Water vapour-induced mesoporous structure collapse observed by VGI and FT-IR spectroscopy

Mesoporous silicas have found much interest since they were first synthesized by Mobil scientists in 1991 [Nature (London) 359 (1992) 710]; using the inherent behaviour of surfactants to self-assemble into rod-like, lamellar or spherical micelles as a template around which the silica can be formed. These materials have considerable potential for use as catalysts and, as such, their interactions with, and stability to, a variety of vapours, including water, is of considerable interest. The vapour generator instrument (VGI) is a novel environmental control cell, designed to fit under a microscope. The VGI controls the temperature and relative humidity (RH) of the sample environment such that the sample can be observed by FF-IR, Raman or optical microscopic techniques, under these controlled conditions. Using the VGI it is possible to follow in situ, by FT-IR spectroscopy, the interactions of the mesoporous silicas with water vapour, with precise RH control, with equilibration times in the range of minutes. In this paper we describe this new technique, demonstrate the correlation of plots of absorbance versus RH with adsorption isotherms. In so doing we provide initial results demonstrating the minimum RH required to induce structural collapse of the silica structure within the mesoporous silicates, and the effect of different templating surfactants on the stability to atmospheric humidity. (C) 2004 Elsevier B.V. All rights reserved.