Molecular dynamics study of the n-hexane–water interface: Towards a better understanding of the liquid–liquid interfacial broadening

Abstract

By molecular dynamics simulations, we have studied the hydrophilic-hydrophobic interface between water and n-hexane liquid phases. For all temperatures studied our computed interfacial tension agrees very well with the experimental value. However, the interfacial width calculated from capillary wave theory systematically overestimates the width obtained from fitting either the total density or composition profile. We rationalize the applicability of capillary wave theory for our system by reconsidering the usual value taken for the correlation length. This is motivated by the presence of order at the interface. Possible implications for recent experimental studies on the structure of model alkane-water interfaces are discussed, including the significance of the intrinsic width parameter.

A molecular dynamics study of an archaeal tetraether lipid membrane: Comparison with a dipalmitoylphosphatidylcholine lipid bilayer

“…First, we have defined the interface position from a fit of the water density profile using an error function (see for instance Ref. 42) and then computed the orientational parameters of the dipole vector and the vector normal to the water molecule plane with respect to the membrane normal. This approach leads to the definition of an averaged position of the interface.…”

Inhomogeneous Monte Carlo simulation of the vapor-liquid equilibrium of benzene between 300 K and 530 K

“…This technique was used in studying the vapor-liquid interfaces in various systems -Lennard-Jones fluids [12], molten salts [13], water [14][15][16], aqueous solutions [17,18] etc. This method can also be modified in order to study the liquid-liquid equilibria between two immiscible liquids [19,20]. The use of boxes with interface in the Gibbs Ensemble Monte Carlo also represents a powerful technique which enables us to study the properties of interfaces between vapor and saturated solution of two immiscible liquids [21][22][23].…”

Simulations of phase separation in quiescent and sheared liquids

“…In many experimental situations this approximation is valid and greatly simplifies the theoretical analysis [62]. Although in simulation work much attention has been given to equilibrium properties of the interface, such as molecular organisation and structure and density profiles [65,71,78,83,94], no direct measurements have been reported of the dynamics of capillary interfacial fluctuations.…”

“…We expect that in most computer simulations of phase separated systems [65,71,78,83,94,102] the dynamics of interfacial fluctuations are influenced by the finite thickness of the liquid layer(s) as well. Indeed, molecular dynamics simulations are usually limited to 10 5 particles, which means that the simulation boxes are usually cubic, with fluid layers of a few nanometers thick.…”

“…Although the interface between two phases appears to be smooth on macroscopic scales, thermal excitations cause the interface to be rough on microscopic scales. The broadening of an interface by these so-called capillary waves [9,13,72] should be distinguished from its intrinsic width caused by partial mixing of the molecules of both phases [65]: for simple fluids interacting via van der Waals forces the local density of one species changes monotonically across the interface from its bulk liquid value to zero [22,43,78]. For ordinary molecular liquids, with surface tensions in the mN/m range, the capillary roughness is relatively small, in the order of a few or tens ofÅngstroms, and can be accessed experimentally by ellipsometry [7,61], X-ray scattering [23,60,75] and neutron [39,55,76] reflectometry.…”