Stability of Sodium Dodecyl Sulfate Micelles in Reline/Water Mixtures: A Molecular Dynamics Study

Abstract

Deep eutectic solvents of type III, such as reline, a mixture of choline chloride and urea, are a relatively new class of green solvents that display physical properties intermediate between conventional salt solutions and ionic liquids. Well-established models used to explain colloidal stability and macromolecule conformation in usual solvents, including the hydrophobic effect and the formation of the electrical double layer, are not applicable to these solvents and their mixtures with water due to strong nonideal behavior and high ionic strength of those solvents. Hence, in this work, molecular dynamics simulations were performed to evaluate the thermodynamic stability of sodium dodecyl sulfate micelles in reline/water mixtures with different compositions by potential of mean force calculations and splitting the contributions from the surfactant ionic head and the hydrophobic tail. The presence of reline reduces the favorable contribution of the surfactant alkyl tail to aggregation, indicating a reduction of the hydrophobic effect, which is explained by a smaller number of water molecules in contact with the surfactant. However, the high adsorption of choline cations over the micelle surface also reduces the repulsion between the charged heads. The second effect dominates over the reduction of the hydrophobic effect, resulting in a more negative aggregation free energy and improved micelle stability in the presence of reline. The electrostatic potential from the anionic micelle was also computed and does not follow the usual electrical double layer models, presenting oscillations away from the micelle surface in mixtures with high reline concentration and with the choline cations almost completely replacing the sodium counterions from the micelle surface.