Transport of Ions and Electrostatic Interactions in Thermoresponsive Poly(N-Isopropylacrylamide-co-acrylic acid) Hydrogels: Electroanalytical Studies

ChengSong Ma, Weimin Zhang, and Malgorzata Ciszkowska*

Department of Chemistry, Brooklyn College, City University of New York, Brooklyn, New York 11210-2889

Abstract:

Poly(N-isopropylacrylacrylamide-co-acrylic acid) hydrogels, NIPA-AA, undergo discontinuous reversible volume phase transitions as a response to temperature increase. Those hydrogels are weakly negatively charged and, therefore, are expected to interact electrostatically with charged species dissolved in those systems. To study transport phenomena and electrostatic interactions in NIPA-AA hydrogels, electroanalytical experiments with two positively charged probes, (ferrocenylmethyl)trimethylammonium, FcTMA⁺, and hexaammineruthenium(III), Ru(NH₃)₆³⁺, cations were performed, and the results compared with those for an uncharged electroactive probe, 1,1'-ferrocenedimethanol, Fc(MeOH)₂. Steady-state voltammetry and chronoamperometry at platinum disk microelectrodes were used to determine diffusion coefficients of those probes. For temperatures below the volume phase transition of a gel, there are not significant differences in the transport behavior of cationic and uncharged probes. After the volume phase transition occurs and the gel collapses, the diffusion coefficients of all probes decrease, the change in diffusion coefficient is more pronounced for cationic probes than for a neutral probe and depends on the charge of the cationic probe. In addition to changes of transport of cationic species in collapsed NIPA-AA hydrogels, changes in their concentration were detected as a result of the volume phase transition.