Van der Waals cohesive forces

The relative volatility or separation factor (a) depends upon the interactions of the solute and the liquid phase, that is, van der Waals cohesive forces. These cohesive forces may be divided into three types ... 

It is in many cases difficult to decide whether a certain adsorption phenomenon belongs to the physical adsorption type or whether it is a case of chemisorption. If we define physical adsorption as the phenomenon which occurs when the molecules are bound to the surface of the adsorbent by van der Waals cohesion forces in the widest sense of the word, hence... 

The sensitivity of the phonon frequencies to temperature shows quite clearly the importance of their anharmonicity.42 The width of the Raman peaks, very small at low temperature ( 1cm-1), evolves in parallel with the frequency shift with temperature, which is still a consequence of the phonon-phonon interactions due to the anharmonicity. The fundamental reason for this strong anharmonicity, as well as the importance of the equilibrium-position shifts between 4 and 300 K,45 resides in the weakness of the van der Waals cohesive forces in the molecular crystal. 

This distance of 3-4.5 A which corresponds to the lattice forces in molecular lattices, may, through its frequent occurrence, account for the fact that interference it produces is especially intense. Thus, for example, in the fatty acids the reflections are especially intense in those planes which contain the chains and, consequently, have the same interplanar distance that also separates the individual chains and corresponds to the lattice forces. In liquids also, where, it is true, the oriented lattice coherence is lost but where the average distances of the molecules are determined by van der Waals cohesion forces, we find very frequently indistinct interference rings when these are interpreted, they show that a lattice plane distance of about 4 A prevails in the liquid also, at least, statistically. From the x-ray diagrams of hydrocarbons, e.g. of liquid isoprene, an average molecular distance of about 4 A can be calculated from the ring. 

Debye, P. (1920) van der Waals cohesion forces Phys. Zeit., 21, 178-187. 

Replacement of gas by the nonpolar (e.g., hydrocarbon) phase (oil phase) has been sometimes used to modify the interactions among molecules in a spread film of long-chain substances. The nonpolar solvent/water interface possesses an advantage over that between gas and water in that cohesion (i.e., interactions between adsorbed molecules) due to dipole and van der Waals s forces is negligible. Thus, at the oil/water interfaces, the behavior of adsorbates is much more ideal, but quantitative interpretation may be uncertain, in particular for the higher chains, which are predominantly dissolved in the oil phase to an unknown extent. The oil phase is poured on the surface of an aqueous solution. Thus, the hydrocarbon, such as heptane or decane, forms a membrane a few millimeters thick. It is thicker than the adsorbed monolayer. Owing to the small difference in dielectric constant between the air and a hydrocarbon oil, the... 

The energy of the combination AB is thus higher than that of 0-5A2 + 0 5jB2. This result can be proved equally well for other combinations. Van der Waals-London forces, therefore, although very important in the mutual cohesion of atoms and molecules, can never lead to an actual chemical combination. ... 

More detailed approaches have made use of Hansen solubility parameters (HSP), based on the realization that the heat of vaporization consists of several components arising from van der Waals dispersion forces (D), dipole-dipole interaction (P), and hydrogen bonding (H). The total cohesive energy density is then the sum of the three contributions, and so the overall solubility parameter is... 

Mutual compatibility of polymers and solvents is one of the important issues of materials engineering not only in the polymer industry but also for pharmaceuticals, where drug delivery and release is closely related to the properties of the individual components. In simple terms, like dissolves like, a principle that can be quantified through the Hildebrand solubility parameter, which is defined as the square root of the cohesive energy density. The basic concept has been further refined by Hansen [1], who introduced individual terms related to the van der Waals dispersion forces, dipole interaction, and hydrogen bonding. The total solubility parameters and... 

The long-range van der Waals interaction provides a cohesive pressure for a thin film that is equal to the mutual attractive force per square centimeter of two slabs of the same material as the film and separated by a thickness equal to that of the film. Consider a long column of the material of unit cross section. Let it be cut in the middle and the two halves separated by d, the film thickness. Then, from one outside end of one of each half, slice off a layer of thickness d insert one of these into the gap. The system now differs from the starting point by the presence of an isolated thin layer. Show by suitable analysis of this sequence that the opening statement is correct. Note About the only assumptions needed are that interactions are superimposable and that they are finite in range. 

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