Liquid crystal phase molecular structure effects

The nature of the lyotropic liquid crystal phase formed by amphiphiles in solution is described at a molecular level by the surfactant packing parameter model, introduced in Section 4.9. Consider the situation where the head group has a larger effective cross-sectional area than the chain. This is the usual situation, and the resulting structures are termed normal structures. If there is a large difference in cross-sectional area between the head group and chain (Ns < ), spherical micelles are formed (Fig. 4.24a). For molecules with less of a mismatch between the effective head and tail cross-sectional areas, rod-like micelles provide a more... 

K. Tu, M. Tarek, M.L. Klein, and D. Scharf. Effects of anesthetics on the structure of a phospholipid bi layer Molecular dynamics investigation of halothane in the hydrated liquid crystal phase of dipalmitoylphosphatidylcholine. Biophys. J., 75 (1998) 2123-2134. 

The rapid rise in computer speed over recent years has led to atom-based simulations of liquid crystals becoming an important new area of research. Molecular mechanics and Monte Carlo studies of isolated liquid crystal molecules are now routine. However, care must be taken to model properly the influence of a nematic mean field if information about molecular structure in a mesophase is required. The current state-of-the-art consists of studies of (in the order of) 100 molecules in the bulk, in contact with a surface, or in a bilayer in contact with a solvent. Current simulation times can extend to around 10 ns and are sufficient to observe the growth of mesophases from an isotropic liquid. The results from a number of studies look very promising, and a wealth of structural and dynamic data now exists for bulk phases, monolayers and bilayers. Continued development of force fields for liquid crystals will be particularly important in the next few years, and particular emphasis must be placed on the development of all-atom force fields that are able to reproduce liquid phase densities for small molecules. Without these it will be difficult to obtain accurate phase transition temperatures. It will also be necessary to extend atomistic models to several thousand molecules to remove major system size effects which are present in all current work. This will be greatly facilitated by modern parallel simulation methods that allow molecular dynamics simulations to be carried out in parallel on multi-processor systems [115]. 

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