Membranes internal water activity

Futerko and Hsing presented a thermodynamic model for water vapor uptake in perfluorosulfonic acid membranes.The following expression was used for the membrane—internal water activity, a, which was borrowed from the standard Flory—Huggins theory of concentrated polymer solutions ... 

A Theory of Membrane Internal Water Activity. From a thermodynamic standpoint, the (water-swollen) equilibrium membrane structure must depend, in part, upon the internal osmotic pres sure which is determined by the water activity, a, within the microscopic cluster regions, a, in turn, should IBe a function of the relative population of unpaired ions and free water molecules in the cluster solution. 

A comparison of theoretically-determined internal water activities for membranes in the monovalent cation forms over a range of water contents is provided in Figure 11. For increasingly large n, differences between cationic forms becomes negligible and a asymptotically approaches unity (dilution effect), w... 

Unphosphorylated functioning according to Fig. 5 catalyzes facilitated diffusion of mannitol across the membrane. The same process has been reported for purified II reconstituted in proteoliposomes [70]. The relevance of this activity in terms of transport of mannitol into the bacterial cell is probably low, but it may have important implications for the mechanism by which E-IIs catalyze vectorial phosphorylation. It would indicate that the transmembrane C domain of Il is a mannitol translocating unit which is somehow coupled to the kinase activity of the cytoplasmic domains. We propose that the inwardly oriented binding site which is in contact with the internal water phase (Ecyt Mtl, see Fig. 5) is the site from where mannitol is phosphorylated when transport is coupled to phosphorylation. Meehan-... 

Ion selective membranes are the active, chemically selective component of many potentiometric ion sensors (7). They have been most successfully used with solution contacts on both sides of the membrane, and have been found to perform less satisfactorily when a solid state contact is made to one face. One approach that has been used to improve the lifetime of solid state devices coated with membranes has been to improve the adhesion of the film on the solid substrate (2-5). However, our results with this approach for plasticized polyvinylchloride (PVC) based membranes suggested it is important to understand the basic phenomena occurring inside these membranes in terms of solvent uptake, ion transport and membrane stress (4,6). We have previously reported on the design of an optical instrument that allows the concentration profiles inside PVC based ion sensitive membranes to be determined (7). In that study it was shown that water uptake occurs in two steps. A more detailed study of water transport has been undertaken since water is believed to play an important role in such membranes, but its exact function is poorly understood, and the quantitative data available on water in PVC membranes is not in good agreement (8-10). One key problem is to develop an understanding of the role of water uptake in polymer swelling and internal stress, since these factors appear to be related to the rapid failure of membranes on solid substrates. 

ATP formation has been demonstrated in the absence of light, if ApH is imposed artificially across the thylakoid membrane [76], or by imposing a A targe enough to supply the energy required [37,71]. In both cases, the activity of the ATP synthase complex is required and ATP synthesis is concomitant with the transfer of protons from the internal water space of the thylakoid lumen to the external bulk phase. 

Multiple emulsions are complex systems of emulsions of emulsions. Both water-in-oil-in-water (W/O/W) and oil-in-water-in-oil (O/W/O) multiple emulsions have potential appHcations in various fields. The W/O/W multiple emulsion may be considered as a water/water emulsion whereby the internal water droplets are separated by an oily layer (membrane). The internal droplets might also consist of a polar solvent such as glycol or glycerol, which may contain a dissolved or dispersed active ingredient (a.i.). The O/W/O multiple emulsion can be considered as an oil/oil emulsion separated by an aqueous layer (membrane). 

A nerve cell or fiber consists of three distinct components a thin (ca. 10 nm), poorly conducting membrane (Fig. 1) separating two electrolyte solutions, the extracellular (outside) and intracellular (inside) fluids (cf. Fig. 2). The water activities and ionic strengths of the outside and inside solutions are equal, but in most other known properties these solutions are different. The extracellular fluid in comparison to the intracellular fluid has high concentrations of Na+ and Cl and a low concentration of K+. The absolute values of these concentrations vary from 0.1 to 0.5 M depending on the animal s environment. The ratios of external to internal concentrations of Na+ and K+ in nerve cells are less variable [Na+]<,/[Na+] 10 [K+]J... 

Transmembrane transfer of water from the external (continuous) phase into the internal (encapsulated) phase (7c. swelling of the emulsion) is an undesirable process. Some of the primary factors which determine the rate of water transfer are the type and concentration of surfactant in the liquid membrane. The direction of the transmembrane transfer of water in an extracting emulsion is determined by the sign of the water activity gradients. 

Figure 1 Schematic structures of micelle and liposome, their formation and loading with a contrast agent, (a) A micelle is formed spontaneously in aqueous media from an amphiphilic compound (1) that consists of distinct hydrophilic (2) and hydrophobic (3) moieties. Hydrophobic moieties form the micelle core (4). Contrast agent (asterisk gamma- or MR-active metal-loaded chelating group, or heavy element, such as iodine or bromine) can be directly coupled to the hydrophobic moiety within the micelle core (5), or incorporated into the micelle as an individual monomeric (6) or polymeric (7) amphiphilic unit, (b) A liposome can be prepared from individual phospholipid molecules (1) that consists of a bilayered membrane (2) and internal aqueous compartment (3). Contrast agent (asterisk) can be entrapped in the inner water space of the liposome as a soluble entity (4) or incorporated into the liposome membrane as a part of monomeric (5) or polymeric (6) amphiphilic unit (similar to that in case of micelle). Additionally, liposomes can be sterically protected by amphiphilic derivatization with PEG or PEG-like polymer (7) [1].

The coexistence of lipid and water solubility in the same molecule is essential for the action of a local anaesthetic drug. Lipophilicity permits the migration of drug across the phospholipid membrane of the nerve cell hydrophilicity is essential for the ionisation of the drug within the nerve. It follows that lipid and water solubility are the external and internal facilitators of local anaesthetic action in the nerve cell. Both within and without the nerve cell the unionised and ionised forms coexist in dynamic equilibrium. Outside the nerve, the active species is the unionised tertiary amine form. Conversely, inside the cell the ionised form predominates. The lower intracellular pH induces a shift in the equilibrium in favour of ionisation (Figure 5.5). 

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