Helium critical constants

If the gas is either hydrogen or helium, determine adjusted critical constants from the empirical formulas... 

These plots give good agreement with the experimental data for most vapors with the exception of hydrogen and helium, both of which have low critical temperatures and pressures. It has been found possible to use the plot for these two gases by using modified critical constants. However, neither gas is particularly important in vapor-liquid equilibria. 

Freund6 found the Eotvos constant abnormally high for high molecular weight oils it increased regularly with the mol. wt. and could be calculated from this. For liquid helium7 the constant is abnormally small, 1 464 the value of o M Iq)213 for liquid helium is a linear function of temperature down to 2-4° K., but approaches a limiting constant value at 1 5°K. On the linear part of the curve the constant is approximately 1, the lowest value known, and for a substance with the lowest critical temperature.8... 

Fluid-fluid phase separations have been observed in many binary mixtures at high pressures, including a large number of systems in which helium is one of the components (Rowlinson and Swinton, 1982). Fluid-fluid phase separation may actually be the rule rather than the exception in mixtures of unlike molecules at high pressures. Fig. 6.4 shows the three-dimensional phase behavior of a binary mixture in schematic form. This diagram includes the vapor pressure curves and liquid-vapor critical points of the less volatile component (1) and the more volatile component (2) in their respective constant-x planes. The critical lines are interrupted one branch remains open up to very high temperatures and pressures. Systems that can be represented by a diagram such as Fig. 6.4, those for which the critical lines always have positive slope in the p — T projection, have been called fluid-fluid mixtures of the first kind. A second class of system, in which the critical line first drops to temperatures below T (l) and then increases, exhibit fluid-fluid equilibrium of the second kind. There is, however, no fundamental distinction between these two classes of fluid mixtures. 

Addition of a solute molecule that repels solvent molecules leads in general to em increase of the pressure at constant volume, that is, there is a positive value of dpldx)yT- Equations (6.7) and (6.8) then tell us that the partial molar volume of the solute should diverge toward -i-oo at the critical point of the solvent if there is such a repulsive interaction. This is actually the case for mercury-helium mixtures as illustrated by the pressure dependence of V shown in Fig. 6.5. This simple mechanistic... 

Abstract Rate constants for charge transfer processes in the interstellar medium are calculated using ab-initio molecular calculations. Two important reactions are presented the recombination of Si + and Si + ions with atomic hydrogen and helium which is critical in determining the fractional abundances of silicon ions, and the C+ + S -> C + S+ reaction, fundamental in both carbon and sulphur chemistry. 

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