Measurement of Bromide Ion Affinities for the Air/Water and Dodecanol/Water Interfaces at Molar Concentrations by UV Second Harmonic Generation Spectroscopy (original) (raw)
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Spectroscopy and modeling of aqueous interfaces
Proceedings of the International School of Physics "Enrico Fermi", 2015
The development of deep-UV second-harmonic generation spectroscopy (SHG) for measuring the strong charge transfer to solvent (CTTS) transitions characteristic of all stable aqueous anions has provided a powerful new probe of water interfaces. By employing suitable models, quantitative thermodynamic results have been obtained for a number of fundamental electrolytes, which are generally in good agreement with theoretical calculations. Details of the experiments and models are described and salient results supporting a novel mechanism for the selective adsorption of ions to the air/water interface are reviewed.
Proceedings of the National Academy of Sciences, 2009
Recent experimental and theoretical results have firmly established the existence of enhanced concentrations of selected ions at the air/water interface. Ion adsorption to aqueous interfaces involving complex organic molecules is relevant to biology in connection with the familiar but incompletely understood Hofmeister effects. Here, we describe resonant UV second harmonic generation (SHG) studies of the strongly chaotropic thiocyanate ion adsorbed to the interface formed by water and a monolayer of dodecanol, wherein the Gibbs free energy of adsorption was determined to be −6.7 ± 1.1 and −6.3 ± 1.8 kJ/mol for sodium and potassium thiocyanate, respectively, coincident with the value determined for thiocyanate at the air/water interface. Interestingly, near 4 M and higher concentrations, the resonant SHG signal increases discontinuously, indicating a structural change in the interfacial region.
Journal of Physical Chemistry C, 2018
Molecular adsorption and orientation of an organic anion, p-nitrophenolate (p-NP −), at the air−aqueous interface have been studied using second harmonic generation techniques and surface tensiometry. It is shown that p-NP − not only adsorbs to the neat air−aqueous interface but also exhibits an orientational rearrangement at the surface. The average p-NP − orientation with respect to the surface normal is found to increase from ∼36 to ∼52°with increasing p-NP − surface coverage. Dipole−dipole and electrostatic interactions between p-NP − adsorbates are not the source of this orientation change. Exploration of this intriguing behavior in the presence of electrolytes revealed that interaction between inorganic ions and p-NP − augments its binding affinity and the observed orientation fluctuation. This study provides a critical understanding of the role halide, and plausibly hydroxide ions, play in influencing adsorbate orientation at an interface and thus paves the way to better elucidate chemical reactivity in sea salt aerosols and related environmental surfaces.
Observation of nitrate ions at the air/water interface by UV-second harmonic generation
Chemical Physics Letters, 2007
Nitrate ions have been observed by resonant second harmonic generation in the air/water interface of sodium nitrate solutions when the nitrate mole fraction exceeds $0.02 at ambient conditions. The surface propensity of nitrate is weak compared to those of other small anions determined by the same methodology, in qualitative agreement with recent simulations, experiments, and the position of nitrate in the Hofmeister anion series. The presence of nitrate ions in the interface of aqueous solutions has potential implications for chemical processes occurring in both natural and anthropogenic aqueous systems.
Characterizing Anion Adsorption to Aqueous Interfaces: TolueneWater versus AirWater
We continue our investigation of the behavior of simple ions at aqueous interfaces, employing the combination of two surface-sensitive nonlinear spectroscopy tools, broadband deep UV electronic sum-frequency generation and UV second harmonic generation, to characterize the adsorption of thiocyanate to the interface of water with toluenea prototypical hydrophobe. We find that both the interfacial spectrum and the Gibbs free energy of adsorption closely match results previously reported for the air−water interface. We observe no relative spectral shift in the higherenergy CTTS transition of thiocyanate, implying similar solvation environments for the two interfaces. Similarly, the Gibbs free energies of adsorption agree within error; however, we expect the respective enthalpic and entropic contributions to differ between the two interfaces, similar to our earlier findings for the air−water versus graphene−water interfaces. Further experiments and theoretical modeling are necessary to quantify the mechanistic differences.
Chemical transformation of bromine chloride at the air/water interface
Journal of Aerosol Science, 2001
The uptake of gaseous bromine chloride (BrCl) on aqueous surfaces was measured, as a function of temperature and liquid composition, using the droplet train technique coupled to a mass spectrometer. The uptake rate on pure water was too slow to be directly measured (i.e., (10\). In order to increase the uptake kinetics above the detection limit of the experimental setup , NaBr, NaI and NaOH were used as scavengers. The uptake kinetics on sodium bromide solutions were complicated by the interference of Cl and Br which are in equilibrium with gaseous BrCl. Therefore, we report results on the other two scavengers. On sodium hydroxide solutions, the uptake is driven by the bulk reactivity of BrCl toward OH\ yielding an average value of the mass accommodation coe$cients of 0.33$0.18 in the temperature range from 270}285 K. On sodium iodide solutions, the uptake kinetics were again driven by the bulk reactivity of the incoming bromine chloride but, in addition a surface reaction channel was found.
Analytical Sciences, 2008
The adsorbed film of 1-decyl-3-methylimidazolium bromide (DeMIMB) at the air/water interface was investigated employing the surface tension measurement and the x-ray absorption fine structure method under the total reflection condition (TRXAFS). From the surface tension measurement, the surface excess concentrations of ions were determined. From the XAFS measurement, two solvation states of bromide ion were found in the adsorbed film, which were assigned to be "free-Br" and "bound-Br". The hydration number of the former was estimated to be 6 while that of the latter was estimated to be 4. The results based on the XAFS analysis provided significant information on the formation of domains in the adsorbed film; the most conceivable situation is that the adsorbed molecules are definitely not homogeneously dispersed, but domains (islands or clusters) are dispersed in the adsorbed film. A regular and rather tight stacking of immidazolium rings may be formed in the domains.
Electron Spectroscopy of Aqueous Solution Interfaces Reveals Surface Enhancement of Halides
Science, 2005
It has been suggested that enhanced anion concentrations at the liquid/vapor interface of airborne saline droplets are important to aerosol reactions in the atmosphere. We report ionic concentrations in the surface of such solutions. Using x-ray photoelectron spectroscopy operating at near ambient pressure, we have measured the composition of the liquid/vapor interface for deliquesced samples of potassium bromide and potassium
In situ investigation of halide co-ion effects on SDS adsorption at air–water interfaces
Co-ions are believed to have a negligible effect on surfactant adsorption, but we show here that they can significantly affect the surfactant adsorption at the air-water interface. Sum frequency generation vibrational spectroscopy (SFG) was employed to examine the effects of three halides (Cl À , Br À and I À ) on the adsorption of an anionic surfactant, sodium dodecyl sulphate (SDS), at the air-water interface. The SFG spectral features of both the interfacial water molecules and the C-H vibrations of the adsorbed surfactant alkyl chains were analysed to characterize the surfactant adsorption. We demonstrate and compare the effects of the three halides, as well as explain the unusual effect of Br À , on the interfacial SDS and water molecules at the air/aqueous solution interface. It was observed that each of the three co-ions has a unique effect on the adsorption and conformation of the interfacial surfactant molecules at low halide concentrations of 10-50 mM, when the effect of halides on the interfacial water structure is believed to be negligible. This discovery implies that not only do they influence surfactant adsorption indirectly via the interfacial water network but also that there is an interaction occurring between these co-ions and the negatively charged head-groups at the interface via hydration by the interfacial water molecules. Even though this interaction/competition is likely to occur only between the surfactant headgroups and the halides, the surfactant hydrophobic tail was also observed to be influenced by the coions. These observed behavioural differences between the co-ions cannot be explained by the variation of charge densities. Therefore, further studies are required to determine the mode of action of halides influencing the adsorption of surfactant which gives Br À such a unique effect.