Diatomic fluids

The most commonly used approach to the problem is to expand the correlation functions and their Fourier transforms in a series of orthogonal functions, usually the spherical harmonics. This approach was pioneered by Chen and Steele in the case of the Percus-Yevick approximation for hard diatomic fluids. More recently, the approach has been generalized to arbitrary... 

Of course, the above considerations may not be relevant to the problem at hand, since in solving the OZ equation, the important functions are y(l, 2), its Fourier transform and B(l,2). ° It is to be expected that y(l,2) will vary less quickly between different orientations and will be continuous even for hard core potentials. Thus, its expansion in spherical harmonics should be better behaved than that of gf(l,2). Computer simulation cannot be used to obtain y(l,2) but Lado has presented some evidence based upon his solution of the RHNC approximation for a hard diatomic fluid using a spherically averaged bridge function that the convergence is good. Nevertheless, the results he presents are, in our view, for a rather short diatomic bond length and may not be conclusive. 

Studies of the thermodynamic properties obtained from solutions of the SSOZ equation have been much more extensive but the success has been mixed. The first such calculations were those of Lowden and Chandler who obtained the pressure of hard diatomic fluids from the RISM (SSOZ-PY) equation. They used two routes to the equation of state a compressibility equation of state in which they integrated the bulk modulus calculated from the site-site correlation functions via... 

The critical properties of several real diatomic fluids show similar behavior. Both the work of Fischer et al. " and McGuigan et al. show clearly how deviations from the principle of corresponding states arise from increases in the diatomic bond length. The most striking effects are an increase in the reduced saturated liquid density and a reduction in the slope of og(F,3p/FJvs. TJT. 

Morriss and Isbister have studied phase equilibrium of polar hard diatomic fluids, and mixtures of polar hard diatomic fluids with nonpolar hard diatomics using the analytic solution of the SSOZ-MSA ° and the energy equation of state. They predict that vapor-liquid phase separation will occur in the polar hard diatomic fluid arising only from the dipolar interactions. They also predict that a mixture of hard diatomic and dipolar hard diatomic fluids will phase separate into two dense fluid phases, one rich... 

The general formulation of density-functional theory for molecular fluids has been described by Smithline et al. They have applied a simplified version of the theory to the freezing of hard core diatomic fluids. A suitable starting point for such theories for rigid nonspherical molecules is the following expression for the grand potential... 

Sullivan et al. and Thompson et al. have studied the structure of hard diatomic fluids in contact with a hard wall and Lennard-Jones 12-6 diatomic fluids interacting with a wall via the Lennard-Jones 9-3 potential. Computer simulations were carried out via the Monte Carlo method for the hard diatomic system and via molecular dynamics for the 12-6 diatomic system. In each case, the simulation results were compared with the results from solutions of the RISM or SSOZ-PY theory adapted to the fluid-wall problem. This adaptation can be achieved by noting that the site density profile for a diatomic fluid in contact with a plane surface can be related to... 

Biggs M, Agarwal P. Mass diffusion of diatomic fluids in random micropore spaces using equilibrium molecular dynamics. Phys Rev E 1994 49 531-537. 

Hsu, C.S., Chandler, D. and Lowden, L.J., 1976, Application of the RISM equation to diatomic fluids The liquids nitrogen, oxygen and bromine, Chem. Phys., 14 213. 

When the above methods fail, estimation methods become important. Schemes based on the Corresponding-States Principle which are particularly important in this respect are described. In order to demonstrate clearly just when the methods of correlation, the theoretical expressions and estimation techniques are applicable, examples are given of transport-property data representation for systems of different complexity simple monatomic fluids, diatomic fluids, polyatomic fluids (specifically, water and refrigerant R134a), nonreacting mixtures and (dilute) alkali-metal vapors as an example of a reacting mixture. 

Moreover, one can also study with GFH the density fluctuations of a homonuclear diatomic fluid under the action of a weak thermalized field by assuming that the field acts on the diatomic molecules in an independent field-atom fashion (L, M, Sese, unpublished results). The process is based on functional derivatives, as explained before. Now, the derivatives are taken with respect to P(x ), with x. being the position vector of the a-A atom (a = 1,2) belonging to molecule j (Fig, 2), The grand partition function is built from the A-particle canonical partition functions which read as... 

Note that by including the atomic positions 4>j generalizes the GFH packet, in which Vj is the position of the COM of the delocalized molecule j, and v. is the atom-a vector from that j center. It is not difficult to show that the total Ihennal-ized linear response from the diatomic fluid can be written as... 

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