Group contribution models values

The group contribution method allows the approximate calculation of solubility by summing up fragmental values associated with substmctural units of the compounds (see Section 7.1). In a group contribution model, the aqueous solubility values are computed by Eq. (12), where log S is the logarithm of solubility, C is the number of occurrences of a substmctural group, i, in a molecule, and is the relative contribution of the fragment i. 

First a database of solute-solvent properties are created in SoluCalc. The database needs the melting point, the enthalpy of fusion and the Hildebrand solubility parameter of the solute (Cimetidine) and the solvents for which solubility data is available. Using the available data, SoluCalc first prepares a list of the most sensitive group interactions and fits sequentially, the solubility data for the minimum set of group interaction parameters that best represent the total data set. For a small set of solvents, the fitted values from SoluCalc are shown in Table 9. It can be noted that while the correlation is very good, the local model is more like a UNIQUAC model than a group contribution model... 

If reliable experimental Pyp values are not experimentally available, the use of Eq. (13) is suggested. In cases when Henry s law constant values are not available, predicted ones by the bond-contribution method of Meylan and Howard [20] can be used. Also, due to uncertainties in the experimental values, e.g. for lindane the experimental values reported from different sources are in the range of 686-12400, while for triflurahn in the range of 1200-13700, Voutsas et al. proposed that predicted values by the group-contribution model of Meylan et al. [25] should be used. 

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. 

This topological approach with connectivity indices has been extended by Bicerano [58] to a point where many of the physical properties of the polymer can be estimated from empirical predictive equations. Bicerano correlated connectivity indices with group contribution values in order to develop a model equation for a specific property [58]. The usefulness of Bicerano s equations is that they can be extended to predict the same property values for new interested polymers. 

Moreover, the objective function obtained by minimizing the square of the difference between the mole fractions calculated by UNIQUAC model and the experimental data. Furthermore, he UNIQUAC structural parameters r and q were carried out from group contribution data that has been previously reported [14-15], The values of r and q used in the UNIQUAC equation are presented in table 4. The goodness of fit, between the observed and calculated mole fractions, was calculated in terms RMSD [1], The RMSD values were calculated according to the equation of percentage root mean square deviations (RMSD%) ... 

The HRC of a polymer is thought to be a fundamental lire property, and empirical molar group contributions to polymer HRC have been determined for a wide range of polymers. These molar group contributions were refined and recalculated for the limited range of polymer chemistry in the development program, in an attempt to obtain better predictive capability. The results of the additive model calculations are shown in Figure 16.4 versus the measured values from MCC. Reasonably... 

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