(243e) Application of Classical Density Functional Theory to the n-Alkane / Water Interface: Accurate Interfacial Properties Using Bulk Phase Parameters

The liquid / liquid interface between oil rich and water rich phases is a fundamental component of many physical and biological systems. Important properties of these interfaces include interfacial tension, width, density profiles and molecular orientation. The interfacial tension is relatively straight forward to measure using pendant drop techniques; however the isolated nature of the liquid / liquid interface makes measurement of structural quantities such as interfacial width and molecular orientations at the interface difficult. The structure of the n – alkane / water interface has been probed experimentally by X – ray reflectivity, neutron scattering and total internal reflection second harmonic generation spectroscopy; however these data are not always in agreement. Also, a number of molecular dynamics (MD) simulation studies have been conducted on the n – alkane / water interface. Calculations of interfacial tension using MD are difficult and fraught with uncertainty. Also, MD simulations have difficulty calculating properties of trace components (small mole fraction); for instance, is there a preferred orientation of alkane molecules in the water phase as the interface is approached? A powerful alternative to these approaches is classical density functional theory (DFT). DFT involves minimizing the grand free energy functional and allows for the calculation of the structure and properties of inhomogeneous fluids. Unlike molecular simulation, DFT is accurate for systems with trace components and is accurate for the calculation of interfacial tension. Computationally, DFT is typically much faster than molecular simulation allowing for a rapid exploration of parameter space. In this work we apply the interfacial statistical associating fluid theory density functional theory (i-SAFT) to the alkane – water interface.  i-SAFT is the extension of the SAFT equation of state for associating chain fluids to interfacial systems. For homogeneous systems the i-SAFT DFT reduces to the SAFT equation of state. We obtain the parameters (alkane chain length, water association energy, molecular sizes and Lennard – Jones well depths) by fitting the SAFT equation of state to bulk phase equilibrium data, and then show how these same parameters can be used to accurately predict the interfacial properties of the oil / water interface (interfacial width, density profiles, interfacial tension etc…). Using the DFT, we are the first to show that alkane molecules in the water phase but close to the interface orient normal to the interface.