Mechanistic aspects of polysaccharide complexation by quaternary ammonium salts,based on the reaction of Xanthan gum with [1-14C]cetyltrimethylammonium bromide (Part II)

Complex formation between cetyltrimethylammonium bromide (CTAB) and xanthan gum in Xanthomonas campestris fermentation broth has been further used to exemplify the reaction between carboxy group — containing polysaccharides and quaternary ammonium salts. The variation of the molar ratio of xanthan gum: CTAB in the supernatant with variation of addition of quaternary ammonium salt is one parameter of the reaction which shows a definite relationship. Although this molar ratio in the supernatant is high (> 1) during the initial stages of the reaction, when little precipitation of the polysaccharide is occurring; it appears that the ratio must be below a critical value of about one before the end-point of the complexation reaction can occur. The molar ratio of xanthan gum:CTA in the complex is initially exaggerated due to precipitation of polysaccharide with uncomplexed carboxy groups, but this ratio attains a constant value during the final stages of the reaction. A surprising observation of this study was the evidence obtained that CTAB added to X. campestris fermentation broth in excess of the complexation reaction end-point requirement is associated with the insoluble xanthan gum-CTA complex and is not present in the supernatant. Several possible explanations have been put forward to account for this observation. On the basis of this and another recent study, a detailed model of the mechanism of the complexation reaction between xanthan gum and CTAB in aqueous solution has been proposed. Discussion has been given to the implications and applicability of this particular complexation reaction mechanism.