The mystery of colloidal crystal formation — novel aspects obtained by ultra-small-angle X-ray scattering

We describe our recent ultra-small-angle X-ray (USAXS) data for the structure of colloidal crystals. Information on the three dimensional ‘bulk’ structure of colloidal crystals can be obtained using USAXS, and the interparticle distance estimated from the Bragg peak in the USAXS profile shows a maximum as a function of added salt concentration (i.e. ionic strength). Since all types of latex particles used in the experiments show a similar trend, this phenomenon, i.e. the existence of ‘a maximum’, can be said to be universal. This maximum position appears at κa=1.3 (κ−1: the Debye length, a: the particle radius). By theoretically analyzing the USAXS profiles, we have quantitatively estimated the distortion of the lattice structure of colloidal crystals. This analysis revealed that the maximum point corresponds to the ‘melting’ point of colloidal crystals. The dependence of interparticle distance on added salt at κa>1.3, where colloidal particles form liquid-like structures, can be explained by the classical theories, such as the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, but that at κa<1.3, where colloidal particles form solid-like crystals, can not be accounted for by current theories and concepts. We also found by USAXS that the colloidal crystal structure is affected largely by the counterion species: when the counterions are changed from H+ to Na+ without changing ionic strength, the interparticle distance and the distortion of crystals increase. These USAXS results, which appear to be quite mysterious but interesting, lead to a possible new mechanism underlying the colloidal crystal formation. Colloidal crystal formation might still have mysterious aspects but our USAXS data will certainly help solve this mystery and will open the door to a new world of colloid science.