Reconsideration of the structure of magnesium n-hexadecylsilicate and other “talc-like” layered magnesium organosilicates

Abstract

The structure of layered magnesium organosilicate materials has been assumed to be “talc-like”: a highly condensed tetrahedral organosilicate network on each face of a magnesium (hydr)oxide sheet, with the presumed formula Mg3Si4R4O8(OH)2 based on isomorphous substitution of talc (Mg3Si4O10(OH)2). However, the arrangement of the organic groups has not been clearly described. For magnesium n-hexadecylsilicate, prepared in this work, XRD and NMR indicate an orthorhombic or hexagonal polyethylene-like structure for the n-hexadecyl chains, which is incompatible with a “talc-like” arrangement of organosilicate tetrahedra. A revised structure and formula (Mg3Si3R3O6(OH)3) are proposed that accommodate the steric and geometric requirements of the organic groups, account for previously overlooked spectroscopic features, and indicate that the conventional synthetic protocol also yields some amorphous organosilica (RSiO1.5). The n-octadecylsilicate, n-propylsilicate, and phenylsilicate derivatives are consistent with the revised model, and previously reported magnesium organosilicate materials are also reconsidered in this context. Revisions to the synthetic protocol for improved efficiency and product uniformity, and modifications to characterization strategies, are proposed.