Mesoscale phenomena in solutions of 3-methylpyridine, heavy water, and an antagonistic salt

TL;DR

This study clarifies mesoscale inhomogeneities in 3-methylpyridine solutions, identifying stable droplets caused by impurities, confirming inherent periodic structures, and classifying the critical behavior within the 3D Ising universality class.

Contribution

It provides a comprehensive analysis combining DLS and SANS to elucidate the origin of mesoscale phenomena and critical behavior in 3MP solutions, resolving previous controversies.

Findings

Mesoscale droplets are caused by hydrophobic impurities in 3MP.

A periodic structure of about 10 nm is inherent to the solution.

Critical behavior follows the 3D Ising universality class.

Abstract

We have investigated controversial issues regarding the mesoscale behavior of 3-methylpyridine (3MP), heavy water, and sodium tetraphenylborate (NaBPh4) solutions by combining results obtained from dynamic light scattering (DLS) and small-angle neutron scattering (SANS). We have addressed three questions: (i) what is the origin of the mesoscale inhomogeneities (order of 100 nm in size) manifested by the "slow mode" in DLS? (ii) Is the periodic structure observed from SANS an inherent property of this system? (iii) What is the universality class of critical behavior in this system? Our results confirm that the "slow mode" observed from DLS experiments corresponds to long-lived, highly stable mesoscale droplets (order of 100 nm in size), which occur only when the solute (3MP) is contaminated by hydrophobic impurities. SANS data confirm the presence of a periodic structure with a periodicity of about 10 nm. This periodic structure cannot be eliminated by nanopore filtration and thus is indeed an inherent solution property. The critical behavior of this system, in the range of concentration and temperatures investigated by DLS experiments, indicates that the criticality belongs to the universality class of the 3-dimensional Ising model.