Small diameter liquid membranes

The apparatus used for this study produced low transport rates for C02, as well as the previously discussed 02, with and without liquid membranes compared with developed oxygenators. The reason for this slow transport is the very large (approximately 0.4 cm) liquid membrane encapsulated bubbles contrasted with the small bubbles of developed oxygenator. A means is needed to produce small fluorocarbon liquid membranes in blood so that the rapid transport achieved in other liquid membrane applications using small diameter liquid membranes can be achieved for transferring gases to and from blood. 

Hollow Fiber-Capillary Hollow Fiber reFers to verv small diameter membranes. The most siiccessFiil one has an outer diameter oF onlv 93 jlrn and is used For reverse osmosis, (iapillarv membranes are larger-diarneter membranes used For liquid separations. The distinction betw een them has blurred to the point where there is a virtual... 

Membrane contactors can be made out of flat sheet membranes, which have some commercial applications. However, the most common commercial membrane contactors are made from small-diameter microporous hollow-fiber (or capillary) membranes with fine pores that run from the inner surface to the outer surface of the hollow-fiber wall. The contactor resembles a tube-in-shell configuration with inlet-outlet ports for the shell side and tube side. These kinds of membranes are typically made of hydrophobic materials such as polypropylene (PP), polyethylene (PE), polytetrafluoroethylene (PTFE), poly(tetrafluoroethylene-co-perfluorovinylether) (PEA), or polyvinylidene fluoride (PVDE). The membrane in a contactor acts as a passive barrier and as a means of bringing two immiscible fluid phases, such as gas and liquid or an aqueous liquid and an organic liquid, into contact with each other... 

The most widely used Ion selective electrode Is that which Is sensitive to the pH of a solution. This electrode is fabricated with a glass membrane which shows a selective permeation for hydronlum Ions. Glass electrodes are also available for many other cations) such as lithium) potassium) sodium) and calcium. Membranes which employ liquid Ion exchangers or neutral carriers) crystal membranes) and gas sensors) have been developed and used successfully for the determination of cations[3]. However) to be useful for in vivo applications) these electrodes must be constructed with micrometer dimensions. The liquid Ion-exchange membranes are particularly useful in this regard[4]. These electrodes are constructed from a glass pipette with a very small tip (1-2 jjm diameter)) and the liquid Ion exchanger Is placed In the tip. 

The actual basis for separation can be the differential solubility in the membrane material or the membrane hole size. When separating gases hollow membrane fibres are popular, as the small diameter of the fibres allows than to withstand the substantial driving pressures involved. Separation of the components in liquid systems usually relies on some form of spirally wound membrane sheet and spacer. Once again, substantial differential pressures are involved across the membrane surface and this conventionally requires the use of some form of spacer to prevent closure of the channels. Not only do these spacers (e.g. coarse woven fabric)... 

Controlled and predictable concentrations over a wide flow rate range are readily achievable. The generator uses a small diameter tube (e.g., 1/4 in. O.D.) made of a suitable membrane material such as polytetrafluoroethylene (pTFE) loaded with the liquid agent. The compound contacts the inside surface of the membrane and the vapor slowly permeates. 

Glass electrodes are used for the analysis of hydrogen ions various other types of ion-selective electrodes are used for the other ions. Electrodes with ion-selective solvent membranes have become very popular. These electrodes are made in the form of thin glass capillaries (about 1 rm in diameter), which in the lower part contain a small volume of a liquid that is immiscible with water the remainder of the capillary is filled with electrolyte solution (e.g., 3M KCl). 

Ultrafiltration processes (commonly UF or UF/DF) employ pressure driving forces of 0.2 to 1.0 MPa to drive liquid solvents (primarily water) and small solutes through membranes while retaining solutes of 10 to 1000 A diameter (roughly 300 to 1000 kDa). Commercial operation is almost exclusively run as TFF with water treatment applications run as NFF. Virus-retaining filters are on the most open end of UF and can be run as NFF or TFF. Small-scale sample preparation in dilute solutions can be run as NFF in centrifuge tubes. 

Figure 4.20.A shows a more recent cell reported by Cobben et al. It consists of three Perspex blocks, of which two (A) are identical and the third (B) different. Part A is a Perspex block (1) furnished with two pairs of resilient hooks (3) for electrical contact. With the aid of a spring, the hooks press at the surface of the sensor contact pads (4), the back side of which rests on the Perspex siuface, so the sensor gate is positioned in the centre of the block, which is marked by an engraved cross as in the above-described wall-jet cell. Part B is a prismatic Perspex block (2) (85 x 24 x 10 mm ) into which a Z-shaped flow channel of 0.5 mm diameter is drilled. Each of the wedges of the Z reaches the outside of the block. The Z-shaped flow-cell thus built has a zero dead volume. As a result, the solution volume held between the two CHEMFETs is very small (3 pL). The cell is sealed by gently pushing block A to B with a lever. The inherent plasticity of the PVC membrane ensures water-tight closure of the cell. The closeness between the two electrodes enables differential measurements with no interference from the liquid junction potential. The differential signal provided by a potassium-selective and a sodium-selective CHEMFET exhibits a Nemstian behaviour and is selective towards potassium in the presence of a (fixed) excess concentration of sodium. The combined use of a highly lead-selective CHEMFET and a potassium-selective CHEMFET in this type of cell also provides excellent results.

Closed bilayer aggregates, formed from phospholipids (liposomes) or from surfactants (vesicles), represent one of the most sophisticated models of the biological membrane [55-58, 69, 72, 293]. Swelling of thin lipid (or surfactant) films in water results in the formation of onion-like, 1000- to 8000-A-diameter multilamellar vesicles (MLVs). Sonication of MLVs above the temperature at which they are transformed from a gel into a liquid (phase-transition temperature) leads to the formation of fairly uniform, small (300- to 600-A-diameter) unilamellar vesicles (SUVs Fig. 34). Surfactant vesicles can be considered to be spherical bags with diameters of a few hundred A and thickness of about 50 A. Typically, each vesicle contains 80,000-100,000 surfactant molecules. 

The so-called bubble point of a membrane - a measure ofthe membrane pore size - can be determined by using standard apparatus. When determining the bubble point of small, disk-shaped membrane samples (47 mm in diameter), the membrane is supported from above by a screen. The disk is then flooded with a liquid, so that a pool of liquid is left on top. Air is then slowly introduced from below, and the pressure increased in a stepwise manner. When the first steady stream of bubbles to emerge from the membrane is observed, that pressure is termed the bubble point. ... 

Liquid crystals, liposomes, and artificial membranes. Phospholipids dissolve in water to form true solutions only at very low concentrations ( 10-10 M for distearoyl phosphatidylcholine). At higher concentrations they exist in liquid crystalline phases in which the molecules are partially oriented. Phosphatidylcholines (lecithins) exist almost exclusively in a lamellar (smectic) phase in which the molecules form bilayers. In a warm phosphatidylcholine-water mixture containing at least 30% water by weight the phospholipid forms multilamellar vesicles, one lipid bilayer surrounding another in an "onion skin" structure. When such vesicles are subjected to ultrasonic vibration they break up, forming some very small vesicles of diameter down to 25 nm which are surrounded by a single bilayer. These unilamellar vesicles are often used for study of the properties of bilayers. Vesicles of both types are often called liposomes.75-77... 

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