Debye-London theory

Intramolecular interactions were introduced for the first time by van der Waals in 1873 he thus attempted to explain the deviation of the real gas from the ideal gas. In order to apply the ideal gas law equation to the behavior of real gases, allowance should be made for the attractive and repulsive forces occurring between molecules. From that time on, the dipole moment theory of Debye (1912) and the dispersion energy or induced dipole theory by London (1930) were the main driving forces of the research about intermolecular interactions. 

Refs. [i] Debye P (1929) Polar molecules. Chemical Catalog, New York [ii] Frohlich H (1949) Theory of dielectrics. Oxford University Press, London [iii] Fawcett WR (2004) Liquids, solutions, and interfaces. Oxford University Press, New York... 

The LSER theory combined with IGC should be applied more in the future because it permits distinction between London, Keesom, and Debye interactions in addition to the acid-base scales. This is not done in the traditional IGC studies in relation to adhesion. 

The basic concepts of this theory were the mutual repulsion consequent upon the interaction of two electro-chemical double layers, and the attraction by the London—Van der Waals forces. The principal facts of stability could be explained by combining these two forces. Among other things, a quantitative explanation of the rule of Schulze and Hardy has been given. For this purpose it was essential to use the unapproximated G o u y—C h a p m a n equations for the double layer. The approximation of Debye and Hiickel, however useful in the theory of electrolytes, appears to have only a very limited applicability in colloid chemistry. 

We have seen, now, that there are three types of interactions that can be involved in the total van der Waals interaction between atoms or molecules dipole-dipole (orientational or Keesome), dipole-induced dipole (induced or Debye), and dispersion (London) interactions. The theories for all three interactions are found (to a first approximation) to involve an inverse 6th power of the distance separating the two interacting centers. The total van der Waals interaction potential, Wvdw(r), can then be written as... 

The thermodynamic model of adhesion, generally attributed to Sharpe and Schonhom [1], is certainly the most widely used approach in adhesion science nowadays. This theory considers that the adhesive will adhere to the substrate because of interatomic and intermolecular forces established at the interface, provided that an intimate contact between both materials is achieved. The most common interfacial forces result from van der Waals (London, Debye and Keesom) and Lewis acid-base interactions. The magnitude of these forces can generally be related to fundamental thermodynamic surface characteristics, such as surface free energies y, of both materials in contact. 

Lifshitz avoided this problem of additivity by developing a continuum theory of van der Waals forces that used quantum field theory. Simple accounts of the theory are given by Israelachvili and Adamson. Being a continuum theory, it does not involve the distinctions associated with the names of London, Debye and Keesom, which follow from considerations of molecular structure. The expressions for interaction between macroscopic bodies (e.g. Eqns. 2-4) remain valid, except that the Hamaker constant has to be calculated in an entirely different way. 

Debye and Keesom forces together with London Dispersion forces are known coiiec-tively as van der Waals forces. See Lifshitz-van der Waals forces for a further discussion. They play a significant role in the Adsorption theory of adhesion and in surface phenomena such as Contact angles and interfacial tension. 

Equation 4.21 includes de Debye and Keesom contributions (first term in square brackets) and the London interactions (integral term). We will not include further details of McLachlan theory here, the reader is referred to the literature (McLachlan 1963a, b, 1965 Mahanty and Ninham 1977 Israelachvili 2010). We will only mention some features that can be deduced by analysis of Equation 4.21 (Israelachvili 2010) ... 

This theory relies on the orientation of surface forces resulting in van der Waals interactiOTi. There are three general types of van der Waals forces (a) Keesom, (b) Debye, and (c) London. 

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