Long-range forces experimental measurements

Computer simulation of molecular dynamics is concerned with solving numerically the simultaneous equations of motion for a few hundred atoms or molecules that interact via specified potentials. One thus obtains the coordinates and velocities of the ensemble as a function of time that describe the structure and correlations of the sample. If a model of the induced polarizabilities is adopted, the spectral lineshapes can be obtained, often with certain quantum corrections [425,426]. One primary concern is, of course, to account as accurately as possible for the pairwise interactions so that by carefully comparing the calculated with the measured band shapes, new information concerning the effects of irreducible contributions of inter-molecular potential and cluster polarizabilities can be identified eventually. Pioneering work has pointed out significant effects of irreducible long-range forces of the Axilrod-Teller triple-dipole type [10]. Very recently, on the basis of combined computer simulation and experimental CILS studies, claims have been made that irreducible three-body contributions are observable, for example, in dense krypton [221]. 

Shear of a liquid-crystalline lipid phase which is in equilibrium with a water phase can result in exposure of hydrocarbon chains to water which results in a strong tendency to fuse with other bilayers with exposed hydrocarbon chains. The formation of liposomes, discussed in the next paragraph, is a consequence of such rupture and fusion behaviour. Recently, the first experimental measurements of the force behind such adhesion, the hydrophobic force, were reported (Israelachvili and Paskey, 1982). It was found to be a long-range force with the same range of existence as the van der Waals interaction and shows an exponential fall-off with a decay length of 10 A. 

Geometric measures of the correctness of the model can also be used. For example, the best dielectric constant will yield a unit cell having minimal distortion of its dimensions, and hydrogen-bond geometries will be closest to the experimental values. Typically, some lattice expansion (e.g., 2%) must be tolerated because of the long-range forces present in the actual crystal but absent in the model. These forces would... 

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. 

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). 

Molecules exhibit relatively long-range attractive forces between themselves which give rise to the cohesive forces in liquids. These forces arise because the electronic distribution in the molecule or atom making up the liquid is not uniform either on a time-averaged basis or with respect to its instantaneous value. Non-uniformity in the time-averaged electronic distribution in a molecule is a well-known phenomenon, and is discussed in terms of the experimentally measured dipole... 

In the absence of a field, previous theoretical predictions based on a long-range calculation predicted that the elastic quadrupolar repulsion force should follow the power law Prepukion° l [4> 7]. Under the present experimental conditions, i.e., in the presence of an electric field, we observe a steeper repulsion as shown by the log-log plot in the inset of Fig. 13. Two reasons might explain the discrepancy between the experimental measurements and the theory. First, since the electric field is likely to distort the ordering of the liquid crystal molecules in the vicinity of the drops, the measured quadrupolar repulsion may intrinsically depend on Eq. Second, short-range effects, not considered in the theoretical approach, may come into play in the experiments. Indeed, the maximum measured separation between two drops is of the order D. 

Polyelectrolyte molecules in highly dilute aqueous solutions exert strong electrical repulsions on each other. These repulsive forces are long range (proportional to l/r ) by comparison with normal dispersion forces (proportional to 1/r ), and as a consequence the intermolecular interactions persist down to the lowest measured concentrations. In osmotic-pressure measurements on polyelectrolytes, the Donnan membrane equilibrium must be satisfied and experimental results indicate that the second virial coefficient in the osmotic-pressure equation (p. 915) becomes very large. 

---