High-density liquids solid-state studies

For chemical applications, vibrational spectroscopy of high-pressure fluid phases, including liquids and compressed gases, is of special importance (Buback, 1991). The fluid, i.e., the non-solid region of a substance, is illustrated in Fig. 6.7-2. The packing density of the circles is approximately proportional to the density of a substance. The bottom left part of Fig. 6.7-2 shows the vapor pressure curve which, up to the critical point, separates the liquid phase from the gas phase. Above the critical temperature (7 ), the density of a substance may change continuously between gaseous and liquid like states vibrational spectroscopic methods make it possible to study the structure and dynamics... 

It is generally accepted that for atomic liquids far from the critical point, the microscopic or nisation is dominated by the repulsive intoactions between the atoms. The longer range attractive forces serve only to maintain the high density of the liquid phase and the temperature acts only as a mechanism with which to vary the density. With this in mind, the hard sphere fluid has become the standard starting point for liquid state themies in which the attractive forces can be introduced, fin example, via a van der Waals approach. Hard sphere systems have been studied extensively using computer simulation, with the numerical data determined for the equation of state used as a substitute for an exactly solvable model for the liquid phase. This is in contrast to the gas and solid phases, for which the ideal gas and harmonic solid provide analytic models, respectively. Indeed, the lack of an analytic model for the liquid phase has meant that many of the current theories rely substantially on the insight obtained from the earliest simulations of the hard sphere fluid [1]. 

There are several indications that a crystalline solid is the most appropriate state to model the protein interior (Chothia, 1984). The very fact that protein structures can be determined to high resolution by X-ray diffraction is indicative of the crystalline nature of the protein. Additionally, the packing density and volume properties of amino acid residues in proteins are characteristic of amino acid crystals (Richards, 1974, 1977). In spite of the apparent crystallinity of the protein interior, most model compound studies have investigated either the transfer of compounds from an organic liquid into water (see, for example, Nozaki and Tanford, 1971 Gill et al., 1976 Fauch-ere and Pliska, 1983), or the association of solute molecules in aqueous solution (see, for example, Schellman, 1955 Klotz and Franzen, 1962 Susi et al., 1964 Gill and Noll, 1972). Both these approaches tacitly assume a liquidlike protein interior. 

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