Expanded mesoporous silicate films grown at the air-water interface by addition of hydrocarbons

We have examined the effect of addition of saturated straight-chain alkanes, C_n (n even and n = 8 8-16), and the aromatics toluene and benzene on the formation of mesoporous silicate films, which form at the air-water interface. The silicate films self-assemble by slowly condensing silicate oligomers attaching to templates formed by the alkyltrimethylammonium surfactants C_nTAX (n even and n = 12 - 18, and X = Cl^- and Br^-) in solution. The collection of X-ray and neutron reflectivity profiles from these solutions shows that there is a chain length dependency to the formation of alkane-swollen hexagonal silicate films, which is manifest as a linear increase of the pore sizes of the films with increasing alkane chain length for C_n, n ≥ 8, up to a limit of a hexagonal cell dimension of a₁₀₀ = 80.8 Å. The expansion is greatest when using decane as expander for all surfactant chain lengths examined. For C₁₂ chain length surfactants a cubic phase results, C₁₄ and C₁₆ chain lengths form expanded hexagonal films, while C₁₈ chain lengths are relatively unaffected by alkanes. There is also anion dependency, which is revealed in larger cell expansion over a wider range of alkanes for bromide syntheses over the chloride analogue. In contrast to the results obtained with MCM materials, the incorporation of aromatics does not produce swollen films. Examination of the structures in bulk solution using small angle neutron scattering (SANS) shows the formation of rodlike micelles in the bulk before film formation is suppressed by adding alkanes. Fitting SANS data to models showed that, in contrast to the case of the sphere to rod transition normally associated with the formation of hexagonal silicate films at the interface, only expanded spherical micellar structures capable of accommodating several alkane molecules per surfactant molecule exist in up to and beyond film formation.