DISPERSE SYSTEMS AS TOPICAL VEHICLES 213 MEASUREMENT OF OXYGEN UPTAKE The oxidation rates of linalool or citral in micellar solutions were assessed by monitoring the molar concentration of dissolved oxygen a lipophilic azoinitiator (AIBN 0.25% w/w) was added to initiate autooxidation. The biological oxygen monitor and the method employed have been described elsewhere (9,10). The reaction vessel was stirred with a rod-shaped magnet, connected to an oxygen electrode and then thermostated at 45.0 ø + 0.1øC to promote decomposition of the azoinitiator. Monitoring was continued for two hours to prevent polarization to the oxygen probe. For each system under study, the measurement was repeated six times. The w/w percentage compositions of the miceliar solutions were as follows: Surfactant Surfactant mixture 3 Hexylene Name mixture 3 without lecithin Water Linalool Citral Ethanol glycol L-Lec 14.40 75.48 5.00 2.27 1.85 LC-Lec 14.40 75.48 4.00 1.00 2.27 1.85 C-Lec 14.40 79.48 1.00 2.27 1.85 LC 14.40 76.48 4.00 1.00 2.27 1.85 L 14.40 75.48 5.00 2.27 1.85 Reference 14.40 81.48 2.27 1.85 Rheology. Rheology steady-state flow experiments were performed at 25.0 ø + 0.1øC by means of a rotational viscometer measuring shear stress as a function of increasing and decreasing shear rates, employing a small adapter chamber with spindle no. 21 (11). The measurements were performed as follows on miceliar solutions containing linalool 5.0% w/w and surfactant mixtures 1, 2, or 3 (hexylene glycol as cosurfactant), with the full compositions given in Table II (miceliar solutions A, B, D): (a) 24 hours after preparation (b) after 15, 30, or 60 minutes evaporation at 25.0 ø + 0.1øC and 63% --- 5% relative humidity and (c) on systems diluted 1:1 with water, then as described in (a) and (b). After this preliminary study, so as to be able to eliminate ethanol from the formulation and to improve the resulting rheological properties, some humectants (xylytol, laureth- 20, methyl gluceth-10, PEG-120 methyl glucose dioleate, and glycerol) were added to miceliar solution D (with surfactant mixture 3). Challenge test. The following miceliar solution of linalool was submitted to a challenge test: surfactant mixture 3 = 14.40% w/w, linalool = 5.00% w/w, hexylene glycol = 1.85% w/w, PEG-120 methylglucose dioleate = 0.50% w/w, PCA = 2.00% w/w, and water = 76.25% w/w. Four mixed cultures, consisting of four different groups of microorganisms, were used. The inoculum level was as follows: Gram(+)ve bacteria Gram(-)ve bacteria Yeasts Molds 1 x 10 7 microorganisms per gram product 1 x 107 microorganisms per gram product 1 x 10 7 microorganisms per gram product 1 x 105 microorganisms per gram product

214 JOURNAL OF COSMETIC SCIENCE The inocula were prepared as described in a previous paper (12) and each microbe strain operation was completed separately. Four separate samples of the miceliar solution were inoculated with one of the four microorganism groups, at a ratio of 1 ml per 100 g of sample, and maintained at room temperature for the duration of the test. The presence of viable microorganisms was investigated by means of the plate count method described elsewhere (12) 24 hours, 7 days, and 30 days after inoculation. Surface and interfacial tension measurements. All measurements were performed at 37.0 ø + 0.1øC using a ring tensiometer. Surface tension measurements were executed on the oils used to formulate the microemulsions, i.e., IPP, n-dodecanol, C•2_•5AB , mineral oil, and caprylic-capric triglyceride, as follows: oil alone 9:1 (w:w) oil:linalool 1:1 (w:w) oil:linalool and 1:1 (w:w) oil:linalool (w:w 1:1), with the addition of increasing per- centages of surfactant mixture 2 (1.0%, 5.0%, and 10.0% w/w). Interfacial tension measurements were done on oil/water systems with the above men- tioned oils. When IPP, C•2_•5 alkylbenzoate, mineral oil, and caprylic-capric triglyc- eride were used as oils, the following ratios applied: oil/water 9:1 (w:w) oil:linalool/ water 1:1 (w:w) oil:linalool/water and 9:1 (w:w) oil:linalool with the addition of increasing percentages of surfactant mixture 2 (0.05%, 0.10%, 0.15%, 0.30%, and 0.50% w/w)/water. When n-dodecanol was used as oil, the following ratios applied: oil/water 9:1 (w:w) oil:linalool/water 1:1 (w:w) oil:linalool/water, and 1:1 (w:w) oil- linalool, with the addition of increasing percentages of surfactant mixture 2 (0.05%, 0.10%, 0.15%, 0.30%, and 0.50% w/w)/water. Dilution of microemulsions. A 4-ml volume of each microemulsion of linalool obtained with surfactant mixture 2, IPP, C•_•5AB, and n-dodecanol was poured into a vessel containing 700 ml water at 45øC to simulate the real-use conditions of bath oils. RESULTS AND DISCUSSION Several miceliar solutions of linalool ranging from 2.8% to 10.6% w/w were obtained using surfactant mixtures 1, 2, or 3 and hexylene glycol as cosurfactant, or surfactant mixtures 4, 5, or 6 and CDCNa as cosurfactant. Although higher percentages of linalool required higher amounts of surfactant to be solubilized, linalool showed some cosurfac- rant effect, probably due to its somewhat amphiphilic structure. The composition of the miceliar solutions containing the lowest and the highest percentages of linalool solubilized by each surfactant mixture (s. m.) are reported in Table I. The pH values of the miceliar solutions varied with the different surfactant mixtures used and were constant over time for miceliar solutions obtained with surfactant mixture 1: pH = 4.83 + 0.02 with surfactant mixture 2: pH = 4.62 + 0.03 with surfactant mixture 3: pH -- 4.79 _+ 0.02 with surfactant mixture 4: pH = 6.06 _+ 0.18 with surfactant mixture 5: pH = 5.74 + 0.18 with surfactant mixture 6: pH -- 5.85 _+ 0.10. The pH values changed little if at all after repeated centrifuging and freeze-thaw cycles. No correction of the pH values was necessary, as they were acceptable for a potential cosmetic application. The miceliar solutions whose mean diameters were investigated before and after three- month stability tests were those containing an intermediate percentage of linalool (5.0% w/w) or, when possible, the maximum percentage (10.0% w/w) (Table II).