to a decrease of 7.1 × 10−4 cc/bar/mole and 4.9 cc/mole for the sodium salt and 0.8 × 10−4 cc/bar/mole and 0.1 cc/mole for the hydrochloride salt. In contrast, the hydrochloride salt of AA-DAM 43 shows an increase of 6.2 × 10−4 cc/bar/mole and 2.2 cc/mole, respectively, while in case of AA-DAM 33, both the hydrochloride salt (56.2 × 10−4 cc/bar/mole and 24.7 cc/mole) and the sodium salt (6.2 × 10−4 cc/bar/mole and 6.2 cc/mole) show an increase. This increase has been ascribed to suppression of dissociation of carboxyl groups by the hydrochloric acid (common ion effect) added for neutralization. However, in all the three amphoteric copolymers, when neutralized with NaOH solution or HCI acid, the viscosity increases over that of the unneutralized copolymer.