Effects of head-group volume on the thermodynamic parameters and species distribution of ionic liquid-based surfactants in water: 1-(n-hexadecyl)-3-alkylimidazolium bromides and chlorides

Abstract

The following series of ionic liquid-based surfactant halides was studied: 1-(n-hexadecyl)-3-Cm-imidazolium bromides and chlorides (C16CmImBr and C16CmImCl, respectively) where Cm = C1-C4. Using isothermal titration calorimetry, we calculated the enthalpy of micelle formation (ΔH⁰mic) at 25 °C for both halide surfactants, and from 15 to 70 °C for C16CmImCl. Values of ΔH⁰mic were then employed to calculate the entropy of micelle formation (ΔS⁰mic) from the corresponding free energy (ΔG⁰mic); the latter was calculated from the critical micelle concentration (cmc) and the degree of counter-ion dissociation (αmic). Values of ∣ΔG⁰mic∣ increase as a function of increasing Cm, although this increase is smaller than that of the previously studied series CnC1ImCl, where Cn = C10 to C16. This is attributed to the difference in dehydration of the surfactant segments that accompanies micelle formation, which is much larger for the increase in the hydrophobic chain (CnC1ImCl) than for the head-group (C16CmImCl). The calculated enthalpograms (ΔH⁰dilution versus [surfactant]) for C16C4ImBr showed deviation from ideal solution behavior, this was attributed to the formation of premicellar aggregates. The latter conclusion was corroborated by Pulsed Field Gradient (PFG)-NMR experiments of C16CmImCl in D2O. Thus, a model based on the presence of the following species in solution fitted satisfactorily the dependence of the observed surfactant diffusion coefficients on [surfactant]: free surfactant molecules; premicellar aggregates and micellized surfactant. The concentration of the premicellar aggregates increases noticeably as a function of increasing Cm, due to hydrophobic interactions between the Cm groups. The solubilization of (anionic) thymol blue indicator by C16CmImBr was studied using UV–Vis spectroscopy. The solubilization power (=moles of solubilized molecule/mole of micellized surfactant) was calculated, and compared with our previous data on the solubilization of (hydrophobic) Sudan IV dye and (less hydrophobic) drug nitrendipine by C16CmImBr. In all cases, the solubilization power decreases as a function of increasing Cm, albeit to different degrees, due to a combination of steric hindrance by the head-group, and decreased micelle size, both acting in the same direction.