Sodium–carboxylate contact ion pair formation induces stabilization of palmitic acid monolayers at high pH

Abstract

Sea spray aerosols (SSA) are known to have an organic coating that is mainly composed of fatty acids. In this study, the effect of pH and salt on the stability and organization of a palmitic acid (PA) monolayer is investigated by surface vibrational spectroscopy and molecular dynamics simulations. Results indicate that alkyl chain packing becomes more disordered as the carboxylic headgroup becomes deprotonated. This is associated with packing mismatch of charged and neutral species as charged headgroups penetrate deeper into the solution phase. At pH 10.7, when the monolayer is ∼99% deprotonated, palmitate (PA) molecules desorb and solubilize into the bulk solution where there is spectroscopic evidence for aggregate formation. Yet, addition of 100 mM NaCl to the bulk solution is found to drive PA molecules to the aqueous surface. Free energy calculations show that PA molecules become stabilized within the interface with increasing NaCl concentration. Formation of contact -COO:Na pairs alters the hydration state of PA headgroups, thus increasing the surface propensity. As salts are highly concentrated in SSA, these results suggest that deprotonated fatty acids may be found at the air-aqueous interface of aerosol particles due to sea salt's role in surface stabilization.

Insights into the Headgroup and Chain Length Dependence of Surface Characteristics of Organic-Coated Sea Spray Aerosols

“…4,25,51 It is likely that the stability and surface activity of these long chain fatty acids decrease upon dissociation of the carboxylic acid proton. 4 However, the stability of these monolayers can be greatly improved by electrostatic attractions or complexation with various sea salts in the aqueous subphase. 49,52 By means of IRRAS, the surface propensity of PA molecules was found to be increased by adding NaCl into the subphase, 4 suggesting that deprotonated fatty acids may be found at the air-aqueous interface due to the role of sea salts in surface stabilization.…”

“…4 However, the stability of these monolayers can be greatly improved by electrostatic attractions or complexation with various sea salts in the aqueous subphase. 49,52 By means of IRRAS, the surface propensity of PA molecules was found to be increased by adding NaCl into the subphase, 4 suggesting that deprotonated fatty acids may be found at the air-aqueous interface due to the role of sea salts in surface stabilization. A study of SA monolayers was carried out on 1, 10 and 100 times diluted ASW.…”

“…[1][2] SSAs formed at the sea surface microlayer through bubble-mediated processes, are commonly composed of a sea salt core coated by a thin organic layer. [3][4][5][6] This organic layer is enriched with various organic species from both biological and anthropogenic sources, among which, surface-active species account for a significant portion. 5,7 Moreover, surface-active species in the sea surface microlayer are expected to be more efficiently transferred into SSA, thus exhibiting much higher concentration than that measured in the sea surface microlayer.…”

“…10,16 Previous measurements suggested that fatty acids make up a large fraction of the organic materials residing at the interface of marine aerosols. 4,7,17 Fatty acids present in sea surface microlayer are released primarily during the lysis of phospholipid cellular membranes of marine organisms. 11,14 Saturated fatty acids, particularly palmitic acid (PA) and stearic acid (SA), contribute significantly to the organic coating of sea salt particles, 11,[18][19] with arachidic acid (AA) showing a relatively lower abundance.…”

“…[25][26][27] There are extensive studies elucidating the microscopic profile of Langmuir films of saturated fatty acids at the air-water interface using IRRAS. 4,[28][29][30] In addition, fatty acid methyl esters are also known to be able to form organized structures at the air-water interface. [31][32][33][34] Previous studies of saturated long chain fatty acid and fatty acid methyl ester monolayers were carried out at the airwater interface with IRRAS technique, by which the conformational order of the monolayers were shown to be increased with increasing alkyl chain-length.…”