Molecular dispersion force

Very recently, Good [17] has extended the theory of interfacial energies [16] to include an explicit accoxmt of the different types of inter molecular forces dispersion, dipole induction, and dipole orientation. In the present paper, we will use the theory, as extended [17], to show how the actual surface free energies of certain solids can be calculated from contact angle data, including Fox and Zisman s critical surface tensions. 

Molecular interactions are the result of intermolecular forces which are all electrical in nature. It is possible that other forces may be present, such as gravitational and magnetic forces, but these are many orders of magnitude weaker than the electrical forces and play little or no part in solute retention. It must be emphasized that there are three, and only three, different basic types of intermolecular forces, dispersion forces, polar forces and ionic forces. All molecular interactions must be composites of these three basic molecular forces although, individually, they can vary widely in strength. In some instances, different terms have been introduced to describe one particular force which is based not on the type of force but on the strength of the force. Fundamentally, however, there are only three basic types of molecular force. 

When thinking about chemical reactivity, chemists usually focus their attention on bonds, the covalent interactions between atoms within individual molecules. Also important, hotvever, particularly in large biomolecules like proteins and nucleic acids, are a variety of interactions between molecules that strongly affect molecular properties. Collectively called either intermolecular forces, van der Waals forces, or noncovalent interactions, they are of several different types dipole-dipole forces, dispersion forces, and hydrogen bonds. 

Ihe boiling points of different molecular substances are directly related to the strength of the intermolecular forces involved. The stronger the intermolecular forces, the higher the boiling point of the substance. In the remainder of this section, we examine the nature of the three different types of intermolecular forces dispersion forces, dipole forces, and hydrogen bonds. 

The effect of molecular interactions on the distribution coefficient of a solute has already been mentioned in Chapter 1. Molecular interactions are the direct effect of intermolecular forces between the solute and solvent molecules and the nature of these molecular forces will now be discussed in some detail. There are basically four types of molecular forces that can control the distribution coefficient of a solute between two phases. They are chemical forces, ionic forces, polar forces and dispersive forces. Hydrogen bonding is another type of molecular force that has been proposed, but for simplicity in this discussion, hydrogen bonding will be considered as the result of very strong polar forces. These four types of molecular forces that can occur between the solute and the two phases are those that the analyst must modify by choice of the phase system to achieve the necessary separation. Consequently, each type of molecular force enjoins some discussion. 

Dispersive forces are more difficult to describe. Although electric in nature, they result from charge fluctuations rather than permanent electrical charges on the molecule. Examples of purely dispersive interactions are the molecular forces that exist between saturated aliphatic hydrocarbon molecules. Saturated aliphatic hydrocarbons are not ionic, have no permanent dipoles and are not polarizable. Yet molecular forces between hydrocarbons are strong and consequently, n-heptane is not a gas, but a liquid that boils at 100°C. This is a result of the collective effect of all the dispersive interactions that hold the molecules together as a liquid. 

Returning to the molecular force concept, in any particular distribution system it is rare that only one type of interaction is present and if this occurs, it will certainly be dispersive in nature. Polar interactions are always accompanied by dispersive interactions and ionic interactions will, in all probability, be accompanied by both polar and dispersive interactions. However, as shown by equation (11), it is not merely the magnitude of the interacting forces between the solute and the stationary phase that will control the extent of retention, but also the amount of stationary phase present in the system and its accessibility to the solutes. This leads to the next method of retention control, and that is the volume of stationary phase available to the solute. 

Why is it that some substances readily mix to form solutions while others do not Whether one substance dissolves in another substance is largely dependent on the inter-molecular forces present in the substances. For a solution to form, the solute particles must become dispersed throughout the solvent. This process requires the solute and solvent to initially separate and then mix. Another way of thinking of this is that the solute particles must separate from each other and disperse throughout the solvent. The solvent may separate to make room for the solute particles or the solute particles may occupy the space between the solvent particles. Determining whether one substance dissolves in another requires examining three different intermolecular forces present in the substances—between the... 

The four main types of HPLC techniques are NP, RP, lEX, and SEC (Section 1.2). The principal characteristic defining the identity of each technique is the dominant type of molecular interactions employed. There are three basic types of molecular forces ionic forces, polar forces, and dispersive forces. Each specific technique capitalizes on each of these specific forces ... 

Further developments with this model were to a large extent initiated by Fowkes ), who started from the assumption that the molecular forces, determining Y could be split up into components, of which only the London-van der Waals or dispersion forces had enough range to penetrate into an adjoining phase. For these forces the Berthelot principle would hold, so in (2.11.16] the geometric mean only involves the dispersion contributions to y and he replaced [2.11.17] by... 

In total, there are three different basic types of molecular force, all of which are electrical in nature. These forces are called dispersion forces, polar forces, and ionic forces. Despite there being many different terms used to describe molecular interactions (e.g., hydrophobic forces, 77-77 interactions, hydrogen-bonding, etc.), all interactions between molecules are the result of composites of these three different types of molecular force. 

As the degree of branching increases for a series of molecules of the same molecular weight, the molecules become more compact. A compact molecule can have fewer points of contact with its neighbors than more extended molecules do. As a result, the total induced dipole forces (dispersion forces) are weaker for branched molecules, and the boiling points of such compounds are lower. This pattern can be seen in the data in Table 27-3 and in Example 27-1. 

Solubility data can be plotted in a three-dimensional system using the 8D, 8p, and SH parameters as coordinates. The solubility regions for various polymers will be spherical if the 8D axis is expanded with a unit distance equal to twice that of the unit distances on the other axes. This effect is attributed to the directional nature of the molecular forces involved in 8P and 8H vs. the omnidirectional nature of the atomic dispersion forces. Description of solubility in terms of a geometrically simple model has many advantages, particularly in computer processing. 

The solubility parameter or cohesive force of an individual solvent is believed to result from its inner molecular forces of attraction. Individual molecular forces characterize and dominate certain molecular regions of the structure. For instance dispersion (or London) forces result from the association between the electron systems of two adjacent molecules and the arrangement of the electrons. These forces are not affected by temperature, they operate within a short distance, they are accumulative, and they are general They reside in all molecules and represent the total attractive force known in saturated aliphatic hydrocarbons. 

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