The effect of the subphase composition on monostearin-distearin mixed monolayers

Abstract

In this work we have studied the behaviour of mixed monostearin and distearin monolayers, both structural characteristics and monolayer molecular loss, at the air-water interface, as a function of temperature, surface pressure, and interfacial and subphase compositions. The subphases were aqueous solutions of ethanol, glucose, and sucrose at 0.5 M. The compression isotherms were obtained using an automated Langmuir-type film balance over a temperature range 10–40°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 the mixed monolayer was observed with sugars in the subphase. These phenomena are indicative of the existence of different forms of association between film-forming components, as a function of both interfacial and subphase composition. This association can be demonstrated by surface pressure-area isotherms and by thermodynamic studies. The magnitude of interactions between film-forming components depends on the variables studied. The destabilization of mixed films has been followed kinetically from relaxation experiments either at constant surface pressure or at the collapse area. Desorption and collapse are the mechanisms that control the monolayer molecular loss, depending on the experimental conditions.