Cleavable Cationic Gemini Surfactants

Abstract: This research project concerns investigations of ester-containing cationic gemini surfactants (esterquat and betaine ester types). These dimeric surfactants are compared with their monomeric counterparts. Series of both stable and ester-containing cationic gemini and monomeric surfactants were synthesized and their physical-chemical properties were investigated. The critical micelle concentration (CMC) values of the ester-containing gemini surfactants were generally 10-20 times smaller than those of the corresponding monomers. The alkyl chain length was decisive of the CMC while the length and the nature of spacer did not affect the CMC much. The low CMC of the gemini surfactants resulted in very high efficiency in covering surfaces in adsorption experiments, in stabilizing emulsions and foams, and in solubilizing hydrophobic dyes. The micellar aggregation number of the surfactants was determined with steady state and with time-resolved fluorescence quenching. Based on calculations of diffusion coefficients obtained from NMR measurements, a growth of the micelles with increasing surfactant concentration was observed for some of the gemini surfactants and a structure of interdigitated micelles was proposed for all the geminis. Studies of the chemical hydrolysis showed that the betaine ester type surfactants were much more susceptible to alkaline hydrolysis than the esterquat type surfactants. It was also found that the ester-containing geminis hydrolyzed more readily than the ester-containing monomeric surfactants, which was attributed to an anchimeric assistance by the second quaternary ammonium head group. In contrast to the results from the chemical hydrolysis the monomeric ester-containing surfactants biodegraded faster than the corresponding gemini surfactants. The low rate of biodegradation of the ester-containing geminis was found to be due to the dicationic species formed on hydrolysis of the two ester bonds being very resistant to further biodegradation.