Importance of the van der Waals forces

The van der Waals forces are important in the study of both surface and colloid phenomena. Of the vdW forces, only the dispersion forces are universal, while polar and induction forces (as well as other forces) are sometimes called specific forces, i.e. they are only present when the substances involved have some specific characteristics, e.g. electric charges, presence of dipoles or hydrogen bonds. 

Of the three van der Waals forces, only the polar ones depend directly on temperature (Equation 2.4a). Moreover, for polar forces, the potential energy depends on the fourth power of the dipole moment for pure polar fluids and is thus quite important for highly polar molecules (having a dipole moment above 1 Debye). 

A comparative evaluation of the van der Waals forces for molecules of same type is shown in Table 2.3. It can be seen that the van der Waals forces are important in all cases and they cannot be ignored, not even for highly polar and hydrogen bonding molecules. For example, the dispersion contribution of water is 15% of the total van der Waals forces at 0 °C and 24% at 298 K. The dispersion forces account for over 80% for water-methane and 60% for ammonia-ammonia interactions. 

The van der Waals forces (either between molecules or between particles) are always attractive when the 

Molecule Dipole moment (D) C-dipole C-induction C-dispersion 

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For a confirmation of these assumptions extensive calculations should be carried on the hydrogen-bonded codon-anticodon pairs. In addition, the importance of the van der Waals forces should be investigated17. 

The specific value includes both polar and hydrograi bonding effects. Noticeably, the dispersion value indicates that approximately 30% of the surface tension value of water is due to dispersion forces, which agrees well with theoretical considerations and the molecular theories mentioned in Chapter 2. (The contribution of dispersion to water s potential energy is around 24% at 298 K.) This illustrates the importance of the van der Waals forces even for strongly hydrogen bonding compounds such as water. The dispersion surface tension value of water from the Fowkes theory can be considCTed to be widely accepted and is therefore used in further calculations. 

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