Forces, attractive repulsive

In addition to pore structure, tlie surface chemistry of the adsorbent may have a strong influence on the adsorption of aromatic compounds. Fig. 7 (from [48]) shows that the Langmuir adsorption capacity of phenol is dependent tai the concentration of acidic surface oxides, as defined by Boehm [49]. Furthermore, for acidic and basic species, the effect of the adsorbent s surface charge combined with that of the pH of the solution is extremely important because it determines the nature of the forces (attractive/repulsive) between the adsorbate and the adsorbent surface. For cationic dyes, adsorption is promoted when the surface charge is negative adsorption capacities are 1.7 to 2.3 times higher at basic pH and adsorption rates are doubled [50]. 

Fig. 3. Attraction—repulsion potentials as a function of distance between particle centers. Curve 1 represents the attractive potential caused by van der Waals forces, curve 2 is the repulsive potential caused by double-layer forces, and curve 3 is the resultant force experienced by the two particles.

Forces Molecules are attracted to surfaces as the result of two types of forces dispersion-repulsion forces (also called London or van der Waals forces) such as described by the Lennard-Jones potential for molecule-molecule interactions and electrostatic forces, which exist as the result of a molecule or surface group having a permanent electric dipole or quadrupole moment or net electric charge. 

The theory presented in this section is based on the grand canonical ensemble formulation, which is perfectly well-suited for the description of confined systems. Undoubtedly, in the case of attractive-repulsive interparticle forces unexpected structural and thermodynamic behavior in partly... 

Both attractive forces and repulsive forces are included in van der Waals interactions. The attractive forces are due primarily to instantaneous dipole-induced dipole interactions that arise because of fluctuations in the electron charge distributions of adjacent nonbonded atoms. Individual van der Waals interactions are weak ones (with stabilization energies of 4.0 to 1.2 kj/mol), but many such interactions occur in a typical protein, and, by sheer force of numbers, they can represent a significant contribution to the stability of a protein. Peter Privalov and George Makhatadze have shown that, for pancreatic ribonuclease A, hen egg white lysozyme, horse heart cytochrome c, and sperm whale myoglobin, van der Waals interactions between tightly packed groups in the interior of the protein are a major contribution to protein stability. 

The crystallographic radius assigned to the ion Fc+++ is comparable with that assigned to the scandium ion Sc+++. The ions K, Ca+t, and Sc+++ have the same number of electrons, and the same closed electronic shells as the argon atom. In aqueous solution there will be electrostatic forces of attraction between Ca++ and Cl, and between 8c+ t+ and Cl- but the quantum-mechanical forces between these ions will be forces of repulsion only. Between Fe+++ and Cl-, on the other hand, there may be quantum-mechanical forces of attraction. In view of the rather intense electrostatic attraction between Fe+++ and a negative ion, a 1 E. Rabinowitch and W. H. Stockmayer, J. Am. Chern. Soc., 64, 341 (1942). 

It is known that electric charges attract or repel each other with a force that is inversely proportional to the square of the distance between them. If two spheres like those in the electrometer (Figure 5-7) are negatively charged, what would be the change in the force of repulsion if the distance between them were increased to four times the original distance ... 

The surface forces apparatus (SEA) can measure the interaction forces between two surfaces through a liquid [10,11]. The SEA consists of two curved, molecularly smooth mica surfaces made from sheets with a thickness of a few micrometers. These sheets are glued to quartz cylindrical lenses ( 10-mm radius of curvature) and mounted with then-axes perpendicular to each other. The distance is measured by a Fabry-Perot optical technique using multiple beam interference fringes. The distance resolution is 1-2 A and the force sensitivity is about 10 nN. With the SEA many fundamental interactions between surfaces in aqueous solutions and nonaqueous liquids have been identified and quantified. These include the van der Waals and electrostatic double-layer forces, oscillatory forces, repulsive hydration forces, attractive hydrophobic forces, steric interactions involving polymeric systems, and capillary and adhesion forces. Although cleaved mica is the most commonly used substrate material in the SEA, it can also be coated with thin films of materials with different chemical and physical properties [12]. 

When electrons are in the region between two nuclei, attractive electrical forces exceed repulsive electrical forces, leading to the stable arrangement of a chemical bond. Remember that electrons are not point charges but are spread out over a relatively large volume. 

The qualitative discussion of solubility has focussed so far on the attractive forces in solute-solvent interactions. However, where water is concerned, it is also important to consider the forces of repulsion due to the so-called hydrophobic interactions that may arise in certain cases (Franks, 1975). These hydrophobic interactions may be explained in terms of thermodynamic concepts. 

Thus, the two bosons have an inereased probability density of being at the same point in spaee, while the two fermions have a vanishing probability density of being at the same point. This eonelusion also applies to systems with N identieal partieles. Identical bosons (fermions) behave as though they are under the influence of mutually attractive (repulsive) forces. These apparent forces are called exchange forces, although they are not forces in the mechanical sense, but rather statistical results. 

The physicochemical forces between colloidal particles are described by the DLVO theory (DLVO refers to Deijaguin and Landau, and Verwey and Overbeek). This theory predicts the potential between spherical particles due to attractive London forces and repulsive forces due to electrical double layers. This potential can be attractive, or both repulsive and attractive. Two minima may be observed The primary minimum characterizes particles that are in close contact and are difficult to disperse, whereas the secondary minimum relates to looser dispersible particles. For more details, see Schowalter (1984). Undoubtedly, real cases may be far more complex Many particles may be present, particles are not always the same size, and particles are rarely spherical. However, the fundamental physics of the problem is similar. The incorporation of all these aspects into a simulation involving tens of thousands of aggregates is daunting and models have resorted to idealized descriptions. 

Combined Electrostatic and Steric Stabilization. The combination of the two mechanisms is illustrated in Figure 4, taken from Shaw s textbook, (13) where the repulsion of the steric barrier during a collision falls off so rapidly as the colliding particles bounce apart that the dispersion force attractions hold the particles together in the "secondary minimum". This is exactly what happens in the system investigated in this paper. 

Figure 4. Potential energy diagrams for a pair of particles with on the left, a steric barrier (V ) and dispersion force attraction (V.) and on the right, with electrostatic repulsion (V ) added. Reproduced with permission from Ref. (13).Copyright 1980, Butterworths.

Coulomb blockade devices, 22 171, 172 Coulomb forces, in adhesion, 21 602 Coulombic attraction/repulsion, 23 804 Coulometry, 9 568, 575 Coulter counter, 12 11 Coumadin, 4 98... 

The interplay between the attractive (e.g.. Van der Waals or capillary forces) and repulsive forces involved during the approach of the tip to different surfaces under different environments are presented in Fig. 5 and discussed here. When the cantilever approaches a hard and non-compressible surface (Fig. 5a), at first the forces are too small to produce any measurable deflection of the cantilever, and therefore the position of the cantilever remains unchanged. At a certain distance the attractive forces overcome the cantilever spring constant and the tip leaps into contact with the specimen surface (Fig. 5b). As the cantilever continues to press down while the tip rests on the surface, the separation between the base of the tip and the sample decreases further, which results in the deflection of the tip with a subsequent increase... 

Particles in all kinds of suspensions or dispersions interact with two different kinds of forces (e.g., attractive forces and repulsive forces). One observes that, lyophobic suspensions (sols) must exhibit a maximum in repulsion energy in order to have a stable... 

Total force = repulsion forces + attraction forces... 

With short chain derivatives, the forces of repulsion are higher than the ones of attraction the curvature is high and spherical micelles are formed at a concentration called the critical micellar concentration (cmc). This concentration can be detected by a change in the physico-chemical properties of the solution (e.g. surface tension, Fig. 3 a). Above a characteristic temperature (referred as Krafft temperature), the tensio-active molecules are infinitely soluble in the form of micelles (Fig. 3 b). 

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