The Joback method, often named Joback–Reid method, predicts eleven important and commonly used pure component thermodynamic properties from molecular structure only. It is named after Kevin G. Joback in 1984 and developed it further with Robert C. Reid. The Joback method is an extension of the Lydersen method and uses very similar groups, formulas, and parameters for the three properties the Lydersen already supported (critical temperature, critical pressure, critical volume). Joback and Reid extended the range of supported properties, created new parameters and modified slightly the formulas of the old Lydersen method.
Basic principles
Group-contribution method
The Joback method is a group-contribution method. These kinds of methods use basic structural information of a chemical molecule, like a list of simple functional groups, add parameters to these functional groups, and calculate thermophysical and transport properties as a function of the sum of group parameters. Joback assumes that there are no interactions between the groups, and therefore only uses additive contributions and no contributions for interactions between groups. Other group-contribution methods, especially methods like UNIFAC, which estimate mixture properties like activity coefficients, use both simple additive group parameters and group-interaction parameters. The big advantage of using only simple group parameters is the small number of needed parameters. The number of needed group-interaction parameters gets very high for an increasing number of groups (1 for two groups, 3 for three groups, 6 for four groups, 45 for ten groups and twice as much if the interactions are not symmetric). Nine of the properties are single temperature-independent values, mostly estimated by a simple sum of group contribution plus an addend. Two of the estimated properties are temperature-dependent: the ideal-gas heat capacity and the dynamic viscosity of liquids. The heat-capacity polynomial uses 4 parameters, and the viscosity equation only 2. In both cases the equation parameters are calculated by group contributions.
Model strengths and weaknesses
Strengths The popularity and success of the Joback method mainly originates from the single group list for all properties. This allows one to get all eleven supported properties from a single analysis of the molecular structure. The Joback method additionally uses a very simple and easy to assign group scheme, which makes the method usable for people with only basic chemical knowledge.
Weaknesses
Newer developments of estimation methods have shown that the quality of the Joback method is limited. The original authors already stated themselves in the original article abstract: "High accuracy is not claimed, but the proposed methods are often as or more accurate than techniques in common use today." The list of groups does not cover many common molecules sufficiently. Especially aromatic compounds are not differentiated from normal ring-containing components. This is a severe problem because aromatic and aliphatic components differ strongly. The data base Joback and Reid used for obtaining the group parameters was rather small and covered only a limited number of different molecules. The best coverage has been achieved for normal boiling points (438 components), and the worst for heats of fusion (155 components). Current developments that can use data banks, like the Dortmund Data Bank or the DIPPR data base, have a much broader coverage. The formula used for the prediction of the normal boiling point shows another problem. Joback assumed a constant contribution of added groups in homologous series like the alkanes. This doesn't describe the real behavior of the normal boiling points correctly. Instead of the constant contribution, a decrease of the contribution with increasing number of groups must be applied. The chosen formula of the Joback method leads to high deviations for large and small molecules and an acceptable good estimation only for mid-sized components.
Formulas In the following formulas Gi denotes a group contribution. Gi are counted for every single available group. If a group is present multiple times, each occurrence is counted separately.
Normal boiling point
T b [ K ] = 198.2 + ∑ T b , i . {\displaystyle T_{\text{b}}[{\text{K}}]=198.2+\sum T_{{\text{b}},i}.}
Melting point
T m [ K ] = 122.5 + ∑ T m , i . {\displaystyle T_{\text{m}}[{\text{K}}]=122.5+\sum T_{{\text{m}},i}.}
Critical temperature
T c [ K ] = T b [ 0.584 + 0.965 ∑ T c , i − ( ∑ T c , i ) 2 ] − 1 . {\displaystyle T_{\text{c}}[{\text{K}}]=T_{\text{b}}\left[0.584+0.965\sum T_{{\text{c}},i}-\left(\sum T_{{\text{c}},i}\right)^{2}\right]^{-1}.}
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