Direct Observation of the Vapor-Liquid Phase Transition and Hysteresis in 2 nm Nanochannels

The characterization of fluid phase transitions in nanoscale pores remains a challenging problem that can significantly affect various applications, such as drug delivery, carbon dioxide storage, and enhanced oil recovery. Previous theoretical and experimental studies have shown that the fluid phase transition changes drastically when the fluid is confined within nanocapillaries with dimensions of <10 nm, potentially due to the dominance of fluid-surface interactions compared to bulk effects. However, due to challenges in performing experiments at the nanoscale, there have been limited experimental observations of the phase transition at this scale. Recent advances in lab-on-a-chip (LOC) technology have enabled the observation of many nanoscale phenomena. In this study, for the first time, we present the direct observation and visualization of n-butane vapor-iquid phase transitions in a 2 nm slit pore using LOC technology. Our experiments, for the first time, measured and directly visualized the deviation of the vapor-liquid phase transition pressure in a 2 nm slit pore compared to the associated unconfined or bulk value. We also measured the liquid-vapor phase transition pressure and observed a significant difference from the vapor-liquid phase transition pressure. We complemented our experimental observations with grand canonical ensemble Monte Carlo molecular simulations to understand the underlying molecular-level mechanisms.

Automated summary

The characterization of fluid phase transitions in nanoscale pores is important for various applications and poses challenges due to the dominance of fluid-surface interactions at this scale. Limited experimental observations have been made due to difficulties in performing experiments at the nanoscale. In this study, for the first time, the vapor-liquid phase transitions of n-butane in a 2 nm slit pore were directly observed and visualized using lab-on-a-chip technology. The experiments measured the deviation of the phase transition pressure compared to the unconfined or bulk value, and revealed significant differences between the liquid-vapor and vapor-liquid phase transition pressures.