The growth of self-assembled titania-based films at the air-water interface

The surfactant-assisted assembly of new titania-based films at the air-water interface has been monitored by time-resolved energy-dispersive reflectometry using X-rays and neutrons. Two films are described. The first, prepared from an aqueous alkaline solution containing Ti(OBuⁿ)₄ and the cationic surfactant tetradecyltrimethylammonium chloride (C₁₄TAC), shows short-range ordering. In moderately alkaline solution (pH ∼ 10). triethanolamine (TEA) was required to complex the titanium alkoxide precursor to prevent immediate heterogeneous precipitation of titania. Reflectometry shows that the assembly is partly structured and comprises only a few layers. The film forms within 30 min of mixing the reagents. No further significant structural changes were detected until, after several hours, severe compression buckling prevented further reflectivity measurement. The second film, obtained from aqueous acidic solution containing Ti(OBuⁿ)₄ and sodium dodecyl sulfate (SDS), contains material that is highly ordered. Strongly acidic conditions (pH < 1) enabled immediate heterogeneous precipitation of titania to be avoided prior to reaction with the dilute aqueous SDS solution. The film prepared by this route displayed a Bragg peak corresponding to a d-spacing of 35 Å. The formation of these films is reconciled in terms of the attraction between the surfactant headgroup and titania above and below the pH of the formal isoelectric point of titania.