One-Dimensional Bonded Fluids

Abstract

A study is made of a linear lattice fluid in which bonds can form between pairs of molecules on second-neighbor sites with an empty site between them. At low temperatures and pressures there is, thus, competition between local open configurations and more close-packed local configurations of higher energy. By using a matrix expression for the grand partition function, the density and effective coordination number are found as functions of the absolute temperature T and one-dimensional pressure p. Equivalent results are shown to follow from a constant-pressure partition function based on an explicit expression for the configuration number. It is found that a pressure p0 exists such that for any p < p0 the open configuration is stable at T = 0, while the density passes through a maximum as T increases. For any p > p0, on the other hand, the close-packed configuration is stable at T = 0 and the density decreases monotonically with T. Using a constant-pressure partition function, we also consider continuous models with the ``bonding'' represented by a potential well separated from the hard core. With a well of parabolic shape, similar results to those of the lattice model are obtained, while with a square well the curve of density against T displays a minimum as well as a maximum for any p < p0.