Cohesion Intermolecular

Cohesion intermolecular attractive force between particles within a substance Colligative Property a property dependent on the number of particles in solution and not on the type of particles, for example, boiling point elevation and freezing point depression... 

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

The transition of a liquid phase to its vapor phase involves the separation of molecules in the liquid and the removal of molecules from the surface of the liquid into the vapor phase. The energy absorbed when a definite quantity of a liquid is vaporized (the latent heat of vaporization) therefore depends on the intermolec-ular attractive forces which have to be overcome in order to separate molecules. According to Trouton s rule, the boiling points of nonassociated liquids, on the absolute-temperature scale, are approximately proportional to their latent heats of vaporization. Hence, the boiling point of a liquid depends on the relative strength of cohesive intermolecular forces. 

If the concentration of junction points is high enough, even branches will contain branches. Eventually a point is reached at which the amount of branching is so extensive that the polymer molecule becomes a giant three-dimensional network. When this condition is achieved, the molecule is said to be cross-linked. In this case, an entire macroscopic object may be considered to consist of essentially one molecule. The forces which give cohesiveness to such a body are covalent bonds, not intermolecular forces. Accordingly, the mechanical behavior of cross-linked bodies is much different from those without cross-linking. 

The intermolecular forces of adhesion and cohesion can be loosely classified into three categories (7) quantum mechanical forces, pure electrostatic... 

Orowan (1949) suggested a method for estimating the theoretical tensile fracture strength based on a simple model for the intermolecular potential of a solid. These calculations indicate that the theoretical tensile strength of solids is an appreciable fraction of the elastic modulus of the material. Following these ideas, a theoretical spall strength of Bq/ti, where Bq is the bulk modulus of the material, is derived through an application of the Orowan approach based on a sinusoidal representation of the cohesive force (Lawn and Wilshaw, 1975). 

The greater the viscosity of a liquid, the more slowly it flows. Viscosity usually decreases with increasing temperature. Surface tension arises from the imbalance of intermolecular forces at the surface of a liquid. Capillary action arises from the imbalance of adhesive and cohesive forces. 

The degree of realism of these model structures can be assessed by comparison of computed properties with experimental ones. The cohesive energy is, by definition, the difference in energy per mole of substance between a parent chain in its bulk environment and the same parent chain in vacuo, i.e., when all intermolecular forces are eliminated. This difference is readily computed from the minimized... 

Every gas changes into a liquid if the pressure is high enough and the temperature is low enough. The atoms or molecules of a liquid or solid stick together in a finite volume rather than expanding, as a gas does, to fill all available space. This cohesiveness comes from electrical forces of attraction between the negative electron cloud of each atom and the positive nuclei of other atoms. We describe intermolecular forces in Chapter 11. 

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. 

In its solid state, however, the basic structural features of ordinary hexagonal ice (ice I) are well established. In this structure (Figure 1.2), each water molecule is hydrogen bonded to four others in nearly perfect tetrahedral coordination. This arrangement leads to an open lattice in which intermolecular cohesion is large. 

The viscosity of a fluid is an important property in the analysis of liquid behavior and fluid motion near solid boundaries. Viscosity is the fluid resistance to shear or flow and is a measure of the adhesive/cohesive or frictional fluid property. The resistance is caused by intermolecular friction exerted when layers of fluids attempt to slide by one another. 

Plasticization has been explained by a variety of theories in an attempt to explain how the plasticizer reduces the rigidity of the final part. All theories rely on the premise that the plasticizer reduces the strength of the intermolecular forces between the polymer chains. The theories fall into two broad categories interference mechanisms and expansion mechanisms. The interference mechanisms state that plasticizer molecules interact only weakly with the polymer chains after separating the chains from one another, thereby reducing the overall cohesion of the material. The expansion mechanisms state that the reduced rigidity arises from an increase in the free volume of the system as the system expands to incorporate bulky,... 

According to Eyring (Moore and Eyring, 1938) and Joly (1956), Newtonian flow in a monolayer is the result of a cohesive attraction between surfactant molecules. For every molecule that flows from higher to lower surface pressure in a motion parallel to the canal walls, there is another molecule ready to fill the hole vacated by the first. The mechanism for this cohesive flow is presumably attractive van der Waals interactions between hydrocarbon chains. This model assumes that the average intermolecular separation in a surface-continuous monolayer does not exceed the cross-sectional area of the molecule as defined by the average molecular area A of the film at the surface pressure n in the pressurized compartment of the viscometer. 

Molecules have forces of attraction between them, and these intermolecular forces are responsible for many of the properties of liquids. There is a cohesion energy that holds the molecules together. The energy necessary to overcome these forces to vaporize a mole of liquid is known as the cohesion energy of the liquid or the energy of vaporization. It is related to the enthalpy of vaporization by the equation... 

We have already mentioned that silver chloride is readily soluble in liquid ammonia. Because it is slighdy less polar than water and has lower cohesion energy, intermolecular forces make it possible for organic molecules to create cavities in liquid ammonia. As a result, most organic compounds are more soluble in liquid ammonia than they are in water. Physical data for liquid ammonia are summarized in Table 10.2. 

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