Van der Waals long-range force

Refinements in the theory of interparticle long-range van der Waals forces (the Landau-Lifshitz theory) are within reach. New techniques are now available for measuring the complex dielectric constants of various media required for the implementation of that theory. 

It is convenient, although artificial to divide intermolecular forces into two types - long-range (van der Waals) forces and short-range (valence or chemical) forces. 

The water molecules that are in immediate contact with dissolved nonpolar groups are partially oriented. They form a cagelike structure around each hydrophobic group. When particles surrounded by such hydration layers are 1-2 nm apart, they sometimes experience either a fairly strong repulsion or an enhanced attraction caused by these hydration layers.21 64 66,72 Direct experimental measurements have shown that these effects extend to distances of 10 nm21,63 and can account for the previously mentioned long-range van der Waals forces. 

There is plenty of force in a long-range van der Waals force, at least for facultatively flat-footed bugs. There is also the important lesson to us humans that the consequences of flattening can be enormous. 

How strong are these long-range van der Waals forces between semi-infinite bodies A and B across a medium m or across a vacuum For the four materials whose dielectric responses are plotted in the preceding section, the corresponding Hamaker coefficients (with the neglect of retardation) make an instructive table. See Table LI. 3. 

V. A. Parsegian, "Long range van der Waals forces," in Physical Chemistry Enriching Topics From Colloid and Interface Science, H. van Olphen and K. J. Mysels, eds. IUPAC 1.6, Colloid and Surface Chemistry (Theorex, La Jolla, CA, 1975), pp. 27-73. 

While van der Waals forces between individual atoms act over very short distances, they can be felt at surprisingly great distances when exerted by large molecules or molecular aggregates. Forces between very smooth surfaces have been measured experimentally at distances as great as 10 nm and even to 300 nm. However, these long-range van der Waals forces" probably depend upon layers of oriented water molecules on the plates (see also Section 5). 

These assumptions are justifiable as the heat of adsorption of the small inert sorbate (e.g., N2 or Ar) is rather low and, hence, differences between sorption sites at the surface will be very small. Similarly, the interaction between the first and the following layers will be close to the heat of condensation, as the effect of polarization by the surface will be small beyond the first layer (screening of the long-range van der Waals forces). From its conception, the BET theory extends the Langmuir model to multilayer adsorption. It postulates that under dynamic equilibrium conditions the rate of adsorption in each layer is equal to the rate of desorption from that layer. Molecules in the first layer are located on sites of constant interaction strength and the molecules in that layer serve as sorption sites for the second layer and so forth. The surface is, therefore, composed of stacks of sorbed molecules. Lateral interactions are assumed to be absent. With these simplifications one arrives at the BET equation... 

Kresin V V, Tikhonov G, Kasperovich V, Wong K and Brockhaus P 1998 Long-range van der Waals forces between alkali clusters and atoms J. Chem. Phys. 108 6660... 

In AFM, several forces contribute to the deflection of the cantilever. The force most commonly associated with AFM is an interatomic force called the van der Waals forces. Figure 3.10 shows the dependence of the short-range repulsive force and the long-range van der Waals forces on the distance between the tip and the sample. 

In NC-AFM, the stiff cantilever oscillates near the surface of the sample at a frequency of 50 000-500 000 cps. The tip has no contact with the sample. The cantilever is held 5-10 nm away from the surface, within the region of the force distance curve where the long-range van der Waals forces are dominant [7]. In this mode of operation, the tip is responding to a force between the tip and the sample and can be several orders of magnitude lower than the force in contact mode. 

The most common and possibly most versatile SPM is the atomic force microscope (AFM). The AFM relies on the principle of the atomic forces exerted between two objects as they are brought close to each other (Fig. 2). At a distance of tens to hundreds of angstroms, the interatomic forces are attractive (predominantly due to long-range van der Waals forces). On approaching the surface to a distance of a few angstroms, the force becomes repulsive. To observe this phenomenon. 

V.A. Parsegian "Long Range van der Waals Forces", in Physical Chemistry Enriching Topics from Colloid and Surface Science, ed. by H. van 01phen,... 

Second, we discuss the energy requited to squeeze the chain, starting from a dilute solution in the same solvent and assuming that chain entropy is the only significant factor (no long-range van der Waals force in the tube). We try to estimate the reduction in entropy A5 due to confmement ... 

On the physical side, there are many complications. As mentioned, single-chain adsorption is never observed. One always reaches a situation where many chains compete for the same portion of surface. Furthermore the single-chain problem may be modified by the existence of long range van der Waals forces between the surface and each monomer. The corresponding potential decreases relatively slowly (as D ) and the attraction energy may not be cast into the form used in eq. (I.S7). 

A practical difficulty may be related to the existence of long range van der Waals forces. When a monomer immersed in a solvent is at a distance z from the wall, it experiences an attractive potential that decreases as z . Then, even if the monomers in the first layer experience repulsion, the region of thickness i near the wall might have its concentration in-... 

The theory of the local Prederiks transition [85, 86] considers the competition of two terms, the surface anchoring energy due to the short range forces W responsible for one type of orientation (homeotropic in the previous example), and the long-range Van der Waals forces U z) integrated over their penetration radius... 

A very useful tool for understanding the stability of colloids is provided by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, which was named after the four scientists responsible for its development. The theory allows for both the forces between electrical double layers (repulsive for similarly charged particles) and long-range van der Waals forces that are usually attractive. 

We have generalized this analysis to statistically seifsimilar structures. We will assume that the spreading coefficient S = yc - (y + y) is negative. For this case, the liquid A wets only partially a smooth surface made of the grain material. Then the cocoon cannot be formed on a smooth surface (Dy 2). On the other hand, in the case of complete wetting (S > 0), an ultra thin film controlled by long-range van der Waals forces will be formed. 

Effects of long range van der Waals forces on the anchoring of a nematic fluid at an interface" J. Colloid Interface Sci.. 57. 403 (1976). 

As already said, the potential curve is the result of two terms the contribution of attractive force at long ranges (Van der Waals force) and the repulsive term at short ranges. The curves indicate the strength of the bond based on the depth of the potential well more deep is the well more stable is the molecule [15]. 

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