Hydrophobic-hydrophilic composite membranes

Khayet, M., Mengual, J.I., Matsuura, T., Porous hydrophobic/ hydrophilic composite membranes Apphcation in desalination using direct contact membrane distdlation, J. Membr. Sci. 252, 101,2005. 

A relatively simple way to fulfill all of the MD requirements, although some of them are mutually contradictory, is to use porous hydrophobic/hydrophilic composite membranes. The top hydrophobic thin layer will prevent the penetration of water into the pores. On the other hand, resistance to the mass transfer is minimized because of the thinness of this hydrophobic layer. Both the hydrophobic and hydrophilic layers will contribute to the overall resistance to the heat transfer. Hence, the heat conductance can be reduced by using a relatively thick hydrophilic sublayer. 

Considering the obvious advantages of the membrane surface modification by SMM and the applicability of this principle in manufacturing hydrophobic/hydro-philic membranes for MD, we synthesized four different types of hydrophobic SMMs (nSMMl, nSMM2, nSMM3, and SMM41). These SMMs were then blended into PEI, which is a hydrophilic polymer, to produce novel hydrophobic/hydrophilic composite membranes by the phase inversion method. Finally, these membranes were tested for the desalination of a 0.5 M NaCl solution by DCMD. 

Khayet, M., Matsuura, T. and Mengual, J.l. 2005a. Porous hydrophobic/hydrophilic composite membranes Estimation of the hydrophobic layer thickness, 266 68-79. 

The majority of solid proton-conducting membranes, most commonly used in contemporary fuel cell technology, are hydrated perfluorinated sulfonic acid ionomers. In recent years, enormous programs in membrane research have explored empirically how various modifications of the benchmark material, viz. Nafion, affect the physical membrane properties. The main modifications include (1) varying the hydrophobic/hydrophilic composition of the polymer, (2) controlling the grafting density and lengths of the sidechains,... 

Qtaishat, M., Khayet, M. and Matsuura, T. 2009c. Novel composite hydrophobic/hydrophilic polysulfone membranes for desalination by direct contact membrane distillation, to-.. 341 139-148. 

M. Qtaishat, M. Khayet, and T. Matsuura, Novel porous composite hydrophobic/hydrophilic polysuhone membranes for desalination by direct contact membrane distillation. Journal of Membrane Science 341 (2009) 139-148. 

J. Xu et al. [283] have shown that immobilization of enzymes can be done using a specially designed composite membrane with a porous hydrophobic layer and a hydrophilic ultrafiltration layer. A polytetrafluoroethylene (PTFE) membrane with micrometer pores as an excellent hydrophobic support for immobilization was employed for the porous hydrophobic layer, and a biocompatible material of polyvinyl alcohol (PVA) which provided a favourable environment to retain the lipase activity was used to prepare the hydrophilic... 

An early generation of composite membranes, developed by Riley, et al. (21), was based on cellulose triacetate (CTA) cast in an ultrathln coat from chloroform on the finely porous surface of a cellulose nitrate/cellulose acetate substrate. These membranes did not reflect a need for a hydrophllic-gel Intermediate layer. Yet, this membrane substrate is much more hydrophilic than the rejecting CTA layer, and high flux as well as high separation were concurrently obtained. This is not the case if the porous substrate is highly hydrophobic. A rejecting layer deposited on such a surface would yield an extremely poor productivity due to the loss of... 

Ad Figure 2.6. A fimdamentally different system is to load a polymer film (e g. a siloxane) with zeolite crystals. Especially the Twente group studied such composite membranes. The zeolite crystals then just contribute to the permeation by acting as selective reservoirs of components. Hydrophobic (silicalite-1) as well as hydrophilic (zeolite A) zeolites have been studied in such a configuration. 

The capillary inside surface also affects the composite membrane performance. For alcohols with hydrophilic OH groups it is desirable to have a hydrophobic capillary surface. 

Figure 2. Concentration profiles in hydrophobic and hydrophilic and composite membranes used in a groundwater extraction nxxiule.

Overall membrane compression Lipid-domain interface fluctuations Free volume fluctuations Local depressions and distortions Transient hydrophobic pores Transient hydrophilic pores Foot-in-the-door hydrophilic pores Composite hydrophilic pores Membrane enzyme changes Membrane macromolecule protrusion changes Rupture and REB not actually described (5) Suggested alternative to transient pores (6) Transport of nonpolar species (7) Possible precursors to hydrophilic pores (8, 9) Possible precursor to hydrophilic pores (10) Key to quantitative descriptions (10-16) Candidate metastable pores" Candidate metastable pores Coupling to membrane macromolecules (17) Candidate signaling change mechanism... 

Hydrophobic/hydrophilic membranes for recovery of biofuel and dehydration, respectively Composite polydimethylsiloxane membrane... 

In membrane extraction of metals, the mass transport of solute from one phase to another occurs by diffusion. It is controlled by phase equilibrium and the resistances of boundary layers in two phases and the membrane material. Both types of materials are used for membrane extraction and stripping, hydrophilic and hydrophobic, and composite hydro-philic/hydrophobic barriers are also developed to avoid the membrane solubilization [122,123]. To enhance separation, the reactive liquids that induce chemical reaction with one of the separated species can be used. In membrane SX of metals, extracting agents, such as tri- -octylphosphine oxide (TOPO), di(2-ethylhexyl)phosphoric acid (D2EHPA), and n-octyl(phenyl)-A,A-diisobutylcarbamoylmethylphosphine oxide (CMPO), and commercial reagents like CYANEX 301, CYANEX 923, LIX622, and LIX622N are applied. 

To account for composite membranes, Prasad and Sirkar [189] presented more complex expressions to include the effect of both the hydrophobic and the hydrophilic moieties of these membranes on the overall mass transfer coefficients, according to Eq. (6) and Eq. (7). 

The objectives of this chapter are to describe the fabrication of novel composite hydrophobic/hydrophilic membranes for DCMD using different surface-modifying macromolecules (SMMs) and a hydrophilic polymer polyetherimide (PEI). The membrane characteristics are related to the DCMD performance. 

Composite porous hydrophobic/hydrophilic membranes, having a very thin hydrophobic layer that is responsible for the mass transfer and a thick hydrophilic layer, the pores of which are filled with water, to prevent heat loss through the overall membrane. This seems to be a relatively simple solution that fulfills all of the above conditions for achieving high permeability and low thermal conductivity. The composite hydrophobic/hydrophilic membranes could be made by blending hydrophobic SMMs into a hydrophilic polymer solution, which is the cornerstone of this work. 

Novel composite hydrophobic/hydrophilic membranes made specifically for the MD process were prepared and successfully tested for desalination application by DCMD. This was achieved through blending different types of SMMs into PEI, a hydrophilic polymer. 

Qtaishat, M., Rana, D., Matsuura, T. and Khayet, M. 2009b. Effect of surface modifying macromolecules stoichiometric ratio on composite hydrophobic/hydrophilic membranes characteristics and performance in direct contact membrane distillation. A10iE. 55(12) 3145-3151. 

Results of methanol permeability at various DS, SiWA, and Si02 are plotted in Figure 13.15. It can be seen that no membrane possesses higher methanol permeability than the Naflon 112 membrane. The corresponding methanol permeability for Nafion 112 was about 15.07 x 10 cm%. The methanol permeabilities of composite membranes based on SPEEK are obviously lower than that of Naflon 112, which is possibly due to the microstructure differences between the SPEEK and the Naflon 112 membrane. Kreuer reported that the Naflon membrane has high hydrophobicity of the perfluorinated backbone and also high hydrophilicity of the sulfonic groups (Kreuer 2001). In the presence of water, this character is more pronounced and consequently increases the hydrophobic/hydrophilic domains of the Naflon membrane. 

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