F ORMULATION AND S TABILITY OF A SCORBIC A CID IN L IQUID AND S EMISOLID P REPARATIONS

Abstract

The present investigation is based on a study of the photodegradation of ascorbic acid (vitamin C) in organic solvents and in oil-in-water cream preparations containing a combination of emulsifying agents and humectants. It also involves the study of the effect of other vitamins (riboflavin, nicotinamide and alpha-tocopherol) and certain compounds acting as stabilizing agents (citric acid, tartaric acid and boric acid) on the rate of photodegradation of ascorbic acid in cream preparations. The photodegradation of ascorbic acid in organic solvents and cream preparations (pH 4.0–7.0) leads to the formation of dehydroascorbic acid which is also biologically active. The kinetics of photodegradation of ascorbic acid alone and in combination with other vitamins in creams has been studied using a UV spectrophotometric method and the official iodimetric method, respectively. These methods were validated in the presence and absence of other vitamins / stabilizing agents under the experimental conditions employed. The recoveries of ascorbic acid in creams are in the range of 90–96% and the reproducibility of the analytical methods is within ± 5%. The apparent first-order rate constants (k obs ) for the photodegradation of ascorbic acid in aqueous / organic solvents (0.29–0.40 × 10 –3 min –1 ) and in creams (0.44–1.42 × 10 –3 min –1 ) have been determined. A linear relationship has been observed between k obs and solvent dielectric constant / reciprocal of solvent viscosity indicating the dependence of the rate of photodegradation on solvent characteristics. In the creams the photodegradation of ascorbic acid appears to be affected by the concentration of other vitamins, pH of the medium, carbon chain length of the emulsifying agents (myristic acid, palmitic acid and stearic acid), viscosity of the ivhumectant (ethylene glycol, propylene glycol and glycerin) and redox potentials of ascorbic acid. The study indicates that the relative polar character of the emulsifying agent and the ionized state and redox potential of ascorbic acid at a particular pH are important factors in the photodegradation of ascorbic acid in creams. The second-order rate constants (k′) (3.20 × 10 –2 – 1.89 M –1 min –1 ) for the photochemical interaction of ascorbic acid and the individual vitamins (riboflavin, nicotinamide, alpha-tocopherol) along with the values of k 0 obtained from the intercepts of the plots of k obs versus vitamin concentration are also reported. The values of k 0 indicate that riboflavin and nicotinamide act as photosensitizing agents, and alpha- tocopherol acts as a stabilizing agent in the photodegradation of ascorbic acid in the creams. The k obs verses pH profiles for the photodegradation of ascorbic acid in creams represents sigmoid type curves indicating the oxidation of the ionized form (AH – ) of ascorbic acid (pKa 1 4.1), with pH. The AH – species appears to be more susceptible to photooxidation than the non-ionized form of ascorbic acid. The effect of stabilizing agents on the photodegradation of ascorbic acid has been found to be in the order of citric acid > tartaric acid > boric acid. The low activity of boric acid may be to some extent due to its interaction with the emulsifying agents and humectants. The polarity of the emulsifying acids also plays a part in the rate of degradation of ascorbic acid. Reaction schemes for the photodegradation of ascorbic acid and its photochemical interaction with riboflavin, nicotinamide and alpha-tocopherol have been presented.