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Molecular Weight Determination of Oils
The molecular weight is a basic quantity needed in many kinds of projects involving the materials that are the subject of this symposium. It is needed to convert weight fractions to mole fractions, which occur in thermodynamic expressions which one uses in the design, evaluation and operation of all kinds of separation equipment and processes. In the examination of natural as well as manufactured oil mixtures, it may be desirable to characterize them by means of a hydrocarbon type analysis, which often requires the direct knowledge of molecular weight. Development of the methods for measuring molecular weight has paralleled the remarkable progress of science during the past decades. These methods are often grouped according to the averaging procedures involved. If a method gives the average of the molecular weights of the individual mole fraction, (where wi is the weight, and Mi the molecular weight of the i-th fraction of each molecular species in the mixture), the resulting value is called the number average molecular weight. Methods that yield the average of the molecular weights of the individual weight fractions, , give a weight average molecular weight. The light scattering method is representative of this group. Other methods, such as the ultracentrifuge, and the viscosimetric procedures, furnish numbers that are more complicated functions of composition and molecular weight. For various reasons the latter procedures, including the weight average methods, are most useful in the high molecular weight range, of the order of 104 and up, as for polymers, natural macromolecules, and even micellar structures. The application of these techniques actually becomes more difficult, and their results less reliable, at the lower levels. While these methods have been used down to molecular weights around 1000, they are not generally practical at this level. Since the oils that are the topic of this symposium have a molecular weight generally of well below 1000, we shall not discuss these methods further.
Stross, F. H.
Shell Development Co., Emeryville, Calif.
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