Micellar properties of surface active ionic liquids: A comparison of 1-hexadecyl-3-methylimidazolium chloride with structurally related cationic surfactants

Ionic liquids, ILs, carrying long-chain alkyl groups are surface active, SAILs. We investigated the micellar properties of the SAIL 1-hexadecyl-3-methylimidazolium chloride, C16MeImCl, and compared the data with 1-hexadecylpyridinium chloride, C16PyCl, and benzyl (3-hexadecanoylaminoethyl)dimethylammonium chloride, C15AEtBzMe2Cl. The properties compared include critical micelle concentration, cmc; thermodynamic parameters of micellization; empirical polarity and water concentrations in the interfacial regions. In the temperature range from 15 to 75 °C, the order of cmc in H2O and in D2O is C16PyCl > C16MeImCl > C15AEtBzMe2Cl. The enthalpies of micellization, ΔHmic°, were calculated indirectly from by use of the van’t Hoff treatment; directly by isothermal titration calorimetry, ITC. Calculation of the degree of counter-ion dissociation, αmic, from conductivity measurements, by use of Evans equation requires knowledge of the aggregation numbers, Nagg, at different temperatures. We have introduced a reliable method for carrying out this calculation, based on the volume and length of the monomer, and the dependence of Nagg on temperature. The Nagg calculated for C16PyCl and C16MeImCl were corroborated by light scattering measurements. Conductivity- and ITC-based ΔHmic° do not agree; reasons for this discrepancy are discussed. Micelle formation is entropy driven: at all studied temperatures for C16MeImCl; only up to 65 °C for C16PyCl; and up to 55 °C for C15AEtBzMe2Cl. All these data can be rationalized by considering hydrogen-bonding between the head-ions of the monomers in the micellar aggregate. The empirical polarities and concentrations of interfacial water were found to be independent of the nature of the head-group.

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