Van de Waals forces

Another separation mechanism that depends primarily on the size of the solute molecules involves the formation of inclusion compounds, clathrates or adducts.7 Stable complexes are formed whereby one (called the guest) is trapped inside the other (called the host). Van de Waals forces are present and help stabilize the complexes, and in some cases hydrogen bonds are involved in forming the cages. These methods are not widely used, but they can produce some exceptional separations. A general discussion can be found in references 8 and 9, and some typical applications are in references 10 (GC), 11 (TLC), and 12 (LC). 

The adsorption of PAHs was a combination of two factors, van de Waals force and a thermodynamic gradient determined by the hydrophobicity driving them out of the solution. Adsorption was dominated by van der Waals attractive forces between instantaneous and induced dipole moments of molecules (Gauthier et al., 1987). Dipole moments depended upon the polarizability and could be estimated by the sum of bond moments in the molecule. An increase in the numbers of C=C double bonds should contribute to molecular polarizability and thereby increase the van der Waals attraction strength. As a result, heavy molecular weight PAHs with more C=C double bonds should be adsorbed more efficiently to sediment particles. 

Cousins, S.K., Brown Jr, R.M. Cellulose I mlcrofibrll assembly computational molecular mechanics energy analysis favours bonding by Van de Waals forces as the Initial step In crystallization. Polymer 36, 3885-3888 (1995)... 

Radiochemical analyses of PWR primary coolant show that the major fraction of the neptunium and plutonium traces in the coolant is usually associated with the corrosion product suspended solids and that it can be removed from the coolant by filtering. Only in cases of very low corrosion product concentrations in the coolant were significant proportions of the transuranium elements observed to be present in a dissolved (i. e. non-filtrable) form. As yet, it is not known whether mixed oxide formation between magnetite-type oxides and these elements is responsible for this behavior or whether the actinide traces are adsorbed by van de Waals forces onto the large surface areas of the finely dispersed suspended solids. Under constantload operation conditions and as long as no additional fuel rod failures occur, the activity ratio Pu Co in the corrosion products remains virtually constant over time, thus indicating a similar behavior of these different elements in the coolant. 

Table 2-1 Van de Waals forces of attraction Fyjy (in 10" N) between a sphere (radius / ) and a half-space at an adhesional distance of 4 A between the adherents, as a function of Lifshitz-van der Waals constant hco (Krupp,1967)...

Coulomb repulsion, thermally fluctuating particles are more likely to come close enough to be attracted via van de Waals forces and aggregate. 

It is important to point out that the Zisman critical surface tension analysis is only limited to low surface energy surfaces and alkane solvents, where van de Waals force is the prime interaction. Many exceptions were found with polar andH-bonding solvents. Despite this limitation, the Zisman analysis remains as one of the useful tools to estimate the surface tension of low surface energy material to date [13]. 

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