(208c) Interfacial Properties of 5CB/Water and 5CB/Vacuum Interfaces: A Molecular Dynamics Simulation Study

Experimentally liquid crystals (LC) have shown strong
anchoring effect at various interfaces. For example, LC comprised of
4-cyano-4'-pentylbiphenyl (5CB) molecules at water interface show reorientation
from planar to homeotropic alignment depending on interface structure and
composition (e.g., presence of surfactants and ionic species).  Such
property enables enormous amount of opportunities for LC based sensors
applications. However, the understanding of LC anchoring mechanisms at
molecular scale remains rather poor. In this work we will discuss the results
of our atomistic molecular dynamics (MD) simulation study of 5CB/water and
5CB/vacuum interfaces which utilized APPLE&P force field for 5CB and
polarizable SWM4 water model. Analysis of these simulations provided the
molecular level insight of correlations between the interfacial structure and
the anchoring of the nematic phase LC. Initial results indicate that at room
temperature, contrary to the nematic ordering found in the distance, the 5CB
molecules at the water interface have assumed planer alignment (Figure 1, top),
which is in agreement  with experiment. In contrast, the 5CB/vacuum system
has shown primarily homeotropic alignment at the vacuum interfaces (Figure 1,
bottom). Self-diffusion coefficients of 5CB molecules at interfaces and in the
bulk 5CB phases are compared to demonstrate the change in mobility of LC
molecules due to surface effects.

Figure 1. System snapshot of
5CB/Water (top) and 5CB/Vacuum (bottom) at 308K.