Silylation of Monolithic Resorcinol-Formaldehyde Xerogels

Abstract

Hydrophobic materials are useful for many applications where liquids are involved. Phenolics represent a class of materials that is intrinsically hydrophilic due to the presence of free aromatic hydroxy groups. Due to their high porosity, phenolic aerogels and xerogels are particularly prone to absorb water and other polar liquids. This behavior includes the most prominent organic gels, based on resorcinol-formaldehyde (RF). However, few strategies to overcome the inherent hydrophobicity of RF gels are reported, and these are often limited to powders. Literature-known examples include the trimethylsilylation of powdered RF resulting in material that retain hydrophobic for a limited time. To this end, we report on the silylation of monolithic RF xerogels, using sterically demanding silyl reagents. Our synthetic studies include the variation of amine base and electronic nature of the silyl counterion. Characterization of the resulting gels by means of elemental analysis, X-ray photoelectron spectroscopy, pycnometry, sorption analysis, and scanning electron microscopy with electron-dispersive X-ray spectroscopy is presented. Furthermore, the wetting behavior of the silylated gels exemplified by their contact angle to water after exposure of the gels to ambient air and various liquids (water, acid and base) is discussed. Though apparently silylated to little extent, these gels retained significant hydrophobic behavior over the course of several months. Moreover, monoliths bearing sterically demanding silyl groups displayed higher stability towards aqueous acid than trimethylsilylated RF gels.