Swelling studies of gelatin. I: Gelatin without additives

The swelling rate and the equilibrium swelling of gelatin (type B) were studied by casting warm gelatin solutions into films, cutting them into short rectangular strips after gelation, drying them, and measuring the weight gain on immersion in buffer solutions as a function of time. The process variables investigated included concentration of the gelatin casting solutions, the thickness, drying conditions, age and residual moisture content of the film strips, the chemical nature and concentration of the buffers in the swelling solutions, and the temperature of these solutions at a constant pH of 7.0 (1.9 pH units above the isoionic point). The swelling kinetics followed a second-order equation. The initial swelling rate and the equilibrium swelling of the amorphous portion of the gelatin strips (which was somewhat smaller than the total observed swelling) were calculated from a linearized form of the rate equation. Of the factors investigated, the equilibrium swelling was increased most strongly when the temperature of the swelling solution was raised from 20 to 25 degrees C. Strip thickness was the predominant factor governing the rate of swelling, which was inversely proportional to the thickness. Conditions leading to slower drying and longer storage times promoted more extensive crystallization, thereby increasing the density of the gelatin strips and reducing their swelling rate.