The self-assembly of an arylazopyrazole-based photosurfactant
(PS),
based on cetyltrimethylammonium bromide (CTAB), and its mixed micelle
formation with CTAB in aqueous solution was investigated by small
angle neutron and X-ray scattering (SANS/SAXS) and UV–vis absorption
spectroscopy. Upon UV light exposure, PS photoisomerizes from E-PS (trans) to Z-PS (cis), which transforms oblate ellipsoidal micelles into
smaller, spherical micelles with larger shell thickness. Doping PS
with CTAB resulted in mixed micelle formation at all stoichiometries
and conditions investigated; employing selectively deuterated PS,
a monotonic variation in scattering length density and dimensions
of the micellar core and shell is observed for all contrasts. The
concentration- and irradiance-dependence of the E to Z configurational transition was established
in both neat and mixed micelles. A liposome dye release assay establishes
the enhanced efficacy of photosurfactants at membrane disruption,
with E-PS exhibiting a 4-fold and Z-PS a 10-fold increase in fluorescence signal with respect to pure
CTAB. Our findings pave the way for external triggering and modulation
of the wide range of CTAB-based biomedical and material applications.
We examine the solution
structures in a mixed surfactant system
of sodium dodecyl sulfate (SDS) and
N
,
N
-dimethyldodecylamine
N
-oxide (DDAO) in water, on
both sides of the two-phase boundary, employing dynamic light scattering,
small-angle neutron scattering, and Fourier transform infrared spectroscopy.
The precipitate phase boundary was accessed by lowering pH to 8, from
its floating pH 9.5 value, and was experimentally approached from
the monomeric and micellar regions in three ways: at fixed DDAO or
SDS concentrations and at a fixed (70:30) SDS:DDAO molar ratio. We
characterize the size, shape, and interactions of micelles, which
elongate approaching the boundary, leading to the formation of disk-like
aggregates within the biphasic region, coexisting with micelles and
monomers. Our data, from both monomeric and micellar solutions, indicate
that the two phase structures formed are largely pathway-independent,
with dimensions influenced by both pH and mixed surfactant composition.
Precipitation occurs at intermediate stoichiometries with a similar
SDS:DDAO ratio, whereas asymmetric stoichiometries form a re-entrant
transition, returning to the mixed micelle phase. Overall, our findings
demonstrate the effect of stoichiometry and solution pH on the synergistic
interaction of mixed surfactants and their impact on phase equilibrium
and associated micellar and two-phase structures.
We investigate the thermodynamics of a highly interacting blend of poly(cyclohexyl methacrylate)/deuterated poly(styrene) (PCHMA/dPS) with small-angle neutron scattering (SANS). This system is experimentally challenging due to the proximity of the blend phase boundary (>200 °C) and degradation temperatures. To achieve the large wavenumber q-range and flux required for kinetic experiments, we employ a SANS diffractometer in time-of-flight (TOF) mode at a reactor source and ancillary microscopy, calorimetry, and thermal gravimetric analysis. Isothermal SANS data are well described by random-phase approximation (RPA), yielding the second derivative of the free energy of mixing (G″), the effective interaction (χ ̅ ) parameter, and extrapolated spinodal temperatures. Instead of the Cahn−Hilliard−Cook (CHC) framework, temperature (T)-jump experiments within the one-phase region are found to be well described by the RPA at all temperatures away from the glass transition temperature, providing effectively near-equilibrium results. We employ CHC theory to estimate the blend mobility and G″(T) conditions where such an approximation holds. TOF-SANS is then used to precisely resolve G″(T) and χ ̅ (T) during T-jumps in intervals of a few seconds and overall timescales of a few minutes. PCHMA/dPS emerges as a highly interacting partially miscible blend, with a steep dependence of G″(T) [mol/cm 3 ] = −0.00228 + 1.1821/T [K], which we benchmark against previously reported highly interacting lower critical solution temperature (LCST) polymer blends.
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