PARTIAL STRUCTURE FACTORS IN COLLOIDAL SILICA MIXTURES DETERMINED WITH SMALL-ANGLE NEUTRON-SCATTERING CONTRAST VARIATION

Small-angle neutron scattering experiments at various contrasts were performed on concentrated mixtures of colloidal spheres differing in size. Both colloidal components consisted of fairly monodisperse silica cores coated with a layer of octadecyl chains. Cyclohexane was used as dispersing medium; variation of the contrast was achieved by using mixtures of H-1-cyclohexane and H-2-cyclohexane. Scattered intensities were measured at three volume fractions up to 0.4, at equal partial volume fractions. The different contrast dependence of the scattering amplitudes of both colloids allowed us to calculate partial structure factors. This was done using a method which has not been reported previously. Describing the intraparticle structures with layered-sphere models, and using a decoupling approximation, three partials were obtained from a system of linear equations. The scattering curves at the various contrasts constitute a consistent data set, at all volume fractions. Although the separate components interact like hard spheres, the partial structure factors in the mixture reflect marked deviations from hard-sphere behavior. Their nature can be qualitatively explained with an attractive interaction between unequal particles. This is confirmed with simple model calculations.