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AN ATMOSPHERIC OXIDATION OF VITAMIN C IN ANIONIC, NON-IONIC AND CATIONIC SURFACTANT SYSTEM
DEPARTMENT OF RADIOCHEMISTRY AND COLLOID CHEMISTRY
Vitamin C (L-ascorbic acid, AA) functions as an antiscorbutic agent and is a cofactor in variety of enzymatic processes. Furthermore, AA is an excellent antioxidant in vivo and in vitro.
It is well known that antioxidant activity of the antioxidant species in homogenous solutions may not be the same as that in heterogeneous media. This matrix dependence, is the reason for investigating vitamin C in micelles, in which it can be arranged in ordered way. In addition to this ordered environment, micelles also provide an interface between a hydrophilic and a hydrophobic region. This interface can be simple chemicals model for study the processes occurring at biological membranes.
In the experiments the kinetics of the ascorbic acid decomposition was determined by ultraviolet spectroscopy in aqueous solution of: anionic surfactant sodium dodecyl sulphate, SDS, of concentration in excess the value of cmc (i.e. 8.0×10-3 M), non-ionic surfactant octylphenol ethylene oxide, Triton X-100, (the value of cmc is 2.0×10-4 M), cationic surfactant hexadecyltrimethylammonium bromide, CTAB, (the value of cmc is 9.6×10-4 M).
The micelle formation of SDS shows a rather insignificant influence on the stability of ascorbic acid against oxidation (Fig. 1).
Fig.1. 0.002% Vitamin C decomposition in SDS aqueous solution.
The slight increase of oxidation rate at the cmc is probably due to premicellar associations of the acid with the surfactant: an assumption supported by the fact that the acid has a significant influence on the microemulsion droplet structure according to the results in presented early [1,2]. With the higher amounts of surfactant at concentrations in excess of cmc, on the other hand, the number of acid molecules in the micelles becomes small and the high polarity of the surfactant molecules causes the micelle to have a compact structure with a small radius: a feature interpreted as less favourable to oxidation because the AA molecules have tendency to locate at the strongly charged surface (vide infra).
Fig. 2. 0.002% Vitamin C decomposition in Triton X-100 aqueous solution
. Fig. 3. 0.002% Vitamin C decomposition in CETAB aqueous solution.
As can be seen at the Figures 2 and 3 the ascorbic acid is easier oxidized in micellar solution of non-ionic and cationic surfactants than in water. An organization of these micellar systems plays role. As a result of interaction of ascorbic anion with surfactants association structures, the polar part of AA is exposed to aqueous solution and subject to the oxidation.
For all kinetic curves presented at the Figures 1-3 an oxidation initiation rate R of ascorbic acid was calculated according to:
with nj as a stoichiometric factor characteristic for each antioxidants determined by Wayner  (n of ascorbic acid equals 2) and d[AO]j/dt as the consumption rate of each antioxidant. Rate R was calculated as a slope of linear regression of each experimental curves all micellar systems.
As expected, a general trend in the oxidation rate changes in a following manner: the micellar solution of non-ionic surfactant Triton X-100 shows faster oxidation (R » 1×10-3 [wt.%/h.]), than that of anionic surfactant SDS (R » 2×10-4[wt.%/h.]), the cationic one - CTAB an even higher ones (R » 1.3×10-3[wt.%/h.]). The more subtle effects were observed with each surfactant concentration changes.
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