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

Abstract

Complex formation between sodium carboxymethylcellulose (NaCMC) and 1-dodecyl-3-methylimidazolium bromide (C 12mimBr) 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 C 12mimBr/NaCMC complexes markedly. At C NaBr < 0.20 M, C 12mimBr forms micelle-like aggregates with the NaCMC polymer chains to form C 12mimBr/NaCMC complexes above the critical aggregate surfactant concentration ( C 1). Free C 12mimBr micelles are formed at C m before the saturation concentration of surfactant on the NaCMC chains ( C 2). However, when C NaBr > 0.20 M, there is no polyelectrolyte/surfactant complex formation because of complete salt screening of the electrostatic attraction between C 12mimBr micelles and the NaCMC chains. Additionally, the addition of salt to a system of 0.01 g/L NaCMC and fixed C 12mimBr concentration induces the formation of C 12mimBr/NaCMC complexes when C NaBr < 0.20 M, and dissociates the complexes at C NaBr > 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).