Membrane fluorocarbon liquid

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. 

Figure 6. Evolution of the membrane structure as a function of water content, 1 (moles of water per mole of sulfonic acid sites). The pictures are cross-sectional representations of the membrane where the gray area is the fluorocarbon matrix, the black is the polymer side chain, the light gray is the liquid water, and the dotted line is a collapsed channel. (Reproduced with permission from ref 89. Copyright 2003 The Electrochemical Society, Inc.)...

The resistance to 02 and C02 transport of the liquid membrane should be minimal. The liquid membranes are typically quite thin. In systems where they have been measured, they are usually 1 or less, which is substantially thinner than polymeric membranes used for tran-port. The thinness of the membranes combined with the high solubility of fluorocarbons (13, 14) for 02 and C02 would be expected to lead to minimal liquid membrane resistance to transport. 

Liquid Membrane Formation. The most important test of liquid-membrane oxygenation was determining whether stable fluorocarbon-... 

Liquids (either solvents or solvents plus surfactants) other than fluorocarbons might be used to form liquid membranes for blood oxygenation. However, the following criteria must be satisfied ... 

Liquid Membrane Stability. While studies of other investigators have indicated that the blood has good compatibility with the liquid fluorocarbon surface, they also indicate that fluorocarbon droplets should not be introduced to the bloodstream of animals (6, 7, 8). Liquid membrane rupture in the oxygenator apparatus could produce droplets from the fluorocarbon which had formed the liquid membrane. These droplets would be entrained and returned to a test animal with the oxygenated blood. As a preliminary test for liquid membrane rupture and droplet formulation, the oxygen flow into apparatus was momentarily stopped, and blood samples were withdrawn for examination. 

The blood samples were centrifuged at 20,000 rpm at a distance of 4.5 inches for 20 minutes in a centrifuge maintained at 20°C. After centrifugation the blood separated into two layers, a top layer of plasma and a bottom layer of red cells. Since the liquid fluorocarbon is immiscible with the blood and is much heavier than the blood, entrainment of fluorocarbon in blood should result in the formation of a small, third layer of the fluorocarbon at the very tip of the pointed centrifuge tubes after such intensive centrifugation. However, no such layer was found in the tubes for all the four blood samples tested. The blood samples were also examined carefully under microscope. No tiny droplets of fluorocarbon were noticed. While it is possible that a few liquid membranes ruptured and escaped detection and more definitive testing would be required before application, instability of the liquid membranes does not seem to be a major problem. 

Liquid membrane of fluorocarbons can be formed encapsulating oxygen bubbles in blood. The transfer of oxygen and carbon dioxide through the liquid membrane to and from the blood, respectively, have been shown. Very similar transfer rates with and without liquid membranes indicate that the resistance of the liquid membranes is small. 

As the membrane becomes more hydrated, the sulfonic acid sites become associated with more water allowing for a less bound and more bulk-like water to form. This is why there is a flattening out of the slope above X = 6 in the uptake isotherm, Figure 5.1. The extreme case is when the membrane is placed in a liquid-water reservoir, where the ionic domains swell and a bulklike liquid-water phase comes into existence throughout the membrane. The way in which water does this is unknown but is probably due to the interfadal properties of the membrane, such as the fluorocarbon-rich skin on the surface of Nafion [26,27] or the removal of a liquid-vapor meniscus at the membrane surface [28]. In essence, the reorganization results in a porous structure with an average pore size between 1 and 2 nm [29]. 

When the membrane is placed in liquid water, rearrangement and a phase-transition occur. This could be due to surface rearrangements wherein the fluorocarbon-rich skin of the membrane is repelled from the interface between the water and membrane. What this means is that in order to minimize the energy of the system the side chains and backbone of the polymer reorient so that the chains are now arranged at the membrane/ water interface. This hypothesis agrees with the data that show that the water contact angle on the membrane surface becomes more hydrophilic after the membrane is placed in liquid water [32]. The presence of liquid water also results in the removal of a vapor-liquid meniscus, which could also aid in the above rearrangements [28]. 

The membrane has two functions. First, it acts as the electrolyte that provides ionic conduction between the anode and the cathode but is an electronic insulator. Second, it serves as a separator for the two-reactant gases. Some sources claim that solid polymer membranes (e.g., sulfonated fluorocarbon acid polymer) used in PEMFC are simpler, more reliable, and easier to maintain than other membrane types. Since the only liquid is water, corrosion is minimal. Pressure balances are not critical. However, proper water management is crucial for efficient fuel cell performance [6]. The fuel cell must operate under conditions in which the by-product water does not evaporate faster than it is produced, because the membrane must be hydrated. Dehydration of the membrane reduces proton conductivity. On the other hand, excess of water can lead to flooding of the electrodes. 

Di Noto group recently also reported on the PFMs based on Naflon, Si02, and a protic ionic liquid (PIL), triethylammonium trifluoromethanesulfonate (TFATF), using vibrational studies of FTIR and FT-Raman. Their vibrational studies showed that (1) the neutralization of membranes by TFA and their impregnation with TFATF influence the conformational composition of fluorocarbon backbone chains... 

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