Spontaneous Transformations between Surfactant Bilayers
of Different Topologies Observed in Mixtures of Sodium Octyl Sulfate
and Hexadecyltrimethylammonium Bromide
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Abstract
The influence of adding salt on the
self-assembly in sodium octyl
sulfate (SOS)-rich mixtures of the anionic surfactant SOS and the
cationic surfactant hexadecyltrimethylammonium bromide (CTAB) have
been investigated with the two complementary techniques, small-angle
neutron scattering (SANS) and cryo-transmission electron microscopy. We are able to conclude that addition of a substantial amount of
inert salt, NaBr, mainly has three effects on the structural behaviors:
(i) the micelles become much larger at the transition from micelles
to bilayers, (ii) the fraction of bilayer disks increases at the expense
of vesicles, and (iii) bilayer aggregates perforated with holes are
formed in the most diluted samples. A novel form factor valid for
perforated bilayer vesicles and disks is introduced for the first
time and, as a result, we are able to directly observe the presence
of perforated bilayers by means of fitting SANS data with an appropriate
model. Moreover, we are able to conclude that the morphology of bilayer
aggregates changes according to the following sequence of different
bilayer topologies, vesicles → disks → perforated bilayers,
as the electrolyte concentration is increased and surfactant mole
fraction in the bilayer aggregates approaches equimolarity. We are
able to rationalize this sequence of transitions as a result of a
monotonous increase of the bilayer saddle-splay constant (<i>k̅</i><sub><i>c</i></sub><sup>bi</sup>) with decreasing influence from electrostatics,
in agreement with theoretical predictions as deduced from the Poisson–Boltzmann
theory