Surface pK_a of octanoic, nonanoic, and decanoic fatty acids at the air–water interface: applications to atmospheric aerosol chemistry

Abstract

There exists large uncertainty in the literature as to the pK of medium-chain fatty acids at the air-water interface. Via surface tension titration, the surface-pK values of octanoic (C), nonanoic (C), and decanoic (C) fatty acids are determined to be 4.9, 5.8, and 6.4, respectively. The surface-pK determined with surface tension differs from the bulk value obtained during a standard acid-base titration. Near the surface-pK of the C and C systems, surface tension minima are observed and are attributed to the formation of surface-active acid-soap complexes. The direction of the titration is shown to affect the surface-pK of the C system, as the value shifts to 5.2 with NaOH titrant due to a higher concentration of Na ions at pH values close to the surface-pK. As the reactivity and climate-relevant properties of sea spray aerosols (SSA) are partially dictated by the charge and surface activity of the organics at the aerosol-atmosphere interface, the results presented here on SSA-identified C-C fatty acids can be used to better predict the health and climate impact of particles with significant concentrations of medium-chain fatty acids.

Metal Ion Interactions with Crude Oil Components: Specificity of Ca2+ Binding to Naphthenic Acid at an Oil/Water Interface

“…Several experimental studies have identified increased pK a values for acid groups present at surfaces and interfaces, including self-assembled monolayers [51], fatty acid micelles [52,53] and, more recently, at the air/water interface for fatty acid monolayers [54], the latter study also identifying the effect of chain length on the magnitude of the pK a change. In the context of the present paper, this will mean that adsorbed NA molecules will remain largely undissociated as the effective (interfacial) pK a is higher than the pH of the aqueous solutions.…”

“…more recently, at the air/water interface for fatty acid monolayers [54], the latter study also identifying the effect of chain length on the magnitude of the pKa change. In the context of the present paper, this will mean that adsorbed NA molecules will remain largely undissociated as the effective (interfacial) pKa is higher than the pH of the aqueous solutions.…”

Collapse Mechanisms of Nascent and Aged Sea Spray Aerosol Proxy Films

“…As a result, the longer fatty acids will dominate the lateral interactions in the SSA film, making the film more rigid due to the greater sum of dispersion interactions [66,73,74]. Carboxylic acid head groups of long chain fatty acids (>C 14 ) are only partially deprotonated at high pH due to their relatively high surface pK a values [75,76]. However, cation-head group interactions slightly moderate the pK a increase associated with increasing alkyl chain length [76].…”

“…Carboxylic acid head groups of long chain fatty acids (>C 14 ) are only partially deprotonated at high pH due to their relatively high surface pK a values [75,76]. However, cation-head group interactions slightly moderate the pK a increase associated with increasing alkyl chain length [76]. As SSA ages in the marine boundary layer (MBL), the particles interact with trace gases that lower the pH of the particle aqueous core.…”

Effect of Alkyl Chain Length in Anions on the Physicochemical Properties of Cellulose-Dissolving Protic Ionic Liquids

“…For the case in which the DpK a value was above 20, the T d exceeded 4508C under an N 2 atmosphere. [10] Table 1 summarises the DpK a values for this study, which were determined via the following equation: DpK a ¼ pK a of DBUH þ (13.28) -pK a of carboxylic acid [13][14][15] (see Table 1). Since the DpK a values of the PILs synthesised in this study are between 6.88 and 9.51, it is likely that proton transfer (neutralisation) insufficiently occurs during the synthesis of PILs.…”