The Jones–Ray Effect Is Not Caused by Surface-Active Impurities

Pure aqueous electrolyte solutions display a minimum in surface tension at concentrations of 2 ± 1 mM. This effect has been a source of controversy since it was first reported by Jones and Ray in the 1930s. The Jones–Ray effect has frequently been dismissed as an artifact linked to the presence of surface-active impurities. Herein we systematically consider the effect of surface-active impurities by purposely adding nanomolar concentrations of surfactants to dilute electrolyte solutions. Trace amounts of surfactant are indeed found to decrease the surface tension and influence the surface chemistry. However, surfactants can be removed by repeated aspiration and stirring cycles, which eventually deplete the surfactant from solution, creating a pristine surface. Upon following this cleaning procedure, a reduction in the surface tension by millimolar concentrations of salt is still observed. Consequently, we demonstrate that the Jones–Ray effect is not caused by surface-active impurities.

The Diverse Nature of Ion Speciation at the Nanoscale Hydrophobic/Water Interface

Halil Ibrahim Okur, Sylvie Roke, Evangelia Zdrali

Many biological systems are composed of nanoscale structures having hydrophobic and hydrophilic groups adjacent to one another and in contact with aqueous electrolyte solution. The interaction of ions with such structures is of fundamental importance. Although many studies have focused on characterizing planar extended (often air/water) interfaces, little is known about ion speciation at complex nanoscale biological systems. To start understanding the complex mechanisms involved, we use a hexadecane nanodroplet system, stabilized with a dilute monolayer of positively charged dodecyltrimethylammonium cations (DTA(+)) groups in contact with an electrolyte solution (NaSCN). Using vibrational sum frequency scattering, second harmonic scattering, zeta-potential measurements, and quantum density functional theory, we find DTA(+)-SCN- ion pairing at concentrations as low as 5 mM. A variety of ion species emerge at different ionic strengths, with differently oriented SCN- groups adsorbed on hydrophilic or hydrophobic parts of the surface. This diverse and heterogeneous chemical environment is surprisingly different from the behavior at extended liquid planar interfaces, where ion pairing is typically detected at molar concentrations and nanoscale system stability is no requirement.

AMER CHEMICAL SOC2019

Trace gas and aerosol interactions in the fully coupled model of aerosol-chemistry-climate ECHAM5-HAMMOZ: 1. Model description and insights from the spring 2001 TRACE-P experiment

Isabelle Bey, Luca Pozzoli

In this paper, we introduce the ECHAM5-HAMMOZ aerosol- chemistry-climate model that includes fully interactive simulations of Ox-NOx-hydrocarbons chemistry and of aerosol microphysics (including prognostic size distribution and mixing state of aerosols) implemented in the General Circulation Model ECHAM5. The photolysis rates used in the gas chemistry account for aerosol and cloud distributions and a comprehensive set of heterogeneous reactions is implemented. The model is evaluated with trace gas and aerosol observations provided by the TRACE-P aircraft experiment. Sulfate concentrations are well captured but black carbon concentrations are underestimated. The number concentrations, surface areas, and optical properties are reproduced fairly well near the surface but underestimated in the upper troposphere. CO concentrations are well reproduced in general while

{O}_{3}

concentrations are overestimated by 10-20 ppbv. We find that heterogeneous chemistry significantly influences the regional and global distributions of a number of key trace gases. Heterogeneous reactions reduce the ozone surface concentrations by 18-23% over the TRACE-P region and the global annual mean

{O}_{3}

burden by 7%. The annual global mean OH concentration decreases by 10% inducing a 7% increase in the global CO burden. Annual global mean

{HNO}_{3}

surface concentration decreases by 15% because of heterogenous reaction on mineral dust. A comparison of our results to those from previous studies suggest that the choice of uptake coe±cients for a given species seems to be the critical parameter that determines the global impact of heterogeneous chemistry on a trace gas (rather than the description of aerosol properties and distributions). A prognostic description of the size distribution and mixing state of the aerosols is important, however, to account for the effect of heterogeneous chemistry on aerosols as further discussed in the second of this two-part series.

2008

Electrolytes induce long-range orientational order and free energy changes in the H-bond network of bulk water

Michele Ceriotti, Yixing Chen, Nikolaos Gomopoulos, Chung-Wen Liang, Carlos Alberto Macias Romero, Halil Ibrahim Okur, Sylvie Roke, Gabriele Tocci, David Mark Wilkins

Electrolytes interact with water in many ways: changing dipole orientation, inducing charge transfer, and distorting the hydrogen-bond network in the bulk and at interfaces. Numerous experiments and computations have detected short-range perturbations that extend up to three hydration shells around individual ions. We report a multiscale investigation of the bulk and surface of aqueous electrolyte solutions that extends from the atomic scale (using atomistic modeling) to nanoscopic length scales (using bulk and interfacial femtosecond second harmonic measurements) to the macroscopic scale (using surface tension experiments). Electrolytes induce orientational order at concentrations starting at 10 μM that causes nonspecific changes in the surface tension of dilute electrolyte solutions. Aside from ion-dipole interactions, collective hydrogen-bond interactions are crucial and explain the observed difference of a factor of 6 between light water and heavy water.

American Association for the Advancement of Science (AAAS)2016

Trace gas and aerosol interactions in the fully coupled model of aerosol-chemistry-climate ECHAM5-HAMMOZ: 1. Model description and insights from the spring 2001 TRACE-P experiment

Isabelle Bey, Luca Pozzoli

{O}_{3}

{O}_{3}

burden by 7%. The annual global mean OH concentration decreases by 10% inducing a 7% increase in the global CO burden. Annual global mean

{HNO}_{3}

2008