New insight into the orientational order of water molecules at the water/1,2-dichloroethane interface: A Monte Carlo simulation study

Abstract

The preferential orientation of the water molecules near the water/1,2-dichloroethane interface is analyzed in detail at different distances from the interface on the basis of a grand canonical ensemble Monte Carlo simulation. The orientation of the individual water molecules is described by the angular polar coordinates of the interface normal vector in a local coordinate frame fixed to the particular water molecule, and the bivariate joint distribution of the two polar angles is calculated. It is found that water molecules have two distinct orientational preferences, and these two preferences exist simultaneously among the water molecules penetrating farthest into the organic phase. In the first preferred orientation the plane of the molecule is parallel to the interface, whereas in the second the molecular plane is aligned perpendicularly to the interface and the molecular dipole vector declines from the plane parallel to the interface by about 30° pointing toward the organic phase. The first of the two preferred orientations is found to be present in the entire interfacial region and also, to a smaller extent, in the subsurface water layer adjacent to the interface. The second orientational preference is only present among the water molecules penetrating farthest into the organic phase. The two orientations correspond to the alignment of a hydrogen bonded pair of water molecules, in which the molecule located toward the aqueous phase has the first, whereas the one on the organic side the second of the two preferred orientations. The obtained picture is in a clear contrast with the findings of previous studies, in which the orientation of the water molecules was described by monovariate distributions of the alignment of one or more selected molecule-fixed vectors. In order to understand the origin of the difference between the present results and earlier findings we also calculate the monovariate distributions of the direction of three of such molecular vectors, i.e., the dipole vector of the water molecule, the vector joining the two H atoms, and the vector perpendicular to the molecular plane. The comparison of the obtained monovariate distributions with the bivariate joint distribution of the two polar angles reveals that the averaging of the bivariate distribution over any of its two angles completely obscures the dual orientational preference. The present study clearly points out the importance of choosing appropriate statistical distributions in the analysis of simulation results and demonstrates the pitfalls of averaging over too many variables.

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

“…Nevertheless, as pointed out by Jedlovszky et al 22 , monovariate distributions of angles are insufficient to completely describe the orientation of the water molecules. Instead, one must employ a bivariate angle distribution, which uniquely defines the orientation of a given water molecule 22 .…”

“…Nevertheless, as pointed out by Jedlovszky et al 22 , monovariate distributions of angles are insufficient to completely describe the orientation of the water molecules. Instead, one must employ a bivariate angle distribution, which uniquely defines the orientation of a given water molecule 22 . In this paper, we apply Jedlovszky's method to determine the orientation of both water and organic molecules as a function of distance to the interface, using both a global and a local definition of the interfacial plane.…”

“…The angles chosen to describe the water molecule are those proposed by Jedlovszky et al 22 publication by Jedlosvsky et al 22 .…”

“…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.…”

Intrinsic Structure of the Interface of Partially Miscible Fluids: An Application to Ionic Liquids

“…To calculate the bivariate statistics of the rings we followed the methodology proposed by Jedlovszky et al: we compute the likelihood of observing any combinations of the angular polar coordinates (P (cos θ, φ)) of the macroscopic surface normal ( n) in a coordinate frame fixed to the imidazolium ring (X',Y',Z'). 76 In this frame axis Z' is chosen to point along the bisector of the vectors pointing from the C2 atom to the two N atoms, N1 and N3 (for the atomic labels see Figure 1). Axis Y' points along the N1→N3 vector while axis X' is perpendicular to Y' and Z' in a way that the (X',Y',Z') triplet satisfies the right-hand rule.…”