Description of the adsorption behaviour of proteins at water/fluid interfaces in the framework of a two-dimensional solution model.

In the framework of the two-dimensional non-ideal solution model, surface layer equations of state, adsorption isotherms and functions of the distribution of protein molecules in respect to different molar area were derived. The thermodynamic analysis was based on Butler's equation for the chemical potentials of the components and a first-order model for the non-ideality of surface layer enthalpy and entropy. For concentrated solutions, aggregation of protein molecules in the surface layer was assumed. The resulting equations satisfactorily describe the measured adsorption and surface pressure isotherms of proteins at liquid/fluid interfaces in terms of a set of constant parameters. The model reflects the well-known differences between proteins and ordinary surfactants: a sharp increase in the surface pressure with concentration beyond a certain protein adsorption; an almost constant surface pressure at higher concentrations and a significant increase in the adsorption layer thickness with increasing adsorption for flexible proteins.