Compaction of porous silica – A photophysical study

Photophysical studies, utilizing pyrene, are used to investigate the compaction of porous silica. Conventional studies show that the volume of this material is markedly reduced (4×) on compression to 1 G Pa. The nature of the processes giving rise to this volume change vary with the logarithm of the applied pressure. Organic molecules in particular pyrene, indicate that the surface of the silica wrinkles and provides new adsorption sites that screen the adsorbate from other molecules such a O2, NH3, and CCl4. At high pressures a portion of the adsorbate is completely trapped in silica, if loaded onto the material prior to compaction. Adsorbates with weak bonds, e.g., bromopyrene are chemically altered on compaction in silica. The data amplify earlier studies on the uncompacted system, and confirm the heterogeneous environment of SiO2 at the sites where organic molecules are adsorbed.