Plasma membrane osmosis

Osmosis, water movement across a semipermeable membrane driven by differences in osmotic pressure, is an important factor in the life of most cells. Plasma membranes are more permeable to water than to most other small molecules, ions, and macromolecules. This permeability is due partly to simple diffusion of water through the lipid bilayer and partly to protein channels (aquaporins see Fig. 11-XX) in the membrane that selectively permit the passage of water. Solutions of equal osmolarity are said to be isotonic. Surrounded by an isotonic solution, a cell neither gains nor loses water (Fig. 2-13). In a hypertonic solution, one with higher... 

The most extensive studies of plasma-polymerized membranes were performed in the 1970s and early 1980s by Yasuda, who tried to develop high-performance reverse osmosis membranes by depositing plasma films onto microporous poly-sulfone films [60,61]. More recently other workers have studied the gas permeability of plasma-polymerized films. For example, Stancell and Spencer [62] were able to obtain a gas separation plasma membrane with a hydrogen/methane selectivity of almost 300, and Kawakami et al. [63] have reported plasma membranes... 

Water tends to move across a semipermeable membrane from a solution of low solute concentration to one of high concentration, a process termed osmosis, or osmotic flow. In other words, since solutions with a high concentration of dissolved solute have a lower concentration of water, water will spontaneously move from a solution of high water concentration to one of lower. In effect, osmosis Is equivalent to diffusion of water. Osmotic pressure is defined as the hydrostatic pressure required to stop the net flow of water across a membrane separating solutions of different compositions (Figure 7-24). In this context, the membrane may be a layer of cells or a plasma membrane that Is permeable to water but not to the solutes. The osmotic pressure Is directly proportional to the difference in the concentration of the total number of solute molecules on each side of the membrane. For example, a 0.5 M NaCl solution is actually 0.5 M Na ions and 0.5 M Cl ions and has the same osmotic pressure as a 1 M solution of glucose or sucrose. 

The plasma membranes of cells are semipermeable in other words they are permeable to water and a select group of small molecules, but are impermeable to large molecules and many ions. If the total concentrations of solutes on either side of a semipermeable membrane are different, water diffuses from the region of low solute concentration to the region of high solute concentration imtil the solute concentrations are the same on both sides. This process is called osmosis, and the difference in water pressiue across the cell membrane is called the osmotic pressure. 

Introduction of a water-soluble ionic substance into the vascular system results in an increase in the number of particles in the bloodstream as the contrast substance dissolves. The body possesses several internal regulation systems and, when perturbed by an injection, attempts to restore the concentrations of substances in the blood to their normal or preinjection levels. To re-equilibrate the system, water from the cells of surrounding body tissue moves into the blood plasma through capillary membranes. This transfer of water is an example of osmosis, the diffusion of a solvent (water) through a semipermeable membrane (the blood vessels) into a more concentrated solution (the blood) to equalize the concentrations on both sides of the membrane. To accommodate the increase in... 

Method D in Table 1 represents a case where dry support films were always used because of the need to employ a vacuum and because of the very nature of plasma deposition processes. Yasuda (12) showed that a wide variety of gas phase reactants could be used in this technique. Not only conventional vinyl monomers were used but also any organic compounds with adequate vapor pressure. Further, copolymers could be prepared by introduction of a second reactant such as nitrogen. Wydeven and coworkers (13,14) showed the utility of this method in preparing reverse osmosis membranes from an allylamine plasma. 

Yasuda, H. "Composite Reverse Osmosis Membranes Prepared by Plasma Polymerization," in "Reverse Osmosis and Synthetic Membranes," Sourirajan, S., Ed., National Research Council, Canada, Ottawa, 1977, p.263. 

Filtration of small (nano) particles from solvent using a filter with extremely small pores (0.001-0.010 micron) finer than ultrafiltration, not as fine as reverse osmosis. Used for the removal of viruses from plasma protein products. See Yaroshchuk, A.E., Dielectric exclusion of ions from membranes, Adv. Colloid Interface Sci. 85,193-230,2000 Rossano, R., D Elia, A., and Riccio, R, One-step separation from lactose recovery and purification of major cheese-whey proteins by hydroxyapatite — a flexible... 

Wu, S., et al. (1997). Plasma modification of aromatic polyamide reverse osmosis composite membrane surface. J. Applied Polymer Science. 64, 10, 1923-1926. 

The deposition of a plasma polymer on an appropriate porous substrate to form a composite reverse osmosis membrane is a typical example of the second case. In both cases, however, the selection of the substrate membrane is the crucially important factor, particularly in the second case. In the application of nanofilm, the pore size of the substrate membrane must have the uniformity of pore size in nanometer scale, which is an extremely difficult requirement. 

The aspect of hole filling by plasma deposition can be demonstrated by the transport characteristics of LCVD-prepared membranes. First, the porosity as porous media calculated from the gas permeability dependence on the applied pressure can be correlated to the salt rejection of the composite membrane as shown in Figure 34.13. The effective porosity s/, where s is the porosity and q is the tortuosity factor, is measured in dry state and may not directly correlate to the porosity of the membranes in wet state. The effective porosity of LCVD-prepared membranes was measured before the reverse osmosis experiment. The decrease of porosity (as porous media) is clearly reflected in the increase in salt rejection in reverse osmosis. 

Figure 34.13 Reverse osmosis characteristics of composite membranes prepared by plasma polymerization of benzene/H20/N2 compared with those from acetylene/H20/ N2 represented by the solid line porous polysulfone film as the substrate, 3.5% NaCl at 1500 psi.

The membrane separates the blood from a dialyzing solution, or dialysate, that is similar to blood plasma in its concentration of needed substances (e.g., electrolytes and amino acids) but contains none of the waste products. Because the concentrations of undesirable substances are thus higher in the blood than in the dialysate, they flow preferentially out of the blood and are washed away. The concentrations of needed substances are the same on both sides of the membrane, so these substances are maintained at the proper concentrations in the blood. The small pore size of the membrane prevents passage of blood cells. However, Na and Cl ions and some small molecules do pass through the membrane. A patient with total kidney failure may require up to four hemodialysis sessions per week, at 3 to 4 hours per session. To help hold down the cost of such treatment, the dialysate solution is later purified by a combination of filtration, distillation, and reverse osmosis and is then reused. 

Studies of the plasma polymerization of tetrafluoroethylene in such a capacltively coupled system are described in another paper presented at this symposium [ 9]. The apparatus has been used to coat polysulfone hollow fibers with pyridine and acetylene + nitrogen plasma polymer to form a composite reverse osmosis desalination membrane. Salt rejections of 90-93% have been achieved at fluxes of 1.5-2.0 g.f.d with a fiber take up rate of 50-100 cm/min. 

The twenty chapters included in this volume can be conveniently divided into the following groups review plasma polymerization of hydrocarbons plasma polymerization of fluorocarbons plasma polymerization of organometallic systems plasma-initiated polymerization and applications of plasma polymerization. Though the emphasis of this Symposium is on the fundamental aspects of plasma polymerization, we should not lose sight of the fact that it is the potential applications of this technique that has stimulated the efforts in basic research. Potential applications for plasma-polymerized films include membranes for reverse osmosis, protective coatings for optical components, and insulating layers for semiconductors. 

A membrane designated "Solrox" made by Sumitomo Chemical Company is closely related to the above plasma polymerized composite membranes. A 1980 report by T. Sano described the Sumitomo process (31). A support film was cast from a polyacrylonitrile copolymer containing at least 40 mole percent acrylonitrile. The support film was dried and exposed to a helium or hydrogen plasma to form a tight cross-linked surface skin on the porous polyacrylonitrile support film. Data in a U.S. Patent issued in 1979 to Sano et al showed that the unmodified support film had a water flux of 87 gfd (145 L/ sq m/hr) at 142 psi (10 kg/sq cm). After the plasma treatment a reverse osmosis test using 0.55 percent NaCl at 710 psi (4895 kPa) showed 10.5 gfd (17.5 L/sq m/hr) flux at 98.3 percent salt rejection (32). This membrane appears to fall between a conventional asymmetric membrane and a composite membrane. If the surface skin is only cross-linked, one might call it a modified asymmetric membrane. However, if the surface skin is substantially modified chemically to make it distinct from the bulk of the membrane it could be considered as a composite type. 

Movements of water are due mainly to osmosis and filtration. In osmosis, water moves to the area of highest solute concentration. Thus, active movement of salts into an area creates a concentration gradient down which water flows passively. In filtration, hydrostatie pressure in arterial blood moves water and nonprotein solutes through specialized membranes to produce an almost protein-free filtrate This process occurs in formation of the renal glomerular filtrate. Filtration also accounts for movement of water from the vascular space into the interstitial compartment, which is opposed by the osmotic (oncotic) pressure of plasma proteins. 

There are several potential routes to the preparation of composite reverse osmosis membranes, whereby the ultrathin semipermeable film is formed or deposited on the microporous sublayer.1 2 The film can be formed elsewhere, then laminated to the microporous support, as was done in the earliest work on this membrane approach. Or it can be formed in place by plasma polymerization techniques. Alternatively, membrane polymer solution or polymer-forming reactants can be applied in a dipcoating process, then dried or cured in place. The most attractive approach from a commercial standpoint, however, has been the formation of the semipermeable membrane layer in situ by a classic "non-stirred" interfacial reaction method. Several examples of membranes made by this last approach have reached commercial status. 

Plasma polymerization has been explored in considerable detail as a potential route to composite reverse osmosis membranes. Yasuda has written an excellent review covering the fundamentals and progress of plasma polymerization through 1974.88 Advances since then have not been particularly promising from a commercial standpoint, with the exception of the Solrox membrane (see below). 

Wydeven and his coworkers examined the plasma polymerization of several amine monomers and obtained the best reverse osmosis membranes using allyl-amine.90/91 Reverse osmosis performances of 98 to 99% salt rejection and 4 to 8 gfd flux were achieved at test conditions of 1.0% sodium chloride, 600 psi, 20 C. ESCA spectra showed the nitrogen groups in the plasma-formed polymer to be nitrile or imine groups, but not amine groups. Oxygen was also present in the ESCA analysis. Elemental analysis showed a membrane stoichiometry of C3H 3 8N 0.9O0.1. 

Yasuda obtained membranes with good reverse osmosis properties by polymerizing gas plasma mixtures of acetylene/water/nitrogen and acetylene/wa-ter/carbon monoxide.92 9J Elemental analysis of the acetylene/water/nitrogen membrane was not given, but it was likely approximately equivalent to the allyl-amine-derived composition. Yasuda has observed membrane performance levels as high as 99% salt rejection and 38 gfd, tested on 3.5% sodium chloride at 1,500 psi. 

Vasuda, H "Composite Reverse Osmosis Membranes Prepared by Plasma... 

---