3.5. Solubility of chitosan and the three chitosan conjugates
3.5.1. Solubility in waterThe solubility of chitosan and the three chitosan conjugates in distilled water at RT is summarized in Fig. 4a. Chitosan is insoluble in water and the aqueous solution at neu-tral pH. However, it ionizes at acidic pH and becomes soluble.The formation of a quaternary salt and hydrophilic groups on to the chiotosan backbone greatly improved its solubility. The results demonstrate that all conjugated chitosan can be dissolved in dis-tilled water at room temperature with various solubility values.After quaternization of chitosan, the solubility of TM-chitosan was 24.0 ± 3.8% in water, based on the initial dried weight. The water solubility of TM-gluconate-chitosan increased from 40.5 ± 5.7% to 84.5 ± 3.2% as the TM-chitosan: GLU (w/w) ratio increased from 1:0.25 to 1:1. After conjugation of gluconate on to the chitosan backbone, the level of internal hydrogen bonding between the amine groups and the crystallinity of the native chitosan were both reduced. Although the TM-GN-chitosan still has a number of OH groups, the water solubility was still high because the GN chains caninterrupted the strong hydrogen bonds between the amine group sofchitosan. With respect to the TM-GN-HT-chitosan, the solubility was numerically slightly, but not statistically significantly, lower than that for the TM-GN-chitosan from the same TM-chitosan: GLU(w/w) ratio of 1:1. Presumably, this is accounted for as that the SHgroup in the TM-GN-HT-chitosan was only slowly and incompletelyassociated with hydrogen bonding and so the TM-GN-HT-chitosanwas less soluble in water than the TM-GN-chitosan.