Swelling ratio driven changes of probe concentration in pH- and ionic strength-sensitive poly(acrylic acid) hydrogels

Poly(acrylic acid) gels are known to swell to different extent (measured by the swelling ratio) depending on pH and ionic strength of the solution. The experimental method employed by us to monitor the concentration and diffusion coefficient of the probes allows the simultaneous determination of their concentration and diffusion coefficient for any value of the ionic strength and pH of the solution trapped in the polymeric network. Two different electroactive probe systems, 1,1′-ferrocenedimethanol and 2,2,6,6-tetramethylpiperidine 1-oxyl, were selected for the examination. Small values of the swelling ratio, obtained either by application of a high ionic strength or a low pH, result in an unexpected high concentration of both probes in the gel samples. Correspondingly, the probes diffusion coefficients were much smaller compared to their values in the swollen gels.