TEMPERATURE AND DENSITY EVOLUTION OF THE ORIENTATIONAL CORRELATIONS IN FLUIDS OF LINEAR-MOLECULES

A general method is proposed for studying temperature and density effects on the microscopic local structure in fluids. The method is based on the centre-centre radial distribution functions, which are divided for partial contributions related to specific microscopic configurations. Fluid iodine is investigated because recent experimental measurements of the atom-atom radial distribution function are available, and a good potential model exists. The introduction of a correlation parameter allows quantitative results to be obtained, which clearly show the different role played by density and temperature in affecting the local order. It is shown that, in the first coordination shell, the density produces a strong variation for those molecules whose centres of mass are in contact and leaves the distribution of the other ones unchanged. This effect starts beyond a characteristic density which nearly corresponds to the critical density. By contrast, an increase in temperature produces an equal loss of orientational correlations for all the molecules in the first coordination shell.