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The behaviors of oil–water–solid interfaces are of basic and applied importance for many fields, such as energy, geochemistry, and biomedical sciences, yet molecular mechanisms are still poorly known. In particular, the traditional adsorption–desorption mechanism based on a single compound does not work to describe the contact line for liquid–liquid–solid systems where the two liquids have similar interactions with the solid surface. Here, we present a series of experiments of water spreading in alkanes, i.e., n-decane, n-dodecane, and n-hexadecane, under various surface wettabilities. The contact line motion is captured by the high-speed camera, allowing us to analyze the friction coefficient, displacement distance and frequency, and activation free energy during the contact line motion via the molecular kinetic theory. We further develop a friction model of contact line motion in liquid–liquid–solid systems by establishing the activation free energy of liquid–liquid–solid systems as a function of the activation free energies of separate one-liquid systems. Results show that the contact line friction coefficients increase with the increasing hydrophobicity (e.g., for n-decane, the coefficient changes from 0.3072 to 2.1967 Pa·s), explained by the increase in the displacement distance and sharp decrease in the displacement frequency of the molecules. Additionally, we find a non-monotonic dependence of the contribution of the surface part to the activation free energy on the wettability, with a maximum percentage of 70%–80% at the intermediate wettability, where the reduced collisions of liquid molecules weaken the molecular adsorption/desorption process. Finally, we find that the correlation coefficient n in our model is affected by the surface wettability, with a maximum at the intermediate wettability, showing a good prediction of the results in the literature. The reason can be attributed to the non-monotonic effects of wettability on the surface part of the activation free energy in two-liquid systems. [ABSTRACT FROM AUTHOR]

The synergistic effect of ions and nanoparticles on the interfacial tension is of great significance for extensive applications in interface-related industrial processes. However, its mechanisms are still unclear owing to a lack of understanding on the interaction between nanoparticles/ions at the interface. Here, we employ the molecular dynamics method to explore the synergistic effect of ions and nanoparticles on reducing the decane–water interfacial tension and reveal the dominant role of the three-phase contact angle and the interaction between nanoparticles. The results show that the reduction of interfacial tension is sensitive to cation species and temperature. The stronger hydration of cations induces an increased three-phase contact angle, weakening the interaction between nanoparticles and water molecules at the interface. Hence, the virial term of interfacial tension decreases. Meanwhile, the potential of mean force between nanoparticles at the interface indicates that the order of interaction strength between nanoparticles for different cations is Ca2+ ＞ Mg2+ ＞ Na+. The strong interaction between nanoparticles restricts the motion of nanoparticles and water molecules at the interface, inducing a reduced kinetic energy term of interfacial tension. Therefore, the interfacial tension decreases after adding the nanoparticles. Besides, as temperature rises, the difference in the adsorption ability of nanoparticles on water molecules causes a falling interfacial tension with a characteristic stage. [ABSTRACT FROM AUTHOR]

Building a connection between the variations in interfacial tension and the microstructure of the oil–water interface is still very challenging. Here, we employ a molecular dynamics method to study the effect of monovalent ions on the decane–water interfacial tension and reveal the relationship between ionic hydration and the variation of interfacial tension. Our results indicate that interfacial tension presents a non-monotonic dependence on the ionic concentrations owing to the distinctive adsorption characteristics of ions. At low ionic concentrations, the hydration of the discrete ions at the interface causes an enhancement in the virial term of the interfacial tension, resulting in an increase of the interfacial tension with increasing ionic concentrations. At high ionic concentrations, the ion pairs at the interface weaken the ionic hydration, thus the virial term of the interfacial tension decreases and the interfacial tension decreases slightly. In addition, the kinetic energy term of interfacial tension increases only with increasing temperature, while the virial term decreases with an increase in either temperature or pressure on account of the weakening ionic hydration; therefore, the increase of temperature and pressure induces different degrees of the decrease in the interfacial tension owing to the major contribution of the virial term, particularly at high ionic concentrations. [ABSTRACT FROM AUTHOR]

We identify the inhibition effect of a non-permeating gas component on gases permeating through the nanoporous graphene membranes and reveal its mechanisms from molecular dynamics insights. The membrane separation process involves the gas mixtures of CH4/H2 and CH4/N2 with different partial pressures of the non-permeating gas component (CH4). The results show that the permeance of the H2 and N2 molecules decreases sharply in the presence of the CH4 molecules. The permeance of the N2 molecules can be reduced to as much as 64.5%. The adsorption of the CH4 molecules on the graphene surface weakens the surface adsorption of the H2 and N2 molecules due to a competitive mechanism, accordingly reducing the permeability of the H2 and N2 molecules. For the N2 molecules with stronger adsorption ability, the reduction of the permeance is greater. On the other hand, the CH4 molecules near the nanopore have a blocking effect, which further inhibits the permeation of the H2 and N2 molecules. In addition, we predict the selectivity of the nanopore by using density functional theory calculations. This work can provide valuable guidance for the application of nanoporous graphene membranes in the separation of the gas mixtures consisting of permeating and non-permeating components with different adsorption abilities. [ABSTRACT FROM AUTHOR]

The movement of the contact line in liquid-liquid-solid systems is a major phenomenon in natural and industrial processes. In particular, n-alkanes are widely occurring in the oil, soil pollution, and chemical industries, yet there is little knowledge on the effects of molecular chain length on the contact line movement. Here, we studied the effects of molecular chain length on the contact line movement in water/n-alkane/solid systems with different surface wettabilities. We used n-heptane (C7), n-decane (C10), and n-hexadecane (C16) as alkanes and α-quartz as the solid surface. We calculated the time-variation contact line moving velocity and also analyzed the jump frequency and the mean distance of the molecular displacement occurring within the contact line zone by molecular-kinetic theory. Molecular dynamics simulation results show that the contact line velocity decreases with increasing the chain length, originally caused by the decreasing the jump frequency and mean distance. These variations with the molecular chain length are related to the more torsions and deformations of the molecules with a longer chain length. In addition, the moving mechanism of the contact line on the same solid surface does not change at different molecular chain lengths, implying that the moving mechanism mainly depends on the three-phase wettability. [ABSTRACT FROM AUTHOR]

Understanding ion transport across interfaces is of fundamental importance in many processes such as liquid-liquid extraction, phase transfer catalysis, enhanced oil recovery and emulsion stabilisation. However, the factors that control ion transport across interfaces are poorly known due to a lack of knowledge of structural changes at interfaces. We studied here the effects of ionic concentration and external force on the transport of ions across the decane-water interface using classical molecular dynamics simulations. The results show that the evolution of interfacial structures during ion transfer across the interface is controlled by hydrogen bonding and ionic interactions at the interface. We also identified a new mode of ion transfer across the interface at low ionic concentrations, involving a 'water ridge', rather that the classical 'water finger'. In the water ridge mode, hydrogen bonds are not broken due to low ion levels, and the water ridge induces gradual interface deformation. Whereas, at high ionic concentrations, hydrogen bonds are broken by the strong ion electrostatic repulsion, thus inducing the formation of a water finger. We also found that the variation of the Gibbs free energy during ion transfer is directly relevant to the ionic concentration. The water ridge at low ionic concentrations, which displaces more water molecules towards the decane phase, induces less free energy variation than the water finger at high ionic concentrations.

The movement of the contact line in liquid-liquid-solid systems is a major phenomenon in natural and industrial processes. In particular, n -alkanes are widely occurring in the oil, soil pollution, and chemical industries, yet there is little knowledge on the effects of molecular chain length on the contact line movement. Here, we studied the effects of molecular chain length on the contact line movement in water/ n -alkane/solid systems with different surface wettabilities. We used n -heptane (C 7 ), n -decane (C 10 ), and n -hexadecane (C 16 ) as alkanes and α -quartz as the solid surface. We calculated the time-variation contact line moving velocity and also analyzed the jump frequency and the mean distance of the molecular displacement occurring within the contact line zone by molecular-kinetic theory. Molecular dynamics simulation results show that the contact line velocity decreases with increasing the chain length, originally caused by the decreasing the jump frequency and mean distance. These variations with the molecular chain length are related to the more torsions and deformations of the molecules with a longer chain length. In addition, the moving mechanism of the contact line on the same solid surface does not change at different molecular chain lengths, implying that the moving mechanism mainly depends on the three-phase wettability.