Interactions between Ionic Surfactants and Polysaccharides in Aqueous Solutions

Methylcellulose (MC), chitosan (CS), and K-carrageenan (KC) are chosen as typical representatives of neutral and positively, and negatively charged polysaccharides, respectively. Sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) are chosen as anionic and cationic surfactants, respectively. From these polymers and surfactants, six combinations of polymer-surfactant are made: MC-SDS, MC-CTAB, CS-SDS, CS-CTAB, KC-SDS, and KC-CTAB. The polymer-surfactant interactions for the six pairs are systematically investigated by isothermal titration calorimetry (ITC). It was observed that MC-SDS exhibited strong hydrophobic interaction, as evidenced by the endothermic peak, followed by a weak ion-dipole interaction. KC-SDS did not show any ionic interaction, as expected, but exhibited weaker hydrophobic interaction than MC-SDS. Strong ionic interaction was found for CS-SDS, followed by hydrophobic interaction between some hydrophobic moieties of CS and SDS tails. Titration of CTAB with MC resulted in a pattern of polymer-surfactant interaction similar to that for the titration of CTAB with CS. KC showed strong interaction with CTAB by largely reducing the high endothermic enthalpy changes caused by the demicellization of CTAB. A general rule that determines the order (or priority) for a surfactant to bind to a polymer has been proposed based on our experimental results and discussion.