Abstract
Deep eutectic solvents (DES) are potentially greener solvents obtained through the complexation of simple precursors which, among other applications, have been investigated in recent years for their ability to support the self-assembly of amphiphilic molecules. It is crucial to understand the factors which influence surfactant solubility and self-assembly with respect to the interaction of the surfactant molecule with the DES components. In this work, small-angle neutron scattering (SANS) has been used to investigate the micellization of cationic (CnTAB) and anionic (SDS) surfactants in a ternary DES comprising choline chloride, urea, and glycerol, where the hydrogen bond donors are mixed in varying molar ratios. The results show that in each case either globular or rodlike micelles are formed with the degree of elongation being directly dependent on the composition of the DES. It is hypothesized that this composition dependence arises largely from the poor solubility of the counterions in the DES, especially at low glycerol content, leading to a tighter binding of the counterion to the micelle surface and giving rise to micelles with a high aspect ratio. This potential for accurate control over micelle morphology presents unique opportunities for rheology control or to develop templated syntheses of porous materials in DES, utilizing the solvent composition to tailor micelle shape and size, and hence the pore structure of the resulting material.
Highlights
Amphiphile self-assembly in ionic liquids (ILs) was first demonstrated in 1982 for ethylammonium nitrate,[1] and well over 30 protic ionic liquids are known to exhibit this behavior.[2]
Previous investigations showed that surfactant behavior in choline chloride/ urea differs significantly from that in choline chloride/glycerol, and as such, the micellization in these mixed environments could be used to tune morphologies between those two extremes
The results presented here show that the formation of these mixed solvents does enable the modulation of micelle morphology, transitioning from elongated micelles at low glycerol content to globular aggregates at higher glycerol contents
Summary
Amphiphile self-assembly in ionic liquids (ILs) was first demonstrated in 1982 for ethylammonium nitrate,[1] and well over 30 protic ionic liquids are known to exhibit this behavior.[2]. The sphere model appropriately fitted the data from globular micelles and the cylinder model appeared suitable for elongated micelles, an ellipsoidal model was chosen to fit all of the data as it covers a wide range of aspect ratios and appropriately describes the scattering from micelles presented in this work This is consistent with the results of previous investigations on similar systems.[31] This model contains structural parameters for the equatorial (req) and polar (rpo) radii, where the equatorial dimension is the radius of the micelle through the rotational axis of the spheroid (see Figure S4 for a diagram). ACalculated as the average molar mass by considering the ratio of components in each DES. bMolar volumes were calculated by considering the density and average molar mass of each sample. cGordon parameters were calculated from surface tension measurements
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