(22g) Uncovering the Thermodynamics of Capillary Phase Transitions Under Non-Isothermal Conditions

Fluids under confinement can exhibit thermodynamic properties, including phase transitions, which are drastically different from their bulk macroscopic properties. Typically, the adsorption of fluids and the associated phase transition from the vapor to the liquid state inside a nanopore has been characterized using adsorption isotherms, where the fluid uptake is measured as a function of the relative humidity at constant temperature. On the other hand, capillary phase transitions can also be thermally driven, such as that encountered in laser-induced phase transitions of water inside carbon nanotubes (CNTs) [1,2]. While temperature changes are crucial for many applications in geochemistry, electrokinetics, catalysis and chemical sensing, significant knowledge gaps remain in our understanding of capillary phase transitions under non-isothermal conditions.

In this work, we use a combination of analytical theory and all-atomistic molecular dynamics (MD) simulations [3] to investigate water confined inside CNTs in contact with bulk water reservoirs on both ends, where the heat from a separate laser source is used to induce liquid to vapor phase transitions. We develop a new theoretical framework to describe this process occurring at a constant pressure of the bulk water reservoir, but under non-isothermal conditions imposed by the laser source. Our approach involves modeling the system in a hybrid statistical mechanical ensemble, where the water molecules confined inside the CNT are in quasi-thermal equilibrium with the laser source (serving as the thermal reservoir) and separately in quasi-chemical equilibrium with the bulk water reservoir (serving as the chemical potential reservoir at ambient temperature and pressure), with the chemical potential and thermal reservoirs operating at different temperatures. To describe the phase transitions, we first formulated an analytical theory of water confined inside CNTs based on a modified Peng-Robinson equation of state, including demonstrating the importance of using the right thermodynamic condition for modeling such non-isothermal processes. Next, we utilized the theory to obtain diameter-dependent thermodynamic properties such as the phase transition temperature and the isosteric heat of adsorption, which are shown to strongly depend on the strength of the water-CNT interactions. Finally, we designed a novel algorithm involving a combination of grand canonical Monte Carlo (GCMC) and canonical MD simulations, enabling accurate prediction of phase transitions in good agreement with the results from the analytical theory. Overall, our study sheds lights on the immense potential of non-isothermal phase transitions for providing thermodynamic insights on confined fluids within CNTs and other 1D/2D nanomaterials.

References

[1] Agrawal, K. V., Shimizu, S., Drahushuk, L. W., Kilcoyne, D. & Strano, M. S. Observation of extreme phase transition temperatures of water confined inside isolated carbon nanotubes. Nature Nanotechnology 12, 267-273 (2017).

[2] Faucher, S., Kuehne, M., Oliaei, H., Misra, R. P., Li, S. X., Aluru, N. R. & Strano, M. S. Observation and Isochoric Thermodynamic Analysis of Partially Water-Filled 1.32 and 1.45 nm Diameter Carbon Nanotubes. Nano Letters 23, 389-397 (2023).

[3] Misra, R. P. & Blankschtein, D. Insights on the Role of Many-Body Polarization Effects in the Wetting of Graphitic Surfaces by Water. The Journal of Physical Chemistry C 121, 28166-28179 (2017).