Joback method
The Joback method, often named Joback–Reid method, predicts eleven important and commonly used pure component thermodynamic properties from molecular structure only.
Joback and Reid extended the range of supported properties, created new parameters and modified slightly the formulas of the old Lydersen 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.
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.
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.
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.
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 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.
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.
In the following formulas Gi denotes a group contribution.
If a group is present multiple times, each occurrence is counted separately.
This critical-temperature equation needs a normal boiling point Tb.
If an experimental value is available, it is recommended to use this boiling point.
It is, on the other hand, also possible to input the normal boiling point estimated by the Joback method.
where Na is the number of atoms in the molecular structure (including hydrogens).
The Joback method uses a four-parameter polynomial to describe the temperature dependency of the ideal-gas heat capacity.
These parameters are valid from 273 K to about 1000 K. But you are able to extend it to 1500K if you don't mind a bit of uncertainty here and there.
The method uses a two-parameter equation to describe the temperature dependency of the dynamic viscosity.
Since the methyl group is present twice, its contributions have to be added twice.
