title: Effects of atmospherically important solvated ions on organic acid adsorption at the surface of aqueous solutions.
abstract: The effects of salts on the solubility of amphiphilic organic molecules are of importance to numerous atmospheric, environmental, and biological systems. A detailed picture of the influence of dissolved atmospheric salts, NaCl and Na(2)SO(4), on the adsorption of hexanoic acid at the vapor/water interface is developed using vibrational sum-frequency spectroscopy and surface tension measurements as a function of time, organic concentration, and solution pH. We have found that for hexanoic acid adsorption at the vapor/water interface, a fast initial adsorption is followed by two considerably slower processes: a reorientation of the polar headgroup and a restructuring of the headgroup solvation shell. The addition of salts affects this restructuring by reducing the range of water--headgroup interactions immediately upon surface adsorption for ion containing solutions. Reorientation of the organic headgroup with time occurs at the surface of both salt-containing and salt-free solutions, but the most stable orientation differs with the added ions. The dissolved salts also enhance the interfacial concentration of hexanoic acid, consistent with the known salting-out behavior of Cl(-) and SO(4)(2-) anions.
authors.affiliation: Department of Chemistry, University of Oregon, 1253 University of Oregon, Eugene, Oregon 97403, USA.
authors.forename: Melissa C Kido
authors.initials: MC
authors.lastname: Soule
authors.forename: Patrick G
authors.initials: PG
authors.lastname: Blower
authors.forename: Geraldine L
authors.initials: GL
authors.lastname: Richmond