Interaction of Nitrate, Barium, Strontium and Cadmium Ions with Fused Quartz/Water Interfaces Studied by Second Harmonic Generation

NaCl, MgCl2, and AlCl3 Surface Coverages on Fused Silica and Adsorption Free Energies at pH 4 from Nonlinear Optics

We employ amplitude- and phase-resolved second harmonic generation experiments to probe interactions of fused silica:aqueous interfaces with Al3+, Mg2+, and Na+ cations at pH 4 and as a function of metal cation concentration. We quantify the second-order nonlinear susceptibility and the total interfacial potential in the presence and absence of a 10 mM screening electrolyte to understand the influence of charge screening on cation adsorption. Strong cation:surface interactions are observed in the absence of the screening electrolyte. The total potential is then employed to estimate the total number of absorbed cations cm-2. The contributions to the total potential from the bound and mobile charges were separated using Gouy-Chapman-Stern model estimates. All three cations bind fully reversibly, indicating physisorption as the mode of interaction. Of the isotherm models tested, the Kd adsorption model fits the data with binding constants of 3-30 and ∼300 mol-1 for the low (<0.1 mM) and high (0.1-3 mM) concentration regimes, corresponding to adsorption free energies of -13 to -18 and -24 kJ mol-1 at room temperature, respectively. The maximum surface coverages are around 1013 cations cm-2, matching the number of deprotonated silanol groups on silica at pH 4. Clear signs of decoupled Stern and diffuse layer nonlinear optical responses are observed and found to be cation-specific.

Transport Behavior of Cd2+ in Highly Weathered Acidic Soils and Shaping in Soil Microbial Community Structure

The mining and smelting site soils in South China present excessive Cd pollution. However, the transport behavior of Cd in the highly weathered acidic soil layer at the lead-zinc smelting site remains unclear. Here, under different conditions of simulated infiltration, the migration behavior of Cd2+ in acid smelting site soils at different depths was examined. The remodeling effect of Cd2+ migration behavior on microbial community structure and the dominant microorganisms in lead-zinc sites soils was analyzed using high-throughput sequencing of 16S rRNA gene amplicons. The results revealed a specific flow rate in the range of 0.3-0.5 mL/min that the convection and dispersion have no obvious effect on Cd2+ migration. The variation of packing porosity could only influence the migration behavior by changing the average pore velocity, but cannot change the adsorption efficiency of soil particles. The Cd has stronger migration capacity under the reactivation of acidic seepage fluid. However, in the alkaline solution, the physical properties of soil, especially pores, intercept the Cd compounds, further affecting their migration capacity. The acid-site soil with high content of SOM, amorphous Fe oxides, crystalline Fe/Mn/Al oxides, goethite, and hematite has stronger ability to adsorb and retain Cd2+. However, higher content of kaolinite in acidic soil will increase the potential migration of Cd2+. Besides, the migration behavior of Cd2+ results in simplified soil microbial communities. Under Cd stress, Cd-tolerant genera (Bacteroides, Sphingomonas, Bradyrhizobium, and Corynebacterium) and bacteria with both acid-Cd tolerance (WCHB 1-84) were distinguished. The Ralstonia showed a high enrichment degree in alkaline Cd2+ infiltration solution (pH 10.0). Compared to the influence of Cd2+ stress, soil pH had a stronger ability to shape the microbial community in the soil during the process of Cd2+ migration.

Ion Depletion in the Interfacial Microenvironment from Cell-Surface Interactions

The nanoscale region immediately adjacent to surfaces, although challenging to probe, is directly responsible for local chemical and physical interactions between a material and its surroundings. Cell-surface contacts are mediated by a combination of electrostatic and acid-base interactions that alter the local environment over time. In this study, a label-free vibrational probe with a nanometer length scale reveals that the electrostatic potential at a silica surface gradually increases in the presence of bacteria in solution. We illustrate that the cells themselves are not responsible for this effect. Rather, they alter the interfacial chemical environment in a manner that is consistent with a reduction of the ionic strength to a level that is roughly four times lower than that of the bulk aqueous phase.

Divalent Ion Specific Outcomes on Stern Layer Structure and Total Surface Potential at the Silica:Water Interface

The second-order nonlinear susceptibility, χ⁽²⁾, in the Stern layer and the total interfacial potential drop, Φ(0)_tot, across the oxide:water interface are estimated from SHG amplitude and phase measurements for divalent cations (Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺) at the silica:water interface at pH 5.8 and various ionic strengths. We find that interfacial structure and total potential depend strongly on ion valency. We observe statistically significant differences between the experimentally determined χ⁽²⁾ value for NaCl and that of the alkali earth series but smaller differences between ions of the same valency in that series. These differences are particularly pronounced at intermediate salt concentrations, which we attribute to the influence of hydration structure in the Stern layer. Furthermore, we corroborate the differences by examining the effects of anion substitution (SO₄^2- for Cl^-). Finally, we identify that hysteresis in measuring the reversibility of ion adsorption and desorption at fused silica in forward and reverse titrations manifests itself both in Stern layer structure and in total interfacial potential for some of the salts, most notably for CaCl₂ and MgSO₄ but less so for BaCl₂ and NaCl.

A New Imaginary Term in the Second-Order Nonlinear Susceptibility from Charged Interfaces

Nonresonant second harmonic generation (SHG) phase and amplitude measurements obtained from the silica-water interface at varying pH values and an ionic strength of 0.5 M point to the existence of a nonlinear susceptibility term, which we call χ_X⁽³⁾, that is associated with a 90° phase shift. Including this contribution in a model for the total effective second-order nonlinear susceptibility produces reasonable point estimates for interfacial potentials and second-order nonlinear susceptibilities when χ_X⁽³⁾ ≈ 1.5χ_water⁽³⁾. A model without this term and containing only traditional χ⁽²⁾ and χ⁽³⁾ terms cannot recapitulate the experimental data. The new model also provides a demonstrated utility for distinguishing apparent differences in the second-order nonlinear susceptibility when the electrolyte is NaCl versus MgSO₄, pointing to the possibility of using heterodyne-detected SHG to investigate ion specificity in interfacial processes.

Molecular Fingerprints of Hydrophobicity at Aqueous Interfaces from Theory and Vibrational Spectroscopies

Hydrophobicity/hydrophilicity of aqueous interfaces at the molecular level results from a subtle balance in the water-water and water-surface interactions. This is characterized here via density functional theory-molecular dynamics (DFT-MD) coupled with vibrational sum frequency generation (SFG) and THz-IR absorption spectroscopies. We show that water at the interface with a series of weakly interacting materials is organized into a two-dimensional hydrogen-bonded network (2D-HB-network), which is also found above some macroscopically hydrophilic silica and alumina surfaces. These results are rationalized through a descriptor that measures the number of "vertical" and "horizontal" hydrogen bonds formed by interfacial water, quantifying the competition between water-surface and water-water interactions. The 2D-HB-network is directly revealed by THz-IR absorption spectroscopy, while the competition of water-water and water-surface interactions is quantified from SFG markers. The combination of SFG and THz-IR spectroscopies is thus found to be a compelling tool to characterize the finest details of molecular hydrophobicity at aqueous interfaces.

What the Diffuse Layer (DL) Reveals in Non-Linear SFG Spectroscopy

Following our recent work [Phys. Chem. Chem. Phys. 20:5190–99 (2018)] that provided the means to unambigously define and extract the three water regions at any charged interface (solid–liquid and air–liquid alike), denoted the BIL (Binding Interfacial Layer), DL (Diffuse Layer) and Bulk, and how to calculate their associated non-linear Sum Frequency Generation Spectroscopy (SFG) χ2(ω) spectroscopic contributions from Density Functional Theory (DFT)-based ab initio molecular dynamics simulations (DFT-MD/AIMD), we show here that the χDL2(ω) signal arising from the DL water region carries a wealth of essential information on the microscopic and macroscopic properties of interfaces. We show that the χDL2(ω) signal carries information on the surface potential and surface charge, the isoelectric point, EDL (Electric Double Layer) formation, and the relationship between a nominal electrolyte solution pH and surface hydroxylation state. This work is based on DFT-MD/AIMD simulations on a (0001) α–quartz–water interface and on the air–water interface, with various surface quartz hydroxylation states and various electrolyte concentrations. The conclusions drawn make use of the interplay between experiments and simulations. Most of the properties listed above can now be extracted from experimental χDL2(ω) alone with the protocols given in this work, or by making use of the interplay between experiments and simulations, as described in this work.

Nitrate Photochemistry at the Air-Ice Interface and in Other Ice Reservoirs

The photolysis of snowpack nitrate (NO₃^-) is an important source of gaseous reactive nitrogen species that affect atmospheric oxidants, particularly in remote regions. However, it is unclear whether nitrate photochemistry differs between the three solute reservoirs in/on ice: in liquid-like regions (LLRs) in the ice; within the solid ice matrix; and in a quasi-liquid layer (QLL) at the air-ice interface, where past work indicates photolysis is enhanced. In this work, we explore the photoformation of nitrite in these reservoirs using laboratory ices. Nitrite quantum yields, Φ(NO₂^-), at 313 nm for aqueous and LLR ice samples agree with previous values, e.g., 0.65 ± 0.07% at -10 °C. For ice samples made via flash-freezing solution in liquid nitrogen, where nitrate is possibly present as a solid solution, the nitrite quantum yield is 0.57 ± 0.05% at -10 °C, similar to the LLR results. In contrast, the quantum yield at the air-ice interface is enhanced by a factor of 3.7 relative to LLRs, with a value of 2.39 ± 0.24%. These results indicate nitrate photolysis is enhanced at the air-ice interface, although the importance of this enhancement in the environment depends on the amount of nitrate present at the interface.

Structural definition of the BIL and DL: a new universal methodology to rationalize non-linearχ(2)(ω) SFG signals at charged interfaces, includingχ(3)(ω) contributions

BIL (Binding Interfacial Layer) and DL (Diffuse Layer) at aqueous interfaces: universal structural definitions, deconvolution of their SFG signals andχ³contribution.

Absorption of Near UV Light by HNO3/NO3(-) on Sapphire Surfaces

We have determined absorption of the near UV light (290-345 nm) by nitric acid (HNO3) deposition on sapphire window surfaces as a function of the HNO3 pressure, by using Brewster angle cavity ring-down spectroscopy. Apparent monolayer HNO3 surface absorption cross sections have been obtained; they range between (1.7 ± 1.1) × 10(-19) and (0.29 ± 0.03) × 10(-19) cm(2)/molecule. When nitric acid cross section values on sapphire surfaces were divided by those on fused silica surfaces for which only molecular HNO3 adsorption was reported, a new absorption band appeared in the 320-345 nm region. The shape of this absorption band is similar to that reported for surface nitrate (NO3(-)) at quartz/water interfaces, but is red-shifted by about 10 nm. Our study suggests that a small percentage (<7%) of adsorbed HNO3 formed by HNO3 deposition on sapphire surfaces is dissociated into surface nitrate on the time scale of about 5-7 min. Background transmission changes in the 320-350 nm region after exposing clean sapphire surfaces with many repeated HNO3 deposition/evacuation cycles are consistent with surface nitrate formation. We obtained nitrate surface absorption cross section data over 320-350 nm range. We also modeled photolysis rates of HNO3/NO3(-) on urban grimes. Atmospheric implications of the results are discussed.

Aqueous proton transfer across single-layer graphene

Proton transfer across single-layer graphene proceeds with large computed energy barriers and is therefore thought to be unfavourable at room temperature unless nanoscale holes or dopants are introduced, or a potential bias is applied. Here we subject single-layer graphene supported on fused silica to cycles of high and low pH, and show that protons transfer reversibly from the aqueous phase through the graphene to the other side where they undergo acid–base chemistry with the silica hydroxyl groups. After ruling out diffusion through macroscopic pinholes, the protons are found to transfer through rare, naturally occurring atomic defects. Computer simulations reveal low energy barriers of 0.61–0.75 eV for aqueous proton transfer across hydroxyl-terminated atomic defects that participate in a Grotthuss-type relay, while pyrylium-like ether terminations shut down proton exchange. Unfavourable energy barriers to helium and hydrogen transfer indicate the process is selective for aqueous protons.

Proton transfer across graphene is associated with large computed energy barriers and is thought to be generally unfavourable. Here, the authors observe aqueous proton transfer through graphene subjected to pH cycling, suggesting that it is due to transfer through rare, naturally occurring atomic defects.

Precipitates of Al(III), Sc(III), and La(III) at the muscovite-water interface

The interaction of Al(III), Sc(III), and La(III) with muscovite-water interfaces was studied at pH 4 and 10 mM NaCl using second harmonic generation (SHG) and X-ray photoelectron spectroscopy (XPS). SHG data for Sc(III) and La(III) suggest complete and/or partial irreversible adsorption that is attributed by XPS to the growth of Sc(III) and La(III) hydroxides/oxides on the muscovite surface. Al(III) adsorption appears to coincide with the growth of gibbsite (Al(OH)3) deposits on the muscovite surface, as indicated by the magnitude of the interfacial potential computed from the SHG data. This interpretation of the data is consistent with previous studies reporting the epitaxial growth of gibbsite on the muscovite surface under similar conditions. The implication of our findings is that the surface charge density of mica may change (and in the case of Al(III), even flip sign from negative (mica) to positive (gibbsite)) when Al(III), Sc(III), or La(III) is present in aqueous phases in contact with heterogeneous geochemical media rich in mica-class minerals, even at subsaturation conditions.

Charge asymmetry at aqueous hydrophobic interfaces and hydration shells

Guilty as charged: Water is often modeled as a dielectric continuum, but the molecular structure of water is asymmetric. Two ions that have a virtually identical size, shape, and structure, but an opposite charge sign have been investigated to see whether charge makes a fundamental difference to water structuring. The spectroscopic data for the hydration and interface structures are found to be remarkably different for opposite charges.

Surface and zeta-potentials of silver halide single crystals: pH-dependence in comparison to particle systems

We have carried out surface and zeta-potential measurements on AgCl and AgBr single crystals. As for particle systems we find that, surprisingly and previously unnoted, the zeta-potential exhibits pH-dependence, while the surface potential does not. A possible interpretation of these observations is the involvement of water ions in the interfacial equilibria and in particular, stronger affinity of the hydroxide ion compared to the proton. The pH-dependence of the zeta-potential can be suppressed at sufficiently high silver concentrations, which agrees with previous measurements in particle systems where no pH-dependence was found at high halide ion concentrations. The results suggest a subtle interplay between the surface potential determining the halide and silver ion concentrations, and the water ions. Whenever the charge due to the halide and silver ions is sufficiently high, the influence of the proton/hydroxide ion on the zeta-potential vanishes. This might be related to the water structuring at the relevant interfaces which should be strongly affected by the surface potential. Another interesting observation is accentuation of the assumed water ion effect on the zeta-potential at the flat single crystal surfaces compared to the corresponding silver halide colloids. Previous generic MD simulations have indeed predicted that hydroxide ion adsorption is accentuated on flat/rigid surfaces. A thermodynamic model for AgI single crystals was developed to describe the combined effects of iodide, silver and water ions, based on two independently previously published models for AgI (that only consider constituent and background electrolyte ions) and inert surfaces (that only consider water and background electrolyte ions). The combined model correctly predicts all the experimentally observed trends.

Counting the number of magnesium ions bound to the surface-immobilized thymine oligonucleotides that comprise spherical nucleic acids

Label-free studies carried out under aqueous phase conditions quantify the number of Mg(2+) ions binding to surface-immobilized T40 sequences, the subsequent reordering of DNA on the surface, and the consequences of Mg(2+) binding for DNA-DNA interactions. Second harmonic generation measurements indicate that, within error, 18-20 Mg(2+) ions are bound to the T40 strand at saturation and that the metal-DNA interaction is associated with a near 30% length contraction of the strand. Structural reordering, evaluated using vibrational sum frequency generation, atomic force microscopy, and dynamic light scattering, is attributed to increased charge screening as the Mg(2+) ions bind to the negatively charged DNA, reducing repulsive Coulomb forces between nucleotides and allowing the DNA single strands to collapse or coil upon themselves. The impact of Mg(2+) binding on DNA hybridization and duplex stability is assessed with spherical nucleic acid (SNA) gold nanoparticle conjugates in order to determine an optimal working range of Mg(2+) concentrations for DNA-DNA interactions in the absence of NaCl. The findings are consistent with a charge titration effect in which, in the absence of NaCl, (1) hybridization does not occur at room temperature if an average of 17.5 or less Mg(2+) ions are bound per T40 strand, which is not reached until the bulk Mg(2+) concentration approaches 0.5 mM; (2) hybridization proceeds, albeit with low duplex stability having an average Tm of 31(3)°C, if an average of 17.5-18.0 Mg(2+) ions are bound; and (3) highly stable duplexes having a Tm of 64(2)°C form if 18.5-19.0 Mg(2+) ions are bound, corresponding to saturation of the T40 strand.

Specific ion effects at two single-crystal planes of sapphire

Experimental results on specific ion effects at the c- and r- single-crystal planes of sapphire obtained by zeta-potential measurements at pH 5.8 are reported. Both crystal planes have negative electrokinetic charge at pH 5.8 and their intrinsic isoelectric points are found close to pH 4. The water structure "making" surface (i.e., r-plane, based on surface diffraction and surface complexation modeling) causes cation specificity in the order Li(+) > Na(+) > K(+) > Rb(+) > Cs(+) in chloride systems while no anion sensitivity occurs in sodium systems (Cl(-), NO3(-), and BrO3(-)) as expected. The cation series concurs with the simple idea of structure making ions being adsorbed more strongly on structure making surfaces and also concurs with the sequence found for particulate alumina for the cation series in nitrate systems. On the structure breaking basal plane (i.e., c-plane, again based on surface diffraction and surface complexation modeling), no cation specific effects are observed in chloride systems, but the structure breaking properties are retrieved in the cation series in nitrate systems. Surprisingly, anion specificity is observed on sapphire-c. Furthermore, the chloride ion shows unexpected behavior that suggests chloride adsorption onto the negatively charged surface. Based on these experimental observations in conjunction with generic results from published MD simulations, the c-plane sapphire aqueous electrolyte interface is a nonpolar surface with negative charge. The nonpolarity finds repercussions in the weak water ordering and the observed ion specific effects. The low isoelectric points of the cuts cannot be explained by the respective surface chemistries of the ideal surfaces. Relation to "inert" surfaces and concomitant dominance of hydroxide ion adsorption is a possible explanation for the low isoelectric points of both cuts. The reported ion specific effects occur at concentrations below 10 mM. Overall, the results support the idea that ion specific effects are largely governed by surface hydration.

Uranyl adsorption at the muscovite (mica)/water interface studied by second harmonic generation

Uranyl adsorption at the muscovite (mica)/water interface was studied by second harmonic generation (SHG). Using the nonresonant χ(3) technique and the Gouy-Chapman model, the initial surface charge density of the mica surface was determined to be -0.022(1) C/m(2) at pH 6 and in the presence of dissolved carbonate. Under these same conditions, uranyl adsorption isotherms collected using nonresonant χ(3) experiments and resonantly enhanced SHG experiments that probe the ligand-to-metal charge transfer bands of the uranyl cation yielded a uranyl binding constant of 3(1) × 10(7) M(-1), corresponding to a Gibbs free energy of adsorption of -52.6(8) kJ/mol, and a maximum surface charge density at monolayer uranyl coverage of 0.028(3) C/m(2). These results suggest favorable adsorption of uranyl ions to the mica interface through strong ion-dipole or hydrogen interactions, with a 1:1 uranyl ion to surface site ratio that is indicative of monovalent cations ((UO(2))(3)(OH)(5)(+), (UO(2))(4)(OH)(7)(+), UO(2)OH(+), UO(2)Cl(+), UO(2)(CH(3)COO(-))(+)) binding at the interface, in addition to neutral uranyl species (UO(2)(OH)(2) and UO(2)CO(3)). This work provides benchmark measurements to be used in the improvement of contaminant transport modeling.

Importance of length and sequence order on magnesium binding to surface-bound oligonucleotides studied by second harmonic generation and atomic force microscopy

The binding of magnesium ions to surface-bound single-stranded oligonucleotides was studied under aqueous conditions using second harmonic generation (SHG) and atomic force microscopy (AFM). The effect of strand length on the number of Mg(II) ions bound and their free binding energy was examined for 5-, 10-, 15-, and 20-mers of adenine and guanine at pH 7, 298 K, and 10 mM NaCl. The binding free energies for adenine and guanine sequences were calculated to be -32.1(4) and -35.6(2) kJ/mol, respectively, and invariant with strand length. Furthermore, the ion density for adenine oligonucleotides did not change as strand length increased, with an average value of 2(1) ions/strand. In sharp contrast, guanine oligonucleotides displayed a linear relationship between strand length and ion density, suggesting that cooperativity is important. This data gives predictive capabilities for mixed strands of various lengths, which we exploit for 20-mers of adenines and guanines. In addition, the role sequence order plays in strands of hetero-oligonucleotides was examined for 5'-A(10)G(10)-3', 5'-(AG)(10)-3', and 5'-G(10)A(10)-3' (here the -3' end is chemically modified to bind to the surface). Although the free energy of binding is the same for these three strands (averaged to be -33.3(4) kJ/mol), the total ion density increases when several guanine residues are close to the 3' end (and thus close to the solid support substrate). To further understand these results, we analyzed the height profiles of the functionalized surfaces with tapping-mode atomic force microscopy (AFM). When comparing the average surface height profiles of the oligonucleotide surfaces pre- and post- Mg(II) binding, a positive correlation was found between ion density and the subsequent height decrease following Mg(II) binding, which we attribute to reductions in Coulomb repulsion and strand collapse once a critical number of Mg(II) ions are bound to the strand.

Divalent metal cation speciation and binding to surface-bound oligonucleotide single strands studied by second harmonic generation

The binding of Sr(II), Ca(II), Mg(II), Ba(II), Mn(II), Zn(II), and Cd(II) to silica/water interfaces functionalized with A(15)T(6) oligonucleotides was quantified at pH 7 and 10 mM NaCl using the Eisenthal χ((3)) technique. The binding free energies range from -31.1(6) kJ/mol for Ba(II) to -33.8(4) kJ/mol for Ca(II). The ion densities were found to range from 2(1) ions/strand for Zn(II) to 11(1) ions/strand for Cd(II). Additionally, we quantified Mg(II) binding in the presence of varying background electrolyte concentrations which showed that the binding free energies changed in a linear fashion from -39.3(8) to -27(1) kJ/mol over the electrolyte concentration range of 1-80 mM, respectively. An adsorption free energy versus interfacial potential analysis allowed us to elucidate the speciation of the bound Mg(II) ions and to identify three possible binding pathways. Our findings suggest that Mg(II) binds as a fully hydrated divalent cation, most likely displacing DNA-bound Na ions. These measurements will serve as a benchmark for computer simulations of divalent metal cation/DNA interactions for geochemical and biosensing applications.

Specific and nonspecific metal ion-nucleotide interactions at aqueous/solid interfaces functionalized with adenine, thymine, guanine, and cytosine oligomers

This article reports nonlinear optical measurements that quantify, for the first time directly and without labels, how many Mg(2+) cations are bound to DNA 21-mers covalently linked to fused silica/water interfaces maintained at pH 7 and 10 mM NaCl, and what the thermodynamics are of these interactions. The overall interaction of Mg(2+) with adenine, thymine, guanine, and cytosine is found to involve -10.0 ± 0.3, -11.2 ± 0.3, -14.0 ± 0.4, and -14.9 ± 0.4 kJ/mol, and nonspecific interactions with the phosphate and sugar backbone are found to contribute -21.0 ± 0.6 kJ/mol for each Mg(2+) ion bound. The specific and nonspecific contributions to the interaction energy of Mg(2+) with oligonucleotide single strands is found to be additive, which suggests that within the uncertainty of these surface-specific experiments, the Mg(2+) ions are evenly distributed over the oligomers and not isolated to the most strongly binding nucleobase. The nucleobases adenine and thymine are found to bind only three Mg(2+) ions per 21-mer oligonucleotide, while the bases cytosine and guanine are found to bind eleven Mg(2+) ions per 21-mer oligonucleotide.

Interactions of Al(III), La(III), Gd(III), and Lu(III) with the fused silica/water interface studied by second harmonic generation

The interactions of the trivalent metal cations Al(III), La(III), Gd(III), and Lu(III) with the silica/water interface were studied using the nonlinear optical technique of second harmonic generation (SHG). Specifically, the Eisenthal chi(3) technique was used to quantify the thermodynamics of trivalent ion adsorption to the bare fused silica surface. SHG adsorption isotherms were measured and fit with the triple layer surface complexation model to obtain adsorption free energies, binding constants, and interfacial charge densities. The adsorption free energy for Al(III) was found to be -37.2(5) kJ/mol, while the adsorption free energies for the three trivalent lanthanide cations ranged from -29.9(9) to -32.2(7) kJ/mol. Despite identical ionic charges, the metals under investigation displayed different affinities for the fused silica/water interface, and this finding is analyzed and interpreted in the context of size-dependent metal cation properties and metal ion speciation. The thermodynamic results from this work are valuable benchmarks for computer simulations of trivalent metal transport in the environment.

Sample cells for probing solid/liquid interfaces with broadband sum-frequency-generation spectroscopy

Two sample cells designed specifically for sum-frequency-generation (SFG) measurements at the solid/liquid interface were developed: one thin-layer analysis cell allowing measurement of films on reflective metallic surfaces through a micrometer layer of solution and one spectroelectrochemical cell allowing investigation of processes at the indium tin oxide/solution interface. Both sample cells are described in detail and data illustrating the capabilities of each are shown. To further improve measurements at solid/liquid interfaces, the broadband SFG system was modified to include a reference beam which can be measured simultaneously with the sample signal, permitting background correction of SFG spectra in real time. Sensitivity tests of this system yielded a signal-to-noise ratio of 100 at a surface coverage of 0.2 molecules/nm(2). Details on data analysis routines, pulse shaping methods of the visible beam, as well as the design of a purging chamber and sample stage setup are presented. These descriptions will be useful to those planning to set up a SFG spectrometer or seeking to optimize their own SFG systems for measurements of solid/liquid interfaces.

DNA at aqueous/solid interfaces: chirality-based detection via second harmonic generation activity

We present electronic spectra of single-strand and duplex DNA oligonucleotides covalently attached to fused quartz/aqueous interfaces and demonstrate that a strong nonlinear optical linear dichroism response is obtained when adenine and thymine bases undergo Watson-Crick base pairing to form a double helix. Complementary chi(3) charge screening studies indicate that the signal originates from 5 x 10(11) strands per square centimeter, or 6 attomoles of surface-bound oligonucleotides. The label-free, molecular-specific nature afforded by nonlinear optical studies of DNA at aqueous/solid interfaces allows for the real-time tracking of interfacial DNA hybridization for the first time.

Thermodynamics, interfacial structure, and pH hysteresis of Rb+ and Sr2+ adsorption at the muscovite (001)-solution interface

The coverage and average height of adsorbed Rb+ and Sr2+ at the muscovite (001)-solution interface were measured with resonant anomalous X-ray reflectivity (RAXR) as a function of cation concentration (10-8 < [Sr2+] < 10(-1) m, 10-6 < [Rb+] < 10(-1) m at pH 5.5 and 3.5) and pH (1.5 to 5.5 at [Me(n+)] = 10(-3) m) without background electrolyte. At pH 5.5, Rb+ uptake approximately follows a Langmuir isotherm with deltaG(Rb)(o) = -23.5 +/- 4.0 kJ x mol(-1) and a saturation coverage of Tmax = 0.94 +/- 0.06 Rb+ per unit cell area, Auc = 46.72 A2, compensating the nominal surface charge density (1 e-/Auc). The Sr2+ isotherm has a saturation coverage of 0.47 +/- 0.05 Sr2+/Auc that also compensates the muscovite's charge, but the adsorption edge is both more abrupt and shifted significantly to lower concentration than that for Rb+. The uptake of Sr2+ is consistent with a Frumkin isotherm with an intrinsic adsorption constant, deltaG(Sr)(o) = -28.8 +/- 6.0 kJ x mol(-1) and a correlation energy, gamma(Sr) = -7.2 +/- 3.7 kJ x mol(-1). The average height of each adsorbed cation, corresponding to inner-sphere dominant Rb+ and coexisting inner- and outer-sphere Sr2+ distributions, was independent of ion coverage at pH 5.5. At pH 3.5, the adsorption edges of both ions shift to higher cation concentration, indicating competition with hydronium, and the shifts are accompanied by an irreversible reduction in the saturation coverage. The inner-sphere dominant mode of Rb+ adsorption did not change at pH 3.5, while that of Sr2+ changed to an outer-sphere dominant distribution. Hysteresis in both the amount and height of the adsorbed ion was observed as a function of the direction in which pH was changed, indicating that the intrinsic surface charge density decreased after reaction with acidic solutions. These results suggest new and unexpected interrelationships among the distribution of adsorbed ions, competitive adsorption of hydronium, and surface charge density at the mineral-solution interface.