Chromatography and the Hotly Debated Enigma of Aqueous Surface's

Article pubs.acs.org/JPCC

Chromatography and the Hotly Debated Enigma of Aqueous Surface’s Acid−Base Character Teresa Cecchi* Accademia delle Scienze, Via Zamboni, 31, 40126 Bologna, Italy ITT Montani, Via Montani 7, 63900 Fermo, Italy ABSTRACT: How charges are accommodated on the water surfaces is a 100 year mystery. In the past decade the question whether the surface of water is acidic or basic has arisen and has deepened the debate about the presence of ions at dielectric boundaries. Experimental results are scarce and scattered. Arguments stemming from macroscopic experiments support a basic interface but are in stark contrast to those probing it at the molecular level. For the very first time we placed chromatography in the armory for tackling the water surface acid−base character. Chromatography is a formidable tool to ascertain the extent to which H+ and OH− adsorb onto an hydrophobic surface. Frontal analysis gave unequivocal quantitative experimental evidence of the higher surface affinity of OH− compared to that of H+. Pulse chromatography independently confirmed this outstanding result. Adsorption isotherm data were obtained from breakthrough curves, and the isotherm parameters were estimated by fitting them to the BET isotherm model. The estimates of the parameters, which have a clear physical meaning, are very reliable.

1. INTRODUCTION Water is surely the most important molecule in our relation to nature. The molecular architecture of water interfaces plays a crucial role in many chemical, biological, physical, and biophysical processes.1,2 The understanding of water interfaces is pivotal: the boundaries between water and air, a protein, or an hydrophobic medium are governed by general physical effects.3−5 The ion-free interface paradigm described by the Onsager− Samaras theory is an outstanding puzzle of physical chemistry: it rationalizes Gibbs adsorption isotherm predictions, but it stands in stark contrast to mounting experimental evidence,2,6 to molecular dynamics predictions concerning the presence of ions at water interfaces,7−9 and to theoretical speculations.10,11 A quantitative description of the driving forces that determine ion adsorption or repulsion at the interface is still elusive.4,12 One of the great debates that stirs the controversy concerning the presence of ions at dielectric boundaries deeper regards the autoionization of surface water which gives rise to its pH, defined using proton concentration in the topmost layer of neat water: its acidity and its basicity were equally claimed.1 Aqueous surface’s acid−base character was the object of intensive scientific scrutiny, but it is an intriguing open question.1 Moreover, there is no agreement on water surface charge and potential. Molecular dynamics simulations strongly supported an acidic water/air interface.13,14 Experimental results obtained via second harmonic generation spectroscopy15 and X-ray photoelectron spectroscopy16 supported a stronger surface propensity for protons, too. Many authors17,18 interpreted their results to indicate that hydrogen ions exist at the air−aqueous © 2013 American Chemical Society

interface, as confirmed via phase-sensitive sum-frequency generation.19 However, other spectroscopic studies were interpreted as demonstrating the presence of both hydronium and hydroxide at the water surface.20 Another experimental proof of an acidic surface was claimed to be the changes in the surface tension of water: some acid solutions have reduced surface tension, and this has been regarded as a proof of hydronium surface excess.2 This view, backed by many research groups, received wide recognition and a little criticism.21,22 Only recently the hypothesis of surface water basicity has been the focus of interest of the debate. Colussi and co-workers recently confirmed the presence of hydroxide ions on the surface of even very acidic water via proton-transfer reactions;23 their experimental study resulted in a loss of scientific certainties as regards aqueous surface’s acid−base character.1 Actually, the strongest evidence of a negatively charged water surface comes from old, but neglected, macroscopic experiments, such as zeta potential and titration measurements.22 The higher surface affinity of anions compared to that of cations was considered responsible for the electrostatic potential difference across the solution−air interface by Frumkin24 and Randles25 and recently confirmed by Klitzing.26 It is also instructive to observe that MD simulations that pioneered the acidic water surface view have recently highlighted a slightly enhanced surface population of hydroxide,27 thereby confirming the crucial importance of Received: September 23, 2013 Revised: November 12, 2013 Published: November 14, 2013 25579

dx.doi.org/10.1021/jp409480t | J. Phys. Chem. C 2013, 117, 25579−25585

The Journal of Physical Chemistry C

Article

firm experimental evidence from a number of different techniques. While aqueous surface’s acid−base character was deeply investigated at the water/air interfaces, investigations of the related water/hydrophobic interfaces are very scarce and, unfortunately, at variance with each other. The preference of the hydrated proton for water−hydrophobic interfaces was reported for the first time via molecular dynamics simulations.28−30 Ab initio simulations showed that both hydronium and hydroxide are “amphiphilic surfactants” that stick to a fully hydrated graphene sheet, but the effect is more pronounced for the hydroxide than for the hydronium.31 Other experimental outcomes highlight that hydrophobic surfaces (oil droplets, solid hydrophobic polymers, hydrophobic assembled structures) in contact with water acquire a net negative charge because hydroxide is preferentially adsorbed over hydronium; in this respect, solid and liquid water interfaces behave similarly to gas−water interfaces,22,32 confirming a structural similarity between all these aqueous boundaries.3−5 We have recently demonstrated that chromatography, a typical separative technique, represents a pristine tool to furnish a quantitative evaluation of the free energy of adsorption of kosmotropes and chotropes at a dieletric boundary and to rank their adsorbophilicities according to the Hofmeister series.33,34 The present paper aims to furnish reliable quantitative evidence concerning the propensity of the most common acid (HCl) and base (NaOH) to the water/hydrophobic interface via an experimental setup up to date unexploited in this thriving research field.

2.3. Pulse Chromatography. A steady stream of a solution of the tested analyte is pumped through the column until equilibrium is reached and the concentration plateau is obtained. At this time, a concentration vacancy is injected (25 μL ultrapure H2O), and the resulting chromatogram is recorded.35 2.4. Statistical Treatment of Data and Error Analysis. Experimental data were fitted via MacCurveFit 1.5.4 (Kevin Raner software, Australia) via a nonlinear least-squares quasiNewton algorithm which minimizes the sum-of-squares of the vertical distances between the experimental data and the corresponding model data points.

3. RESULTS AND DISCUSSION Design of the Experimental Setup. First, we had to select a proper chromatographic stationary phase since it represents the aqueous dielectric boundary that we aimed to study. We selected a PGC column since its production process provides a perfectly nonpolar material. The surface of PGC is crystalline and highly reproducible. Its fully porous spherical particles are gigantic aromatic molecules with fully satisfied valence, without active sites for secondary interactions.36,37 Moreover, from a merely chromatographic point of view, PGC is an intriguing stationary phase.38,39 However, the fundamental reason for selecting PGC is the fact that its essential role in the nanoage is well recognized: it is the precursor of graphene, carbon nanotubes, and fullerenes; graphitic carbons play a crucial role in physics, chemistry, material science, biology, and medicine.40 Hence, our findings may be exploited in materials science and engineering that represent a blooming field of knowledge. However, it has to be pointed out that there is a twofold difference between the PGC/aqueous interface and the air/water interface: first, the PGC surface is rigid, which was shown to alter water arrangement in the vicinity compared to fluctuating surface as water/vapor or water/oil;30 more importantly, the aromatic character of PGC can alter solute binding due to the presence of π-electrons. As regards analyte selection, it can be recalled that HCl and NaOH are the most common acid and base; they are both strong and completely dissociated. They are appealing model compounds since H+ and OH− counterions have similar ionic specificities: actually, NaCl is almost central in the Hofmeister series, thereby indicating a similar behavior of sodium and chloride.5 It follows that the extent to which HCl and NaOH adsorb onto PGC reflects the adsorbophilicities of hydronium and hydroxide, respectively. In the most diluted solutions (10 μM HCl or NaOH) the proton and the hydroxide are only 2 orders of magnitude higher that in neat water; hence, our results can be extrapolated to predict the behavior of neat water. As long as the chromatographic technique concerns, we mainly selected frontal analysis (FA) because it is the most common basis for assessing the behavior of putative adsorbers since it gives the lowest error in deriving the experimental data points from the chromatographic records compared to alternative conventional procedures.35 Pulse chromatography was used to independently confirm FA results. In FA, a solution of known concentration of the studied compound (feeding solution) is percolated through the column, and the breakthrough curve is recorded until the eluate has the same composition of the feeding solution. The mass of studied compound adsorbed at equilibrium is calculated from the retention time of the front shock of the breakthrough curve, tshock. There are three ways to estimate it:

2. METHODS 2.1. Instruments. Analyses were performed via an ICS1600 Standard Integrated IC System equipped with an UltiMate 3000 Thermostated Column Compartment (Dionex, Milan, Italy), a 25 μL injection loop, and an electrochemical detector operated at constant temperature (35 °C). Porous graphitic carbon (PGC) column (Hypercarb TM 150 × 4.6 mm i.d., 5 μm particle diameter, 250 Å median pore diameter, 120 m2 g−1 specific surface area) was purchased from Superchrom (Milan, Italy). The material can withstand pressure up to 400 bar and the entire pH and organic modifier concentration ranges. 2.2. Frontal Analysis. 18.2 MΩ·cm at 25 °C ultrapure water was produced via a Simplicity Water Purification Systems (Millipore, Milan, Italy), and it was used for the preparation of all HCl and NaOH solutions in the 10 μM−10 mM range. Reagent grade HCl and NaOH were used. Since our goal was to investigate the comparative evaluation of the interaction between a completely apolar surface and HCl or NaOH, we decided to keep our system as simple as possible: the mobile phase comprises only water and HCl or NaOH at the proper concentration. The column was thermostated at the chosen temperature for at least 1 h before FA was run. The flow rate was 1 mL min−1. The column hold-up time (0.73 min) was obtained by the recording of the first movement of the baseline via the injection of ultrapure water. The extra-column hold-up time was obtained via FA of HCl 10 mM without the analytical column. Each solution was analyzed via FA, at least in triplicate, thereby obtaining the breakthrough curves. The equivalent area to evaluate the elution time of tshock was estimated via the Chromeleon software (version 6.50).35 The adsorption isotherms were recorded at 25 °C in the concentration range 10−10 000 μM. 25580

dx.doi.org/10.1021/jp409480t | J. Phys. Chem. C 2013, 117, 25579−25585

The Journal of Physical Chemistry C

Article

from the curve inflection point, from the elution time of the half-height of the plateau, and using the equivalent area method. We used the third method because it is the most accurate.34 The stationary phase concentration of the analyte, Cs (μmol/ g), in equilibrium with the feeding solution concentration, C (μM), is derived from tshock according to the relationship35 Cs = CF(tshock − text − t0)/(1000G)

(1) −1

where F is the mobile phase flow rate (mL min ), text (min) the extra-column hold-up time (measured from the elution time of the inflection point of the same breakthrough curve obtained with no column), t0 (min) the hold-up time, and G (g) the grams of stationary phase inside the column. Since tshock increases with decreasing concentration, we had frontal chromatograms that lasted up to ca. 10 h at the lowest concentrations. This method is time-consuming, but we did not trade off accuracy for simpleness because we knew that our goal was tough and elusive. The equilibrium constant (Kads) associated with the passage of the solute from the bulk solution to the adsorbed phase is K ads = Cs/C

Figure 2. Experimental adsorption isotherm of NaOH onto PGC. Experimental data (○) were obtained via FA of NaOH solutions in the concentration range 10 μM−10 mM at 298 K; Cs were calculated according to eq 1. The solid line represents the fitting of experimental data according to eq 3.

curves have a plateau that represents “first degree saturation” of the surface, i.e., the condition in which all possible sites in the original surface are filled. The formation of this “monolayer” does not necessarily imply that it is a close-packed layer of single molecules or ions, since solvent molecules may also be present. The plateau length increases with increasing energy barrier to be overcome before additional adsorption can occur on new sites. A complete saturation of the new surface may not be always realizable.42 The isotherm parameters were to be estimated by fitting experimental data to some adsorption models. Isotherms can be classified into four kinds, according to the behavior of the adsorbate (ideal or not) and to the surface adsorption sites homogeneity or heterogeneity. We explored the Langmuir, Jovanović, bi-Langmuir, Tóth, Freundlich, Moreau, and BET isotherms. The BET adsorption isotherm was originally developed on the basis of reasonable physical assumptions regarding multilayer adsorption and the nonideal behavior of adsorbate on homogeneous surfaces at constant temperature. It was clear that the only isotherm model able to fit the whole range of experimental data was just the BET isotherm that is able to track a reversal in isotherm convexity at high concentrations due to adsorbate interactions (nonideal adsorption onto a homogeneous surface); the BET isotherm is

(2)

When the units of measure of both Cs and C are the same, that is, μmol g−1 (1 mM corresponds to 1 μmol g−1, assuming unitary density of the solutions), an adimensional constant was obtained; it is worth noting that it approaches the thermodynamic equilibrium constant, since the more is the dilution, the more concentrations approach the activities. HCl and NaOH Adsorption Isotherms. The adsorption isotherms measurement is an important task since it is our main source of information on the mechanisms of adsorption processes. From FA, we obtained the adsorption isotherm data, using eq 1 to calculate Cs corresponding to each C, as detailed in Figures 1 and 2. The isotherm shape let us classify it

Cs = Cs,satbsC /((1 − b lC)(1 − b lC + bsC))

(3)

where bs and bl are respectively fitting parameters related to the equilibrium constants for solute adsorption onto either the bare surface of the adsorbent or on the layer of a solute already adsorbed. If bl is very low, the Langmuir isotherm is obtained; it is not negligible if adsorbate interactions are operating and lead to further adsorption, after the “first degree saturation” of the surface. Fitting of experimental data according to eq 3 are shown in Figures 1 and 2. Table 1 details the best BET isotherm parameters estimates and their standard deviations, the correlation coefficients (R), and the sum of squares of errors (SSE) obtained via a nonlinear least-squares quasi-Newton fitting algorithm. When Cm is so low that adsorption takes place only onto the pristine PGC surface, in eq 3 the denominator equals one and the following relationships holds true:

Figure 1. Experimental adsorption isotherm of HCl onto PGC. Experimental data (○) were obtained via FA of HCl solutions in the concentration range 10 μM−10 mM at 25 °C; Cs were calculated according to eq 1. The solid line represents the fitting of experimental data according to eq 3.

as a type II adsorption van der Waals isotherm41 that corresponds to a L3 isotherm, according to the isotherms classification by Giles and co-workers.42 Here the initial curvature shows that as more sites in the substrate are filled, it becomes increasingly difficult for a bombarding solute molecule to find a vacant site available. All sufficiently complete 25581

dx.doi.org/10.1021/jp409480t | J. Phys. Chem. C 2013, 117, 25579−25585

The Journal of Physical Chemistry C

Article

concentration is 8.434. From the estimates of Kads,i NaOH and Kads,i HCl in Table 1 (that represent each constant at infinite dilution according to eq 4) we have that their ratio is 33.653. Therefore, it is clear that this ratio increases with decreasing concentration, and it can be predicted that 33.653 can be taken as a reasonable estimate at the neat water/PGC interface. This experimental behavior is at variance with partition coefficients for H+ and OH− at the air−water surface calculated according to a salt ion partitioning model46 but rewardingly confirms robust experimental results concerning the water/ hydrophobic interface22 and computer simulations concerning a hydrophobic surface (fully hydrated graphene) similar to PGC.31 Noteworthy the infinite dilution Kads,NaOH/Kads,HCl ratio we found is also qualitatively and semiquantitatively compatible with the ratio of hydroxide to hydronium ions at water interface, calculated according to a modified Poisson− Boltzmann model.43 It is worth noting that the estimated monolayer capacity corresponds graphically to the beginning of the isotherm plateau. Taking the specific stationary phase surface into account (120 m2 g−1), the area available for a single HCl or NaOH unit can be estimated, as it can be observed in Table 1. It is clear that the monolayer is not compact, probably due to the presence of solvent molecules. At higher concentrations, a second rise of the isotherm can be attributed to adsorption onto the exposed parts of the layer already present. At the highest concentration the area available for a single HCl or NaOH molecules is still much wider than that occupied by a single molecule; rewardingly, for NaOH, it compares well with that reported by Beattie and co-workers (300 Å2).22 It follows that computer simulation of electrolyte solutions at hydrophobic interface should use slabs as large as possible in order to capture the effects created by the adsorbed solute, since the area available for each solute should be at least comparable to the area of the simulation slab. In this context it is worth noting that water surface was predicted to be acidic according a computer simulation that made use of 72 waters in a box 13 × 16 × 11 Å3.13 Slabs of larger dimension are only recently being studied,31 and in this case the interface affinity of hydroxide was found to be higher than that of the hydronium. Interestingly, the pore size is large enough to avoid all finite size effects47 so that the adsorption calculated with this stationary phase will be the same as that on an extended thermodynamic interface made of the same material. Figure 3 allows a quick evaluation of the stronger adsorbophilicity of NaOH compared to that of HCl. In order to demonstrate the specificities of both hydroxide and hydronium ions, we compared their adsorption to that of other sodium and chloride salts, respectively; the mobile phase concentration of each electrolyte is 10 μM (trace conditions). Figure 3 clearly singles out HCl and NaOH from other chloride and sodium electrolytes, respectively, thereby confirming their “amphiphilic” character highlighted by ab initio simulations.31 Actually, if we consider different sodium salts, we notice that their adsorption increases with increasing chaotropicity of the anion. This behavior parallels that of chloride salts: the higher the chaotropic character of the cation, the stronger their adsorption is.34 Interestingly, NaCl, which is the salt made of H+ and OH− counterions (in NaOH and HCl, respectively), is less adsorbed than both NaOH and HCl. Given the importance of our findings, we decided to obtain further independent experimental evidence of the relative adsorbophilicity of the proton and of the hydroxide via pulse

Table 1. Best Isotherm Parameters Estimates and Their Standard Deviations (St Dev), the Correlation Coefficients (R), and the Sum of Squares of Errors (SSE) Obtained via a Nonlinear Least-Squares Quasi-Newton Algorithm of Isotherm Data in Figures 1 and 2; Calculated Kads,i and ΔG° for HCl and NaOH Adsorption onto PGC and Their Standard Deviations, Areas Available for a Single HCl or NaOH Molecule at First Degree Saturation (A 1st degree), and Minimum Areas Available for a Single HCl or NaOH Molecule at the Highest Cs (A min) Cs,sat (μmol g−1) Cs,sat st dev bs (mM−1) bs st dev bl (mM−1) bl st dev SSE R Kads,i Kads,i st dev ΔG° (kJ mol−1) ΔG° st dev A 1st degree (Å2) A min (Å2)

K ads,i = Cs,satbs

HCl

NaOH

7.278 0.210 20.548 4.034 0.041 0.002 0.629 0.998 149.548 33.679 −12.401 0.558 2738 1617

19.295 0.211 260.829 30.401 0.036 0.001 1.029 0.999 5032.804 641.588 −21.108 0.316 1033 663

(4)

Kads,i represents Kads at infinite dilution, that is, the true thermodynamic equilibrium constant. Hence ΔG° = −RT ln K ads,i

(5)

The estimates in Table 1 deserve to be commented upon. From the product of Cs,sat and bs, according to eq 4 we obtain Kads,i at 25 °C that, in turn, allows one to estimate the standard free energy of adsorption according to eq 5. From values detailed in Table 1 it is clear that ΔG° for NaOH adsorption onto PGC is ca. twice that of HCl. The negative sign of ΔG° witnesses the spontaneity of both the adsorption processes, which was already highlighted.20,31,43 These ΔG°s are much higher than that for the adsorption of a methylene group onto hydrocarbon/water interface, that is, −3.2 kJ mol−1, or for the adsorption of butylsulfonate (ΔG° = −7.9 kJ mol−1)44 onto silica-based reversed phase materials. For HCl ΔG° compares well to that for the adsorption of hexylsulfonate (ΔG° = −11.3 kJ mol−1)44 onto silica-based reversed phase materials. It is striking to observe that our estimates of ΔG° compare well also with a nonspecific (dehydration independent) favorable free energy of ion partitioning (ca. −5 kJ mol−1) to the apolar surface whose source has the yet to be identified5 and with the estimated free energy of adsorption of a proton at the air/water interface (ca. −7.43 kJ mol−1).45 The best fit of the experimental points also result in an estimate of bl 3 orders of magnitude lower than bs as easily predicted and expected from the presence of the isotherm plateau that translates into an energy barrier for further adsorption after the first degree of saturation was reached. From isotherm data in Figures 1 and 2, it is possible to calculate Kads for each experimental point via eq 2; hence, the Kads,NaOH/Kads,HCl ratio can be easily obtained. At the highest concentration it is 2.438; the same ratio at the lowest 25582

dx.doi.org/10.1021/jp409480t | J. Phys. Chem. C 2013, 117, 25579−25585

The Journal of Physical Chemistry C

Article

From Figure 4 it is clear that interface affinity of NaOH is higher than that of HCl at both 5000 and 10 000 μM. This is an important qualitative confirm of quantitative results obtained via FA.

4. CONCLUSIONS Both the acidic and the basic surface communities have their disciples who interpreted differently similar data. Our experimental results provide a further piece of evidence to support a basic water surface, which is certainly not as simple as often assumed. The quality of the present results is witnessed by the fact that the fitting parameters have an unambiguous physical meaning, and their estimates are very reliable because they compare to estimates obtained via nonchromatographic techniques for similar physical systems. A number of consequences follow from this groundbreaking experimental evidence: 1. It highlights that both the proton and the hydroxide are characterized by interface affinity even if the former is less adsorbophilic that the latter. 2. It is at variance with the Onsanger−Samaras theory. 3. It confirms33,34 a not separative aim of chromatography that is able to furnish not only a qualitative demonstration of the adsorption but also a quantitative estimate of its the free energy that represents a challenging experimental target.48 HPLC is a sound and powerful tool to put to the test computer simulations predictions, and it is much simpler than than sophisticated spectroscopic methods, such as second harmonic generation spectroscopy, X-ray photoelectron spectroscopy, and phase-sensitive sum-frequency generation, which were used to shed light on the aqueous surface’s acid−base character. 4. It gives quantitative indications for the reasonable dimensions of molecular dynamics simulation slabs. 5. It could provide future practical applications in the nanotechnology research field. Nowadays, it appears as though graphite has an essential new role in the Nano Age since it is the precursor of graphene (a one-atom-thick layer of graphite), carbon nanotubes (graphene wrapped into a seamless cylinder), and fullerenes; graphitic carbons represent the place where physics, material science, biology, and medicine easily meet.40 These materials have potential as superconductors, catalysts, scanning tunneling microscope tips, diagnostic tools, and drug

Figure 3. Stationary phase concentrations of chloride electrolytes (left) and sodium electrolytes (right) obtained by frontal analysis of each 10 μM electrolyte solutions. Raw data for chloride salts and sodium salts are taken from ref 34.

chromatography.35 A steady stream of a solution of the tested analyte is pumped through the column until equilibrium is reached and the concentration plateau is obtained. At this time, a concentration vacancy is injected (25 μL of ultrapure H2O), and the resulting chromatogram is recorded. The negative concentration pulse propagates at a velocity that depends on the tangent to the isotherm:35 the retention time of the concentration vacancy is longer if the affinity of the analyte for the stationary phase is stronger. The retention time of the vacancies are 1.45 and 1.44 for NaOH 5000 and 10 000 μM, respectively, and 1.42 and 1.39 for HCl 5000 and 10 000 μM in that order. It is rewarding to observe that the stronger NaOH vacancy retention parallels the steeper adsorpion isotherm in Figure 2 compared to that in Figure 1 both at 5000 and10 000 μM, according to the vacancy retention time expression35 t = t0(1 + F dCs/dC)

(6)

where F is phase ratio of the column (the ratio between the stationary phase volume and the mobile phase volume).

Figure 4. Pulse chromatography: injection of 25 μL of ultrapure H2O on the concentration plateau of each analyte. Black: water vacancy, column equilibrated with NaOH 5000 μM; blue: water vacancy, column equilibrated with HCl 5000 μM; pink: water vacancy, column equilibrated with NaOH 10 000 μM; brown: water vacancy, column equilibrated with HCl 10 000 μM. 25583

dx.doi.org/10.1021/jp409480t | J. Phys. Chem. C 2013, 117, 25579−25585

The Journal of Physical Chemistry C

Article

(19) Mucha, M.; Frigato, T.; Levering, L. M.; Allen, H. C.; Tobias, D. J.; Dang, L. X.; Jungwirth, P. Unified Molecular Picture of the Surfaces of Aqueous Acid, Base, and Salt Solutions. J. Phys. Chem. B 2005, 109, 7617−7623. (20) Tian, C.; Ji, N.; Waychunas, G.; Shen, Y. R. Interfacial Structures of Acidic and Basic Aqueous Solutions. J. Am. Chem. Soc. 2008, 130, 13033−13039. (21) Beattie, J. K. Comment on Autoionization at the Surface of Neat Water: Is the Top Layer pH Neutral, Basic, or Acidic? Phys. Chem. Chem. Phys. 2008, 10, 330−331. (22) Beattie, J. K.; Djerdjev, A. M.; Warr, G. G. The Surface of Water is Basic. Faraday Discuss. 2009, 141, 31−39. (23) Mishra, H.; Enami, S.; Nielsen, R. J.; Stewart, L. A.; Hoffmann, M. R.; Goddard, W. A.; Colussi, A. Brønsted Basicity of the Air−Water Interface. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 18679−18683. (24) Frumkin, A. N. Phase Interface Powers and Adsorption on the Segregative Surface Air-Solution of Anorganic Electrolyte. Z. Phys. Chem. Stoechiom. Verwandtschaftsl. 1924, 109, 34−48. (25) Randles, J. E. B. Structure at the Free Surface of Water and Aqueous Electrolyte Solutions. Phys. Chem. Liq. 1977, 7, 107−179. (26) Klitzing, R. Effect of Interface Modification on Forces in Foam Films and Wetting Films. Adv. Colloid Interface Sci. 2005, 114−115, 253−266. (27) Mundy, C. J.; Kuo, I. F. W.; Tuckerman, M. E.; Lee, H. S.; Tobias, D. J. Hydroxide Anion at the Air−Water Interface. Chem. Phys. Lett. 2009, 481, 2−8. (28) Chen, H.; Xu, J.; Voth, G. A. Unusual Hydrophobic Interactions in Acidic Aqueous Solutions. J. Phys. Chem. B 2009, 113, 7291−7297. (29) Iuchi, S.; Chen, H.; Paesani, F.; Voth, G. A. Hydrated Excess Proton at Water−Hydrophobic Interfaces. J. Phys. Chem. B 2009, 113, 4017−4030. (30) Vacha, R.; Zangi, R.; Engberts, J. B. F. N.; Jungwirth, P. Water Structuring and Hydroxide Ion Binding at the Interface between Water and Hydrophobic Walls of Varying Rigidity and van der Waals Interactions. J. Phys, Chem. C 2008, 112, 7689−7692. (31) Kudin, K. N.; Car, R. Why Are Water-Hydrophobic Interfaces Charged? J. Am. Chem. Soc. 2008, 130, 3915−3919. (32) Zimmermann, R.; Dukhin, S.; Werner, C. Electrokinetic Measurement Reveal Interfacial Charge at Polymer Films Caused by Simple Electrolyte Ions. J. Phys. Chem. B 2001, 105, 8544−8549. (33) Cecchi, T. Chromatography and the Hundred Year Mystery of Inorganic Ions at Aqueous Interfaces: First Evidence of the Presence of a Kosmotrope at the Graphite/Electrolyte Solution Interface. J. Phys. Chem. C 2013, 117, 19002−19009. (34) Cecchi, T.; Marcotulli, F. Chromatography and the Hundred Year Mystery of Inorganic Ions at Aqueous Interfaces: Adsorption of Inorganic Ions at the Porous Graphitic Carbon Aqueous Interface Follows the Hofmeister Series. J. Chromatogr., A 2013, 1314, 106−114. (35) Guiochon, G.; Felinger, A.; Shirazi, S.; Katti, A. Fundamentals of Preparative and Nonlinear Chromatography; Elsevier: Amsterdam, 2006; Chapter 3. (36) Hypercarb HPLC Columns Technical Guide. (37) Pereira, L. Porous Graphitic Carbon as a Stationary Phase in HPLC: Theory and Applications. J. Liq. Chromatogr. Relat. Technol. 2008, 31, 1687−1731. (38) Cecchi, T. Ion-Pair Chromatography and Related Techniques; CRC Press, Taylor & Francis Group LLC: Boca Raton, FL, 2010; Chapters 5, 6, and 16. (39) Cecchi, T. Retention Mechanism for Ion-pair Chromatography with Chaotropic Reagents: from Ion-Pair Chromatography towards a Unified Salt Chromatography. Adv. Chromatogr 2011, 49, 1−35. (40) Mukhopadhyay, P.; Gupta, R. K. Graphite, Graphene, and Their Polymer Nanocomposites; CRC Press, Taylor and Francis Group LLC: Boca Raton, FL, 2013. (41) Brunauer, S.; Deming, S. L.; Deming, W. E.; Teller, E. On a Theory of the van der Waals Adsorption of Gases. J. Am. Chem. Soc. 1940, 62, 1723−1732.

delivery vessels; hence, we believe that our results could attract the attention of scientists belonging to many different research fields.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], tel +39 0734 622632, fax +39 0734 621229. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Support from Margherita Bonanni and Fondazione Cassa di Risparmio di Fermo is gratefully aknowledged.

■

REFERENCES

(1) Saykally, R. J. Two Sides of the Acid−Base Story. Nat. Chem. 2013, 5, 1−2. (2) Jungwirth, P. Ions at Aqueous Interfaces. Faraday Discuss. 2009, 141, 9−30. (3) Willard, A. P.; Chandler, D. Coarse-Grained Modeling of the Interface between Water and Heterogeneous Surfaces. Faraday Discuss. 2009, 141, 209−220. (4) Horinek, D.; Netz, R. R. Specific Ion Adsorption at Hydrophobic Solid Surfaces. Phys. Rev. Lett. 2007, 99, 226104. (5) Pegram, L. M.; Record, M. T., Jr. Hofmeister Salt Effects on Surface Tension Arise from Partitioning of Anions and Cations between Bulk Water and the Air-Water Interface. J. Phys. Chem. B 2007, 111, 5411−5417. (6) Otten, D. E.; Onorato, R.; Michaels, R.; Goodknight, J.; Saykally, R. J. Strong Surface Adsorption of Aqueous Sodium Nitrite as an Ion Pair. Chem. Phys. Lett. 2012, 519−520, 45−48. (7) Netz, R. R.; Horinek, D. Progress in Modeling of Ion Effects at the Vapor/Water Interface. Annu. Rev. Phys. Chem. 2012, 63, 401−418. (8) Jungwirth, P.; Tobias, D. J. Specific Ion Effects at the Air/Water Interface. Chem. Rev. 2006, 106, 1259−1281. (9) Chang, T. M.; Dang, L. X. Recent Advances in Molecular Simulations of Ion Solvation at Liquid Interfaces. Chem. Rev. 2006, 106, 1305−1322. (10) Boström, M.; Williams, D. R. M.; Ninham, B. W. Surface Tension of Electrolytes: Specific Ion Effects Explained by Dispersion Forces. Langmuir 2001, 17, 4475−4478. (11) Dos Santos, A. P.; Levin, Y. Surface and Interfacial Tensions of Hofmeister Electrolytes. Faraday Discuss. 2013, 160, 75−87. (12) Wang, R.; Wang, Z. G. Effects of Ion Solvation on Phase Equilibrium and Interfacial Tension of Liquid Mixtures. J. Chem. Phys. 2011, 135, 014707. (13) Buch, V.; Milet, A.; Vacha, R.; Jungwirth, P.; Devlin, J. P. Water Surface is Acidic. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 7342−7347. (14) Vacha, R.; Buch, V.; Milet, A.; Devlin, J. P.; Jungwirth, P. Autoionization at the Surface of Neat Water: Is the Top Layer pH Neutral, Basic, or Acidic? Phys. Chem. Chem. Phys. 2007, 9, 4736− 4747. (15) Peterson, P. B.; Saykally, R. J. Is the Liquid Water Surface Basic or Acidic? Macroscopic vs. Molecular-Scale Investigations. Chem. Phys. Lett. 2008, 458, 255−261. (16) Winter, B.; Faubel, M.; Vacha, R.; Jungwirth, P. Reply to Comments on Frontier Article “Behavior of Hydroxide at the Water/ Vapor Interface”. Chem. Phys. Lett. 2009, 481, 19−21. (17) Levering, L. M.; Sierra-Hernández, M. R.; Allen, H. C. Observation of Hydronium Ions at the Air-Aqueous Acid Interface: Vibrational Spectroscopic Studies of Aqueous HCl, HBr, and HI. J. Phys. Chem. C 2007, 111, 8814−8826. (18) Tian, C.; Ji, N.; Waychunas, G. A.; Shen, Y. R. Interfacial Structures of Acidic and Basic Aqueous Solutions. J. Am. Chem. Soc. 2008, 130, 13033−13039. 25584

dx.doi.org/10.1021/jp409480t | J. Phys. Chem. C 2013, 117, 25579−25585

The Journal of Physical Chemistry C

Article

(42) Giles, C. H.; Smith, D.; Huitson, A. A General Treatment and Classification of the Solute Adsorption Isotherm I. Theoretical. J. Colloid Interface Sci. 1974, 47, 766−778. (43) Manciu, M.; Ruckensteinb, E. Ions Near the Air/Water Interface. II: Is the Water/Air Interface Acidic or Basic? Predictions of a Simple Model. Colloids Surf., A 2012, 404, 93−100. (44) Cecchi, T. Extended Thermodynamic Approach to IonInteraction Chromatography: a Thorough Comparison with the Electrostatic Approach and Further Quantitative Validation. J. Chromatogr., A 2002, 958, 51−58. (45) Kathmann, S. M.; Kuo, I. F. W.; Mundy, C. J.; Schenter, G. K. Understanding the Surface Potential of Water. J. Phys. Chem. B 2011, 115, 4369−4377. (46) Pegram, L. M.; Record, M. T., Jr. Quantifying Accumulation or Exclusion of H+, HO−, and Hofmeister Salt Ions near Interfaces. Chem. Phys. Lett. 2008, 467, 1−8. (47) Weber, S. G. Theoretical and Experimental Studies of Electrostatic Effects in Reversed Phase Liquid Chromatography. Talanta 1989, 36, 99−106. (48) Garrett, B. Ions at Air/Water Interface. Science 2004, 303, 1146−1147.

25585

dx.doi.org/10.1021/jp409480t | J. Phys. Chem. C 2013, 117, 25579−25585