Insight into the Ionizing Surface Potential Method and Aqueous Sodium Halide Surfaces

Abstract

Complementing the microscopic picture of the surface structure of electrolyte solutions set out by previous theoretical and experimental studies, the ionizing surface potential technique offers a unique approach to quantifying the impact of aqueous inorganic ions upon the interfacial electric field of the air–aqueous interface. In this Feature Article, we review the vulnerability of theoretical and empirically derived χwater values as a normative reference for aqueous ion surface potentials. Instead, we recognize and evaluate aqueous ion surface potentials relative to well-known ionic surfactants cetyl­tri­methyl­ammonium bromide (CTAB) and sodium dodecyl sulfate (SDS). Additionally, we also explore factors that impact the magnitude of the measured surface potentials using the ionizing method, particularly in the type of reference electrode and ionizing gas environment. With potential measurements of sodium halide solutions, we show that iodide has a dominant effect on the air–aqueous electric field. Compared to chloride and bromide, iodide is directly observed with a net negatively charged surface electric field at all salt concentrations measured (0.2 to 3.0 mol/kg water). Also, above the 2 M region, bromide is observed with a net negatively charged surface. Although several scenarios contribute to this effect, it is most likely due to the surface enrichment of bromide and iodide. While the results of this study are pertinent to determining the specific interfacial reactivity of aqueous halides, these anions seldom transpire as single-halide systems in the natural environment. Therefore, we also provide an outlook on future research concerning surface potential methods and more complex aqueous electrolyte systems.

Discerning Poly- and Monosaccharide Enrichment Mechanisms: Alginate and Glucuronate Adsorption to a Stearic Acid Sea Surface Microlayer

“…99% ACROS Organics, Lot: A0397856) were used as received from the same batch to maximize homogeneity in molecular composition. All aqueous solutions were prepared with nanopure water with a resistivity of 18.2 MΩ·cm (Milli-Q Advantage A10, EMD Millipore) in glassware cleaned with a piranha acid bath. ,, The aqueous solutions consisted of constant marine water concentrations of 476 mN NaCl and 10 mM CaCl 2 and a varying concentration of alginate or glucuronate ranging from 0 to 100 mg/L to a total volume of 1 L. The ionic composition was selected to model the concentration of the most abundant and enriched cations in seawater, without complicating spectral interpretation, while maintaining high ionic strength as in seawater. The nanopure water has a pH of 5.6 ± 0.1 due to atmospheric CO 2 acidification. , Atmospheric CO 2 also acidifies the proxy aqueous solution, so the solutions were initially pH adjusted to 8.7 ± 0.1 to ensure that the pH will be approximately 8.2 ± 0.1 throughout the experimental measurements.…”

“…All aqueous solutions were prepared with nanopure water with a resistivity of 18.2 MΩ•cm (Milli-Q Advantage A10, EMD Millipore) in glassware cleaned with a piranha acid bath. 54,58,59 The aqueous solutions consisted of constant marine water concentrations of 476 mN NaCl and 10 mM CaCl 2 and a varying concentration of alginate or glucuronate ranging from 0 to 100 mg/L to a total volume of 1 L. The ionic composition was selected to model the concentration of the most abundant and enriched cations in seawater, without complicating spectral interpretation, while maintaining high ionic strength as in seawater. The nanopure water has a pH of 5.6 ± 0.1 due to atmospheric CO 2 acidification.…”

Low Density Interior in Supercooled Aqueous Nanodroplets Expels Ions to the Subsurface

“…The related question of the propensity of ions in a liquid–vapor planar interface has been studied over several decades. − It is emphasized that the forces that determine the location of multiple ions in a planar interface are not the same as those in charged droplets regardless of their size. The reason is that the conductivity of the aqueous droplets drives the excess ions nearer to the surface and the counterions toward the interior .…”

Hydration and Hydrogen Bond Order of Octadecanoic Acid and Octadecanol Films on Water at 21 and 1 °C

“…The hydrophobic tail, polar headgroup, and composition of the underlying aqueous subphase all strongly influence the surface potential measurements. ,− ,− Surface potential variation during monolayer compression is often discussed in terms of changes in dipole moment alignment due to tail orientation relative to the water surface; however, increasing molecular density and polar headgroup orientation and dehydration during monolayer compression are also important contributors to the overall surface potential . Quadrupole contributions have also been indicated in some systems . The headgroup hydration shells screen the polar headgroup dipole moments, thereby reducing the surface potential at large MMAs.…”

“…65 Quadrupole contributions have also been indicated in some systems. 66 The headgroup hydration shells screen the polar headgroup dipole moments, thereby reducing the surface potential at large MMAs. Monolayer compression to low MMAs dehydrates the polar headgroups, reduces the dipole moment screening, and the increases surface potential.…”