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AN ATMOSPHERIC OXIDATION OF VITAMIN C AND E IN THE SURFACTANT SYSTEM

 

MARTA SZYMULA

DEPARTMENT OF RADIOCHEMISTRY AND COLLOID CHEMISTRY

 

 

        Vitamin E (a-tocopherol, a-T) is the major, if not the only, antioxidant found in membranes, whereas vitamin C (ascorbate, AA) is one of the major water-soluble antioxidants. Several studies have shown that the association of ascorbic acid and their derivatives with a-tocopherol enhances the antioxidant capability of the two vitamins. The reason for such synergy has been ascribed to the protecting role that the ascorbic acid ring plays toward tocopherols in phospholipid vesicles, trough a hydrogen-transfer mechanism that involves the oxidation of a-tocopheryquinone back to a-tocopherol. The another opinion is that vitamin E and vitamin C act as antioxidants independent of each other and protect cells when compared to cells lacking both vitamins C and E. The ascorbic acid derivative / tocopherol mixtures are therefore a very promising and important tool for the stabilisation and protection of many different systems, such as biological materials, food, and artificial membranes used as drug carriers, from both the scientific and industrial point of view.

        It is well known that micelles mimic natural biomembranes. This is a reason why much interest has been shown in behaviour of ascorbic acid and a-tocopherol in surfactant systems.

        The results of vitamin C and E oxidation presented in this paper are the continuation of previous one in which the comparison of atmospheric and electrochemical oxidation of vitamin C in surfactant system was studied [1]. Our investigations are based on the well-known micellar system pentanol / water / sodium dodecyl sulfate as an anionic surfactant. Vitamin C can be solubilized up to 60% wt. into a microemulsion region of the system.

        It is known from literature that vitamin E can act as an antioxidant when it is solubilized in SDS solutions.  It was observed that adding the vitamin into surfactant solution at concentration above the micellar dissociation concentration (the value of MDC for SDS is 0.24% wt.) simply by shaking by hand could bring about dissolution of vitamin. As a result emulsion with small particle size of 50 nm (i.e. microemulsion size) but not stable against oxidation in air can be obtained.

        The kinetics of the ascorbic acid (Figure 1a) and a-tocopherol (Figure 1b) decomposition were determined in the systems which composition corresponded to one line of SDS concentration (6% wt.) in microemulsion region of the phase diagram. This line is running through the inverse micellar solution, the basis for W/O microemulsions, a bicontinuous part, and the aqueous micellar solution, which forms the basis for the O/W microemulsions. The initial concentration of vitamins was dictated by experimental conditions i.e. by reasonable value of absorbance in UV measurements. For vitamin C it was 0.002% wt.- lmax of 265 nm, for vitamin E 0.015% wt. - lmax of 292 nm.

Fig. 1. Remainder of vitamine C (a) and E (b) in the system SDS/water/pentanol.

        The comparison of ascorbic acid oxidation kinetics in presence (presented results) and absence [1] of a-T in the system shows that there are not tangible differences in the time of ascorbic acid decomposition. The main effect is an enhancement of AA oxidation with increasing pentanol concentration in the system. Ascorbic anion interacts with sodium dodecyl sulphate association structures, changing W/O microemulsion towards O/W system. The polar part of AA is exposed to aqueous solution and subjected to the oxidation.

        Ascorbic acid is stimulating of a-T decomposition as long as it is in non-oxi-dised form. After some time, when vitamin C is completely decomposed, all these micellar systems reveal more stability with regard to vitamin E - for ex. Figure 2.

Fig. 2. Vitamin E decomposition in W/O microemulsion

system (SDS/water/pentanol).

In Figure 2 additional two absorption peaks at the wavelengths of 260 nm and 269 nm corresponding to vitamin E oxidation products - tocopherylquinones can be observed.

Genrally, the acting of two vitamins in micellar system is similar to each other. They are both good antioxidants in ivestigated W/O micro-emulsions. In SDS micelles, ascorbic acid and a-toco-pherol are shown to have an additive effect, not a synergistic effect, as antioxidants.

 

References:

[1] M. Szymula, J. Szczypa and S.E. Friberg, J.Dispersion Science and Technology, in press, (1999).