Solute Retention Properties

Also included are sections on how to analyze mechanisms that affect flux feature models for prediction of micro- and ultrafiltration flux that help you minimize flux decline. Descriptions of cross-flow membrane filtration and common operating configurations clarify tf e influence of important operating parameters on system performance. Parameters irdlucnc irxj solute retention properties during ultrafiltration arc identified and discussed or treated in detail. 

Membranes for Reverse Osmosis. The first commercially successful membrane was the anisotropic or asymmetric structure invented by Loeb and Sourirajan (1960 cited by Sourirajan, 1970). It is made of cellulose acetate and consists of a dense layer 0.2-0.5 jjim diameter. The thin film has the desired solute retention property while offering little resistance to flow, and the porous substructure offers little resistance to flow but provides support for the skin. The characteristics of available membranes for reverse osmosis and ultrafiltration are listed in Tables 19.2 through 19.4. 

Different membrane materials with similar or identical MWCO value may show different solute retention properties under otherwise similar operating conditions. If adsorption effects are negligible, such a result can be attributed primarily to the differences in their pore size distributions. This is illustrated in Fig. 3. It can be seen that, although the two membranes are rated by the same MWCO value, their retention characteristics are distinctly different (sharp versus diffuse). 

Membrane Pore Diameter or Molecular Weight Cutoff. The value of membrane pore diameter will have a major influence on the permeation and separation characteristics for most process filtration applications. The intrinsic membrane permeability is related to the pore diameter for many microfiltration membranes whereas, for ultrafiltration membranes, it is typically indicative of the solute retention properties. Tables 8 and 9 provide typical permeability and retention data for many common MF and UF membranes, respectively.f t ... 

It is not within the scope of this chapter to provide a comprehensive discussion of silica gel chemistry. An excellent treatise is available (77). The parameters that most significantly affect bonding chemistries and solute retention properties are surface area, pore volume, pore diameter, trace metal impurities, and thermal pretreatments. Both Sander and Wise (90) and Sands et al. (91) have studied the effect of pore diameter and surface treatment of the silica on bonding reactions. Boudreau and Cooper (92) have studied the effects of thermal pretreatments at 180, 400, and 840°C on the subsequent chemical modification of silica gel, and showed that thermal pretreatment at temperatures >200°C can produce more homogeneous distribution of active silanols which are available for subsequent derivatization. 

Concentration of Chloroform in n-Heptane %w/v In contrast, the interactions with the stationary phase are becoming weaker as the surface becomes covered with chloroform. Thus retention is reduced by both the increased interactions in the mobile phase and reduced interaction with the stationary phase. When the concentration of chloroform in the solvent mixture is in excess of 50%, then the interactive properties of the stationary phase no longer change as the surface is now covered with a mono-layer of chloroform. However, solute retention will continue to decrease due to the increased interactions of the solute with the higher concentrations of chloroform in the mobile phase. It is clear that even with this simple example the dependence of retention on solvent composition is quite complex. 

As in isocratic mode, the estimate of log P is indirect and based on the construction of a linear retention model between a retention property characteristic of the solute (logkw) and a training set with known logP ci values. To assess the most performing procedures, the three hydrophobicity indexes (( )o, CHI and logkw) were compared on the basis of the solvation equation [41]. These parameters were significantly inter-related with each other, but not identical. Each parameter was related to log P with values between 0.76 and 0.88 for the 55 tested compounds fitting quality associated with the compound nature. 

The retention properties of ultrafiltration membranes are expressed as Molecular Weight Cutoff (MWCO). This value refers to the approximate molecular weight (MW) of a dilute globular solute (i.e., a typical protein) which is 90% retained by the membrane. However, a molecule s shape can have a direct effect on its retention by a membrane. For example, linear molecules like DNA may find their way through pores that will retain a globular species of the same molecular weight. 

The influence of the bonded organic moiety on solute retention has not yet been elucidated and only a very small number of papers discuss the properties and use of such phases so far. The numerous advantages of chemically bonded phases make the application of polar chemically bonded phases with nonpolar eluents quite attractive even if the standardization of these phases may pose problems 106) similar to those encountered in the standardization of aidsorbents as well as of polymeric liquid phases in gas chromatography. A detailed discussion of the properties and chromatographic use of bonded stationary phases is given by Melander and Horvath (this volume). 

The physical theories for electric charges have been incorporated in colloid and surface chemistry for many years. In the treatment presented here, these theories have been selected, adapted, and applied to describe the retention of ionic solutes. The approximations made in these models are well known and have limitations. Here, they are chosen from four requirements they should have a meaningful physico-chemical interpretation, be easy to use, be easy to understand, and give practical and useful results. This implies that the presented models are useful starting points for describing and understanding the retention properties for the type of systems that are discussed here. When the experimental results deviate from the model, it may be possible to extend it within its framework. In other situations, empirically based functions may complement the model, or it may be necessary to resort to more sophisticated models. 

The factors that control separation and dispersion are quite different. The relative separation of two solutes is solely dependent on the nature and magnitude of the Interactions between each solute and the two phases. Thus, the relative movement of each solute band would appear to be Independent of column dimensions or particle geometry and be determined only by the choice of the stationary phase and the mobile phase. However, there is a caveat to this statement. It assumes that any exclusion properties of the stationary phase are not included in the term particle geometry. The pore size of the packing material can control retention directly and exclusively, as in exclusion chromatography or, indirectly, by controlling the access of the solute to the stationary phase in normal and reverse phase chromatography. As all stationary phases based on silica gel exhibit some exclusion properties, the ideal situation where the selective retention of two solutes is solely controlled by phase interactions is rarely met in practice. If the molecular size of the solutes differ, then the exclusion properties of the silica gel will always play some part in solute retention. 

The thermodynamic dead volume includes those static fractions of the mobile phase that have the same composition as the moving phase, and thus do not contribute to solute retention by differential interaction in a similar manner to those with the stationary phase. It is seen that, in contrast to the kinetic dead volume, which by definition can contain no static mobile phase, and as a consequence is independent of the solute chromatographed, the thermodynamic dead volume will vary from solute to solute depending on the size of the solute molecule (i.e. is dependent on both ( i )and (n). Moreover, the amount of the stationary phase accessible to the solute will also vary with the size of the molecule (i.e. is dependent on (%)). It follows, that for a given stationary phase, it is not possible to compare the retentive properties of one solute with those of another in thermodynamic terms, unless ( ), (n) and (fc) are known accurately for each solute. This is particularly important if the two solutes differ significantly in molecular volume. The experimental determination of ( ), (n) and( ) would be extremely difficult, if not impossible In practice, as it would be necessary to carry out a separate series of exclusion measurements for each solute which, at best, would be lengthy and tedious. 

In previous chapters, liquid chromatography column theory has been developed to explain solute retention, band dispersion, column properties and optimum column design for columns that are to be used for purely analytical purposes. The theories considered so far, have assumed that solute concentrations approach (for all practical purposes) infinite dilution, and, as a consequence, all isotherms are linear. In the specific design of the optimum preparative column for a particular preparative separation, initially, the same assumptions will be made. 

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