The effect of chain length compatibility on the micellar stability
was investigated for mixtures of sodium
dodecyl sulfate (SDS) and alkyltrimethylammonium bromides
(C
n
TAB, n = 8, 10, 12, 14, 16)
at a 20/1 molar
ratio. It was observed that the surface properties of surfactant
solutions were influenced by the chain
length compatibility of the surfactant molecules. The slow
micellar relaxation time (τ2), which
directly
correlates to the micellar stability, was determined using the
pressure-jump technique. The addition of
C
n
TAB to SDS solutions results in a
significant increase in the relaxation time compared to SDS
alone,
showing a maximum for the SDS/C12TAB system. The high
micellar stability results in minimum surface
tension, maximum surface viscosity, maximum micellar lifetime, maximum
foam stability, and minimum
foamability. The higher micellar stability as observed for
SDS/C12TAB is attributed to both the
chain
length compatibility and the Coulombic interaction of the surfactant
head groups, resulting in an optimum
packing of molecules in micelles and at the air/water
interface.
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