Adiabatic compressibility of polyelectrolytes: The influence of solvents on ionic and nonionic polymers

The adiabatic compressibility of two ionic polymers; namely, poly(acrylic acid) (PAA) and poly(N-dimethylaminoethyl methacrylate) (PDAM) in methanol and dioxane solutions and of a nonionic polymer, poly(vinyl pyrrolidone) (PVP) in aqueous, methanol, and dioxane solutions has been studied. The φV2 and φK2 values for the three polymers and their corresponding monomers in methanol and dioxane solutions are found to be concentration independent. There is a marked difference in φK02 and φV02 values between monomer and polymer in all three solvents. In aqueous solution, the difference in φV02 is, on an average, 16.1 cm3/mol, while in methanol and dioxane solution, the same is 24.0 and 20.5 cm3/mol (average), respectively. All three monomers in dilute aqueous solution show a contraction of volume and decrease of adiabatic compressibility which are comparatively small in methanol and dioxane solutions. The φV02 for PAA, PDAM, and PVP were found to have increased by 0.8, 11.0, and 1.5 cm3/mol, respectively, in dioxane solution over that of the value of the aqueous solution. It is interesting to note that in methanol solution, PAA, PDAM, and PVP show a decrease of φK02 and φV02 values by 68.8 cm3/bar/mol and 8.2 cm3/bar/mol, 32.7 cm3/bar/mol, and 4.3 cm3/mol, and 36.6 cm3/bar/mol and 5.8 cm3/mol, respectively, compared to the values obtained from aqueous solution. This has been ascribed to geometric effect since the void space around the molecules is smaller in methanol than in water.