A simple lattice model for the microstructure of neat alcohols: application to liquid methanol.

Simple lattice model for self-associating molecules such as methanol or tert-butanol is proposed and studied in mean-field (MF) approximation in the case of methanol. In addition to the isotropic van der Waals interaction, the hydrogen bonding is present in this model when the neighboring alcohol molecules are in appropriate orientations. The orientation of the polar molecule is given by the unit vector n parallel to the vector connecting the center of the tail group with the center of the head group of the molecule. Stability region of the uniform fluid phase against gas-liquid separation and order-disorder transition is obtained for neat methanol in MF approximation. In order to describe the self-association patterns in the liquid, we consider the grand-canonical ensemble average of the scalar product of the orientations of the molecules <n(x) * n(x + Delta x)> as a function of the vector Delta x describing the separation between the centers of the molecules. For methanol we find in MF oscillatory decay of <n(x) * n(x + Delta x)> for Delta x perpendicular n(x) and for Delta x parallel n(x); the wavelength is somewhat less than two molecular diameters in both cases, and the decay length is larger in the perpendicular direction. This indicates that on average alternating antiparallel and parallel orientations of the second molecule are found for increasing separation from the first molecule in both directions. Such local orientational ordering of the molecules is consistent with association into zigzag chainlike clusters found in recent spectroscopic measurements and computer simulations. In Fourier representation the above structure function assumes maximum for the wave number that coincides with the prepeak position for site-site correlations found in simulations. We argue that <n(x) * n(x + Delta x)> can provide a useful tool for discriminating between different local arrangements of any polar molecules.