The study of phase equilibria is historically one of the most important sources of information about the nature of intermolecular forces in non-electrolyte liquids and their mixtures. Many of the main features of vapour-liquid and liquid-liquid phase behaviour were already well characterized experimentally during the early part of this century, but the theoretical explanation of phase equilibria for a wide variety of substances and over a large range of pressures and temperatures has lagged far behind. This paper presents theoretical studies of phase equilibria in binary mixtures obeying the van der Waals equation, especially liquid-liquid equilibria that can occur at high pressures. The variety of fluid phase behaviour that occurs in binary mixtures can be qualitatively discussed in terms of the changes in thermodynamic properties near critical points. Upper critical solution temperatures (UCSTs) occur when a heterogeneous (two-phase) system becomes a homogeneous (one-phase) system when the temperature is raised. The maximum temperature along the temperature-mole fraction ( T, x ) coexistence curve for constant pressure is the UCST at this pressure. Lower critical solution temperatures (LCSTs) occur when a homogeneous system becomes a two-phase system when the temperature is increased. The LCST is at the minimum of the T, x coexistence curve. Thermodynamic considerations of critical points yield requirements for the curvature of the mixing functions plotted against x .

The van der Waals equation of state, in spite of its oversimplifications, gives useful qualitative information about mixtures over a wide range of temperatures and pressures. When the Bronsted principle of congruence is used to evaluate the parameter a12 for the mixture, a wide range of properties can be predicted : excess functions (including temperature and composition dependence) and phase eqiulibria (including lower critical solution phenomena at high temperatures), in good qualitative agreement with experimental properties of mixtures of n-alkanes.Mixtures of n-alkanes serve, in a certain sense, as low molecular weight analogues of polymer solutions and the extensive experimental data can be used to test various theoretical models. General qualitative features of alkane mixtures, which any satisfactory model must produce, include : (a) the molar excess volume YE is negative at all temperatures, increasingly so at higher temperatures, and with a minimum skewed in the direction of mixtures rich in the smaller component. (b) The molar excess enthalpy is small and positive at low temperatures and becomes negative at higher temperatures ; in a short intermediate range, the curve of RE against mol fraction is S-shaped. (c) For certain mixtures (e.g., CH4 + n-C6HI4), lower critical solution phenomena occur near the gas-liquid critical point of the more volatile (smaller) component.Many of these properties can be interpreted in terms of the principle of congruence an extended theory of corresponding ~t a t e s , ~' ~ or the new Flory equation of state.6* For several years we have been investigating the properties of van der Waals mixtures at elevated temperatures and pressures, particularly with respect to phase behaviour,8 and have found that this model also reproduces-with a reasonable choice of parameters-the qualitative behaviour of n-alkane mixtures. Because of the simplicity of the van der Waals equation and the physical reasonableness of its Q: and b parameters, it is useful to show how it leads to the observed behaviour of hydrocarbon mixtures.

Our investigation indicates that a state of reduced natriuretic peptide level exists in the obese individual with heart failure.

Volatile anesthetics like halothane and enflurane are of interest to clinicians and neuroscientists because of their ability to preferentially disrupt higher functions that make up the conscious state. All volatiles were once thought to act identically; if so, they should be affected equally by genetic variants. However, mutations in two distinct genes, one in Caenorhabditis and one in Drosophila, have been reported to produce much larger effects on the response to halothane than enflurane [1, 2]. To see whether this anesthesia signature is adventitious or fundamental, we have identified orthologs of each gene and determined the mutant phenotype within each species. The fly gene, narrow abdomen (na), encodes a putative ion channel whose sequence places it in a unique family; the nematode gene, unc-79, is identified here as encoding a large cytosolic protein that lacks obvious motifs. In Caenorhabditis, mutations that inactivate both of the na orthologs produce an Unc-79 phenotype; in Drosophila, mutations that inactivate the unc-79 ortholog produce an na phenotype. In each organism, studies of double mutants place the genes in the same pathway, and biochemical studies show that proteins of the UNC-79 family control NA protein levels by a posttranscriptional mechanism. Thus, the anesthetic signature reflects an evolutionarily conserved role for the na orthologs, implying its intimate involvement in drug action.