Adsorption on Membranes

Effects of fish protein adsorption on membrane flux... 

Immobihzation of lipases due to adsorption on membrane surfaces allows to control specificity and influence the direction of reversible enzymatic reations [147] and to realize processes using biphastic (water/organic) media. 

A wide range and a number of purification steps are required to make available hydrogen/synthesis gas having the desired purity that depends on use. Technology is available in many forms and combinations for specific hydrogen purification requirements. Methods include physical and chemical treatments (solvent scmbbing) low temperature (cryogenic) systems adsorption on soHds, such as active carbon, metal oxides, and molecular sieves, and various membrane systems. Composition of the raw gas and the amount of impurities that can be tolerated in the product determine the selection of the most suitable process. 

Second, most membrane materials adsorb proteins. Worse, the adsorption is membrane-material specific and is dependent on concentration, pH, ionic strength, temperature, and so on. Adsorption has two consequences it changes the membrane pore size because solutes are adsorbed near and in membrane pores and it removes protein from the permeate by adsorption in addition to that removed by sieving. Porter (op. cit., p. 160) gives an illustrative table for adsorption of Cytochrome C on materials used for UF membranes, with values ranging from 1 to 25 percent. Because of the adsorption effects, membranes are characterized only when clean. Fouling has a dramatic effect on membrane retention, as is explained in its own section below. 

The most important physical methods are physical and ionic adsorption on a water-insoluble matrix, inclusion and gel entrapment, and microencapsulation with a liquid or a solid membrane. The most important chemical methods include covalent attachment to a water-insoluble matrix, cross-hnking with the use of a multifunctional, low-molecular weight reagent, and co-cross-linking with other neutral substances, for example proteins. 

Membranes offer a format for interaction of an analyte with a stationary phase alternative to the familiar column. For certain kinds of separations, particularly preparative separations involving strong adsorption, the membrane format is extremely useful. A 5 x 4 mm hollow-fiber membrane layered with the protein bovine serum albumin was used for the chiral separation of the amino acid tryptophan, with a separation factor of up to 6.6.62 Diethey-laminoethyl-derivatized membrane disks were used for high-speed ion exchange separations of oligonucleotides.63 Sulfonated membranes were used for peptide separations, and reversed-phase separations of peptides, steroids, and aromatic hydrocarbons were accomplished on C18-derivatized membranes. 

Because membrane filtration is the only currently acceptable method of sterilizing protein pharmaceuticals, the adsorption and inactivation of proteins on membranes is of particular concern during formulation development. Pitt [56] examined nonspecific protein binding of polymeric microporous membranes typically used in sterilization by membrane filtration. Nitrocellulose and nylon membranes had extremely high protein adsorption, followed by polysulfone, cellulose diacetate, and hydrophilic polyvinylidene fluoride membranes. In a subsequent study by Truskey et al. [46], protein conformational changes after filtration were observed by CD spectroscopy, particularly with nylon and polysulfone membrane filters. The conformational changes were related to the tendency of the membrane to adsorb the protein, although the precise mechanism was unclear. 

Equation (6.4.4) is valid when the coverage of the electrolyte-membrane interface is small. At higher concentrations of transferred ion, the ion transfer is retarded by adsorption on the opposite interface, so that the dependence of G0 on c is characterized by a curve with a maximum, as has been demonstrated experimentally. 

Can the loss of useful material in the purge be avoided or reduced by additional separation on the purge The roles of refrigerated condensation, low-temperature distillation, absorption, adsorption and membranes in this respect have already been discussed. 

Distinguishing between adsorption on to the cell surface and the actual transfer across the cell membrane into the cell may be difficult, since both processes are very fast (a few seconds or less). For fish gills, this is further complicated by the need to confirm transcellular solute transport (or its absence) by measuring the appearance of solutes in the blood over seconds or a few minutes. At such short time intervals, apparent blood solute concentrations are not at equilibrium with those in the entire extracellular space, and will need correcting for plasma volume and circulation time in relation to the time taken to collect the blood sample [30]. Nonetheless, Handy and Eddy [30] developed a series of rapid solution dipping experiments to estimate radiolabelled Na+... 

In equation (34), n is the number of cells and Na is Avogadro s number, and Rt is the total carrier concentration (including both bound and free carriers). Solute depletion can be especially important in laboratory experiments, since large numbers of cells are generally employed at low solute concentrations that are typical of trace elements in natural waters. On the other hand, at high solute concentrations corresponding with carrier saturation, nonspecific adsorption to membrane components other than the carriers becomes important, and thus interpretation is much more difficult. 

The Ag2 S ISE has Nemstian response dE/d log a( = 0.0296 V in the sulphide concentration range 10" to 10" M and silver ions from 10 to 10 M if the solutions are prepared from pure salts, as a further concentration decrease is prevented by adsorption on the glass (see p. 76 and [87, 163]). After prolonged use, the limit of the Nemstian behaviour shifts to about 10" m [130] as a result of formation of mixed potentials on accumulation of metallic silver in the membrane surface. An analogous deterioration in the membrane function in the presence of iodine results from surface oxidation [23]. Cyanide interferes only at large concentrations the equilibrium constant of the reaction... 

Isotope effects of this kind are relevant for an understanding of the isotope composition of clay minerals and absorption of water on mineral surfaces. The tendency for clays and shales to act as semipermeable membranes is well known. This effect is also known as ultraliltration . Coplen and Hanshaw (1973) postulated that hydrogen isotope fractionations may occur during ultraliltration in such a way that the residual water is emiched in deuterium due to its preferential adsorption on the clay minerals and its lower diffusivity. 

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