Orientation‐Induced Adsorption of Hydrated Protons at the Air–Water Interface

The surface tension of the air—water interface increases upon addition of inorganic salts, implying a negative surface excess of ionic species. Most acids, however, induce a decrease in surface tension, indicating a positive surface excess of hydrated protons. In combination with the apparent negative charge at pure air–water interfaces derived from electrokinetic experiments, this experimental observation has been a source of intense debate since the mid‐19th century. Herein, we calculate surface tensions and ionic surface propensities at air–water interfaces from classical, thermodynamically consistent molecular dynamics simulations. The surface tensions of NaOH, HCl, and NaCl solutions show outstanding quantitative agreement with experiment. Of the studied ions, only H₃O⁺ adsorbs to the air–water interface. The adsorption is explained by the deep potential well caused by the orientation of the H₃O⁺ dipole in the interfacial electric field, which is confirmed by ab initio simulations.     

Electrochemistry of TEMPO: an assessment of the water diffusion constant in the aqueous liquid/vapor interfacial region

TEMPO, 2,2,6,6-tetramethylpiperidnyl-1-oxy, is a weak surfactant exhibiting a reversible redox activity: a one-electron oxidation to its oxonium cation. In the course of our earlier work (ref. Wu et al. in J Am Chem Soc 127:4490–4496, 2005; Glandut et al. in J Phys Chem B 110:6101–6109, 2006; Glandut et al. in Langmuir 22:10697–10704, 2006), we developed a full understanding of TEMPO’s electrochemistry at line microband electrodes. In these experiments TEMPO diffuses to the line electrode residing in the plane of the air/water interface in two coupled media, bulk aqueous phase with D?=?7.7?×?10^?6 cm²/s and along the 2D air/water interface with at least an order of magnitude greater surface diffusion constant, D _surf. The magnitude of the TEMPO oxidation current depends jointly on D _surf and on the rate of surface partitioning expressed by the desorption rate constant, k _des. The population of TEMPO partitioned to the air/water interface is largely unsolvated and couples to the aqueous solution by hydrogen bonding to predominately one water molecule. Our experimental methodology allows us to simultaneously determine D _surf and k _des by recording TEMPO voltammetric curves with line microband and barrier microband electrodes. In this report, we present a new methodology of producing and characterizing barrier microband electrodes using vapor-deposited SiO and introduce additional measures such as aspiration of the air/water interface designed to substantially reduce if not eliminate negative error due to surface impurities. These investigations generated a more accurate value of D _surf of 1.0?±?0.3?×?10^?4 cm²/s which we discuss in terms of the dynamic properties of water in the air/water interfacial region.     

Pressure and isotope effects on the proton jump of the hydroxide ion at 25°C

The limiting molar conductances Λ⁰ of potassium deuteroxide KOD in D₂O and potassium hydroxide KOH in H₂O were determined at 25°C as a function of pressure to disclose the difference in the proton-jump mechanism between an OH^? (OD^?) and a H₃O⁺ (D₃O⁺) ion. The excess conductance of the OD^? ion in D₂O λ _E ^O (OD ^-), as estimated by the equation $$\lambda _E^O (OD^ - ) = \Lambda ^O (KOD/D_2 O) - \Lambda ^O (KCl/D_2 O)$$ increases a little with pressure as well as the excess conductance of the OH^? ion in H₂O $$\lambda _E^O (OH^ - ) = \Lambda ^O (KOH/H_2 O) - \Lambda ^O (KCl/H_2 O)$$ However, their rates of increase with pressure are much smaller than those of the excess deuteron and proton conductances, λ _E ^O (D ⁺) and λ _E ^O (H ⁺). With respect to the isotope effect on the excess conductance, λ _E ^O (OH ^-)/λ _E ^O (D ⁺) decreases with presure as in the case of λ _E ^O (H ⁺)/λ _E ^O (D ⁺), but the value of λ _E ^O (OH ^-)/λ _E ^O (OD ^-) itself is much larger than that of λ _E ^O (H ⁺)/λ _E ^O (D ⁺) at each pressure. These results are ascribed to the difference in the pre-rotation of water molecules, which is brought about by the difference in the intial orientation of the rotating water molecule adjacent to the OH^? (OD^?) or the H₃O⁺ (D₃O⁺) ion.     

Global Minimum‐Energy Structure and Spectroscopic Properties of I2.−⋅n H2O Clusters: A Monte Carlo Simulated Annealing Study

The vibrational (IR and Raman) and photoelectron spectral properties of hydrated iodine‐dimer radical‐anion clusters, I₂^.? ? n H₂O (n=1–10), are presented. Several initial guess structures are considered for each size of cluster to locate the global minimum‐energy structure by applying a Monte Carlo simulated annealing procedure including spin–orbit interaction. In the Raman spectrum, hydration reduces the intensity of the I? I stretching band but enhances the intensity of the O? H stretching band of water. Raman spectra of more highly hydrated clusters appear to be simpler than the corresponding IR spectra. Vibrational bands due to simultaneous stretching vibrations of O? H bonds in a cyclic water network are observed for I₂^.? ? n H₂O clusters with n≥3. The vertical detachment energy (VDE) profile shows stepwise saturation that indicates closing of the geometrical shell in the hydrated clusters on addition of every four water molecules. The calculated VDE of finite‐size small hydrated clusters is extrapolated to evaluate the bulk VDE value of I₂^.? in aqueous solution as 7.6 eV at the CCSD(T) level of theory. Structure and spectroscopic properties of these hydrated clusters are compared with those of hydrated clusters of Cl₂^.? and Br₂^.?.     

Solvation of Calcium–Phosphate Headgroup Complexes at the DPPC/Aqueous Interface

The effects of sodium (Na⁺) and calcium (Ca²⁺) cations on model zwitterionic dipalmitoylphosphatidylcholine (DPPC) monolayers spread on metal chloride salt solutions are investigated by means of vibrational sum frequency generation (VSFG) and heterodyne‐detected (HD)‐VSFG spectroscopy. VSFG and HD‐VSFG spectra in the OH stretching region reveal cation‐specific effects on the interfacial water′s H‐bonding network, knowledge of which has been limited to date. It is found that low‐concentrated Ca²⁺ more strongly perturbs interfacial water organization relative to highly concentrated Na⁺. At higher Ca²⁺ concentrations, the water H‐bonding network at the DPPC/CaCl₂ interface reorganizes and the resulting spectrum closely follows that of the bare air/CaCl₂ interface up to ～3400 cm^?1. Most interesting is the appearance of a negative band at ～3450 cm^?1 in the DPPC/CaCl₂ Im χ_s⁽²⁾ spectra, likely arising from an asymmetric solvation of Ca²⁺–phosphate headgroup complexes. This gives rise to an electric field that orients the net OH transition moments of a subset of OH dipoles toward the bulk solution.     

Transport Processes at α‐Quartz–Water Interfaces: Insights from First‐Principles Molecular Dynamics Simulations

Car–Parrinello molecular dynamics (CP–MD) simulations are performed at high temperature and pressure to investigate chemical interactions and transport processes at the α‐quartz–water interface. The model system initially consists of a periodically repeated quartz slab with O‐terminated and Si‐terminated (1000) surfaces sandwiching a film of liquid water. At a temperature of 1000 K and a pressure of 0.3 GPa, dissociation of H₂O molecules into H⁺ and OH^? is observed at the Si‐terminated surface. The OH^? fragments immediately bind chemically to the Si‐terminated surface while Grotthus‐type proton diffusion through the water film leads to protonation of the O‐terminated surface. Eventually, both surfaces are fully hydroxylated and no further chemical reactions are observed. Due to the confinement between the two hydroxylated quartz surfaces, water diffusion is reduced by about one third in comparison to bulk water. Diffusion properties of dissolved SiO₂ present as Si(OH)₄ in the water film are also studied. We do not observe strong interactions between the hydroxylated quartz surfaces and the Si(OH)₄ molecule as would have been indicated by a substantial lowering of the Si(OH)₄ diffusion coefficient along the surface. No spontaneous dissolution of quartz is observed. To study the mechanism of dissolution, constrained CP–MD simulations are done. The associated free energy profile is calculated by thermodynamic integration along the reaction coordinate. Dissolution is a stepwise process in which two Si? O bonds are successively broken. Each bond breaking between a silicon atom at the surface and an oxygen atom belonging to the quartz lattice is accompanied by the formation of a new Si? O bond between the silicon atom and a water molecule. The latter loses a proton in the process which eventually leads to protonation of the oxygen atom in the cleaved quartz Si? O bond. The final solute species is Si(OH)₄.     

Comparison of temperature,pressure and isotope effects on the proton jump between the hydroxide and oxonium ion

The limiting molar conductances ° of potassium deuteroxide KOD in D₂O and potassium hydroxide KOH in H₂O were determined at 5 and 45°C as a function of pressure to clarify the difference in the temperature, pressure and isotope effects on the proton jump between an OD^– (OH^–) and a D₃O⁺ (H₃O⁺) ion. The excess conductances of the OD^– ion in D₂O and the OH^– ion in H₂O, _E ⁰ (OD^-) and _E ⁰ (OH^-), increase with increasing temperature and pressure as in the case of the excess deuteron and proton conductances, _E ⁰ (D⁺) and _E ⁰ (H⁺). However, the temperature effect on the excess conductance is larger for the OD^–(OH^–) ion than for the D₃O⁺ (H₃O⁺) ion but the pressure effect is much smaller for the OD^– (OH^–) ion than for the D₃O⁺ (H₃O⁺) ion. These findings are correlated with larger activation energies and less negative activation volumes found for the OD^– (OH^–) ion than for the D₃O⁺ (H₃O⁺) ion. Concerning the isotope effect, the value of _E ⁰ (OH^-)/ _E ⁰ (OD^-) deviates considerably from at each temperature and pressure in contrast with that of _E ⁰ (H⁺)/ _E ⁰ (D⁺), although both of them decrease with increasing temperature and pressure. These results are discussed mainly in terms of the difference in repulsive force between the OD^– (OH^–) or the D₃O⁺ (H₃O⁺) ion and the adjacent water molecule, the difference in strength of hydrogen bonds in D₂O and H₂O, and their variations with temperature, pressure, and isotope.     

Na+ transport across the beta alumina-propylene carbonate interface

Interfacial Na⁺ ion transport between polycrystalline beta alumina and propylene carbonate has been studied using a galvanostatic transient technique which separates interfacial overpotential from bulk resistivity effects. No interfacial polarization is detected during ion entry into beta alumina and exit from beta alumina across a dry interface from 30–1000 μA cm^?2. Transport across an interface contaminated with adsorbed water follows Tafel-type i/E behavior with a transition coefficient (α) of 0.24 and exchange current (i₀) of 3.0×10^?6 A cm^?2 at 23°C. Interfacial transport appears to take place through an intermediate state in which the mobile ion is adsorbed on the interface. Large increases in interfacial polarization occur at both dry and hydrated interfaces for ionic currents exceeding the rate of adsorption or desorption.     

Adsorption of uranyl species onto the rutile (110) surface: a periodic DFT study

To model the structures of dissolved uranium contaminants adsorbed on mineral surfaces and further understand their interaction with geological surfaces in nature, we have performed periodic density funtional theory (DFT) calculations on the sorption of uranyl species onto the TiO₂ rutile (110) surface. Two kinds of surfaces, an ideal dry surface and a partially hydrated surface, were considered in this study. The uranyl dication was simulated as penta‐ or hexa‐coordinated in the equatorial plane. Two bonds are contributed by surface bridging oxygen atoms and the remaining equatorial coordination is satisfied by H₂O, OH^?, and CO₃^2? ligands; this is known to be the most stable sorption structure. Experimental structural parameters of the surface–[UO₂(H₂O)₃]²⁺ system were well reproduced by our calculations. With respect to adsorbates, [UO₂(L1)_x(L2)_y(L3)_z]ⁿ (L1=H₂O, L2=OH^?, L3=CO₃^2?, x≤3, y≤3, z≤2, x+y+2z≤4), on the ideal surface, the variation of ligands from H₂O to OH^? and CO₃^2? lengthens the U? O_surf and U? Ti distances. As a result, the uranyl–surface interaction decreases, as is evident from the calculated sorption energies. Our calculations support the experimental observation that the sorptive capacity of TiO₂ decreases in the presence of carbonate ions. The stronger equatorial hydroxide and carbonate ligands around uranyl also result in U?O distances that are longer than those of aquouranyl species by 0.1–0.3 Å. Compared with the ideal surface, the hydrated surface introduces greater hydrogen bonding. This results in longer U?O bond lengths, shorter uranyl–surface separations in most cases, and stronger sorption interactions.     

Modulating the Solubilities of Ionic Liquid Components in Aqueous–Ionic Liquid Biphasic Systems: A Q‐NMR Investigation

Aqueous–ionic liquid (A–IL) biphasic systems have been examined in terms of deuterated water, acid, and IL cation and anion mutual solubilities in the upper (water‐rich, in mole fraction) and lower phase of aqueous/IL biphasic systems at ambient temperature. The biphasic mixtures were composed of deuterated acids of various concentrations (mainly DCl, DNO₃, and DClO₄ from 10^?2 to 10^?4 M ) and five ionic liquids of the imidazolium family with a hydrophobic anion (CF₃SO₂)₂N^?, that is, [C₁C_nim][Tf₂N], (n=2, 4, 6, 8 and 10). The analytical techniques applied were ¹H NMR, ¹⁹F NMR, Karl–Fischer titration, pH potentiometry for IL cations and anions, and water and acid determination. The effects of the ionic strength (μ=0.1 M NaCl and NaNO₃ as well as μ=0.1 M , 0.2 M and 0.4 M NaClO₄, according to the investigated acid), the nature of the IL cation, and the nature of the mineral acid on the solubilities of the (D₂O, D⁺, Tf₂N^?, C₁C_nim⁺) entities in the lower or upper phases were determined. The addition of sodium perchlorate was found to enhance the Tf₂N^? solubility while inhibiting the solubility of the ionic liquid cation. Differences in IL cation and anion solubilities of up to 42 mM were evidenced. The consequences for the characterization of the aqueous biphasic system, the solvent extraction process of the metal ions, and the ecological impact of the ILs are discussed.     

Theoretical Study on the Spectroscopic Properties of CO3.−.nH2O Clusters: Extrapolation to Bulk

Vertical detachment energies (VDE) and UV/Vis absorption spectra of hydrated carbonate radical anion clusters, CO₃^.?.n H₂O (n=1–8), are determined by means of ab initio electronic structure theory. The VDE values of the hydrated clusters are calculated with second‐order Moller–Plesset perturbation (MP2) and coupled cluster theory using the 6‐311++G(d,p) set of basis functions. The bulk VDE value of an aqueous carbonate radical anion solution is predicted to be 10.6 eV from the calculated weighted average VDE values of the CO₃^.?.n H₂O clusters. UV/Vis absorption spectra of the hydrated clusters are calculated by means of time‐dependent density functional theory using the Becke three‐parameter nonlocal exchange and the Lee–Yang–Parr nonlocal correlation functional (B3LYP). The simulated UV/Vis spectrum of the CO₃^.?.8 H₂O cluster is in excellent agreement with the reported experimental spectrum for CO₃^.? (aq), obtained based on pulse radiolysis experiments.     

Reactions of Hydrated Singly Charged First‐Row Transition‐Metal Ions M+(H2O)n (M=V,Cr, Mn,Fe, Co,Ni, Cu,and Zn) toward Nitric Oxide in the Gas Phase

Reactions of M⁺(H₂O)_n (M=V, Cr, Mn, Fe, Co, Ni, Cu, Zn; n≤40) with NO were studied by Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometry. Uptake of NO was observed for M=Cr, Fe, Co, Ni, Zn. The number of NO molecules taken up depends on the metal ion. For iron and zinc, NO uptake is followed by elimination of HNO and formation of the hydrated metal hydroxide, with strong size dependence. For manganese, only small HMnOH⁺(H₂O)_n?1 species, which are formed under the influence of room‐temperature black‐body radiation, react with NO. Here NO uptake competes with HNO formation, both being primary reactions. The results illustrate that, in the presence of water, transition‐metal ions are able to undergo quite particular and diverse reactions with NO. HNO is presumably formed through recombination of a proton and ³NO^? for M=Fe, Zn, preferentially for n=15–20. For manganese, the hydride in HMnOH⁺(H₂O)_n?1 is involved in HNO formation, preferentially for n≤4. The strong size dependence of the HNO formation efficiency illustrates that each molecule counts in the reactions of small ionic water clusters.     

Transport properties of separating membranes MF-4SK during alkaline electrolysis of water

The transport properties of separating membranes MF-4SK are studied during electrolysis of H₂O in solutions of KOH. The effective diffusion coefficients of molecules of KOH and H₂O and the transfer coefficients of ions K⁺ and OH^? and molecules of H₂O are measured at KOH concentrations reaching 11 M, currents reaching 0.31 A cm^?2, at ambient temperature and at 80°C. In contact with a KOH solution in the concentration interval 0.1 to 11 M, the membranes that initially swelled in H₂O lose a considerable fraction of water that was present in them and the overall volume of clusters and solution-filled channels in them noticeably decreases. The coefficients of transfer by current of ions K⁺ out of anodic compartment into cathodic and the OH^? ions in the reverse direction, respectively, happen to be equal to about 0.6 and 0.4 at ambient temperature and 0.8 and 0.2 at 80°C. The coefficients of transfer of water molecules out of the anodic volume into the cathodic volume in the process of electrolysis happen to be in the limits 1.6–1.9 at ambient temperature and in the limits 2.2–2.8 at 80°C. The effective diffusion coefficients of molecules of KOH and H₂O at moderate concentrations of KOH (5.6 M) amount to ～2.6 × 10^?7 and 30 × 10^?7 cm²s^?1 at ambient temperature and ～4 × 10^?7 and 61 × 10^?7 cm²s^?1 at 80°C, respectively. At a high concentration of KOH (～10 M) these quantities substantially diminish.     

Unique Hydrogen‐Bonded Complex of Hydronium and Hydroxide Ions

H₃O⁺ and OH^?, formed by the self‐ionization of two coordinating water molecules during the crystal growing of a host molecule [1,3,5‐tris(hydroxymethyl)2,4,6‐triethylbenzene ( 1 )], could be effectively stabilized by hydrogen‐bonding interactions with the preorganized hydroxy groups of three molecules of 1. The binding motifs observed in the complex ( 1 )₃?H₃O⁺?HO^? show remarkable similarity to those postulated for the hydrated hydronium and hydroxide ion complexes, which play important roles in various chemical, biological, and atmospheric processes, but their molecular structures are still not fully understood and remain a subject of intensive research.     

ToF‐SIMS studies of the oxidation of Fe by D2O vapour: comparison with XPS

The oxidation of iron (Fe) by water (D₂O) vapour at low pressures and room temperature was investigated using time‐of‐flight (ToF) SIMS. The results supported those found previously using XPS and the QUASES^? program in that a duplex oxide structure was found containing a thin outer surface hydroxide (Fe(OD)₂) layer over an inner oxide (FeO) layer. The extraordinary depth resolution of the ToF‐SIMS profiles assisted in identifying the two phases; this resolution was achieved by compensation for surface roughness. A substantial concentration of deuterium was found in the subsurface oxide layer. This observation confirmed previous assessments that the formation of FeO was from the reaction of Fe(OD)₂ with outward‐diffusing Fe, leaving deuterium as a reaction product. Copyright © 2005 John Wiley & Sons, Ltd.     

Vibrational branching ratios for the A3Πi → X3Σ− system of OH+ and OD+

Near-thermal charge exchange between He⁺ and H₂(D₂O) is used as a source of OH⁺(OD⁺) A³H_i→ X³Σ^? emission. A comparison between experimental emission branching ratios and those calculated in the r-centroid approximation suggests that the electronic transition moment varies as a function of the r-centroid.     

Luminescence in near-thermal charge exchange. II. He+ + H2O and He+ + D2O

An ICR spectrometer fitted with synchronous photon counting equipment is used to study the emission produced by near-thermal (? 0.1 eV) collisions between He⁺ and H₂O (D₂). Within the investigated wavelength region, 185 to 500 nm, the only significant emission features are the A³Π (υ' ? 3) → X³Σ^? bands in OH⁺ and OD⁺, and the A²Σ⁺ → X²Π(0.0) band in OH and, possibly, in OD. The corresponding excitation rate constants represent only ? 2% of the total He⁺/H₂O (D₂O) charge transfer. The resonant electron-jump model for thermal-energy charge exchange is discussed in the light of recent information on the He⁺/H₂O reaction and on the excited states of H₂O⁺ and their excitation by electron and photon impact on H₂O (D₂O).     

Investigations of electrode surfaces in acetonitrile solutions using surface-enhanced Raman spectroscopy

The discovery of surface-enhanced Raman scattering (SERS) has resulted in a new tool for the elucidation of phenomena occurring at the electrode/solution interface. Our long term objective is to use this technology to identify species adsorbed at the electrode surface and to determine the dependence of the Raman intensity from these species on such variables as solvent, concentration, pH, electrode potential and laser exciting wavelength. The phenomenon is described in the introduction, to provide background for those not familiar with this topic. This is followed by a presentation of recent data from our laboratory, with a focus on nonaqueous solvents. Bands of the solvated Li⁺ and Na⁺ cations at the electrode surface are reported. Bands from trace H₂O (D₂O and HOD) and OH^– (OD^–) have been observed. At sufficiently negative potentials CN^– is generated from acetonitrile. Evidence is presented for the occurrence of photoelectrochemical reduction. Some preliminary results are given for propylene carbonate solvent which support this interpretation.Session lecture, IX International Conference on Non-Aqueous Solutions, Pittsburgh, PA, August 1984.     

When It's Heavier: Interfacial and Solvation Chemistry of Isotopes in Aqueous Electrolytes for Zn-ion Batteries

The electrochemical effect of isotope (EEI) of water is introduced in the Zn-ion batteries (ZIBs) electrolyte to deal with the challenge of severe side reactions and massive gas production. Due to the low diffusion and strong coordination of ions in D₂O, the possibility of side reactions is decreased, resulting in a broader electrochemically stable potential window, less pH change, and less zinc hydroxide sulfate (ZHS) generation during cycling. Moreover, we demonstrate that D₂O eliminates the different ZHS phases generated by the change of bound water during cycling because of the consistently low local ion and molecule concentration, resulting in a stable interface between the electrode and electrolyte. The full cells with D₂O-based electrolyte demonstrated more stable cycling performance which displayed ∼100 % reversible efficiencies after 1,000 cycles with a wide voltage window of 0.8–2.0 V and 3,000 cycles with a normal voltage window of 0.8–1.9 V at a current density of 2 A g⁻¹.     

Polyelectrolyte complexes. VI. Polycation structure,polyanion molar mass,and polyion concentration effects on complex nanoparticles based on poly(sodium 2‐acrylamido‐2‐methylpropanesulfonate)

The formation and characterization of some interpolyelectrolyte complex (IPEC) nanoparticles based on poly(sodium 2‐acrylamido‐2‐methylpropanesulfonate) (NaPAMPS), as a function of the polycation structure, polyanion molar mass, and polyion concentration, were followed in this work. Poly(diallyldimethylammonium chloride) and two polycations (PCs) containing (N,N‐dimethyl‐2‐hydroxypropyleneammonium chloride) units in the backbone (PCA₅ and PCA₅D₁) were used as starting polyions. The complex stoichiometry, (n^?/n⁺)_iso, was pointed out by optical density at 500 nm (OD₅₀₀), polyelectrolyte titration, and dynamic light scattering. IPEC nanoparticle sizes were influenced by the polycation structure and polyanion molar mass only before the complex stoichiometry, which was higher for the more hydrophilic polycations (PCA₅ and PCA₅D₁) and for a higher NaPAMPS molar mass, and were almost independent of these factors after that, at a flow rate of the added polyion of about 0.28 mL × (mL PC)^?1 × h^?1. The IPEC nanoparticle sizes remained almost constant for more than 2 weeks, both before and after the complex stoichiometry, at low concentrations of polyions. NIPECs as stable colloidal dispersions with positive charges in excess were prepared at a ratio between charges (n^?/n⁺) of 0.7, and their storage colloidal stability, as a function of the polycation structure and polyion concentration (from 0.8 to ca. 7.8 mmol/L), was demonstrated. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2495–2505, 2004