Effect of N-Sulfonation on the Colloidal and Liquid Crystal Behavior of Chitin Crystallites

Abstract

Chitin crystallites were heterogeneously N-sulfonated in an aqueous medium using triethylamine/sulfur trioxide (TEA/SO3) or pyridine/sulfur trioxide. The extent of N-sulfonation of the crystallites has been controlled by the amount of TEA/SO3added in the reaction. The concentration of sulfur in the crystallites after N-sulfonation was quantified using conductimetric titration and elemental analysis. The ratio of N-sulfonated amino groups to amino groups (S/N) was calculated based on the titration data. The presence of N-S bonds assumed to be at crystallite surfaces was demonstrated by X-ray photoelectron spectroscopy (XPS). After N-sulfonation, the crystallites have two ionizable groups at their surface: —NH3+and —NHSO3H(Na). The former is pH dependent. The colloidal properties of the N-sulfonated crystallites having different S/N were investigated by plotting the zeta potential as a function of the pH of the suspension. The isoelectric point was found to change with the level of N-sulfonation. Transmission electron microscopy shows that the aggregation of crystallites depends strongly on the extent of N-sulfonation. Above a certain concentration, the original chitin crystallites form tactoids (chiral nematic domains) in an aqueous medium. This phenomenon was not observed for the crystallites with a low extent of surface N-sulfonation (below 70%). At about 80% N-sulfonation, the formation of tactoids was once again observed.