Stockmayer Fluid with a Shifted Dipole: Interfacial Behavior

Abstract: We investigate the properties of the liquid-vapor interface in the shifted Stockmayer fluid using molecular dynamics simulations in the canonical ensemble. We study the role of the dipole moment strength and the degree of asymmmetry on equilibrium interfacial characteristics, including density profiles, polar order, nematic order, interfacial polarization, electric field, and electrostatic potential. In addition, we compute angular distribution functions across the interface to gain insight into how the dipole shift affects the molecular orientation. We find that the shift significantly effects angular distribution functions by altering the polar order while leaving the nematic order relatively unaffected, in comparison to the reference symmetric Stockmayer fluid. We find that these results are consistently explained using an image-dipole construction that has been previously applied to symmetric Stockmayer fluids but has never been extended to the shifted model. We find remarkable agreement between the simple theory and the simulations in the qualitative shape of the distribution functions for both the liquid and vapor phases in proximity to the interface. Unexpectedly, the spontaneous polarization at the interface, and therefore the generated electric field, changes sign as the dipole moment strength increases. This also leads to an inversion of the sign of the potential difference across the interface.