Enthalpy interaction parameter

Differentiating the enthalpy equation with respect of N, will give the partial molar heat of mixing of component i, which in turn is related to the enthalpy interaction parameter,... 

Figure 2.2. Enthalpy interaction parameter in charge-unsymmetrical molten binary systems.

It is obvious from the foregoing discussion that the enthalpies of mixing for charge-unsymmetrical systems do not follow the simple conformal solution theory. When the anion in a strontium halide-alkali metal halide mixture from chloride to bromide and from bromide to iodide is changed, the enthalpy of mixing is decreasing. For all systems, the enthalpy interaction parameter, k, is a linear function of 512 with the usual exception for lithium-containing systems. Two important features of the k versus 512 plot should be emphasized ... 

The enthalpy interaction parameter xh is obtained from the partial molar heat of mixing. For component 1 this has the form... 

FIG. 1. Enthalpies of mixing, AH, and enthalpy interaction parameters, AHM/N1N2, in liquid potassium nitrate-lithium nitrate mixtures at 3 5 C. (From Kleppa and Hersh ). 

FIG. 2. Plot of enthalpy interaction parameters (evaluated at Ni = No = 0.5) against 6 = [(d - d2)/(di + d2)]- for binary liquid alkali nitrate mixtures. (From Kleppa and HershlO). 

FIG. 3. Plots of enthalpy interaction parameters for lithiimi and sodi-um containing common anion mixtures. 

Note that for these systems the enthalpy interaction parameters are all positive (endothermic). Note also that for a given common alkali cation, the larger the difference in the sizes of the two anions being mixed, the more endothermic is the enthalpy of mixing. All asymmetries (b) are quite small and may be positive, zero, or negative. Since the asymmetries are smaJ.1, the value of the interaction parameter at the equimolar composition, UAH (0.5)j well represents the whole range of compositions for each binary system. 

It is of considerable interest to consider in greater detail plots of the enthalpy interaction parameters for the alkali chloride-magnesium chloride systems. These are shown in Fig. 8, taken from the work of Kleppa and McCarty. Note in particiilar the sharp dip in X near N qi2 0.33 for the systems containing KCl, RbCl, and CsCl. This correlates with the "anomalous partial entropy curves for these mixtures, and undoubtedly reflects the tendency of these systems to form the complex anion MgCl. Note also that the dip is completely absent in the silver chloride and lithium chloride system, and occurs only as a very broad minimum in sodium chloride-magnesium chloride. This is consistent with the nearly ideal entropy found in these systems. 

More fundamental treatments of polymer solubihty go back to the lattice theory developed independentiy and almost simultaneously by Flory (13) and Huggins (14) in 1942. By imagining the solvent molecules and polymer chain segments to be distributed on a lattice, they statistically evaluated the entropy of solution. The enthalpy of solution was characterized by the Flory-Huggins interaction parameter, which is related to solubihty parameters by equation 5. For high molecular weight polymers in monomeric solvents, the Flory-Huggins solubihty criterion is X A 0.5. 

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... 

Disposing the Flory-Huggins modified equation, including the free entropy of mixing per total volume, AS , as a function of conversion and the enthalpy term expressed with the interaction parameter [66-68,72] ... 

Excess enthalpy is parameterized through interaction parameter W ... 

Interaction parameter, employed in the Floiy-Huggins theory, to account for the contribution of the noncombinatorial entropy of mixing and the enthalpy of mixing to the Gibbs energy of mixing. 

Table 1. Changes in the enthalpy (Ah) and the entropy (As) parameters appearing in the polymer-solvent interaction parameter % of NIPA-water system...

The interaction parameter / can also be estimated by the use of solubility parameters (8) / is proportional to (8M — 8TS)2. But this approach has a considerable error (A8 in the range of 0.4 MPa1/2), and considers only the excess free enthalpy. For these reasons it is better to determine the miscibility window experimentally. 

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