Van der Waals surfaces

Van der Waals forces, although very weak, operate in all adsorbent-adsorbate interactions, and result from short-range dipole-dipole, dipole-induced dipole, or induced dipole-induced dipole attractions. Although van der Waals interactions are forces acting universally, they assume particular importance in the adsorption of nonionic and non-polar molecules or portions of molecules on similar sites of the adsorbent molecule [17,159]. These forces are additive, and thus their contribution increases with the size of the molecule and with its capacity to adapt to the adsorbent surface. Van der Waals attractions have often been invoked in case of difficulties in explaining adsorption of an organic pollutant onto SPHS, but the experimental evidence has not always been convincing. 

Equation 2.16 contains contributions from the translational entropy of the mobile species, the conformational entropy of polymer chains, the free energy associated with the different chemical equilibria in the system, the polymer-polymer and polymer-surface van der Waals (vdW) interaction energies, the electrostatic interaction energies and the repulsive interactions between all the different molecular species. The expressions for each of these terms are shown in Table 2.2, while the definition of the symbols is given in Appendix. Note that in Table 2.2, the densities. 

For (sub-) monolayer coverage 9 = adsorbed fraction, w = pair Interaction parameter. 2 coordination number. a , = molecular cross section or site area, a = surface Van der Waals constant, /) = [l - 40(1- 0) l-exp(-io/fcT) ]. ... 

N /JV°(metx) 0 is reduced surface coverage as explained in the text, w = pair interaction energy, z = co-ordination number (for lattices) a = surface Van der Waals constant, B°[T) = second surface virial coefficient = l-40(l-e) l-exp(-u)/fcT)) = depth of... 

Steric descriptors and/or -> size descriptors representing the volume of a molecule. The volume of a molecule can be derived from experimental observation such as the volume of the unit cell in crystals or the molar volume of a solution or from theoretical calculations. In fact, analytical and numerical approaches have been proposed for the calculation of molecular volume where the measure depends directly on the definition of - molecular surface-, -> van der Waals volume and -> solvent-excluded volume are two volume descriptors based on van der Waals surface and solvent-accessible surface, respectively. 

Non contact mode - with distances greater than 10 A between the tip and the sample surface, Van der Waals, electrostatic, magnetic or capillary forces produce the surface images. This generally provides lower resolution than contact mode. 

Potential energy surface, van der Waals energy spectrum, and vibronic transitions in an r-triazine-argon complex have been studied by ab initio methods <2006JCP044310>. 

One may use different types of molecular cavities and surfaces definitions (e.g. equipotential surfaces, equidensity surfaces, van der Waals surfaces). Among them there is a subset that shares a common trait they consider that a molecule may be represented as a set of rigid interlocking spheres. There are three such surfaces a)... 

Murray, J. S. Politzer, P. 2009. Molecular surfaces, van der Waals radii and electrostatic potentials in relation to noncovalent interactions. Croat. Chem. Acta 82 267-275. 

We consider a medium between two planar, parallel surfaces separated by a distance x. The medium is supposed to be in contact with a large reservoir and we neglect edge effects. In the interfadal zones dose to the surfaces, the properties of the medium can be different from its bulk properties. For example, surface charges change the ion concentration of water dose to a charged surface. Van der Waals forces can impose a dielectric property different from the bulk. 

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