AUTO-OXIDATION OF LINOLEIC ACID 293 M•10 4 2.20 2.10 0 360 720 1'080 ' 1•40 ' 1'800 ' sec, Figure 1. Oxidation of linoleic acid in microemulsion 8 (Table Ill) in the absence of initiator: molar oxygen concentration in the microemulsion vs time (sec). hexanediol) did not affect the oxidation rate significantly. The rates of oxygen uptake, Rp, for the microemulsions containing 1-butanol or 2-ethyl-l,3-hexanediol as cosur- factants, at increasing concentrations of linoleic acid, are reported in Table VII. The rate of oxygen uptake, Rp, showed a linear dependence on the linoleic acid concen- Table VII Rate of Oxidation of LH in Microemulsions with 2-Ethyl-l,3-Hexanediol and 1-Butanol as Cosurfactants, at Different Concentrations of Linoleic Acid LH ButOH HexOH AIBN Rp X 109 Microemulsion (M X 102) (% w/w) (% w/w) (M X 103) (M/s) 1 -- 11.94 -- 2.42 2 2.32 11.86 -- 3.00 4 4.61 11.79 -- 3.86 6 6.87 11.71 -- 5.86 8 9.09 11.64 -- 6.38 12 -- -- 13.51 -- 2.44 13 2.32 -- 13.43 -- 3.20 15 4.61 -- 13.34 -- 5.11 17 6.87 -- 13.26 -- 6.98 19 9.09 -- 13.17 -- 7.84 3 2.32 11.86 -- 2.46 4.12 5 4.61 11.79 4.80 8.44 7 6.87 11.71 -- 6.87 12.28 9 9.09 11.64 9.47 15.83 14 2.32 -- 13.43 2.46 3.84 16 4.61 -- 13.34 4.61 5.64 18 6.87 -- 13.26 6.87 9.56 20 9.09 -- 13.17 9.47 11.34

294 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS tration. The linear dependence was observed in the absence and in the presence of the azo-initiator, and with both the cosurfactants. The data of Rp are not appreciably influenced by the nature of the cosurfactant. Therefore, we conclude that the structure of cosurfactants is not very important in the protection of linoleic acid from auto- oxidation. An appreciable difference is observed, however, if the inhibitor is present or absent. The rates of oxygen uptake, in the presence of D,L-tx-tocopherol as inhibitor, Rinh, at two concentrations are reported in Table VIII the inhibitor effect is higher at the lower concentrations. This may suggest that tx-T can exhibit two opposite effects, in aqueous media, on linoleic acid auto-oxidation, either pro-oxidant or antioxidant, depending on its concentration. In fact, the rate of inhibition, Rinh, in the microemulsions with butanol, for the highest concentration of c•-T (5.0 X 10 -2 M), is higher than the Rp. Studies of Cillard and Cillard (21,23) on the behavior of c•-T on the oxidation of linoleic acid emphasied that tocopherols can exhibit opposite effects on the linoleic auto- oxidation rate in aqueous media. According to these authors, the effect observed in their studies depended on the tocopherol concentration and the kind of tocopherol used. After adding a concentration 1.56 x 10-4 M ofc•-T to the microemulsions with butanol, the Rin h was three to five times lower than Rp (see Table VIII). By using 2-ethyl-l,3- hexanediol as cosurfactant, the decrease of Rin h was less significant. c•-T did not greatly affect these systems, likely as a consequence of the microemulsion structure that pro- tected the linoleic acid from auto-oxidation. The rate of oxygen uptake, Rp, of the microemulsions containing a mixture of 1,2- propanediol and water as continuous phase, and 2-ethyl-1,3-hexanediol as cosurfactant in the presence or in the absence of azo-initiator, and the Ri• h values in the presence of c•-T are reported in Table IX. The presence of 1,2-propanediol in the external phase did not significantly affect the rate of reaction, and the values of Rp and Rich of the microemulsions in the absence of 1,2-propanediol were about the same. On the con- trary, a lowering of Tween 20 and of 2-ethyl-1,3-hexanediol concentration produced an increase of the oxidation rate, probably as a consequence of the different distribution of the surfactant and the cosurfactant among the three phases. From the literature data the dependence of the inhibition of auto-oxidation of linoleic acid upon the alcohol concentration is known (25,26). Likely the increase of oxidation Table VIII Rp and Rin h of Linoleic Acid, in the Absence and in the Presence of c•-T, with 2-Ethyl-1,3-Hexanediol and 1-Butanol as Cosurfactants LH ButOH HexOH AIBN c•-T Microemulsion (M X 10 2) (% w/w) (% w/w) (M X 10 3) (M) Rp X 10 9 Rin h X 10 9 (M/s) (M/s) 9 9.09 11.64 9.47 -- 15.83 -- 10 9.09 11.64 9.47 1.56 x 10 -4 -- 2.67 11 9.09 11.64 -- 9.47 6.11 x 10 -4 -- 7.20 11a 9.09 11.64 9.47 5.00 X 10 -2 -- 25.00 20 9.09 -- 13.17 9.47 -- 11.34 -- 21 9.09 -- 13.17 9.47 1.56 X 10 -4 -- 4.44 22 9.09 m 13.17 9.47 6.11 x 10 -4 -- 10.30