Effect of Symmetrical Tetraalkylammonium Salts on Cloud Point of Nonionic Surfactants

The salting in and salting out of the nonionic surfactant octoxynol NF by halides of ammonium and the four lowest symmetrical tetraalkylammonium cations were investigated by measuring their effect on the cloud point at various salt concentrations. The chloride anion tended to salt the surfactant out, lowering its cloud point. The iodide anion tended to salt it in, raising the cloud point, while the bromide ion had no pronounced effect. The ammonium and tetramethylammonium cations tended to lower the cloud point, the latter more extensively than the former. The tetraethylammonium cation had no pronounced effect, while the tetra-n-propylammonium and tetra-n-butylammonium cations tended to raise the cloud point, the latter more extensively than the former. The salt effect on the cloud point generally increased with increasing salt concentration. Tetramethylammonium chloride was the most efficient salt in lowering the cloud point (by 4° at the 0.10-molal level and by 20° at the 1.00-molal level), while tetrabutylammonium halides were the most efficient salts in raising the cloud point (by 18–19° at the 0.10-molal level for chloride and bromide). Cloud point increases were attributed to the formation of mixed micelles or to hydrotropy. The salting-out efficiency of tetramethylammonium chloride and bromide was ascribed to their ability to increase the structure of water.