Complexes between sodium alginate and cetyltrimethylammonium <i>p</i> -toluenesulfonate: a multi-technique investigation of their formation in solution as a function of pH

The interaction between the cationic surfactant cetyltrimethylammonium p-toluenesulfonate (CTAT) and a carbohydrate-based biopolymer (sodium alginate) has been investigated at several pH values using surface tension measurements, isothermal calorimetry, turbidimetry, and fluorescence probing experiments. The results indicate that CTAT interacts strongly with sodium alginate at a surfactant concentration that is at least an order of magnitude lower than the critical micelle concentration of pure CTAT in water. Further addition of CTAT leads to fully saturated complexes at a concentration that is not significantly dependent on the solution pH and is surprisingly not much different than the critical micelle concentration (CMC) value of pure CTAT. The excess CMCs (CMC e values) reflect the concentration at which the saturated complex equilibrates with free surfactant and pure surfactant micelles; these values, determined from isothermal calorimetry and surface tension measurements, increase when the solution pH is increased consistent with the expected changes in the degree of ionization of the biopolymer as pH rises. In agreement with our previous paper, the data presented here indicate that the condensation of the surfactant onto the anionic polymer is a complex processes encompassing strong electrostatic interactions, cooperative binding of the amphiphile ions to the polymer backbone, and multiple equilibria among spherical micelles, rod-like micelles, free surfactant ions, and the polymer–surfactant complex.