Abstract
Deep eutectic solvents have been demonstrated to support amphiphile self-assembly, providing potential alternatives as structure-directing agents in the synthesis of nanostructures, and drug delivery. Here we have expanded on this recent research to investigate the self-assembly of alkyltrimethylammonium bromide surfactants in choline chloride:malonic acid deep eutectic solvent and mixtures of the solvent with water. Surface tension and small-angle neutron scattering were used to determine the behavior of the amphiphiles. Surfactants were found to remain active in the solvent, and surface tension measurements revealed changes in the behavior of the surfactants with different levels of hydration. Small-angle neutron scattering shows that in this solvent the micelle shape depends on the surfactant chain length, varying from globular micelles (aspect ratio ∼2) for short chain surfactants to elongated micelles (aspect ratio ∼14) for long chain surfactants even at low surfactant concentration. We suggest that the formation of elongated micelles can be explained through the interaction of the solvent with the surfactant headgroup, since ion-ion interactions between surfactant headgroups and solvent may modify the morphology of the micelles. The presence of water in the deep eutectic solvents promotes an increase in the charge density at the micelle interface and therefore the formation of less elongated, globular micelles.
Highlights
The micellization of surfactant molecules in water, due to the thermodynamically preferred separation of hydrophilic and hydrophobic phases, is among the most well-established phenomena in physical science
We have found that the traditional hardsphere model is not sufficient to account for the interactions between micelles, principally C12TAB in choline chloride:malonic acid and in choline chloride:glycerol.[20]
Micellization of surfactants in carboxylic acid-based Deep eutectic solvents (DES) has been demonstrated. These systems are of wide interest since the formation of DES using these compounds as hydrogenbond donors has been closely related to the formulation of solvents with nonharmful, organic compounds present in nature
Summary
The micellization of surfactant molecules in water, due to the thermodynamically preferred separation of hydrophilic and hydrophobic phases, is among the most well-established phenomena in physical science. Very few nonaqueous, “classic” molecular solvents that support this process have been identified because the typically weak intermolecular interactions in such solvents do not offer a sufficient driving force for micellization.[1] finding new solvents capable of forming micelles remains an important goal because surfactant self-assembly has important applications such as the templated production of functional nanomaterials with enhanced properties.[2,3] Ionic liquids (ILs) are molten salts, formed from bulky, asymmetrical cations and anions,[4] and were first found to support amphiphile self-assembly in 1982.5 More recently, interest has surged, and ILs are known to support the self-assembly of a wide range of amphiphiles in varied ILs.[1,6] Like water, the solvophobic effect drives ILs to promote micellar phases[7−10] and the formation of lyotropic phases and microemulsions.[11,12]. Because DES necessarily have a Received: September 15, 2017 Revised: November 27, 2017 Published: November 28, 2017
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