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

Abstract

We present results of molecular dynamics simulations of the interface between water and 2-nitrophenyloctyl ether (NPOE). This system is analyzed in detail using a procedure to calculate intrinsic profiles of several important properties (density, radial distribution functions, hydrogen bonds, molecular orientation, self-diffusion). The interface was found to be molecularly sharp, but corrugated by thermal fluctuations. Using a method based on capillary wave theory, we have estimated the interfacial tension and obtained good agreement with values calculated from the virial route. The results were compared to simulations of the water/nitrobenzene interface. The presence of an alkyl chain in NPOE introduces an added degree of hydrophobicity, which causes an increase in the interfacial tension. Furthermore, interfacial NPOE molecules are less organized than nitrobenzene and show a distinct dynamic response. These results shed light on the observed differences between these two organic liquids in electrochemical studies.

Reactivity and Dynamics at Liquid Interfaces

“…These intrinsic orientational profiles can better identify the orientational ordering at the interface. 140,142,143,151,176,177,179 Complementary (and sometimes consistent) experimental information has been provided in recent years mainly by non linear spectroscopic methods (second harmonic and sum frequency generation), discussed below.…”

“…14 suggests that a plot of < ζ 2 > vs. ln A should be a straight line with a slope of k B T/4πγ. 142,143,148 A more direct test of the underlying assumptions of CW theory can be done by computing the spectral representation of the fluctuating surface z = ζ(s). This surface is…”

“…For example, by showing that the probability distribution of the surface height (defined as the maximum value of ζ(s) when s is sampled over a grid of varying sizes 125,135,142,143 ) is a…”

“…93,125,132,135,138,140,142,143,151,[174][175][176][177][178][179] It is interesting to note that the orientational profiles of the water dipole at the water liquid/vapor interface and at the interface between water and a non-polar liquid all exhibit similar behavior: The water dipole tends to lie parallel to the interface (possibly with a slight tilt toward the bulk) but with a broad distribution. At this orientation, the water molecule is able to maximize its ability to hydrogen bond with other water molecules.…”

“…The significant reduction in the first peak without a significant change in its position has been demonstrated to some degree for water when a non-polar solute is near the Gibbs surface. 135,142,143 This reduction reflects the diminished density of the solvent. If one normalizes the local g(r) by the local density 135 (using the density profile), any observed variation in …”