Van der Waals forces origin

The considerations on the intermolecular interactions can be conveniently reduced to considerations of atom-atom nonbonded interactions. Although these interactions can be treated by nonempir-ical quantum mechanical calculations, empirical and semi-empirical approaches have also proved useful in dealing with them. In the description of the atom-atom nonbonded interactions it is supposed that the van der Waals forces originate from a variety of sources. 

The van der Waals force originates in charge fluctuations in the electron shell of the atoms. Slightly simplified, it can be imagined that the charge distribution of an atom is not static because the outer electrons move about. 

This is a generic result and in this form valid for all bodies. If the bodies approach each other because of the so-called attractive van der Waals force originating from this effective potential and come too close, the electronic clouds of the bodies will repel each other. For this reason, the attractive potential (1.33) cannot be valid below a certain finite distance of the bodies (see Fig, 1,12). Therefore, it is reasonable to add a repulsive term that accommodates volume exclusion at short distance. It is convenient but rather arbitrary to choose... 

It is worthwhile to highlight an inconsistency in the literature. Specifically, one often finds that authors attribute the origins of van der Waals forces solely to dispersion forces. Although this is not strictly correct, in light of the present discussion, one can see that assuming that London and van der Waals forces are synonymous is often not a bad approximation. However, the reader is cautioned that this is not quite correct and can lead to erroneous conclusions under some circumstances. 

Work on the production and oxidation of SWNT samples at SRI and other laboratories has led to the observation of very long bundles of these tubes, as can be seen in Fig. 2. In the cleanup and removal of the amorphous carbon in the original sample, the SWNTs self-assemble into aligned cable structures due to van der Waals forces. These structures are akin to the SW nanotube crystals discussed by Tersoff and Ruoff they show that van der Waals forces can flatten tubes of diameter larger than 2.5 nm into a hexagonal cross-sectional lattice or honeycomb structure[17]. 

Binnig et al. [48] invented the atomic force microscope in 1985. Their original model of the AFM consisted of a diamond shard attached to a strip of gold foil. The diamond tip contacted the surface directly, with the inter-atomic van der Waals forces providing the interaction mechanism. Detection of the cantilever s vertical movement was done with a second tip—an STM placed above the cantilever. Today, most AFMs use a laser beam deflection system, introduced by Meyer and Amer [49], where a laser is reflected from the back of the reflective AFM lever and onto a position-sensitive detector. 

The van der Waals forces result from several sources among which the dispersion forces make the most important contribution. The origin of the dispersion forces may be understood as follows A dipole may appear instantaneously on a... 

Althongh van der Waals forces are present in every system, they dominate the disjoining pressnre in only a few simple cases, such as interactions of nonpolar and inert atoms and molecnles. It is common for surfaces to be charged, particularly when exposed to water or a liquid with a high dielectric constant, due to the dissociation of surface ionic groups or adsorption of ions from solution, hi these cases, repulsive double-layer forces originating from electrostatic and entropic interactions may dominate the disjoining pressure. These forces decay exponentially [5,6] ... 

In filtration, the particle-collector interaction is taken as the sum of the London-van der Waals and double layer interactions, i.e. the Deijagin-Landau-Verwey-Overbeek (DLVO) theory. In most cases, the London-van der Waals force is attractive. The double layer interaction, on the other hand, may be repulsive or attractive depending on whether the surface of the particle and the collector bear like or opposite charges. The range and distance dependence is also different. The DLVO theory was later extended with contributions from the Born repulsion, hydration (structural) forces, hydrophobic interactions and steric hindrance originating from adsorbed macromolecules or polymers. Because no analytical solutions exist for the full convective diffusion equation, a number of approximations were devised (e.g., Smoluchowski-Levich approximation, and the surface force boundary layer approximation) to solve the equations in an approximate way, using analytical methods. 

Attractive or repulsive forces between molecular entities or groups within the same molecular entity (i.e., both intermolecular and intramolecular) not due to bond formation or to electrostatic interactions of ions or ionic groups with one another or with neutral molecules. The origin of van der Waals forces is in electric polarization of uncharged atoms, groups, or molecules and includes dipole-dipole interactions, dipole-induced dipole interactions, and London forces (induced dipole-induced dipole interactions). 

The LB monolayers of dimethyldioctyadecylammonium ions on molecularly smooth muscovite mica surfaces were investigated. Direct measurements of the interaction between such surfaces were carried out using the surface force apparatus. A long-range attractive force considerably stronger than the expected van der Waals force was measured. Studies on the electrolyte dependence of this force indicate that it does not have an electrostatic origin but that the water molecules were involved in this. 

Bonds and Forces - These properties are the mediators affecting the changes in size and conformation. Van der Waal forces, ionic bonds, hydrogen bonds, covalent bonds, and hydrophobic bonds all play a part in the original protein structure as well as in the modifications leading to altered functionality. Adequate correlations of these with functional properties are the subjects of "Functional Evaluations" 3). 

Our objectives in this chapter are to look into the origin of van der Waals forces, see how they affect macroscopic behavior and properties of materials, and establish relations for scaling up the molecular-level forces to forces between macroscopic bodies. 

Phenomena such as the ones described above are usually (and conveniently) described in terms of macroscopic properties such as surface tension, contact angle, and so on. This is what was done in Chapters 6 and 7. In the present chapter, we probe the molecular origin of van der Waals forces, go into some of the details of how they scale up in the case of macroscopic bodies, and illustrate their importance in molecular as well as macroscopic phenomena. 

The focus of this chapter is to present a fairly comprehensive introduction to the van der Waals forces. In addition to giving an introduction to the origin of these forces, we also illustrate the prevalence of these forces through examples of their role in some of the common macroscopic phenomena that are not necessarily directly related to colloidal problems. 

---