Behavior of colloids in supercritical water: an attempt to study diffusion coefficients using dynamic light scattering

This paper deals with studying the diffusion coefficients of polystyrene and colloidal gold particles dispersed in water above its critical point (374.0 degreesC and 22.064 MPa) using dynamic light scattering (DLS). The possible contribution from pure water to the total intensity of the scattered light and to the intensity autocorrelation function (ACF) measured by DLS has been examined at the conditions of particle size measurements. The results show that in the case of colloidal dispersions the ACF reflects mainly the particle diffusion even at thermodynamic states characterized by long-range density fluctuations of the pure solvent because the dynamics of the latter is much slower. In addition the diffusion coefficient of the samples has been studied as a function of temperature and/or pressure in the range 24-374 degreesC. The experimental results collected so far are promising for applying DLS to probing colloidal behavior in hot water under high pressure including supercritical area.