Vitamin C/Stearic Acid Hybrid Monolayer Adsorption at Air-Water and Air-Solid Interfaces.

Abstract

Because of the antioxidant activity of vitamin C (Vit C) polar heads, they can be used as a protective agent for fatty acids. Hence, the study on the growth of Vit C/stearic acid (SA) mixed binary films at air–water interface (known as Langmuir monolayer) and air–solid interface (known as Langmuir–Blodgett films) is of paramount interest. Although Vit C is situated at subsurface beneath SA molecules and interacts via hydrogen bonding between the hydroxyl groups of Vit C and SA, several Vit C molecules may infiltrate within SA two-dimensional matrix at the air–water interface. The increased mole fraction of Vit C (0.125–0.5) and the reduction of temperature (from 22 to 10 °C) of the subphase water result in an increase in the amount of adsorbed Vit C at the air–water interface. The surface pressure (π)–area (A) isotherms illustrate that such inclusion of Vit C provokes a spreading out of Vit C/SA binary monolayers, which leads to an alteration of different physicochemical parameters such as elasticity, Gibbs free energy of mixing, enthalpy, entropy, interaction energy parameter, and activity coefficient. However, being polar in nature, the transfer of pure Vit C on substrates gets affected. It can be transferred onto substrate by mixing suitably with SA as confirmed by infrared spectra. Their structures, extracted X-ray reflectivity, and atomic force microscopy (topography and phase imaging) are found to be strongly dependent on the nature of the substrate (hydrophilic and hydrophobic).