Homologous series classifications

While .. . homology provides for the chemist a convenient system of structural classification, inherent in that system are differences as well as similarities in the properties and reactions of the members of any homologous series. A chemist, and it is from the standpoint of a chemist skilled in this art that the question of obviousness must be resolved, would consider the differences as well as the similarities in the properties and reactions of the members in any given homologous series. .. ... 

Like dissolves like a substance is most soluble in that solvent to which it is most closely related in structure. This statement serves as a useful classification scheme for all organic molecules. The solubility measurements are done at room temperature with 1 drop of a liquid, or 5 mg of a solid (finely crushed), and 0.2 mL of solvent. The mixture should be rubbed with a rounded stirring rod and shaken vigorously. Lower members of a homologous series are easily classified higher members become more like the hydrocarbons from which they are derived. 

With this background, the reader should be ready to consider the trends in phase behavior for a number of experimental systems. Several compilations are available for experimental systems and their type classifications. These are typically in terms of homologous series, similar to the experimental programs themselves. We prefer to organize the list according to the type behavior. The type behavior plays a larger role the chemical processing. 

In commercial practice, these distinctions separate most foams into discrete classifications. In theoretical analysis, it is difficult to use a single process to produce a complete spectrum of structures, and it is difficult to relate properties to the complete spectrum of structure in a single unambiguous homologous series. It is hoped that such continuous analysis will be more feasible in the future. 

Makovicky, E. (1989). Modular classification of sulphosalts - current status. Definition and application of homologous series , N. Jahrb. Miner. Abh. 160, 269-297. 

Consideration of these characteristics makes it clear that only very special liquid pairs could conceivably form ideal solutions. It would be necessary that the molecules of the constituents be very similar in every respect, for example in structure, size, and chemical nature. Thus, solutions of optical isomers, adjacent members of an homologous series, and similar mixtures would be expected to be nearly ideal, but actually all solutions can at best only approach ideality as a limit. Solutions which form immiscible liquid phases are of necessity extremely nonideal, and extraction operations depend upon this. The extent to which solutions depart from ideality is manifested by deviations of the properties of the solutions from the characteristics listed above, and a study of these deviations will permit us to some extent to predict their behavior in extraction operations. The most useful characteristics of the ideal solution for these purposes is that of vapor pressure, since considerable information has now been accumulated for many mixtures on this and related properties such as boiling points of solutions, azeotropism, and vapor-liquid equilibria. Classifications of compounds according to the effect of intermolecular forces on properties of mixtures also provide much useful material, but the second and third characteristics in the list above are of limited value owing to lack of experimental data to which we can refer. 

The usefulness of the classification given in Fig. 4 has been demonstrated for many solvent-solute families . In such a family one component (e.g., the solvent 1) is maintained constant, whereas the solute 2 is varied systematically within a homologous series. 

The adsorption of snrfactants at liquid interfaces and their applications were described in many pnbhcations. Also books have been dedicated to this subject. For example in [1] not only the general classification of snrfactants and the synthesis and application of the main types of surfactants were discnssed but also the adsorption characteristics for a large number of homologous series of surfactants were snmmarized. hi various technologies the action of snrfactants is essential, snch as in the formation for foams and emulsions [2]. 

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