Emulsions of hydrophobic deep eutectic solvents stabilised with poloxamer surfactants and stearate salts

Small angle neutron scattering and small angle X-ray scattering methods were used alongside droplet size analysis to determine the nanoscale effects of the addition of stearate salts of sodium, calcium and magnesium at the interface of a 30:70 oil : water emulsion. The dispersed phase consisted of hydrophobic dodecanoic acid : menthol deep eutectic solvent with up to 2.5 wt.% stearate salt. The continuous phase consisted of an aqueous solution of non-ionic, poloxamer surfactant. The scattering data shows the presence of sharp smooth interfaces, indicated by the Q^-4 scattering decay, while undulations in the scattering in the Porod region indicate the presence of aggregated surfactant with a lamellar structure at the interface of the two phases. The nanoscale structural characteristics, such as layer thickness and d-spacing of these surfactant assemblies are modified by the presence of stearate salts. This effect is demonstrated especially with sodium stearate, which improves the emulsion storage stability and reduces the polydispersity of the emulsion droplets. This increase in stability is due to the ability of sodium salts to alter the surfactant structure and promote the formation of gel networks on the emulsion interface.