Electron Transfer Mechanism at the Oil/Water Interface Revealed by Multidimensional Free Energy Calculations

Boundary effects and quadrupole contribution in sum frequency generation spectroscopy

Calculation of time correlation functions is a primary task in the computational analysis of sum frequency generation spectroscopy. This paper resolved basic issues to extract interface signals from the calculation. These issues stem from the boundary to restrict the bulk region, which renders the practical computation feasible at a finite and affordable cost. The boundary is found to have significant influences on the time correlation functions, which is closely related to the quadrupole contribution in the nonlinear susceptibility. Thus, we thoroughly examined these influences to establish a proper treatment in performing reliable spectroscopic analysis. We elucidated the distinction of the present boundary effects from the quadrupole contribution and also established a proper center of molecule to minimize the quadrupole effect in the time correlation functions. In the case of liquid water, the proper center was found to be close to the center of mass of a water molecule.

A hydrogen bond-modulated soft nanoscale water channel for ion transport through liquid-liquid interfaces

Ion transport through interfaces is of ubiquitous importance in many fields such as electrochemistry, emulsion stabilization, phase transfer catalysis, liquid-liquid extraction and enhanced oil recovery. However, the knowledge of interfacial structures that significantly affect ion transport through liquid-liquid interfaces is still lacking due to the difficulty of observing nanoscale interfaces. We studied here the evolution of interfacial structures during ion transport through the decane-water interface under different ionic concentrations and external forces using molecular dynamics simulations. The roles of hydrogen bonds in ion transport through interfaces are revealed. We identified a soft nanoscale channel during ion transport through liquid-liquid interfaces and the decane phase under specific external force. The stability of the water channel and the ion transport velocity both increase with ionic concentration due to the layered ordering structures of the water near the channel surface. We observed that the stability and connectivity of the water channel in the decane phase are remarkably improved both by the high increase of the number of hydrogen bonds in the water channel with increasing ionic concentration, and by the conformational change in water molecules near the water channel surface. Our discovery of a soft nanoscale water channel by molecular simulations implies that there is a potential stable passage for ion transport through liquid-liquid interfaces.

Recent progress in simulating microscopic ion transport mechanisms at liquid-liquid interfaces

Transport of ions through liquid-liquid interfaces is of fundamental importance to a wide variety of applications. However, since it is quite challenging for experimentalists to directly and selectively observe molecules at the interfaces, microscopic mechanisms of ion transport have been largely presumed from kinetic information. This Perspective illustrates recent examples that molecular dynamics simulations with proper free energy surfaces clarified mechanistic pictures of ion transport. The key is a proper choice of coordinates and defining/calculating free energy surfaces in multidimensional space. Once the free energy surfaces for realistic systems are available, they naturally provide new insight into the ion transport in unprecedented details, including water finger, transient ion pairing, and electron transfer.