Retention enthalpy

If the retention enthalpies of the two sites differ, curvature may be observed in the plots. Moreover, if the enthalpies are opposite in sign, a minimum will occur in the van t Hoff plot at a temperature where the ratio of the retention foctors for the two mechanisms equals the absolute value of the reciprocal of the ratio of the corresponding enthalpies. Most frequently, however, less dramatic curvature would be expected. Such behavior may be anticipated in the RPC of amines with- arge nonpolar moieties which could be retained by silmiophilic interactions with surface silanols and by solvophobic interactions with nonpolar ligates of a reversed phase with low surface coverage. Recently an lihalysis of this behavior has been reported 93). 

We reinforce the concept that this approach is not acceptable because it does not account for the charge density of the stationary phase verified by extensive experimental evidence. The disregarded electrostatic potential that develops depends on column temperature, as explained above. Hence a sound theoretical description of the influence of temperature on retention enthalpy is still needed. The concept of the weighted... 

Calculate Liquid retentate enthalpy and Vapor permeate Enthalpies in each cell and liquid feed... 

Equation (1) can be viewed in an over-simplistic manner and it might be assumed that it would be relatively easy to calculate the retention volume of a solute from the distribution coefficient, which, in turn, could be calculated from a knowledge of the standard enthalpy and standard entropy of distribution. Unfortunately, these properties of a distribution system are bulk properties. They represent, in a single measurement, the net effect of a large number of different types of molecular interactions which, individually, are almost impossible to separately identify and assess quantitatively. 

Summarizing, the greater the forces between the molecules, the greater the energy (enthalpy) contribution, the larger the distribution coefficient, and the greater the retention. Conversely, any reduction in the random nature of the molecules or any increase in the amount of order in the system reduces the distribution coefficient and attenuates the retention. In chromatography, the standard enthalpy and standard entropy oppose one another in their effects on solute retention. Experimentally it has... 

Different portions of the standard free energy of distribution can he allotted to different parts of a molecule and, thus, their contribution to solute retention can be disclosed. In addition, from the relative values of the standard enthalpy and standard entropy of each portion or group, the nianner in which the different groups interact with the stationary phase may also be revealed. 

Finally, another important and interesting fact is established from the data treated in this manner. All the examples given confirm that the standard entropy term tends to increase with the standard enthalpy term. Consequently, the increase in retention is not as great as that which would be expected from the increase in standard enthalpy alone. 

It is seen that if the enthalpies and entropy s differ for two enantiomer pairs, there will always be a temperature where they elute coincidentally and cannot be separated. From the curves and intercepts given in Figure 15, the temperature for coincident retention of the two phenyl ethanol enantiomers is 432°K or 159°C and for the methylpiperidine enantiomers is 433°K or 160°C, which agrees excellently with the... 

The net retention volume and the specific retention volume, defined in Table 1.1, are important parameters for determining physicochemical constants from gas chromatographic data [9,10,32]. The free energy, enthalpy, and. entropy of nixing or solution, and the infinite dilution solute activity coefficients can be determined from retention measurements. Measurements are usually made at infinite dilution (Henry s law region) in which the value of the activity coefficient (also the gas-liquid partition coefficient) can be assumed to have a constant value. At infinite dilution the solute molecules are not sufficiently close to exert any mutual attractions, and the environment of each may be considered to consist entirely of solvent molecules. The activity... 

When retention ordering can be established, the theoretical peak capacity could be effectively utilized in a multidimensional separation system in a far more efficient manner. However, one is reminded that with the exception of synthetic polymers and a few other special cases of small molecules, real samples have almost random retention time distributions. It is rare when the free energy, enthalpy, and entropy of interaction are determined in LC for molecules utilized in retention mechanism studies. However, the retention energetics have been determined in GC studies by Davis et al. (2000) who found that many complex samples will exhibit Poisson distributions of retention times due to a Poisson distribution in enthalpy and a compensating distribution in entropy. 

Since members of a homologous series have incremental boiling point differences and if the amount of any homolog in the moving gas phase is related to vapor pressure at the temperature of the experiment, plots of log k vs. carbon number should also be a straight line. (The enthalpy of vaporization increases monotonically with carbon number.) This in fact is observed in gas-liquid equilibrium separation systems. It is the basis of retention index systems pioneered by Kovats for qualitative identification. 

The van der Waals volume can be related to the hydrophobicity of the solutes, and retention of molecular compounds can be predicted from their van der Waals volumes, 7r-energy, and hydrogen-bonding energy effects [72-74], It should be noted that the isomeric effect of substituents cannot be predicted with good precision because this is not simply related to Hammett s a or Taft s other hand, the hydrophobicity is related to enthalpy [75], Retention times of non-ionizable compounds were measured in 70 and 80% acetonitrile/water mixtures on an octadecyl-bonded silica gel at 25-60°C and the enthalpy values obtained from these measurements. 

Enthalpy-entropy compensation has been investigated in reversed-phase HPLC with octylsilica stationary phase [77]. The compensation temperatures were determined for this system, and the results show that their change with the composition of the mobile phase is almost similar to that with octadecylsilica stationary phase. It can be concluded that the retention mechanisms of the separation of alkyl benzenes is the same in both systems with the mobile phase exceeding 20% water content. 

An increase in the number of methylene units in alkyl benzene did not significantly affect the 7t-energy effect on their retention, but the enthalpy effect increased dramatically [80]. This means that a hydro-phobic compound can be adsorbed directly onto an octadecyl-bonded silica gel. The value of enthalpy effect of a methylene unit in alkyl benzene was calculated to be 500 cal/mol. 

Obviously, the magnitude of the temperature effect on retention depends on the difference in the enthalpy of the solute in either phase, and is specific for each solute. Therefore, it also changes the column selectivity. There is no retention and no temperature effect for AH=0. 

The same is true for the chiral polysiloxanes described here. Their use as stationary phases in gas chromatography allows the calculation of the differences in enthalpy and entropy for the formation of the diaste-reomeric association complexes between chiral receptor and two enantiomers from relative retention time over a wide temperature range. Only the minute amounts of the polysiloxanes required for coating of a glas capillary are necessary for such determinations. From these numbers further conclusions are drawn on the stereochemical and environmental properties required for designing systems of high enantio-selectivity in condensed liquid systems. 

The higher the enthalpy change (AH), the longer the retention, and log k shows a linear correlation to enthalpy.33 As seen in Figure 6.8, a number of groups of compounds exhibit such a linear relationship between enthalpy and log A , although it is not perfect. The AH values were independent of their van der Waals volumes, as seen in Figure 6.9. 

Figure 6.7 Measurement of enthalpy using chromatography for the relationship between absolute temperature and retention factor.

The enthalpy of methylphenols was about 2.0 kcal mol- and that of chlorophenols varied from 2.0 to 2.4 kcal mol -1 in the case of pentachlorophe-nol, indicating that the retention difference depended not upon the size but on the 7r-electron density.39 A similar result was obtained for alkylated and halogenated aromatic acids, whose enthalpies were nearly equal, but whose retention factors were different.40 The AH values may depend on the type of stationary phase used and the water content of the eluent.41... 

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