Membranes binary mixtures

W. Kujawski, Pervaporative removal of organics from water using hydrophobic membranes. Binary mixtures. Separation Science and Technology 35 (2000) 89-108. 

The liquid membrane (thickness 0.2 cm) was separated from the aqueous solutions by two vertical cellophane films.The electrode compartments were filled with 0.05 M sulfuric acid solutions and were separated by the solid anion-exchange membranes MA-40. Binary mixtures contained, as a mle, 0.04 M Cu(II) and 0.018 M Pt(IV) in 0.01 M HCl. 0.1 M HCl was used usually as the strip solution. 

Once synthesized several factors influence the particular leaflet of the membrane lipid bilayer where the lipids reside. One is static interactions with intrinsic and extrinsic membrane proteins which, by virtue of their mechanism of biosynthesis, are also asymmetric with respect to the membrane. The interaction of the cytoplasmic protein, spectrin with the erythrocye membrane has been the subject of a number of studies. Coupling of spectrin to the transmembrane proteins, band 3 and glycophorin 3 via ankyrin and protein 4.1, respectively, has been well documented (van Doit et al, 1998). Interaction of spectrin with membrane lipids is still somewhat conjectural but recent studies have characterized such interactions more precisely. O Toole et al. (2000) have used a fluorescine derivative of phosphatidylethanolamine to investigate the binding affinity of specttin to lipid bilayers comprised of phosphatidylcholine or a binary mixture of phosphatidylcholine and phosphatidylserine. They concluded on the basis... 

Fig. 2. Phase diagram describing lateral phase separations in the plane of bilayer membranes for binary mixtures of dielaidoylphosphatidylcholine (DEPC) and dipalmitoyl-phosphatidylcholine (DPPC). The two-phase region (F+S) represents an equilibrium between a homogeneous fluid solution F (La phase) and a solid solution phase S presumably having monoclinic symmetry (P(J. phase) in multilayers. This phase diagram is discussed in Refs. 19, 18, 4. The phase diagram was derived from studies of spin-label binding to the membranes.

As indicated in my report, we now know the rates of lateral diffusion of phospholipids in lipid bilayers in the fluid state, and in a few cases the rates of lateral diffusion of proteins in fluid lipids are also known. At the present time nothing is known about the rates of lateral diffusion of phospholipids in the crystalline, solid phases of the substances. As mentioned in my report, there are reasons to suspect that the rates of lateral diffusion of phospholipids in the solid solution crystalline phases of binary mixtures of phospholipids may be appreciable on the experimental time scale. Professor Ubbelohde may well be correct in pointing out the possibility of diffusion caused by defects. However, such defects, if present, apparently do not lead to significant loss of the membrane permeability barrier, except at domain boundaries. 

The 13C nuclear resonance studies in my report provide some informations on lipid membrane fluctuations in binary mixtures. Totally unsolved problems include an appropriate two-dimensional Debye-Huckel theory for membranes, and theoretical treatments of boundary free energies (between proteins and lipids, and between solid and fluid phase lipids). 

Because of its hydrophilic nature even unmodified BC shows great potential to separate azeotropes such as EtOH/FbO. It adsorbs seven times more water than ethanol. This selectivity and a reasonable flux increase with growing temperature and thinning of the membrane. In addition, the BC membranes also show a high water affinity in aqueous binary mixtures of organic solvents. 

The volumetric flux of a binary mixture of solvent and solute, jy, through a membrane can be expressed as... 

La liquid-ordered phase bilayers prepared from binary mixtures of the same phosphatidylcholine and cholesterol but is much slower ( 10 s ) in Lx liquid-ordered phase membranes prepared from sphingomyelin and cholesterol (55). The activation free energy for the process, which corresponds to the energy necessary to put the translocating lipid molecule at the bilayer mid-plane, is 100kJmol In contrast, the rate constant for transmembrane translocation of cholesterol may be 1 s (56). 

On the other hand, actual binary mixture tests using porous alumina and glass membranes show separation factor values for helium recovery from oxygen that are lower than what Knudsen diffusion provides, as indicated in Table 7.15. Only Koresh and Soffer [1983a 1983b] show an ideal separation factor of 20 to 40 with a low permeability of 1.2x10 barrers when molecular sieve membranes with a reported pore diameter of 0.3 to 0.5 nm are used. 

In this paragraph shortly the permeation measurement method is introduced, followed by various examples of permeation through a silicalite-1 membrane on a sintered stainless steel support. This includes unary and binary mixtures as a function of partial pressure, composition and temperature. Finally the present state of modelling permeation through silicalite-1 membranes is reviewed. 

Y.D. Chen and R.T. Yang, Preparation of carbon molecular sieve membrane and diffusion of binary mixtures in the membrane, Ind. Eng. Chem. Res. 55 3146 (1994). 

Brun JP, Larchet C, Melet R, and Bulvestre G. Modelling of the pervaporation of binary mixture through moderately swelling, non-reacting membranes, J. Memb. Sci. 1985 23(3) 257-283. 

For a binary mixture of components A and B, the flux can be expressed for the entire permeate (J, total flux) or each component and u, the flux of component A and B, respectively), having dimensions of mass/(area x time). The flux can be calculated provided the mass of the permeating component, the membrane area and the time of measurement are known. To this end, the following expression can be used ... 

Sorption and diffusion in polymers are of fundamental and practical concern. However, data acquisition by conventional methods is difficult and time consuming. Again, IGC represents an attractive alternative. Shiyao and co-workers, concerned with pervaporation processes, use IGC to study adsorption phenomena of single gases and binary mixtures of organic vapors on cellulosic and polyethersulfone membrane materials (13). Their work also notes certain limitations to IGC, which currently restrict its breadth of application. Notable is the upper limit to gas inlet pressure, currently in the vicinity of 100 kPa. Raising this limit would be beneficial to the pertinent use of IGC as an indicator of membrane-vapor interactions under conditions realistic for membrane separation processes. 

For simplicity of presentation, we assume here membranes containing binary mixtures of acidic and neutral (zwitterionic) lipids. The temporal evolution of the spatially varying charged-lipid compositions on the membrane upper (u) and lower (1) leaflets are linked to the Laplace-Beltrami (LB) operators acting on the corresponding electrochemical potentials through two CH equations (one for each leaflet) each of the form [28] ... 

Figure 4 Adsorption of lysine-13 polypeptide onto ternary phosphatidylcholine (PC)/ phosphatidylserine (PS)/PIP2 lipid membrane with 74 25 1 composition (panels A and B), and onto binary PC/PS lipid membrane with 71 29 composition (panel C). (a) Normalized local fraction of PIP2 lipids in the ternary system, (b) Local PS lipid fractions in the ternary system, (c) Local PS lipid fraction in the binary mixture. All plots shown for f=0.5 ts after beginning of propagation. For these calculations lysine-13 was placed near the membrane, such that the minimum distance between van der Waals radii of lysine-13 and membrane atoms was 3 A, and the peptide was oriented with its major (long) axis parallel to the bilayer plane.

The Dusty Gas Model (DGM) is one of the most suitable models to describe transport through membranes [11]. It is derived for porous materials from the generalised Maxwell-Stefan equations for mass transport in multi-component mixtures [1,2,47]. The advantage of this model is that convective motion, momentum transfer as well as drag effects are directly incorporated in the equations (see also Section 9.2.4.2 and Fig. 9.12). Although this model is fundamentally more correct than a description in terms of the classical Pick model, DGM/Maxwell-Stefan models )deld implicit transport equations which are more difficult to solve and in many cases the explicit Pick t)q>e models give an adequate approximation. For binary mixtures the DGM model can be solved explicitly and the Fickian type of equations are obtained. Surface diffusion is... 

SEPARATION OF BINARY MIXTURES IN SIMPLE MESOPOROUS MEMBRANES... 

The parameter to describe the separation efficiency for a binary mixture is the separation factor a which is a measure of the enrichment of a gas component after it has passed the membrane. 

As discussed in Section 9.4.1, the contribution of Knudsen diffusion to the total flux decreases with decreasing pore radius of the membrane material. Initially the selectivity of binary mixtures of gases is constant and equal to the Knudsen value. 

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