Point-particle interaction with substrate

However, two important points should be noted. Firstly, that when one of the ions of an added electrolyte can interact with the surfactant to form an insoluble salt this reaction can remove the adsorbed layer from the particle (see Table I). Secondly, the hydrophobic chain of the surfactant must be compatible with the particle surface. This point is illustrated in Figure 10 which shows the adsorption of dodecanoic acid on to PTFE particles. The adsorption of the C11H23 hydrocarbon chain to PTFE is clearly much less favourable than the adsorption of the, C7F15, fluorocarbon chain in fact, a C7H15 chain, in the form of octanoic acid showed no apparent adsorption on to a PTFE surface. Thus, although both acids have hydrophobic chains, there is clearly a remarkable difference between their affinities for the substrate. 

Why should the atom-cylinder interaction energy go as the energy of a point particle with a plane With the l-A-size atom, the 50-nm impact parameter, and the l-cm cylinder radius, there is a clean separation of sizes. The atom sees a substrate of infinite radius along the cylinder and effectively infinite radius compared to atom-substrate distance. The interaction is then effectively that of a plane and a point particle. 

The latter point brings us to an important question in the field of catalysis by supported metal particles to which extent is the chemical reactivity of a (sub-) nanocluster affected by the interaction with the substrate Very few theoretical studies were dedicated to this problem, and most of them are related to the surface of MgO, an oxide which interacts weakly widi the supported particle, as shown above. Still, the knowledge accumulated in the course of the years on the structure of surface defects and morphology of the MgO surface allows one to analyze some of the mechanisms which can modify the chemical properties of a supported cluster as a function of the site where nucleation has occurred. 

By analogy with friction, we may distinguish static and kinetic adhesion. Static adhesion is measured by the force of resistance to the onset of detachment, and kinetic adhesion is measured by the interaction between particles and surface in the course of detachment. In order to detach particles, the force of static adhesion is the primary barrier that must be overcome since the kinetic adhesion is always smaller than the static. This situation attracted the attention of G. I. Fuks, who pointed out that static friction is measured by the force directed tangential to the substrate [12]. 

---