Partition coefficient ionic interactions

Solute Flux Solute partitioning between the upstream polarization layer and the solvent-filled membrane pores can be modeled by considering a spherical solute and a cylindrical pore. The equilibrium partition coefficient 0 (pore/bulk concentration ratio) for steric exclusion (no long-range ionic or other interactions) can be written as... 

Due to their better biomimetic properties, phospholipids have been proposed as an alternative to 1-octanol for lipophiiicity studies. The use of immobilized artificial membranes (lAM) in lipophiiicity determination was recently reviewed and we thus only briefly summarize the main conclusions [108]. lAM phases are silica-based columns with phospholipids bounded covalently. lAM are based on phosphatidylcholine (PC) linked to a silica propylamine surface. Most lipophiiicity studies with lAM were carried out using an aqueous mobile phase with pH values from 7.0 to 7.4 (log D measurements). Therefore, tested compounds were neutral, totally or partially ionized in these conditions. It was shown that the lipophiiicity parameters obtained on I AM stationary phases and the partition coefficients in 1-octanol/water system were governed by different balance of intermolecular interactions [109]. Therefore the relationships between log kiAM and log Poet varied with the class of compounds studied [110]. However, it was shown that, for neutral compounds with log Poet > 1, a correspondence existed between the two parameters when double-chain lAM phases (i.e., lAM.PC.MG and IAM.PC.DD2) were used [111]. In contrast, in the case of ionized compounds, retention on lAM columns and partitioning in 1 -octanol / water system were significantly different due to ionic interactions expressed in lAM retention but not in 1-octanol/water system and due to acidic and basic compounds behaving differently in these two systems. 

The stationary phases play an important part in Liquid Chromatography using micellar mobile phases. They interact with both the surfactant and with solutes. To study the interactions with surfactants, adsorption isotherms were determined with two ionic surfactants on five stationary phases an unbonded silica and four monomeric bonded ones. It seems that the surfactant adsorption closely approaches the bonded monolayer (4.5 pmol/m2) whatever the bonded stationary phase-polarity or that of the surfactant. The interaction of the stationary phase and solutes of various polarity has been studied by using the K values of the Armstrong model. The KgW value is the partition coefficient of a solute between the... 

This change Is caused by the interaction between the aromatic hydrocarbon and the polar group of the surfactant. This phenomenon has been clarified for normal micelles of ionic surfactants (24) for Inverse micelles material Is available only for nonlonlc surfactants. Christenson and collaborators (25-27) made an extensive study using NMR and calorimetry. The results of both studies agree showing a partition coefficient for benzene between the surfactant polar group and Its hydrocarbon chain of approximately 3. 

The SEC partition coefficient [6] (.K sec) was measured on a Superose 6 column for three sets of well-characterized symmetrical solutes the compact, densely branched nonionic polysaccharide, Ficoll the flexible chain nonionic polysaccharide, pullulan and compact, anionic synthetic polymers, carboxylated starburst dendrimers. All three solutes display a congruent dependence of K ec on solute radius, R. In accord with a simple geometric model for SEC, all of these data conform to the same linear plot of i sEc versus R. This plot reveals the behavior of noninteracting spheres on this column. The mobile phase for the first two solutes was 0.2M NaH2P04-Na2HP04, pH 7.0. In order to ensure the suppression of electrostatic repulsive interactions between the dendrimer and the packing, the ionic strength was increased to 0.30M for that solute. 

Supports for SEC of proteins are designed to be neutral and very hydrophilic to avoid disruption of protein structure and interaction of the solutes with the support by ionic or hydrophobic mechanisms. The base matrix can be either silica or polymer efforts are made to totally mask its properties with a carbohydratelike stationary phase. The pore structure is critical to successful SEC. Not only must the total pore volume (F,) be adequate for separation, the pore diameter must be consistent and nearly homogeneous for attainment of maximum resolution between molecules with relatively small differences in molecular size (radius of gyration or molecular weight). A twofold difference in size is usually required for separation by SEC. Pore homogeneity can be assessed from the slope of the calibration curve of the logarithm of the molecular weight versus the retention time or the partition coefficient (Kd) = (F - Fq)/F , where F is... 

Reverse miceUes have been applied in the separation of amino acids and proteins. The separation is based on the balance between electrostatic forces and hydrophobic interactions [120]. The pH value is a crucial parameter determining this balance. If reversed miceUes are applied in LMs, then the underlying interactions are determined by interfacial partition coefficients of the amino acids/proteins separated, that is, hydrophobicity of the compounds separated, ionic strength of the feed and stripping solutions, the chemical nature of the electrolytes present, and the intertacial curvature of the amphiphilic film [121]. Changing the above-mentioned conditions, the overaU charge of the reverse miceUe can be altered, and so the separation conditions can be manipulated [122]. 

Naggar et al. [147] investigated the solubilization of the zwitterionic antibiotics tetracycline and oxytetracycline by polysorbate 20 and 80 at pH 5 and assumed that the interactions were due to some form of complexation which seems unlikely. A wider range of tetracyclines and their interactions with a non-ionic, anionic and cationic surfactant were studied by Ikeda et al [148] over a pH range of 2.1 to 5.6. It is unlikely that surfactant solutions of tetracycline are required, but the results are relevant in discussing tetracycline-surfactant interactions that could influence the activity of the antibiotic. Apparent partition coefficients of four tetracyclines in a polyoxyethylene (Brij 35) are shown in Table 6.14 obtained from dynamic dialysis measurements the interactions with ionic surfactants (Table 6.15) appear more complex. 

The practicality of both approaches is open to question in view of the potential toxicity of the co-solvents and electrolytes. The order of interaction of prostaglandins with the non-ionic surfactant C12E23 was PGEi > PGE2 > PGF2 [220] which corresponds to the order of their cyclohexane/water partition coefficients. 

The first microscopic theory for ionic friction in polar solvents was proposed by Wolynes, " in which the ion—solvent interactions were partitioned into short-range repulsive and long-range attractive components. The friction coefficient in the Wolynes model is simplified into the following two terms ... 

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