Mixtures of stearoyl-6-O-ascorbic acid and alpha-tocopherol: A monolayer study at the gas/water interface

In the recent past the antioxidant properties of vitamin C, vitamin E, and their derivatives have been extensively studied, particularly for their peculiar activity as radical scavengers. Furthermore, the association of vitamin C with vitamin E shows an interesting synergic effect in preventing the oxidation of unsaturated fatty compounds such as linoleic acid, with vitamin C acting as a protective shield for vitamin E in solution against oxidizing agents. Vitamin C derivatives, such as its alkanoyl-6-O-ascorbic acid esters, act even better than ascorbic acid itself, because they possess the amphiphilic structure that allows the molecule to enter the hydrophobic region of a micelle or of a liposome. In order to deepen the understanding of the vitamin C/vitamin E mixed system and of the role played by ascorbic acid in restoring tocopherols, the miscibility properties of ascorbic acid derivatives and of tocopherols need to be studied in all states of aggregation (Langmuir monolayers, micellar dispersions, vesicles). In this paper we report a detailed study of the properties and behavior of monomolecular films of stearoyl-6-O-ascorbic acid (ascorbyl stearate), and alpha-(+/-)-tocopherol at different temperatures and pH, and of their mixtures at 20 degrees C at the gas/water interface. Our results show that ascorbyl stearate produces monolayers with both liquid-expanded and liquid-condensed phases, while alpha-tocopherol films are in the liquid-expanded phase only. The two pure components are completely miscible at the gas/water interface in all ratios, and indicate the presence of repulsive interactions between the hydrocarbon chains of the surfactants.