Comparative Study of Adsorbed and Spread β-Casein Monolayers at the Water−Air Interface with the Pendant Drop Technique

A Langmuir-type pendant drop film balance has been applied in the study of protein monolayers at the water−air interface in a comparative study of adsorbed and spread β-casein layers. Trurnit's method for spreading proteins at the air−water interface has been modified and thoroughly adapted to the requirements of the pendant drop technique. Adsorbed β-casein layers have been obtained by means of double coaxial capillary that enables a subphase exchange, once the desired amount of protein attains the interface. Precisely, this becomes one of the most practical aspects of this technique, allowing a direct comparison between spread and adsorbed layers. Moreover, it provides an alternative to conventional spreading methods for obtaining protein layers and promises to be specially useful in the study of globular protein monolayers, which are very difficult to obtain with conventional methods. Comparison between spread and adsorbed π− A isotherms and static elasticity offers an excellent concordance, which increases in successive compression−expansion cycles, suggesting that a similar structural configuration is finally adopted at the interface independently of the technique (spreading or adsorption) used to form the layer.