Computational Investigation of the n-Alkane/Water Interface with Many-Body Potentials: The Effect of Chain Length and Ion Distributions

Abstract

Molecular dynamics simulations with polarizable potentials were carried out to investigate the n-alkane/water interface, including sodium-halide ion distributions. A new polarizable force field was developed for n-alkanes, which gave good agreement with experiment for liquid densities and heats of vaporization for different n-alkanes at different temperatures. Also, good agreement with experiment was found for alkane/water interfacial tensions for a variety of alkanes. Alkane/water interfaces with linear alkanes of different chain lengths had fairly similar properties, except that with longer alkanes, the interfacial width decreased. Water induced dipoles were reduced at the alkane/water interface in comparison with the bulk, but their induced dipoles were greater at the alkane/water interface than at the air/water interface. Furthermore, the water structure expanded at the alkane/water interface in comparison with the bulk, but this expansion was not as significant as at the air/water interface. Sodium-halide concentrated solutions at the n-octane/water interface were simulated and compared with the air/water interface. Iodide had a similar interfacial concentration at both interfaces, while bromide and chloride showed significantly reduced interfacial concentration at the alkane/water interface. Iodide’s differing behavior was linked to the fact that it has favorable hydrophobic interactions with alkanes that were not as strong for bromide and chloride with the alkanes.

Computational study on the effect of alkyl chain length on alkane–water interfacial width

“…This may be a systems size effect due to the interfacial width in the z-direction, but as can be observed it is rather small. There are density oscillations present near the interface for the alkane phases, as described in our previous work [16]. Of interest is how the different systems behave at the interface.…”

“…Also, k B is the Boltzmann constant and c is the interfacial tension. We used the interfacial tension calculated from previous work [16], which are very similar between n-hexane/water and n-nonane/water, but slightly higher for the n-nonane/water system, as is found experimentally [36]. Table 1 gives the values extracted from Eq.…”

“…There are also partial positive charges at the hydrogen atomic positions, and both a negative partial charge and point polarizability located at an m-site, which is along the bisector of the oxygen-hydrogen bonds. Alkanes were simulated using a previously developed polarizable model [16]: the model included LJ, point charges, and point polarizabilties on all atomic sites. The model had similar parameters (with differing charges) for the methyl and methylene carbons and the same parameters for all hydrogen atoms.…”

“…Previous X-ray reflectivity experiments have found that the interfacial width increases with n-alkane chain length [13][14][15]. However, recent work by us has found an opposite trend, that the longer the chain length, the smaller the interfacial width [16]. The molecular simulations results only investigated the separate water and alkane interfacial widths extracted from specific densities.…”

Nanoscale Structure of the Oil-Water Interface

“…The exact nature, and origin, of this layer remains controversial, and assigned either to water depletion [13][14][15] or to enrichment by the lowerdensity, hydrogen-rich terminal CH 3 groups of the monolayer [15,18]. Since both CH 3 and CH 2 groups are expected to be present at the n-alkane/water interface [2,19] the depletion behavior may differ compared to the SAMs [20]. To address these issues, we employed XR measurements and MD simulations to probe and model the interface of water with liquid normal-alkanes (CH 3 (CH 2 ) n−2 CH 3 , denoted C n , with n = 6, 12, and 16).…”