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.

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.

The treatment of hazardous wastes using cement-based solidification–stabilization (S–S) is of increasing importance as an option for remediating contaminated sites. Indeed, among the various treatment techniques, S–S is one of the most widely used methods for treating inorganic wastes. To enhance the application of S–S and to further develop this technology for site remediation, particularly for organic contaminants, it is important to have a better understanding of the mechanisms involved in the process. The primary objective of this review is to survey the current knowledge in this subject, focusing on (i) cement chemistry, (ii) the effects of inorganic (heavy metals) and organic compounds on cement hydration, and (iii) the mechanisms of immobilization of different organic and inorganic compounds. For heavy metals, cement-based S–S technology has been shown to be effective in immobilizing the contaminants, even without any additives. In applying cement-based S–S for treating organic contaminants, the use of adsorbents such as organophilic clay and activated carbon, either as a pretreatment or as additives in the cement mix, can improve contaminant immobilization in the solidified–stabilized wastes. The concept of degradative solidification–stabilization, which combines chemical degradation with conventional solidification–stabilization, seems promising, although further study is required to assess its technical and economic feasibility.Key words: cement, contaminated soil, immobilization, organics, precipitation, adsorption.

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.