Organic cations: hydrophobic coatings

In 1959, William Grubbs used organic cation exchange polymers in fuel cells. As shown in Fig. 1.9, the cell consists of polymer electrolyte sandwiched between platinum impregnated porous electrodes. Teflon is coated on the electrodes to make them hydrophobic. The operation temperature of PEFMC can be within 80-200°C. The low operation temperature of PEMFC offers quick start-up but platinum catalysts are required to promote the electrochemical reaction. Platinum particles are deposited very finely onto carbon powders to attain the maximum surface area. The contact between gas, electrolyte and platinum catalyst should be good for the uninterrupted function of PEFMC. 

Researchers found that zeolite particles can be treated with high-molecular-weight surfactants [such as hexadecyltrimethylammonium (HDTMA)]. HDTMA does not penetrate into the internal pore structure of the zeolite, but coats the outer surface. The outer surface then develops hydrophobic anionic exchange properties, while the inner surface retains the capacity to adsorb cations. Researchers claim that SMZ can be used for the three major classes of water contaminants inorganic cations, inorganic anions, and nonpolar organics. 

In principle, the adsorption of organic environmental chemicals on layer silicates proceeds extremely slowly so that it may take several weeks to establish an equilibrium state [22]. In contrast, if the layer silicate surface is coated with cationic surfactants in different quantities [22] or of different hydrophobicity, e.g., with DTAB (dodecyl trimethyl ammonium bromide) or DDDMAB (didoceyl dimethyl ammonium bromide) [23] then equilibrium in both systems is reached within 30 min, i.e., a drastic increase in the adsorption rate of organic pollutants results. Their residence time or probability of residence in the soil electrolyte is therefore drastically reduced and their bioavailability thus also drops. 

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