Investigation of acetic acid-induced gelation of CaO colloids in alcohol: Role of alcohol in the gelation process

Our earlier research demonstrated that gels with various structures, such as homogeneous gels composed of calcium acetate nanofibers, and xerogels in which CaO particles are connected by calcium acetate, are obtainable by adding small amounts of acetic acid to CaO colloids in ethanol. For this study, we investigated gelation behavior dependence on the alcohol species and elucidated novel information related to the gelation mechanism. Because similar gelation was observed in CaO colloids in 1-propanol, it has been suggested that alcohol itself plays no important role in gelation unless it dissolves calcium acetate. However, results obtained from this study revealed that gelation does not occur in 2-propanol or tert-butanol, whereas it does occur in 1-butanol, indicating that gelation occurs in primary alcohols but not in secondary or tertiary alcohols. Based on the relation between the pKa values of acetic acid and the class of alcohol, we propose that the acidity of acetic acid in alcohol is a crucially important factor affecting gelation. Alcohol plays a fundamentally important role in the gelation process, particularly in calcium acetate formation, by changing the effective acidity of acetic acid. The small amount of water contained in 2-propanol induces gelation, which supports our findings because water increases the acetic acid acidity.