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

Hayes, Patrick L.; Malin, Jessica N.; Konek, Christopher T.; Geiger, Franz M.

doi:10.1021/jp076976g

Cited by 73 publications

(174 citation statements)

References 66 publications

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“…The negative TPB À ions form strong p-H bonds to water (as shown in Figure 2) and thus are expected to preferentially orient water hydrogen atoms towards the oil/water interface (Figure 3 C, top left). Moreover, the DC electric field produced by adsorption of negative TPB À ions will favor a similar dipolar orientation of water, as expected [38,[44][45][46] (Figure 3 C, top right). In contrast, the weak p-H bonds between water and TPA + will tend to slightly favor orienting water hydrogen atoms towards the interface, while the DC electric field Figure 3.…”

Section: Methodssupporting

confidence: 63%

Charge Asymmetry at Aqueous Hydrophobic Interfaces and Hydration Shells

et al. 2014

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The solvation of ions in a dielectric continuum is symmetric with respect to the sign of the ionic charge. Yet, many phenomena show indirect evidence that water may have a preference for negative charge: the vast majority of biological membrane interfaces are neutral or negatively charged, [1] water reorientation dynamics around anions and cations is different, [2] and the zero charge point of water is usually well below pH 7 at hydrophobic/water interfaces. [3] There is undoubtedly a link to the asymmetry of the water molecule. Here, we spectroscopically quantify differences between the structures of hydration shells and hydrophobic/ water interfaces induced by ions of opposite charge but essentially identical molecular structure. We show that these two ions, tetraphenylborate (TPB À ) and tetraphenylarsonium (TPA + ), interact dramatically differently with water and its interface: The anion is preferentially hydrated and induces greater orientational order to water near hydrophobic interfaces. In contrast, the cation forms far fewer and weaker p À H bonds than the anion and strongly reduces the orientational order of water near a hydrophobic interface.The vast majority of lipid (cell) membranes, macromolecules, and their interfaces are either neutral or negatively charged. [1] The hydrophobic-water surface carries a negative charge, and the point of zero charge of most interfaces occurs at very low pH values. [4][5][6] The hydration free energies [7][8][9] and entropies [8,10] of anions are calculated to be significantly more negative than those of cations of similar size. Moreover, the dynamics of water reorientation and hydrogen-bond exchange is quite different around positive and negative ions. [2,11] Both experiments and simulations indicate that ions of different size, structure, and charge partition differently at aqueous interfaces. [12,13] In addition to ion polarization, the surface potential at an air-water interface has been proposed to be one of the driving forces that favors anion adsorption. [9,14,15] Although specific ion (i.e. Hofmeister) effects exist for both negative and positive ions, and although they typically correlate with differences in size, polarizability, and/or charge density, [16] specific ion effects are generally smaller for positive ions than for negative ions, [17] thus suggesting an inherent asymmetry in the interaction of water with positive and negative ions. Yet, water is often treated as a dielectric continuum. [18] The implied charge asymmetry is undoubtedly related to waters dipolar character and highly directional hydrogenbonding proclivity. Although electrostatic interactions are symmetric with respect to charge exchange, hydrogen-bonding interactions are not symmetric with respect to solute charge, as water preferentially donates hydrogen bonds to anions (OÀH···X À ). Moreover, hydrogen bonding is highly cooperative (non-additive) in the sense that the average energy per hydrogen bond in water clusters increases nonlinearly with the total number of hydrogen bonds. [19...

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Section: Methodssupporting

confidence: 63%

Charge Asymmetry at Aqueous Hydrophobic Interfaces and Hydration Shells

et al. 2014

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“…To this end, we carried out a charge screening experiment at pH 7 using methods described previously. 53,59,64,86 Briefly, we collected the I SHG as a function of NaCl concentration up to 0.4 M at pH 7. SHG intensity normalization, referencing, and averaging was carried out on data obtained from triplicate measurements as described previously.…”

Section: Resultsmentioning

confidence: 99%

Interaction of Magnesium Ions with Pristine Single-Layer and Defected Graphene/Water Interfaces Studied by Second Harmonic Generation

et al. 2014

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This work reports thermodynamic and electrostatic parameters for fused silica/water interfaces containing cm(2)-sized graphene ranging from a single layer of pristine graphene to defected graphene. Second harmonic generation (SHG) measurements carried out at pH 7 indicate that the surface charge density of the fused silica/water interface containing the defected graphene (-0.009(3) to -0.010(3) C/m(2)) is between that of defect-free single layer graphene (-0.0049(8) C/m(2)) and bare fused silica (-0.013(6) C/m(2)). The interfacial free energy of the fused silica/water interface calculated from the Lippmann equation is reduced by a factor of 7 in the presence of single-layer pristine graphene, while defected graphene reduces it only by a factor of at most 2. Subsequent SHG adsorption isotherm studies probing the Mg(2+) adsorption at the fused silica/water interface result in fully reversible metal ion interactions and observed binding constants, Kads, of 4(1) - 5(1) × 10(3) M(-1) for pristine graphene and 3(1) - 4(1) × 10(3) M(-1) for defected graphene, corresponding to adsorption free energies, ΔGads, referenced to the 55.5 molarity of water, of -30(1) to -31.1(7) kJ/mol for both interfaces, comparable to Mg(2+) adsorption at the bare fused silica/water interface. Maximum Mg(2+) ion densities are obtained from Gouy-Chapman model fits to the Langmuir adsorption isotherms and found to range from 1.1(5) - 1.5(4) × 10(12) ions adsorbed per cm(2) for pristine graphene and 2(1) - 3.1(5) × 10(12) ions adsorbed per cm(2) for defected graphene, slightly smaller than those of for Mg(2+) adsorption at the bare fused silica/water interface ((2-4) × 10(12) ions adsorbed per cm(2)), assuming the magnesium ions are bound as divalent species. We conclude that the presence of defects in the graphene sheet, which we estimate here to be around 1.3 × 10(11) cm(2), imparts only subtle changes in the thermodynamic and electrostatic parameters quantified here.

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“…42 Experimental studies have been performed with X-ray photolelectron spectroscopy of ions situated within several nanometers of the air/water interface (as determined by the penetration depth of the probe) of a liquid jet. 25 Second harmonic (SH) generation measurements of the surface potential of polystyrene particles and of emulsion droplets and liposomes 43,44 have been performed as well as resonant SH reflection measurements 34 and sum frequency generation measurements 35 of nitrate ions at the quartz/water interface. These studies showed that it is possible to measure the resonant response of the interfacial ions, and reported how this response is affected by different counterions.…”

Section: ■ Introductionmentioning

confidence: 99%

Stern Layer Formation Induced by Hydrophobic Interactions: A Molecular Level Study

et al. 2013

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The molecular ionic surface structure and charge of the electric double layer around a nanodroplet and its structural change induced by hydrophobic effects are measured using vibrational coherent surface scattering spectroscopy, second harmonic scattering, and electrokinetic mobility measurements. Tetraalkylammonium chloride salts were added to negatively charged nanoscopic oil droplets in water. When we vary the alkyl chain length of the cation from CH3 to C4H10, both the size of the cation and its hydrophobic interaction are increased. We find that tetramethylammonium ions change the electrokinetic potential and the water structure but do not detectably adsorb to the interface. Tetrapropylammonium and tetrabutylammonium ions clearly adsorb to the interface. The corresponding (Stern) layer appears to be a mixed monolayer of anions and cations. An estimate of the amount of cations in the Stern layer is also made.

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

Cited by 73 publications

References 66 publications

Charge Asymmetry at Aqueous Hydrophobic Interfaces and Hydration Shells

Charge Asymmetry at Aqueous Hydrophobic Interfaces and Hydration Shells

Interaction of Magnesium Ions with Pristine Single-Layer and Defected Graphene/Water Interfaces Studied by Second Harmonic Generation

Stern Layer Formation Induced by Hydrophobic Interactions: A Molecular Level Study

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