Polyampholytic Hydrogel Swelling Transitions: Limitations of the Debye−Hückel Law

Abstract

This study has examined polyampholytic hydrogel swelling transitions over a wide range of bath salt concentrations. Copolymer 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS−H) and [3-(methacryloylamino)propyl] trimethylammonium chloride (MAPTA−C1) hydrogels were prepared with a total monomer concentration of 1.4 M near the charge balance point. Hydrogel equilibrium swelling measurements were made in aqueous baths having sodium chloride concentrations ranging from 1 μM to 1 M. The observed swelling transitions have been interpreted using a series of thermodynamic models based on successively higher order Mayer ionic solution theory corrections. Collapse transitions at low and intermediate bath salt concentrations observed experimentally were successfully predicted using only a zero order Mayer, or Donnan model, approximation. First order Mayer corrections, equivalent to the Debye−Hückel limiting law (DHLL), failed to produce physically meaningful results at the polymer ion and bath salt concentrations of interest in this study. However, a second order Mayer correction, equivalent to the Debye−Hückel limiting law + second virial coefficient (DHLL + B2), was in qualitative agreement with our experimental results. Further improvements based on the straightforward extension to higher order Mayer approximations are discussed.