Water-carbon dioxide mixtures at high temperatures and pressures: local order in the water rich phase investigated by vibrational spectroscopy

Atomic-Scale Insights into the Phase Behavior of Carbon Dioxide and Water from 313 to 573 K and 8 to 30 MPa

We performed molecular dynamics (MD) simulations of CO2 + H2O systems by employing widely used force fields (EPM2, TraPPE, and PPL models for CO2; SPC/E and TIP4P/2005 models for H2O). The phase behavior observed in our MD simulations is consistent with the coexistence lines obtained from previous experiments and SAFT-based theoretical models for the equations of state. Our structural analysis reveals a pronounced correlation between phase transitions and the structural orderliness. Specifically, the coordination number of Ow (oxygen in H2O) around other Ow significantly correlates with phase changes. In contrast, coordination numbers pertaining to the CO2 molecules show less sensitivity to the thermodynamic state of the system. Furthermore, our data indicate that a predominant number of H2O molecules exist as monomers without forming hydrogen bonds, particularly in a CO2-rich mixture, signaling a breakdown in the hydrogen bond network's orderliness, as evidenced by a marked decrease in tetrahedrality. These insights are crucial for a deeper atomic-level understanding of phase behaviors, contributing to the well-grounded design of CO2 injection under high-pressure and high-temperature conditions, where an atomic-scale perspective of the phase behavior is still lacking.

Molecular Mechanism of Conformational Crossover of Mefenamic Acid Molecules in scCO2

In this work, we studied conformational equilibria of molecules of mefenamic acid in its diluted solution in scCO₂ under isochoric heating conditions in the temperature range of 140-210 °C along the isochore corresponding to the scCO₂ density of 1.1 of its critical value. This phase diagram range totally covers the region of conformational transitions of molecules of mefenamic acid in its saturated solution in scCO₂. We found that in the considered phase diagram region, the equilibrium of two conformers is realized in this solution. In the temperature range of 140-180 °C, conformer I related to the first, most stable polymorph of mefenamic acid prevails. In the temperature range of 200-210 °C, conformer II, which is related to the second metastable polymorph becomes dominant. Based on the results of quantum chemical calculations and experimental IR spectroscopy data on the mefenamic acid conformer populations, we classified this temperature-induced conformational crossover as an entropy-driven phenomenon.

Near Infrared Spectroscopy As an Effective Way of Studying Hydrogen Bonding in a LiCl–H2O–CO2 Ternary Mixture

Abstract

The possibility of using near IR spectroscopy to analyze the effect isobaric heating has on hydrogen bonding in an aqueous solution of LiCl in equilibrium with supercritical carbon dioxide (a LiCl–H2O–scCO2 ternary mixture) is demonstrated in a wide range of electrolyte concentrations. It is shown that this approach is highly efficient when studying ion and molecular systems with different types of interparticle interactions. The use of near IR spectroscopy allows distinguishing spectral contributions from hydrogen bonded n-mers of bulk water and water molecules in the solvation shells of ions or in ion–water chains like those formed in solutions with extremely high electrolyte concentrations that do not contain bulk water. It is shown for the studied ternary mixture that raising the concentration of electrolyte completely neutralizes the destructive effect of carbon dioxide on the formation of a hydrogen bonded structure of water. The latter is stabilized under the influence of an ion field, which also substantially weakens the temperature effect.

Self-diffusion of water-cyclohexane mixtures in supercritical conditions as studied by NMR and molecular dynamics simulation

The self-diffusion coefficients of water (D_w) and cyclohexane (D_ch) in their binary mixtures were determined using the proton pulsed field gradient spin-echo method from medium to low densities in subcritical and supercritical conditions. The density (ρ), temperature (T), and water mole fraction (x_w) are studied in the ranges 0.62-6.35 M (M = mol dm^-3), 250-400 °C, and 0.109-0.994, respectively. A polynomial fitting function was developed for a scaled value of Ξ = ρDT^-1/2 with ρ, T, and x_w as variables in combination with a comprehensive molecular dynamics (MD) simulation. The NMR and MD results agree within 5% for water and 6% for cyclohexane, on average. The differences between D_w and D_ch in the dependence on ρ, T, and x_w are characterized by the activation energy E_a and the activation volume ΔV_Ξ ^‡ expressed by the scaled fitting function. The decrease in the ratio D_w/D_ch and the increase in the E_a of water with increasing x_w are related to the increase in the number of hydrogen bonds (HBs). The D_w value for a solitary water molecule at a low x_w is controlled by the solvation shell, most of which is occupied by nonpolar cyclohexane molecules that provide less friction as a result of weaker interactions with water. A microscopic diffusion mechanism is discussed based on an analysis of the HB number as well as the first-peak height of the radial distribution functions that are taken as measures of the potential of the mean field controlling self-diffusion.

Structure-Property Relationships in CO2-philic (Co)polymers: Phase Behavior, Self-Assembly, and Stabilization of Water/CO2 Emulsions

This Review provides comprehensive guidelines for the design of CO2-philic copolymers through an exhaustive and precise coverage of factors governing the solubility of different classes of polymers. Starting from computational calculations describing the interactions of CO2 with various functionalities, we describe the phase behavior in sc-CO2 of the main families of polymers reported in literature. The self-assembly of amphiphilic copolymers of controlled architecture in supercritical carbon dioxide and their use as stabilizers for water/carbon dioxide emulsions then are covered. The relationships between the structure of such materials and their behavior in solutions and at interfaces are systematically underlined throughout these sections.

Competition between lanthanides in extraction with tri-n-butyl phosphate in supercritical CO2 from solid nitrates

Competition extraction of lanthanide nitrates with tri-n-butyl phosphate (TBP) in supercritical CO₂ (SC-CO₂) is investigated by monitoring the absorption spectra of Ln(iii) in UV-Vis region.

Supercritical carbon dioxide and its potential as a life-sustaining solvent in a planetary environment

Supercritical fluids have different properties compared to regular fluids and could play a role as life-sustaining solvents on other worlds. Even on Earth, some bacterial species have been shown to be tolerant to supercritical fluids. The special properties of supercritical fluids, which include various types of selectivities (e.g., stereo-, regio-, and chemo-selectivity) have recently been recognized in biotechnology and used to catalyze reactions that do not occur in water. One suitable example is enzymes when they are exposed to supercritical fluids such as supercritical carbon dioxide: enzymes become even more stable, because they are conformationally rigid in the dehydrated state. Furthermore, enzymes in anhydrous organic solvents exhibit a "molecular memory", i.e., the capacity to "remember" a conformational or pH state from being exposed to a previous solvent. Planetary environments with supercritical fluids, particularly supercritical carbon dioxide, exist, even on Earth (below the ocean floor), on Venus, and likely on Super-Earth type exoplanets. These planetary environments may present a possible habitat for exotic life.

Hypothesis: origin of life in deep-reaching tectonic faults

The worldwide discussion on the origin of life encounters difficulties when it comes to estimate the conditions of the early earth and to define plausible environments for the development of the first complex organic molecules. Until now, the role of the earth's crust has been more or less ignored. In our opinion, deep-reaching open, interconnected tectonic fault systems may provide possible reaction habitats ranging from nano- to centimetre and even larger dimensions for the formation of prebiotic molecules. In addition to the presence of all necessary raw materials including phosphate, as well as variable pressure and temperature conditions, we suggest that supercritical CO2 as a nonpolar solvent could have played an important role. A hypothetical model for the origin of life is proposed which will be used to design crucial experiments for the model's verification. Because all proposed processes could still occur in tectonic faults at the present time, it may be possible to detect and analyse the formation of prebiotic molecules in order to assess the validity of the proposed hypothesis.