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

Abstract

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 (Cm), and the saturation concentration of surfactant on the NaCMC chains (CS) 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 (CP) and the onset of a redissolution concentration (CR) 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.