Intermolecular forces van der Waals

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... 

Adsorption of a gas stream passing through a bed of absorbent is by two discrete processes Physical adsorption involves intermolecular forces (van der Waals) and condensation of gases within the solid materials. The amount of material adsorbed depends on the amount of solid but it is not directly related to the surface area. The process is reversible and desorption can occur by raising temperature or lowering pressure. Chemisorption involves the reaction of the gas with the solid adsorbent to form a bond and is influenced by temperature and pressure. The process is usually irreversible and confined to a single layer of molecules on the solid surface. 

The nature of the problem, as well as the solution to it, is represented in Fig. 1. The figure shows the circumstances in which adhesive joints are normally made with the substrate and adhesive surrounded by air. In the making of adhesive joints, the presence of air is seldom even considered, for the perfectly good reason that it does not represent a problem. However, the fact is that aU surfaces are contaminated by the permanent gases, but they are only weakly adsorbed (see Adsorption theory of Adhesion) and are readily displaced by adhesive, which then spreads freely and spontaneously over the substrate surface. In this way, intimate contact is achieved between liquid adhesive and solid substrate and adhesion results. Various Theories of adhesion are discussed elsewhere, but all require that adhesive and adherend are in intimate contact. For example, attractive intermolecular forces (van der Waals, see Dispersion forces. Polar forces) can operate only over short ranges ( 1 nm) and chemical interaction between adhesive and adherend also reqnires the two reactants to be in close contact. 

Microemulsions. The shape of the amphiphile, embodied in the v/aT packing parameter concept, along with a subtle interplay of intermolecular forces (van der Waals, electrostatic, hydration, polarizability, specific ion effects, etc) modulate the amphiphilic polar-apolar interface. Exactly as in the L.C. state not only o/w or w/o microdomains exist in Li and L2 microemulsion phases. Indeed a variety of microstructures from discrete to bicon-tinuous nanoaggregates have been identified. Moreover the development and the use of NMR techniques has provided a wide and unique insight of knowledge in the characterization of microstructural features and transitions in microemulsion systems. As reported in the previous volume, the transition of the dispersed phase from a disconnected to an interconnected domain. 

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). 

Increasing intermolecular forces van der Waals < dipole-dipole < hydrogen bonding... 

---