pH difference in polyelectrolyte gels and their solutions

The pH inside polyacrylic acid gels is measured using a series of pH sensitive dyes. The intensities of the optical absorbance at two wavelengths, corresponding to the acid and base forms of the indicators, allow the internal pH to be determined independent of dye concentration. The technique was verified by measurements on polyacrylic acid linear polymer solutions where both direct pH electrode measurements as well as the spectral technique could be applied. In 0.15 M NaCl solutions gels and their solutions have similar pH values at 0, 20, and 40 mole% degree of neutralization of the acrylic acid groups. However, for higher degrees of ionization in the salt solution, and for all of the measurements in deionized water the pH values in the gel are below the values in the external polymer solution. The difference in pH is as large as 1 pH unit. The change in local H⁺ concentration in the vicinity of a negatively charged cylinder results in a spatially varying pH in the gel. Counterion condensation onto the polymer backbone also makes the average pH inside the gel network lower than the value assumed from the amount of added base to neutralize the acrylic acid groups on the gel. The large difference observed between the pH in gel and in its solution indicates that these measurements may prove a sensitive probe of internal gel structure.