6 research outputs found

    Chemical Composition and Properties of the Liquid–Vapor Interface of Aqueous C1 to C4 Monofunctional Acid and Alcohol Solutions

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    The liquid–vapor 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 liquid–vapor 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 solution–air 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 Solution–Air Interface as Seen from Ozone Uptake Kinetics and X‑ray Photoelectron Spectroscopy

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    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>–</sup> is 57 ± 10 M<sup>–1</sup> s<sup>–1</sup>. On mixed NaBr–citric 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 × 10<sup>–3</sup>–1.0 × 10<sup>–4</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 solution–air 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

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    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 −Si–O–Si–O–Si–, cyclic siloxane unit (Si–O)<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

    Liquid–Vapor Interface of Formic Acid Solutions in Salt Water: A Comparison of Macroscopic Surface Tension and Microscopic in Situ X‑ray Photoelectron Spectroscopy Measurements

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    The liquid–vapor 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 liquid–vapor 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 liquid–vapor interface. XPS measurements over an experimental probe depth of 51 Å 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 −0.2 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

    X‑ray Reflectivity Studies on the Mixed Langmuir–Blodgett Monolayers of Thiol-Capped Gold Nanoparticles, Dipalmitoylphosphatidylcholine, and Sodium Dodecyl Sulfate

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    Langmuir–Blodgett 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

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    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 (∌9 Å from the membrane surface), not lying on the membrane–water 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 toxin–VS binding interface
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