Photostabilization of ascorbic acid with citric acid, tartaric acid and boric acid in cream formulations.

This study involves the evaluation of the effect of certain stabilizers, that is, citric acid (CT), tartaric acid (TA) and boric acid (BA) on the degradation of ascorbic acid (AH(2) ) in oil-in-water cream formulations exposed to the UV light and stored in the dark. The apparent first-order rate constants (0.34-0.95 × 10(-3) min(-1) in light, 0.38-1.24 × 10(-2) day(-1) in dark) for the degradation reactions in the presence of the stabilizers have been determined. These rate constants have been used to derive the second-order rate constants (0.26-1.45 × 10(-2) M(-1) min(-1) in light, 3.75-8.50 × 10(-3) M(-1) day(-1) in dark) for the interaction of AH(2) and the individual stabilizers. These stabilizers are effective in causing the inhibition of the rate of degradation of AH(2) both in the light and in the dark. The inhibitory effect of the stabilizers is in the order of CT > TA > BA. The rate of degradation of AH(2) in the presence of these stabilizers in the light is about 120 times higher than that in the dark. This could be explained on the basis of the deactivation of AH(2) -excited triplet state by CT and TA and by the inhibition of AH(2) degradation through complex formation with BA. AH(2) leads to the formation of dehydroascorbic acid (A) by chemical and photooxidation in cream formulations.

Photochemical interaction of ascorbic acid with riboflavin, nicotinamide and alpha-tocopherol in cream formulations.

The present work is based on a study of the effect of some vitamins such as riboflavin (RF), nicotinamide (NA) and alpha-tocopherol (TP) on the photodegradation of ascorbic acid (vitamin C) (AH2) in oil-in-water cream formulations using a UV irradiation source. A UV spectrophotometric and the official iodimetric methods have been used for the assay of AH2 in cream formulations. These methods have been validated in the presence of RF, NA and TP before their application to the creams. The recoveries of AH2 in the creams are in the range of 90-95% and the reproducibility of the method is within ±5%. The apparent first-order rate constants (k(obs) ) for the photodegradation of AH2 in the presence of RF, NA and TP, individually, in the creams have been obtained. The second-order rate constants for the photochemical interaction of AH2 and the vitamins RF, NA and TP have been determined from the plots of k(obs) for AH2 photolysis versus the individual vitamin concentration along with the values of k0 from the intercept on the vertical axis. The values of k0 in the presence of RF and NA are lower than those of the k(obs) , indicating that these vitamins act as photosensitizers for the degradation of AH2 in creams. On the contrary, the value of k0 in the presence of TP is higher than that of the k(obs) , suggesting a stabilizing effect of this vitamin on the degradation of AH2 in creams. The mode of interaction of the individual vitamins with AH2 on photolysis has been discussed.