Surface chemical functionalities in bioactive glasses. The gas/solid adsorption of acetonitrile

Abstract

The room temperature adsorption of acetonitrile (ACN) on two sol–gel glassy systems for biomedical applications (58S and 77S; composition: SiO₂–CaO–P₂O₅) was studied by in situ IR spectroscopy/adsorption microcalorimetry, and the influence of the chemical composition on the surface properties of the glasses was investigated. In order to highlight the role played by Ca and P species in modifying the surface properties of a silica matrix, some model systems were also investigated, namely: (i) a pure commercial amorphous silica; (ii) the same silica system, surface modified with dosed amounts of Ca and/or P species. Different effects were observed, depending on the metal (Ca) or non-metal (P) nature of the hetero-atoms. The contact of (perdeuterated) ACN with Ca surface species yielded a ν_CN IR band at ∼2185 cm⁻¹, assigned to Ca²⁺ ← NCA coordinative interactions. The spectral position of the ν_CN band is independent of the sample origin (either bioactive glass or Ca-doped silica), whereas population and energy distribution of the species formed depend (strongly) on both sample origin and Ca loading. Surface acidic properties of all systems increase significantly with Ca content, as witnessed by both adsorbed amounts and adsorption enthalpies. The zero-coverage values of the latter quantity are quite high for the 58S and 77S bioactive glasses (∼95 and ∼80 kJ mol⁻¹, respectively) and for medium–high Ca-loaded silicas (∼80 and >100 kJ mol⁻¹ for 8 and 16% CaO, respectively). ACN adsorption enthalpy on surface SiOH groups is close to/lower than the enthalpy of ACN liquefaction (30 kJ mol⁻¹). In the case of the highest Ca surface loading, the increased ionic nature of the system induces a significant surface reactivity, never observed in the case of bioactive glasses. By contrast, P species do not affect significantly the surface acid/base properties of all silica-based systems, especially when Ca species are present also (in excess).