Variation in Average Molecular Orientation of an Organic Anion at the Air–Aqueous Interface: Influence of Halide Ions (original) (raw)
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The Journal of Chemical Physics, 2009
Here we report the polarization dependent non-resonant second harmonic generation (SHG) measurement of the interfacial water molecules at the aqueous solution of the following salts: NaF, NaCl, NaBr, KF, KCl, and KBr. Through quantitative polarization analysis of the SHG data, the orientational parameter D,(D= cosθ / cos 3 θ) value and the relative surface density of the interfacial water molecules at these aqueous solution surfaces were determined. From these results we found that addition of each of the six salts caused increase of the thickness of the interfacial water layer at the surfaces to a certain extent. Noticeably, both the cations and the anions contributed to the changes, and the abilities to increase the thickness of the interfacial water layer were in the following order: KBr > NaBr > KCl > NaCl ∼ NaF > KF. Since these changes can not be factorized into individual anion and cation contributions, there are possible ion pairing or association effects, especially for the NaF case. We also found that the orientational parameter D values of the interfacial water molecules changed to opposite directions for the aqueous solutions of the three sodium salts versus the aqueous solutions of the three potassium salts. These findings clearly indicated unexpected specific Na + and K + cation effects at the aqueous solution surface. These effects were not anticipated from the recent molecular dynamics (MD) simulation results, which concluded that the Na + and K + cations can be treated as small non-polarizable hard ions and they are repelled from the aqueous interfaces. These results suggest that the electrolyte aqueous solution surfaces are more complex than the currently prevalent theoretical and experimental understandings.
The journal of physical chemistry letters, 2016
The surface-active ions tetraphenylarsonium (Ph4As(+)) and tetraphenylboron (Ph4B(-)) have a similar structure but opposite charge. At the solution-air interface, the two ions affect the surface tension in an identical manner, yet sum-frequency generation (SFG) spectra reveal an enhanced surface propensity for Ph4As(+) compared with Ph4B(-), in addition to opposite alignment of interfacial water molecules. At the water-oil interface, the interfacial tension is 7 mN/m lower for Ph4As(+) than for Ph4B(-) salts, but this can be fully accounted for by the different bulk solubility of these ions in the hydrophobic phase, rather than inherently different surface activities. The different solubility can be accounted for by differences in electronic structure, as evidenced by quantum chemical calculations and NMR studies. Our results show that the surface propensity concluded from SFG spectroscopy does not necessarily correlate with interfacial adsorption concluded from thermodynamic measur...
The Journal of Physical Chemistry C, 2010
Recent experimental and theoretical work has demonstrated that certain anions can exhibit enhanced concentrations at aqueous interfaces and that the adsorption of bromide is particularly important for chemical reactions on atmospheric aerosols, including the depletion of ozone. UV second harmonic generation resonant with the bromide charge-transfer-to-solvent band and a Langmuir adsorption model are used to determine the affinity of bromide for both the air/water and dodecanol/water interfaces. The Gibbs free energy of adsorption for the former is determined to be-1.4 kJ/mol with a lower 90% confidence limit of-4.1 kJ/mol. For the dodecanol/water interface the data are best fit with a Gibbs free energy of +8 kJ/mol with an estimated lower limit of-4 kJ/mol.
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.
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.
The Journal of Physical Chemistry C, 2015
We report on halide ion (Cl − , Br − , I − ) adsorption from the subphase water to a cationic Langmuir monolayer consisting of 1,2-dipalmitoyl-3-trimethylammonium-propane (DPTAP) molecules at the air/water interface. Reductions in the water OH signal of sum-frequency spectra and the surface pressure of the π−A isotherm follow the order of the anion size, indicating preferable adsorption of the larger anions to better screen the surface charge of the DPTAP monolayer. Complementary X-ray fluorescence measurements of DPTAP on Cl − and I − reveal that the integrated number of adsorbed ions within the probing depth (6−8 nm) is the same for both ions. Incorporating the above outcomes leads to the contrasting adsorption structures that the larger halide anions (I − ) are directly adsorbed to the headgroup strata, while the Cl − ions form a more diffusive distribution contiguous to the monolayer. Our study shows that the length scale over which ions neutralize a charged interface varies significantly and specifically even for monovalent ions.
2012
We demonstrate that the driving forces for ion adsorption to the air−water interface for point charge models result from both cavitation and a term that is of the form of a negative electrochemical surface potential. We carefully characterize the role of the free energy due to the electrochemical surface potential computed from simple empirical models and its role in ionic adsorption within the context of dielectric continuum theory. Our research suggests that the electrochemical surface potential due to point charge models provides anions with a significant driving force for adsoprtion to the air−water interface. This is contrary to the results of ab initio simulations that indicate that the average electrostatic surface potential should favor the desorption of anions at the air−water interface. The results have profound implications for the studies of ionic distributions in the vicinity of hydrophobic surfaces and proteins.