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

Author(s)

Keppeler, Nicolas ; Galgano, Paula D. ; Schoenhoff, Monika ; Malek, Naved I. ; El Seoud, Omar A.

• Document type: Journal article
• Source: JOURNAL OF MOLECULAR LIQUIDS; v. 362, p. 10-pg., 2022-07-05.

Abstract

The following series of ionic liquid-based surfactant halides was studied: 1-(n-hexadecyl)-3-C-m-imidazolium bromides and chlorides (C(16)C(m)ImBr and C(16)C(m)ImCl, respectively) where C-m = C-1-C-4. Using isothermal titration calorimetry, we calculated the enthalpy of micelle formation (delta H-mic(omicron)) at 25 ? for both halide surfactants, and from 15 to 70 ? for C(16)C(m)ImCl. Values of AH(mic)(0) were then employed to calculate the entropy of micelle formation (delta S-mic(omicron)) from the corresponding free energy (delta G(mic)(omicron)); the latter was calculated from the critical micelle concentration (cmc) and the degree of counter-ion dissociation (alpha(mic)). Values of |delta G(mic)(omicron)| increase as a function of increasing C-m, although this increase is smaller than that of the previously studied series C(n)C(1)ImCl, where C-n = C-10 to C-16. 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 (C(n)C(1)ImCl) than for the head-group (C(16)C(m)ImCl). The calculated enthalpograms (delta H-dilution(omicron) versus [surfactant]) for C(16)C(4)ImBr 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 C(16)C(m)ImCl 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 C-m, due to hydrophobic interactions between the C-m groups. The solubilization of (anionic) thymol blue indicator by C(16)C(m)ImBr 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 C(16)C(m)ImBr. In all cases, the solubilization power decreases as a function of increasing C-m, 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. (C) 2022 Elsevier B.V. All rights reserved. (AU)

• FAPESP's process: 14/22136-4 - Use of green solvents and their mixtures for optimizing chemical processes
• Grantee: Omar Abou El Seoud
• Support Opportunities: Research Projects - Thematic Grants
• FAPESP's process: 17/06394-1 - Muti-user equipment approved in grant 2014/22136-4: isothermal titration calorimeter (ITC) for the determination of thermodynamic parameters of molecular interactions
• Grantee: Omar Abou El Seoud
• Support Opportunities: Multi-user Equipment Program