Group interaction parameters

Fig. 4. UNIFAC group interaction parameter matrix, the ISi represents parameters fit and parameters not available (168). A represents an aromatic...

In contrast to the NRTL-SAC model, the UNIFAC model developed by Fredenslund et. al. [29] divides each molecule into a set of functional groups that interact with each other on a binaiy basis and whose interactions are combined together to describe the global liquid phase interaction between molecules. Because the segments in UNIFAC are based on functional groups it is possible to model a system provided that all of the molecular structures are known. The problem with pharmaceutical sized molecules is that existing UNIFAC parameter tables do not contain many of the group interaction parameters that are necessary, and even when they do, the interactions are fitted to a database of chemicals that are much smaller and simpler than pharmaceuticals, and typically fail to represent them adequately. 

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... 

All GCMs suffer from two major problems of principle. First, they can only handle molecules and mixtures for which all required group parameters or group-interaction parameters have been fitted before. This limitation is more severe for the two-dimensional methods because parameters for all combinations of groups have to be available. This is more unlikely than just... 

Table 2 UNIFAC group interaction parameters for prediction of vapour-liquid equilibria at temperatures between 250 and 425 K 539...

Note all group-interaction parameters that do not exist are set to zero. Interactions between the same group are equal to zero. 

It should be pointed out that for MHV1 and MR1, the UNIFAC group interaction parameter were obtained with VLE data for C02/n-aikanes up to n-C10FI22 (Holderbaum and Gmehling, 1991), while for LCVM, VLE data up to n-C28H58 were used(Boukouvalas et al., 1994). 

Coefficients in the group-group interaction parameter defined by eq (6)... 

The group interaction parameter function y/mn is determined for each possible binary group pair, m and n, by the following equation. 

Step 13 Determine the group interaction parameter function, y/mn, for every possible binary group pair, corresponding to each group interaction parameter, amn, obtained in Step 3, using Equation (3C-12). 

This method is to be used to predict the activity coefficient of a low molecular weight solvent in a solution with a polymer. This procedure requires the structure of all components in the mixture, group reference volumes for all groups in the solution as well as the group interaction parameters for all groups. Use of the number average molecular weight of the polymer is recommended. 

When Oi deviates significantly from unity, as would be the case when not all of the compounds are completely miscible, then separate computations for Oi are required. This can be accomphshed by using some of the recently estabhshed models (55) which compute the a/s by using group interaction parameters obtained from data reduction. 

Xm is the mole fraction of subgroup m in the mixture. i/>nm is the group interaction parameter between groups n and m and is dependent on the temperature ... 

The group interaction parameter anm is found from the large sets of VLE and LLE data in the literature, which are tabulated for many subgroups. It is worth noting that a m a. There are some modifications to the original UNIFAC equation in order to make the model robust for some complex systems. In the UNIFAC-DM method, the modification is made on the combinatorial part ... 

TABLE 2-357 UNIFAC-VISCO Group Interaction Parameters... 

A number of methods based on regular solution theory also are available. Only pure-component parameters are needed to make estimates, so they may be applied when UNIFAC group-interaction parameters are not available. The Hansen solubility parameter model divides the Hildebrand solubility parameter into three parts to obtain parameters 8d, 5p, and 5 accounting for nonpolar (dispersion), polar, and hydrogenbonding effects [Hansen,/. Paint Technot, 39, pp. 104-117 (1967)]) An activity coefficient may be estimated by using an equation of the form... 

Hwang SM, Lee JM, and Lin H. New Group-interaction Parameters of the UNIFAC Model Aromatic Methoxyl Binaries. Ind Eng Chem Res 2001 40 1740-1747. 

UNFI2.DTA CONTAINS UNIFAC BINARY GROUP INTERACTION PARAMETER INFORMATION. 

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