Physicochemical Response of Gelatin in a Coulombic Soup of Monovalent Salt: A Molecular Simulation and Experimental Study.

The effect of salt on the static properties of aqueous solution of gelatin is studied by molecular dynamics simulation at pH = 1.2, 7, and 10. At the isoelectric point (pH = 7), a monotonic increase in size of the polymer is obtained with the addition of sodium chloride ions. In the positive polyelectrolyte regime (pH = 1.2), collapse of gelatin is observed with increase in salt concentration. In the negative polyelectrolyte regime, we observe an interesting collapse-reexpansion behavior. This is due to the screening of repulsion between the excess charges followed by the screening of attraction of oppositely charged ions as the salt concentration is increased. This mechanism is very different from the charge inversion mechanism which causes the reexpansion in the presence of multivalent ions. The location of salt concentration corresponding to the minimum size of the chain is comparable to the theoretical estimate. The shift in the peak of radial distribution function calculated between monomers and salt ions confirms this spatial reorganization. The predictions from the simulation are verified by dynamic light scattering(DLS) and small-angle X-ray scattering (SAXS) experiments. The size of the hydrodynamic "clusters" obtained from DLS confirms the simulation predictions. Persistence length of the gelatin is calculated from SAXS to get single chain statistics, which also agrees well with the simulation results.