Surfactant Aggregation in DESs

Deep eutectic solvents (DESs) have attracted significant attention for a wide range of applications including metal deposition, natural product extraction, metal recycling and catalysis. In general they are good at solubilising polar and charged solutes and hence their properties can be tuned. This study aims to investigate how the properties of DESs can be modified by the addition of surfactants. One aim is to understand how and why surfactants aggregate and how their surface activity differs from aqueous solutions. Firstly, the physical properties of three DESs are characterised containing different surfactants. The critical micelle concentrations, CMC are analysed together with the extent of aggregation. It was found that CMC values of SDS in Reline and Glyceline are smaller than in aqueous systems while in Ethaline the value are similar to those in water. Dynamic light scattering and viscosity data show that supramolecular aggregates of SDS in Ethaline change from cylindrical to liquid crystalline phases at about 3 times the CMC concentration. Moreover, the thermodynamic parameters of the micellization indicated that SDS aggregation was enthalpy controlled. The pattern of the micelle aggregation was different to that observed in water. Surfactant aggregates are found to form despite the high ionic strength due to that large choline cations having a low charge density. The interface properties of surfactants were studied and it was found that the aggregation was favoured in media with a higher surface energy as this disfavoured the solubilisation of monomers. The surfactants were found to be less surface active than they were in aqueous solutions. This was proposed to be due to the high ionic strength of the DESs meaning that charge-charge interactions between the interface and the surfactant were less important. The implication of this is demonstrated when surfactants were tested as brighteners in copper electroplating solutions. It was found that surfactants did not affect the deposit morphology as significantly as molecules which specifically adsorbed on the copper surface which indicated the method by which brighteners functioned in DESs was not primarily through charge-charge interactions.