Probing static structure of colloid-polymer suspensions with multiply scattered light

Time-dependent measurements of light propagation were conducted in aqueous dispersions of 523 nm diameter polystyrene at concentrations between 0.1 an d 0.4 solids volume fraction in order to assess how particle correlation is influenced by depletion interactions arising from the addition of soluble polyethyleneoxide (PEO). In the absence of polymer, the transport scatterin g length can be predicted from Mie scattering theory and the Percus-Yevick (P-Y) model: for static structure of a dense hard-sphere colloidal solution . Depletion forces arising from the addition of PEO of varying molecular we ights influenced the spatial ordering of the dispersion and caused a furthe r increase in the transport scattering length beyond that predicted by hard -sphere static structure factor but similar to that predicted by the mean s phere approximation. (MSA) to the P-Y model described by Ye et al. (1996). Onset of flocculation occurred with increased PEO addition and correlated w ith PEO molecular weight. Phase separation was noted by no further change i n the transport scattering length, except when flocculation was Induced by the highest molecular weight PEG. The use of time-dependent measurements of light propagation in dense systems provides an alternative to small-angle light, neutron, and X-ray scattering characterization of interaction potent ials in dense, multiply scattering samples and promises further fruitful in vestigation of colloidal particle interactions in suspensions. (C) 1999 Aca demic Press.