Cohesion, molecular

A few quantitative data have already been given regarding the magnitude of the intermolecular forces, which show that, generally speaking, secondary valence forces exist of the order of 500-5000 cal. per atom or atomic group, while the chemical primary valences are found in the region of 20,000-200,000 cal. per mol. 

A very interesting paper by M. DunkeP will now be cited which is of similar importance to the secondary valences as is Table 51 for the primary valence.  

We should like to see a table prepared also for the intermolecular forces, to which the term molecular cohesion has been applied by K. H. Meyer because of their importance to the structure of the solid and liquid phase in which the secondary valence forces radiating from the principal groups are tabulated, at least, according to the order of magnitude of their energy dimensions. 

For the data in Table 18 the dissociation energies of the primary valence bond, i.e. the heats of combustion had to be used, but now we must consider the energy of overcoming molecular cohesion, which is measured by the heats of sublimation and vaporization or by the van der Waals constant a. For the latter, and for the Sutherland constant closely allied to it, van Laar has already established certain additive relationships. These are expressed even more definitely in the well known Traube rule for the heats of adsorption of homologous series, which, according to Eucken, are closely related to the heats of vaporization. 

Heats of vaporization doubtless constitute the best experimental material for deducing such relationships. M. Dunkel has collected data for some years and has estimated from them the molar cohesion of the principal typical groups of organic molecules. The difficulty which arises here in contrast to the Fajans-v. Weinberg calculation of the primary valence forces is due to the fact that, because of the large absolute values in the first case, the effect of temperature on these forces, i.e. the influence of specific heats and of chemical constants, can be ignored, while the experimental vapor pressure curves would lead to completely erroneous heats of vaporization if the specific heats were neglected. Measurements at 

This argument should not be interpreted to mean that the formation of a chemical bond is a classical process. It means rather that the quantum- 

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The reason for the activity of the above named classes of liquids is not fully understood but it has been noted that the most active liquids are those which reduce the molecular cohesion to the greatest extent. It is also noticed that the effect is far more serious where biaxial stresses are involved (a condition which invariably causes a greater tendency to brittleness). Such stresses may be frozen in as a result of molecular orientation during processing or may be due to distortion during use. 

The internal pressure is a differential quantity that measures some of the forces of interaction between solvent molecules. A related quantity, the cohesive energy density (ced), defined by Eq. (8-35), is an integral quantity that measures the total molecular cohesion per unit volume. - p... 

The development of theoretical chemistry ceased at about 1930. The last significant contributions came from the first of the modern theoretical physicists, who have long since lost interest in the subject. It is not uncommon today, to hear prominent chemists explain how chemistry is an experimental science, adequately practiced without any need of quantum mechanics or the theories of relativity. Chemical thermodynamics is routinely rehashed in the terminology and concepts of the late nineteenth century. The formulation of chemical reaction and kinetic theories take scant account of statistical mechanics and non-equilibrium thermodynamics. Theories of molecular structure are entirely classical and molecular cohesion is commonly analyzed in terms of isolated bonds. Holistic effects and emergent properties that could... 

The cohesive pressure c is a measure of the total molecular cohesion per unit volume, given by eqn. 3.11... 

Differences in the frictional properties of most plastics can be explained in terms of the ratio of shear strenghth to hardness. Shooter and Tabor observed that the coefficients of friction for polytetrafluoroethylene are 2—3 times lower than anticipated by this calculation. It is believed that this discrepancy is caused by the inherently low cohesive forces between adjacent polymer chains and is responsible for the absence of stick-slip. The large fluorine atoms effectively screen the large carbon-fluorine dipole, reducing molecular cohesion so that the shear force at the interface is low. The shear strength of the bulk material is higher because of interlocking molecular chains. 

Well-defined products from the chaotic turmoil, which is a chemical reaction, result from a balance between external thermodynamic factors and the internal molecular parameters of chemical potential, electron density and angular momentum. Each of the molecular products, finally separated from the reaction mixture, is a new equilibrium system that balances these internal factors. The composition depends on the chemical potential, the connectivity is determined by electron-density distribution and the shape depends on the alignment of vectors that quenches the orbital angular momentum. The chemical, or quantum, potential at an equilibrium level over the entire molecule, is a measure of the electronegativity of the molecule. This is the parameter that contributes to the activation barrier, should this molecule engage in further chemical activity. Molecular cohesion is a holistic function of the molecular quantum potential that involves all sub-molecular constituents on an equal basis. The practically useful concept of a chemical bond is undefined in such a holistic molecule. 

The key to molecular cohesion and chemical binding is the valence state -a truly non-classical state of matter. It arises from excitation of a valence electron to the point at which it decouples from the parent atom but remains associated with it because of environmental confinement. It can be likened... 

Debating the relative merits of the partitioning schemes is a sterile exercise. Molecular cohesion is of strictly quantum-mechanical origin and therefore holistic. When this interaction is reduced to classically recognizable features the required abstraction destroys the holistic picture. Any abstraction is a classical approximation to a non-classical system. The essential difference between classical and non-classical descriptions of a molecule arises from the non-local basis of the latter (compare 3.4) which simultaneously depends on all particle coordinates. It is shaped by the complex wave function, which cannot be simulated by the superposition of real (classical) functions. The argument is therefore settled by this essential difference between quantum and classical theories [218]. The former is defined in the complex plane and the latter in real space, as shown in figure 2.7. 

An important measure of the total molecular cohesion per unit volume of liquid is the cohesive pressure c (also called cohesive energy density), which characterizes the energy associated with all the intermolecular solvent/solvent interactions in a mole of the solvent. The cohesive pressure is defined as the molar energy of vapourization to a gas at zero pressure, Af/y, per molar volume of the solvent, V, according to Eqs. (3-5) and (5-76) in Sections 3.2 and 5.4.2, respectively [93, 94]. The cohesive pressure c is related to the internal pressure n cf. Eq. (3-6) and Table 3-2 in Section 3.2. 

Walden i supposed the molecular cohesion Ma for compounds of carbon,... 

The influence of substituents on the solubility of molecules in water can be due to their effect on the properties of the solid or liquid (for example, on its molecular cohesion) or to the effect of the substituent on its interaction with water molecules. It is not easy to predict what effect a particular substituent will have on crystal properties, but as a guide to the solvent interactions, substituents can be classified as either hydrophobic or hydrophilic, depending on their polarity (see Table 5.4). The position of the substituent on the molecule can influence its effect, however. This can be seen in the aqueous solubilities of o-, m- and p-dihydroxy-benzenes as expected, all are much greater than that of benzene, but they are not the same, being 4, 9 and 0.6 mol dm, respectively. The relatively low solubility of the para compound is due to the greater stability of its crystalline state. The melting points of the derivatives indicate that is so, as they are 105°C, 111°C, and IZO C, respectively. In the case of the ortho... 

Other interesting properties, related to molecular cohesion in condensed phases, are the enthalpies of vaporization and sublimation. In the classical MD approach, the vaporization enthalpy of a liquid or of a solid is ... 

The molecular cohesive energy Ecoh is defined as the energy reduction upon forming a crystal from isolated molecules, i.e., Ecoh = Emoi - Ehlln /2. where Ebuik and Emoi denote the total energies of the bulk and the isolated molecule, respectively. The factor 2 takes into account the number of molecules in the unit cell. By this definition the cohesive energy is positive for any stable crystal. 

Period II lasted 7 years oil yield was 4,500,000 tons and the amount of formation water produced was 2,900,000 tons. Formation pressure declined from 40 kg/cm to 15 kg/cm. Characteristically for this period, the water in the liquids recovered increased, then, towards the end of the period, decreased again. Gas dissolved in the oil provided the basic drive during this period. Forces of molecular cohesion between the oil and the reservoir rocks are weakened and oil mobility enhanced when water cut stays within the 35-50% range. 

The wettability characteristics of an adhesive/adherend pair are determined by the relative values of surface tension of the adhesive and adherend. Surface tension of a liquid is a direct measurement of intermolecular forces and is half of the free energy of molecular cohesion. Surface tension is commonly represented by -q (gamma), and is measured in dynes/cm. The value of the surface tension of the solid substrate, or adherend, is called the critical surface tension, To ensure that the surface of the adherend will be wetted by an adhesive, an adhesive whose surface tension is less than the critical surface tension should be selected, so that... 

An important property of the surfaces of droplets is the surface tension that expresses the cohesion of water molecules (Chapter 2.5.1.1). On molecules existing close to the droplet surface, forces are directed to the inner of the droplet. Therefore, each liquid has the tendency to form spherical particles (if they are not counteracting forces such as gravitation and other outer forces). The reason is simple a sphere of a given volume has the smallest surface of all bodies. Thus, a growing droplet needs to overcome the molecular cohesion. There are two equivalent definitions of surface tension ... 

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