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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 Teresa Cecchi*,† Accademia delle Scienze dell’Istituto di Bologna, Via Zamboni, 31, 40126 Bologna, Italy ABSTRACT: Recent theoretical speculations and computer simulations question the validity of the Onsager−Samaras theory concerning the ion-free interface between an electrolyte solution and a hydrophobic surface. For the very first time we used chromatography not for a separative goal but as a tool to detect the adsorption of LiF, strongly expected to shun any dielectric boundary, onto porous graphitic carbon. Frontal analysis gave unequivocal experimental evidence to this unexpected phenomenon: the van’t Hoff relationship and the adsorption isotherm furnished reliable thermodynamic quantities, such as the adsorption equilibrium constants, the monolayer capacity, and the standard free energy, enthalpy, and entropy of adsorption. From the influence of pressure on adsorption a very reasonable difference of the partial molar volume of LiF in the adsorbed and bulk phases was obtained. The contribution due to LiF adsorption to the interface potential difference was estimated. The main new physical insight provided by this article is however the ascertained groundbreaking adsorption of a kosmotrope onto a graphitic surface, a material that is the focus of interest of many thriving research fields in the nanoage.

1. INTRODUCTION

increased surface propensity for soft ions (chaotropes) compared to hard ions (kosmotropes). Notwithstanding a tremendous amount of work in the past ten years, a quantitative description of the driving forces that determine ion adsorption or repulsion at the interface is still missing.3,5,11 It follows that first-hand experimental data play a crucial role to bring an up-to-date account of the prestigious endeavor that is seeking an explanation of the nature of water interfaces, the key of many processes on the Earth.3 While the air/water interface was deeply investigated, ion specificity, the so-called Hofmeister effect, at solid surfaces, is unexplained and has been hardly debated so far,12 and this prompted us to explore this subject. To the best of our knowledge, chromatography has never been deliberately exploited to ascertain the presence of kosmotropes at the water/hydrophobic stationary phase interface.13 The general impression gleaned from the chromatographic literature is the lack of interest in the quantification of inorganic electrolyte adsorption onto the stationary phase, even if chaotropic salts are common eluent additives.14,15 Actually the chromatographic interface represents a dielectric boundary, hence chromatography represents an unexplored formidable tool to test the latest theoretical finding. Since no such idea has ever been entertained before, we decided to use chromatography to provide the adsorption of a simple inorganic electrolyte onto a completely apolar stationary phase with experimental evidence.

Textbook knowledge of the interface between an electrolyte solution and a hydrophobic medium mandates that the interface is devoid of ions, as a necessary consequence of the Gibbs adsorption equation, which relates the increase in surface tension observed for solutions of inorganic salts to a negative surface excess of ions.1 Onsager and Samaras provided the theoretical rationalization: image charges repel ions from the interface dielectric boundary.2 The decorations of this primitive model are endless but still predict an ion-free outermost liquid layer:3 the fact that ions shun the interface has become common wisdom. In clear violation of the Onsager and Samaras model, in the 1930s, experiments revealed that some salts did not produce the monotonic increase in surface tension that had become the expected behavior, but actually exhibited a minimum at millimolar concentrations. This “Jones−Ray effect” that underscores ionic surface activity was recently verified and rationalized.4,5 Recent molecular dynamics (MD) simulations, at variance with the Onsager and Samaras theory, highlighted an increased concentration of some anions at aqueous interfaces.3,6−8 MD simulations however have drastic computational demands for a realistic representation of interfaces, and their ability to capture the large-scale whole picture of the interface depends on the dimensions of the simulation slabs and the model assumptions; hence, they need to be validated carefully to ensure their legitimacy. Recent theoretical efforts9−11 shed new light on computer simulation predictions. Common theoretical evidence is an © 2013 American Chemical Society

Received: June 9, 2013 Revised: July 26, 2013 Published: September 9, 2013 19002

dx.doi.org/10.1021/jp405702r | J. Phys. Chem. C 2013, 117, 19002−19009

The Journal of Physical Chemistry C

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Figure 1. Breakthrough curve by frontal analysis, LiF concentration step from 0 to 10 μM at 298 K.

solutions in the 10 μM−10 mM range. All solutions and dilutions were prepared gravimetrically. Since our goal was to investigate the nature of the interaction between a supposed completely apolar surface (the graphitic surface) and ionic species, we decided to keep our system as simple as possible: the mobile phase comprises only water and LiF at the proper concentration. The column was thermostatted at the chosen temperature for at least 1 h before frontal analysis20 was run. The flow rate was 1 mL min−1. The column hold-up time 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 frontal analysis of LiF solutions without the analytical column. Each electrolyte solution was analyzed via frontal analysis in triplicate, thereby obtaining the breakthrough curves. The equivalent area to evaluate the elution time of the front shock of the breakthrough curve (tshock) was estimated via the Chromeleon software (version 6.50 copyright 1994−2002). The adsorption isotherm was recorded at 298 K in the concentration range 10−10 000 μM. The van’t Hoff plot was obtained in the temperature range 283−328 K under trace conditions (LiF 10 μM). To confirm the exothermicity of LiF adsorption onto PGC, the column was equilibrated with a solution of LiF 10 μM at 298 K. After 850 min the temperature of the thermostatted column was raised from 298 to 313 K; the temperature was subsequently lowered to the initial temperature at 1100 min. The flow rate (1 mL min −1 ), the feeding solution concentration, and the detector temperature were held constant to rule out any other possible sources of detector signal variation. To study the influence of pressure on adsorption, the column was equilibrated with a solution of LiF 1.0 mM, at 298 K. We deliberately manipulated the system pressure via a backpressure regulator valve, inserted, via polyether ether ketone (PEEK) fittings, between the column outlet and the detector (see Figure 5 captions for details). Again, the flow rate (1 mL min−1), the feeding solution concentration, and the detector temperature were held constant to pinpoint the exact cause of the investigated phenomenon.

Porous graphitic carbon (PGC) was selected as the hydrophobic phase because: (i) it is supposed to be a perfectly apolar material free from active sites available for secondary interactions,16 (ii) it is an intriguing chromatographic stationary phase;16,17and (iii) graphite has an essential role in the nanoage since it is the precursor of graphene, carbon nanotubes, and fullerenes; graphitic carbons represent the place where physics, chemistry, material science, biology, and medicine meet.18 As a probe analyte we selected LiF since it is expected to show the least surface affinity,3,11,19 among singly charged inorganic electrolytes, due to its strong kosmotropicity. Providing its adsorption onto PGC with experimental evidence would be a groundbreaking, unprecedented result. As a chromatographic technique we selected frontal analysis because it was demonstrated to give the lowest error in deriving adsorption data points from the chromatographic records, when the equal area retention time definition procedure is used.20 The adsorption of a kosmotrope onto PGC, qualitatively and quantitatively demonstrated in the following, prompts a paradigm shift in interface science.

2. METHOD 2.1. Instruments. Analyses were performed via an ICS1600 Standard Integrated IC System equipped with an UltiMate 3000 Thermostatted Column Compartment (Dionex, Milan, Italy), a 25 μL injection loop, and an electrochemical detector operated at constant temperature (308 K). To study the influence of pressure on adsorption, the pressure was modified via a back-pressure regulator (1500 psig, ALLTECH part n. 39024) at constant flow rate. The PGC column (Hypercarb 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. The surface of PGC is crystalline and highly reproducible, and there are no active sites for secondary interactions.16,17 2.2. Frontal Analysis. 18.2 MΩ·cm at 298 K ultrapure water was used for the preparation of all the reagent-grade LiF 19003



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Figure 2. Breakthrough curve by frontal analysis, LiF concentration step from 7.5 to 10.0 mM at 298 K.

Figure 3. van’t Hoff plot for LiF 10 μM in the temperature range 283−328 K.

2.3. 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.

column hold-up time; and G (g) is the amount of stationary phase inside the column. We decided to work with the lowest LiF concentration (10 μM) because we avoided nonidealities due to nontrace conditions. Since the adsorption of LiF, according to the literature, was expected to be vanishing,3,11,19 we predicted very low tshock; according to eq 1, tshock increases with decreasing analyte concentration in the feeding solution, hence the LiF concentration was kept as low as possible also to maximize tshock. In Figure 1 we can observe that a gradual sorption process results in an increase of the electrolyte concentration in the eluate with increasing feeding solution volume passing through the column. When the column effluent has the same concentration as the feeding solution, a plateau of the detector signal is obtained, and it corresponds to a dynamic equilibrium condition in which the sorbed electrolyte equates the solubilized amount. If LiF did not adsorb onto the PGC apolar stationary phase, as soon as the feeding solution reaches the detector (text + t0, 4.3 min in our case), the detector signal should immediately reach the plateau.

3. RESULTS AND DISCUSSION 3.1. Unequivocal Qualitative Experimental Evidence of LiF Adsorption onto PGC. In frontal analysis, a feeding solution of known analyte concentration is percolated through the column, and the breakthrough curve is recorded. It represents the concentration vs time curve of the effluent. The stationary phase concentration of the analyte, Cs (μmol g−1), in equilibrium with the feeding solution concentration, C (μM), is derived from the following relationship20 Cs = C·F(tshock − text − t0)/(1000·G)

(1) −1

where F is the mobile phase flow rate (mL min ); tshock (min) is the elution time of the front shock of the breakthrough curve; text (min) is the extra-column hold-up time; t0 (min) is the 19004



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Figure 4. Influence of temperature variation on the detector signal: at 850 min the temperature of the thermostatted column was raised from 298 to 313 K; at 1100 min the temperature was lowered to 298 K.

constant was obtained. It is worth noting that under these infinite dilution conditions the concentration ratio is approximately equal to the activity ratio, hence it approaches the thermodynamic equilibrium constant. At each temperature, Cs was calculated according to eq 1. The best straight line describing the experimental points in Figure 3 is ln Kads = 1398.90 ± 205.22·(1/T) − 0.92 ± 0.47 (R = 0.9804). The quite good linearity of the plot demonstrates an invariant retention mechanism in the range of the explored temperatures. The linearity is not perfect probably because, strictly speaking, the obtained experimental data exhibit the combined influence of temperature and pressure (see below). However, the estimates of the standard adsorption enthalpy (ΔH°) and entropy (ΔS°) from, respectively, the slope and the intercept of the best straight line are only affected by a negligible error since its magnitude is significant only for high molecular weight compounds.20 We have ΔH°= −11.63 ± 1.71 kJ mol−1 and ΔS° = −8 ± 1 J mol−1 K−1. It is strongly rewarding to observe that they compare well to ΔH° and ΔS° estimates obtained via a nonchromatographic technique (resonant UV second harmonic generation spectroscopy) for the adsorption of aqueous thiocyanate ions from bulk solution to the liquid/vapor interface.22 The negative ΔH° and the negative ΔS° confirm a robust observation in computer simulations:3 in particular, entropy always disfavors the presence of ions at water interfaces. At 298 K the standard free energy change for adsorption of LiF onto PGC is ΔG° = −9.35 ± 2.13 kJ mol−1. The magnitude of ΔG° is almost 3-fold compared to the adsorption of a methylene group onto the hydrocarbon/water interface, that is, −3.2 kJ mol−1,23 and it is midway between ΔG°s relative to the adsorption of butylsulfonate (ΔG° = −7.9 kJ mol−1)24 and hexylsulfonate (ΔG° = −11.3 kJ mol−1)24 onto chromatographic apolar phases. It is striking to observe that the order of magnitude of our estimate of ΔG° is the same as that of (i) a nonspecific (dehydration independent) free energy of ion partitioning (ca. −5 kJ mol−1) to the apolar surface25 and (ii) the estimated free energy of adsorption of a proton at the air/ water interface (ca. −7.43 kJ mol−1).26 These quantitative

It is striking to observe that this is not the case: the steadystate was not reached before 400 min, thereby unequivocally demonstrating that the column was adsorbing LiF entering it; the equilibration time is very long. The breakthrough curve shape shown in Figure 1, compared to those obtained at higher concentrations (Figure 2) that show the classical breakthrough profile,20 is characterized by a dual shock breakthrough. This rare phenomenon was related to adsorption kinetics,21 and it parallels the “split peak” phenomenon in elution chromatography that is observed when the mass transfer kinetics is very slow: in both cases a fraction of the analyte does not have an opportunity to interact with the stationary phase;20 this fraction is not retained and elutes sharply almost at the void time, while the rest of the sample elutes with low efficiency at much longer times. Very slow kinetics, under these trace conditions, are also witnessed by the long time (>10 h) needed to obtain a LiF-free effluent, during column washing with ultrapure water. The dual shock breakthrough does not influence the estimate of the adsorbed LiF since the equivalent area method was used to estimate the amount of adsorbed analyte.20 The quantitative details of the LiF adsorption are given in the following. 3.2. Influence of Temperature on LiF Adsorption onto PGC and Quantitative Estimates of Thermodynamic Properties. Frontal analysis was performed at different temperatures to obtain the well-known van’t Hoff relationship ln K ads = −ΔH °/RT + ΔS°/R

(2)

that was graphically detailed in Figure 3. The equilibrium constant (Kads) associated with the passage of the solute from the bulk solution to the adsorbed phase, obtained at each temperature, under trace conditions (10 μM) is K ads = Cs/C

(3)

where the units of measure of both Cs and C are the same, that is μmol g−1 (10 μM corresponds to 0.01 μmol g−1, assuming unitary density for the LiF solution); this way an adimensional 19005



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Figure 5. Influence of a pressure change on the signal of the effluent from the column equilibrated with a solution of LiF 1 mM, 298 K. At 17.6 min the pressure changes from 148 bar (previously obtained closing the back-pressure regulator valve 30 min before the experiment) to 88 bar; at 23.1 min the original pressure was restored closing the back-pressure regulator valve.

surface concentration remaining at 313 K is 0.3675 μmol g−1 that compares very well with the value directly obtained by the subsequent frontal analysis, independently performed to obtain the van’t Hoff data point at 313 K, that is 0.3786 μmol g−1 (Kads = 37.86, ln Kads = 3.63, Figure 3). 3.4. Influence of Pressure on LiF Adsorption onto PGC. The influence of pressure on adsorption and on the adsorption equilibrium constant (Kads) has never been investigated as regards electrolyte interface affinity since it is usually small even if not always negligible. The following classical fundamental thermodynamic relationship holds true20

comparisons validate our findings.3 Noteworthy, chromatography was able to furnish thermodynamic quantities that represent very rare and challenging experimental targets.27 3.3. Unequivocal Qualitative and Quantitative Confirmation of the Exothermicity of LiF Adsorption onto PGC. Given the remarkable finding of the adsorption of the strongest kosmotropic electrolyte at the solution/PGC interface, we decided to obtain further experimental evidence of this phenomenon. We decided to monitor the detector signal, when the breakthrough of LiF 10 μM was complete (plateau region) during a column temperature change: if LiF is actually adsorbed onto the stationary phase and if the process is actually exothermic (as found from the van’t Hoff relationship), when the temperature is raised (decreased), a desorption (adsorption) should take place, and it should show as a peak (negative peak); any other sources of detector signal variations are ruled out by the experimental condition setup: constant flow (1 mL min−1), constant detector temperature, and constant feeding solution concentration (LiF 10 μM). Figure 4 provides our predictions with experimental evidence. At 850 min the temperature of the thermostatted column was raised from 298 to 313 K; the temperature was subsequently lowered to the initial temperature at 1100 min. The broad desorption peak that starts at 850 min and the broad adsorption negative peak that starts at 1100 min confirm both the presence of adsorbed analyte and the van’t Hoff estimate of the exothermicity for LiF adsorption. It is rewarding to observe that their areas are, respectively, 20.5711 and 19.7614 μS·min, thereby ruling out hysteresis phenomena. The slow kinetics (which resulted in the anomalous breakthrough curve shape, see Figure 1 and discussion above) may be responsible for the poor peak shape of both the positive and the negative peaks. From a peak area calibration with LiF standard solutions, the positive peak corresponds to a desorption of 0.0653 μmol g−1 as a consequence of the temperature increase. Since the adsorbed equilibrium concentration, at 298 K, is 0.4328 μmol g−1 (Kads = 43.28, ln Kads= 3.77, Figure 3), the adsorbed LiF

ΔG° = − RT ln K ads = ΔE° + P ΔVm° − T ΔS°

(4)

The difference of the standard partial molar volume ΔV°m of the solute associated with its passage from the bulk solution to the adsorbed phase is related to the adsorption equilibrium constant according to the following relationship20 ΔVm° = −RT(δ(ln K ads)/δP)T

(5)

While it is a common practice to increase system pressure via a flow rate increase, we used the back-pressure regulator to avoid combined effects that would have resulted from a pressure manipulation as a consequence of the flow rate change. The profile of the detector signal can be observed in Figure 5. It is clear that when the pressure was released solute adsorption shows as a negative peak, while when the pressure was increased to the original value analyte desorption manifests as a positive peak. The negative and positive peak areas are, respectively, 0.3497 and 0.3517 μS·min. The peak shapes are not good, but they are interestingly specular, thereby ruling out hysteresis phenomena. From a peak area calibration the positive peak corresponds to a desorption of 0.0025 μmol g−1. Since the adsorbed concentration, in equilibrium with a mobile phase LiF concentration of 1 mM, at 298 K and 88 bar, is 1.7178 μmol g−1 (Figure 6), Kads at both system pressures can be calculated and are, respectively, Kads,88 = 1.7178 and Kads,148 = 1.7152. From eq 5 an approximate estimate of ΔVm ° gives ca. 0.60 mL 19006



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and to capture a reversal in isotherm convexity at high concentrations, due to adsorbate interactions. According to this model the solute molecules can adsorb from the solution onto either the bare surface of the adsorbent or the layer of a solute already adsorbed with adsorption equilibrium constants related, respectively, to bs and bl. Cs,sat is the saturation capacity (μmol g−1) It is instructive to observe that the following relationship holds true

K ads = Cs,satbs

The BET isotherm model was fitted to experimental data via a nonlinear least-squares quasi Newton algorithm; the best estimates of the fitting parameters of eq 6: Cs,sat = 1.43 × 1000 ± 5.56 × 10−02 μmol g−1; bs = 3.60 × 1001 ± 9.60 × 1000 mM−1; bl = 7.23 × 10−02 ± 9.80 × 10−04 mM−1, R = 0.9985, SSE = 0.0071. The estimates of the bs and Cs,sat parameters deserve to be commented upon since their product, according to eq 7, corresponds to Kads at 298 K, that in turn leads to ΔG° = −9.76 ± 0.95 kJ mol−1. This value perfectly compares to the estimate of ΔG° independently obtained above from the van’t Hoff relationship, and this self-consistency is strongly rewarding. The best fit of the experimental points also results in an estimate of bl 3 orders of magnitude lower than bs, as easily predicted from the presence of the isotherm plateau that translates into an energy barrier for further adsorption after the first degree of saturation was reached. It is worth noting that the estimated monolayer capacity, 1.43 μmol g−1, corresponds graphically to the beginning of the isotherm plateau. At the highest concentration the area available for a single LiF unit was 3.86 × 1003 Å2, 2 orders of magnitude higher than the surface of a unit cell of LiF (18.49 Å2): this confirms the positive sign of ΔVm ° (see above) and the presence of hydration molecules; computer simulation of electrolyte solutions at the hydrophobic interface would be futile if the sampled slabs were those often used since they would not be large enough to capture the area occupied by even one LiF molecule. Slabs of almost sufficient dimension (6 nm × 6 nm × 30 nm) are only recently being studied.19 Fluoride adsorption onto an apolar surface was never quantitatively experimentally studied; actually fluoride was often dropped out of simulations of halides at interfaces.12,27,30 In this context, our results are noteworthy since they fill in a lack of literature results and emphasize an urgent research need. Li+ and F− are, respectively, critical members of the alkali and halide ions since they are the only ones which do not show dorbital involvement in bonding interactions. They share a high charge to radius ratio, hence, according to the Hofmeister hermeneutics, they are both kosmotropes, able to break the hydrogen bonded structure of water and to order solvent molecules through their charge. The energy of ion−dipole attractive interactions overbalances the losses of lattice and water structure energies, and a net hydrophilic hydration energy results. In water solutions, at low concentrations (c < 0.06 M, that is in our case) the free ion formation predominates, then dipole formation becomes favorable, as witnessed by the increasing of solution permittivity with increasing LiF concentration.31 The uncertainty about the motive force that drives LiF onto the PGC/aqueous interface parallels the dubiety that characterizes frontier research on the presence of ions at interfaces.3 Why are image charge repulsions predicted by the Onsager and

Figure 6. Experimental adsorption isotherm of LiF onto PGC. Experimental data (ρ) were obtained via frontal analysis of LiF solutions in the concentration range 10 μM−10 mM at 298 K; Cs were calculated according to eq 1.

mol−1, which is a very reasonable value for such a small solute.20 The positive sign of ΔVm ° implies that the standard partial molar volume of LiF is larger in the adsorbed phase than in the bulk phase; it follows that the adsorbed electrolyte does not assume a compact configuration, and this gives some indication of the likelihood of the presence of solvent in an adsorbed layer. It is also noteworthy that from the positive sign of ΔV°m increased adsorption with decreasing system pressure follows, as confirmed in Figure 5. 3.5. Registration of the LiF Adsorption Isotherm from Frontal Analysis. From eq 1 the Cs corresponding to each C was calculated. The qualitative isotherm shape that can be observed in Figure 6 lets us classify it as a Type II van der Waals isotherm.20 The initial curvature shows that as more sites in the substrate are filled it becomes increasingly difficult for a bombarding solute to find a vacant site available. All sufficiently complete curves have a plateau that represents ″first degree saturation″ of the surface. The formation of this ″monolayer″ does not necessarily imply that it is a close-packed layer of analyte since solvent molecules may also be present, and the positive variation of molar volume discussed above indicates that this is actually the case. 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,28 and this part of the curve was not explored for the sake of safety owing to the limited LiF solubility.29 The isotherm parameters were to be estimated by fitting experimental data to some adsorption models. Isotherms can be broadly sorted out into four kinds, according to the behavior of the adsorbate species (ideal or not) and to the surface adsorption sites homogeneity or heterogeneity. We explored the Langmuir, Jovanović, biLangmuir, Tóth, Freundlich, Moreau, and BET isotherms. It was easily predicted that the only isotherm model able to fit the whole range of experimental data was the BET isotherm20 Cs = Cs,satbsC /((1 − b1C)(1 − b1C + bsC))

(7)

(6)

since it is able to describe nonideal adsorption (due to ionic interactions) onto a homogeneous surface such as PGC16,17 19007



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Chromatography is a pristine tool to quantitatively study, in a nonseparative mode, the presence of ions at interfaces; experimental findings may in turn be exploited for new separative aims. Noteworthy, chromatography is able to furnish estimates of thermodynamic quantities that represent very rare and challenging experimental targets, and it is a simple technique, much more widespread than sophisticated spectroscopic methods, such as second harmonic generation spectroscopy, X-ray photoelectron spectroscopy, and phase-sensitive sum-frequency generation, that pioneered the experimental proof of the presence of ions at aqueous interfaces. Molecular dynamics simulations of the up-to-date unexplored chromatographic interface are endorsed since simulation predictions, central to interface sciences, could be easily correlated with experimental chromatographic observables and validated. The unexpected adsorption of kosmotropes onto PGC prompts a paradigm shift in interface science. Graphite-based materials have potential as superconductors, catalysts, scanning tunneling microscope tips, diagnostic tools, and drug delivery vessels, hence we believe that our unpredicted results could attract the attention of scientists belonging to many different research fields in this nanoage, as well as provide future practical nanotechnology applications. Each scientist may wonder what he/she could do if he/she had a “charged graphite sheet”. We have quantitatively demonstrated that “charged graphite sheets” may be very easily obtained exposing graphite to electrolyte solutions.

Samaras theory not operating? It has to be borne in mind that (i) the relative static permittivity of graphite is 10−1532 and (ii) the often neglected dielectric saturation of water molecules adsorbed on the interface,33 confirmed by recent theoretical studies, involves a variable permittivity profile starting at εr ≈ 4 at the interface and increasing to εr = 80 about 10 nm from it.34 It follows that ions approaching the interface by thermal motion would experience a spatially varying dielectric medium; image charge repulsions (recently questioned from a theoretical point of view for an extended set of point charges, as in our case35) would be negligible because the difference between the dielectric constants of the two phases is negligible at the interface. If one takes into account that the ion hydration shell itself was regarded as a dielectric sphere of low permittivity,36 we speculate that this rationalization could be a part of the story and could be exploited in further theoretical speculations about the hundred year mystery of inorganic ions at aqueous interfaces. Similarly to water molecules, whose preferred orientations near an interface result in an electrostatic potential difference across the boundary, the difference in the solvation energy between cations and anions was demonstrated to lead to a charge separation, an electrostatic double layer with anions in the outermost liquid layer, and a finite electrostatic potential difference.6 Experimental results and theoretical predictions of water interface potentials are extremely scattered6,7 but are of the same order of magnitude that can be obtained for the potential difference (Ψ°) between the interface and the bulk solution for our system due to the presence of LiF at the interface: according to the classical Gouy−Chapman model for a semi-infinite geometry37 (justified by the PGC pore size to Debye length high ratio38) we have 2

2

0.5

Ψ° = 2RT /F ln(σ /f + (σ /f + 1) )

■

*E-mail: [email protected]. Present Address

(8)

†

ITIS MONTANI Via Montani 7 Fermo, IT 63023. Tel.: +39 0734 622632. Fax +39 0734 622912.

−2

where σ is the surface charge density (C m ) and f is the following constant f = (8ε0εrRTC)0.5

AUTHOR INFORMATION

Corresponding Author

Notes

The authors declare no competing financial interest.

■

(9)

ACKNOWLEDGMENTS Technical support from Federica Marcotulli and Margherita Bonanni and financial support from Fondazione Cassa di Risparmio di Fermo are gratefully acknowledged.

where ε0 is the vacuum permittivity; R is the gas constant; and T is the absolute temperature. At the highest LiF concentration (C = 10 mM), the corresponding stationary phase concentration was 5.2 μmol g−1 (Figure 6), and the surface charge density, taking into account the specific surface area of PGC (120 m2 g−1), is σ = 4.2 × 10−03 C m−2, that gives, according to eq 9 Ψ° = 18.0 mV, negatively charged on the PGC side and positively charged on the side of the bulk. This is a rough estimate of the surface potential that neglects dipole formation, but it can be concluded that a successful model of aqueous interfaces must begin with a charged interface, not a neutral one: adsorption may continue until the electrostatic repulsion of the impinging ions by those already adsorbed becomes appreciable.

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REFERENCES

(1) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces; Wiley: New York, 1997; Ch. III. (2) Onsager, L.; Samaras, N. N. T. The Surface Tension of DebyeHückel Electrolytes. J. Chem. Phys. 1934, 2, 528−536. (3) Netz, R. R.; Horinek, D. Progress in Modeling of Ion Effects at the Vapor/Water Interface. Annu. Rev. Phys. Chem. 2012, 63, 401−418. (4) Petersen, P. B.; Saykally, R. J. Adsorption of Ions to the Surface of Dilute Electrolyte Solutions: The Jones-Ray Effect Revisited. J. Am. Chem. Soc. 2005, 127, 15446−15452. (5) Wang, R.; Wang Z.-G. Effects of Ion Solvation on Phase Equilibrium and Interfacial Tension of Liquid Mixtures. J. Chem. Phys. 2011, 135, 014707 (11 pages). (6) Jungwirth, P.; Tobias, D. J. Specific Ion Effects at the Air/water Interface. Chem. Rev. 2006, 106, 1259−1281. (7) Chang, T.-M.; Dang, L. X. Recent Advances in Molecular Simulations of Ion Solvation at Liquid Interfaces. Chem. Rev. 2006, 106, 1305−1322. (8) Mundy, C. J.; Kuo, I.-F. First-Principles Approaches to the Structure and Reactivity of Atmospherically Relevant Aqueous Interfaces. Chem. Rev. 2006, 106, 1282−1304.

4. CONCLUSIONS The presence of ions at aqueous interfaces is an issue in stark contrast to textbook knowledge. It is crystal clear that adsorption of LiF onto PGC takes place, at variance with Onsager and Samaras theory predictions. This is a cornerstone evidence: not only chaotropes but also kosmotropes are able to adsorb onto apolar surfaces. We qualitatively and quantitatively demonstrated the adsorption of even the most kosmotropic electrolyte onto PGC. 19008



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