Polymers, transport across membrane

Reverse osmosis membranes are characterized by an MWCO of -100 Da, and the process involves transmembrane pressures (TMP) of 10-50 bar (1000-5000 kPa), which are 5-10 times higher than those used in UF [11,36]. Unlike UF, the separation by RO is achieved not by the size of the solute but due to a pressure-driven solution-diffusion process [36]. Like UF membranes, RO membranes are uniquely stmctured films from synthetic organic polymers and consist of an ultrathin skin layer superimposed on a coarsely porous matrix [3]. The skin layer of the RO membrane is nonporous, which may be treated as a water-swollen gel, and water is transported across membrane by dissolving in this gel and diffusing to the low-pressure side... 

Note that in the component mass balance the kinetic rate laws relating reaction rate to species concentrations become important and must be specified. As with the total mass balance, the specific form of each term will vary from one mass transfer problem to the next. A complete description of the behavior of a system with n components includes a total mass balance and n - 1 component mass balances, since the total mass balance is the sum of the individual component mass balances. The solution of this set of equations provides relationships between the dependent variables (usually masses or concentrations) and the independent variables (usually time and/or spatial position) in the particular problem. Further manipulation of the results may also be necessary, since the natural dependent variable in the problem is not always of the greatest interest. For example, in describing drug diffusion in polymer membranes, the concentration of the drug within the membrane is the natural dependent variable, while the cumulative mass transported across the membrane is often of greater interest and can be derived from the concentration. 

The monomers of the polymer must be provided externally and transported across the membrane boundary to support the replication process. Other small molecules or ions needed for biosynthetic reactions must be delivered from the environment... 

I. Rubinstein, Effects of deviation from local electroneutrality upon electrodiffusional ionic transport across a cation-selective membrane, Reactive Polymers, 2 (1984), pp. 117-131. 

Two types of solid ionic conductors are of special interest—those in which metallic cations such as lithium ions can be transported across the polymer membrane, and others in which protons can move from one side of the membrane to the other. The first... 

Kozlowski, C.A., Kozlowska, J., Pellowski, W. and Walkowiak, W. (2006) Separation of cobalt-60, strontium-90, and cesium-137 radioisotopes by competitive transport across polymer inclusion membranes with organophosphorous adds. Desalination, 198, 141. 

The solid polymer electrolyte (SPE) fuel cell makes use of the high stability and the cation selectivity of Nafion, a Teflon-like material that has been modified by the incorporation of sulfonic groups. The membrane is coaled with a porous catalyst on both sides. Hydrogen is oxidized on one side of the membrane and the formed in the process is transported across the membrane to the other side, where it interacts with (OH) ions formed by the reduction of oxygen, to form water. This water is removed from the cell by capillary action with the use of a... 

In these devices polymer materials containing specific ingredients constitute the backbone of the film covering the electrochemical transducer. Here we deal with a liquid membrane, because the organic solvent provides the medium in which the ions permeate across the membrane. The polymer membrane ion-selective electrodes (ISE) and their ion transport across the membrane function similarly as the ion transport across the membranes of living cells (Figure 8.29). We follow the presentation given by Widmer (1993). 

Chisty MNA, Bellantone RA, Taft DR, Plakogiannis FM. In vitro evaluation of the release of albuterol sulfate from polymer gels effect of fatty acids on drug transport across biological membranes. Drug Dev Ind Pharm 2002 28(10) 1221—1229. 

Fig. 3.1 Schematic illustration of a targeted drug-delivery principle that involves accumulation of polymer-covered liposomal drug carriers in porous diseased tissue where secretory sPLAj subsequently acts as a local trigger for liposomal drug release and drug transport across the target cell membrane.

SILP systems have proven to be interesting not only for catalysis but also in separation technologies [128]. In particular, the use of supported ionic liquids can facilitate selective transport of substrates across membranes. Supported liquid membranes (SLMs) have the advantage of liquid phase diffusivities, which are higher than those observed in polymers and grant proportionally higher permeabilities. The use of a supported ionic liquid, due to their stability and negligible vapor pressure, allow us to overcome the lack of stability caused by volatilization of the transport liquid. SLMs have been applied, for example, in the selective separation of aromatic hydrocarbons [129] and CO2 separation [130, 131]. 

Brown PR, Hallman JL, Whaley LW, Desai DH, Pugia MJ, Bartsch RA, Competitive, proton-coupled, alkah metal cation transport across polymer-supported hquid membranes containing sym-(decyl)-dibenzo-16-crown-5-oxyacetic acid Variation of the alkyl 2-mtrophenyl ether membrane solvent. J. Membr. Sci. 1991 56, 195-206. Michaels AS, Membranes, membrane processes, and their apphcations Needs, unsolved problems, and challenges of the 1990s. Desahnation, 1990 77, 5-34. 

Summary. We have shown that ion transport in "Nafion" per-fluorinated membrane is controlled by percolation, which means that the connectivity of ion clusters is critical. This basically reflects the heterogeneous nature of a wet membrane. Although transport across a membrane is usually perceived as a one-dimensional process, our analysis suggests that it is distinctly three-dimensional in "Nafion". (Compare the experimental values of c and n with those listed in Table 7.) This is not totally unexpected since ion clusters are typically 5.0 nm, whereas a membrane is normally several mils thick. We have also uncovered an ionic insulator-to-conductor transition at 10 volume % of electrolyte uptake. Similar transitions are expected in other ion-containing polymers, and the Cluster-Network model may find useful application to ion transport in other ion containing polymers. Finally, our transport and current efficiency data are consistent with the Cluster-Network model, but not the conventional Donnan equilibrium. 

The nucleotide sugar compounds are hydrophilic and it is necessary to transfer them to the sites of the synthases within the endomembrane system. Their transport across the membrane is an obvious control point. Not only is the rate of polysaccharide synthesis controlled by this mechanism but in addition the qualitative nature of the polymer found in the membrane compartment may be determined. Thus although the synthases may be present in a particular part of the membrane system e.g. endoplasmic reticulum, the assembly of the polysaccharide does not occur at this level if the nucleotide sugar cannot enter the lumen of the compartment (13). The main assembly of the polysaccharides occurs at the Golgi apparatus. Figure 1. 

Gibbs-Donnan equilibrium determines the concentration difference across simple membranes made of polymers, porous ceramic media, and other ultrafiltration devices. However, the difference of ion concentrations across the membranes of living cells and nerves is more complicated because of the existence of ion pumps as a result of carrier-mediated or facilitated diffusion, so that the concentrations of some ions are not in thermodynamic equilibrium. For example, there is a much higher sodium con- centration outside cells than there is inside, while the reverse is true for potassium ions. This occurs because there is a carrier (probably a lipoprotein) that binds with a sodium ion inside the cell, transports the ion across membrane, and then releases it into the fluid outside the cell. The carrier is then transformed and binds with a potassium ion, which is then transported into the cell. This mechanism is discussed in courses... 

C.Y. Zhu and R.M. Izatt, Macrocyclic-mediated separation of Eu2+ from trivalent lanthanide cations in a modified thin-sheet-supported liquid membrane system, J. Membr. Sci., 1990, 50, 319 P.R. Brown, J.L. Hallman, L.W. Whaley, D.H. Desai, M.J. Pugia and R.A. Bartsch, Competitive, proton-coupled, alkali metal cation transport across polymer-supported liquid membranes containing s>yn(decyl-dibenzo-16-crown-5-oxyacetic acid) Variation of the alkyl 2-nitrophenyl ether membrane, ibid., 1991, 56, 195. 

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