Structural effects of polydispersity in charged colloidal dispersions

Abstract

Experimental information on the microscopic structure of charged colloidal dispersions is usually extracted from the intensity of scattered light l(k) which can be linked to the ‘measured structure factor’S^M(k). A reinterpretation of given experimental results for S^M(k) is given in terms of a size and charge polydisperse model and the systematic analysis of the effects of polydispersity on S^M(k). In our model, the interaction between macroions is assumed to be of a Yukawa type and the polydispersity is characterised by histograms with standard deviations from 10 to 40% with up to 10 components. The partial structure factors S_αβ(k) are evaluated by solving the multicomponent Ornstein–Zernike (OZ) equations in connection with the thermodynamically self-consistent closure of Rogers–Young (RY). The accuracy of the RY approximation is demonstrated by comparing the results with simulation data on monodisperse Yukawa systems. The results for S^M(k) for polydisperse systems show significant differences from results obtained by treating the systems as being monodisperse. For S^M(k) a large increase is found at small k as well as a shift in the main peak. These features are discussed in terms of the fluctuation and scattering abilities of each component of the dispersion. The role played by the charge and the size polydispersity is also analysed by introducing the generalised Bhatia–Thornton structure factors. Finally, S^M(k) is compared with scattering data. Quantitative agreement is found for all k values and, in particular, in the range of small k in which all one-component models are particularly inaccurate. The difference between S^M(0) for a polydisperse system and the isothermal osmotic compressibility is emphasised.