Sequestration of carbon dioxide (CO(2)) in deep, geological formations involves the injection of supercritical CO(2) into depleted reservoirs containing fluids such as brine or oil. The interfacial tension (IFT) between supercritical CO(2) and the reservoir fluid is an important contribution to the sequestration efficiency. In turn, the IFT is a complex function of the reservoir fluid phase composition, the molecular structure of each reservoir fluid component, and environmental conditions (i.e., temperature and pressure). Molecular dynamics simulations can be used to probe the dependence of the IFT on these factors, since the IFT can be calculated directly from the simulated atomic forces and velocities at system equilibrium using the mechanical definition of the IFT. Here, we examine the contribution of each type of atomic force to the IFT, including bonded and nonbonded forces, as quantified by the anisotropy of the atomic virial tensor. In particular, we first examine a supercritical CO(2)-pure liquid water interface, at typical reservoir conditions (temperature of 343 K and pressure of 20 MPa), as a reference state against which CO(2)-brine systems can be compared. In this system, we note that the interactions between water molecules and between CO(2) molecules ("self" interactions) contribute positively to the IFT, while the interactions between water and CO(2) molecules ("cross" interactions) contribute negatively to the IFT. We find that the magnitude of the water "self" interactions is the dominant contribution. In terms of specific types of forces, we find that nonbonded electrostatic (QQ), bonded angle-bending, and bonded bond-stretching interactions contribute positively to the IFT, while nonbonded Lennard-Jones (LJ) interactions contribute negatively to the IFT. We also find that the balance between the LJ interactions and the bond-stretching interactions, in particular, plays a significant role in determining the magnitude of the IFT. Using orientational probability distribution functions to study molecular ordering about the interface, we find that the CO(2) molecules prefer to lie parallel to the interface at the Gibbs dividing surface (GDS) and that both the CO(2) and the water molecules are more ordered at the GDS than in the bulk. Finally, we present an initial study of a CO(2)-brine system with CaCl(2) as the model salt at a concentration of 2.7 M. We quantify the effect of the salt on the molecular orientation of water, and show that this effect leads to an increase in the IFT, relative to the CO(2)-water system, which is consistent with experimental measurements.

For geological CO2 storage in deep saline aquifers, the interfacial tension (IFT) between supercritical CO2 and brine is critical for the storage security and design of the storage capacitance. However, currently, no predictive model exists to determine the IFT of supercritical CO2 against complex electrolyte solutions involving various mixed salt species at different concentrations and compositions. In this paper, we use molecular dynamics (MD) simulations to investigate the effect of salt ions on the incremental IFT at the supercritical CO2-brine interface with respect to that at the reference supercritical CO2-water interface. Supercritical CO2-NaCl solution, CO2-CaCl2 solution and CO2-(NaCl+CaCl2) mixed solution systems are simulated at 343 K and 20 MPa under different salinities and salt compositions. We find that the valence of the cations is the primary contributor to the variation in IFT, while the Lennard-Jones potentials for the cations pose a smaller impact on the IFT. Interestingly, the incremental IFT exhibits a general linear correlation with the ionic strength in the above three electrolyte systems, and the slopes are almost identical and independent of the solution types. Based on this finding, a universal predictive formula for IFTs of CO2-complex electrolyte solution systems is established, as a function of ionic strength, temperature, and pressure. The predicted IFTs using the established formula agree perfectly (with a high statistical confidence level of ∼96%) with a wide range of experimental data for CO2 interfacing with different electrolyte solutions, such as those involving MgCl2 and Na2SO4. This work provides an efficient and accurate route to directly predict IFTs in supercritical CO2-complex electrolyte solution systems for practical engineering applications, such as geological CO2 sequestration in deep saline aquifers and other interfacial systems involving complex electrolyte solutions.

Amine-based CO 2 chemisorption has been a longstanding motif under development for CO 2 capture applications, but large energy penalties are required to thermally cleave the N-C bond and regenerate CO 2 for subsequent storage or utilization. Instead, it is attractive to be able to directly perform electrochemical reactions on the amine solutions with loaded CO 2. We recently found that such a process is viable in dimethyl sulfoxide (DMSO) if an exogenous Li-based salt is present, leading to formation of CO 2-derived products through electrochemical N-C bond cleavage. However, the detailed influence of the salt on the electrochemical reactions was not understood. Here, we investigate the role of individual electrolyte salt constituents across multiple cations and anions in DMSO to gain improved insight into the salt's role in these complex electrolytes. While the anion appears to have minor effect, the cation is found to strongly modulate the thermochemistry of the amine-CO 2 through electrostatic interactions: 1 H NMR measurements show that post-capture, pre-reduction equilibrium proportions of the formed cation-associated carbamate vary by up to five-fold, and increase with the cation's Lewis acidity (e.g. from K + → Na + → Li +). This trend is quantitatively supported by DFT calculations of the free energy of formation of these alkali-associated adducts. Upon electrochemical reduction, however, the current densities follow an opposing trend, with enhanced reaction rates obtained with the lowest Lewis-acidity cation, K +. Meanwhile, molecular dynamics simulations indicate significant increases in desolvation and pairing kinetics that occur with K +. These findings suggest that, in addition to strongly affecting the speciation of amine-CO 2 adducts, the cation's pairing with-COOin the amine-CO 2 adduct can significantly hinder or enhance the rates of electrochemical reactions at moderate overpotentials. Consequently, designing electrolytes to promote fast cation-transfer appears important for obtaining higher current densities in future systems.

In the carbon dioxide (CO 2 )-enhanced oil recovery (EOR) process and the subsequent geological CO 2 sequestration, a ternary system consisting of CO 2 , crude oil and brine exists in the reservoir due to the common practice of injecting CO 2 together with brine. In this paper, we carried out molecular dynamics simulations to study the interfacial properties of the ternary CO 2 , hexane and 1.52 mol/L sodium chloride (NaCl) solution system under 330 K and 20 MPa with different CO 2 compositions at the supercritical state, which are very important for the efficiency of the EOR and CO 2 sequestration processes. We observed that CO 2 mixes well with hexane and a clear interface separates the CO 2 -hexane mixture with the NaCl solution. The interfacial roughness increases with the CO 2 composition, indicating deeper molecular penetrations and shorter capillary wave lengths, which leads to the reduced interfacial tension. Interestingly, the surface excess of CO 2 reaches maximum at a CO 2 molar fraction of 62.5% (or a weight fraction of 46%), which implies the amphiphilic feature of CO 2 , acting like surfactants, towards the hexane-brine interface. The orientational preferences of CO 2 , hexane and water molecules at the interface are more random at higher CO 2 compositions, as a result of the increased absolute amount of CO 2 and the absence of hexane at the interface.

Block copolyelectrolytes are solid-state singleion conductors which phase separate into ubiquitous microdomains to enable both high ion transference number and structural integrity. Ion transport in these charged block copolymers highly depends on the nanoscale microdomain morphology; however, the influence of electrostatic interactions on morphology and ion diffusion pathways in block copolyelectrolytes remains an obscure feature. In this paper, we systematically predict the phase diagram and morphology of diblock copolyelectrolytes using a modified dissipative particle dynamics simulation framework, considering both explicit electrostatic interactions and ion diffusion dynamics. Various experimentally controllable conditions are considered here, including block volume fraction, Flory−Huggins parameter, block charge fraction or ion concentration, and dielectric constant. Boundaries for microphase transitions are identified based on the computed structure factors, mimicking small-angle X-ray scattering patterns. Furthermore, we develop a novel "diffusivity tensor" approach to predict the degree of anisotropy in ion diffusivity along the principal microdomain orientations, which leads to highthroughput mapping of phase-dependent ion transport properties. Inclusion of ions leads to a significant leftward and upward shift of the phase diagram due to ion-induced excluded volume, increased entropy of mixing, and reduced interfacial tension between dissimilar blocks. Interestingly, we discover that the inverse topology gyroid and cylindrical phases are ideal candidates for solid-state electrolytes in metal-ion batteries. These inverse phases exhibit an optimal combination of high ion conductivity, well-percolated diffusion pathways, and mechanical robustness. Finally, we find that higher dielectric constants can lead to higher ion diffusivity by reducing electrostatic cohesions between the charged block and counterions to facilitate ion diffusion across block microdomain interfaces. This work significantly expands the design space for emerging block copolyelectrolytes and motivates future efforts to explore inverse phases to avoid engineering hurdles of aligning microdomains or removing grain boundaries.