IDENTIFICATION OF SURFACE ACTIVE AGENTS 791 times in both chromatograms. These peaks are assigned to the di-esters present in diethylene glycol monostearate. These compounds have no available hydroxyl groups and, therefore, do not react with hexamethyl- disilazane. The fact that these components are identical in the two chromatograms confirms the fact that this is a mixture of the di-esters derived from diethylene glycol. The mono-ester components, peaks B and C, which still contain a hydroxyl group do show a considerable change on trimethylsilylation. Peak A is due to free diethylene glycol. Figure 3 shows a comparison of the chromatograms for ethylene glycol monostearate before and after reaction with hexamethyldisilazane. In this case, the di-ester peaks C, D, and E are identical in both chromato- grams. Peaks A and B, due to the mono-ester, are also almost the same in both cases. This is due to the relatively non-polar characteristic of the mono-ester of ethylene glycol monostearate. However, there are changes that have occurred in the mono-ester peaks, before and after trimethylsilylation. The retention times of the two unreacted compo- nents are slightly longer than those of their counterparts in the bottom chromatogram. In addition, the reacted mono-esters show a considera- bly greater response for the same sample size. Even though the peaks have not undergone the considerable changes shown in the other exam- pies, it is still apparent that some reaction has taken place. There is no free ethylene glycol peak, since the derivative of this component is so low boiling it is not resolved from the pyridine used as a solvent. The three glycol stearates shown thus far can be easily distinguished by their chromatograms. Not only is there a difference between their response before and after trimethylsilylation, but in addition there are differences in the retention times of the various components which provide for absolute identification. It can be readily seen that through the use of calibration standards it should be possible to analyze these glycol stearates quantitatively for free glycol, mono-esters, and di-ester concentration. Presently used methods for glycol and mono-ester are rather lengthy wet chemical procedures. There is no direct quantitative method for di-ester content commonly in use. The di-ester content could be an important factor in the performance of a surface active agent, such as glyceryl monostearate Other glycol esters that have been studied by this procedure include diethylene glycol mono-oleate, propylene glycol monostearate, diethyl- ene glycol monolaurate, diglycerol monostearate, triglycerol mono- stearate, and decaglycerol monostearate. In each case the surface

792 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS