Mixtures of Monomeric and Dimeric Cationic Surfactants

Compounds with the formula C m H2m+1(CH3)2N+(C s H2 s )N+(CH3)2C m H2 m +1·2Br-, abbreviated as m-s-m according to a previous convention, are considered to be dimeric cationic surfactants. The Krafft temperatures of five of these dimeric surfactants (12-2-12, 14-2-14, 16-2-16, 16-4-16, and 16-6-16) and that of a monomeric surfactant, C16H33(CH3)3N+·Br- (hexadecyltrimethylammonium bromide, CTAB, where C stands for cetyl), have been determined from conductivity measurements. The values are 14.4, 32.8, 45.0, 34.0, 41.0, and 24.0 °C, respectively. The conductivities of these surfactants and the binary mixtures of the dimeric surfactants with CTAB were studied at 46.5 °C. A new mathematical treatment has been used to analyze the conductivity data to obtain the critical micelle concentration (cmc) of each binary mixture. Through the study of the cmc's, the nonideality of the binary mixtures was determined, and the trends are 16-2-16 ≫ 14-2-14 > 12-2-12, and 16-2-16 > 16-4-16 ≈ 16-6-16. It is suggested that the nonideality is due to the tendency of the dimeric surfactants to form ropelike micelles, especially with the increase in the length of the terminal chains and the decrease in the length of the spacer, whereas the monomeric surfactant CTAB prefers to form spherical micelles.