Ionic liquid nanostructure enables alcohol self assembly.

Thomas Murphy,Silvia Imberti,Robert Hayes,Rob Atkin,Gregory G Warr

doi:10.1039/c6cp01739h

Abstract

Weakly structured solutions are formed from mixtures of one or more amphiphiles and a polar solvent (usually water), and often contain additional organic components. They contain solvophobic aggregates or association structures with incomplete segregation of components, which leads to a poorly defined interfacial region and significant contact between the solvent and aggregated hydrocarbon groups. The length scales, polydispersity, complexity and ill-defined structures in weakly structured solutions makes them difficult to probe experimentally, and obscures understanding of their formation and stability. In this work we probe the nanostructure of homogenous binary mixtures of the ionic liquid (IL) propylammonium nitrate (PAN) and octanol as a function of composition using neutron diffraction and atomistic empirical potential structure refinement (EPSR) fits. These experiments reveal why octanol forms weakly structured aggregates in PAN but not in water, the mechanism by which PAN stabilises the octanol assemblies, and how the aggregate morphologies evolve with octanol concentration. This new understanding provides insight into the general stabilisation mechanisms and structural features of weakly structured mixtures, and reveals new pathways for identifying molecular or ionic liquids that are likely to facilitate aggregation of non-traditional amphiphiles.

Highlights

Surfactants and other amphiphilic species dissolve in water because the hydrophilic head group confers solubility on the hydrophobic tail
Micelles are welldefined structures whose shapes depend on the amphiphile packing geometry, which in turn depends on solvent conditions such as temperature, electrolyte and surfactant concentration; the near-spherical structures usually present at low concentrations may become elongated as the concentrations is increased, and transform into various lyotropic liquid crystals at high concentrations
The final water content of the ionic liquid (IL) was determined to be o0.1% by Karl–Fischer titration. d3-propylammonium nitrate (PAN) was synthesised by performing the acid–base reaction in deuterium oxide (D2O 99% Sigma Aldrich), dried, and subsequently washed with three molar equivalents of D2O before the excess D2O was removed by rotary evaporation for several hours at 45 1C, and dried a final time following the same procedure as for H-PAN. 1H-NMR shows that, on average, 2.85 out of 3 amino hydrogen atoms are replaced with deuterium

Summary

Introduction

Surfactants and other amphiphilic species dissolve in water because the hydrophilic head group confers solubility on the hydrophobic tail. Neutronbeam techniques enable contrast variation via isotopic substitution that exploits the different scattering cross-sections of hydrogen and deuterium This allows contributions to the scattering from different components or even individual functional groups in hydrogen-rich and organic mixtures to be amplified or isolated; here, labelling of cation and alkanol alkyl tails and polar groups will be used to establish the liquid nanostructure from the molecular scale up to that of alkanol aggregates. Its dissolution and self-assembly in PAN is examined to probe this issue

Materials and methods

Results and discussion

Conclusions