Salt effect on the complex formation between 1-dodecyl-3-methylimidazolium bromide and sodium carboxymethylcellulose in aqueous solution

Complex formation between sodium carboxymethylcellulose (NaCMC) and 1-dodecyl-3-methylimidazolium bromide (C12mimBr) has been studied by isothermal titration microcalorimetry (ITC), turbidimetric titration, and the surface tension measurements at varied concentrations of NaBr. The addition of salt is found to influence the formation of C12mimBr/NaCMC complexes markedly. At CNaBr < 0.20 M, C12mimBr forms micelle-like aggregates with the NaCMC polymer chains to form C12mimBr/NaCMC complexes above the critical aggregate surfactant concentration (C1). Free C12mimBr micelles are formed at Cm before the saturation concentration of surfactant on the NaCMC chains (C2). However, when CNaBr > 0.20 M, there is no polyelectrolyte/surfactant complex formation because of complete salt screening of the electrostatic attraction between C12mimBr micelles and the NaCMC chains. Additionally, the addition of salt to a system of 0.01 g/L NaCMC and fixed C12mimBr concentration induces the formation of C12mimBr/NaCMC complexes when CNaBr < 0.20 M, and dissociates the complexes at CNaBr > 0.20 M. The salt effect on complex formation is explained as the result of a competition between the screening interaction (the addition of NaBr screens the electrostatic attraction between surfactant headgroups and stabilizes surfactant micelles) and the increasing interaction (the addition of NaBr compresses the diffusive electric double layers and hence reduces the repulsion between the surfactant heads and carboxylate groups).