Spectroscopy and modeling of aqueous interfaces (original) (raw)
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The Journal of Physical Chemistry B, 2006
The existence of polarizable anions at the outermost layer of electrolyte solutions has received much recent attention from both theory and experiment, but remains controversial. Anions can be probed directly in the UV via their strong charge-transfer-to-solvent (CTTS) transitions. We have recently described experimental characterizations of enhanced concentrations of several anions at the air-water interface, using the surfacespecific technique of second harmonic generation. Here we present a detailed description of the experimental design and methodology used in these experiments, as well as a proof of principle experiment with the known surfactant tetrabutylammonium iodide (TBAI), yielding surface enhancements in excellent agreement with surface tension measurements. Furthermore, we analyze the observed increase in the nonresonant contribution to the SHG response from the water background of alkali halide solutions. The observed change in the water structure of alkali halide (except iodide) solutions is linear in concentration and correlates with the fractional saturation concentration of the salt and with the ionic volume fraction. Finally, the surface adsorption of iodide at high bulk concentrations is analyzed, but it is not possible to differentiate between a Gibbs free energy of adsorption of zero (surface concentration proportional to the bulk) or -0.8 kcal/mol, as predicted by recent molecular dynamics simulations. . He is a member of the National Academy of Sciences, the recipient of over 50 honors and awards, the coauthor of over 300 publications, and has mentored over 50 PhD students.
On the Nature of Ions at the Liquid Water Surface
Annual Review of Physical Chemistry, 2006
Key Words surface enhancement, electrolytes, interfacial ions, second harmonic generation, Jones-Ray effect, Hofmeister series ■ Abstract A qualitatively new understanding of the nature of ions at the liquid water surface is emerging. Traditionally, the characterization of liquid surfaces has been limited to macroscopic experimental techniques such as surface tension and electrostatic potential measurements, wherein the microscopic picture then has to be inferred by applying theoretical models. Because the surface tension of electrolyte solutions generally increases with ion concentration, all inorganic ions have been thought to be repelled from the air-water interface, leaving the outermost surface layer essentially devoid of ions. This oversimplified picture has recently been challenged: first by chemical kinetics measurements, then by theoretical molecular dynamics simulations using polarizable models, and most recently by new surface sensitive experimental observations. Here we present an overview of the nature of the interfacial structure of electrolyte solutions and give a detailed description of the new picture that is emerging.
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 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.
Elucidating the mechanism of selective ion adsorption to the liquid water surface
Proceedings of the National Academy of Sciences, 2012
Adsorption of aqueous thiocyanate ions from bulk solution to the liquid/vapor interface was measured as a function of temperature by resonant UV second harmonic generation spectroscopy. The resulting adsorption enthalpy and entropy changes of this prototypical chaotrope were both determined to be negative. This surprising result is supported by molecular simulations, which clarify the microscopic origins of observed thermodynamic changes. Calculations reveal energetic influences of adsorbed ions on their surroundings to be remarkably local. Negative adsorption enthalpies thus reflect a simple repartitioning of solvent density among surface, bulk, and coordination regions. A different, and much less spatially local, mechanism underlies the concomitant loss of entropy. Simulations indicate that ions at the interface can significantly bias surface height fluctuations even several molecular diameters away, imposing restrictions consistent with the scale of measured and computed adsorpti...
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.
Confirmation of enhanced anion concentration at the liquid water surface
Chemical Physics Letters, 2004
The textbook view of the liquid electrolyte surfaces as being devoid of ions have recently been challenged by molecular dynamics simulations, which predict a surface enhancement of highly polarizable anions. Here we present the first direct experimental verification of this prediction. Enhanced azide ðN À 3 Þ concentrations were measured at the liquid surface by femtosecond second harmonic generation (SHG) experiments exploiting the charge-transfer-to-solvent (CTTS) resonance of N À 3 , yielding a surface excess free energy of À9.9 ± 0.3 kJ/mole. Such surface-enhanced concentrations of anions could have important consequences for the chemical reactions taking place on atmospheric aerosols and at the ocean-air interface.
Polarity of Liquid Interfaces by Second Harmonic Generation Spectroscopy
The Journal of Physical Chemistry A, 1997
A spectroscopic method, based on the interface selectivity of second-harmonic generation, is used to obtain the polarity of liquid interfaces. In this paper the second-harmonic measurement of the spectrum of the polarity indicator molecule N,N′-diethyl-p-nitroaniline (DEPNA) at the air/water interface demonstrates the method. Two different approaches are used to measure the intramolecular charge transfer (CT) absorption band position of DEPNA at the air/water interface. The DEPNA CT band blue-shifts from 429 nm in bulk water (polar solvent) to 359 nm in bulk hexane (nonpolar solvent) and 329 nm in the gas phase (no solvent). At the air/water interface, the charge transfer peak band maximum occurs at 373 nm, which indicates that the polarity of the air/water interface is similar to those of the bulk solvents carbon tetrachloride and butyl ether. The DEPNA results together with the results from another solvatochromic polarity indicator molecule, ET-(30), which will be reported elsewhere, show that the polarity results of the air/water interface are general.
1993
Resonant optical second harmonic generation (SHG) is obtained from monolayers of 4-n-octyloxybenzoic acid (OBA) and noctyl-4-hydroxybenzoate (OHB) adsorbed at the water/ 1,2dichloroethane interface. The polarization dependence of the SHG from pure monolayers of OBA and OHB indicates that the molecular axis of the OBA molecules is tilted 6" closer to the surface normal than that of the OHB molecules. The SHG from a series of mixed monolayers of OBA and OHB exhibits interference effects that demonstrate that the dipole moments of the coadsorbed molecules are oriented in opposite directions at the interface.