Observation of nitrate ions at the air/water interface by UV-second harmonic generation
Graphical abstract
The observation of nitrate anions within the air/water interface is reported through the use of UV-second harmonic generation spectroscopy.
Introduction
A growing body of recent experimental and theoretical results is challenging the traditional view that small inorganic ions are excluded from the air/water interface [1]. This rapidly evolving picture of interfacial electrolyte partitioning is developing through the interpretation of increasingly sophisticated simulations [2], [3], [4], [5], [6], and through the application of more selective and innovative experimental techniques [7], [8], [9], [10], [11]. Recent simulations and experiments indicate both the presence and concentration enhancement of specific anions, including I−, , SCN−, , and even hydronium, at the air/solution interface [9], [10], [12], [13], [14]. In some remarkable cases (e.g. I− and , strong interfacial adsorption appears even in the dilute (millimolar) regime where one expects the Wagner–Onsager–Samaras (WOS) theory to hold, as well as at higher (molar) concentrations, which are technically beyond the scope of WOS theory [12], [13]. We refer the reader to recent reviews [15], [16], [17], [18] for further details.
While this departure from classical electrolyte behavior is undoubtedly due to the neglect of specific molecular properties (e.g. size, polarizability, dehydration energy, surface charge density, etc.) in WOS theory and its adaptations, the strongest current arguments as to which specific effects drive ions to an interface against the repulsion of image charges are derived predominantly from the microscopic pictures generated through computational modeling. Jungwirth and Tobias have demonstrated the propensity of highly polarizable anions for the outermost liquid layer [3], [19], where induction forces are implicated as playing the significant role [20]. Archontis and Leontidis have also suggested that hydrophobic (steric exclusion) forces are also significant in driving larger atomic ions such as iodide to the interface [21], while more recent work points to dehydration energy and/or surface charge density as an alternative order parameter for surface activity [11], [22]. It is noted by Pegram and Record in their analysis that this order parameter is valid only across the anion series and take this as evidence that anion and cation surface propensities are thus driven by different mechanisms. Much of the more recent results and conclusions on this topic are reviewed by Collins et al. [23].
While the investigation of chemical forces driving ion surface enhancement is ongoing, efforts in our group are also focused on expanding the experimental data to test some of the most ubiquitous and environmentally relevant ions as to their surface activity. To this end, we report here the results of a study of the interfacial aqueous nitrate anion that, like chloride, bromide and sulfate, is among the most abundant charged species present in atmospheric aerosols and terrestrial water sources. Excepting the vibrational sum-frequency generation (VSFG) of Schnitzer et al. [24], and the mass-spectrometric/electro-spray study of Cheng et al. [11], little experimental investigation of the nitrate anion at the air/water interface has been performed, and to the best of our knowledge, no direct experimental detection of the nitrate anion at the air/water interface has previously been performed. Two polarizable MD simulations have produced conflicting results concerning the propensity of the nitrate anion to the interface [5], [25]. Dang et al.’s model suggests low surface affinity, while Salvador et al.’s model suggests a higher propensity. The differing results are possibly attributable to the different localization and magnitudes of polarizable centers used in the two simulations [25]. A very recent simulation by Thomas et al. [26] concludes that the nitrate ion resides primarily in the bulk, but does exhibit a low probability to exist in the interfacial region. In the present study, we directly probe the relative number density of the aqueous nitrate anion in the interfacial region as a function of bulk sodium nitrate concentration as a means to establish the surface affinity of the nitrate anion in water via resonant UV-SHG experiments.
Section snippets
Experimental
Our procedure follows that described elsewhere [12], and only the salient and altered details are presented here. The samples were prepared fresh from >99% purity sodium nitrate dissolved in 18.2 MΩ water from a Millipore Milli-Q A-10 system with <4 ppb total organic content. The water was sparged with nitrogen for at least 1 h before use. The various concentrations studied were generated by successive dilutions of either stock 6 M or 1 M sodium nitrate solutions, following a rigorous protocol for
Results and discussion
The intensity of the second harmonic response is described in terms of the number of nitrate molecules present at the surface by [13]wherein Ni is the number of species i, and 〈βi〉 is the molecular hyperpolarizability of species i averaged over molecular orientations. As an even-order process under the dipole approximation, this average is zero for centrosymmetric media, such as within the bulk solution [28]. At the air/water interface, this inversion
Conclusions
Qualitatively, this study establishes that appreciable concentrations of the nitrate anion do reside in the interfacial region at bulk nitrate mole fractions greater than 0.02, and thus provides another example of a small inorganic anion that is present at the air/water interface. While our results demonstrate that the nitrate anion concentration is not strongly enhanced at the aqueous interface, and the presence of nitrate in the interface significantly perturbs the interfacial water
Acknowledgments
This work is supported by the Experimental Physical Chemistry Division of the National Science Foundation. P.B.P. was also funded by the Danish Research Training Council.
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