212 JOURNAL OF COSMETIC SCIENCE (Eppendorf, Hamburg) digital tensiometer K10PST (Krtiss, Hamburg) oxygen moni- tor Model YS 153 equipped with polarographic electrodes (Yellow Spring Instruments Co.) and HPLC apparatus consisting of an UV detector SP-2A, a pump unit control LC 6A, and a C-R 3A chromatopac integrator (Shimadzu, Japan). FORMULATION OF MICELLAR SOLUTIONS OF LINALOOL Miceliar solutions of linalool were prepared at room temperature using the following surfactant mixtures: 1) Dodecyl polyglucose-cocoamide propylbetaine 56.5% w/w--lecithin 43.5% w/w. 2) Decyl polyglucose 54.5% w/w--lecithin 45.5% w/w. 3) Decyl polyglucose 40.6% w/w--caprylyl-capryl glucoside 17.7% w/w--lecithin 41.7% w/w. 4) Dodecyl glucoside cocoamide propylbetaine 60.0% w/w--lecithin 40.0% w/w. 5) Decyl polyglucose 60.0% w/w--lecithin 40.0% w/w. 6) Decyl polyglucose 39.9% w/w--caprylyl-capryl glucoside 16.6% w/w--lecithin 43.5% w/w. Hexylene glycol was used as cosurfactant with surfactant mixtures 1, 2, and 3. CDCNa was used as cosurfactant with surfactant mixtures 4, 5, and 6. Increasing amounts of linalool (up to 10.0% w/w) were dispersed in water, and then fixed amounts of cosur- factant and ethanol were added and finally brought to transparency with an appropriate quantity of the chosen surfactant mixture. FORMULATION OF MICELLAR SOLUTIONS OF LINALOOL AND CITRAL Miceliar solutions containing both odorous molecules were prepared by partially replac- ing linalool with citral in those miceliar solutions previously obtained containing 5.0% w/w linalool, employing the surfactant mixtures 1, 2, or 3 (hexylene glycol was used as cosurfactant). FORMULATION OF MICROEMULSIONS OF LINALOOL Microemulsions were prepared with a fluid lipid (IPP, n-dodecanol, C•2_•sAB, mineral oil, cyclomethicone, and caprylic-capric triglyceride), replacing half the amount of linalool (10.0% w/w) used for the miceliar solutions containing surfactant mixtures 1, 2, or 3. Lecithin was dispersed in a water-ethanol solution, hexylene glycol was added, and finally it was titrated to transparency with the chosen surfactant mixture. CHARACTERIZATION OF MICELLAR SOLUTIONS AND MICROEMULSIONS WITH LINALOOL Stability tests. Miceliar solutions and microemulsions were tested for stability by applying repeated centrifuging and freeze-thaw cycles: the systems were centrifuged from 2000 rpm to 13000 rpm for 20 min, then stored at -20øC for three weeks and heated to 40øC for four hours. Each cycle was repeated for three months (7). Mean diameters. The mean diameters of the micelles and of the droplets in the micro- emulsions were determined at 25.0 ø + 0.1øC by means of photocorrelation laser spec- troscopy as described in a previous paper (8).

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