Attraction between molecules summary

The attractive component of the DLVO force balance arises from van der Waals or dispersion interactions. In summary, all molecular species (apart from the proton) have, via the interaction of the electron cloud with the background electromagnetic field, the ability to induce dipoles in nearby molecules, and to be so affected themselves. This dipole-dipole interaction leads to an attractive force between molecules, and indeed between macroscopic bodies (although it is important to point out that dispersion forces are not, in general, pair-wise additive). 

This is detailed in previous chapters, but for the sake of completion of this section a summary is given here. The attraction between atoms or molecules is of three types Dipole-dipole (Keesom force), dipole-induced-dipole (Debye force) and London dispersion force. 

In summary, two mechanisms have been proposed to explain the results. One assumes that the ion pairs are formed in the mobile phase and behave as nonionic moities similar to other polar molecules in RPLC. The other rests on the belief that the counterions selectively sorb in the stationary phase and attract and retard the analyte ions by an ion exchange mechanism this mechanism requires that the ion-pair reagent have a hydrophobic end that would be attracted to the alkyl chain on the bonded phase and an ionic site on the other end. Perhaps both mechanisms are partially correct, and the predominant mechanism may depend on the operating conditions. In any case the following discussion will not attempt to distinguish between the mechanisms or justify either one. 

In summary the results observed in these studies [160] of poly(Sty-co-DVB) swelling in aromatic liquids serve to show that the method of measuring a is so sensitive that it can detect an effect caused by even the smallest modification in the molecular geometry of attached substituents, and that these differences correlate qualitatively with expectation based on the known principles of physico-organic chemistry of aromatic compounds. Since the observed a is the net effect of electronic attraction and steric hindrance between the sorbed molecule and the adsorption site, i.e. the monomer unit of the polymer, it would be impossible to separate quantitatively the electronic and steric contributions of a particular substituent. The ability to make such a differentiation, however, appears to be more promising with liquids that comprise homologous series of the type Z(CH2)nH (where Z is a phenyl, chloro, bromo or iodo substituent), since the added electronic contribution to Z by each additional methylene group is well known to be extremely small when n becomes >3 [165],... 

More specific discussion will focus on relationships between molecular structure and physical properties for the class of compounds as a whole. In this chapter a brief summary of the kinds of forces that attract molecules to each other is presented. Alkanes, lacking charged atoms or highly polarized bonds, do not exhibit either ionic or dipolar forces. As nonpolar molecules, alkane molecules are attracted to each other by only the rather weak London forces. These can be understood fairly simply. In even a totally unpolarized bond, the electrons are always moving. Even though the average location of the electron pair is exactly half-way between the atoms, at any particular moment in time, the electrons may be closer to one atom or the other ... 

Originally intended for application to substances whose cohesion arose from dispersion forces, the parameter seem to be of limited use with polymers, which generally decompose before vaporization enthalpies can be determined. The concept now has been greatly expanded. The overall 6 can be divided into dispersion and polar contributions. Often non-polar homomorphs of polar molecules can provide values of 5, and polar contributions, can then be obtained from differences between 6 and Further refinements due to Hansen " have introduced a three-component solubility parameter, which separates non-dispersive contributions into polar and hydrogen bond components. This has been applied to organic liquids, and to some polymers. Calculations of b for macromolecules also can be made from tabulated values of molar attraction constants, and extensive summaries of b and of other cohesion parameters are readily available to the potential user. Ultimately, however, the application of b to polymer systems is impeded for the following reasons ... 

On the contrary, the interfadal tension between fluid phases usually decreases at higher process pressures. This reveals an attractive feature for spray processes at high pressures [13]. Following Eq. (2.6), this means an enhancement of molecule concentration within the interfacial volume (adsorption) and in summary a volume decrease of the total system at constant pressure appears. 

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