Membrane retention rate

Yamashita et al. [82] also studied the effect of BSA on transport properties in Caco-2 assays. They observed that the permeability of highly lipophilic molecules could be rate limited by the process of desorption off the cell surface into the receiving solution, due to high membrane retention and very low water solubility. They recommended using serum proteins in the acceptor compartment when lipophilic molecules are assayed (which is a common circumstance in discovery settings). 

While a number of dendritic catalysts have been described, catalyst recyclization in most cases is an unsolved problem. Diaminopropyl-type dendrimers bearing Pd-phosphine complexes have been retained by ultra- or nanofiltration membranes, and the constructs have been used as catalysts for allylic substitution in a continuously operating chemzyme membrane reactor (CMR) (Brinkmann, 1999). Retention rates were found to be higher than 99.9%, resulting in a sixfold increase in the total turnover number (TTN) for the Pd catalyst. 

Figure 6.3 Ultrafiltration membranes are rated on the basis of nominal molecular weight cut-off, but the shape of the molecule to be retained has a major effect on retentivity. Linear molecules pass through a membrane, whereas globular molecules of the same molecular weight may be retained. The table shows typical results obtained with globular protein molecules and linear polydextran for the same polysulfone membrane [8]...

Figure 6.2 Effect of membrane pore diameter on permeate flux of skimmed milk and protein retention rate [Adapted from Attia et al., 1991b]...

Also shown in Figure 6.2 is the protein retention rate for the above two membranes. As expected, the smaller pore (0.2 pm) membrane shows a higher retention capacity than the larger pore membrane. 

Parametric studies of the effects of TMP, temperature and crossflow velocity on the permeate flux and protein retention rate have been conducted using 0.8 pm alumina membranes at a pH of 4.4. The maximum steady state flux is observed at a TMP of 3 bars. As expected, a higher crossflow velocity increases the steady state permeate flux, as illustrated in Figure 6.3 under the condition of 50 C, TMP of 5 bars and pH of 4.40 [Attia et al., 1991b]. The protein retention rate also improves with the inciease in the crossflow velocity. The permeate flux reaches 175 L/hr-m, accompanied by a protein retention rate of 97.5% when the crossflow velocity is at 3.8 m/s. This improvement in the flux corresponds to a reduction in the thickness of the external fouling layer. 

Shown in Figure 6.4 are the effects of the process stream temperature (35 to 50X) on the permeate flux at a crossflow velocity of 3 m/s and a TMP of 5 bars for a feed pH of 4.40 [Attia et al., 1991b]. The steady state flux increases from 20 to 130 L7hr-m as the temperature increases from 35 to 50 C. Correspondingly, the protein retention rate decreases from 98.6 to 95.6 %. This increase in the flux and decrease in the protein retention may be explained by the reduced viscosity, increased diffusion coefficients and increased solubility of the constituents in the membrane and solution as a result of raising the temperature. 

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). 

The alignment of polymer chains in shear flow through the spinneret may explain why hollow fibers generally have lower permeabilities than flat-sheet membranes with the same retention rating. 

Table 4.1. Pure water permeability of NE-1812 and the retention rate of monosaccharide and sodium chloride by NE-1812 membrane under different operating pressures [12].

By increasing the pressure, membrane pores become smaller because of the compaction effect, resulting in the increase of retention rate. Sarney etal. [13] and Aydogan et al. [14] also reported that the effect of pressure on the retention rate of sugar component was not significant. Bo Cui found that the pressure can be maintained at about 0.5 MPa, as long as the pressure is in the range of membrane tolerance [12[. 

Coefficient of retention (R) It measures the retention rate (%) of a determined solute by the membrane during filtration. Retention (R) can be defined by the following equation R = 100(1 - (CPICR)), where CP is the solute concentration in permeate, and CR is the concentration of the solute in the retentate. 

Physicochemical profiling has gained considerable importance in the last years as most companies realized that inappropriate physicochemical properties could lead to compound withdrawal later in development. The basic physicochemical parameters of interest for drugability prediction are solubility and permeability, the two components of the Biopharmaceutical Classification Scheme. However, these two fundamental parameters are in turn influenced by other physicochemical parameters worth considering, particularly in the lead optimization phase. For example, permeability is influeneed by lipophilicity (induces membrane retention) and pH (ioniz-able compounds), solubility is influenced by pH (ionizable compounds), and dissolution rate is linked to particle size, polymorphism, and wettability. 

The parallel artificial membrane assay (PAMPA) was first described by Kansy et al. The assay consists in measuring the rate of transfer of compounds from a donor to an acceptor compartment that is separated by a porous filter coated with a mixture of phospholipids in dodecane. The exact nature of the membrane is not known and is unlikely to be a well-defined phospholipid bilayer. Interesting correlations between kinetics of transfer and fraction absorbed in humans were, however, observed. The technology was further improved by taking into account membrane retention and the derivation of differential equations to convert flux ratios into permeability values.The system is now available in the form of a commercial instrument (PSR evolution series, pION, Inc www.pion-inc.com). Improved correlations with GIT permeability were oteerved using different lipid compositions designed to closer mimic the nature of native membranes. ... 

Membrane retention Advantageous for allosteric enzymes. The enzyme and the derivatized coenzyme are retained by the membrane. Drawback problems at high shear rate. 38... 

The membrane separation process was initially conducted in degumming vegetable oil and then was adapted for the recovery of carotenoids. Dense polymeric membranes are employed in this system and are very effective in the separatirm of xanthophylls, phospholipids, and chlorophyll, with retention of 80-100 %, producing an oil rich in carotenes [72,73]. This process, however, requires an additional step of hydrolysis or transesterification. Chiu, Coutinho, and Gruigalves examined the membrane technology as an alternative to concentrate carotenoids from crude palm oil in detriment of ethyl esters. A flat sheet polymeric membrane constituted by polyethersulfone was used and obtained a retention rate of 78.5 % [74]. Damoko and Cheryan obtained similar results using nanofiltration with 2.76 MPa and 40 °C in red palm methyl esters [75]. Whereas Tsui and Cheryan combined ultraiiltration with nanofiltration to separate zein and xanthophylls from ethanolic com extract [76]. 

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