Ionic liquid-induced changes in properties of aqueous cetyltrimethylammonium bromide: a comparative study of two protic ionic liquids with different anions.

In this work we have shown a comparative study of changes in physicochemical properties of an aqueous solution of a common cationic surfactant cetyltrimethylammonium bromide (CTAB) upon addition of two protic ionic liquids N,N-dimethylethanolammonium [corrected] hexanoate (DAH) and N,N-dimethylethanolammonium [corrected] formate (DAF). The aim of this manuscript is to offer a comparative study and establish the role of the alkyl chain length of the anion of the added protic ionic liquids on the physicochemical properties of aqueous solution of CTAB. At lower concentration (i.e., ≤ 30 mM) both ionic liquids show the same trend in modifying the properties of aqueous CTAB solution, but DAH as an additive shows a more dramatic increase in aggregation number and size of the CTAB micelle compared to that of DAF as an additive. At higher concentrations of additives (DAF and DAH), the properties of aqueous CTAB solution change in an entirely different way. The size of the CTAB micelle was found to be 0.9 nm. With the addition of 215 mM DAH, the size of the CTAB micelle increases to 25.0 nm, whereas with the addition of 215 mM DAF it increases to only 5.6 nm. Zeta potential, electrical conductance, microviscosity, and dipolarity measurements were performed to gain insight into this abrupt size change in the case of DAH. It is proposed that the formate and hexanoate anions undergo Coulombic attractive interaction with cationic head groups of the CTAB micelle at all concentrations. In the case of DAH, the presence of a hexyl chain on the hexanoate ion allows it to align with the tail part of CTAB, whereas in the case of DAF the absence of an alkyl chain in the formate ion is apparently unable to align the formate anion with the tail part of CTAB. So this difference in the location of the anions of DAF and DAH is responsible for the different size changes and different behaviors of the two ionic liquids.