The results of adiabatic compressibility measurements for two amphoteric polyelectrolytes, acrylic acid–4-vinyl-N-n-butylpyridinium bromide copolymer and methacrylic acid–4-vinyl-N-n-butylpyridinium bromide copolymer, and their completely neutralized potassium salts have been described. The two amphoteric polyelectrolytes are strongly acidic, and the strength of the acidic group has been increased by the presence of the neighboring basic group. The reduced viscosity for these polyampholytes is extremely low (∼0.03–0.07 dl/g) which may be on account of the mutual interaction of acid and base groups making the polyelectrolyte chain highly coiled and compact. The apparent molal volume ΦV2 and apparent molal compressibility ΦK2 are concentration independent. Their potassium salts showed the lowest ΦV2 and ΦK2 values as the electrostriction is highest here; these values showed a slight increase in the presence of dilute KBr solution as the dissociation is somewhat reduced. However, in excess KBr solution, the acrylic acid-containing copolymer having comparatively more carboxyl groups (21.7%) behaves more or less similar to poly(acrylic acid), whereas the methacrylic acid-containing copolymer, with only 15.1% carboxyl groups, behaves closely to poly(4-vinyl-N-n-butylpyridinium bromide). These polyelectrolytes showed absorption in the 2700 to 3800 Å region in the presence of chloride, bromide, and iodide ions which are due to charge transfer complex formation between the halide ions and the polymer nucleus. The magnitude of extinction coefficient for these two polyampholytes is much less than that of poly(4-vinyl-N-n-butylpyridinium bromide). The limiting values of apparent molal volume, ΦV20, for acrylic acid- and methacrylic acid-containing copolymers are 183.9 and 184.6 cc/mole, respectively. These experimentally determined values fall short of 9.2 and 6.2 cc/mole compared to the calculated values (if there was no interaction between acid and base groups) which are definitely due to mutual interaction of acid and base groups in the polymer chain.
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