Modifying nanoscale silica with itself: a method to control surface properties of silica aerogels independently of bulk structure

A nanoscale morphological composite of silica can be prepared in which the bulk properties (density, transparency) of the rough colloidal silica network are retained while the surface properties are dictated by a ramified form of nanoscale acid-catalyzed silica. This silica-modified silica composite is created by chemically modifying a base-catalyzed silica gel with a silica sol prepared by acid-catalyzed hydrolysis and condensation. Such structural nanocomposites of base-catalyzed, acid-modified (BCAM) silica gels can be supercritically dried to create BCAM aerogels in which the nanoscale mesoporous material is transparent to visible light while the surface of the silica network adopts the chemical nature (silanol-rich, hydrophilic) typical of an acid-catalyzed silica aerogel. Allowing an acid-catalyzed silica sol to gel about a base-catalyzed silica gel creates BCAM gels-in-gels that retain high porosity (>85%), yet which permit non-covalent entrapment of molecules at the base-catalyzed interface under conditions in which they would otherwise freely diffuse out of the gel or aerogel structure. The BCAM approach may be extended to create morphological nanocomposites other than SiO2/SiO2 and is demonstrated for a titania-modified silica gel and aerogel.