Dynamic light scattering study of polystyrene latex suspended in water at high temperatures and high pressures

The stability of monodispersive polystyrene spheres suspended in water has been studied at high pressure and temperature in the range of 0.1-28 MPa and 24-325 degreesC. The diffusion coefficient of the latex was measured with dynamic light scattering using a special experimental cell for performing measurements at elevated pressure and temperature. It is found out that the influence of the increased pressure on the measured results is negligibly small. The elevation of temperature significantly increases the particle diffusion coefficient due to the viscosity decrease of the medium, but its dependence is well described by the theoretical predictions for the diffusion coefficients of the particles with constant size calculated at 24-275 degreesC. This result means that the polystyrene spheres do not change in size and the suspension remains stable below 275 degreesC at least in the time scale of our experiments. The change in the size of latex was observed at 300 and 325 degreesC. The measured decrease of the diffusion coefficient with time could be related to the increase in the average particle size due to the beginning of the coagulation process, but some other effects as swelling of polystyrene might be also possible. (C) 2001 Elsevier Science B.V. All rights reserved.