The NaCl salt-solution interface often serves as an example of an uncharged surface. However, recent laser-Doppler electrophoresis has shown some evidence that the NaCl crystal is positively charged in its saturated solution. Using molecular dynamics (MD) simulations, we have investigated the NaCl salt-solution interface system, and calculated the solubility of the salt using the direct method and free energy calculations, which are kinetic and thermodynamic approaches, respectively. The direct method calculation uses a salt-solution combined system. When the system is equilibrated, the concentration in the solution area is the solubility. In the free energy calculation, we separately calculate the chemical potential of NaCl in two systems, the solid and the solution, using thermodynamic integration with MD simulations. When the chemical potential of NaCl in the solution phase is equal to the chemical potential of the solid phase, the concentration of the solution system is the solubility. The advantage of using two different methods is that the computational methods can be mutually verified. We found that a relatively good estimate of the solubility of the system can be obtained through comparison of the two methods. Furthermore, we found using microsecond time-scale MD simulations that the positively charged NaCl surface was induced by a combination of a sodium-rich surface and the orientation of the interfacial water molecules.

Based on molecular dynamics simulations of eight ions (Na, K, Rb, Cs, Mg, Ca, Sr, and Ba) on muscovite mica surfaces in water, we demonstrate that experimental data on the muscovite mica surface can be rationalized through a unified picture of adsorption structures including the hydration structure, cation heights from the muscovite surface, and state stability. These simulations enable us to categorize the inner-sphere surface complex into two different species: an inner-sphere surface complex in a ditrigonal cavity (IS1) and that on top of Al (IS2). By considering the presence of the two inner-sphere surface complexes, the experimental finding that the heights of adsorbed cations from the muscovite surface are proportional to the ionic radius for K and Cs but inversely proportional to the ionic radius for Ca and Ba was explained. We find that Na, Ca, Sr, and Ba can form both IS1 and IS2; K, Rb, and Cs can form only IS1; and Mg can form only IS2. It is suggested that the formation of IS1 and IS2 is governed by the charge density of the ions. Among the eight ions, we also find that the hydration structure for the outer-sphere surface complexes of divalent cations differs from that of the monovalent cations by one adsorbed water molecule (i.e., a water molecule located in a ditrigonal cavity).

The design and evaluation of a novel decahydroisoquinolin scaffold as an inhibitor for severe acute respiratory syndrome (SARS) chymotrypsin-like protease (3CL(pro)) are described. Focusing on hydrophobic interactions at the S2 site, the decahydroisoquinolin scaffold was designed by connecting the P2 site cyclohexyl group of the substrate-based inhibitor to the main-chain at the α-nitrogen atom of the P2 position via a methylene linker. Starting from a cyclohexene enantiomer obtained by salt resolution, trans-decahydroisoquinolin derivatives were synthesized. All decahydroisoquinolin inhibitors synthesized showed moderate but clear inhibitory activities for SARS 3CL(pro), which confirmed the fused ring structure of the decahydroisoquinolin functions as a novel scaffold for SARS 3CL(pro) inhibitor. X-ray crystallographic analyses of the SARS 3CL(pro) in a complex with the decahydroisoquinolin inhibitor revealed the expected interactions at the S1 and S2 sites, as well as additional interactions at the N-substituent of the inhibitor.

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In this study, we investigated the stability of asphaltene adsorption structures at the oil–water interface, focusing on the role of heteroatoms, by molecular dynamics simulations. We employed an oil (1:1 mixture of heptane and toluene, by volume)–water system and used 13 types of asphaltene molecules. Two sets of asphaltene models with the alkyl side chain at different locations were considered. For each set, six models were employed, which have essentially the same structures but with different heteroatoms (such as nitrogen, oxygen, and sulfur) on the aromatic ring (i.e., heteroaromatic ring). Besides 12 models, an additional asphaltene molecule with a carboxyl group at the end of the alkyl side chain was included. We evaluated the asphaltene adsorption Gibbs free energy at the oil–water interface using potential of mean force calculations. It is found that the basic pyridine-type nitrogen-containing asphaltene presents the highest adsorption Gibbs free energy among six asphaltene molecules for both sets. The heteroatom of the asphaltene molecule forms a hydrogen bond with the water molecules so that it can stabilize asphaltene adsorption at the oil–water interface. The strength of the hydrogen bond depends on the negative charge of the heteroatom, with the basic pyridine-type nitrogen being the highest, and the highest adsorption Gibbs free energy. Furthermore, it is found that the acidic pyrrole-type nitrogen-containing asphaltene has the most significant weak hydrogen bonding between the heteroaromatic ring and water molecules due to the charge of the carbon atom in that ring being higher than others. The thiophene-type sulfur-containing asphaltene has the most significant van der Waals interaction; the adsorption Gibbs free energy shows a significant value for both sets. The carboxyl asphaltene molecule has the highest affinity to the oil–water interface among 13 models because it has two heteroatoms. The detailed understanding of the asphaltene adsorption behavior presented in this study would be useful to solve the stability issue of oil–water emulsions in crude oil production.