Catanionic and chain-packing effects on surfactant self-assembly in the ionic liquid ethylammonium nitrate

Saffron J Bryant; Charl J Jafta; Rob Atkin; Michael Gradzielski; Gregory G Warr

doi:10.1016/j.jcis.2019.01.048

Catanionic and chain-packing effects on surfactant self-assembly in the ionic liquid ethylammonium nitrate

J Colloid Interface Sci. 2019 Mar 22:540:515-523. doi: 10.1016/j.jcis.2019.01.048. Epub 2019 Jan 14.

Authors

Saffron J Bryant¹, Charl J Jafta², Rob Atkin³, Michael Gradzielski⁴, Gregory G Warr⁵

Affiliations

¹ School of Chemistry and University of Sydney Nano Institute, The University of Sydney, NSW 2006, Australia.
² Helmholtz Zentrum Berlin, D-14109 Berlin, Germany.
³ School of Molecular Sciences, The University of Western Australia, WA 6009, Australia.
⁴ Institute of Chemistry, Technical University Berlin, Strasse des 17. Juni 124, D-10623 Berlin, Germany.
⁵ School of Chemistry and University of Sydney Nano Institute, The University of Sydney, NSW 2006, Australia. Electronic address: gregory.warr@sydney.edu.au.

PMID: 30669108
DOI: 10.1016/j.jcis.2019.01.048

Abstract

Hypothesis: Micelle formation, particularly the formation of compact, globular micelles in the high ionic strength environment of an ionic liquid, seems to be at odds with the principle of opposing forces and the concept of the surfactant packing parameter. Here we examine how interactions between polar head-groups, with solvent ions, and chain packing affect self-assembly structure in ionic liquids.

Experiments: The self-assembly of pure cationic and anionic surfactants, their catanionic mixtures, and the effect of the cosurfactant tetradecanol in the ionic liquid ethylammonium nitrate (EAN) is investigated by small-angle neutron scattering (SANS).

Findings: The critical micelle concentrations of cationic surfactants are much higher and micelles much smaller in EAN than in water, whereas anionic micelles are less dramatically affected. Catanionic surfactant mixtures form small micelles in EAN at all compositions, unlike their counterparts in water which form vesicle dispersions or precipitates near 1:1 mol ratios. This contrasts with the behavior of cationic surfactants when mixed with double-chained analogs or alkanol cosurfactants, which induce micelle growth and shape transitions to rod-like and bilayer aggregates through chain packing effects similar to aqueous systems. This enables the aggregate morphology in the ionic liquid to be controlled through the composition of the surfactant mixtures.

Keywords: Catanionic surfactant; Cosurfactant; Ionic liquid; Mixed micelle; Neutron scattering.