Direct Evaluation of Solid–Liquid Equilibria by Molecular Dynamics Using Gibbs-Duhem Integration

Room temperature electrofreezing of water yields a missing dense ice phase in the phase diagram

Water can freeze into diverse ice polymorphs depending on the external conditions such as temperature (T) and pressure (P). Herein, molecular dynamics simulations show evidence of a high-density orthorhombic phase, termed ice χ, forming spontaneously from liquid water at room temperature under high-pressure and high external electric field. Using free-energy computations based on the Einstein molecule approach, we show that ice χ is an additional phase introduced to the state-of-the-art T–P phase diagram. The χ phase is the most stable structure in the high-pressure/low-temperature region, located between ice II and ice VI, and next to ice V exhibiting two triple points at 6.06 kbar/131.23 K and 9.45 kbar/144.24 K, respectively. A possible explanation for the missing ice phase in the T–P phase diagram is that ice χ is a rare polarized ferroelectric phase, whose nucleation/growth occurs only under very high electric fields.

Water can crystallize in different ice polymorphs according to temperature and pressure conditions. Here the authors predict by molecular dynamics simulations a new ice phase spontaneously forming at room temperature under high pressure and high electric field.

Modeling and Simulation Environments for Sustainable Low-Carbon Energy Production – A Review

This paper reviews research trends in modeling for low-carbon energy production. The focus is on two currently significant low-carbon energy processes; namely, bioenergy and post-combustion carbon capture (PCC) processes. The fundamentals of these two processes are discussed and the role of modeling and simulation tools (MSTs) is highlighted. The most popular modeling software packages are identified and their use in the literature is analyzed. Among commercially available packages, it is found that no single software package can handle all process development needs such as, configuration studies, techno-economic analysis, exergy optimization, and process integration. This review also suggests that optimal modeling results reported in literature can be viewed as optimal at the individual plant level, but sub-optimal for plant superstructure level. This review has identified key gaps pertinent to developing hybrid models that describe integrated energy production processes. ASPEN Plus is found to be dominant for modeling both bioenergy and PCC processes for both steady-state and dynamic modes respectively.

A new phase diagram of water under negative pressure: The rise of the lowest-density clathrate s-III

Ice and ice clathrate are not only omnipresent across polar regions of Earth or under terrestrial oceans but also ubiquitous in the solar system such as on comets, asteroids, or icy moons of the giant planets. Depending on the surrounding environment (temperature and pressure), ice alone exhibits an exceptionally rich and complicated phase diagram with 17 known crystalline polymorphs. Water molecules also form clathrate compounds with inclusion of guest molecules, such as cubic structure I (s-I), cubic structure II (s-II), hexagonal structure H (s-H), tetragonal structure T (s-T), and tetragonal structure K (s-K). Recently, guest-free clathrate structure II (s-II), also known as ice XVI located in the negative-pressure region of the phase diagram of water, is synthesized in the laboratory and motivates scientists to reexamine other ice clathrates with low density. Using extensive Monte Carlo packing algorithm and dispersion-corrected density functional theory optimization, we predict a crystalline clathrate of cubic structure III (s-III) composed of two large icosihexahedral cavities (8(6)6(8)4(12)) and six small decahedral cavities (8(2)4(8)) per unit cell, which is dynamically stable by itself and can be fully stabilized by encapsulating an appropriate guest molecule in the large cavity. A new phase diagram of water ice with TIP4P/2005 (four-point transferable intermolecular potential/2005) model potential is constructed by considering a variety of candidate phases. The guest-free s-III clathrate with ultralow density overtakes s-II and s-H phases and emerges as the most stable ice polymorph in the pressure region below -5834 bar at 0 K and below -3411 bar at 300 K.