Hydrophobic-hydrophilic composite

A neutral molecule solubilized in the micelle can be located in several positions or microenvironments. As early as the 1930s it was suggested by Lawrence that the site of a solubilized molecule would be dependent on the hydrophobic/hydrophilic composition of the solubilizate. Two extremes are easily identified the core of the micelle providing a hydrocarbon-like microenvironment, and the palisade layer providing an aqueous or water-rich interfacial environment. It seems logical to assume, then, that nonpolar solutes like alkanes would prefer the micellar core and that polar molecules would be anchored at the surface. However, this is an oversimplification available data tend to contradict it. First, the solubility of alkanes in micelles is significantly lower than expected if compared to solubility in hydrocarbon solvents. Second, the size of a micelle is normally such that part of the solute would be close to the surface at any time. Sepulveda et al. state that for SDS micelles at least half of the solute will be within 4 to 5 A of the surface. We should also consider the timescale of the experiments, as the timescale for intramicellar migration is short. The rate constants of entry and exit of molecules to and from micelles is of the order 1(F and... 

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. 

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,... 

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. 

Fig. 18.12 (a-f) SEM images and wettability of PU/c-PVA composite fibrous film, (a) Cross section of PU/c-PVA film, which is of two-layer structure, (b, c) PU microfiber layer with high water repellency. (d-f) c-PVA nanofibers layer with hydrophilic property, (g, h) Unidirectional water-penetration phenomenon of the hydrophobic/hydrophilic composite film, (g) Water drops onto the upward hydrophobic PU side and penetrates spontaneously, (h) Water drops onto the upward hydrophilic c-PVA film side and spreads instead of penetration (Reproduced ftom Ref. [45] by permission of The Royal Society of Chemistry)... 

The hydrophilic/hydrophobic SIN composition of PDMS with poly(HEMA) and poly(AAC) were proposed as a potential apphcation for high-permeability soft contact lenses. Other sUicone-containing IPNs for contact lenses include polymerization of MMA in the presence of polymerized methacryloxypropyl trimethoxysilane, the cross-linking of a polymeric hydrogel of a copolymer of NVP during the final compression or injection-moulding process. 

Further elaboration of these dendrigraft principles allowed the synthesis of a variety of core-shell type dendrigrafts, wherein elemental composition as well as the hydrophobic/hydrophilic character in the core can be controlled independently. 

If both continuous and dispersed phases of highly concentrated emulsions contain monomeric species, it is possible to obtain hydrophilic/hydrophobic polymer composite materials. Polyacrylamide/polystyrene composites have been prepared in this manner [180], from both w/o and o/w HIPEs containing aqueous acrylamide and a solution of styrene in an organic solvent. 

The "onion skin" growth mechanism is supported by filming experiments in which film formation is greatly effected by the nature of the monomer composition added last in the polymerization. In power feed examples, as well as in staged feeds, hard and hydrophobic compositions hinder film formation while softer and more hydrophilic compositions aid film formation. Curiously, in this respect, it was found that the filming characteristics of all-acrylic latexes responded to non-uniform polymerization techniques much more dramatically than did their styrene-acrylic counterparts. 

It allows the composition, hydrophobicity-hydrophilicity, and chemical properties of the mesoporous host to be tuned using chemistry. 

In the methodology developed by us [24], the incompatibility of the two polymers was exploited in a positive way. The composites were obtained using a two-step method. In the first step, hydrophilic (hydrophobic) polymer latex particles were prepared using the concentrated emulsion method. The monomer-precursor of the continuous phase of the composite or water, when that monomer was hydrophilic, was selected as the continuous phase of the emulsion. In the second step, the emulsion whose dispersed phase was polymerized was dispersed in the continuous-phase monomer of the composite or its solution in water when the monomer was hydrophilic, after a suitable initiator was introduced in the continuous phase. The submicrometer size hydrophilic (hydrophobic) latexes were thus dispersed in the hydrophobic (hydrophilic) continuous phase without the addition of a dispersant. The experimental observations indicated that the above colloidal dispersions remained stable. The stability is due to both the dispersant introduced in the first step and the presence of the films of the continuous phase of the concentrated emulsion around the latex particles. These films consist of either the monomer-precursor of the continuous phase of the composite or water when the monomer-precursor is hydrophilic. This ensured the compatibility of the particles with the continuous phase. The preparation of poly(styrenesulfonic acid) salt latexes dispersed in cross-linked polystyrene matrices as well as of polystyrene latexes dispersed in crosslinked polyacrylamide matrices is described below. The two-step method is compared to the single-step ones based on concentrated emulsions or microemulsions. 

Single-step preparations of composite polymers have been examined in previous sections. The volume fraction of the continuous phase was, however, relatively small in those cases. In contrast, the present method allows us to prepare composites with larger volume fractions of the continuous phase. Composites with large volume fractions of the continuous phase can also be obtained in a single-step by polymerizing an emulsion or a microemulsion [24]. An emulsion of a hydrophobic (hydrophilic) monomer in another hydrophilic (hydrophobic) monomer can be extremely stable (even thermodynamically stable, and then it is called a microemulsion) if a sufficiently large amount of surfactant is introduced into the system. For an emulsion to be thermodynamically stable, a cosurfactant is in most cases needed besides the surfactant. The latter method was used to prepare composites by employing acrylamide... 

The main conclusion from the above experimental facts and theoretical concepts to be emphasized is that the hydration energy or the hydrophobicity (hydrophilicity) of a solute depends upon the chemical composition of a given aqueous medium. It seems reasonable to assume that this dependence plays a regulating role in living systems and therefore it should be taken into account while studying the hydropobicity of biological molecules. 

The layered structure of a cast film is controlled by the surface properties during evaporation. Significant compositional gradients can be generated by making use of the natural tendencies of one polymer to migrate toward the air-polymer Interface and the other toward the substrate. Hydrophobicity/hydrophilicity of macromolecules is often cited as the driving force (27, 28). 

Ruckenstein, E., and J. S. Park, Hydrophilic-Hydrophobic Polymer Composites, J. Poly. Sci, Part C Pofym.Lett.,26, 529 (1988). 

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