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A new method of preparing thermoresponsive hydrogels consisting of N-isopropylacrylamide (NIPA) and acrylic acid (AA), with a well-defined concentration of an electroactive probe, 1,1'-ferrocenedimethanol (Fc(MeOH)2), is described, and a comparison of the physical and electrochemical properties of NIPA-AA gels with those of aqueous solutions is presented. The NIPA-AA gels undergo a discontinuous volume phase transition at 45 degrees C; this transition results in a release of approximately 40% of the solution mass from the gel phase. Characterization of hydrogels with electroactive probes is performed using electroanalytical techniques and FTIR and UV/vis spectroscopies. Steady-state voltammetry and chronoamperometry at platinum disk microelectrodes are used to measure the diffusion coefficient of Fc(MeOH)2 in gels under a wide range of experimental conditions. Similar diffusion coefficients for Fc(MeOH)2 in NIPA-AA gels are obtained by either electroanalytical technique at temperatures lower than 20 degrees C. The uncertainty in the Fc(MeOH)2 concentration in the gels, resulting from the discontinuous volume change transition, necessitated the use of concentration-independent chronoamperometric data (i.e. the chronoamperometric response divided by the steady-state current obtained at sufficiently long times) to obtain reliable diffusion coefficient values for Fc(MeOH)2. For temperatures above the volume phase transition, changes of concentration of Fc(MeOH)2 are detected in a copolymeric collapsed phase.

Weak acids such as acetic, ascorbic, and salicylic acids are easily reduced at platinum microelectrodes in the absence of the supporting electrolyte. The current is mass transport controlled, and the reduction of proton is preceded by dissociation of the acid in the reaction layer. Since these acids are only slightly dissociated, transport should not be enhanced by migration and the heights of their voltammetric waves should be nearly independent of supporting electrolyte concentration. However, transport-limited currents diminish by 50% when a small amount of a supporting electrolyte is added to the solution. Thus the wave height in the absence of electrolyte exceeds that with electrolyte present by a factor of 2, as expected for oneelectron reduction of a singly charged reactant. This change in the wave height is connected with the small amount of hydrogen ion in the solution arising from the dissociation of the acid. Since the equilibrium concentration of hydrogen ion, which is the only cation available in the solution, is much lower than that of undissociated acid, only a small ratio of the concentration of the supporting electrolyte to that of the acid is needed to eliminate this effect. Dihydrogen phosphoric acid, which is negatively charged, behaves differently. Taking into account this outcome and the limited dissociation of weak acids, the diffusion coefficients of acetic, ascorbic, salicylic, and dhydrogen phosphoric acids were determined at 20 OC. They are 0.97 X 5.6 X 10-6, 7.7 X loa, and 6.4 X loa cm2 s-1, respectively.It has beendemonstrated recently that the heights of steadystate voltammetric waves of strong and weak acids at platinum microelectrodes depend linearly on the total or formal acid concentration over a very wide range of concentration.' Apparently this dependence can be used for analytical purposes. Examples in which measuring the voltammetric waves could be competitive with use of the glass electrode include measurements in media containing charged colloidal particles (where standard pH electrodes cannot be used2 ), probing proton concentration in a small region (e.g., near a large electrode), or any situation where one cannot wait for the potential of a pH electrode to stabilize. Reduction of hydrogen ion also has the potential to be an excellent test system for electrochemical examination of microelectrodes, due to the large and well-known diffusion coefficient value of the hydrogen ion. Perchloric acid is superior as a test system since both hydrogen ion and perchlorate anion are easily rinsed from most surfaces, and carry-over of perchlorate would not interfere in most measurements.The height of the wave for reduction of hydrogen ion in solutions of strong acid was found to depend on the concentration of the supporting electrolyte' as predicted by the theory for transport assisted by m i g r a t i~n .~-~ For uncharged substrates the theory predicts no dependence on the electrolyte concentration. For weak acids of the type HA, which are only slightly dissociated, it could be expect...

Voltammetric reduction of perchloric, phosphork, acetk, and ascorbic acid was investigated under steady-state comlitknr at platinum-and goiddlrk microelectrodes. The ckp.nd.nco of the wave height of proton in perchloric acid on the concentratlon of supporting electrolyte (lithium perchlorate) was compared with the theory for current8 1lmIt.d by migration and dMurlon. The wave height depend8 linearly on hydrog.n ion concentration wlthout and with wpportlng electrolyte up to 0.04 and 0.08 M, respectively. A b , a w t k and ascorbk acids exhibit linear caiibratlon plots with current proportional to concentration of undissociated acid up to 0.1 M. Thk k as would be predlcted for reduction of proton wbwquonl to fad dkrociation of the acid. The fairly wdl-ddhd lknnlng current for dihydrogen phosphate indlcates that weak adds with pK, In excess of 7 are accomible to voltammotrlc investigation.

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Steady-state voltammetry and chronoamperometry at microelectrodes were used to study mass transport properties of temperature sensitive poly(N-isopropylacrylamide-co-acrylic acid), NIPA-AA, hydrogels. 1,1‘-Ferrocenedimethylanol, Fc(MeOH)2, and 2,2,6,6-tetramethyl-1-piperidinyloxy, TEMPO, were used as electroactive probe molecules. The activation energy of diffusion of Fc(MeOH)2 in aqueous solutions and in NIPA-AA hydrogels was found to be in the range of 17−19 kJ/mol, which suggests that the local microscopic viscosity does not change significantly because of the gelation process, although the macroscopic viscosity of the gels is extremely large. It was found that the diffusion coefficients of Fc(MeOH)2 and TEMPO in NIPA-AA hydrogels in their swollen state are approximately 20%−50% smaller than those in aqueous solutions, and that the diffusion coefficient of probe molecules in these gels is inversely proportional to the concentration of copolymer in the hydrogels. The “obstruction effect” and “hydration effect” were used to explain this phenomenon, and experimental results were compared with predictions of the model.

The transport of uncharged electroactive probes, 1,1‘-ferrocenedimethanol and 4-hydroxy-TEMPO, and electroactive cations, Tl+, was studied in polyacrylate hydrogels using steady-state voltammetry at platinum and mercury film microelectrodes. It was found that, for concentrations of polymer less than 1.5%, the diffusion coefficient of uncharged probes does not differ significantly from that observed in aqueous solutions without a polymeric network. For the probe cation, strong electrostatic interactions were observed between Tl+ and anionic polymeric networks; those interactions resulted in a significant decrease in the diffusivity of Tl+ cations. Experimental data for Tl+ transport in sodium polyacrylate gels were compared with predictions of Manning's theory for polyelectrolyte solutions. Electrostatic interactions between Tl+ and anionic polyacrylate three-dimensional gel network were found stronger than those predicted for solutions of an equivalent polyelectrolyte. Electrostatic effects for gels were found even stronger when Tl+ cations served as counterions in thallium polyacrylate gels; the transport of Tl+ counterion was more suppressed in those gels than for a Tl+ probe in sodium polyacrylate, especially for small values of charge separation distance in polymeric units. The mobility of a counterion forming a gel, Na+, was also studied using conductance measurements, and appropriate expressions for conductivity as a function of poly(acrylic acid) neutralization degree were developed based on Manning's line charge model. Experimental conductivity data for Na+ agreed with predictions of the model.

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