Solute partition

The most important class of separation techniques is based on the selective partitioning of the analyte or interferent between two immiscible phases. When a phase containing a solute, S, is brought into contact with a second phase, the solute partitions itself between the two phases. 

A separation in which solutes partition between a mobile and stationary phase. 

J. A. Marques, K. A. Dill. Solute partitioning into chain molecule interphases monolayers, bilayer membranes and micelles. J Chem Phys 55 434—444, 1986. 

Solid phase micro extraction (SPME) is a techniques in which a silica fiber coated with a thin film of polymer is brought into contact with an aqueous matrix where the organics in solution partition onto the fiber. The fiber is subsequently placed into the injector of a GC where the heat causes the release of analyte onto the column. This has been applied to endosulfan (a- and (3-) and endosulfan sulfate in water with limits of detection of less than 0.3 pg/L reported (Magdic and Pawliszyn 1996). 

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... 

Given that = k o/ ow is Ihe solute partition coefficient, then the expression for the total flux for this particular case can be written as... 

The solubility-diffusion theory assumes that solute partitioning from water into and diffusion through the membrane lipid region resembles that which would occur within a homogeneous bulk solvent. Thus, the permeability coefficient, P, can be expressed as... 

Seiler [250] proposed a way of estimating the extent of hydrogen bonding in solute partitioning between water and a lipid phase by measuring the so-called A log P parameter. The latter parameter is usually defined as the difference between the partition coefficient of a solute measured in the octanol-water system and that measured in an inert alkane-water suspension AlogP = log Kp oet — log Kp aik. 

The cosolvent may increase the aqueous solubility of the sparingly soluble molecules, which would lower the membrane-donor solution partition coefficient. According to Eq. (2.3), Pe will decrease. Since %R is related to lipophilicity (Section 7.7.2), the retention is also expected to decrease. 

Ong, S., Pidgeon, C., Thermodynamics of solute partitioning into immobilized artificial membranes, Anal. Chem. 67, 2119-2128 (1995). 

Figure 12 Theoretical predictions of solute partitioning in a gel with a fiber radius of 0.4 nm for solutes of different hydrodynamic radii, based on the size exclusion theory of Eq. (30). (From Ref. 174.)...

Obviously, the availability of large enough cavities decreases strongly with the size of the solute. We have already discussed another explanation of the relationship between permeation rate and solute size, based completely on solubility, i.e. the size-selectivity of solute partitioning in the ordered lipid-... 

Mitragotri, S., Johnson, M. E., Blankschtein, D. and Langer, R. (1999). An analysis of size selectivity of solute partitioning, diffusion, and permeation across lipid bilayers, Biophys. J., 77, 1268-1283. 

However, the temperature also affects the solute partitioning between the mobile and stationary phase and therefore also the chromatographic retention. The distribution of the solute between the mobile and stationary phases is a function of (i) its solubility in the liquid phase and (ii) its adsorption on the solid phase. This is characterized by the distribution ratio K defined as the ratio of the concentration of the solute in the stationary phase to its concentration in the mobile phase. The higher this ratio, the longer the retention. According to the Van t Hoff equation... 

Surfactant solutions critical micelle concentration distribution of reactants among particles surfactant aggregation numbers interface properties and polarity dynamics of surfactant solutions partition coefficients phase transitions influence of additives... 

The impact of salt concentration on the formation of micelles has been reported and is in apparent accord with the interfacial tension model discussed in Sect. 4.1, where the CMC is lowered by the addition of simple electrolytes [ 19,65, 280,282]. The existence of a micellar phase in solution is important not only insofar as it describes the behavior of amphipathic organic chemicals in solution, but the existence of a nonpolar pseudophase can enhance the solubility of other hydrophobic chemicals in solution as they partition into the hydrophobic interior of the micelle. A general expression for the solubility enhancement of a solute by surfactants has been given by Kile and Chiou [253] in terms of the concentrations of monomers and micelles and the corresponding solute partition coefficients, giving... 

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