Interaction Between 1-Dodecyl-3-Methylimidazolium Bromide and Sodium Carboxymethylcellulose in Aqueous Solution: Effect of Polymer Concentration

Effect of the concentration of water-soluble polyanion (sodium carboxymethylcellulose, NaCMC) on the interaction between a cationic surfactant (1-dodecyl-3-methylimidazolium bromide, C₁₂mimBr) and NaCMC in aqueous solution has been studied by isothermal titration microcalorimetry (ITC), conductivity, surface tension, and rheological measurements. From the surfactant/polymer interacting enthalpy, it can be deduced that the electrostatic attraction between the cationic surfactant and anionic polyelectrolyte causes an endothermic process, and the C₁₂mimBr monomers binding to the NaCMC chains to form micelle-like aggregates through hydrophobic interaction is an exothermic process. Increasing the NaCMC concentration causes the interaction between C₁₂mimBr and NaCMC to decrease, and the characteristic surfactant concentrations, including the critical aggregation surfactant concentration (CAC), the surfactant concentration to form free micelles (C_m), and the saturation concentration of surfactant on the NaCMC chains (C_S) to increase. Because of the strong electrostatic interaction between C₁₂mimBr and NaCMC, the formation of C₁₂mimBr/NaCMC complexes can lead to precipitation or redissolution depending on solution composition, so the critical precipitation concentration (C_P) and the onset of a redissolution concentration (C_R) has been determined by the electrical conductivity. The rheological results reveal a dramatic increase in solution viscosity around the CAC, attributed to interpolymer cross-linking through the formation of mixed micelles involving the carboxylic acid groups of NaCMC and the surfactant.