Effects of molecular size and orientation on the interfacial properties and wetting behavior of water/n-alkane systems: a molecular-dynamics study.

Abstract

Molecular dynamics simulations (MD) are performed to study the interfacial structure/tension and wetting behavior of water/n-alkane systems (water/nC5 to water/nC16 where nCx = CxH(2x + 2)). In particular, we study complete-to-partial wetting transitions by changing the n-alkane chain length (NC) at a constant temperature, T = 295 K. Simulations are carried out with a united-atom TraPPE model for n-alkanes and the TIP4P-2005 model of water. Simulation results are in excellent agreement with the initial spreading coefficients and contact angles calculated using experimental values of the surface and interfacial tensions. In addition, it has been determined that water/(nC5-nC7) and water/(nC8-nC16), respectively, exhibit complete and partial initial wetting modes. Simulations show that the interfacial structures of water/(nC5-nC7) are different from water/(nC8-nC16) systems. In the latter, water preferentially orients near the interface to increase the number of hydrogen bonds and the charge and mass densities. Moreover, the orientation of n-alkane molecules at water/(nC8-nC16) interfaces has a long-range persistence, resulting in layered structures that increase with NC. In addition, simulation results of the orientational order parameter Sz show alignment behavior of the n-alkane molecules with respect to the interfaces. Simulations predict that the central segments of n-alkane are strongly packed in the interfaces while the end segments (methyl groups) form smaller peaks in the outer edge of the layer. This observation confirms the "horseshoe" or "C-shaped" structure of n-alkane molecules in the water/n-alkane interfaces. At constant temperature, the interface widths of both water and the n-alkanes decrease with increasing n-alkane molecular length. These results suggest that increasing the n-alkane chain length affects the water/n-alkane interfacial properties in a manner similar to that of cooling.