Morphology Modulation of Ionic Surfactant Micelles in Ternary Deep Eutectic Solvents

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 (C n TAB) 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.

Long-Range Electrostatic Colloidal Interactions and Specific Ion Effects in Deep Eutectic Solvents

“… 62 Similarly, other specific charge interactions with hydrotropes and solvent ions have been shown to affect the morphology of micelles in DES. 47 , 63 − 65 …”

“…This has been shown for the self-assembly of surfactants in ternary DES, where the solubility of the counterion modulates micelle morphology. 65 There, the reduction of the solubility was shifting the energetic balance toward the condensed state at the micelle interface. This was investigated here by comparing the long-range interactions in the counterion-exchanged solvents.…”

“…These effects have previously been reported for anionic surfactant micelles in DES, where the substitution of the counterion in dodecylsulfate surfactant solutions resulted in changes in micelle morphology and CMC . Similarly, other specific charge interactions with hydrotropes and solvent ions have been shown to affect the morphology of micelles in DES. ,− …”

“…As the ion-pair interaction relies on a balance between the counterion–headgroup interactions (i.e., ion condensation free energy) and the counterion–solvent interactions (i.e., ion solvation free energy), the resulting counterion condensation will be defined by the interplay between these two ionic perturbations. This has been shown for the self-assembly of surfactants in ternary DES, where the solubility of the counterion modulates micelle morphology . There, the reduction of the solubility was shifting the energetic balance toward the condensed state at the micelle interface.…”

Martini 3 Coarse-Grained Model for Type III Deep Eutectic Solvents: Thermodynamic, Structural, and Extraction Properties

“…This will, in turn, reduce the repulsion between the dodecyl sulfate headgroups, allowing for elongation. 116 Our models reproduce this, with the exception of the Stern layer being primarily occupied by the choline cations instead of sodium. Another contradiction is observed when the micelle is solvated in water.…”