Chemical Composition and Properties of the Liquid–Vapor Interface of Aqueous C1 to C4 Monofunctional Acid and Alcohol Solutions

Abstract

The liquid-vapor interface is playing an important role in aerosol and cloud chemistry in cloud droplet activation by aerosol particles and potentially also in ice nucleation. We have employed the surface sensitive and chemically selective X-ray photoelectron spectroscopy (XPS) technique to examine the liquid-vapor interface for mixtures of water and small alcohols or small carboxylic acids (C1 to C4), abundant chemicals in the atmosphere in concentration ranges relevant for cloud chemistry or aerosol particles at the point of activation into a cloud droplet. A linear correlation was found between the headgroup carbon 1s core-level signal intensity and the surface excess derived from literature surface tension data with the offset being explained by the bulk contribution to the photoemission signal. The relative interfacial enhancement of the carboxylic acids over the carboxylates at the same bulk concentration was found to be highest (nearly 20) for propionic acid/propionate and still about 5 for formic acid/formate, also in fair agreement with surface tension measurements. This provides direct spectroscopic evidence for high carboxylic acid concentrations at aqueous solution-air interfaces that may be responsible for acid catalyzed chemistry under moderately acidic conditions with respect to their bulk aqueous phase acidity constant. By assessing the ratio of aliphatic to headgroup C 1s signal intensities XPS also provides information about the orientation of the molecules. The results indicate an increasing orientation of alcohols and neutral acids toward the surface normal as a function of chain length, along with increasing importance of lateral hydrophobic interactions at higher surface coverage. In turn, the carboxylate ions exhibit stronger orientation toward the surface normal than the corresponding neutral acids, likely caused by the stronger hydration of the charged headgroup.

The opposing effect of butanol and butyric acid on the abundance of bromide and iodide at the aqueous solution–air interface

“…46 This simple photoelectron attenuation law neglects elastic scattering effects, which are likely too small to become relevant in view of the uncertainty of all parameters involved, as also discussed further below. Note that for values of d/l { 1, the expression on the right hand side approaches AG, which corresponds to the case where all photoelectrons from carbon atoms from molecules constituting G are detected without attenuation, as assumed in our previous study by Lee et al, 52 which seemed a reasonable approximation as d/l doesn't exceed 0.25 for C1 to C4 organics. For the cases in this study, the difference to eqn (2) is around up to 15%.…”

“…Amphiphilic organics tend to minimize solvation of the hydrophobic tails and orient themselves in a way that the butyl chains reside above the interface while hydrating the hydrophilic head group. 52,68,69 The surface excess was derived from surface tension data reported by Granados et al (2006) for Bu-acid and by Donaldson and Anderson 60 for Bu-ol, as described previously, 52 in both cases for solutions not containing halide salts. This allows us to calculate the photoemission signal with a simple model.…”

“…A complete description of the equilibrated nature of a liquid microjet under the conditions of photoemission experiments has been given in the literature previously. 49,62 In the context of interest in this study, it is important that the surface active organic species (1-butanol and butyric acid) can reach their equilibrium concentration at the surface until the jet is hit by the X-ray beam, at 100 ms. For the 0.1 M/0.002 mole fraction solution range relevant for this study, the diffusional time scale for the formation of a monolayer equivalent on the surface is less than 1 ms. 52,53,62,63 For bromide containing solutions, spectra for obtaining core-level spectra of Br 3d, Na 2s, C 1s and O 1s at about the same kinetic energy were not taken each one after the other with setting the beamline energy to the appropriate energy value in between. We set the primary photon energy for the detection of Br 3d and Na 2s to 229 eV, resulting in a photoelectron KE of ca.…”

“…Our recent study showed a correlation between the solute photoemission signals of aqueous solutions of C1 to C4 alcohols and carboxylic acids with the surface excess derived from surface tension measurements, thus nicely demonstrating that XPS probes a locally equilibrated interface with respect to bulk -surface equilibrium. 52 Therefore, LJ-XPS is the ideal tool for furthering our understanding of the halide ions co-existing with organic matter at the aqueous solution-vapor interface.…”

Selective evaporation of a butanol/water droplet by microwave irradiation, a step toward economizing biobutanol production

“…For 1-butanol, it has been verified that the surface tension of its aqueous solution can be reduced to 25 mN/m at the solubility of 6 wt.%(Cheng and Park, 2017). Hence, it is expected that alkanols are strongly adsorbed at the air/water surface(Lee et al, 2016) and have selective evaporation(Walz et al, 2015), even at low concentration conditions of fermentative broth.…”