Calorimetric study of adsorption of alcohols on silicas

This work describes adsorption and wetting characterisation of hydrophobic ordered mesoporous silicas (OMSs) with the SBA-15 and SBA-16 type. A synthetic approach to prepare hydrophobic SBA-15 and SBA-16 silicas was explored grafting with covalently-attached monolayers (CAMs) of C_nH_2n+1Si(CH₃)₂N(CH₃)₂, labelled in this research as SBA-15H and SBA-16H, respectively. The materials prepared were characterised by nitrogen adsorption and FTIR analysis. The surface properties of the materials were assessed by BET C constants. For hydroxylated bare SBA-15 silica and SBA-16, C constants were ≈100–150 and 70–90 respectively, indicating high-energy, polar surfaces of Si–OH groups. The C constants of SBA-15H and SBA-16H grafted with CAMs of alkyldimethylsilanes were in the range of ≈15–25 and ≈35–50, demonstrating non-polar, low-energy organic surfaces of closely packed alkyl groups. The hydrophobic character in this study was evidenced by studying the adsorption of alcohols of chain lineal (C1–C5), where the modified silicas showed that the adsorption capacity of less polar alcohols increased, as in the case of pentanol, butanol and propanol. Immersion calorimetry proved to be a useful and versatile technique for analysing the structural changes of silicas.