PARTITIONING OF AMPHIPHILIC ADDITIVES BETWEEN THE MICELLES OF NORMAL-ALKYLTRIMETHYLAMMONIUM BROMIDES AND THE SURROUNDING AQUEOUS-SOLUTION AS A FUNCTION OF SURFACTANT CHAIN-LENGTH

The partitioning of 1-propanol, 2-propanol, 1-butanol, 2-butanol, 2-methyl-2-propanol, 1-butylurea, tert-butylurea, cyclohexanol, 6-caprolactam, 1-hexanol, 1-octanol, and 1-decanol between the micelles of dodecyl- (C12), tetradecyl- (C14), and hexadecyl-(C16) trimethylammonium bromides and the surrounding aqueous phase has been determined at 25°C. It is found that the ability to depress the critical micelle concentration, the corresponding ability to increase the micellar degree of ionization, and the coaggregation (solubilization) tendency all tend to decrease with increasing surfactant chain length. This is in accord with the results obtained previously for the partitioning of a large number of various amphiphilic additives between the micelles of sodium alkyl sulfates and water. The results are analyzed in terms of the factors contributing to the standard free energy of coaggregation, and the general trend observed is interpreted as due to the tendency of the amphiphilic additive to be coaggregated in the outer rough region of the micelle in contact with water composed of the ionic heads plus portions of the hydrocarbon chains and whose extent decreases with increasing surfactant chain length. Finally, comparative published data on the partitioning of amphiphilic additives in n-alkyltrimethylammonium bromides have also been discussed. © 1990 American Chemical Society.