THE EFFECT OF THE SUBPHASE COMPOSITION ON MONOSTEARIN-DISTEARIN MIXEDMONOLAYERS

In this work we have studied the behaviour of mixed monostearin and di stearin monolayers, both structural characteristics and monolayer mole cular loss, at the air-water interface, as a function of temperature, surface pressure, and interfacial and subphase compositions. The subph ases were aqueous solutions of ethanol, glucose, and sucrose at 0.5 M. The compression isotherms were obtained using an automated Langmuir-t ype him balance over a temperature range 10-40 degrees C. The results show a contraction in the structure of mixed films spread on water or aqueous ethanol solution. However, an expansion in the structure of th e mixed monolayer was observed with sugars in the subphase. These phen omena are indicative of the existence of different forms of associatio n between film-forming components, as a function of both interfacial a nd subphase composition. This association can be demonstrated by surfa ce pressure-area isotherms and by thermodynamic studies. The magnitude of interactions between him-forming components depends on the variabl es studied. The destabilization of mixed films has been followed kinet ically from relaxation experiments either at constant surface pressure or at the collapse area. Desorption and collapse are the mechanisms t hat control the monolayer molecular loss, depending on the experimenta l conditions.