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.