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.
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