Buffered partition systems

Buffered partition systems are used to separate weak acids and bases. The degree of ionization is kept constant by impregnation of the paper with a suitable buffer and/or by adding acid or base to the aqueous-organic solvent [31,32]. 

The lipid-aqueous partition coefficient of a drug molecule affects its absorption by passive diffusion. In general, octanol/pH 7.4 buffer partition coefficients in the 1-2 pH range are sufficient for absorption across lipoidal membranes. However, the absence of a strict relationship between the partition coefficient of a molecule and its ability to be absorbed is due to the complex nature of the absorption process. Absorption across membranes can be affected by several diverse factors that may include the ionic and/or polar characteristics of the drug and/or membrane as well as the site and capacity of carrier-mediated absorption or efflux systems. 

For working of this technique, prediction of the optimum combination of stationary phase and mobile liquid phase is not always possible since the partitioning system may involve mixed solvents, solutions of salts, buffers, or complexing agents, and determination of the proper conbination of stationary phase and solvent system in partition chromatography is not unoften a matter of trial and error. 

The toxicity and the affinity of solvents to cell structures increase with hydrophobic properties of solvents, e.g., high toxicities with P, values of l-S. The partition coefficient correlates with the membrane-buffer partition coefficient between membrane and aqueous system. They also depend on membrane characteristics. 

Ionization of the solutes to be separated greatly influences their chromatographic behaviour. If the effective partition coefficient has to be concentration-independent. ionization should be wholly suppressed or increased to a maximum or, alternatively, maintained at a steady level. Acids, alkalis and buffers are therefore often used as components of the partition system for ionizable substances. The addition of acid to a system with an aqueous stationary and organic mobile phase will decrease the relative migration Rp value) of an alkaloid. A base such as ammonia will increase the migration of an alkaloid in a comparable system. 

Ottiger, C. Wunderli-Allenspach, H., Partition behavior of acids and bases in a phosphatidylcholine liposome-buffer equilibrium dialysis system, Eur. J. Pharm. Sci. 5, 223-231 (1997). 

Fig. 37), suggesting that desolvation of the polar bonds in the molecule is a major determinant of permeability. Consistent with this, good correlations were found between the permeabilities of these peptides and their partition coefficients between heptane-ethylene glycol (r2 = 0.87) or the differences in partition coefficients between n-octanol-buffer and isooctane-buffer (r2 = 0.82) both these buffer systems provide experimental estimates of hydrogen-bonding potential. These results are qualitatively identical with those described earlier for the permeability of these peptides across Caco-2 cell monolayers. 

Monkiedje et al. [10] investigated the fate of niclosamide in aquatic system both under laboratory and field conditions. The octanol/watcr partition coefficient (Kaw) of niclosamide was 5.880 x 10 4. Adsorption isotherm studies indicated that the Freundlich parameters (K, n) for niclosamide were 0.02 and 4.93, respectively, for powder activated carbon (PAC), and 9.85 x 10 5 and 2.81, respectively, for silt loam soil. The adsorption coefficient (Aoc) for the drug was 0.02 for PAC, and 4.34 x 10-3 for the same soil. Hydrolysis of niclosamide occurred in distilled water buffer at pH above 7. No photolysis of the drug was observed in water after exposure to long-wave UV light for 4 h. Similarly, neither chemically volatilized from water following 5 h of sample aeration. Under field conditions, niclosamide persisted in ponds for over 14 days. The half-life of niclosamide was 3.40 days. 

Martinez, F. and Gomez, A. (2002). Thermodynamics of partitioning of some sulfonamides in 1-octanol-buffer and liposome systems, J. Phys. Org. Chem., 15, 874-880. 

Lipophilicity in particular, as reflected in partition coefficients between aqueous and non-aqueous media most commonly water (or aqueous buffer) and Z-octanol,has received much attention [105,141,152,153,176,199,232,233]. Logic )W for the octanol-water system has been shown to be approximately additive and constitutive, and hence, schemes for its a priori calculation from molecular structure have been devised using either substituent tt values or substructural fragment constants [289, 299]. The approximate nature of any partition coefficient has been frequently emphasized and, indeed, some of the structural features that cause unreliability have been identified and accommodated. Other complications such as steric effects, conformational effects, and substitution at the active positions of hetero-aromatic rings have been observed but cannot as yet be accounted for completely and systematically. Theoretical statistical and topological methods to approach some of these problems have been reported [116-119,175,289,300]. The observations of linear relationships among partition coefficients between water and various organic solvents have been extended and qualified to include other dose-response relationships [120-122,160,161,299-302]. 

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