Mixing enthalpy, determination

Most multicomponent systems undergo phase separation because of their positive mixing enthalpies coupled with low entropy of mixing. Morphological features have been central to the study of multicomponent systems, because domain sizes, shapes, and interfacial bonding characteristics determine the mechanical properties. A proper understanding of these features often allow synergistic behavior to be developed. 

Calorimetry constitutes a powerful tool to investigate materials. It is a measurement technique that enables us to obtain values of the thermodynamic quantities of substances. The methods used for the characterization of thermodynamic properties of molten salts include temperature, enthalpy, and heat capacity measurements as mixing enthalpy and phase diagram determinations for their mixtures. 

A survey of different experimental techniques used in mixing calorimetry was given very recently by Gaune-Escard (2002). Calorimetry for the determination of mixing enthalpy can be divided into two groups for temperatures up to 1200 K and above 1200 K. Several experimental techniques may be used for the determination of mixing enthalpies. The main problem met in all of them is the elimination of all side effects arising from ... 

Only solid solutions were present under the conditions of the investigation of the Fe-Co [242], Ni-Cu [250], and SnTe-PbSe [262] systems given in Table 7. The thermodynamic activities, excess Gibbs energies, mixing enthalpies and excess entropies were determined by the use of the ion intensity ratio method for the Fe-Co and Ni-Cu systems as described for the melts. The partial pressure of the molecules SnTe, SnSe, PbTe, and PbSe were obtained for different compositions of the quasi-binary system SnTe-PbSe using the isothermal evaporation technique. 

In the combined mode I h- II of operation, the inlet temperatures can be determined from the mixing enthalpy balance equation. 

As discussed earlier, experimentally determined activity coefBcienls may be e.xtrapolated to other temperatures making use of the temperature dependence of the partial molar mixing enthalpy. This temperature dependence of the enthalpy shows small enthalpy changes for moderate temperature changes. Thus, if the expression of heat of mixing... 

JAB Jablonski, P., Miiller-Blecking, A., and Borchard, W., A method to determine mixing enthalpies by DSC, J. Therm. Anal Calorim., 74,779,2003. 

Conventionally, AS ix < 0. According to (4.3), the dissolution of polymers is mainly determined by the temperature and the mixing enthalpy. In the microscopic level, the latter mainly originates from the change of inter-molecular interactions. Of course, the practical dissolution is determined by the critical condition of thermodynamic equilibrium between two solution phases, as introduced in Sect. 9.1. 

The partial molar quantities of mixing were determined for normal and branched alkanes (O5 — Cio), cyclohexane, benzene and tetrachloromethane in polyisobutylene [57]. Partial molar enthalpies of mixing were measured for normal alkenes in low and high density polyethylene, polypropylene, polybutene-1, polystyrene, poly(methyl acrylate), poly(vinyl chloride), polyCN-isopropyl-acrylamide), ethylene-vinyl acetate copolymer, ethylene-carbon oxide copolymer [88] normal, branched and cyclic alkanes, benzene, n-butylbenzene, ois- and ra s-decalin, tetraline and naphthalene in polystyrene at 183, 193 and 203°C [60] these solutes in poly (methyl acrylate) [57] n-nonane, n-dodecane and benzene in polystyrene in the range 104.8 — 165.1 C [71] O7—C, C12 normal alkanes and aromatic hydrocarbons in polystyrene at an average temperature of 204.9°C [72], C7—Cg normal alkanes in poly(ethylene oxide) at an average temperature of 66.5 "C [72] normal alkanes in ethylene oxide—propylene oxide block copolymers (Pluronics L 72, L 64 and F 68) at the same average temperature [72]. 

Calculated mixing enthalpies of a, 0, /, , and ry—phases of the Cu-Zn system are shown in Fig. 3. As standard states we used the pure fee Cu and hep Zn with theoretically determined c/a ratio 1.82, which compares well with experimental c/a= 1.856 . One can see that calculations correctly reproduce the principal features of the Cu-Zn phase diagram . Considering the random alloys alone we observe that for the Cu-rich alloys the fee structure is stable. When Cu concentration increases, the fee alloys are almost degenerate with the hep alloys, followed by the region where bcc alloys are more... 

Figure 3. Calculated mixing enthalpies of completely random fee (circles, full line), bee (squares, dashed line), and hep (triangles, dot-dashed line) Cu-Zn alloys, as well as the ordered B2 CuZn compound (diamond). The pure fee Cu and hep Zn with theoretically determined c/a ratio 1.82 are used as standard states, and the so-called ground state lines (long-dashed... 

Witl, 1995Wit2] Isothermal calorimetry at 1880°C was carried out in three sections with xpe cr as 0.25 0.75, 0.50 0.50 and 0.75 0.25 The whole range of existence of homogeneous liquid phase, from Cr-Fe binary up to 32 at.% B, enthalpies of mixing were determined. 

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