Self-diffusion of water in gelatin gels: 2. Quasi-elastic neutron scattering data

The self-diffusion of water in gelatin gels of different concentrations was studied by quasi-elastic neutron scattering (QENS). The total self-diffusion coefficients and their components corresponding to the contributions from collective (Lagrange type) and single-particle (Frenckel type) mechanisms of molecular motions were determined. From the data obtained one can conclude that the self-diffusion of free water proceeds by a single-particle mechanism. The growth of a bound water fraction in the system with gelatin leads to the gradual pumping of single-particle diffuse modes into collective modes. Purely collective diffuse modes accompanied by complete retardation of macroscopic water diffusion are observed at relatively low gelatin volume fractions ( 0.4) compared to the expected volume fractions ( 0.7) at which water binding to polymer by hydrogen bonds would lead to the same effect. This is associated with the fact that gels show the microspace effect favourable for the existence of bound water.