The influence of the fluid–wall interaction potential on the structure of a simple fluid in a narrow slit

Abstract

The structure of a Lennard-Jones fluid in a narrow planar slit is studied for different kinds of wall–fluid interaction potentials by applying anisotropic integral equation theories. Density profiles and in some cases the force between the walls as a function of the slit width are calculated. It is found that when the state of the fluid in the slit is close to the liquid–vapor coexistence line, there must exist a distinct attractive minimum in the wall potential in order to induce an oscillatory density profile in the slit. Then, the force as a function of the slit width is also oscillatory. The repulsive part of the wall potential has little influence on the oscillatory behavior. When the wall potential consists of a very soft repulsive part close to the wall followed by a diffuse attractive minimum further out, the layer structure disappears completely. However, when the pressure of the fluid is raised above the vapor pressure, the oscillatory structure slowly reappears. These results are utilized to discuss certain experimental surface force measurements where solid, molecularly smooth walls have been modified by adsorption of amphiphiles. It is suggested that the main reason why the oscillatory forces do not appear in such systems is that the attractive part of the wall potential becomes diffuse and not that the repulsive wall becomes soft.