6 research outputs found
Chemical Composition and Properties of the LiquidâVapor Interface of Aqueous C1 to C4 Monofunctional Acid and Alcohol Solutions
The liquidâvapor interface
is playing an important role in aerosol and cloud chemistry in cloud
droplet activation by aerosol particles and potentially also in ice
nucleation. We have employed the surface sensitive and chemically
selective X-ray photoelectron spectroscopy (XPS) technique to examine
the liquidâvapor interface for mixtures of water and small
alcohols or small carboxylic acids (C1 to C4), abundant chemicals
in the atmosphere in concentration ranges relevant for cloud chemistry
or aerosol particles at the point of activation into a cloud droplet.
A linear correlation was found between the headgroup carbon 1s core-level
signal intensity and the surface excess derived from literature surface
tension data with the offset being explained by the bulk contribution
to the photoemission signal. The relative interfacial enhancement
of the carboxylic acids over the carboxylates at the same bulk concentration
was found to be highest (nearly 20) for propionic acid/propionate
and still about 5 for formic acid/formate, also in fair agreement
with surface tension measurements. This provides direct spectroscopic
evidence for high carboxylic acid concentrations at aqueous solutionâair
interfaces that may be responsible for acid catalyzed chemistry under
moderately acidic conditions with respect to their bulk aqueous phase
acidity constant. By assessing the ratio of aliphatic to headgroup
C 1s signal intensities XPS also provides information about the orientation
of the molecules. The results indicate an increasing orientation of
alcohols and neutral acids toward the surface normal as a function
of chain length, along with increasing importance of lateral hydrophobic
interactions at higher surface coverage. In turn, the carboxylate
ions exhibit stronger orientation toward the surface normal than the
corresponding neutral acids, likely caused by the stronger hydration
of the charged headgroup
Competition between Organics and Bromide at the Aqueous SolutionâAir Interface as Seen from Ozone Uptake Kinetics and Xâray Photoelectron Spectroscopy
A more detailed understanding of
the heterogeneous chemistry of
halogenated species in the marine boundary layer is required. Here,
we studied the reaction of ozone (O<sub>3</sub>) with NaBr solutions
in the presence and absence of citric acid (C<sub>6</sub>H<sub>8</sub>O<sub>7</sub>) under ambient conditions. Citric acid is used as a
proxy for oxidized organic material present at the ocean surface or
in sea spray aerosol. On neat NaBr solutions, the observed kinetics
is consistent with bulk reaction-limited uptake, and a second-order
rate constant for the reaction of O<sub>3</sub> + Br<sup>â</sup> is 57 ± 10 M<sup>â1</sup> s<sup>â1</sup>. On
mixed NaBrâcitric acid aqueous solutions, the uptake kinetics
was faster than that predicted by bulk reaction-limited uptake and
also faster than expected based on an acid-catalyzed mechanism. X-ray
photoelectron spectroscopy (XPS) on a liquid microjet of the same
solutions at 1.0 Ă 10<sup>â3</sup>â1.0 Ă 10<sup>â4</sup> mbar was used to obtain quantitative insight into
the interfacial composition relative to that of the bulk solutions.
It revealed that the bromide anion becomes depleted by 30 ± 10%
while the sodium cation gets enhanced by 40 ± 20% at the aqueous
solutionâair interface of a 0.12 M NaBr solution mixed with
2.5 M citric acid in the bulk, attributed to the role of citric acid
as a weak surfactant. Therefore, the enhanced reactivity of bromide
solutions observed in the presence of citric acid is not necessarily
attributable to a surface reaction but could also result from an increased
solubility of ozone at higher citric acid concentrations. Whether
the acid-catalyzed chemistry may have a larger effect on the surface
than in the bulk to offset the effect of bromide depletion also remains
open
Shape and Confinement Effects of Various Terminal Siloxane Groups on Supramolecular Interactions of Hydrogen-Bonded Bent-Core Liquid Crystals
To investigate the shape and confinement
effects of the terminal
siloxane groups on the self-assembled behavior of molecular arrangements
in hydrogen-bonded (H-bonded) bent-core complexes, four H-bonded bent-core
complexes <b>S1</b>, <b>P1</b>, <b>C4</b>, and <b>P8</b> with string-, ring-, and cage-like siloxane termini (i.e.,
linear siloxane unit âSiâOâSiâOâSiâ,
cyclic siloxane unit (SiâO)<sub>4</sub>, and silsesquioxane
unit POSS, respectively) were synthesized and investigated. By X-ray
diffraction measurements, different types of SmCG (B8) phases and
leaning angles were controlled by the shape effect of the string-
and cage-like siloxane termini for <b>S1</b> and <b>P1</b> (with only one arm of H-bonded bent-core), respectively. In addition,
the confinement effect of <b>P1</b>, <b>C4</b>, and <b>P8</b> (accompanied by increasing the numbers of attached H-bonded
bent-core arms) resulted in higher transition temperatures and the
diminishing of mesophasic ranges (even the disappearance of mesophase
in <b>P8</b>). Moreover, AFM images showed the bilayer smectic
CG phases of <b>S1</b> and <b>P1</b> were aligned to reveal
highly ordered thread-like structures by a DC field. By spontaneous
polarization measurements within the mesophasic ranges, <b>S1</b> and <b>P1</b> showed ferroelectric transitions but <b>C4</b> displayed antiferroelectricity. Finally, the electro-optical performance
of B8 phases could be optimized through binary mixtures of <b>S1</b> and <b>P1</b>, and a well aligned modulated ribbon phase could
be formed via specific molar ratios of the binary mixtures
LiquidâVapor Interface of Formic Acid Solutions in Salt Water: A Comparison of Macroscopic Surface Tension and Microscopic in Situ Xâray Photoelectron Spectroscopy Measurements
The liquidâvapor interface
is difficult to access experimentally
but is of interest from a theoretical and applied point of view and
has particular importance in atmospheric aerosol chemistry. Here we
examine the liquidâvapor interface for mixtures of water, sodium
chloride, and formic acid, an abundant chemical in the atmosphere.
We compare the results of surface tension and X-ray photoelectron
spectroscopy (XPS) measurements over a wide range of formic acid concentrations.
Surface tension measurements provide a macroscopic characterization
of solutions ranging from 0 to 3 M sodium chloride and from 0 to over
0.5 mole fraction formic acid. Sodium chloride was found to be a weak
salting out agent for formic acid with surface excess depending only
slightly on salt concentration. In situ XPS provides a complementary
molecular level description about the liquidâvapor interface.
XPS measurements over an experimental probe depth of 51 Ă
gave
the C 1s to O 1s ratio for both total oxygen and oxygen from water.
XPS also provides detailed electronic structure information that is
inaccessible by surface tension. Density functional theory calculations
were performed to understand the observed shift in C 1s binding energies
to lower values with increasing formic acid concentration. Part of
the experimental â0.2 eV shift can be assigned to the solution
composition changing from predominantly monomers of formic acid to
a combination of monomers and dimers; however, the lack of an appropriate
reference to calibrate the absolute BE scale at high formic acid mole
fraction complicates the interpretation. Our data are consistent with
surface tension measurements yielding a significantly more surface
sensitive measurement than XPS due to the relatively weak propensity
of formic acid for the interface. A simple model allowed us to replicate
the XPS results under the assumption that the surface excess was contained
in the top four angstroms of solution
Xâray Reflectivity Studies on the Mixed LangmuirâBlodgett Monolayers of Thiol-Capped Gold Nanoparticles, Dipalmitoylphosphatidylcholine, and Sodium Dodecyl Sulfate
LangmuirâBlodgett
monolayers of thiolated gold nanoparticles
mixed with dipalmitoylphosphatidylcholine/sodium dodecyl sulfate (DPPC/SDS)
were investigated by combining the X-ray reflectivity, grazing-incident
scattering, and TEM analyses to reveal the in-depth and in-plane organization
and the 2D morphology of such mixed monolayers. It was found that
the addition of a charged single-tail surfactant to the thiolated
Au nanoparticle monolayer helps to stabilize the Au nanoparticle monolayer
and to strengthen the mechanical property of the mixed monolayer film.
For mixing with lipids, it was found that the thiolated gold nanoparticles
could be pushed on top of the lipid monolayer when the mixed monolayer
is compressed. At a typical comparable total surface area ratio of
gold nanoparticle to lipid, the thiolated gold nanoparticles could
form a uniform domain on top of the DPPC monolayer. When there are
more thiolated gold nanoparticles than that could be supported by
the lipid monolayer, domain overlapping could occur to form bilayer
gold nanoparticle domains at some regions. At low total surface area
ratio of thiolated gold nanoparticle to lipid, the thiolated gold
nanoparticles tend to form a connected threadlike aggregation structure.
Evidently, the morphology of the thiolated gold nanoparticle monolayer
is highly depending on the total surface area ratio of the thiolated
gold nanoparticle to lipid. SDS is found to have a dispersion power
capable of dispersing the originally uniform Au-8C nanoparticle domain
of the mixed Au-8C/DPPC monolayer into a foamlike structure for the
mixed Au-8C/SDS/DPPC monolayer. It is evident that not only the concentration
ratio but also the size and shape of the template formed by the amphiphilic
molecules and their interaction with the thiolated gold nanoparticles
can all have great effects on the organizational structure as well
as morphology of the thiolated gold nanoparticle monolayer
The Penetration Depth for Hanatoxin Partitioning into the Membrane Hydrocarbon Core Measured with Neutron Reflectivity
Hanatoxin (HaTx)
from spider venom works as an inhibitor of Kv2.1
channels, most likely by interacting with the voltage sensor (VS).
However, the way in which this water-soluble peptide modifies the
gating remains poorly understood as the VS is deeply embedded within
the bilayer, although it would change its position depending on the
membrane potential. To determine whether HaTx can indeed bind to the
VS, the depth at which HaTx penetrates into the POPC membranes was
measured with neutron reflectivity. Our results successfully demonstrate
that HaTx penetrates into the membrane hydrocarbon core (âŒ9
Ă
from the membrane surface), not lying on the membraneâwater
interface as reported for another voltage sensor toxin (VSTx). This
difference in penetration depth suggests that the two toxins fix the
voltage
sensors at different positions with respect to the membrane normal,
thereby explaining their different inhibitory effects on the channels.
In particular, results from MD simulations constrained by our penetration
data clearly demonstrate an appropriate orientation for HaTx to interact
with the membranes, which is in line with the biochemical information
derived from stopped-flow analysis through delineation of the toxinâVS
binding interface