Surfactants (Soaps and Detergents) and Physicochemical Properties - Surface and Colloid Chemistry: Principles and Applications - page 50

SDS
SD
Na
Micelle
To tal SDS
FIGure 3.9 Variation of concentration of different ionic species for SDS solutions (Na + ,
SD − , SDS micelle ).
Below CMC, the SDS molecules in water are found to dissociate into SD − and Na +
ions. Conductivity measurements show that
a. SDS behaves as a strong salt, and SD − and Na+ ions are formed (same as
one observes for NaCl).
b. A break on the plot is observed at SDS concentration equal to the CMC.
This clearly shows that the number of ions decreases with concentration.
The latter indicates that some ions (in the present case, cations, Na + ) are partially
bound to the SDS-micelles, which results in a change in the slope of the conductiv-
ity of the solution. Similar behavior is observed for other ionic detergents such as
cationic (CTAB) surfactants.
At CMC, micelles (aggregates of SD − with some counterions, such as Na + ) are
formed, and some Na + ions are bound to these, which is also observed from con-
ductivity data. In fact, these data analyses have shown that approximately 70% Na +
ions are bound to SD − ions in the micelle. The surface charge was estimated from
conductivity measurements (Birdi, 2002). Therefore, the concentration of Na + will
be higher than SD − ions after CMC. A large number of reports are found in the litera-
ture, in which the transition from the monomer phase (before CMC) to the micellar
phase (after CMC) have been analyzed.
This is also true in the case of cationic surfactants. Thus, in the case of CTAB
solutions, there are CTA + and Br − ions below CMC, and above CMC, there are, addi-
tionally, CTAB micelles. In these systems, the counterion is Br − .
3.2.1.2.1 Analyses of CMC
CMC will be dependent on different factors, including both the alkyl part and polar
part. Further, the interaction of the detergent with the solvent will also have an effect
on the CMC.