Silica Structures Templated on Fibers of Tetraalkylphosphonium Salt Gelators in Organogels

Abstract

Phosphonium cations (18(n)RP(+)) consisting of three or four n-octadecyl chains and R = PhCH(2) or C(m)H(2)(m+1) (m = 1-5 or 12) when n = 3 and with iodide, bromide, chloride, fluoride, or perchlorate anions are used to gelate and polymerize solutions of 2-10 wt % tetraethyl orthosilicate in ethanol, benzene, tetrahydrofuran, and dimethyl sulfoxide using acid or base catalysis and under hydrolytic or nonhydrolytic conditions. These are the simplest low-molecular-mass organic gelator structures of which we are aware that have been able to template silica. The silica objects that are obtained after the hydrolytic sol-gel process include porous, spherulitic, and tubular objects in the size range of several micrometers to tens of nanometers. Their specific shapes and sizes depend on the specific conditions of the hydrolytic sol-gel process, including the nature of the catalyst. The electrostatic interaction between silicate intermediates and gelator strands is the driving force for templating. The template effect is strongly influenced by several factors, including (1) the competition between silicate/solvent and silicate/template interactions, (2) the period of the sol-gel process, (3) the hardness of the anion of the gelator salt, (4) the surface tension of the solvent, (5) and the sequence of drying and template removal processes. The nature of the R group influences the stability of the molecular gels but appears to have little effect on the silica morph obtained. In addition, it is shown in one case, where a direct comparison is possible, that the fibers of one of our phosphonium salts are a much more efficient template for silica than those of the corresponding ammonium salt (with its "harder" cationic center). The specific nature of the objects and the conditions under which they can be formed are discussed.