DESTABILIZATION OF MONOGLYCERIDE MONOLAYERS AT THE AIR-AQUEOUS SUBPHASE INTERFACE .2. THE ROLE OF FILM ELASTICITY

The aim of this work is to establish a quantitative relationship betwe en the destabilization of a monoglyceride (monomyristin) monolayer at the air-water interface as a function of subphase composition (ethanol and sugar solutions), temperature, and surface pressure. The destabil ization of monomyristin films has been followed kinetically by observi ng changes in the film area with time at constant surface pressure. It has been observed that the film stability is a phenomenon related to the cohesive forces in the film and to the interactions in the subsurf ace region. Film elasticity-expressed by the modulus (-d pi/dA)-is pro posed as the parameter which describes this relationship. Its value ha s been related to the mechanisms that control film destabilization. Th e most common relaxation mechanisms found are desorption caused by dis solution and diffusion in the subphase and collapse caused by nuclei f ormation and further growth. The diffusion-controlled mechanism only a ppears when the him has a condensed structure and with low values of t he elasticity modulus. For monolayers with a liquid-condensed or liqui d-expanded structure-with high values of the elasticity modulus-the re laxation mechanism of collapse with nuclei formation appears. The valu es of the kinetic parameters for the destabilization can be discussed in terms of interactions between subphase and film molecules.