Abstract
In this study, classical molecular dynamic simulations have been used to examine the molecular properties of the water-alkane interface at various NaCl salt concentrations (up to 3.0 mol/kg). A variety of different force field combinations have been compared against experimental surface/interfacial tension values for the water-vapour, decane-vapour and water-decane interfaces. Six different force fields for water (SPC, SPC/E, TIP3P, TIP3Pcharmm, TIP4P & TIP4P2005), and three further force fields for alkane (TraPPE-UA, CGenFF & OPLS) have been compared to experimental data. CGenFF, OPLS-AA and TraPPE-UA all accurately reproduce the interfacial properties of decane. The TIP4P2005 (four-point) water model is shown to be the most accurate water model for predicting the interfacial properties of water. The SPC/E water model is the best three-point parameterisation of water for this purpose. The CGenFF and TraPPE parameterisations of oil accurately reproduce the interfacial tension with water using either the TIP4P2005 or SPC/E water model. The salinity dependence on surface/interfacial tension is accurately captured using the Smith & Dang parameterisation of NaCl. We observe that the Smith & Dang model slightly overestimates the surface/interfacial tensions at higher salinities (>1.5 mol/kg). This is ascribed to an overestimation of the ion exclusion at the interface.
Highlights
The liquid-liquid interface plays an important role in many physical, chemical and biological processes
The results show that the three tested force fields accurately reproduce the overall trend of surface tension at various temperatures
Our results show that both TraPPE-UA and OPLS-AA marginally underestimate the liquid density of decane at 293.15 K, and both underestimate the surface tension
Summary
The liquid-liquid interface plays an important role in many physical, chemical and biological processes. Mugele et al examined the three-phase wettability of a mineral-oil-water system, measuring contact angle variations between mica, water and alkane as a function of salt concentration and composition. The interface between water and alkane has been experimentally measured using x-ray reflectivity, as in the study of Mitrinovic et al, who measured the width of the water-hexane interface as 3.5 Å15 and Tikhonov et al, who measured the interfacial width of water-docosane (C 22 H 46) as 5.7 Å16 Such advances in experimental techniques have been mirrored in the computational realm. Partay et al were able to examine the intrinsic interface of the water-vapour system[21], whilst Hantal et al examined the interface between water and CCl422 Using such techniques improves the clarity of the molecular structuring at the interface, and is seeing increasing use when studying the water-alkane interface[23,24]. The examination of the intrinsic density profile frequently leads to more insightful observations from the simulations
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