RELATION OF WATER AND NEUTRAL ORGANIC-COMPOUNDS IN THE INTERLAYERS OFMIXED CA TRIMETHYLPHENYLAMMONIUM-SMECTITES/

Organoclays were prepared by exchanging Ca2+ in a Ca2+-saturated smect ite partially or fully with trimethylphenylammonium (TMPA) cations. Th e mechanistic function of these organoclays as adsorbents for neutral organic compounds in aqueous solution was examined. TMPA cations were found to take a random distribution on the surfaces of mixed Ca/TMPA-s mectites. The presence of TMPA, and its random distribution, resulted in water associated with the clay surfaces being held more weakly. App arently, the interspersing of TMPA and Ca2+ ions prohibits the formati on of a stable network of water molecules around Ca2+. Water molecules associated with the siloxane surface in mixed Ca/TMPA-clays are remov ed during the adsorption of neutral organic compounds from bulk water, leaving only similar to 11 strongly held water molecules around each Ca2+, as opposed to similar to 58 water molecules in homoionic Ca2+-sm ectite. These results demonstrate that the amount of water associated with the clay surfaces and interlayers depends on the nature of the ex change cation(s), and not on the amount of available siloxane surface area by itself. We conclude that in TMPA-smectites the TMPA cations fu nction as nonhydrated pillars, and sorption of organic solutes occurs predominantly on the adjacent siloxane surfaces, which are hydrophobic in nature. The water molecules around Ca2+ in mixed Ca/TMPA-smectites obscures some of the siloxane surfaces. This diminishes sorption capa city, in an amount roughly equivalent to the fraction of the CEC occup ied by Ca2+, because organic solutes cannot displace the waters of hyd ration of Ca2+.