Aggregation behavior of nonionic surfactants in ionic liquid mixtures

We investigated the aggregation behavior of polyoxyethylene (POE)-type nonionic surfactants in ionic liquid mixtures composed of 1-ethyl- and 1-hexyl-3-methylimidazolium tetrafluoroborates (emimBF4 and hmimBF4, respectively) by means of 1H NMR chemical shift analysis and dynamic light-scattering measurements. The surfactants do not aggregate in hmimBF4, whereas they are essentially immiscible with emimBF4. That is, the surfactants are highly solvophilic to hmimBF4, while are highly solvophobic to emimBF4. In mixtures of emimBF4 and hmimBF4 micellization was observed. The critical micelle concentration (cmc) decreased and the mean hydrodynamic diameter of micelles, and hence, the micellar aggregation number, increased with increase in mole fraction of emimBF4 in the ionic liquid mixture. 1H NMR chemical shift analysis revealed that hmimBF4 interacts with surfactant molecules preferentially in the ionic liquid mixture through interaction of hexyl groups with the surfactant hydrocarbon chains. The present work demonstrates that solvent quality can be controlled by mixing two ionic liquids to induce self-aggregation of amphiphilic molecules.

Graphical abstractThe nonionic surfactant C12E6 forms micelles in emimBF4/hmimBF4 mixed ionic liquid, where the cmc decreases and the micellar size increases with the increase in the mole fraction of emimBF4 in the ionic liquid mixture..Figure optionsDownload full-size imageDownload high-quality image (49 K)Download as PowerPoint slideHighlights► Polyoxyethylene (POE)-type nonionic surfactants do not aggregate in the ionic liquid hmimBF4. ► POE-type nonionic surfactants are immiscible with ionic liquid emimBF4. ► POE-type nonionic surfactants form micelles in mixed emimBF4/hmimBF4 solvents depending on composition. ► The solvent quality of ionic liquids for amphiphilic molecules can be tuned by mixing the two ionic liquids.