Nature and effects of nonexponential correlation functions in probe diffusion experiments by quasielastic light scattering

Abstract

The shape of intensity autocorrelation functions measured by dynamic light scattering in a ternary probe/matrix/solvent system is considered in detail. The probe is 907 Å polystyrene latex, and the matrix is a high molecular weight hydroxypropylcellulose. Experiments were performed with and without adding surfactant to prevent surface interactions between the probe and matrix which lead to probe clustering. In either case, the correlation functions were dramatically more nonexponential than those we reported earlier for lower molecular weight hydroxypropylcellulose ( J. Colloid Interface Sci. 122, 120 (1988)). Two independent Laplace inversion algorithms and a discrete multiple exponential analysis find the correlation functions to be bimodal. The slower mode is definitely diffusive, and is closely associated with the self-diffusivity of the latex probe. It is not easy to identify the physical significance of the weaker, faster mode. The nonexponentiality is sufficient that, if it were not accounted for properly, serious misinterpretations of the data could result. For example, in the absence of surfactant, analysis by simple cumulants methods leads one to the conclusion that the Stokes-Einstein law fails dramatically. After detailed consideration of the nonexponentiality, the failure is much less, though still significant relative to experimental uncertainty.