Study of spatial correlations in a supercooled molecular system

Thermodynamic properties of a molecular dipolar liquid using the two-phase thermodynamic approach

A modified version of the original two-phase thermodynamic approach has been extended to evaluate the thermodynamic properties of molecular systems. Its basic assumption states that the density of states can be decomposed into solid-like and gas-like components. The solid part has been approximated by that of a set of harmonic oscillators, whereas a subset composed of rough hard spheres has been considered for the gas part. In this new approach, molecules have been modelled as rotating hard spheres that experience elastic collisions. The technique has been tested on a system made up of dipolar diatomic molecules, and it leads to very good results for total entropy, potential energy reference and heat capacity. Translation and rotation solid components of the overall spectra have been compared to the real part of the instantaneous normal mode vibrational densities of states. Similarities between them reinforce the validity of the two-phase thermodynamic approach.

Free energy and entropy of a dipolar liquid by computer simulations

Thermodynamic properties for a system composed of dipolar molecules are computed. Free energy is evaluated by means of the thermodynamic integration technique, and it is also estimated by using a perturbation theory approach, in which every molecule is modeled as a hard sphere within a square well, with an electric dipole at its center. The hard sphere diameter, the range and depth of the well, and the dipole moment have been calculated from properties easily obtained in molecular dynamics simulations. Connection between entropy and dynamical properties is explored in the liquid and supercooled states by using instantaneous normal mode calculations. A model is proposed in order to analyze translation and rotation contributions to entropy separately. Both contributions decrease upon cooling, and a logarithmic correlation between excess entropy associated with translation and the corresponding proportion of imaginary frequency modes is encountered. Rosenfeld scaling law between reduced diffusion and excess entropy is tested, and the origin of its failure at low temperatures is investigated.

Rotational dynamics of a dipolar supercooled liquid

We study the rotational dynamics of a supercooled molecular liquid by means of molecular dynamics simulations. The system under investigation is composed of rigid diatomic molecules with an associate dipole moment. At room temperature, orientational correlations decrease rapidly with increasing distances. Upon cooling, angles between dipole moments of molecules within the first coordination shell decrease. As for the dynamical properties, rotational diffusion coefficients decrease with temperature at a smaller rate than translational diffusion coefficients do, and the critical temperature associated with the former is lower than the one corresponding to their translational counterparts. Translation and rotation about an inertial axis are uncorrelated, whereas some coupling between translation and dipole reorientation is obtained.

Microscopic dynamics of supercooled low weight alcohols

Dynamical properties of low weight alcohols have been analyzed both in the liquid and the supercooled states. Realistic interatomic potential models for methanol and ethanol have been used. The influence of temperature on the hydrogen-bonded structure has been undertaken. Remarkable similarities have been obtained in both systems. Velocity autocorrelation functions have been evaluated for molecules participating in zero, one, and two hydrogen bonds at a wide range of temperatures. A backscattering area preceded by a shoulder has been identified as a signature of this function when evaluated for the subset of molecules that participate in two hydrogen bonds. Memory functions have also been evaluated. Their initial decay depends only slightly upon temperature. A more marked temperature dependence is observed for the nonassociated molecules. For them, reasonable agreement with the mode-coupling approach predictions has been encountered.