The significant information for colloidal crystal formation revealed by ultra-small-angle synchrotron X-ray and neutron scattering

Colloidal crystals are formed in dispersions at very low ionic strengths. Various techniques have been used to study the mechanism of colloidal crystal formation, such as the ultra-small-angle technique, whose usefulness has been demonstrated previously. Recently, the use of ultra-small-angle X-ray scattering (USAXS) showed that the nearest interparticle distance in colloidal crystals has a maximum as a function of added salt concentration. This finding could not be explained by classical colloidal theories and was not consistent with the observations based on the microscopic technique. In the present study, we applied the USAXS technique using synchrotron radiation, which provides monochromatic X-rays with a high flux density, to investigate the effect of gravity on colloidal crystal formation. The high flux density of the synchrotron radiation enables one to investigate the structure of colloidal crystals in polystyrene (PS) latex dispersions with a mixture of H2O and D2O as a medium designed to match the density of the PS latex particle. The synchrotron USAXS study also revealed that the effect of gravity on the colloidal crystal formation is negligible for poly(methylmethacrylate) (PMMA) latex dispersions, in which we observed the novel behavior of the interparticle distance as a function of added salt concentration. We also found the destruction of colloidal crystals due to the continuous irradiation of synchrotron radiation. In addition, we applied ultra-small-angle neutron scattering (USANS) to study colloidal alloy structures by incorporating the contrast matching method. The USANS study confirmed that the structure of colloidal crystals in a mixture of two latex dispersions changed drastically with the physicochemical condition of the mixture.