Physical properties, estimation methods group contributions

Physical property estimation methods may be classified into six general areas (1) theory and empirical extension of theory, (2) corresponding states, (3) group contributions, (4) computational chemistry, (5) empirical and quantitative structure property relations (QSPR) correlations, and (6) molecular simulation. A quick overview of each class is given below to provide context for the methods and to define the general assumptions, accuracies, and limitations inherent in each. 

Critica.1 Properties. Several methods have been developed to estimate critical pressure, temperature, and volume, U). Many other properties can be estimated from these properties. Error propagation can be large for physical property estimations based on critical properties from group contribution methods. Thus sensitivity analyses are recommended. The Ambrose method (185) was found to be more accurate (186) than the Lyderson (187) method, although it is computationally more complex. The Joback and Reid method (188) is only slightly less accurate overall than the Ambrose method, and is more accurate for some specific substances. Other methods of lesser overall accuracy are also available (189,190) (T, (191,192) (T, P ),... 

Group contribution techniques are based on the concept that a particular physical property of a compound can be considered to be made up of contributions from the constituent atoms, groups, and bonds the contributions being determined from experimental data. They provide the designer with simple, convenient, methods for physical property estimation requiring only a knowledge of the structural formula of the compound. 

In addition to matching bulk physical properties as already mentioned, it is also necessary to consider the activity coefficients to insure that the molecular interactions between the solutes and the solvent in the original and the substitute are generally similar. This insures that proposed substitute solvents will likely dissolve the same solutes and have similar effects to those of the original solvent. However, it is important to match only the activity coefBcients of the solutes in the solvents at in te dilution (zero solute concentration), so as not to include solute-solute interactions. The authors matched the activity coefficients at infinite dilution of a representative from six chemical families alcohols, ethers, ketones, water, normal alkanes, and aromatics, i.e., they have matched these activity coefficients in the solvent to be replaced to those in the replacement solvent. The particular components used are ethanol, diethyl ether, acetone, water, normal octane, and benzene. However, one could conceivably use different compounds successfully. Activity coefficients can be estimated from group contribution methods (77). 

The normal boiling point is an easily accessible physical property and has been measured for a large number of substances. In case that it is not available, the normal boiling point can be estimated with group contribution methods, for example, Joback and Reid [4] and Constantinou and Gani [5], analogously to the estimation of the critical point. The estimation formula for the Joback method is... 

An area that has used chemical stmctures for predictive purposes quite successfully is the estimation of thermophysical properties of compounds. There has been an extensive compilation of estimation methods (81), and prediction of physical properties has been automated using these techniques (82). More recendy, the use of group contribution techniques to design new molecules that have specified properties has been described (83). This approach to compound design is being used to develop replacement materials for chloroduorocarbons. 

In general, procedures for estimating physical and thermodynamic properties and functions can be divided into two categories, namely, group contribution methods and semi-empirical correlations. It is usually difficult, if not impossible, to employ a semi-empirical correlation for predicting the properties of a new material or those of an existing material at a condition different from that under which the available data were obtained. In contrast, the group contribution method, which is based on the assumption that the property of a material is contributed from... 

A group contribution method, specifically, the second-order law, Is adopted In this work for estimating the physical and thermodynamic properties and functions of organic chemicals. 

Group contribution methods, such as the one discussed above, have been reasonably successful for estimating many physical and thermodynamic properties of pure substances and mixtures, especially when each molecule contains no more than one nonalkyl functional group. These methods dissect a molecule into functional groups that are assumed to be independent of each other. Tliat is, a functional group is assumed to behave the same in its interactions with other functional groups independent of the molecule of which it is... 

Theoretical approaches to molecular structure design require accurate estimates of physical and transport properties. These are derived commonly iiom the principles of thermodynamics and transport phenomena, and using molecular simulations. Since the literature abounds with estimation methods, reference books and handbooks are particularly useful sources. One of the most widely used. Properties of Gases and Liquids (Poling et al., 2001), provides an excellent collection of estimation methods and data for chemical mixtures in the vapor and liquid phases. For polymers. Properties of Polymers (van Krevelen, 1990) provides a collection of group-contribution methods and data for a host of polymer properties. 

Group contribution methods have been applied to the problem of estimating the solubility parameter without physical measurements [110-117,39,118,119]. Small [39] was one of the first to recognize the additive properties of... 

TABLE 1 gives the reported values or ranges of the physical-chemical properties of chlorobenzenes (CBs), polychlorinated biphenyls (PCBs) and polychlorinated dibenzo-p-dioxins (PCDDs). Fugacity ratios were obtained from a single estimated entropy of fusion of 56 J mol °K (Yalkowsky 1979), molar volumes were calculated by the Le Bas method, an additive group contribution method (Reid et al. 1977). Total surface area (TSA) values were obtained from Yalkowsky et al. (1979 a,b). Solubilities, vapour pressures and octanol/water partition coefficients (Andren et al. 1986 Shiu and Mackay 1986 Bobra et al. 1985) are also tabulated. Henry s law constants were calculated as PSl/C and the octanol solubility Q as C Kq, . 

Physical properties such as density, viscosity, thermal conductivity, and heat capacity are generally not a serious problem in simulation. The group-contribution methods are reasonably good, and simulator databank include experimental heat capacity data for more than a thousand substances. Although these correlations have random and systematic errors of several percent, this is close enough for most purposes. (However, they are not sufficient when you are paying for a fluid crossing a boundary based on volumetric flowrate.) As noted in Section 13.2.2. one should always be aware of which properties are estimated and which are from experimental measurements. 

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