Abstract
The acidity of alginate aerogel films has been investigated by infrared spectroscopy of adsorbed NH3. Supercritical drying of the alginate provided samples with a surface area of several hundred m2 g-1, in which the probe molecule could reach all acidic sites. Free carboxylic groups were studied on acid-gelled alginates and behaved as effective Brønsted acid sites. Ionotropic alginate gels formed by alkaline earth cations presented only the Lewis acidity of the cations. Ionotropic gels formed by transition metal cations presented both Lewis and Brønsted sites, due to the presence of a fraction of free carboxylic groups. The incomplete salification was correlated to the pH of the gelling solutions.