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Shifted Equilibria of Organic Acids and Bases in the Aqueous Surface Region† Cite This: Phys

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Shifted Equilibria of Organic Acids and Bases in the Aqueous Surface Region† Cite This: Phys PCCP View Article Online PAPER View Journal | View Issue Shifted equilibria of organic acids and bases in the aqueous surface region† Cite this: Phys. Chem. Chem. Phys., 2018, 20,23281 ab b a Josephina Werner, * Ingmar Persson, Olle Bjo¨rneholm, Delphine Kawecki,‡a Clara-Magdalena Saak, a Marie-Madeleine Walz, §a Victor Ekholm, a Isaak Unger,a Corina Valtl,a Carl Caleman, ac Gunnar O¨ hrwall d and Nønne L. Prisle ef Acid–base equilibria of carboxylic acids and alkyl amines in the aqueous surface region were studied using surface-sensitive X-ray photoelectron spectroscopy and molecular dynamics simulations. Solutions of these organic compounds were examined as a function of pH, concentration and chain length to investigate the distribution of acid and base form in the surface region as compared to the aqueous bulk. Results from these experiments show that the neutral forms of the studied acid–base pairs are strongly enriched in the aqueous surface region. Moreover, we show that for species with at Creative Commons Attribution-NonCommercial 3.0 Unported Licence. least four carbon atoms in their alkyl-chain, their charged forms are also found to be abundant in the surface region. Using a combination of XPS and MD results, a model is proposed that effectively describes the surface composition. Resulting absolute surface concentration estimations show clearly that the total organic mole fractions in the surface region change drastically as a function of solution Received 23rd March 2018, pH. The origin of the observed surface phenomena, hydronium/hydroxide concentrations in the Accepted 30th August 2018 aqueous surface region and why standard chemical equations, used to describe equilibria in dilute bulk DOI: 10.1039/c8cp01898g solution are not valid in the aqueous surface region, are discussed in detail. The reported results are of considerable importance especially for the detailed understanding of properties of small aqueous rsc.li/pccp droplets that can be found in the atmosphere. This article is licensed under a Introduction which ions may be present, and even enriched, at the surface.2,3 Also the possibly changed protolytic properties of the aqueous Open Access Article. Published on 31 August 2018. Downloaded 10/4/2021 4:43:40 AM. Acid–base chemistry at the aqueous interface is of great importance surface region and its underlying mechanisms have been subject for many processes e.g. in atmospheric chemistry and biochemistry. to intensive discussions. These protolytic surface properties have The different physico-chemical properties of the aqueous surface been investigated using different experimental and theoretical region in comparison to the bulk have been investigated throughout approaches, with rather contradictory outcomes.4–14 the last decades. For inorganic salts, the classic electrostatic model Discrepancies among experimental results and modeled with an ion-free surface region,1 has been replaced by a model in systems regarding the spatial distribution of hydronium and hydroxide ions at the air–water interface have been debated intensively.4,6–8,10,15 These ions are rather small species with a Department of Physics and Astronomy, Uppsala University, Box 516, relatively high charge densities. They are strongly hydrated as SE-751 20 Uppsala, Sweden. E-mail: [email protected] DH b Department of Molecular Sciences, Swedish University of Agricultural Sciences, can be seen from their large hydration enthalpies, hydr, + À À1 16 Box 7015, SE-750 07 Uppsala, Sweden of À1091 (H )andÀ460 (OH )kJmol , respectively. The c Center for Free-Electron Laser Science, Deutsches Elektronen-Synchrotron, effect of the hydration behavior of these chemical species on their Notkestraße 85, DE-22607 Hamburg, Germany residence in the aqueous surface region, their general role in d MAX IV Laboratory, Lund University, Box 118, SE-22100 Lund, Sweden surface chemistry and the influence of the bulk equilibria on the e Faculty of Science, Nano and Molecular Systems Research Unit, University of Oulu, Box 3000, FI-90014, Finland chemical speciation in the surface region are often discussed f Department of Physics, Division of Atmospheric Science, University of Helsinki, aspects in this context. Box 64, FI-00014, Finland Numerous studies draw conclusions about the pH, i.e. the † Electronic supplementary information (ESI) available. See DOI: 10.1039/c8cp01898g local hydronium and hydroxide concentrations, in the aqueous ‡ Present address: Technology and Society Laboratory, Empa – Swiss Federal surface region upon investigation of indicator species and their Laboratories for Materials Science and Technology, St. Gallen, Switzerland. § Present address: Department of Cell and Molecular Biology, Uppsala University, ability for protonation/deprotonation at the aqueous surface 6,8,10,11,13,17 Box 596, SE-75124 Uppsala, Sweden. region. Although acid–base equilibria in the aqueous This journal is © the Owner Societies 2018 Phys. Chem. Chem. Phys., 2018, 20, 23281--23293 | 23281 View Article Online Paper PCCP surface region have been investigated for several chemical of water in the surface region, but the observed distribution compounds by various experimental and theoretical techniques, between neutral and charged species strongly indicates that it is no study has been conducted investigating proton acceptors and lowerinthisregionthaninthebulk,possiblysufficientlylowto donors using the same approach. Results of separate studies on hinder dissociation of salts. A consequence of this is that it is individual compounds show that neutral forms of carboxylic difficult for single ions, which are dissolved in the aqueous bulk acids7 and amines10 are more abundant at the surface region in solution, to reach the surface region and even more so with comparison to their respective charged forms, i.e. carboxylate increasing charge density of the ion. However, ion-pair formation and ammonium ions. It was found that some ionic species are offers one possibility for charged species to reach to the surface depleted from the interfacial region, due to the reduced possibility region, or at least to be close to it. One such documented example 10,17 + to be fully hydrated. In another study, a thermodynamic is the two-dimensional guanidinium ion, (C(NH2)3) ,whichcan approach was used to explain the different degree of protonation reside very close to the air–water interface, when it is accompanied in the aqueous surface region to be caused by the different by chloride ions.21 Guanidinium ions were shown to reach hydration enthalpies of protonated and deprotonated species.12 significantly closer to the water surface than in systems without Whether these studies allow general conclusions on changed a counter-ion that is able to reside in the surface region.22 In abundances of hydronium/hydroxide ions in the interfacial another study it was shown that ammonium ions, which are region as compared with the bulk water is a major concern usually depleted from the surface region, are strongly attracted to and should be carefully validated. theaqueoussurfaceregionbyco-dissolved carboxylate ions.23 In this paper we show that the pH-dependent speciation and The dissociation of an acid (HA) in aqueous solution is distribution of the acid and base forms are quite different in quantified by a dissociation constant Ka (or its negative logarithm: the surface region as compared to the bulk and that it is not pKa), defined as the ratio of the activities of the reaction’s products possible to draw conclusions on the interfacial hydronium or (H+ and AÀ) and the reactant (HA). For weak acids and their hydroxide concentration by only examining a neutral species conjugated bases and for dilute solutions, concentrations (denoted Creative Commons Attribution-NonCommercial 3.0 Unported Licence. and its respective conjugated charged form. We refer to the as [species]) can be used instead of activities (a) by assuming the surface region as the air–water interface and the number of water activity coefficients to be unity. molecule layers below (surface-near-bulk region), required to HA + H O # H O+ +AÀ (1) reach bulk solution properties, throughout this work. The depth 2 3 þ À þ À of this region is difficult to specify, and it is likely dependent on aH3O Á aA ½H3O ½A Ka ¼ (2) temperature, ionic strength of the aqueous bulk and the specific aHA ½HA dissolved solutes. The surface region is thus characterized by distinctly different physico-chemical properties, i.e. solute con- The effect of the pH on the actual concentrations of acid and centrations, structure, dynamics etc., in comparison with the bulk base form of a conjugated acid–base pair in the bulk is given by This article is licensed under a solution. the so-called Henderson–Hasselbalch equation:24 Previous XPS studies have shown that the concentration of ½AÀ neutral compounds is significantly higher in the aqueous surface pH ¼ pKa þ log (3) ½HA Open Access Article. Published on 31 August 2018. Downloaded 10/4/2021 4:43:40 AM. region as compared to the bulk, also in cases where the 7,18,19 compound is miscible with water. It can be assumed that Rearranging eqn (3), we find that the ratio R of the acid and the physico-chemical properties of water at the air–water inter- base concentration in the bulk of the solution can be expressed as face and some water molecule layers below the interface are ½HA significantly different from bulk water. One example for the R ¼ ¼ 10ðÞpKaÀpH (4) deviating properties of water in the bulk and in the surface ½AÀ region is its relative permittivity. In bulk water it amounts to and the bulk acid fraction F of the solution as e = 78.520 and promotes dissociation of salts and hydration of A r À1 inorganic ions, and polar or charged functional groups. As ½HA ðÞpHÀpKa FA ¼ ¼ 10 þ 1 ; (5) discussed above, neutral molecules are more abundant in the ½HAþ½AÀ surface region than their charged conjugates. This suggests that the relative permittivity of water in the surface region is significantly which will be used later in the analysis.
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