Observation of a cubic phase in mesoporous silicate films grown at the air/water interface

X-Ray diffraction is reported from mesoporous silicate films grown at the a ir/water interface. The films were studied both as powdered films, and orie nted on silicon or mica sheets. At early stages of growth we observe Bragg diffraction from a highly ordered cubic phase, with both long and short d-s pacing peaks. We have assigned this as a discontinuous micellar Pm3n phase in which the silica is partly ordered. Later films retain only the known he xagonal p6m peaks and have lost any order both at short d-spacings and the longer d-spacing Bragg peaks characteristic of the cubic structure. The sil ica framework is considerably expanded from that in bulk amorphous silica, average Si Si distances are some 30% greater. Incorporation of glycerol or polyethylene glycol preserves the earlier cubic structure. To be consistent with earlier, in situ, X-ray and neutron reflectivity data we infer that b oth structures are produced after a phase transition from a less-ordered hi m structure late in the induction phase. The structural relations between t he film Pm3n and p6m phase(s) and the known bulk SBA-1 and MCM-41 phases ar e briefly discussed.