The nonlinear fittings in Figures 1 and 2 with correlations≥ 0.98 allow the determination of the pvalues by reproducing the titration curves of acetic and pyruvic acids in bulk solutions with <5.0% error relative to respective literature values of 4.75 and 2.39. (23) Any deviations from the literature pvalues, possibly due to electric fields on the surface of water, (32) are therefore negligible and can be disregarded. This statement is corroborated by the fact thatgas-phase bases (like trimethylamine)acids are protonated and deprotonated on the surface of water at pH values lower than their p’s in bulk water. Electric fields do not perturb the charge balance of the double layer; otherwise, they should have had opposite effects on the titration of acids and bases on the surface of water. (1,3) Therefore, this procedure is valid for determining the pvalues of low-molecular-weight carboxylic acids, in agreement with previous observations made for-hexanoic and-octanoic acids with a different ESI-MS setup. (3) Thus, the measured pvalues imply that standard eqs 3 and 4 also govern the deprotonation behavior registered for acetic and pyruvic acids, respectively (3) whereandare the slopes of the respective sigmoid curves, which take unity value in traditional titration methods, and the dissociation fractions clearly depend on the difference (p– pH) rather than only on pH. Therefore, the observed titration curves reflect functions in which neither pnor pH has practically shifted relative to their bulk values. (3) Thus, it can be deduced that the pH of the experiments in Figures 1 and 2 cannot be distinguished from those in bulk solution.

The dissociation fraction of acetate (and the representative normalizedvalue) spans from α= 0 for the bulk solution with only undissociated acid up to α= 1 when only the conjugate base is present. The inflection point of the characteristic sigmoid curve fitting the dataset in Figure 1 marks p, which indicates at which bulk acid strength the concentrations of the acid and base forms are equal. More specifically, the inflection point is located at≅ α= 0.5, where pH = p= 4.55 and the asymptote tends to 1.

It is important to highlight that before this study, only one experimental report had tackled the investigation of pshifts by observing a transfer of protons initiated from the air side of the air–water interface. (1) It should be emphasized that Bronsted acidity and basicity are relative concepts that describe the extent of proton transfer of acids and bases with the local solvent. Herein, we demonstrate that the surface of water is more basic than the bulk, meaning that it is a better proton acceptor than bulk water. Vibrational sum frequency spectroscopy (VSFS) experiments on interfaces of aqueous solutions of acetic acid (of unreported pH) revealed the sole presence of undissociated acid, (38) even at the lowest acid concentrations, which exceeded those used in our experiments by a factor of 3. VSFS experiments were apparently unable to detect acetate, (38) possibly due to limitations on the alignment of moieties at the interface. (39) Furthermore, we wish to point out the absence of VSFS experiments on acetate solutions. (38) We consider that VSFS data are moot regarding the extent of dissociation of acetic acid on the surface of its aqueous solutions. (38) In short, acidity is a property that depends on the medium and is defined by the local equilibrium constant for the process AH + HO ⇄ A+ H. This equilibrium may have different pvalues on the surface versus in the bulk because the acid and water may have different acid–base properties as a result of the differential hydration of reactants and products, as discussed in the next section.

The high number of [CHCOCOOH(g)] = 3.28 × 10molecules cmin the experiment of Figure 5 also exceeds the expected plateau for Langmuir adsorption of low-molecular-weight carboxylic acids. (1) A reasonable estimate for the reactive uptake coefficient of gaseous pyruvic acid by water of γ= 0.06 can be retrieved after rearranging eq 5 for this molecule as followswhere the mean thermal velocity used is that of CHCOCOOH(g) at 298 K, υ= 2.91 × 10cm s= 7.39 × 10molecules cm (1) and τ= 1 × 10s for this system, (29) and the value= 3.25 × 10HOcmis obtained above using acetic acid.

Figure 5. (A) OESI-MS titration curve for pyruvic acid (from a solution in water) deposited on the surface of water for microdroplets exposed to 1.40 ppmv (≡3.28 × 10 13 molecules cm –3 under the experimental conditions) for a contact time τ c = 1 μs. (○)Normalized ion count for the acetate anion at m / z 87, I 87 , as a function of pH fitted with a (red line) sigmoid curve with the coefficient of correlation r 2 = 0.967. (B) First derivative of the sigmoid curve in (A), where the maximum corresponds to the p K a (blue line).

When a gas stream of pyruvic acid, generated by a flow of N(g) through a solution in water, encounters the interface of the pH-adjusted aqueous microdroplets, pyruvate is immediately formed by dissociation ( Figure 5 ). The sigmoid fitting of Figure 5 A for gaseous pyruvic acid deposited on the surface of water at variable pH has an inflection point at p= 0.65 matching the maxima for its first derivative in Figure 5 B. By comparison, the measured pfor CHCOCOOH(g) deposited on the surface of water ( Figure 5 ) drops 1.78 pH units relative to CHCOCOOH(aq) in bulk water ( Figure 2 ). Thus, the air side of the interface senses CHCOCOOH(g) to be 55-times stronger as a Bronsted acid than CHCOCOOH(aq) on the water side of the interface.

The experiments in Figures 3 and 4 employed a high mixing ratio of 2.00 ppmv CHCOOH(g), which is larger than the maximum 300 ppbv (≡7.39 × 10molecules cm) of organic acid needed to reach the plateau for Langmuir adsorption in this kind of system. (1) Therefore, the available water surface of generated microdroplets is fully covered by impinging acetic acid molecules. Under these conditions, it is clear that gaseous acetic acid does not transfer its proton to neutral water molecules directly but to the interfacial dangling basic groups, (33) HO, which limit the amount of CHCOOions generated and detected. The surface density of HOtoward the air side of the aqueous microdroplet interface,, available to interact with CHCOOH(g) molecules can be estimated from the product of two terms, the frequency of collisions provided by the kinetics theory of gases and τ= 1 μswhere the reactive uptake coefficient for acetic acid is γ= 0.05, (34) the mean thermal velocity of gaseous acetic acid at 298 K is υ= 3.52 × 10cm s, and the number density for 300 ppbv acetic acid is= 7.39 × 10molecules cm. By the substitution of the previous values in eq 5 , an estimate of= 3.25 × 10HOcmis obtained for the experiments in Figures 3 and 4 , which is equivalent to a surface charge density= 0.52 nC cm. The reportedand its associatedvalue in our aqueous microdroplet setup are in excellent agreement with those reported from experiments with hexanoic acid sparged on the surface of a water jet during 10 μs contact time. (1) Previous work has demonstrated that anions can populate the interface of water with air at depths that are inversely proportional to the square of their size, as represented by the ionic radius, even for dissimilar concentrations affecting the sublayers. (27,35,36) Thus, the fact thatis 2 × 10times smaller than that observed from ζ-potential measurements in the electrophoresis of bubbles and oil droplets in water (33,37) is logically explained as reflecting experiments that have tested the basicity of water at different depths. (1)

Figure 4. (A) OESI-MS titration curve for acetic acid (from a solution in cyclohexanol) deposited on the surface of water for microdroplets exposed to 2.00 ppmv (≡4.55 × 10 13 molecules cm –3 under the experimental conditions) for a contact time τ c = 1 μs. (○)Normalized ion count for the acetate anion at m / z 59, I 59 , as a function of pH fitted with a (red line) sigmoid curve with the coefficient of correlation r 2 = 0.987. (B) First derivative of the sigmoid curve in (A), where the maximum corresponds to the p K a (blue line).

Figure 3. (A) OESI-MS titration curve for acetic acid (from a solution in water) deposited on the surface of water for microdroplets exposed to 2.00 ppmv (≡4.55 × 10 13 molecules cm –3 under the experimental conditions) for a contact time τ c = 1 μs. (○)Normalized ion count for the acetate anion at m / z 59, I 59 , as a function of pH fitted with a (red line) sigmoid curve with the coefficient of correlation r 2 = 0.986. (B) First derivative of the sigmoid curve in (A), where the maximum corresponds to p K a (blue line).

Dependence of the pK a Shifts on the Selective Hydration of Acid–Base Pairs

K a shift of 2.75 units from the bulk phase to the surface of water for acetic acid agrees well with the observations for hexanoic and octanoic acids.C sp2 oriented perpendicularly to the surface plane. Simultaneously, the hydrophobic −CH 3 of acetic acid is repelled into the air, aligning the axis defined by the C–C bond close to the surface normal (with a tilt ≤15° from the surface normal).– group The pshift of 2.75 units from the bulk phase to the surface of water for acetic acid agrees well with the observations for hexanoic and octanoic acids. (1,3) Similar to simple monocarboxylic acids, the impinging gaseous acetic acid molecule has its −COOH group oriented in an upright position with respect to the surface. (40) In other words, when gaseous acetic acid reaches the local plane of the interface, its −COOH group points to the aqueous core with the plane established by the −COOH group with aoriented perpendicularly to the surface plane. Simultaneously, the hydrophobic −CHof acetic acid is repelled into the air, aligning the axis defined by the C–C bond close to the surface normal (with a tilt ≤15° from the surface normal). (40) A similar assumption could be made for the corresponding −COOgroup (41) from quickly formed acetate after proton loss from the acid form that has accommodated at the interface.

– group formed at the interface, changing the amphiphilic balance of the molecule.–]/[−COOH] sensed from the air side of the air–water interface is larger than in the bulk. In other words, the interfacial ratio [−COO–]/[−COOH] cannot be predicted by bulk pK a values. Additional water molecules can contribute to hydrating the −COOgroup formed at the interface, changing the amphiphilic balance of the molecule. (42) On this basis, photoelectron spectroscopy and VSFS measurements have proposed that enrichment of acetic acid relative to acetate occurs at the interface. (38,42) However, we interpret the previous spectroscopy studies of acetic acid solutions to have sampled the interior layer of the interface of acetic acid solutions. Instead, in our work, the external layers of the interface are directly sampled, as gaseous acetic acid collides with water, showing that the acid has a larger capacity to transfer its proton to surface water (that behaves as a base) than in the bulk. Therefore, acetic acid behaves as a stronger acid at the interface, and the ratio [−COO]/[−COOH] sensed from the air side of the air–water interface is larger than in the bulk. In other words, the interfacial ratio [−COO]/[−COOH] cannot be predicted by bulk pKvalues.

K a shift of 1.78 units is observed, which is in agreement with the shift of 1.84 pK a units computed at the quartz–water interface using a combination of ab initio molecular dynamics simulations. For pyruvic acid, a smaller pshift of 1.78 units is observed, which is in agreement with the shift of 1.84 punits computed at the quartz–water interface using a combination of ab initio molecular dynamics simulations. (2) Whereas the likelihood of natural hydrophobic quartz surfaces is low (covered with siloxane Si–O–Si bridges), Parashar et al. modeled the most common hydrophilic quartz surface with −OH groups from the silanols (43) directly in contact with water. (2) Because the orientation of pyruvic acid at the air–water interface remains unexplored, the quartz–water model serves here as a first-order approximation for visualizing the behavior of the hydrophilic −OH groups from the terminal silanols in contact with water. Related VSFS measurements show that the interfacial potential at the quartz–water and air–water interfaces for the surface adsorption of lysozyme is controlled by the protonation/deprotonation of the protein’s groups. (39) Whereas at the air–water interface, some lysozyme residues are organized at the hydrophobic air surface, major changes are only observed on the hydrophilic quartz–water interface for pH > 8.0, when a net negative charge is developed. (39) Therefore, such an extrapolation of the interfaces provides an initial model to interpret the mechanism by which gaseous pyruvic acid interacts with the surface of water under the experimental pH conditions explored in Figure 5

3 group of the acid to be in contact with the surface of water. Therefore, the different surface propensity of pyruvic acid relative to acetic acid also affects the interfacial abundance of each acid–base pair.K a shift is associated with the molecular orientation of pyruvic acid/pyruvate at the air–water interface as well as the local structure and proton-accepting properties of interfacial water. At variance with the perpendicular geometry for acetic acid, the −COOH group of pyruvic acid is pictured to be practically plane parallel to the surface of water (as shown in the table of content graphic). Remarkably, radial distribution functions for pyruvic acid and pyruvate with the oxygen and hydrogen atoms of water, respectively, (2) suggest that the acid remains localized, with C═O accommodated on the surface of water while getting hydrated. (24) The previous configuration constrains the hydrophobic −CHgroup of the acid to be in contact with the surface of water. Therefore, the different surface propensity of pyruvic acid relative to acetic acid also affects the interfacial abundance of each acid–base pair. (42) Pyruvate also plays an important role favoring the dissociation of pyruvic acid at the interface. On the surface, pyruvate not only forms hydrogen bonds with water from the first monolayer, but as it gets buried deeper than the neutral molecule, it also gains additional stabilization by H bonds with the contiguous layer of water. (2) Thus, the enhanced acidity of pyruvic acid at the interface is ascribed to the differential microsolvation of the acid versus its conjugated base. The pshift is associated with the molecular orientation of pyruvic acid/pyruvate at the air–water interface as well as the local structure and proton-accepting properties of interfacial water. (2)

K a,R1 = 1.78 × 10–2 and K a,R2 = 2.24 × 10–1, respectively: (R1) (R2) The results presented in the previous section for gaseous acetic and pyruvic acids follow the thermodynamically favorable interfacial proton transfer behavior (1) dictated by reactions R1 and R2 with dissociation constants= 1.78 × 10and= 2.24 × 10, respectively: