Progress in the Chemistry of Fats and Other Lipids,

Abstract

The solubility of tristearin has been determined quantitatively in hexane, benzene, carbon tetrachloride, chloroform, ethyl acetate, acetone and ordinary unhydrogenated cottonseed oil. The solubility of this triglyceride is decreased markedly by increased polarity of the solvents. In the presence of solvents, tristearin exhibits three distinct solubility curves corresponding to the three crystalline forms established previously by melting point determinations and X-ray measurements. The occurrence of a fourth crystalline form reportedly melting at 70° is not confirmed by the solubility studies. The polymorphic behavior of the triglycerides has received more attention than all other aliphatic compounds. Probably because of differences in reported results, particular emphasis has been placed on investigation of the saturated, single-acid triglycerides. Malkin,2-3 Button4-6 and others7·8 have reported numerous attempts to characterize the several crystalline modifications of these tri-glycerides by correlating their observed melting points with the X-ray diffraction measurements of the respective crystal lattices. Until recently, there has been some question as to whether three or four basic configurations of triglycerides occur, and the various investigators have differed further over correlation of observed melting points with the respective crystal forms revealed by the X-ray diffraction studies. It now appears that three distinct crystal forms of the saturated, single-acid triglycerides have been established quite definitely .9· 10 With the exception of a report on solubilities in aceto-and butyroglycerides,11 no solubility studies comparable to those on other aliphatic compounds have been reported for the triglycerides. This paper reports the solubility of tristearin in hexane, benzene, carbon tetrachloride, chloroform, ethyl acetate, acetone and ordinary unhydrogenated cottonseed oil. The observed polymorphic behavior is correlated with the known thermal characteristics and crystalline structure of tri-stearin. Experimental The triste arm used in this investigation was prepared by Dr. R. R. Allen by the interesterification of triacetin with highly purified methyl stearate using sodium methoxide as the catalyst. The product was recrystallized repeatedly from acetone. Cooling and heating curves were obtained by the usual (1) Presented before the Division of Paint, procedure used in this Laboratory .11 Fifteen-gram samples of tristearin were placed in a jacketed Pyrex test-tube equipped with a mercury thermometer and Nichrome wire stirrer. The thermometer which was graduated in 0.1 ° intervals had been calibrated by the National Bureau of Standards. The stirrer was actuated uniformly by means of an oscillating device operated by an electric motor controlled by a Variac transformer. Samples were stirred during cooling only until crystals appeared; the samples were not stirred Fig. 1.-Cooling curve of tristearin showing plateaus at the a and ß' freezing points. seconds. Fig. 2.-Heating curve of tristearin after minimum holding time showing a plateau at the ß' melting point and a change of slope at the ß melting point.