Thermodynamics, structure and electrostatics of adsorbates at geochemical interfaces determined by second harmonic and vibrational sum frequency generation
Solid-water interfaces are ubiquitous in nature where they dramatically impact industrial technologies and the environmental fate of pollutants. Thus, a fundamental understanding of the interactions occurring at solid-water interfaces is necessary to accurately model pollutant mobility and optimize surfactant-based technologies. Given this need, second harmonic generation (SHG) and vibrational sum frequency generation (SFG) are utilized to investigate a surfactant and several pollutants at model environmental interfaces.
Initial SHG studies focus on the adsorption of the antibiotic oxytetracycline to four model environmental interfaces: methylamide functionalized silica as well as amide-linked carboxylic acid-, benzoic acid-, and benzyl-terminated silica. The amide-linked benzoic acid-terminated silica displays the highest adsorption free energy among the systems surveyed. A method of using contact angle measurements to predict oxytetracycline mobility across environmental interfaces is presented.
The interaction of nitrate with the silica/water interface is also studied. The binding constant and free energy of adsorption are determined. The electronic spectrum for interfacial nitrate is measured, and a new absorption band is observed that cuts further into the solar spectrum than that of bulk aqueous or solid-state nitrate. The experiments demonstrate that nitrate is surface active and adsorption to silica-rich mineral dust particles may increase the photoactivity of nitrate.
An off-resonance SHG method, termed “the χ(3) technique”, is applied to investigate metal ions at the silica/water and carboxylic acid functionalized silica/water interface. Adsorption isotherms are obtained, and adsorbate surface densities are determined. It is found that at the silica/water interface a water shell encloses the adsorbed metal, whereas for the carboxylic acid functionalized interface acid deprotonation occurs along with inner-shell adsorption.
Theχ(3) technique is also utilized to investigate the adsorption of the surfactant cetyltrimethylammonium (CTA) at the silica/water interface for varying ionic strengths. Results indicate that significant counter-ion co-adsorption occurs, and that hydrophobic interactions between surfactant hydrocarbon chains drive adsorption. SFG is utilized to probe the structure of adsorbed CTA. At low salt concentrations the surfactant adsorbs as micelle-like structures across a range of bulk surfactant concentrations. At higher salt concentrations isolated surfactant molecules adsorb at low CTA surface coverage and then rearrange into aggregates at high surface coverage.