A. Moncho-Jordá and M. Quesada-Pérez
We use a generalized nonlinear Poisson-Boltzmann cell model that includes excluded-volume effects to investigate whether the effective charge (Zeff) of charged thermosensitive hydrogel particles increases or decreases upon the hydrogel thermal collapse. We find the existence of a crossover charge, Z∗, that separates two regimes. For hydrogel bare charges below Z∗ the system shows a behavior consistent with theories based on linear approximations, i.e., Zeff increases in the collapsed state. However, for bare charges above Z∗, the system enters an anomalous regime, in which Zeff decreases in the collapsed state. We show that diluted hydrogel suspensions at low ionic strength are more likely to follow the anomalous behavior. Our theory provides a full physical justification for the controversial theoretical and experimental results reported in this regard, and describes how the interplay between electrostatic, excluded-volume and entropic effects affects this crossover.