Can Polarity-Inverted Surfactants Self-Assemble in Nonpolar Solvents

TL;DR

This study uses molecular dynamics simulations to explore the self-assembly of a polarity-inverted surfactant in water and cyclohexane, revealing stable inverted micelles in water but not in cyclohexane, and providing insights into phase behavior.

Contribution

It demonstrates the formation of inverted micelles by a novel surfactant with an adamantane head and triethylene glycol tail, validated by experimental data, and explores conditions for stable micelle formation.

Findings

Stable inverted micelles form in water but not in cyclohexane.

Simulations align with experimental diffusion and scattering data.

Mixture resides in supercritical phase with co-existing aggregated and free surfactants.

Abstract

We investigate the self-assembly process of a surfactant with inverted polarity in water and cyclohexane using both all-atom and coarse grained hybrid particle-field molecular dynamics simulations. Unlike conventional surfactants, the molecule under study, proposed in a recent experiment, is formed by a rigid and compact hydrophobic adamantane moiety, and a long and floppy triethylene glycol tail. In water, we report the formation of stable inverted micelles with the adamantane heads grouping together into a hydrophobic core, and the tails forming hydrogen bonds with water. By contrast, microsecond simulations do not provide evidence of stable micelle formation in cyclohexane. Validating the computational results by comparison with experimental diffusion constant and small-angle X-ray scattering intensity, we show that at laboratory thermodynamic conditions the mixture resides in the supercritical region of the phase diagram, where aggregated and free surfactant states co-exist in solution. Our simulations also provide indications about how to escape this region, to produce thermodynamically stable micellar aggregates.