Organic enrichment at aqueous interfaces studied with non-linear spectroscopy: cooperative adsorption of soluble saccharides to lipid monolayers

Abstract

Field measurements of sea spray aerosols have reported high concentrations of soluble organic material that are in excess of the concentration of soluble organics in the ocean. The studies described in this dissertation investigated a possible mechanism for this increase deemed cooperative adsorption. The cooperative adsorption mechanism describes an interaction between an insoluble Langmuir monolayer at the aqueous/vapor interface and soluble organic molecules that would not normally be enriched at the surface. In this model, the soluble organics are drawn to the surface through non-covalent interactions with the lipid surfactant. This mechanism was investigated with the surface specific nonlinear optical technique, vibrational sum frequency generation spectroscopy. These optical measurements were coupled with surface tension measurements and differential scanning calorimetry measurements. To study cooperative adsorption, model systems were used; these were composed of a phosphatidylcholine lipid surfactant, DPPC, and soluble saccharides including glucosamine, glucuronic acid, and trehalose. Glucosamine, in both a positive and neutral state, induced ordering in both expanded and condensed DPPC monolayers, supporting cooperative adsorption as a mechanism. Glucuronic acid, an anion, ordered lipid monolayers in the limits that the lipid DPPC was moderately packed and there were no competing ions in solution. Trehalose, a larger, uncharged saccharide showed, through ordering the DPPC monolayer, indications of cooperative adsorption in moderately packed DPPC when the trehalose concentration was sufficiently high. These results support cooperative adsorption as a mechanism for the accumulation of soluble organics in sea spray aerosols with some limitations.