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Molecular Forces All intermolecular forces are electrical in nature. The three different types are termed dispersion forces, polar forces and ionic forces. All interactions between molecules are composites of these three forces. Dispersion Forces Dispersion forces were first described by London (3), and for this reason were originally called 'London's dispersion forces'. London's name has now been dropped and they are now simply referred to as 'dispersion' forces. They arise from charge fluctuations throughout a molecule resulting from electron/nuclei vibrations.   Glasstone (4) suggested that dispersion forces could be best described as follows,   "although the physical

when mixed phases are considered. Solute/phase interactions result from three basic types of intermolecular force, all of which are electrical in nature. Although it is theoretically possible that magnetic and even gravimetric forces may be also present, they will have no significance compared with those of electrical origin. The three types of molecular interactive force are dispersive, polar and ionic giving rise to dispersive interactions, polar interactions and ionic interactions. Polar forces have been further divided into sub groups ranging from 'strong dipole-dipole interactions' (hydrogen bonding) to 'weak dipole-dipole interactions ((p)-(p) interactions). This type of division is questionably useful as it tends to 'confuse' more than 'explain' when dealing with chromatographic retention. Division into the two groups, dipole-dipole interactions and dipole-induced dipole interactions, however, is appropriate, as it describes two physically different types of polar

cases retention has both "energetic" and "entropic" components which, by careful adjustment, can be made to achieve very difficult and subtle separations. Thermodynamics show that there are two processes controlling distribution but does not indicate how the distribution can be managed or controlled. To do this, it is necessary to identify how the magnitude of (K) and (Vs) are controlled. In general, (K) is usually determined by the nature and strength of the intermolecular forces between the solute and the two phases. The availability of the stationary phase (the magnitude of (Vs)) is largely determined by the geometry of the stationary phase. Factors Affecting the Magnitude of the Distribution Coefficient (K) The magnitude of (K) is determined by the relative affinity of the solute for the two phases. Those solutes interacting more strongly with the stationary phase will exhibit a larger distribution coefficient and will

nbsp;   and (DSo) is the standard entropy. The standard enthalpy and standard entropy represent two distinctly different portions of the energy associated with distribution and are related to quite different parts of the distribution processes.   The enthalpy term represents the energy involved when the solute molecules break their interactions with the mobile phase and interact with, and enter, the stationary phase. These interactions result from intermolecular forces that are electrical in nature (see book 7 for details) and are accompanied by the absorption or evolution of heat.   However, when the solute interacts with the stationary phase, because the interactive forces between the solute and the stationary phase molecules are stronger than those between the solute molecules and the mobile phase, the solute molecules are held more tightly and, consequently, are more restricted. This motion restriction, reduced freedom of

Thus, the standard energy change is made up of an actual energy or standard enthalpy change resulting from the intermolecular forces between solute and stationary phase and a standard entropy change that reflects the resulting restricted movement, or loss inrandomness, of the solute while preferentially interacting with the stationary phase. From, equations (1) and (2), substituting for (DGo )                        RT ln (K)  =  -DGo = -DHo+ TDSo      &

This means that the intermolecular forces are stronger and, thus, the stationary phase molecules hold the solute molecules more tightly. In turn, this indicates that the freedom of movement and the random nature of the solute molecule are also more restricted, which results in a larger change in standard entropy. It follows that, unless other significant retentive factors are present, any increase in standard enthalpy would be expected to be accompanied by a corresponding increase in standard entropy. The simple