MONO-ESTER CONTENT OF PROPYLENE GLYCOL MONOSTEARATE Apparatus Pyrex glass tube • in. internal diameter, 16 in. long, with glass wool plug and tap at one end. Dropping funnel. Buchner flask. Distillation apparatus. Preparation of Column Dry about 60 grams of silica gel in an oven at about 105øC. for about 3 hours. The silica gel is sufficiently dry when the surface of a test tube half-filled with cold water does not mist when held close to the surface of the gel. Silica gel dried in this manner has been found to lose 2.0- 3.5}/0 of its weight on ashing at 700øC. It should be noted that this preliminary drying of the silica gel is very important, as evidence suggests that a gel containing more than 10% water volatile at 105øC will effect no separation of the two esters. A thick slurry is then prepared by pouring about 100 ml of benzene on to the hot silica gel. In this manner, very little air is trapped in the gel, so that the slurry can be transferred to the glass tube within 10 minutes. The slurry is then carefully added to the tube which is about half full of the solvent. Once the column has been prepared, about 2 centimetre of depth of solvent is allowed to remain above the top of the gel. Experimental Procedure About one gram of material is dissolved in 10 to 20 ml of solvent and added to the column together with several washings. Solvent is allowed to drop into the column from the tap funnel, and at the same time is drawn through the column by aid of gentle suction into the Buchner Flask. The eluate is collected, the solvent is evaporated and the non-volatile residue is weighed. Preliminary Experimental Observations The following facts were established :-- 1. When the column is eluted with benzene the di-ester only is desorbed. 2. When the column is eluted with diethyl ether, mono-ester and fatty acids are desorbed. 3. Propylene glycol is strongly absorbed and is not desorbed by diethyl ether. Figure 1 depicts typical elution curves of the di-ester with benzene solvent. Curve (i) was obtained using a 1 gram sample of P.G.M.S. on a

16 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 12 in. column. Curves (ii) and (iii) show the effect of column length on rate of elution using same quantity of P.G.M.S. It is obvious that the 3 in. column was of insufficient size to retain all of the mono-ester, fatty acids and glycol. ioo q0 •o • 7o • •ø zo Io o o 7.00 400 Goo 60o I.OOO i,loo IRoo Number of ml of solvent passed through the column As in Quinlin and Weiser's I work, we have found that a slight increase in polarity of the solvent is sufficiently high to influence the weak polar attraction of the mono-ester to the gel. 9o 0 7o Figure - Eluti.. •it I0•o Ether 90% B•nzen• Hi•r• -- - I / D, E•r Con•e.r C*lculQ• [ro• I IO o o 200 400 boo •oo i,ooo I•.OO 1,4oo 1.6oo 1,8oo 2, ooo Number of ml of solvent passed through column Figure 2 shows that a 10% diethyl ether in benzene solution is suffi- ciently polar to slowly desorb mono-ester.