Van der Waals forces intermolecular forces

When thinking about chemical reactivity, chemists usually focus their attention on bonds, the covalent interactions between atoms within individual molecules. Also important, hotvever, particularly in large biomolecules like proteins and nucleic acids, are a variety of interactions between molecules that strongly affect molecular properties. Collectively called either intermolecular forces, van der Waals forces, or noncovalent interactions, they are of several different types dipole-dipole forces, dispersion forces, and hydrogen bonds. 

Physisorption (or Physical Adsorption) is adsorption in which the forces involved are intermolecular forces (van der Waals forces) of the same kind as those responsible for the imperfection of real gases and the condensation of vapours, and which do not involve a significant change in the electronic orbital patterns of the species involved. The term van der Waals adsorption is synonymous with physical adsorption, but its use is not recommended. 

For example, the melting and boiling points of the alkanes shown in Table 14.1 gradually increase. This is due to an increase in the intermolecular forces (van der Waals forces) as the size and mass of the molecule increases (Chapter 3, p. 49). 

Intermolecular forces van der Waals forces Dipolar attractions Hydrogen bonding... 

It is seen from Fig. 6.70 that the structures of activated carbon derived from different carbon materials are different. The activated carbon derived from coal has an obvious characteristic peak of graphite. From the crystal structure analysis of graphite, we can see that there is a hexagonal comby plane layer (A-B-A in Fig. 6.71) lattice structure formed via bonding the sp" hybrid orbit with three neighboring atoms. There is still one 2p electron in the 2p orbit in per carbon atom. These p orbits parallel each other and perpendicular to sp" hybrid orbital plane, and therefore form a big tt bond. Thus these tt electrons can move on throughout the whole carbon plane, which is similar to metallic bond. The interaction between carbon layers with horizontal structure via intermolecular force (van der Waals force) forms graphite crystal (Fig. 6.71). 

Chemguide. Intermolecular Bonding—van der Waals Forces. Available online. URL http //www.chemguide.co.uk/atoms/ bonding/vdw.html. Accessed on August 6, 2007. 

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

Several theories have been developed to account for the observed characteristics of the plasticization process Daniels has recently published a review of plasticization mechanisms and theories [8]. Although most mechanistic studies of plasticization have focused on PVC, much of this information can be adapted to other polymer systems. The lubricating theory of plasticization holds that plasticizers act as molecular lubricants to facilitate polymer chain movement when a force is applied to the plastic. It starts with the assumption that the unplasticized polymer chains do not move freely because of surface irregularities and van der Waals attractive forces. As the system is heated and mixed, the plasticizer molecules diffuse into the polymer and weaken the polymer-polymer interactions. Portions of the plasticizer molecule are strongly attracted to the polymer while other parts of the plasticizer molecule can shield the polymer chain and act as a lubricant. This reduction in intermolecular or van der Waals forces among the polymer chains increases the flexibility, softness, and elongation of the polymer. 

The main intermolecular or van der Waals force is the induced dipole-induced dipole interaction. It is largest between molecules of high polarizability those with electron clouds that are easily distorted by an external charge, and have low ionization energies. 

Boyle s law At constant temperature the volume of a given mass of gas is inversely proportional to the pressure. Although exact at low pressures, the law is not accurately obeyed at high pressures because of the finite size of molecules and the existence of intermolecular forces. See van der Waals equation. 

The next point of interest has to do with the question of how deep the surface region or region of appreciably unbalanced forces is. This depends primarily on the range of intermolecular forces and, except where ions are involved, the principal force between molecules is of the so-called van der Waals type (see Section VI-1). This type of force decreases with about the seventh power of the intermolecular distance and, consequently, it is only the first shell or two of nearest neighbors whose interaction with a given molecule is of importance. In other words, a molecule experiences essentially symmetrical forces once it is a few molecular diameters away from the surface, and the thickness of the surface region is of this order of magnitude (see Ref. 23, for example). (Certain aspects of this conclusion need modification and are discussed in Sections X-6C and XVII-5.)... 

Hutson J M 1990 Intermolecular forces from the spectroscopy of van der Waals molecules Ann. Rev. Phys. Chem. 41 123... 

Hutson J M 1990 Intermolecular forces from the spectroscopy of Van der Waals molecules Ann. Rev. Phys. Chem. 41 123-54 Huston J M 1991 An introduction to the dynamics of Van der Waals molecules Adv. Mol. Vibrat. Coll. Dyn. 1A 1-45... 

Halgren T A 1996b. Merck Molecular Force Field II MMEF94 van der Waals and Electrostatic Parameters for Intermolecular Interactions. Journal of Computational Chemistry 17 520-552. 

Van der Waals forces (Section 2 17) Intermolecular forces that do not involve ions (dipole-dipole dipole/mduced dipole and induced dipole/induced dipole forces)... 

Polymer alloys are physical mixtures of structurally different homopolymers or copolymers. The mixture is held together by secondary intermolecular forces such as dipole interaction, hydrogen bonding, or van der Waals forces. 

Hydrophobic Interaction. This is the tendency of hydrophobic groups, especially alkyl chains such as those present in synthetic fibers, and disperse dyes to associate together and escape from the aqueous environment. Hydrophobic bonding is considered (7) to be a combination of van der Waals forces and hydrogen bonding taking place simultaneously rather than being a completely new type of bond or intermolecular force. 

The atoms of a molecule are held together by primary bonds. The attractive forces which act between molecules are usually referred to as secondary bonds, secondary valence forces, intermolecular forces or van der Waals forces. 

In a thermoplastic material the very long chain-like molecules are held together by relatively weak Van der Waals forces. A useful image of the structure is a mass of randomly distributed long strands of sticky wool. When the material is heated the intermolecular forces are weakened so that it becomes soft and flexible and eventually, at high temperatures, it is a viscous melt. 

In many cases, pressurized gases in vessels do not behave as ideal gases. At very high pressures, van der Waals forces become important, that is, intermolecular forces and finite molecule size influence the gas behavior. Another nonideal situation is that in which, following the rupture of a vessel containing both gas and liquid, the liquid flashes. 

A substance exists as a liquid rather than a gas because attractive forces between molecules (intermolecular attractive forces) are greater in the liquid than in the gas phase. Attractive forces between neutral species (atoms or molecules, but not ions) are referred to as van der Waals forces and may be of three types ... 

In the case of nonionic but polar compounds such as sugars, the excellent solvent properties of water stem from its ability to readily form hydrogen bonds with the polar functional groups on these compounds, such as hydroxyls, amines, and carbonyls. These polar interactions between solvent and solute are stronger than the intermolecular attractions between solute molecules caused by van der Waals forces and weaker hydrogen bonding. Thus, the solute molecules readily dissolve in water. 

MF < MC1 < MBr < MI . By contrast for less-ionic halides with significant non-coulombic lattice forces (e.g. Ag) solubility in water follows the reverse sequence MI < MBr < MC1 < MF . For molecular halides solubility is determined principally by weak intermolecular van der Waals and dipolar forces, and dissolution is commonly favoured by less-polar solvents such as benzene, CCI4 or CS2. 

The intermolecular forces operative in nonpolar compounds are also electrostatic-in nature. These weak van der Waals forces involve attraction between nonbonded atoms and are effective over short ranges only. 

---