Inorganic anions and cations are ubiquitous in environmental chemistry. Here, we use second harmonic generation to track the interaction of the environmentally important metal cations barium, strontium, and cadmium and the nitrate anion with fused quartz/water interfaces at pH 7. Using a dynamic flow system, we assess the extent of reversibility in the binding process and report the absolute number density of adsorbed cations, their charge densities, and their free energies of adsorption. We also present resonantly enhanced second harmonic generation experiments that show that nitrate is surface active and report the free energies and binding constants for the adsorption process. The second harmonic generation spectrum of surface-bound nitrate shows a new adsorption band that cuts further into the solar spectrum than nitrate in the aqueous or solid state. The results that we obtain for all four inorganic ions and the implications for tropospheric and aquatic chemistry as well as geochemistry are discussed in the context of fundamental science as well as pollutant transport models.

The adsorption of Ca, Zn, and Cd ions at the solid/water interface was studied using second harmonic generation (SHG). Specifically, we used the SHG χ (3) technique to assess and evaluate metal adsorption at a bare fused quartz/water interface, as well as at a carboxylic acid functionalized fused quartz/water interface. Adsorption isotherms were obtained for the metals at each interface and were fit with the diffuse layer and triple layer models to obtain adsorption free energies, interfacial potentials, absolute adsorbate number densities, and interfacial charge densities. The carboxylic acid functionalized interface displayed lower adsorption free energies but higher adsorbed metal number densities than the bare fused quartz/water interface. We suggest that metal binding at the bare fused quartz/water interface is driven by the initial number of deprotonated surface sites and that it proceeds through an outer-sphere mechanism. In contrast, metal binding at the carboxylic acid functionalized interface is associated with carboxylic acid deprotonation at a substantial free energy cost and proceeds directly through metal-carboxylate interactions. † Part of the special section "Physical Chemistry of Environmental Interfaces".

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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.

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.