Neutron scattering of colloidal particle dispersions; contrast variation with homogeneous and granular solvents

The well known method of contrast variation is based on the assumption that the low-molecular solvent may be considered as a homogeneous background. The colloidal dispersion is described as a quasi- one component system. In this paper this assumption is analyzed in detail. The system is described as a two-component one and use has been made of our recently derived new equation for the scattering of mixtures. It is found that when solvents are condensed liquids, this assumption is accurate when the colloid particles are not too small, say larger than 3 nm. In this limiting case the partial structure factors colloid–solvent, S12, and solvent–solvent, S22, are dependent on the partial structure factor of the colloid, S11. When the solvent compressibility is not negligible, or when extra components—such as depletion agents are present, deviations occur. This last situation is considered in detail for a special type of depletion agent of which the size may be comparable to that of the colloid particles. It is found that the usual contrast variation procedure cannot be applied unless the depletion agent is fully matched with the low-molecular solvent. When this is not the case the procedure must be adapted with the bonus that the (often negative) adsorption of the depletion agent by the colloid can be obtained. Two examples from the literature are given of light scattering from mixtures of colloids and polymers, where both components contribute to the scattering. This case, however, is in fact limited to low K. For higher K, SANS or SAXS experiments are needed. As a special feature it is revealed how the thermodynamics-of-mixing properties obtained from scattering can be exactly expressed in the total- and the osmotic compressibilities.