Conformations of gelatin in trivalent chromium salt solutions: Viscosity and dynamic light scattering study

Abstract

An investigation of the influences of pH, salt type, and salt concentration on the conformations of gelatin molecules in trivalent chromium salt solutions was performed by viscosity and dynamic light scattering (DLS) techniques. It was found that the viscosity behaviors as polyelectrolytes or polyampholytes depended on the charge distribution on the gelatin chains, which can be tuned by the value of pH of the gelatin solution. The intrinsic viscosity of gelatin in basic chromium sulfate aqueous solution at pH = 2.0 first decreased and then increased with increasing Cr(OH)SO4 concentration, while a monotonic decrease of the intrinsic viscosity of gelatin was observed in CrCl3 solution. However, the intrinsic viscosity of gelatin at pH = 5.0 was found to be increased first and then decreased with an increase in salt concentration in Cr(OH)SO4 solution, as well as in CrCl3 solution. We suggested that the observed viscosity behavior of gelatin in trivalent chromium salt solutions was attributed to the comprehensive effects of shielding, overcharging, and crosslinking (complexation) caused by the introduction of the different counterions. In addition, the average hydrodynamic radius (R h ) of gelatin molecules in various salt solutions was determined by DLS. It was found that the change trend of R h with salt concentration was the same as the change of intrinsic viscosity. Based on the results of the viscosity and DLS, a possible mechanism for the conformational transition of gelatin chains with external conditions including pH, salt concentration, and salt type is proposed.