title: Hydrogen-bonding molecular ruler surfactants as probes of specific solvation at liquid/liquid interfaces.
abstract: Resonance-enhanced, second harmonic generation (SHG) is used to measure the electronic structure of solutes sensitive to specific solvation adsorbed to liquid/liquid and liquid/solid interfaces. Here, specific solvation refers to solvent-solute interactions that are directional and localized. N-methyl-p-methoxyaniline (NMMA) is a solute whose first allowed electronic transition wavelength remains almost constant (approximately 315 nm) in non-hydrogen-bonding solvents regardless of solvent polarity. However, in hydrogen-bond-accepting solvents such as dimethylsulfoxide, NMMA's absorbance shifts to longer wavelengths (320 nm), whereas in hydrogen-bond-donating solvents (e.g., water), the absorbance shifts to shorter wavelengths (approximately 300 nm). SHG experiments show that at alkane/silica interfaces, surface silanol groups serve as moderately strong hydrogen-bond donors as evidenced by NMMA's absorbance of 307 nm. At the carbon tetrachloride/water interface, NMMA absorbance also shifts to slightly shorter wavelengths (298 nm) implying that water molecules at this liquid/liquid interface are donating strong hydrogen bonds to the adsorbed NMMA solutes. In contrast, experiments using newly developed molecular ruler surfactants with NMMA as a model hydrophobic solute and a hydrophilic, cationic headgroup imply that, as NMMA migrates across an aqueous/alkane interface, it carries with it water that functions as a hydrogen-bond-accepting partner.
authors.affiliation: Department of Chemistry and Biochemistry, University of Maryland, College Park, College Park, MD 20742, USA.
authors.forename: A Renee
authors.initials: AR
authors.lastname: Siler
authors.forename: Michael R
authors.initials: MR
authors.lastname: Brindza
authors.forename: Robert A
authors.initials: RA
authors.lastname: Walker
mesh_terms: D000473:Alkanes / Q000737:chemistry; D000814:Aniline Compounds / Q000737:chemistry*; D002251:Carbon Tetrachloride / Q000737:chemistry; D004121:Dimethyl Sulfoxide / Q000737:chemistry; D005456:Fluorescent Dyes / Q000737:chemistry*; D006860:Hydrogen Bonding; D012822:Silicon Dioxide / Q000737:chemistry; D012997:Solvents / Q000737:chemistry; D013501:Surface-Active Agents / Q000737:chemistry*; D014867:Water / Q000737:chemistry
chemical_list: D000473:Alkanes; D000814:Aniline Compounds; D005456:Fluorescent Dyes; C546461:N-methyl-p-methoxyaniline; D012997:Solvents; D013501:Surface-Active Agents; D014867:Water; D012822:Silicon Dioxide; D002251:Carbon Tetrachloride; D004121:Dimethyl Sulfoxide