Surface Electric Fields of Aqueous Solutions of NH₄NO₃, Mg(NO₃)₂, NaNO₃, and LiNO₃: Implications for Atmospheric Aerosol Chemistry

Abstract

Ammonium (NH 4 + ), magnesium (Mg 2+ ), sodium (Na + ), and nitrate (NO 3 − ) ions are common constituents of ocean waters and found in abundance in marine atmospheric aerosols. Revealing the surface propensity (surface activity) of relevant ions provides insight into heterogeneous aerosol processes and potential impact on atmospheric chemistry. However, there is sparse surface data on NH 4 + , Mg 2+ , and little consensus for nitrate's surface activity. Phase-resolved vibrational sum frequency generation (VSFG) experiments are highly well-suited for this task; yet, only aqueous NaNO 3 salt has been studied by this technique to date. Here we investigate NH 4 + , Mg 2+ , and Na + , in addition to lithium (Li + ), to fully understand the effect of countercations on nitrate but also to elucidate the surface propensity of the countercations themselves as this is the first phaseresolved heterodyne-detected (HD-)VSFG study to address nitrate with countercations beyond sodium. We show the presence of an interfacial electric field generated by surface-active NO 3 − and its counterions, including NH 4 + , a cation that is commonly identified with nitrate in tropospheric aerosol generated in urban regions. We find that NH 4 + , Li + , Na + , and Mg 2+ are less surfaceactive relative to NO 3 − , with Mg 2+ being the least so. The cation identity and the ion concentration determine the magnitude of this electric field, which decreases in the order of Mg 2+ > Na + > Li + > NH 4 + at the ∼2 M NO 3 − concentration (highly relevant to low water content aerosol). Important to note is that the surface charge density does not completely dictate the trend observed as one might expect.

Specific cation effects at aqueous solution−vapor interfaces: Surfactant-like behavior of Li + revealed by experiments and simulations

“…Moreover, MD simulations have shown that, with few exceptions [e.g., SO 4 2− in (NH 4 ) 2 SO 4 (23)], anions adsorb more strongly to the solution−air interface than their counter cations, and, consequently, electrical double layers are formed near the interface, with the anions residing in or near the topmost layer of the solution, and the cations residing below the anions (14,24,25). Surface potentials (26), phase-sensitive vibrational sum frequency generation (PS-VSFG) spectra (22,27,28), and X-ray photoelectron spectroscopic (XPS) data (19,(29)(30)(31)(32) are consistent with the double layer picture.Compared with anion-specific effects, cation-specific effects at the solution−air interface are generally observed to be relatively weak. For example, the concentration dependence of the STIs of LiCl, NaCl, and KCl are very similar (33).…”

“…In one of the few studies that directly determined cation-specific effects on ion distributions in the interfacial region, XPS spectra and MD simulations revealed that Na + approaches the solution−air interface more closely than Rb + , and that the interfacial population of Cl − is greater in NaCl vs. RbCl solutions (31). PS-VSFG measurements, which provide indirect information on interfacial ion distributions via surface electric fields inferred from the imaginary part of the nonlinear susceptibility, have provided evidence of cation-specific effects on the strength of the electric double layer at the solution−air interfaces of nitrate, sulfate, and halide salt solutions (22,27,28).In almost all aqueous salt solutions of salts containing alkali metal cations, the cations are excluded from the topmost layer of the solution (14). It has been suggested, based on MD simulations (34)(35)(36), that Li + may be an exception.…”

Bulk Contributions Modulate the Sum-Frequency Generation Spectra of Interfacial Water on Model Sea-Spray Aerosols

“…[25][26][27][28][29][30][31][32][33][34][35][36] More recent efforts have focused on gaining molecular-level insights into complex interfaces by determining specific hydrogen bonding patterns, surface propensity and binding motifs of dissolved species, and effects of interfacial electric fields. [37][38][39][40][41][42][43][44][45] These studies demonstrated that interfacial water molecules reorient to point their hydrogen atoms toward negatively charged surfaces and away from positively charged surfaces. 29 Zwitterionic headgroups of phospholipid monolayers on aqueous subphases reorient water molecules to a lesser extent, but along the same direction as negatively charged headgroups.…”

Interfacial and bulk properties of concentrated solutions of ammonium nitrate

“…They revealed the presence of an electric double layer formed by cations and anions induced by the relative attraction to the surface of different ions, in decreasing order of I À , NO 3 À , NH 4 + , Cl À , K + , Na + , and SO 4 2À . Hua et al 28 also studied the distributions of interfacial ions of some nitrate salt solutions (LiNO 3 , NaNO 3 , NH 4 NO 3 , and Mg(NO 3 ) 2 ) using the same experimental technique. They confirmed the appearance of an electric double-layer structure with the greater abundance of NO 3 À ions at the surface as compared with their counterions.…”