Critical surface tension hydrophobic materials

Surface characteristics of the membranes Two of the key characteristics are the hy-drophobicity and the surface charge. Hydrophobic surfaces (or water-hating surfaces) are those where the critical surface tensions of the membrane material... 

Our studies showed that the inner surface of the cardiovascular System was hydrophobic surface with Yc(Zis.)=29 dyne/cm. We could easily select polytetrafluoroethylene as the material with a smaller critical surface tension than 29 dyne/cm to make test materials. Figure-3 shows the surface properties of the inner surface of the cardiovascular system and polytetrafluoroethylene in relation to wetting properties viewed from Zisman s plots. How should we treat polytetrafluoroethylene to minimize the difference in wetting properties between the surface of polytetrafluoroethylene and the inner surface of the cardiovascular system. We tried to improve the surface properties of polytetrafluoroethylene by using the graft copolymerization method and expansion method. 

Flotation separation utilizes differences in surface wettability of materials. This process is being widely used by mining industry and has been proposed for the separation of plastics waste [15], [16]. A schematic of flotation column is shown in Fig 4. In the process the materials to be separated are treated with various chemicals which make some fractions preferentially wettable. The separation occurs in a column where the mixture to be separated is contacted with water in which air bubbles are dispersed. Air bubbles attach themselves to the hydrophobic particles floating them to the surface while hydrophilic particles sink to the bottom. In another version of this process, when both of the materials to be separated are either hydrophilic or hydrophobic but exhibit different critical surface tensions of wetting (defined as a surface tension of a liquid in which the solid exhibits transition from hydrophilic to hydrophobic behaviour), a liquid medium can be selected such that... 

Of course, in many cases, e.g. umbrellas, raincoats or novel non-wetting or self-cleaning materials, we want to have hydrophobic or even super-hydrophobic surfaces. The contact angles should have the maximum possible values in these cases. Alternatively we wish to have as low critical surface tensions as possible. There are many application examples of such novel surfaces, by TCNANO and others. Nature can provide inspiration for making such superhydrophobic materials, as illustrated by Scientific American (March and September 2003) about waterproof coats. 

While Teflon is a polymer that is particularly difficult to wet with a low critical surface tension, as we see in this problem, other fluoro-materials such as perfluoro-acid surfaces are equally or even harder to wet as they have very low values of the critical surface tension. This can be seen in Figure 6.3. Thus these materials ean form the basis of super-hydrophobic and potential self-cleaning surfaces, graphically illustrated in Figure 6.4. 

For a hydrophobic porous material with contact angle greater than 90°, the APc is >0 and depends on the liquid surface tension and the membrane pore size. As an example, considering water-air-polypropylene system, one can calculate that for a dry membrane with a pore size of 0.03 pim (30 nm) the critical entry pressure of water is more than 300 psi (>20 bar). 

The critical micelle concentrations of mixtures of POE nonionic surfactants are of particular interest, since the synthesis of such materials on a commercial basis will always produce a rather broad range of POE chain lengths. Because they contain no electrostatic contribution to the free energy of micelle formation, they can be treated theoretically with a simpler relationship between composition and cmc. In a mixture of nonionic surfactants in which the average POE chain lengths are approximately the same and the hydrocarbon chains different, there was a smooth decrease in the cmc of the mixture as the mole fraction of the more hydrophobic material (lower cmc) was increased, reminiscent of the surface tension-mole fraction curves found for miscible organic materials mixed with water. 

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