Investigation of factors affecting the adsorption of functional molecules onto gel silicas

Flow microcalorimetry and infrared spectroscopy were used to study the surface structure and adsorptive properties of a series of calcined and uncalcined porous silicas. The adsorbates dl-menthol, (r)-(+)-limonene, (+/−)-citronellal and carvone were selected for their functionality, that included carbonyl, vinylic and hydroxyl groups. The amounts of probe retained by the silicas together with the energy exchange involved in the adsorption/desorption process were determined by flow microcalorimetry. The functional groups involved in these interactions were studied by means of infrared spectroscopy. It was observed that the strongest interactions with the silica surface took place through hydrogen bonding onto the surface silanol. The most retentive probes were found to be those with a carbonyl group in their structure. Adsorption onto calcined silicas was found to be less energetic than onto the equivalent calcined samples. The adsorption densities were compared with theoretical predictions based on molecular models. In all cases apart from citronellal monolayer coverage were not observed due to steric effects.