While society strives towards more sustainable lifestyles, technological applications and products that use green solvents are coming up slowly. Deep eutectic solvents (DES) are the epitome of green neoteric solvents. They can be synthesised using simple and cheap organic precursors (e.g. polyols, carboxylic acids, amino acids…), where the mixture of two solid compounds at the eutectic ratio leads to the formation of a liquid at room temperature due to a dramatic change in the system’s entropy compared to the that of the individual precursors. DES are therefore non-toxic, biodegradable and even biocompatible. Since their emergence, the role of DES replacing traditional solvents in myriad technological applications has been studied, such as enzymatic catalysis, electrodeposition and synthesis of nanostructured materials, among others.
The formation of worm-like micelles (WLM) often leads to strong modifications of the rheological behaviour of liquids, resulting in non-Newtonian, viscoelastic fluids which are of utter importance in formulation technology and as response materials. Recently, the self-assembly of surfactants in DES have been reported, showing that the possibility of varying the characteristics of the solvent leads to a rich phase behaviour. However, the self-assembly into WLM in DES remained unexplored.
The co-assembly of ionic surfactants with hydrotropes into WLM in aqueous solution has been reported elsewhere and we hypothesised that the same phenomenon could occur in DES. As such, we synthesised a DES-soluble hydrotrope, choline salicylate, at the Deuteration and Macromolecular Crystallisation platform (ESS) in its protiated and deuterated versions. The effect of that on the micellisation of a cationic surfactant, hexadecyltrimethylammonium chloride, in pure and hydrated 1:2 choline chloride:glycerol DES was investigated.
Contrast variation small-angle neutron scattering (SANS) was used to characterise the micellar structure and to study the mechanism of micellar growth. SANS measurements were performed on the vSANS instrument at the NIST Center for Neutron Research (NCNR, US) using two detector carriages at 4.5 and 18 m sample-to-detector distances and a neutron wavelength of 6.7 Å (dλ/λ= 12%). Data reduction was performed using the standard protocols of the beamline and data were analysed using the form-factor models implemented in SasView 4.2.2.
The microscopic characterization of the system showed that the micelle-hydrotrope interaction in pure and hydrated deep eutectic solvents resulted in a significant increase in micelle elongation. The interaction of the hydrotrope with the surfactant leads to the formation of worm-like micelles where the morphology and flexibility of those can be tuned using the hydrotrope-to-surfactant ratio and the level of hydration. The contrast variation approach revealed that the condensation of the hydrotrope on the micelle alters the effective monomer packing and promotes micellar growth (Figure).
Figure 1.The condensation of the hydrotrope on the micelle, which alters the effective monomer packing, leads to the formation of worm-like micelles with tuneable morphology and flexibility in a deep eutectic solvent.
Now we aim to explore the dynamics and rheological behaviour of WLM in DES. The micelle dynamics will be studied using neutron spin echo and dynamic light scattering, and the non-Newtonian character and viscoelastic properties of these systems will be studied using rheology. These results will be combined with the structural characterisation to elaborate a detailed picture of the structure-dynamics-function relationship for WLM in these solvents. Ultimately, the knowledge gained through these investigations will enable the development of rheology-modified DES in the field of cosmetic and drug formulation technology.
This work resulted from a joint collaboration between Adrian Sanchez-Fernandez (Lund University), Anna Leung (DEMAX, ESS), Andrew Jackson (ESS), and Elizabeth Kelley (NCNR). These activities were financed by the Vinnova – Swedish Governmental Agency for Innovation Systems and the Crafoord Foundation. Experiments were performed at the NIST Center for Neutron Research.
Check out the full article at the Journal of Colloid and Interface Science, 581 (2021) 292-298.