# Missing Eutectic Transition in Electrolyte Solutions in Confinement Due to Ion Accumulation in the Interfacial Layer

## TL;DR

This study explains why eutectic transitions are missing in confined electrolyte solutions due to ion accumulation at the interfacial layer.

## Contribution

The paper identifies ion accumulation in interfacial layers as the key factor inhibiting eutectic transitions in nanoconfined electrolytes.

## Key findings

- Insufficient ion supply leads to unsaturated interfacial layers and no eutectic transition.
- Sufficient ion supply allows for simultaneous salt and ice crystallization at the eutectic point.
- Interfacial layers strongly influence ion distribution and phase behavior in nanoconfinement.

## Abstract

The anomalous properties
of aqueous electrolytes under nanoconfinement
are of wide concern due to their widespread occurrence in both natural
and industrial processes. Recent studies suggest that the eutectic
transition of dilute solutions in nanopores is inhibited. In this
work, we propose that the preferential accumulation of dissolved ions
in the interfacial layer and the resulting depletion of the ions in
the core of the pore solution are the key factors contributing to
the absence of salt crystallization at the eutectic point. Systematic
calorimetric measurements were conducted with CaCl2 and
NaCl solutions confined in nanoporous silica. The results demonstrate
that the occurrence of the eutectic transition is controlled by three
main factors: salt concentration, pore size, and pore filling degree.
In confined solutions with an insufficient supply of ions, the interfacial
layer remains unsaturated, thus leading to the crystallization of
solely ice in the core and the absence of a eutectic transition. Conversely,
in more concentrated solutions or if the supply of ions is sufficient,
the interfacial layer becomes saturated, and the presence of excess
bulk-like ions in the core facilitates simultaneous salt and ice crystallization
at the eutectic point. Supply for the complete saturation of the interfacial
layer can stem either from a sufficiently large core volume, thus,
in large pores, or from diffusion from a reservoir outside of an overfilled
pore. These findings highlight the critical role of interfacial layers
influencing the ion distribution and phase behavior of electrolyte
solutions in nanoconfinement, specifically confirming sorption and
strong enrichment of ions in the interfacial layer.