Insights into the behavior of nonanoic acid and its conjugate base at the air/water interface through a combined experimental and theoretical approach

Abstract

<p>The surface partitioning of a medium chain fatty acid and its conjugate base has been investigated through a combined experimental and theoretical approach of the multi-equilibria involved in the bulk phase and at the air/water interface.</p>

Calcium Bridging Drives Polysaccharide Co-Adsorption to a Proxy Sea Surface Microlayer

“…At pH 5.8, palmitic acid is mostly protonated, and alginate carboxylate groups remain deprotonated. Salts can deprotonate carboxylic acids at pH values significantly below the pKa, 61,95,96 however, so a d33-cetyl alcohol (d33-hexadecanol, CH3(CH2)15OH) monolayer was used as a control to study alginate coadsorption to a fully protonated monolayer. Alginic acid is insoluble in water, so a lower pH value was not tested instead to avoid significant changes in solubility.…”

The surface composition of amino acid – halide salt solutions is pH-dependent

“…Motivated by similar considerations, a few other groups have recently begun to investigate the connection between organic molecules, ionorganic ions and pH at aqueous surfaces and our findings here can be considered to be an extension of their results, despite using a different experimental method. [18][19][20]38 Studies such as ours should be expanded to alkaline solutions, divalent cations and anions such as Ca 2+ and SO 2− 4 , and other organic species. We have used amino acids as a proxy for several kinds of organic species here, since they exhibit two polar moieties with opposite protonation behaviour.…”

“…The pH of the sample solutions was adjusted with concentrated HCl such that the bulk solution pH is below the pK a of the carboxyl functional group of the respective amino acid. Note that the deprotonation behaviour of organic acids at the liquid-gas interface can differ from the bulk and depends on the presence of co-solvated cations 18,19,38 and the concentration of the organic molecules. 39 Measurements were also taken from samples without any further pH adjustments, usually resulting in a pH around 6.…”

“…The interplay of organic surfactants and pH on the surface composition of mixed ionic solutions has been addressed only recently. [18][19][20]25 Our work attempts to further fill this gap and expand our knowledge to the role amino acids play. We chose to use amino acids as organic component in our study as a surrogate for organic solutes in aerosol for several reasons.…”

Vibrational exciton delocalization precludes the use of infrared intensities as proxies for surfactant accumulation on aqueous surfaces

“…Soluble surfactant adsorption and film surface density are measured through vibrational band peak intensities in which hydrocarbon C-H and C-D modes are most commonly tracked. [28][29][30][31][32] The orientation of surfactant terminal methyl groups is frequently assessed via 1D VSFG and by calculating ratios of CH 3 peak intensities obtained by probing the system with different polarizations of light. 33,34 An analogous analysis using the CH 2 peak intensities from 1D IRRAS spectra is used to assess hydrocarbon chain conformational order at interfaces.…”