Acidity of alginate aerogels studied by FTIR spectroscopy of probe molecules

Supercritical drying of alginate gels is an efficient way to prepare aerogels with high surface area (> 300 m(2)center dot g(-1)). FTIR spectroscopy allows to monitor the adsorption of NH3 from the gas phase onto the acid sites of the alginate. Free carboxylic groups are effective Bronsted sites, whereas the divalent cations used in the ionotropic gelation present the properties of Lewis sites. The ratio between Bronsted and Lewis sites provides infomation on the role of pH in alginate gelation and suggests that non-buffered gelation by transition-metal cations is a mixed ionotropic-acid process.