Flexible Molecules

The study of flexible molecules has been one of the main areas of interest during the last years. Molecules as n-butane do not have a rigid permanent shape. In fact they may adopt a number of different shapes or conformations. For instance n-butane can be in the spacial configuration denoted as trans (where all C atoms are in a plane) or in the spatial configuration denoted as gauche (where one of the C atoms is out of the plane). In a liquid sample of a fluid with flexible molecules as for instance (n-hexane) some molecules adopt the all trans configuration, other adopt a configuration with all bonds in trans configuration but one in the gauche configuration etc. The molecular shape is dynamic in the sense that a given molecule adopt a number of spatial configurations in time. This type of molecules are denoted as flexible molecules (alkanes are a typical example of a flexible molecule). When the number of bonds in the chain is big they are denoted as polymers. In the area of flexible molecules we have considered several different problems:

Critical properties of linear and branched alkanes

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    We have developped an equation of state (based on thermodynamic perturbation theory) to predict the changes in critical properties of alkanes (linear and branched). In the figure we show the critical temperature of more than 40 different alkanes, different in molecular weigth and/or structure (isomers). The theory is able to account for most of the features found experimentally.
L.G. MacDowell y C. Vega
J.Chem.Phys. 109, 5681 (1998)

Computer simulations of branched alkanes.

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    We have developped a computer simulation program that allows to determine the equation of state of any branched alkane. In the figure the compressibility factor as obtained from computer simulation is compared to a theoretical equation of state proposed by ourselves (a modification of Wertheim's TPT1 theory). Results are shown for several isomers of octane.
L.G. MacDowell, C. Vega y E. Sanz
J.Chem.Phys. 115, 6220 (2001)

Critical properties of polymers

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    Equation of state for polymers formed by several tangent Lennard-Jones monomers. Simulation and theoretical results are shown. In the figure the chemical potential as a function of the density is presented for a supercritical isotherm (temperature larger than the critical temperature) and subcritical isotherm ( temperature lower than the critical temperature).
L.G. MacDowell, M. Muller, C. Vega y K. Binder
J.Chem.Phys. 113, 419 (2000)

Phase separation of polymer-solvent systems.

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    An equation of state for mixture of a long chain (polymer) and a short chain (solvent) is proposed. In the model the interaction between monomer units is of LJ type. Simulations for the same model were performed. In the figure the phase diagram as obtained from theory and from simulation is presented (in the pressure-composition plane and in the pressure-density plane).
L.G. MacDowell, P. Virnau, M. Muller, K. Binder
J.Chem.Phys. 117, 6360 (2002)