Micellization and clouding behaviour of an ionic surfactant in a deep eutectic solvent: A case of the reline-water mixture

Abstract

The micellization and clouding phenomenon are studied across a wide range of reline (Choline chloride (ChCl)-Urea, 1:2 mol ratio) - water composition using fluorescence spectroscopy. Experiments have also been performed to determine the physical properties (specific conductance (κ) and zero-shear viscosity (η0)) of water in reline and reline in water. κ and η0 vary in opposite ways as water composition of the water in reline decreases. Further, pure reline and the reline-water mixtures have been found Newtonian in nature. Critical micelle concentration (CMC) data of Sodium dodecylsulphate (SDS, an anionic surfactant) suggest three regions of CMC variations with water content in the water in reline and reline in water mixtures. Reline-urea-water motifs (H-bonded) or molecular solutions of components (ChCl and urea) dictate CMC in water in reline/reline in water region. Cloud point (CP) data was acquired using SDS + tetra n-butyl ammonium bromide (TBAB) in water in reline or reline in water and compared with pure water. CP plots are constructed with respect to [SDS] or [TBAB]. It has been noted that the increase in CP is directly dependent on [SDS] and reversely on [TBAB]. A clouding mechanism for SDS (+TBAB) in reline -water mixture has been interpreted on the basis of competition between tetra-n-butylammonium (TBA+) and cholinium (Ch+) counter ion for the micellar surface. CP data have also been acquired in the presence of metal salt (cadmium chloride, CdCl2 or zinc sulphate, ZnSO4). These two salts affect CP oppositely, which is explained on the basis of hydrating capacity of metal ions. SDS + TBAB + reline + water form a system that can be potentially used for biphasic extraction (LLPS) of various metals from waste streams or battery waste.