Ionic solutes, distribution coefficient

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. 

The review of Martynova (18) covers solubilities of a variety of salts and oxides up to 10 kbar and 700 C and also available steam-water distribution coefficients. That of Lietzke (19) reviews measurements of standard electrode potentials and ionic activity coefficients using Harned cells up to 175-200 C. The review of Mesmer, Sweeton, Hitch and Baes (20) covers a range of protolytic dissociation reactions up to 300°C at SVP. Apart from the work on Fe304 solubility by Sweeton and Baes (23), the only references to hydrolysis and complexing reactions by transition metals above 100 C were to aluminium hydrolysis (20) and nickel hydrolysis (24) both to 150 C. Nikolaeva (24) was one of several at the conference who discussed the problems arising when hydrolysis and complexing occur simultaneously. There appear to be no experimental studies of solution phase redox equilibria above 100°C. 

I. 46. The magnitude of the coefficient reflects the electric charge distribution of the ionic species. A 0.1 molal solution of Al2(S04)3 has an activity coefficient of only 0.035. It should also be noted that, in dilute solutions, activity coefficients of electrolytes decrease in magnitude with increasing concentration. A minimum is reached and the coefficient then increases with concentration. See Activity Debye-Huckel Law Biomineralization... 

Due to the ionic nature of cephalosporin molecules, the interfacial chemical reaction may in general be assumed to be much faster than the mass transfer rate in the carrier facilitated transport process. Furthermore, the rate controlling mass transfer steps can be assumed to be the transfer of cephalosporin anion or its complex, but not that of the carrier. The distribution of the solute anion at the F/M and M/R interfaces can provide the equilibrium relationship [36, 69]. The equilibrium may be presumably expressed by the distribution coefficients, mf and m at the F/M and M/R interfaces, respectively and these are defined as... 

In gel chromatography, the distribution coefficient K is little affected by the concentration of solutes, pH, ionic strength, and so on, and is considered to be constant. Therefore, the results obtained in. Section 11.6.2 for constant K can be applied to evaluate the performance of gel chromatography. Figure 11.9 shows the increase in Hs with the liquid velocity in gel chromatography packed with gel particles of 44 pm diameter [4]. The values of Hs increase linearly with the velocity, and the slopes of the lines become steeper with an increase in molecular weights, as predicted by Equation 11.20. 

Often the measure of hydrophobicity desired is an apparent log P of an ionizable solute at a pH where it is partly ionized and thus more hydrophilic. This is most often termed a distribution coefficient and reported as Tog D. These can be calculated from a measured pKa and the neutral log P, if it assumed that the octanol phase contains a negligible portion of the ionic species. This is a good assumption if the pKa of the solute is no more than 3.0 log units on the ionized side of the pH of measurement. 

They are often plasticized PVC matrices, which occlude an ionophore as the key selective element, a chromoionophore or a fluoroionophore as the chemical-optical transducer and, sometimes, ionic additives to maintain electroneutrality. Such optodes follow ion-exchange mechanisms between the membrane and the aqueous solution and the analytical response originates from the ratio of the concentration of ions in the solution or from their product (Fig. 3). Moreover, selectivity is ruled by the ion distribution coefficients between both phases and by the formation constants of complexes within the membrane. 

According to eqn.(3.90), k is proportional to the concentration of the pairing ion, with the proportionality constant being determined by the distribution coefficient for the neutral molecule (Kxy) and by the dissociation constant for this molecule into the two separate ions X and Y (K ). The first factor is affected by the same parameters as retention in the LC of non-ionic solutes (section 3.2). The latter factor will be determined by the nature of the solute ion and the pairing ion and by the composition (ionic strength, pH, modifier content) of the aqueous phase. 

Debye-Hiickel developed a theory for the activity coefficients of an ionic solution at a molecular level. A selected ion in the ideally diluted solution is statistically well distributed and there are no interactions between ions present in the solution. In contrast, the ion in the concentrated solution is surrounded by the excess of counter ions in the vicinity of the ion, as the counter ions are attracted by Coulombic forces, while ions of the same charge are repelled. Thus, ion atmosphere is created. As a result, there is a difference in reversible work between the concentrated wrev and dilute solutions wrev ideal ... 

Apart from the hydrophobic interactions provided by the alkyl part of the molecule, octanol has also hydrogen-bond acceptor and donor functions like lipid membranes have. This property of n-octanol made the octanol-water distribution coefficient that widely used. However, n-octanol or reversed phase materials cannot mimic the interfacial character of the bilayer structure. The ionic interactions between membrane phospholipids and solute are also not represented in the properties of octanol or reversed phase materials. To overcome this issue, alternative stationary phases... 

In conclusion, we can state that additions of small amounts of phosphoric acid increase the rate of Am (III) oxidation by Ag + ions without drastically changing the distribution coefficients of the species involved, because no complexation of Am occurs in these solutions, contrary to what occured in the extensive studies of Myassoeodov et al. (12, 13) in more concentrated phosphoric acid, where the kinetic effects must be attributed to changes in americium and silver (II) ionic species. In our Am-Cm separations we added phosphoric acid, which accelerates the oxidation rates of Am (III), solubilizes argentic oxide, and makes it possible to obtain Am (VI) quantitatively and rapidly at room temperature. 

In the study of the interface with two immiscible electrolyte solutions (ITIES), considerable attention has been focused on the estimation of the Galvani potential difference at the water oil interface on the basis of a reasonable extra-thermodynamic assumptions. The discussion of these estimates is often made in terms of the ionic distribution coefficient, which is defined on the basis of equations (8.9.5) and (8.9.6). Generalizing this equation for the ot P interface at which ion i with charge z,- is transferred, one may write... 

Ion-exchange chromatography involves more variables than other forms of chromatography. Distribution coefficients and selectivities are functions of pH, solute charge and radius, resin porosity, ionic strength and type of buffer, type of solvent and temperature. The number of experimental variables makes lEC a very versatile technique but a difficult one because of the effort needed to optimise a separation. 

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