Coadsorbed film, model

Radioisotopic tracer techniques were applied to study the coadsorption of n-octadecane and stearic acid on a metal surface immersed in a n-octadecane solution of stearic acid. Dual labeling was employed for determining the surface concentrations of both n-octadecane and stearic acid. n-Octadecane was labeled with tritium and stearic acid with carbon-14. The results of half-hour adsorption experiments provide direct proof of coadsorption of polar and nonpolar materials on iron, copper, silver, and platinum surfaces. The films produced on silver and copper by 19-hour adsorption consisted of approximately one molecular layer of stearic acid and two molecular layers of octadecane. A new model is proposed to describe the structure of this thick coadsorbed film. 

The results reported in this paper support the conclusion of Zis-man [2, 5] and others [1, 4], that films produced at comparatively short times contain both the polar solute and the nonpolar solvent, but that coadsorbed films are also formed at longer adsorption times. The proposed model describes the primary adsorbed layer next to the metal surface as consisting of nearly 100% adsorbed stearic acid at long adsorption times. 

Here, we investigate the stress response to large uniaxial deformations of model single-protein films and protein plus surfactant mixed films. We show that the general structure of a compressed (expanded) protein film is very sensitive to the breakability of the protein-protein bonds. We then study the structural changes and mechanical response of a protein plus surfactant mixed film to large compression (expansion). We show that the nature of the protein-protein bond parameters also affects the overall displacement behavior of the coadsorbed surfactant during compression. 

A revised picture is needed to describe the structure of films composed of coadsorbed polar solute molecules and nonpolar solvent molecules. The close-packed model of Reis implies that the coadsorbed solvent should be relatively tightly bound. That this is apparently not the case is suggested by the ease with which the n-octadecane was removed from all the films. The data show that n-octadecane is more firmly attached to the surfaces on which stearic acid is adsorbed than to surfaces on which no acid is adsorbed. This means that the coadsorbed n-octadecane found is not present in the film as relatively large aggregates of randomly oriented solvent molecules on the surface. These aggregates, if they exist on the surface initially, should have been removed by the 5-second rinse. This is based on the fact that solvent was removed from the silver mirrors which had been immersed in pure n-octadecane containing no stearic acid. 

---