Solute Orientational Dynamics and Surface Roughness of Water/Hydrocarbon Interfaces

Abstract

Molecular dynamics simulations of two model water/alkane interfaces are used to sort out the effect of molecular shape on the structure and dynamics of the neat interfaces and the orientational dynamics of a solute adsorbed at the interface. The two 1iquidAiquid interfaces are the watednonane interface and the one between water and a single atom representation of the nonane molecule whose potential energy function is selected to give properties close to that of nonane. We find that although the thermodynamic properties and relevant structural properties of the two interfaces are very similar, the reorientation dynamics of the solute probe are faster by about a factor of 2 in the water/"spherical"-nonane system due to different molecular packing and shape.

Molecular Dynamics Study of the Interface between Water and 2-Nitrophenyl Octyl Ether

“…This means that the interface is relatively sharp on the molecular level. The picture of the water/NPOE interface arising from our simulation results is qualitatively similar to the water/NB interface 14,30 , as well as to other water/organic interfaces 9,10,13,15,[17][18][19][20][21] .…”

“…This is not straightforward. Several previous studies 10, [13][14][15][16][17][18] have computed the total width of the interface, usually from the mean square deviation of the interface position, and applied it directly in equation (6) together with rough estimates of ξ (ranging from 0.4 to 0.9 nm). The rather loose interpretation of these parameters has led to frequent disagreement with interfacial tensions calculated by the virial route.…”

“…This method has been applied to the angle between the interface normal and the water dipole vector 10,13-15,17-21,23 , the water H-H vector 10 and the water O-H vector 14,17,19,21 . In some studies, the procedure was also applied to organic molecules 9,10,[13][14][15][17][18][19] .…”

“…This picture was confirmed in nearly all subsequent simulation studies of realistic liquid/liquid interfaces, and led to attempts to model these systems using capillary wave theory (CWT) 11,12 . Several authors 10,[13][14][15][16][17][18] have attempted to calculate the interfacial tension based on the original form of CWT, but quantitative agreement with experiment and with values calculated from the simulations by alternative routes was not always obtained.…”

“…The most obvious example of this is the density profile, which exhibits a transition from one bulk density to the other across a relatively narrow interfacial region 9,10,[13][14][15][16][17][18][19][20][21] . Other properties have been calculated for several water/organic interfaces, including radial distributions functions (RDFs) 10,15,17,19 , hydrogen bonds 9,10,[13][14][15][17][18][19]21 , molecular orientations 9,10,[13][14][15][17][18][19][20][21][22][23][24] and self-diffusion coefficients 10,[13][14][15]23 . For this purpose, the simulation box is usually divided in slices parallel to the interfacial plane and properties are computed as a function of distance to the interface.…”