Natural materials with high surface area. Physisorption methods for the characterization of the texture and surface of polysaccharide aerogels

Polysaccharide hydrogels are open networks of naturally functionalized hydrocarbon materials. In this study, gels of several polysaccharides (alginate, alginic acid, kappa-carrageenan, agar, chitin and chitosan) have been prepared by various gelling methods. Supercritical drying allows to form aerogels with surface areas as high as 600 m(2) g(-1) and active sites with acid (alginate or carrageenan) or basic (chitosan) properties, which confer them effective catalytic properties. The macroporous volume of polysaccharide aerogels can reach 40 cm(3) g(-1) and provide an excellent accessibility of the active sites. Physisorption of N-2 at 77 K has allowed to assess a secondary mesoporosity attributed to contacts between polymer fibrils. The enthalpies of physisorption have been analyzed through the BET method and the measurement of isosteric heat of adsorption of Ar. Comparison plots have been drawn on the basis of a reference isotherm exploiting the absence of microporosity of fumed silica and the absence of mesoporosity of Lichrospher. Deviations of the low-pressure part of the comparison plots have been exploited to evaluate a variation of density of the physisorbed monolayers in good correlation with enthalpies of adsorption. The molecular area of adsorbed N-2 is an inverse function of the energetics of physisorption. The adsorption heat increases with the polarity of the polymer in the order of chitin<<chitosan<agar<carrageenan similar to alginic acid alginate, where acetylated chitin is the less polar term and polymers with acidic groups or metal cations are the most polar terms of the series. (C) 2010 Elsevier Inc. All rights reserved.