Forces, attractive cohesion

Molecules in contact with the surface of their container experience two sets of intermolecular forces. Cohesive forces attract molecules in the liquid to one another. In addition, adhesive forces attract molecules in the liquid to the molecules of the container walls. 

Capillary action, the rise of liquids up narrow tubes, occurs when there are favorable attractions between the molecules of the liquid and the tube s inner surface. These are forces of adhesion, forces that bind a substance to a surface, as distinct from the forces of cohesion, the forces that bind the molecules of a substance together to form a bulk material. By finding a relation between the height to which a liquid climbs in a tube of known diameter, we can use the height to determine the surface tension. 

Surface tension is responsible for several interesting phenomena. For example, consider how water beads up on a freshly waxed car. The water is a highly polar substance, and the wax is highly nonpolar. Since the polar water is not attracted to the nonpolar wax, there are few adhesive forces between the water and the waxed surface. Adhesive forces cause different substances to stick to each other. However, the surface tension in water results in significant cohesive forces between the water molecules. Cohesive forces, or cohesion, cause a substance to be more attracted to itself. The height of the bead depends on the wax surface and the surface tension of water. 

In the case of a liquid-gas interface, molecules of the liquid in the boundary can only develop attractive cohesive forces with molecules situated below and adjacent to them. They can develop attractive adhesive forces with molecules of the gaseous phase. However at the gas-liquid interface, these adhesive forces are quite small. The net effect is that molecules at the surface of the liquid have potential energies greater than those of similar molecules in the interior of the liquid and experience an inward force toward the bulk of liquid. This force pulls the molecules of the interface together and the surface contracts. 

Matter in general is made possible by the inseparable bond between attraction and repulsion. Because the proportion of these forces may vary, the possibility of an infinite variety of chemical substances is given. Hence, the first causes are not mechanical (as in atomist theories), but physico-dynamic.69 The interaction between the omnipresent ether and the two originary forces creates the specific differences between types of matter. Each chemical substance is characterized by a quantity of ether and three forces [universal attractive force, proper repulsive force, and proper attractive force (chemical cohesion)]. Hence, the ether is the ultimate origin of the variety of substances (together with the two moving forces). 

The force of cohesion, i.e. the maximum value of attractive force between the particles, may be determined by a direct measurement of force, F required to separate macroscopic (sufficiently large) particles of radius r, brought into a contact with each other. Such a measurement yields the free energy of interaction (cohesion) in a direct contact, A (h0) = Ff n r,. Due to linear dependence of F on r, one can then use F, to evaluate the cohesive force F2 = (r2/r )Fx, acting between particles in real dispersions consisting of particles with the same physico-chemical properties but of much smaller size, e.g. with r2 10 8 m (i.e. in the cases when direct force measurements can not be carried out). At the same time, in agreement with the Derjaguin equation... 

A convenient practical measure of the intermolecular forces between polymer chains (hat might be used in selecting solvents for specific polymers is the solubility parameter. Solubility parameter 8 is related to the heat of vaporization, A//vap (cal/cm ), also a measure of the same intermolecular attractive force. The cohesive energy density c of a liquid is the energy needed to overcome intermolecular forces to separate the molecules from each other and is given by the expression... 

Monosaccharides and disaccharides are generally very soluble in water and many form supersaturated solutions (syrups) during thickening. Similarly to dissolution of other solids, the forces holding the individual molecules in the solid state should be disrupted during the dissolution of carbohydrates. Such forces are cohesive forces of crystals that are, unlike the salt crystals, composed of neutral molecules. Intermolecular attractive forces are due to non-bonding interactions, usually to attractive forces between permanent dipoles. However, other non-bonding interactions, such as van der Waals forces, may also be present. 

However, in liquids, repulsive forces also operate at short intermolecular distances to balance the attractive, cohesive intermolecular forces. This leads to... 

The long-range van der Waals interaction provides a cohesive pressure for a thin film that is equal to the mutual attractive force per square centimeter of two slabs of the same material as the film and separated by a thickness equal to that of the film. Consider a long column of the material of unit cross section. Let it be cut in the middle and the two halves separated by d, the film thickness. Then, from one outside end of one of each half, slice off a layer of thickness d insert one of these into the gap. The system now differs from the starting point by the presence of an isolated thin layer. Show by suitable analysis of this sequence that the opening statement is correct. Note About the only assumptions needed are that interactions are superimposable and that they are finite in range. 

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