Mechanisms Underlying the Freezing and Melting Behavior of Water Confined in Silica Nanopores as a Function of Pore Size and Pore Filling

Abstract

Uncovering the mechanisms of freezing and melting behavior in nanoconfined fluids can unlock fundamental insights into the fate and transport of fluids in soils present in cold climates. From a scientific perspective, the structural and thermodynamic behavior of confined and interfacial water has sparked significant discussions, particularly regarding the characteristics of phase transitions and spatial heterogeneity as a function of temperature and pressure. Observations frequently report interfacial unfrozen liquid layers on hydrophilic surfaces, distorted ice crystals and suppressed freezing and melting points in confined water compared to bulk water. These effects are often attributed to the restricted molecular mobility and the influence of pore surfaces. However, the exact nature of these phase transitions and the specific characteristics of the layered arrangement remain uncertain. In this study, we present an approach to elucidate the layered structural arrangement and phase transitions of confined water through integrated thermal analysis and molecular dynamics (MD) simulations. By employing Differential Scanning Calorimetry (DSC) on samples with precisely controlled pore fillings achieved through the vapor loading method, we reveal evidence of the formation of sequential water layers and estimate the individual layer widths. The observed reduction in the enthalpy of fusion during freezing relative to bulk water, alongside distinct heat flow peaks, indicates the potential occurrence of weak first – order transitions in the frozen layers. Experimental findings are further supported by classical MD simulations conducted on analogous systems confined in amorphous silica slit pores with widths ranging from 4 to 8 nm. The combined influence of cooling rate, surface hydrogen bonding, and non-bonding interactions between silica and confined water critically impact the development of interfacial ice polymorphs.