Nitrogen sorption as a tool for the characterisation of polysaccharide aerogels

Supercritically dried aerogels of several polysaccharides (chitin, chitosan, alginate, alginic acid, k- carrageenan, and agar) have been characterised by physisorption ofN2. Surface areas as high as 570m2 g−1 have been measured. The nature of the functional groups of the polysaccharide significantly influences the adsorption of N2 on the surface of the aerogel. The net enthalpy of adsorption increases with the polarity of the surface groups of the polymer, in the order chitin < agar≤chitosan < carrageenan < alginic acid∼alginate. The surface area and the mesopore distribution of the aerogels depend both on the dispersion of the parent hydrogel and on the behaviour of each polymer in the drying treatment. Aerogels which retain the dispersion of the parent hydrogel are mainly macroporous (pores larger than 50 nm) while materials liable to shrink upon solvent exchange form mesoporous structures

Pillaring effects in macroporous carrageenan-silica composite microspheres

The impregnation of a carrageenan gel by a silica sol is an efficient method to form a composite material which can be conveniently activated by CO2 supercritical drying. The textural properties of the solids have been characterized by nitrogen adsorption-desorption at 77K and their composition by thermogravimetric analysis and EDX microprobe. Morphology was examined by SEM. The silica-carrageenan composites present an open macroporous structure. Silica particles retained inside the gel behaved as pillars between the polysaccharide fibrils and form a stick-and-ball network. The stiffening of the carrageenan gel by silica prevented its shrinkage upon drying. The nature of the alkali cations affected the retention of silica particles inside the gel. In the absence of silica, carrageenan fibrils rearrange under supercritical drying and form an aerogel with cavities in the mesopore range