One-Dimensional Bonded Fluids

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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.