SOLUBILITY OF CHOLESTEROL 81 Per cent by weight of cholesterol in isopropyl myristate (starting mixture) Per cent by weight of cholesterol soluble in isopropyl myristate (supernatant solution) 589 nm 365 nm 5.5 5.14 5.28 6.0 5.37 5.39 8.0 5.26 5.15 Average 5.26 5.27 This procedure would enable one to determine unknown cholesterol concentrations in isopropyl myristate accurately using the Perkin Elmer Model 141 Polarimeter (or equivalent) at least one order of magnitude below that reported for the lowest concentration shown in Table I. An 0.05 per cent solution of cholesterol in isopropyl myristate chloroform (1:1) has a rotation of -0.017 ø at 589 nm and -0.057 ø at 365 nm. Incidentally, at 365 nm, there is still sufficient readability of the rotation to go down to a 1 to 5 dilution. Table I Observed Rotation a Percent by Weight of Cholesterol in Solution Number Isopropyl Myristate • 589 • 365 1 1.0 -0.163 ø -0.562 ø 2 2.0 -0.366 ø -1.217 ø 3 3.0 -0.540 ø -1.760 ø 4 4.0 -0.695 ø -2.382 ø 5 5.0 -0.868 ø -2.920 ø 6 5.5 b -0.897 ø• - 3.103 ø• 7 6.0 b -0.938 øe -3.168 • 8 8.0 b -0.918 ø½ -3.028 • aRotation of cholesterol/isopropyl myristate solution diluted by an equivalent volume of chloroform. Read- ings were taken at 24øC. bCrystals of cholesterol present. CSupernatant solution assayed. CONCLUSION The estimation of cholesterol solubility in isopropyl myristate by the classical approach .::::of visually ascertaining when the system composition can no longer support a homo- geneous condition requires a large number of experiments in order to put upper and lower limits on solubility. The use of optical rotation yielded a precise solubility figure with a minimum of effort. In principal, one need only examine the supernatant of a single isopropyl myristate/cholesterol system for cholesterol content which contains solid in equilibrium with solution. The simple optical rotation procedure indicates cholesterol is soluble in isopropyl myristate to the extent of 5.26 per cent (w/w). This technique conceivably can be

82 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS extended to include the deter•nination of the solubility of cholesterol, cholesterol esters, and other optically active cholesterol congeners in isopropyl myristate and other optically inactive solvents. The optical rotation procedure described above is particularly attractive as compared to other analytical procedures (4) used to determine cholesterol content quantitatively with respect to speed and simplicity of the measure- ment. REFERENCES (1) E. S. Lower, "Cholesterol in Cosmetic Formulations--A Review," Drug Cosmet. Ind., 116, 54 (1975). (2) E. S. Lower, "Cholesterol in Cosmetic Formulations--A Review," Drug Cosmet. Ind., 116, 48 (1975). (3) E. S. Lower, "Cholesterol in Cosmetic Formulations--A Review," Drug Cosmet. Ind., 116, 57 (1975). (4) T. Higuchi and E. Brochmann--Hanssen, PharmaceuticalAnalysis, Interscience Publishers, New York, 1961, Pp. 119-25.