Molecular dynamics investigation of the various atomic force contributions to the interfacial tension at the supercritical CO2-water interface.

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.

Molecular dynamics simulation study of water surfaces: comparison of flexible water models.

Molecular dynamics (MD) simulation has been used extensively to study water surfaces. Nevertheless, the quantitative prediction of water surface tension has been controversial, since results from different simulation studies using the same water model may differ considerably. Recent research has suggested that bond flexibility, long-range electrostatic interactions, and certain simulation parameters, such as Lennard-Jones (LJ) cutoff distance and simulation time, may play an important role in determining the simulated surface tension. To gain better insight on the MD simulation of water surfaces, particularly on the prediction of surface tension, we examined seven flexible water models using a consistent set of simulation parameters. The surface tensions of the flexible, extended simple point charge (SPCE-F) model and the flexible three-center (F3C) model at 300 K were found to be 70.2 and 65.3 mN/m, respectively, in reasonable agreement with the experimental value of 71.7 mN/m. More importantly, however, detailed analysis of the interfacial structure and contributions from various interactions have revealed that the surface tension of water is determined by the delicate balance between intramolecular (bond stretching) and intermolecular (LJ) interactions, which reflects both the molecular orientation in the interfacial region and the density variation across the Gibbs dividing surface (GDS). In addition, the water molecules on the liquid side of the GDS were found to lie almost parallel to the surface, which helps to clarify the dual-layer structure suggested by sum-frequency generation spectroscopy. By correlating the simulated surface tensions of the seven water models with selected molecular parameters, it was found that the partial charge distribution in the water molecule is likely a key factor in determining the near-parallel alignment of water molecules with the surface.

Simulation study of charged nanoparticles confined in a rectangular tube with discrete wall charges.

The development of novel nanomaterials has been a subject of intense interest in recent years. An interesting structure among these materials is the so-called "pea pods" (i.e., nanoparticles confined in nanotubes). To facilitate the development and commercialization of these materials, it is important that we have an in-depth understanding of their behavior. The study of confined charged particles is particularly challenging because of the long-ranged nature of electrostatic interaction, and both interparticle and particle-confinement interactions are likely to play a role in determining the system behavior. The primary objective of this study is to develop a better understanding of the behavior of charged nanoparticles in a charged tubular confinement using Monte Carlo simulation, with particular focus on the effect of electrostatic interactions on the structure of the particles. Simulation results have shown that (i) the structuring of confined particles is associated with the asymmetry of the long-ranged interaction and (ii) factors such as confinement geometry and particle charge and size asymmetry can be manipulated to produce different particle structures. The present study represents the first step in an attempt to gain further insight into the behavior of confined nanosystems, with the ultimate objective of exploiting these characteristics, particularly the interactions between the confined particles and their external environment, in developing novel nanomaterials.

A simulation study of electrostatic effects on mixed ionic micelles confined between two parallel charged plates.

Confined colloidal systems have been the subject of extensive theoretical and experimental research, and the recent observation of long-range like-charge attraction in such systems has only highlighted their peculiar behavior. On the other hand, surfactant solutions are often used in small confined space, yet their behavior in confinement has received relatively little attention. A distinct feature of confined self-assembling systems is that the aggregates are capable of adjusting their composition, size, and shape in response to their external environment, which may lead to very different phase characteristics compared to bulk solutions. The primary objective of this study is to explore the effects of varying micelle composition on the structural behavior of a confined mixed ionic micellar solution. Mesoscale canonical Monte Carlo simulations were used to probe the structure of the confined solution, while a molecular-thermodynamic model was used to systematically account for the change in micelle size as we varied its composition. Significant micelle ordering was found under certain conditions, which implies that large deviations from the minimum-energy micelle configuration may not be entropically favorable. Accumulation of micelles along the midplane was observed when the confining walls are weakly charged, suggesting that micelle shape transformation should be considered in more detail. On the other hand, with high wall charge density, apparent attraction was found between like-charged micelles and wall. These findings point to the need for a more quantitative theoretical treatment in describing surfactant self-assembly in confined geometries.

Molecular-Thermodynamic Modeling of Mixed Cationic/Anionic Vesicles.

Vesicles are widely used as model cells in biology and medicine and are also potentially useful as drug carriers and other industrial encapsulating devices. To facilitate the practical implementation of vesicles, as well as to gain a fundamental understanding of the process of vesicle formation, we have developed a molecular-thermodynamic theory to describe the formation of two-component mixed vesicles in aqueous solutions. The central quantity in this theory is the free energy of vesiculation, which is calculated by carefully modeling the various free-energy contributions to vesiculation. In particular, we (i) estimate the surfactant-tail packing free energy using a mean-field approach that accounts for the conformations of the surfactant tails in the vesicle hydrophobic region, (ii) adopt a more accurate equation of state in the calculation of the surfactant-head steric repulsions, and (iii) utilize the nonlinear Poisson-Boltzmann equation to calculate the electrostatic interactions in the case of mixed cationic/anionic charged vesicles. Particular attention has also been paid to issues such as the location of the outer and inner steric-repulsion surfaces in a vesicle and the curvature correction to the interfacial tensions at the outer and inner hydrocarbon/water vesicle interfaces. By knowing only the molecular structures of the surfactants involved in vesicle formation and the solution conditions, our theory can predict a wealth of vesicle properties, including vesicle size and composition distribution, surface potentials, surface charge densities, and compositions of vesicle leaflets. More importantly, this theory enables us to gain an understanding of (i) the underlying mechanisms of stabilization in mixed cationic/anionic vesicular systems, (ii) the effect of the interplay between the various intravesicular free-energy contributions on vesiculation, and (iii) the role of the distribution of surfactant molecules between the two vesicle leaflets in vesicle formation. As an illustration, the theory has been applied to describe vesicle formation in an aqueous mixture of cetyltrimethylammonium bromide (CTAB) and sodium octyl sulfate (SOS). In this system, the vesicles are found to be stabilized entropically, with a predicted mean radius of about 1200 Å for a mixture containing 2 wt % surfactant and a CTAB/SOS weight ratio of 3/7, a value which compares well with the experimentally measured value of 1300 Å. In addition, the predicted outer surface potential of -72 mV is consistent with the measured ζ potential value. The effect of added salt on vesicle properties has also been studied using this theory, and the predicted results conform well to experimental observations.