Efforts have been under way for about a decade to develop the knowledge necessary to safely and efficiently sequester CO2 in geologic formations [1,2]. However, most published research on these topics has focused on field scale models [3-5] and experiments [6,7]. There is a mounting body of evidence that microscropic-scale phenomena play important roles in determining the behavior of CO2 in geologic systems. These include, for example, details of multiphase flow in pore networks , the coupling of pore-scale multiphase flow and mechanics , the properties of residual water films in geologic media invaded by CO2 , the adsorption and diffusion of CO2 in caprocks , the dehydration of clays induced by “dry” CO2 , CO2- water interfacial tension [8,12,13], wetting angles [8,14], and capillary breakthrough pressures in geologic conditions [10,15], the chemical reactivity of CO2- saturated water with nanoporous clay caprocks [15,16], the properties of CO2 hydrate clathrates in marine sediments [17,18], the phase properties of CO2- brine systems in geologic media , the molecular-scale properties of HCO3- and CO32- , and many more. All these phenomena must be understood at the molecular level in order to accurately predict, and eventually control, the fate of geologic CO2.
It turns out that microscopic-scale simulations and experiments already have provided many insights into certain phenomena relevant to CO2 sequestration. For example, microscopic-scale techniques have been used successfully to determine the thermodynamics, structure, and dynamics of clays, an important component of shale or mudstone caprocks that may be used to trap CO2 in geologic formations [1,2,11]. Important issues such as clay swelling, its hysteresis and its dependence on the counterions [21,22], the structure and dynamics of clay interlayers [23,24], the adsorption of volatile molecules , ion sorption and the properties of water at external surfaces of clay particles [26-28], ion exchange thermodynamics and mechanism [29-30], or the properties of clay edges [30,31] have been investigated using ab-initio or classical simulations. A fruitful dialog between experiments and simulations has driven many advances, such as improvements in force fields [32,33] and interpretations of the relationships between micro- and macroscopic scale properties .
Many of the microscopic-scale phenomena listed above for CO2 sequestration, however, have only recently begun to be scrutinized through experimental and simulation methods. For example, although CO2- brine interfacial tension is an important microscopic-scale property that strongly influences the capillary breakthrough pressure of CO2 in porous media, this property has been very scarcely studied. Experimental techniques for the measurement of CO2- water interfacial tension are still being perfected , and the results of different groups differ, for example, on the temperature-dependence of CO2- water interfacial tension [8,14]. Few molecular dynamics  or Monte Carlo simulation  estimates of CO2- water interfacial tension exist, and these have not yet addressed topics such as the sensitivity of simulation results to the choice of interatomic potential parameters or the relationship between interfacial tension and molecular-scale structure at the interface. Very few experimental data  and no simulation results have yet been reported on the influence of salinity on interfacial tension.