Distribution coefficients interactions

Salt Effects. The definition of a capacity factor k in hydrophobic interaction chromatography is analogous to the distribution coefficient, in gel permeation chromatography ... 

The capacity factors of SN-SiO, for metal ions were determined under a range of different conditions of pH, metal ions concentrations and time of interaction. Preconcentration of Cd ", Pb ", Zn " and CvS were used for their preliminary determination by flame atomic absorption spectroscopy. The optimum pH values for quantitative soi ption ai e 5.8, 6.2, 6.5, 7.0 for Pb, Cu, Cd and Zn, respectively. The sorption ability of SN-SiO, to metal ions decrease in line Pb>Cu> >Zn>Cd. The soi ption capacity of the sorbent is 2.7,7.19,11.12,28.49 mg-g Hor Cd, Zn, Pb, andCu, respectively. The sorbent distribution coefficient calculated from soi ption isotherms was 10 ml-g for studied cations. All these metal ions can be desorbed with 5 ml of O.lmole-k HCl (sorbent recovery average out 96-100%). 

The extent to which two solutes are separated depends exclusively on the relative magnitudes of their individual distribution coefficients (K(a)) and (K(b)) and the amount of stationary phase with which they can interact, (V(a)) and (V(b)). Consequently, for them to be separated, either their distribution coefficients must differ (choose appropriate phase systems), the amount of stationary phase with which they interact must differ (choose a stationary phase with exclusion properties), or a subtle combination of both. 

The distribution coefficient is an equilibrium constant and, therefore, is subject to the usual thermodynamic treatment of equilibrium systems. By expressing the distribution coefficient in terms of the standard free energy of solute exchange between the phases, the nature of the distribution can be understood and the influence of temperature on the coefficient revealed. However, the distribution of a solute between two phases can also be considered at the molecular level. It is clear that if a solute is distributed more extensively in one phase than the other, then the interactive forces that occur between the solute molecules and the molecules of that phase will be greater than the complementary forces between the solute molecules and those of the other phase. Thus, distribution can be considered to be as a result of differential molecular forces and the magnitude and nature of those intermolecular forces will determine the magnitude of the respective distribution coefficients. Both these explanations of solute distribution will be considered in this chapter, but the classical thermodynamic explanation of distribution will be treated first. 

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. 

Molecular Interactions and Their Influence on the Magnitude of the Distribution Coefficient (K)... 

Katz et al. tested the theory further and measured the distribution coefficient of n-pentanol between mixtures of carbon tetrachloride and toluene and pure water and mixtures of n-heptane and n-chloroheptane and pure water. The results they obtained are shown in Figure 17. The linear relationship between the distribution coefficient and the volume fraction of the respective solvent was again confirmed. It is seen that the distribution coefficient of -pentanol between water and pure carbon tetrachloride is about 2.2 and that an equivalent value for the distribution coefficient of n-pentanol was obtained between water and a mixture containing 82%v/v chloroheptane and 18%v/v of n-heptane. The experiment with toluene was repeated using a mixture of 82 %v/v chloroheptane and 18% n-heptane mixture in place of carbon tetrachloride which was, in fact, a ternary mixture comprising of toluene, chloroheptane and n-heptane. The chloroheptane and n-heptane was always in the ratio of 82/18 by volume to simulate the interactive character of carbon tetrachloride. 

In contrast molecular interaction kinetic studies can explain and predict changes that are brought about by modifying the composition of either or both phases and, thus, could be used to optimize separations from basic retention data. Interaction kinetics can also take into account molecular association, either between components or with themselves, and contained in one or both the phases. Nevertheless, to use volume fraction data to predict retention, values for the distribution coefficients of each solute between the pure phases themselves are required. At this time, the interaction kinetic theory is as useless as thermodynamics for predicting specific distribution coefficients and absolute values for retention. Nevertheless, it does provide a rational basis on which to explain the effect of mixed solvents on solute retention. 

Solute retention, and consequently chromatographic resolution, is determined by the magnitude of the distribution coefficients of the solutes with respect to the stationary phase and relative to each other. As already suggested, the magnitude of the distribution coefficient is, in turn, controlled by molecular forces between the solutes and the two phases. The procedure by which the analyst can manipulate the solute/phase interactions to effect the desired resolution will also be discussed in chapter 2. 

Thus, to achieve the required separation either (1) the distribution coefficient (K) of all the solutes must be made to differ, or (2) the amount of stationary phase (V s) available to each component of the mixture interacts must be made to differ. A further alternative (3)... 

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. 

Chemical forces are normally irreversible in nature (at least in chromatography) and thus, the distribution coefficient of the solute with respect to the stationary phase is infinite or close to infinite. Affinity chromatography is an example of the use of chemical forces in a separation process. The stationary phase is formed in such a manner that it will chemically interact with one unique solute present in the sample and thus, exclusively extract it from the other materials... 

It is the solvent, in this case methanol, that is responsible for reducing the distribution coefficients of the solutes with respect to the stationaiy phase (and consequently, their retention) by increasing the solute-solvent interactions in the mobile phase. Bearing this in mind, then the curves shown in figure 11 can explain some of the unique characteristics of methanol water mixtures when used as the mobile phase in reversed phase LC. 

It has been shown that, in LC, the size of the distribution coefficient of a solute between the two phases determines the extent of its retention. As a consequence, the difference between the distribution coefficients of two solutes establishes the extent of their separation. The distribution coefficients are controlled by the nature and strength of the molecular interactions that takes place between the solutes and the two phases. Thus it is the choice of the phase system that primarily determines the separation that is achieved by the chromatographic system. 

The lower isotherm represents the overload condition that can occur in liquid/liquid or gas/liquid systems under somewhat unique circumstances. If the interactions between solute molecules with themselves is stronger than the interactions between the solute molecules and the stationary phase molecules, then, as the concentration of solute molecules increases, the distribution coefficient of the solute with respect to the stationary phase also increases. This is because the solute molecules interact more strongly with a solution of themselves in the stationary phase than the stationary phase alone. Thus, the higher concentrations of solute in the chromatographic... 

MD simulations in expHcit solvents are stiU beyond the scope of the current computational power for screening of a large number of molecules. However, mining powerful quantum chemical parameters to predict log P via this approach remains a challenging task. QikProp [42] is based on a study [3] which used Monte Carlo simulations to calculate 11 parameters, including solute-solvent energies, solute dipole moment, number of solute-solvent interactions at different cutoff values, number of H-bond donors and acceptors (HBDN and HBAQ and some of their variations. These parameters made it possible to estimate a number of free energies of solvation of chemicals in hexadecane, octanol, water as well as octanol-water distribution coefficients. The equation calculated for the octanol-water coefficient is ... 

Barton, P. Davis, A. M. McCarthy, D. J. Webbom, P. J. H., Drug-phospholipid interactions. 2. Predicting the sites of drug distribution using n-octanol/water and membrane/water distribution coefficients. J. Pharm. Set 86, 1034—1039 (1997). 

Holten Liitzhoft, H.-C., Yaes, W. H. J., Freidig, A. P., Halling-Sorensen, B. and Hermens, J. L. M. (2000). 1-Octanol/water distribution coefficient of oxolinic acid influence of pH and its relation to the interaction with dissolved organic carbon, Chemosphere, 40, 711-714. 

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