Permeability of responsive poly (N-isopropylacrylamide) gel to solutes

Abstract

The diffusion (D) and partition (K) coefficients of dissolved solutes within a thermally responsive hydrogel, poly(N-isopropylacrylamide) (NIPAAm), were determined by independent measurement to give the permeability, defined as the product of D and K. For acetaminophen, the Yasuda et al. free volume theory successfully predicted the decrease in D as the swelling of the gel decreased with increasing temperature. In the collapsed gel, however, this theory overestimated D by 35 times. Ideal size exclusion theory failed to correlate observed partition coefficients (ratio of solute concentration in the gel to that in solution) for dilute solutions of acetaminophen, norethindrone and methyl orange. Examination of K as a function of solute character and the presence of structure breaking and structure forming salts demonstrates that hydrophobic interactions are often dominant in these systems. In the presence of ammonium sulfate, partition coefficients of nearly 200 were determined for methyl orange in collapsed gels, despite their relatively low water content (40%). As a result, permeability does not necessarily decrease with a decline in gel swelling or drop sharply upon gel collapse.