Group contribution models molar volume values

The subunits defined in the model are listed in Table I. Also shown are the group molar volume and the group contribution to the COj permeability and the CO2/CH4 permselectivity the best fit solution to the matrix of linear equations. The gas permeability of each polymer in the dataset was calculated firom the resultant subunit permeability indicated in Table I and the normalized structural equation for each polymer. The CO2 and CH4 permeability (in Barrers) predicted by the group contribution model is compared to experimental values in Figure 1. An excellent correlation is evident for both gases. The correlation between model predicted and experimental CO2/CH4 selectivity is shown in Figure 2. 

The van Laar, Wilson, and NRTL models require only binary mixture information to obtain values of the parameters, whereas the UNIQUAC model also requires pure component molar volumes as well as surface area and volume parameters. These latter parameters for the UNIQUAC model are usually obtained using a group contribution method in which a molecule is considered to be a collection of functional groups and the surface area and volume of the molecule are the sum of like quantities over all groups in the molecule. 

From a consideration of detailed results on the conformational equilibria of aldopentopyranose derivatives, it has been pointed out92- 9 that a more sophisticated model is required before conformational populations can be reliably predicted, at least with acylated derivatives. Even with adjustment of the original parameters in order to take revised values for the anomeric equilibrium of D-lyxopyranose tetraacetate and the conformational equilibrium of /3-D-arabinopyranose tetraacetate into account, the observed data cannot be accommodated within the framework of this model, except on a very broad, qualitative basis. Other possible factors that should be considered " include polar contributions from substituents other than that on C-1, attractive interactions between syn-diaxial acyloxy groups, non-bonded interactions between atoms that have unshared pairs of electrons,repulsive interactions between gauche-vicinal groups, the effect of solvent pressure, and differences between the molar volume of conformers. 

ILs, fluoroalkyl-functionalized ILs exhibited a higher separation for CO2/CH4 and a lower one for CO2/N2. The authors tried to correlate the selectivity with the molar volume and the solubility parameters 5 derived from chemical group contributions [74] and concluded that the model was able to predict trends in CO2 separations however, no absolute values could be calculated via this method. 

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