TGA and NMR relaxation measurement of nonmesoporous silica to investigate the amount of hydrolysis product in acetylsalicylic acid adsorbed on silica

This study investigated a crucial surface property of silica that contributes to the chemical stability of acetylsalicylic acid (ASA) physically adsorbed on silica. Hydrophilic nonmesoporous types of silica were selected, and the number of hydroxyl groups on silica (N_(OH)) was evaluated using thermogravimetric analysis (TGA). The ASA-containing silica was stored at 40 °C in drying conditions, and the amount of ASA degradation was quantified based on salicylic acid. From the scatterplots between the number of hydroxyl groups per unit weight (specific surface area (SSA) × N_(OH)) and the amount of ASA degradation, it was clarified that in ASA adsorbed on silica, the ASA chemical stability was determined by the formula (the SSA × N_(OH)). In addition, a time-domain nuclear magnetic resonance measurement verified the N_(OH) result by estimating the interaction between the silica surface and water in an aqueous silica suspension. The N_(OH) result was found to be reasonable.