Effects of fluoride and urea on the crystal growth of a hectorite-like layered silicate on a silica surface

Abstract Dissolution of the silica substrate has been considered to be the initial reaction step in the in situ crystallization of hectorite-like layered silicate (Ht) on a silica surface in an aqueous urea solution at 373 K, occurring before the reactions with LiF and MgCl 2 . However, the effects of the quantities of fluoride and urea on Ht crystal growth are unclear. Here, magnetite particles encapsulated inside silica microcapsules were used as an indicator of silica dissolution to elucidate the mechanism of Ht growth. When large quantities of LiF and urea were used, the amount of crystallized Ht increased but the magnetization (at 12 kOe) of the sample was reduced, showing erosion of the occluded magnetite particles after dissolution of a certain proportion of the hollow encapsulating silica grains. Hence, the observed reduction in magnetization was adopted as a means to estimate the degree of dissolution of silica during Ht formation. Appropriate concentrations of urea and fluoride were used in the aqueous dispersions to control the amount of Ht formed in the heterogeneous nucleation reactions without the need for excess quantities of silicate, Li + , and Mg 2 + .