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J. Phys. Chem. B 2002, 106, 2330-2336
Structure Development in Octadecyl Trimethylammonium Templated Silicate Films Grown at the Air/Water Interface Stephen A. Holt,† Jeremy L. Ruggles,† John W. White,*,† and Richard F. Garrett‡ Research School of Chemistry, Australian National UniVersity, Canberra, ACT 0200, Australia, and Australian Nuclear Science and Technology Organization, PMB 1, Menai, NSW 2234, Australia ReceiVed: NoVember 6, 2001
The mechanism of growth of silicate films at the air/liquid interface has been investigated in situ by a series of grazing incidence diffraction experiments using a 20 × 25 cm2 imaging plate as the detector. C18TAX (X ) Br- or Cl-) has been used as the film templating surfactant. The formation of a layered phase, prior to growth of the hexagonal mesophase in C18TABr templated films, has been seen. This layered structure has a significantly shorter d spacing compared to the final hexagonal film (43 versus 48 Å, respectively). The correlation lengths associated with the development of the hexagonal in-plane diffraction spots are much longer in-plane than perpendicular to the air/liquid interface (300 Å versus 50 Å). This implies that the film forms via the growth or aggregation of islands that are initially only a micelle or two thick, which then grow down into the solution.
Introduction The self-assembly of oriented mesoporous silicated films at interfaces has been demonstrated by a number of workers.1-5 These films have been grown using a template directed synthesis either at the solid/liquid or air/liquid interface of the solution. The typical synthesis is based upon that developed for the production of particulate MCM-41,6 in which cetyl trimethylammonium chloride/bromide (C16TACl/Br) has been used surfactant template. For film growth the predominant variation in the synthesis used here is the low pH, and the reduction of the silicate source concentration to a molar ratio of approximately 1:1 with the templating surfactant. Structure development at the air/water interface, initial propagation into solution, and the subsequent development of the film to micron thickness are the key questions addressed here. Harvested films2,4,7,8 rather than in situ growth have been extensively studied. Neither these nor many in situ experiments at the air/water interface using reflectometry,9-12 Brewster angle microscopy,13 polarized optical microscopy,3 and grazing incidence X-ray diffraction (GIXD)14 have fully resolved the sequential mechanism of film development. Typically, film growth takes place after an extended induction period, during which reflectometry shows that there is little structural development at the interface. Toward the end of this induction period the interface becomes more structured10 with about seven silicates detected at the air/water interface. From this stage onward the film structure develops quite quickly, with hundreds to thousands of layers assembling very rapidly under this initial structure. GIXD has revealed that the structure develops as a hexagonal array of rods with the long axis parallel to the interface, which becomes detectable during the latter stages of the induction period.14 The present paper on in situ film growth extends the previous studies by using GIXD to examine film growth at different temperatures with C18TAX (x ) Cl, 8r) as the templating species. * To whom correspondence should be addressed. E-mail: John.White@ anu.edu.au. Fax: +61 2 6125 4903. † Australian National University. ‡ Australian Nuclear Science and Technology Organization.
Recently, experiments11 performed at the air/liquid interface with the chain length of the hydrophobic surfactant tail varied from 12 to 18 carbon atoms demonstrated that films grow in a manner similar to that seen for the 16 carbon chain length surfactant but with a linear increase in d spacing for the increasing chain length. For the C18 system, lamellar crystalline phases were observed when the solution temperature was within the micellar solution region and also when the temperature was decreased into the gel region15-17 of the phase diagram. The d spacings observed for films grown from C18TABr synthesis were consistently larger than that from a C18TACl synthesis.11 Anion specific phenomena as well as the late induction phase film growth are studied here. In each case the surfactant was at the same concentration relative to its CMC as used for the C16TAX surfactants. The GIXD method employed enables diffraction from either the very near surface regions,
