Buffer Effects on Swelling Kinetics in Polybasic Gels

The swelling kinetics of polybasic gels consisting of copolymers of methyl methacrylate and dimethylaminoethyl methacrylate are studied in solutions at various acidic pH values, with monoacidic derivatives of acetic acid added as buffers. The effects of solution pH, as well as buffer p K_a and concentration, on swelling rate are assessed. Gel swelling rate shows a nearly linear dependence on the concentration of nonionized buffer in the solution, as determined by the Henderson-Hasselbach equation. This result is explained in terms of the increased availability of protons that are carried by the nonionized buffer to bare amines on the gel. In fact, the so-called pH sensitivity of these gels, under these conditions, can be attributed mainly to the effect of pH on the nonionized buffer concentration. A practical consequence is that these gels may not reliably mediate pH-sensitive swelling-controlled release in oral applications, since the levels of buffer acids in the stomach (where swelling and release are expected to occur) generally cannot be controlled. However, the gels may be useful as mediators of pH-triggered release when precise rate control is of secondary importance.