The adiabatic compressibility of poly(acrylic acid) and polyacrylamide in aqueous solution

The results of adiabatic compressibility measurements of poly(acrylic acid) and polyacrylamide along with their corresponding monomers and two poly(sodium acrylates) obtained by neutralizing the polyacid 25% and 100% with sodium hydroxide have been described. The total adiabatic compressibility of poly(acrylic acid) solution is higher than that of the corresponding salt solutions or of polyacrylamide solutions. The unneutralized acid does not dissociate much, even in dilute solution, and the magnitude of electrostriction in polyamide is greater than in acid. The Φ V 2 and Φ K 2 values for monomers and polymers are seen to be almost concentration independent, and so are the sodium salts of the polyacid. Poly(acrylic acid) and poly(acrylamide) are structurally closely related polymers, and water must be bound to them through polar groups either by hydrogen bonding or by dipole attraction. The hydrophobic part of the solute, because of compact orientation of water and solute in the boundary region, causes a decrease in solvent volume and therefore in the values of Φ V 2 and Φ K 2 . On the other hand, intermolecular hydrogen bonding between the polar groups increases the volume and counterbalances the hydrophobic effect. Because of these two counteracting effects, the observed Φ V 2 and Φ K 2 values are seen to be concentration independent. Contrary to the observation with poly(methacrylic acid) 1 and its sodium salts, the solvated counter‐ions in case of poly(sodium acrylates) make no special contribution in the dilute region. In 100% neutralized polyacid, the dissociation of counterions is complete, and the magnitude of electrostriction is highest in this case. Accordingly, lowest Φ V 2 and Φ K 2 values (37.0 cc/mole and −50.50 × 10 −3 cc bar −1 mole −1 ) are observed. However, the dissociation and therefore the magnitude of electrostriction are somewhat reduced in the presence of 1.0 M NaCl solution; and, accordingly, the values increase to 42.80 cc/mole and −33.0 × 10 −4 cc bar −1 , mole −1 , respectively. The limiting values for the apparent molal volume and the apparent molal compressibility for the polymers show a considerable decrease over those of the monomers. The values of Φ V 2 0 and Φ K 2 0 per methyl group are less in the polymers than in the monomers, and this has been attributed to water clusters that become stronger and better formed as the molecules grow larger and larger. The molar volumes of acrylic acid and methacrylic acid are decreased, while those of acrylamide and methacrylamide are increased when dissolved in water to form an infinitely dilute solution.