Swelling and de-swelling kinetics of gelatin hydrogels in ethanol-water marginal solvent.

Controlled osmotic swelling and de-swelling measurements have been performed on gelatin, a polyampholyte, hydrogels suspended in water-ethanol marginal solvent at room temperature (20 degrees C) where the alcohol concentration was changed from 0 to 100% (v/v). The change in gel mass was monitored as function of time until osmotic equilibrium was established with the surrounding solvent. It was observed that osmotic pressure of polymer-solvent mixing, pi(m)<<pi(el) (pi(el) being the osmotic pressure due to network elasticity). The osmotic pressure arising from ionic contributions, pi(ion) was found to play a significant role in controlling volume phase transitions. For gelatin hydrogels, total swelling pressure of gel, pi(tot) could be related to gelatin volume fraction phi(2), relaxed volume of network V(0), and crosslink density v(e) as: pi(tot)/RT approximately -(v(e)/V(0))[phi(2)(1/3)-phi(2)/2]+(rho(0)exp(phi(2)/2)) independent of gel pH and swelling time (rho(0) is the reference gelatin charge density at phi(2)=0.01). The entire experimental data generated collapsed onto a single master plot. Asymmetric volume transition was observed about iso-electric pH. Results are compared with those obtained for strong bio-polyelectrolyte gels, and chemically crosslinked gelatin gels.