The reactions between silica and water are central to a spectrum of diverse problems ranging from the subtle interactions of molecular/cluster species in hydrothermal silicate synthesis and biomineralisation to geochemical phenomena involving erosion and deposition at a macroscopic level. The scale and structure of the respective water and silica species involved in such processes, and further how these two factors are are influenced by external conditions, is critical to a full understanding of reactive silica-water systems. Computational modelling methods are now well established to deal with many aspects of water and silica as separate systems and such approaches are also increasingly contributing to our our detailed knowledge about interacting water-silica systems. The important question of the relevance of the scale and structure in interacting silica-water systems is, however, largely unexplored by computational means.
Considering scale, molecular silica is soluble in excess water yet nucleates to form silica materials from solution. Structurally, such nucleating silica species may be induced to form symmetric clusters and crystals, whilst bulk crystalline silica surfaces may in turn induce ordered arrays of absorbed water molecules. In these examples, water can facilitate the rearrangement and ordering of silica species and in turn, it can be ordered by the silica substrate. Furthermore, depending on the surface structure of bulk silica, it may act as a strong dissociating absorber of water or hardly interact at all. The tendency of both water and silica, when strongly interacting, to organise in mutually compatible ways is likely linked to a more fundamental propensity of both systems to display tetrahedral order in their respective multifarious bulk phases. Formally regarding bulk silica and water of as built up from corner-sharing tetrahedral units (SiO4 and OH4 respectively) at once helps to rationalise the links between the two systems in terms of their physical properties but also naturally gives rise to a common structural heritage via considering corner-sharing tetrahedral interactions. Both systems, for example, display analogous crystal structures; the clathrate hydrate water ices and the all-silica clathrates perhaps being the most well known. More remarkably is the possibility of crystals from frameworks consisting of water and silica species to form hybrid hydrate-silicate frameworks.
Stefan T. Bromley
(University of Barcelona)