Controlling
the Mesostructure Formation within the Shell of Novel Cubic/Hexagonal
Phase Cetyltrimethylammonium Bromide–Poly(acrylamide-acrylic
acid) Capsules for pH Stimulated Release
The self-assembly of ordered structures
in mixtures of oppositely charged surfactant and polymer systems has
been exploited in various cleaning and pharmaceutical applications
and continue to attract much interest since their discovery in the
late twentieth century. The ability to control the electrostatic and
hydrophobic interactions that dictate the formation of liquid crystalline
phases in these systems is advantageous in manipulation of structure
and rendering them responsive to external stimuli. Nanostructured
capsules comprised of the cationic surfactant, cetyltrimethylammonium
bromide (CTAB), and the diblock copolymer poly(acrylamide-acrylic
acid) (PAAm-AA) were prepared to assess their potential as pH responsive
nanomaterials. Crossed-polarizing light microscopy (CPLM) and small-angle
X-ray scattering (SAXS) identified coexisting <i>Pm</i>3<i>n</i> cubic and hexagonal phases at the surfactant–polymer
interface. The hydrophobic and electrostatic interactions between
the oppositely charged components were studied by varying temperature
and solution pH, respectively, and were found to influence the liquid
crystalline nanostructure formed. The lattice parameter of the mesophases
and the fraction of cubic phase in the system decreased upon heating.
Acidic conditions resulted in the loss of the highly ordered structures
due to protonation of the carboxylic acid group, and subsequent reduction
of attractive forces previously present between the oppositely charged
molecules. The rate of release of the model hydrophilic drug, Rhodamine
B (RhB), from nanostructured macro-sized capsules significantly increased
when the pH of the solution was adjusted from pH 7 to pH 2. This allowed
for immediate release of the compound of interest “on demand”,
opening new options for structured materials with increased functionality
over typical layer-by-layer capsules
