The thermodynamics of multi-phase multi-component equilibrium in nanoconfinement

Abstract

The thermodynamics of the coexistence and the inter-conversion between different phases of water are important in many natural and technological con-texts such as the formation of ice clouds catalyzed by aerosols, water adsorption and transport in porous materials, and crystal nucleation and growth in porous rocks. The study of these processes is inherently complicated by the presence of multiple phases, complex pore geometries, and the transport phenomena at both small spatial and temporal scales. To tackle these challenges, we conducted experimental studies of model pore materials with well-defined geometries in order to better understand the underlying thermodynamics and transport kinetics. In this thesis, I first present an experimental study of freezing transition on nanoporous substrates with well-characterized pore geometries in the context of cloud formation. Building on the conceptual foundation of Pore Condensation Freezing (PCF) proposed by Fukuta and Marcolli, I conducted freezing experiments on a series of nano-porous silicon substrates that share similar surface chemistry but differ in surface structures. We found that the deposition freezing mechanism could not provide a consistent explanation for the observed data; while PCF offered an adequate explanation for the distinct freezing behaviors observed, but outstanding questions remain about the detailed mechanism of the emergence of bulk ice within pore space.Secondly, I present an experimental study on the sorption hysteresis and trans-port dynamics of water in nanochannels. I conducted an experimental study with a simple nanofluidic device to directly observe phase equilibrium and phase transformation of water in well-defined channel geometries. I documented direct evidence of sorption hysteresis due to the pore-blocking effect that was well described by the Kelvin-Laplace equation. During desorption, I observed two distinct desorption mechanisms: desorption by meniscus recession and desorption by cavitation; we studied the emptying dynamics of liquid water within these channels and found that the transport dynamics is highly dependent on the channel geometries. Finally, I present a set of preliminary studies on direct observation of solid-liquid equilibrium using the nanofluidic device aiming to provide some guide-line for the future study of crystal nucleation and growth in a porous material. In these studies, I demonstrated different experimental strategies and their ability to directly study the solid-liquid equilibrium between ice and water, and for the crystal-solution system. However, these observations appeared to be confounded by several experimental challenges that further investigation is required. In conclusion, I present different experimental efforts in probing the thermodynamics and transport of phase equilibrium and transformation in well-characterized and well-defined nanoconfinement. These experiments together provide evidence of nanoconfinement effects on phase equilibrium predicted by classical thermodynamics.