SOME PHYSICAL CHEMICAL PROPERTIES OF STEARIC ACID* By C. C. TILLOTSON The Procter & Gamble Company, Cincinnati 17, Ohio IT IS WELL KNOWN that the most widely used stearic acid of com- merce is not pure stearic acid. It is a mixture consisting of approximately 55 per cent palmitic acid and 45 per cent stearic acid. Also, it contains other fatty acids primarily oleic, linoleic and myristic in amounts varying with the grade and manufacturing method used. This ratio of palmitic to stearic acid was not originally arrived at through experimentation and selection but was provided by nature in tallow and used because the early methods of manufacturing resulted in this particular combination of fatty acids. A typical composition of fatty acids found in tallow is: Myristic ...................................... 2.0% Palmitic ...................................... 28.0 Stearic ....................................... 21.0 Oleic ......................................... 45.0 Linoleic ...................................... 4.0 Grease is also a source of commercial stearic acid. However, it contains a greater proportion of pahnitic to stearic acid than tallow and also con- tains a somewhat higher percentage of oleic and linoleic acids. For clarity of definition, the term "commercial stearic acid" will be used when referring to the combination of approximately 55/45 palmitic to stearic acid. The term "stearic acid" then will mean the saturated fatty acid, C•7H35COOH. Before discussing some of the physical chemical properties of stearic acid and their importance in cosmetic formulations, it would be well to consider briefly the methods of manufacture for the various types of stearic acid available. 1. Fat Splitting. All oils and fats used to manufacture fatty acids first go through a saponification or fat-splitting process which may be done by * Presented at the December 9, 1954, Meeting, New York City. 4O

PHYSICAL CHEMICAL PROPERTIES OF STEARiC ACID 41 either of two methods--one a batch, the other a continuous process. The batch method is the well-known Twitchell process. The continuous method employs a hydrolyzer and is carried out at high temperature under pressure. Each method results in converting the glycerides into corre- sponding fatty acids and glycerin. 2. Separation. After saponification or fat splitting, the fatty acids are separated from the glycerin and purified to obtain the desired fatty acids by one or more of the following processes: (a) Pan and Press. This method is normally used with fatty acid mixtures obtained from tallow and grease. The higher molecular weight saturated fatty acids, i.e., stearic and palmitic, will solidify leaving the un- saturated and the lower molecular weight fatty acids (oleic, linoleic, and myristic) in the liquid phase. The particular combination of 55/45 palmitic to stearic acid gives a large-size crystal structure which upon chilling permits the most complete separation of the liquid and solid fatty acids during the pressing operation. Briefly, this consists of chilling the mixture of fatty acids obtained after saponification to about 36-40øF. The solid cakes obtained upon crystallization are then pressed to remove a large portion of the liquid fatty acids. To remove the remaining liquid fatty acids entrained during cold pressing, the pressed cakes are remelted, allowed to crystallize, and repressed in a hot pressing operation at about 100øF. The number of pressings or amount of time in the hot press is the basis for the terminology single, double, and triple pressed grades of commercial stearic acid. (b) Solvent Crystallization. This consists simply of fractional crystal- lization of solid fatty acids from a solvent solution. It may be carried out over a successive number of crystallizations depending upon the degree of purity desired in the final product. This method produces stearic acid with the approximate 55/45 ratio of palmitic to stearic when tallow fatty acids are used. However, different ratios may be obtained depending upon the mixture of palmitic-stearic acids present in the original fatty acid after fat splitting. (c) FractionalDistillation. The fatty acids obtained after fat splitting may be separated into fatty acids of different chain lengths by fractional distillation. This process does not separate saturated from unsaturated fatty acids of the same chain length. For example, oleic has the same chain length as stearic and both will distill at the same temperature. Therefore, to obtain pure saturated fatty acids it is important to separate the saturated from the unsaturated fatty acids by either solvent crystalliza- tion or pressing before fractional distillation. Hydrogenation permits the conversion of unsaturated fatty acids to the corresponding saturated fatty acids. By a combination of hydrogenation and fractional distillation nearly pure saturated fatty acids can be ob-