Kelly E. Anderson, PhD, Roanoke College
Many lubricants are made of petroleum-derived components and a common class of components is hydrocarbons, or molecules made completely of carbon and hydrogen. Experimental studies have shown that hydrocarbon liquids tend to form solid-like layers next to solid surfaces. This behavior has been observed in several simulation studies as well and is not limited to hydrocarbon liquids. Our previous work has shown that solid-like layers form in binary mixtures and that the longer alkane is preferentially adsorbed at the surface even when the two alkanes vary by one methylene unit. We have also shown that molecular architecture plays a role in adsorption at the surface. In mixtures, the molecule with the longer straight chain segment is more concentrated next to the surface. This year, we built on work started last summer as we continue to examine how the presence of a solid substrate influences the structural properties of a liquid film in the vicinity of a surface.
During the summer of 2011, we began simulations of binary mixtures of linear ethers with linear alkanes. Initial results indicated equimolar mixing at the solid substrate, but further investigations found deficiencies in the original model. These systems were reparameterized during the summer of 2012 to more accurately reflect the interaction of the lone pair electrons on the ether oxygen atoms with the substrate. While these simulations are still ongoing, preliminary results indicate a very strong preference for ether molecules near the substrate as a result of enhanced interactions of the oxygen atom with the substrate. We are particularly exploring the effect of alkane chain length on preferential adsorption. As chain length increases, initial results suggest that there is a point where the additional interactions from the methylene groups in the longer alkane compete effectively with the strong interactions of the oxygen atom with the substrate.
To explore the effects of competitive adsorption more explicitly, this summer, we completed a series of simulations of homologous alkane-perfluoroalkane mixtures in which we varied the strength of the attractive interaction of the perfluoromethylene (-CF2-) groups. We varied this parameter such that the heat of adsorption of each CF2 group ranged from 4.3 kJ/mol to 5.1 kJ/mol. As expected, we found a strong correlation with the strength of the attractive interaction and preferential adsorption at the surface. Previous work showed that the perfluoroalkane was preferentially adsorbed at both the substrate-liquid interface, as well as the vapor-liquid interface. Our simulations did not show a preference for the fluorinated species at the solid surface. Instead, there was a very strong preference for the alkane at the substrate. Increasing the strength of the attraction of the CF2 group shifted the mole fraction of molecules at the surface toward the perfluoroalkane, although the CH2 attraction parameter was always stronger than the CF2 parameter. While this series of simulations was undertaken as a proof of concept experiment, further analysis of the results, and further validation against experimental findings, may bring forward particular points of interest.
Work on this project continues. The simulations of ether-alkane mixtures are ongoing. Additionally, work this year will include examining surface architecture effects on liquid structure as well as examining multicomponent mixtures, particularly alkane solvents with low concentrations of heteroatom additives or branched and cyclic alkanes.