Surface-treatment chemistry in the manufacture of aerogels: computational modelling of cyclic and linear siloxanes

An essential feature of some aerogel manufacturing processes is the `end-cappingʹ of reactive silanol groups in the undried silica gel. Whilst the chemistry associated with the monofunctional trimethylsilanol-based processes is relatively straightforward, alternative chemistry involving multifunctional silylation reagents is not. An example of a multifunctional reagent is dimethyldichlorosilane, which rapidly hydrolyzes in aqueous environments, and in the hydrolyzed form, can condense with itself, yielding linear silanols. The resulting silanols may then further condense to form cyclic siloxanes. Such reactions will inevitably occur under typical end-capping conditions, so it is necessary to consider these when addressing the details of such treatments. We have used a variety of computational methods, including semi-empirical AM1 and PM3 as well as quantum mechanical HF/MP2 at the 6-31+G* and 6-311++G** levels of theory, to investigate some of the siloxane species as isolated molecules. These calculations permit us to compare the enthalpies of formation of the various molecules. Furthermore, we are able to calculate various geometric attributes of the siloxanes. The calculations indicate that in the isolated state, the initial hydrolyses are nearly isoenthalpic, so other mechanisms are needed to explain the strong exothermicity observed in the laboratory. Chain extension of the linear diols is seen to be mildly exothermic with each step. Of the cyclic siloxanes, formation of the trimer, tetramer and pentamer are enthalpically most favored.