Onion structures induced by hydrophilic and hydrophobic ions in a binary mixture

TL;DR

This paper reports the discovery of onion-like multilamellar structures in a binary mixture of D₂O and 3-methylpyridine with salt, where hydrophilic and hydrophobic ions induce phase transitions and mesophases, characterized by neutron scattering.

Contribution

It demonstrates how asymmetric ion interactions induce onion structures in a binary mixture, revealing new mesophase behavior driven by solvation effects.

Findings

Phase transition between disordered and onion phases observed.

Small-angle neutron scattering shows characteristic peaks indicating mesophase formation.

Onion structures form only at low 3MP volume fractions in D₂O-rich solvent.

Abstract

Phase transition is observed between one-phase disordered phase and an ordered phase with multi-lamellar (onion) structures in an off-critical mixture of D$_2$O and 3-methylpyridine (3MP) containing a salt at 85mM. The salt consists of hydrophilic cations and hydrophobic anions, which interact asymmetrically with the solvent composition fluctuations inducing mesophases. The structure factor of the composition distribution obtained from small-angle neutron scattering has a peak at an intermediate wave number in the disordered phase and multiple peaks in the ordered phase. Lamellar layers forming onions are composed of solvation-induced charged membranes swollen by D$_2$O. The onion phase is realized only for small volume fractions of 3MP (in D$_2$O-rich solvent).