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

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.