8 research outputs found
Influence of Film Composition on the Morphology, Mechanical Properties, and Surfactant Recovery of Phase-Separated Phospholipid-Perfluorinated Fatty Acid Mixed Monolayers
Monolayer surfactant films composed of a mixture of phospholipids
and perfluorinated (or partially fluorinated) surfactants are of potential
utility for applications in pulmonary lung surfactant-based therapies.
As a simple, minimal model of such a lung surfactant system, binary
mixed monolayer films composed of 1,2-dipalmitoyl-<i>sn</i>-glycero-3-phosphocholine (DPPC) and perfluorooctadecanoic acid (C18F)
prepared on a simplified lung fluid mimic subphase (pH 7.4, 150 mM
NaCl) have been characterized in terms of mixing thermodynamics and
compressibility (measured through π–<i>A</i> compression isotherms), film morphology (via atomic force, fluorescence,
and Brewster angle microscopy), as well as spreading rate and hysteresis
response to repeated expansion–contraction cycles for a variety
of compositions of mixed films. Under all mixing conditions, films
and their components were found to be completely immiscible and phase-separated,
though there were significant changes in the aforementioned film properties
as a function of composition. Of particular note was the existence
of a maximum in the extent of immiscibility (characterized by Δ<i>G</i><sub>ex</sub><sup>π</sup> values) and enhanced surfactant recovery during hysteresis experiments
at χ<sub>C18F</sub> ≥ 0.30. The latter was attributed
to the relatively rapid respreading rate of the perfluorinated amphiphile
in comparison with DPPC alone at the air–water interface, which
enhances the performance of this mixture as a potential pulmonary
lung surfactant. Further, monolayer film structure could be tracked
dynamically as a function of compression at the air–water interface
via Brewster angle microscopy, with the C18F component being preferentially
squeezed out of the film with compression, but returning rapidly upon
re-expansion. In general, addition of C18F to DPPC monolayers resulted
in improvements to mechanical, structural, and respreading properties
of the film, indicating the potential value of these compounds as
additives to pulmonary lung surfactant formulations
Influence of Film Composition on the Morphology, Mechanical Properties, and Surfactant Recovery of Phase-Separated Phospholipid-Perfluorinated Fatty Acid Mixed Monolayers
Monolayer surfactant films composed of a mixture of phospholipids
and perfluorinated (or partially fluorinated) surfactants are of potential
utility for applications in pulmonary lung surfactant-based therapies.
As a simple, minimal model of such a lung surfactant system, binary
mixed monolayer films composed of 1,2-dipalmitoyl-<i>sn</i>-glycero-3-phosphocholine (DPPC) and perfluorooctadecanoic acid (C18F)
prepared on a simplified lung fluid mimic subphase (pH 7.4, 150 mM
NaCl) have been characterized in terms of mixing thermodynamics and
compressibility (measured through π–<i>A</i> compression isotherms), film morphology (via atomic force, fluorescence,
and Brewster angle microscopy), as well as spreading rate and hysteresis
response to repeated expansion–contraction cycles for a variety
of compositions of mixed films. Under all mixing conditions, films
and their components were found to be completely immiscible and phase-separated,
though there were significant changes in the aforementioned film properties
as a function of composition. Of particular note was the existence
of a maximum in the extent of immiscibility (characterized by Δ<i>G</i><sub>ex</sub><sup>π</sup> values) and enhanced surfactant recovery during hysteresis experiments
at χ<sub>C18F</sub> ≥ 0.30. The latter was attributed
to the relatively rapid respreading rate of the perfluorinated amphiphile
in comparison with DPPC alone at the air–water interface, which
enhances the performance of this mixture as a potential pulmonary
lung surfactant. Further, monolayer film structure could be tracked
dynamically as a function of compression at the air–water interface
via Brewster angle microscopy, with the C18F component being preferentially
squeezed out of the film with compression, but returning rapidly upon
re-expansion. In general, addition of C18F to DPPC monolayers resulted
in improvements to mechanical, structural, and respreading properties
of the film, indicating the potential value of these compounds as
additives to pulmonary lung surfactant formulations
Morphology and Composition of Structured, Phase-Separated Behenic Acid–Perfluorotetradecanoic Acid Monolayer Films
The phase separation
of immiscible surfactants in mixed monolayer
films provides an approach to physically manipulate important properties
of thin films, including surface morphology, microscale composition,
and mechanical properties. In this work, we predict, based upon existing
miscibility studies and their thermodynamic underpinnings described
in the literature, the miscibility and film morphology of mixed monolayers
comprised of behenic acid (C<sub>21</sub>H<sub>43</sub>COOH) and perfluorotetradecanoic
acid (C<sub>13</sub>F<sub>27</sub>COOH) in various molar ratios. Predictions
are tested using a combination of experimental surface characterization
methods for probing miscibility and film morphology at the solid/air
and air/water interfaces. Film components were immiscible and phase-separated
into chemically well-defined domains under a variety of experimental
conditions, with monolayer morphology consistent with initial predictions.
The extensibility of these basic predictions to other systems is discussed
in the context of using these works for different perfluorinated surfactant
molecules
Efficiency of Noncoherent Photon Upconversion by Triplet–Triplet Annihilation: The C60 Plus Anthanthrene System and the Importance of Tuning the Triplet Energies
As
part of a continuing effort to find noncoherent photon upconversion
(NCPU) systems with improved energy conversion efficiencies, the photophysics
of the blue emitter, anthanthrene (An), and the fullerene absorber–sensitizer,
C<sub>60</sub>, have been examined by both steady-state and pulsed
laser techniques. An is a promising candidate for NCPU by homomolecular
triplet–triplet annihilation (TTA) because its triplet state
lies ∼800 cm<sup>–1</sup> below the triplet energy of
the C<sub>60</sub> donor (thereby improving efficiency by reducing
back triplet energy transfer), and its fluorescent singlet state lies
in near resonance with double its triplet energy (thus minimizing
thermal energy losses in the annihilation process). In fluid solution,
efficient triplet–triplet donor–acceptor energy transfer
is observed, and rate constants for homomolecular TTA in the An acceptor
are estimated to approach the diffusion limit. NCPU is also observed
in An + C<sub>60</sub> in polyÂ(methylmethacrylate) thin films
Photophysics of Zinc Porphyrin Aggregates in Dilute Water–Ethanol Solutions
Dimeric and multimeric aggregates
of a model metalloporphyrin,
zinc tetraphenylporphyrin (ZnTPP), have been produced in a controlled
manner by incrementally increasing the water content of dilute aqueous
ethanol solutions. Steady state absorption, fluorescence emission,
and fluorescence excitation spectra have been measured to identify
the aggregates present as a function of solvent composition. The dynamics
of the excited states of the aggregates produced initially by excitation
in the Soret region have been measured by ultrafast fluorescence upconversion
techniques. Only the monomer produces measurable emission from S<sub>2</sub> with a picosecond lifetime; all Soret-excited aggregates,
including the dimer, decay radiationlessly on a femtosecond time scale.
The S<sub>1</sub> state is the only significant product of the radiationless
decay of the S<sub>2</sub> state of the excited monomer, and the aggregates
also produce substantial quantum yields of S<sub>1</sub> fluorescence
when initially excited in the Soret region. The resulting fluorescent
aggregates all decay on a subnanosecond time scale, likely by a mechanism
that involves dissociation of the excited monomer from the excitonic
multimer. The ZnTPP dimers excited at their ground state geometries
in the Soret region exhibit a dynamic behavior that is quite different
from those produced following noncoherent triplet–triplet annihilation
under the same conditions. The important implications of these observations
in determining the aggregation conditions promoting efficient photon
upconversion by excitonic annihilation in a variety of media are thoroughly
discussed
Mixing Behavior in Binary Anionic Gemini Surfactant–Perfluorinated Fatty Acid Langmuir Monolayers
The
miscibility and film structure of mixed Langmuir monolayer
films composed of an anionic gemini <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-dialkyl-<i>N</i>,<i>N</i>′-diacetate ethylenediamine surfactant
(Ace(12)-2-Ace(12)) with perfluoroÂtetradecanoic acid (C<sub>13</sub>F<sub>27</sub>COOH; PF) have been investigated using a variety
of thermodynamic and structural characterization methods. The two
film components were found to be miscible in monolayers at the air–water
interface over a range of compositions and at all but the lowest surface
pressures, with attractive interactions occurring between the two
components. While pure PF monolayers formed crystalline lattices with
hexagonal symmetry and with the surfactant tails oriented normal to
the underlying water subphase, the pure gemini surfactant formed amorphous
films with little tendency to orient at the subphase. In mixed films
with mole ratios of PF:Ace(12)-2-Ace(12) < 2.5, the miscibility
of the two components resulted in a nearly complete loss of crystallinity
of the PF, though films at higher mole fractions of PF showed some
residual crystallinity, albeit with lattice structures that were significantly
different from that of pure PF. Miscibility and film structure in
this mixed system are discussed in comparison with other mixed gemini
surfactant systems in the literature as well as binary mixtures of
phospholipids or monomeric fatty acids with PF
Spectroscopic and Structural Studies of a Surface Active Porphyrin in Solution and in Langmuir–Blodgett Films
Controlling aggregation of the dual
sensitizer–emitter (S-E)
zinc tetraphenylporphyrin (ZnTPP) is an important consideration in
solid state noncoherent photon upconversion (NCPU) applications. The
Langmuir–Blodgett (LB) technique is a facile means of preparing
ordered assemblies in thin films to study distance-dependent energy
transfer processes in S-E systems and was used in this report to control
the aggregation of a functionalized ZnTPP on solid substrates. This
was achieved by synthetic addition of a short polar tail to one of
the pendant phenyl rings in ZnTPP in order to make it surface active.
The surface active ZnTPP derivative formed rigid films at the air–water
interface and exhibited mean molecular areas consistent with approximately
vertically oriented molecules under appropriate film compression.
A red shift in the UV–vis spectra as well as unquenched fluorescence
emission of the LB films indicated formation of well-ordered aggregates.
However, NCPU, present in the solution phase, was not observed in
the LB films, suggesting that NCPU from ZnTPP as a dual S-E required
not just a controlled aggregation but a specific orientation of the
molecules with respect to each other
Photophysics of Soret-Excited Tin(IV) Porphyrins in Solution
The photophysics of low-chlorin tinÂ(IV)
tetraphenylporphyrin dihydroxide,
a core building block for axially substituted supramolecular tin porphyrin
constructs, has been studied in a variety of hydrogen-bonding, nonpolar,
and aprotic polar solvents using steady-state, nanosecond, and femtosecond
time-resolved emission, and femtosecond time-resolved absorption methods.
In hydrogen-bonding solvents the metalloporphyrin exists as solvated
monomers, and its Soret-excited S<sub>2</sub> state in these solvents
exhibits the expected linear energy gap law relationship with first-order
population decay times in the 0.8 to 1.7 ps range. Evidence is presented
that this metalloporphyrin aggregates in other solvents at the concentrations
typically used for these ultrafast measurements and yields species-averaged
time-resolved data. Cw laser excitation in the Q-band under deaerated
conditions produces weak S<sub>2</sub>–S<sub>0</sub> fluorescence
(photon upconversion) as a result of triplet–triplet annihilation