Ultra-small-angle neutron scattering study of colloidal alloys.: 1.: Contrast variation experiments for mixtures of hydrogenated and deuterated polystyrene latices in H2O/D2O

Ultra-small-angle neutron scattering (USANS) experiments on one- and two-component aqueous colloidal dispersions were carried out at the Bonse-Hart double crystal diffractometers installed at the High Flux Isotope Reactor (Oak Ridge, TN) and at the JRR-3M reactor (Tokai, Japan). The USANS curves from low-concentration suspensions of hydrogenated poly(methyl methacrylate) (h-PMMA) latices in D2O show scattering patterns typical for isolated solid spheres, which change dramatically after adding the ion-exchange resin to remove ionic impurities. This additive significantly enhances electrostatic interaction between particles, and thus the diffraction pattern from 2.5% h-PMMA latices in D2O + ion-exchange resin corresponds to a colloidal crystal with the fee structure. The USANS from hydrogenated and deuterated component of the colloidal alloy of h- and d-polystyrene (h-PS and d-PS) latices was measured separately by matching the scattering length density of h-PS and d-PS particles in H2O/D2O mixture. It was found that the minority component of large diameter latices were randomly distributed in the alloy dispersion, whereas the majority component of small diameter particles formed colloidal crystals.