Van der Walls forces

At short interparticle distances, the van der Walls forces show that two metallic particles will be mutually attracted. In the absence of repulsive forces opposed to the van der Walls forces the colloidal metal particles will aggregate. Consequently, the use of a protective agent able to induce a repulsive force opposed to the van der Walls forces is necessary to provide stable nanoparticles in solution. The general stabihzation mechanisms of colloidal materials have been described in Derjaguin-Landau-Verway-Overbeck (DLVO) theory. [40,41] Stabilization of colloids is usually discussed... 

Adsorption/chemisorption at the surface (adsorption of gas molecules occurs on the solid surface because of attractive forces between them). Gas molecules approaching the surface may lose some of their momentum (in the component normal to the surface) and become trapped in the potential well. The energy required to overcome the attractive potential barrier of the surface and the attraction of neighbouring molecules is the heat of adsorption (Van der Wall forces) and several monolayers may be adsorbed. However, if there is some interaction or electron transfer between the gas molecule and the surface (forming, e.g., a surface compound), it is defined as chemisorption. The heat of chemisorption is usually greater than the heat of adsorption. The extent of chemisorption depends upon the specific nature of the solids and gases. 

Adsorption Theory of Adhesion. The adsorption theory states that adhesion results from molecular contact between two materials and the surface forces that develop. Adhesion results from the adsorption of adhesive molecules on the substrate and the resulting attractive forces, usually designated as secondary or van der Walls forces. For these forces to develop, the respective surfaces must not be separated more than 5 angstroms (A) in distance. Therefore, the adhesive must make intimate, molecular contact with the substrate surface. 

After intimate contact is achieved between adhesive and adherend through wetting, it is believed that permanent adhesion results primarily through forces of molecular attraction. Four general types of chemical bonds are recognized as being involved in adhesion and cohesion electrostatic, covalent, and metallic, which are referred to as primary bonds, and van der Walls forces, which are referred to as secondary bonds. 

String-wound These cartridge filters consist of a string of polypropylene (or cotton, nylon, jute, polyester, and so forth) wound around a central core. String-wound cartridge filters rely on Van der Walls forces to capture small particles. These filters suffer from the potential to unload particles at higher pressure drops. Additionally, a slower flow rate is recommended for these filters, about 2 -3 gpm per 10-inch equivalent. 

In the theory developed by Derjaguin and Landau (24) and Verwey and Overbeek (25.) the stability of colloidal dispersions is treated in terms of the energy changes which take place when particles approach one another. The theory involves estimations of the energy of attraction (London-van der Walls forces) and the energy of repulsion (overlapping of electric double layers) in terms of inter-oarticle distance. But in addition to electrostatic interaction, steric repulsion has also to be considered. 

Adsorption may be characterized as either physical or chemical. Physical adsorption consists mainly of van der Walls forces and is reversible. This occurs when the molecular forces of attraction between the adsorbent and the solute are greater than the forces of attraction between the solvent and the solute. Chemical adsorption, as the name implies, involves a chemical reaction between the adsorbent and the adsorbate. Physical adsorption is generally a much stronger type of adsorption and is often times irreversible. 

Long and short-range forces at the molecular level between surfaces have been directly measured using the surface forces microbalance (SFM) first developed by Israelachvili and Tabor and Klein. The SFM utilizes the simultaneous measurement of separation distance and surface force. Electrostatic repulsion forces and van der Walls forces between surfaces in liquids are discussed by Israelachvili and McGuiggan (1988). 

Gas adsorption is one of the most widely used techniques for characterization of porous materials. In this chapter wc arc focused on physical adsorption (physisorption), which in contrast to chemisorption, occurs due to the Van der Walls forces. The amount adsorbed, a, expressed per unit mass of solid (adsorbent) is dependent on the gas (adsorbate) pressure, p, temperature, T, properties of the adsorbent and the nature of the gas-solid interactions [55]. Thus, for a given adsorbate adsorbed on a particular adsorbent one can write ... 

Presence of structural groups that are capable of producing lateral intermolecular bonds (van der Wall forces) and regular, periodic arrangement of such bonds. 

Nanoparticles are generally found in a metastable state. The force of attraction (van der Walls force) between any two particles is inversely proportional to the square of the distance between them. Therefore, in the absence of a barrier to overcome this attraction, particles attract each other and start to grow and eventually coagulate. Two primary types of stabilization exist with nanoclusters, namely electrostatic and steric stabilization. 

Besides the crystalline and porous structure, an active carbon surface has a chemical structure as well. The adsorption capacity of active carbons is determined by their physical or porous structure but is strongly influenced by the chemical structure. The decisive component of adsorption forces on a highly ordered carbon surface is the dispersive component of the van der Walls forces. In graphites that have a highly ordered crystalline surface, the adsorption is determined mainly by the dispersion component due to London forces. In the case of active carbons, however, the disturbances in the elementary microcrystalline structure, due to the presence of imperfect or partially burnt graphitic layers in the crystallites, causes a variation in the arrangement of electron clouds in the carbon skeleton and results in the creation of unpaired electrons and incompletely saturated valences, and this influences the adsorption properties of active carbons, especially for polar and polarizable compounds. 

Depending upon the nature of the forces involved, the adsorption is of two types physical or van der Walls adsorption, and chemisorption or chemical adsorption. In the case of physical adsorption, the adsorbate is bound to the surface by relatively weak van der Walls forces identical with molecular forces of cohesion that are involved in the condensation of vapors into liquids. Chemisorption, on the other hand, involves exchange or sharing of electrons between the adsorbate molecules and the surface of the adsorbent, resulting in a chemical reaction. The bond formed between the adsorbate and the adsorbent is essentially a chemical bond and is thus much stronger than in physical adsorption. 

The van der Walls force between a sphere and plate is modeled by ... 

Most micro self-assembly processes are performed in a fluid medium and use gravity and/or surface tension for assembly. A prime example is self-assembly of small (submillimeter) flat parts that can be formed into a crystallike structure on a fluid surface caused by minimization of interfacial surface tension (otherwise known in layperson s terms as the Cheerios effect). Another prime example is the utilization of gravitational forces to flow parts over a surface, where shape-recognition is used to place parts and surface tension and van der Walls forces hold the parts in place during subsequent processing. 

---