Effects of organic salt additives on the behavior of dimeric ("gemini") surfactants in aqueous solution

The effects of a series of aromatic anions, so-called hydrotropes, on characteristic solution properties of a family of ammonium gemini surfactants with dodecyl chains were explored. The stoichiometric addition of the organic salts to the geminis can result in clear solutions or in phase separation/precipitation, depending on the detailed nature of the added counterions and on the spacer group of the gemini surfactant. Many organic anions induce synergistic effects, strongly reducing the critical micellization concentration (cmc) and the surface tension at the cmc. Furthermore, a number of combinations of organic anions and geminis exhibit thickening of their aqueous solutions. The effects of the added salts are strongly enhanced for the gemini surfactants compared to the monomeric analogue N-dodecyl-N,N,N-trimethylammonium chloride. Even anions such as benzoate may be effective for thickening, and viscoelastic solutions can be obtained with salicylate despite the relatively short alkyl chains.