SPECTROSCOPIC INVESTIGATION OF ASSOCIATION BETWEEN AND ASCORBIC ACID Marta Szymula and Department of Radiochemistry *Department of M. Curie-Skłodowska University,

OF THE FORMATION EQUILIBRIUM PROPYL GALLATE IN MICROEMULSIONS Stanisław Radzki* and Colloid Chemistry Inorganic Chemistry M.C. Skłodowska Sq. 2, Lublin, Poland

PURPOSE The main purpose of our studies included examination of oxidation of water-soluble antioxidants i.e. ascorbic acid (vitamin C, H 2 A, AA) and propyl gallate (PG) in the surfactant systems. The problem of mutual action of two or more antioxidants is very important because a mixture of antioxidants has a stronger action than a single compound has. It was found that activity of antioxidants depends not only on their molecular structure but also on the reaction medium. We chose micellar solution as a medium of antioxidants action studies, having in mind the fact that many substances of hydrophilic and hydrophobic character (e.g. antioxidants) are solved/ solubilized in such a system. We also took into account the idea that the design of microscopic molecular assemblies (e.g. micelles), which mimic the microenvironment present in the biological system can contribute a great deal to the understanding of naturally occurring processes.

Ascorbic acid (Fig.1) and propyl gallate are easily dissolved in the SDS micellar solution and in the microemulsionof SDS/ pentanol/ water system (i.e. in the O/W microemulsion, in the bicontinous system and in the W/O microemulsion). Fig. 1. Ascorbic acid can be solubilized in up to 60% in the microemulsion region of the SDS/ pentanol/ water system.

Stoichiometry of propyl gallate and ascorbic acid associates, de Job's experiment Control of complex formation and determination of their stoichiometry were carried out according to Job's method of continuous variation, This method is based on spectrophotometric analysis of mixture of equimolar solutions in the ratio of x volumes of first compound solutions to 1-x of the second compound solution (in which the ratio of propyl gallate and ascorbic acid varies in increments from 10:0 up to 0:10). Stoichiometry of the associates formed between propyl gallate and ascorbic acid is shown in Fig. 2 at λ 265 nm for which absorbance values corresponds to λ max at u.v. spectrum of ascorbic acid. In the Job's plots for each analysed system, two straight lines crossing on the plot for molar fraction of propyl gallate = 0.5 can be noticed, which corresponds to 1:1 ascorbic acid and propyl gallate associate stoichiometry. It also can be noticed that the associates formed were decomposed with time. After 8 hrs the formation of associates disappear in water and microemulsion.

Determination of equilibrium constants for propyl gallate binding to ascorbic acid Typical evolutions of the absorption spectra of ascorbic acid occurring upon titration with propyl gallate (the first one in the bicontinuous microemulsion) and propyl gallate titrated with ascorbic acid (the second one in the W/O microemulsion) are shown in Fig. 3. The initial concentration of titrated AA and PG solutions was 0.002%. The evolution of the propyl gallate spectra during titration with ascorbic acid and vice versa indicated that the process of molecular complex formation took place, however, the interactions were rather weak inasmuch as formation of new bands was not observed. The conclusion is supported by presence of isosbestic points and deviation from the linearity of Beer- Lambert's law plots.

CALCULATIONS

The values of the association constants K [L mol -1 ] for ascorbic acid with propyl gallate associates in various microemulsions. Solution composition SystemK [L·mol -1 ] water aqueous solution ± 8% 6% SDS micellar solution ± 3% 6% SDS, 5% pentanol 89% waterO/W ± 10% 6% SDS, 22% pentanol 72% waterbicontinous ± 21% 6% SDS, 39% pentanol 55% waterW/O ± 27%

CONCLUSIONS Propyl gallate forms 1:1 molecular complexes with ascorbic acid, which appears to be due to the electron donor-acceptor forces and where perhaps propyl gallate plays the role of the acceptor. The associate formed were decomposed with time. After 8 hrs the formation of associates disappear in water and in microemulsion. Evolution of absorption spectra during the study of molecular complex formation goes through well- defined isosbestic points. The association constants were calculated using the curve-fitting procedure (Fig.4). The interactions observed are the strongest in reverse SDS micelles.

L-Ascorbic acid L-Dehydroascorbic acid -2H + -2e - +2H + +2e -

Propyl Gallate