Swelling and contraction driven mass transfer processes during osmotic dehydration of uncharged hydrogels

The processes of swelling or contraction act as driving forces and thus play important roles in mass transfer during osmotic dehydration of a hydrophilic gel, such as polyacrylamide (PAAm). These driving forces seem to be determined by the interaction between the low molecular solute and the gel polymeric matrix. In a solution of sugars or sodium chloride, the gel undergoes an initial rapid mass loss until it reaches a turning point where the mass transfer process is reversed and the gel swells to a significantly larger extent than in water. However, in solutions of relatively high concentration of sodium citrate or sodium acetate, the gel undergoes only a contraction process and is incapable of regaining any of its lost mass. Empirically, the swelling process follows first order kinetics, which seems to become slower the larger the solution viscosity.