Pure component intermolecular potential parameters

The pure component intermolecular potential parameters used in this study are shown in Table I. They were obtained as follows for carbon dioxide, we fitted the experimental critical temperature and pressure (12) using data from ( ) for the critical constants of the Lennard-Jones (U) system (T - 1.31, - 0.13). For acetone, a... 

Table I. Pure component intermolecular potential parameters...

Although accurate values of mixed coefficients are slightly more difficult to obtain than pure-gas values, they are attractive theoretically for two reasons. Firstly, by careful choice of components the interaction terms in PI2 and Up can be simplified, and secondly, mixed ririals provide a stiff test of the intermolecular potential parameters determined from pure-gas virials. 

Estimating Pure-Component Lennard-Jones Parameters for the 6-12 Intermolecular Potential... 

Tbe basic scheme for modeling the phase behavior of binary mixtures is first to input the pure component characteristic parameters Tc, Pc, and to, and then determine the binary mixture parameters, kj. and iij., by fitting data such as pressure-composition isotherms. Normally k.. and tIj. are expected to be lie between 0.200. If the two species are close in chemical ize and intermolecular potential, the binary mixture parameters will have values very close to zero. In certain cases a small value of either of these two parameters can have a large influence on the calculated results. 

We will now discuss the problem of determining effective or optimal diameters for use with the HSE theory for real fluids when both the form of the intermolecular potential and its parameters are unknown but accurate equations of state which represent the PVT behavior over an extensive range are available for the pure components. 

Combination Rules.—The relation between simple empirical potential functions and the more fundamental and more detailed descriptions of intermolecular interactions is tenuous. Most simple potentials provide a convenient means by which the gross details of the interactions can be specified. As a result, one should not expect that the combination rules that provide the best representation for the dispersion or repulsive interactions will prove the most satisfactory for a given potential. A variety of empirical rules for combining pure-component potential parameters has been proposed the most widely used have been... 

The same intermolecular potential-energy functions may be applied to correlate interaction virial coefficients. In the case where there are no, or insufficient, experimental data, but adequate data exist for the pure components, it may be possible to estimate with useful accuracy the unlike interaction parameters from combining rules such as ... 

In real systems, nonrandom mixing effects, potentially caused by local polymer architecture and interchain forces, can have profound consequences on how intermolecular attractive potentials influence miscibility. Such nonideal effects can lead to large corrections, of both excess entropic and enthalpic origin, to the mean-field Flory-Huggins theory. As discussed in Section IV, for flexible chain blends of prime experimental interest the excess entropic contribution seems very small. Thus, attractive interactions, or enthalpy of mixing effects, are expected to often play a dominant role in determining blend miscibility. In this section we examine these enthalpic effects within the context of thermodynamic pertubation theory for atomistic, semiflexible, and Gaussian thread models. In addition, the validity of a Hildebrand-like molecular solubility parameter approach based on pure component properties is examined. 

If no experimental data are available for the implementation of this approach in its entirety then the use of a model intermolecular pair potential with scaling parameters derived from some other property or by estimation is to be preferred above any other means of evaluation for the dilute-gas state. In the dense fluid state it is at present necessary to make use of a procedure based upon an approximate theory. The Enskog theory in one of its forms is usually the best procedure of this kind. However, its application requires the availability of some experimental data for the property of interest at least for pure components so that again its application is limited. 

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