Small molecule - Silica interactions in porous silica structures

A series porous silica frameworks with different pore sizes (SSZ-59, MCM-41 and SBA-15) was studied by immersion calorimetry in various solutions (water, ethanol, triethylamine, NaCl brine and Na₂CO₃/NaHCO₃ buffer) at 25°C to determine the interaction energies between inorganic silica frameworks and various small molecules which may be of importance in the context of CO₂ sequestration, carbonate mineralization, and environmental remediation. The enthalpies of immersion for the calcined porous silica samples range from -0.7 to -9.6kJ per mole of SiO₂ in water, -3.4 to -10.5kJ per mole of SiO₂ in ethanol, -5.7 to -15.1kJ per mole of SiO₂ in triethylamine, -1.5 to -11.4kJ per mole of SiO₂ in NaCl brine (1M) and -0.5 to -8.0kJ per mole of SiO₂ in a Na₂CO₃/NaHCO₃ buffer solution (1M). The enthalpies of interaction between porous silicas and water, ethanol and triethylamine vary from -0.3 to -2.3kJ per mole of H₂O, from -49.1 to -62.5kJ per mole of ethanol and from -65.5 to -95.6kJ per mole of triethylamine. The enthalpies of interaction suggest that guest-host interactions in silica-based porous media are governed both by pore size and concentration of surface hydroxyl groups. The generally hydrophobic surfaces of porous silica materials bond more strongly to organic molecules (ethanol and triethylamine) than to aqueous solutions and the energetics of such functionalized organic - rich silica surfaces could play a significant role in geologic CO₂ sequestration, enhanced oil recovery, and geochemistry in low-temperature diagenetic and sedimentary environments.