4484
2007, 111, 4484-4486 Published on Web 03/03/2007
Oriented Thiocyanate Anions at the Air-Electrolyte Interface and Its Implications on Interfacial Water - A Vibrational Sum Frequency Spectroscopy Study P. Viswanath† and H. Motschmann* Max Planck Institute of Colloids and Interfaces, Am Mu¨hlenberg 1, 14424, Golm/Potsdam, Germany ReceiVed: January 12, 2007; In Final Form: February 7, 2007
We used infrared-visible sum frequency spectroscopy (IR-VIS SFG) to study the interfacial composition and structure of aqueous potassium thiocyanate electrolyte solutions. The IR-VIS SFG spectra reveal an enrichment of the thiocyanate ions at the air-electrolyte interface. They also give access to the vibrational features of the interfacial water, which are affected by the presence of the ions. Polarization-dependent measurements have been used for a determination of the orientation of the pseudohalide anion. The combined data gives a picture of the interfacial architecture on a molecular scale.
Ion-water interactions at an interface play a decisive role in various physicochemical1-3 and biological processes.4 Before addressing the role of ions at the interface, it is desirable to understand their behavior in bulk. Femtosecond infrared pump probe,5 dielectric relaxation,6 and X-ray absorption spectroscopy7 experiments suggest that the bulk hydrogen-bonded network is not affected by structure-making or structure-breaking ions in solution. In contrast, this picture does not seem to hold at an interface. The traditional picture of the interface of aqueous electrolyte solutions is based on a thermodynamic analysis of the equilibrium surface tension isotherm. Usually, an electrolyte increases the surface tension, which is then interpreted as an interfacial zone depleted by ions.8 Recently, this picture has been challenged by molecular dynamics (MD) simulations using polarizable force fields, which predicted that soft ions such as halides are enriched at the interface with a nonmonotonic ion concentration profile.9 This finding is also supported by certain atmospheric reactions.3 The key to an understanding of this apparent contradiction lies in a reconsideration of the meaning of thermodynamics. There is no a priori prediction of a profile, and thermodynamics can accommodate several conflicting interfacial models provided that the integral excess or depletion is in accordance with the Gibbs equation. Therefore, direct experimental observations of molecular structure and energetics of ions in the interfacial region are required. Electrolytes at the interface are also the most simple system for studying ionspecific effects as reflected in the surface tension,10 surface potential,11 and bubble coalescence12 experiments. Considerable progress has been made recently on the experimental side, which was basically focused toward establishing the interface enrichment of the ions and its influence on the solvent structure (special issue Chem. ReV. 2006, 106). Vibrational sum frequency generation is a versatile nonlinear spectroscopy technique to study air-liquid interfaces. Under electric dipole approximation, the resonant signal arises from * Corresponding author. E-mail:
[email protected]. Fax: +49 331 567 9202. † E-mail:
[email protected].
10.1021/jp0702649 CCC: $37.00
Figure 1. Vibrational sum frequency spectra of water and the 1 M potassium thiocyanate solution under ssp polarization. The points and continuous lines represent the experimental data and fits, respectively.
Figure 2. Vibrational sum frequency spectra showing the CN stretch of the thiocyanate anion for the 1 M potassium thiocyanate solution under different polarization combinations. The points and continuous lines represent the experimental data and fits, respectively.
the noncentrosymmetric arrangement of the molecules at the interface and is absent in the centrosymmetric bulk. This has been used so far as a indirect probe to access the vibrational features of the solvent (water), which are perturbed in the presence of ions.13-16 To address and to correlate the effect of ions on water structure and vice versa at the interface, it is highly © 2007 American Chemical Society
Letters
J. Phys. Chem. C, Vol. 111, No. 12, 2007 4485
Figure 3. Dependence of simulated intensity ratio with respect to the orientation of the thiocyanate anion under different polarization configurations for a Gaussian distribution of different widths. The flat line denotes the experimentally measured value.
desirable to directly probe the ion and the interfacial water simultaneously, which exactly defines the goal of this paper. We have used IR-VIS SFG to establish the interface enrichment of the thiocyanate ions; polarization-dependent measurements yield their orientation along with the interfacial water signatures. The details of our IR-VIS SFG experimental setup, data fitting, and orientation analysis are described in the Supporting Information. Figure 1 shows the vibrational features of water and 1 molar (M) KSCN solution under ssp polarization. The sum frequency spectrum of water shows a broad region between 3000 and 3600 cm-1 and a narrow region around 3700 cm-1. The peaks at 3200 and 3450 cm-1 were attributed to ordered and disordered hydrogen-bonded networks resembling ice-like and liquid-like environments, respectively.17 The noticeable feature for the case of 1 M KSCN solution is the decrease of 3200 cm-1 and slight increase of 3400 cm-1 bands as reported for large and strongly polarizable anions.14,15 In addition, we observe a new feature at 3320 cm-1. This peak is attributed to tetrahedral configuration of water.15 The sharp feature around ∼3700 cm-1 is widely accepted to originate from the free or dangling OH facing toward air. The spectral assignment of hydrogen-bonded interfacial water is still an intense area of research, and two schools exist: one that relies on bulk sensitive spectroscopy techniques like ATR-IR and Raman,14 and the other uses IR-VIS SFG itself by exploiting the decoupling nature of OH from the OD stretch for H2O + D2O mixtures.15 For the purpose of this work, we have used three peaks to fit the spectra in the water range whose details are provided in the Supporting Information. To correlate the perturbation of solvent features due to the presence of ions at the interface, we investigate the vibrational signature from the anion. Figure 2 shows the CN vibrational stretch of the thiocyanate anion in all polarization combinations around 2065 cm-1 confirming its enrichment at the interface. Also, the intensity in ppp polarization is relatively stronger than ssp or sps polarizations, indicating constructive interference between the relevant nonlinear susceptibility components. Because ssp (sps) polarization probes the perpendicular (parallel) component of the transition dipole qualitatively, one can infer that the orientation of thiocyanate should lie intermediate at the air-electrolyte interface. By choosing different polarization combinations, we thus could deduce the orientation of the thiocyanate anion.18,19 Figure 3 shows the intensity ratio obtained for both ssp to ppp (Figure 3a) and ssp to sps polarizations (Figure 3b) in comparison with the experimentally measured value. On the basis of that, the orientation of the thiocyanate anion is found to be about 45°. To the best of our knowledge, this is the first report that demonstrates the applicability of the IR-VIS SFG technique on air-electrolyte solutions to probe directly the presence of an oriented anion in addition to its
implications on interfacial water signature. Petersen et al. studied thiocyanate at the air-water interface using second harmonic generation (SHG) exploiting the charge transfer to solvent transition and MD simulations.20 The combined analysis reveals the interface enrichment of thiocyanate and the existence of a Gibbs minimum assuming the orientation of thiocyanate to be flat at the interface. Our studies agree concerning the surface excess of the anion but strongly disagree on its orientation. These results can also be correlated directly with the surface potential, which becomes increasingly negative with increasing concentration of thiocyanate and also with the surface tension measurements where the slope is of lower magnitude when compared with other monovalent salts.11 Further studies exploring the concentration dependency of the anion signal and water features will be reported elsewhere. In summary, we have demonstrated the interface enrichment of thiocyanate by probing its vibrational features using the VSFG technique in addition to the interfacial water signature. Our polarization measurements and subsequent analysis yield the orientation of the thiocyanate ion to be around 45° at the interface. Our current study contributes toward better understanding of this biologically relevant chaotropic ion and water interactions at the interface. Further our work shows that the orientation of the anion is relevant and needs to be taken into account to get a full picture on the interfacial architecture. Acknowledgment. PV is grateful to a French-German network program on “Complex Fluids: From 3 to 2 dimensions” for a postdoctoral research fellowship. We thank Prof. Mo¨hwald for his continuous and steady support and stimulating discussions and G. Wienskol for the purification of the salts. Supporting Information Available: Experimental details, curve fitting, and orientation analysis. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Brown, G. E., Jr.; Henrich, V. E.; Casey, W. H.; Clark, D. L.; Eggleston, C.; Felmy, A.; Goodman, D. W.; Gratzel, M.; Maciel, G.; McCarthy, M. I.; Nealson, K. H.; Sverjensky, D. A.; Toney, M. F.; Zachara, J. M. Chem. ReV. 1999, 99, 77. (2) Kathmann, S. M.; Schenter, G. K.; Garrett, B. C. Phys. ReV. Lett. 2005, 94, 116104. (3) Finlayson-Pitts, B. J. Chem. ReV. 2003, 103, 4801. (4) Kunz, W.; Lo Nostro, P.; Ninham, B. W. Curr. Opin. Colloid Interface Sci. 2004, 9, 1. (5) Omta, A. W.; Kropman, M. F.; Woutersen, S.; Bakker, H. J. Science 2003, 301, 347. (6) Wachter, M.; Kunz, Buchner, R.; Hefter, G. J. Phys. Chem. A 2005, 109, 8675. (7) Naslund, L.; Edwards, D. C.; Wernet, P.; Bergmann, U.; Ogasawara, H.; Pettersson, L. G. M.; Myneni, S.; Nilsson, A. J. Phys. Chem. A 2005, 109, 5995.
4486 J. Phys. Chem. C, Vol. 111, No. 12, 2007 (8) Onsager, L.; Samaras, N. N. T. J. Chem. Phys. 1934, 2, 529. (9) Jungwirth, P.; Tobias, D. J. Phys. Chem. B 2001, 105, 10468. (10) Weissenborn, P. K.; Pugh, R. J. J. Colloid Interface Sci. 1996, 184, 550. (11) Jarvis, N. L.; Scheiman, M. A. J. Phys. Chem. 1968, 72, 74. (12) Craig, V. S. J.; Ninham, B. W.; Pashley, R. M. Nature 1993, 364, 317. (13) Shultz, M. J.; Schnitzer, C.; Simonelli, D.; Baldelli, S. Int. ReV. Phys. Chem. 2000, 19, 123. (14) Liu, D.; Ma, G.; Levering, L. M.; Allen, H. C. J. Phys. Chem. B 2004, 108, 2252.
Letters (15) Raymond, E. A.; Richmond, G. L. J. Phys. Chem. B 2004, 108, 5051. (16) Mucha, M.; Frigato, T.; Levering, L. M.; Allen, H. C.; Tobias, D. J.; Dang, L. X.; Jungwirth, P. J. Phys. Chem. B 2005, 109, 7617. (17) Du, Q.; Freysz, E.; Shen, Y. R. Phys. ReV. Lett. 1994, 72, 238. (18) Zhuang, X.; Miranda, P. B.; Kim, D.; Shen, Y. R. Phys. ReV. B 1999, 59, 12632. (19) Wang, H.-F.; Gan, W.; Lu, R.; Rao, Y.; Wu, B.-H. Int. ReV. Phys. Chem. 2005, 24, 191. (20) Petersen, P. B.; Saykally, R. J.; Mucha, M.; Jungwirth, P. J. Phys. Chem. B 2005, 109, 10915.
