J. Phys. Chem. C 2009, 113, 17795–17802
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Free Energy Relationships in the Electric Double Layer and Alkali Earth Speciation at the Fused Silica/Water Interface Jessica N. Malin, Joseph G. Holland, and Franz M. Geiger* Department of Chemistry, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208 ReceiVed: June 23, 2009; ReVised Manuscript ReceiVed: August 31, 2009
The adsorption of divalent Sr ions at the fused silica/water interface was studied using the Eisenthal χ(3) technique to quantify Sr2+ adsorption at neutral pH as a function of screening electrolyte concentration. We determine binding constants, adsorption free energies, absolute adsorbate number densities, and interfacial charge densities and examine the relationships between the measured adsorption free energies and the electric double layer interfacial potential present at each electrolyte concentration. Our results provide the first direct experimental investigation into the widely used additive adsorption free energy expression in which the observed free energy is modeled as the sum of an electrostatic free energy and an intrinsic chemical free energy. At screening electrolyte concentrations of 10 mM and lower, the free energy for Sr2+ adsorption to the fused silica/water interface depends directly on the interfacial potential, while the observed adsorption free energy becomes independent of the interfacial potential at higher electrolyte concentrations. This change in the free energy/interfacial potential relationship indicates that the charge of adsorbing strontium species changes from +2 to +1 at screening electrolyte conditions exceeding 10 mM. This finding is consistent with the formation of ion pairs which are thermodynamically favored at the interface but not in the aqueous bulk. Additional experiments using bromide and iodide show anion polarizability effects contribute about 10% to the chemical free energy. I. Introduction Electrostatics tells us that the speciation of metal ions is a key determinant for the free energy gained upon adsorption.1-3 For instance, Coulomb’s law states that divalent cations should interact more strongly with the charged surface sites of common oxides than monovalent cations. Given that solution thermodynamics predicts alkali earth cations to be divalent unless electrolyte concentrations approach those of brines,4 they are commonly treated to be divalent when interacting with surfaces as well. However, charge repulsion among the adsorbed cations may lead to changes in the charge state of the adsorbed alkali earth cations from +2 to +1. A reduction in interfacial charge density may occur through the formation of metal cation: electrolyte anion pairs in the form of M2+(H2O)nX- ions, which are thermodynamically favored at the interface but not in the aqueous bulk. This scenario is reminiscent of ion pairs and complexes at electrolyte/air interfaces and of shifts in interfacial acid-base equilibria by multiple pKa units from their corresponding bulk solution values.5-14 Given the importance of metal ion adsorption at aqueous/solid interfaces in environmental, materials, biological, industrial, and chemical processes,15-24 fundamental studies regarding the speciation, i.e., charge states, of metal cations adsorbed to fluid/solid interfaces are necessary for understanding, controlling, and predicting interfacial phenomena found in nature. Here, we focus on quantifying free energy relationships in the electrical double layer set up at fused silica/aqueous interfaces through an experimental laboratory model study that allows us to evaluate the speciation of strontium cations adsorbed to fused silica/water interfaces. Specifically, we determine the binding constants, free adsorption energies, absolute surface coverage, and ion speciation for Sr2+ adsorption at fused silica/water interfaces as a function of background
electrolyte concentration while keeping the bulk solution at pH 7. 90Sr is one of the major fission products found in nuclear waste.25-28 As a result, the soils and groundwater surrounding many DOE sites, such as Hanford, Savannah River, and Oak Ridge, have become contaminated with 90Sr from waste tank leakage.25,26,28,29 Strontium contamination is hazardous to public health because 90Sr is a beta emitter with a half-life of 29.1 years that is highly mobile groundwater systems.25,26,30-32 If ingested through drinking water, 90Sr is processed in the body through the same mechanisms as Ca, leading to bioaccumulation in bones and teeth. Once incorporated into bone, the β-emissions that result from 90Sr decay can lead to cancer of the bone and surrounding soft tissue. The U.S. Environmental Protection Agency has set a Maximum Contaminant Level (MCL) of 8 pCi per liter of drinking water for beta emitters such as 90Sr.32 Unfortunately, 90Sr groundwater concentrations near several DOE sites exceed this MCL by several orders of magnitude, making research into the environmental mobility of strontium a top priority.26,33,34 Numerous techniques have been used to study the adsorption behavior of Sr ions at varying soil/water and mineral oxide/ water interfaces, including batch and column experiments,28,31,35-38 X-ray spectroscopies,26,29,39-41 and surface complexation modeling of experimental adsorption data.25,30,42,43 Sverjensky reported a detailed surface complexation analysis of alkaline earths, including Sr2+, interacting with silica, and predicted strontium to be an outer-sphere species at pH 7 and low ionic strength.44 Laboratory studies reveal low partitioning coefficients (Kd) and retardation factors (Rf) for Sr, indicating that Sr is highly mobile in the environment. Extended X-ray adsorption fine structure (EXAFS) and X-ray adsorption near edge structure (XANES) spectroscopies work has determined that Sr2+ is weakly associated with alumina-silicate minerals, and that the surface-bound
10.1021/jp905881h CCC: $40.75 2009 American Chemical Society Published on Web 09/21/2009
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ions are fully hydrated, which is consistent with an adsorption process that follows an outer-sphere mechanism.26,31 However, there is limited precedent in the literature investigating radionuclide adsorption over a range of background electrolyte concentrations to date. Considering the high mobility of Sr2+ in groundwater, it is likely that Sr pollution will travel through areas of varying electrolyte concentration when spreading from nuclear waste point sources. Therefore, research focusing on how screening electrolyte concentrations control Sr2+ surface complexation reactions is needed to ensure that predictive models are calibrated appropriately for groundwater ionic strength. Yoshida and Suzuki studied Sr2+ adsorption to quartz in a column experiment at high ionic strengths (100 mM) and found that the extent of Sr2+ adsorption was decreased when compared to the column retention at low ionic strength (1 mM).37 Likewise, Chen et al. observed the same decrease in Sr2+ adsorption at high ionic strength in their batch studies performed with silica.31 However, a systematic and quantitative study of Sr interactions with oxide/aqueous interfaces as a function of electrolyte concentration has not yet been reported. To fill this void, we present an analysis of the relationship between Sr2+ adsorption free energies and the interfacial potential at a given electrolyte concentration. Our measurements explicitly explore the additive free energy relationship in the electric double layer that is commonly employed in adsorption models.17,45,46 We present evidence that Sr2+ exhibits one of two binding modes depending on the screening electrolyte concentration. This result is crucial to the calibration of surface complexation models used in predicting Sr pollutant mobility through groundwater systems of varying background electrolyte concentration. II. Background and Theory The adsorption of divalent metals from an electrolyte solution onto a mineral oxide can be driven by electrostatic and Coulombic forces, redox chemistry, entropy and interfacial tension minimization, and hydrophobic expulsion, among others.17,19 In this study, the fused silica substrate is redox inert, and carries a negative charge under a pH 7 aqueous phase. Electrostatic and Coulombic interactions are thus expected to dominate the interaction of Sr2+ with the fused silica/water interface. A metal ion adsorbing from a bulk electrolyte solution onto a charged mineral oxide must pass through the electric double layer set up in the interfacial region. For an ion moving through the electric double layer, the following additive expression can be used to describe the observed adsorption free energy:16,17,19,45,47,48
∆Gads ) ∆Gelectrostatic + ∆Gchemical
(1)
In this formulation, the observed adsorption free energy (∆Gads) is assumed to be the sum of the free energy due to the purely Coulombic electrostatic interactions (∆Gelectrostatic) and the intrinsic chemical free energy (∆Gchemical) associated with the adsorption process (i.e., hydrogen bonding, dipole-dipole interactions, etc.). Further, the Coulombic contribution to the adsorption free energy can be expressed as the work required to bring a charged particle in contact with a charged surface:16,17,45
∆Gads ) F∆zΦ0 + ∆Gchemical
(2)
where Φ0 is the interfacial potential, ∆z is the change in charge of the surface species associated with adsorption, and F is Faraday’s constant. As seen in eq 2, the observed adsorption free energy should show a linear dependence on the interfacial potential. Gouy-Chapman theory describes the exponential decay of the interfacial potential from a charged surface through an electric double layer. The following Gouy-Chapman expression describes the interfacial potential (Φ0) as a function of the surface charge density (σ) and the electrolyte concentration Celec:16,17,19,49
Φ0 )
(
2kBT sinh-1 σ ze
π 2εε0TCelec
)
(3)
where kB is the Boltzmann constant, T is the temperature, z is the charge on the electrolyte ion, e is the charge on an electron, ε0 is the permittivity in a vacuum, and ε is the dielectric constant for water at 25 °C. By substitution of eq 3 into eq 2, one sees that the change in the surface species charge upon adsorption (∆z) can be determined by measuring the observed adsorption free energies as a function of the background electrolyte concentration (Celec) at a constant pH. The surface specificity, submonolayer analyte sensitivity, and real-time capabilities of second harmonic generation (SHG) make it a well-suited technique for such adsorption measurements. Specifically, SHG is ideal for studying metal adsorption as a function of interfacial potential.50-53 The use of SHG to probe interfacial potential is known as the χ(3) technique, which was pioneered by Eisenthal and co-workers.49,54 SHG is a nonlinear optical technique that is forbidden in centrosymmetric media, making it a powerful tool for studying interfacial phenomena.55-59 During the SHG process, two photons of incident frequency ω, upon interacting with aligned dipoles at the interface of two bulk media, combine to produce one photon at twice the frequency 2ω. The SHG electric field (ESHG) is proportional to the induced second-order polarization at the interface (P2ω) as follows:49,54,56,59
ESHG ∝ P2ω ) χ(2)EωEω
(4)
In the above expression, Eω refers to the incident electric field and χ(2) is the second-order susceptibility of the interface. In the case of our negatively charged fused silica under pH 7 water,50 there is a static electric field present at the interface, which sets up an interfacial potential. The presence of an interfacial potential leads to an additional, third-order; contribution to the second harmonic E-field:49,54,59
ESHG ∝ P2ω ) χ(2)EωEω + χ(3)EωEωΦ0
(5)
Again, Φ0 is the interfacial potential defined as the integration of the static electric field from 0 to ∞ in the direction perpendicular to the charged surface,49 and χ(3) is the third-order susceptibility of the interface. The incident electric field is held constant throughout the experiments, and the second- and thirdorder susceptibilities are constants related to the intrinsic properties of the interface, which undergo a negligible change throughout the SHG experiments. Therefore, eq 5 can be reduced to the following practical form:49
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√ISHG ) ESHG ) A + BΦ0
(6)
Equation 6 illustrates that any changes in interfacial potential will result in a direct change in the SHG E-field and thus the observed SHG signal intensity (ISHG). The sensitivity of the Eisenthal χ(3) technique to changes in the interfacial potential is ideal for exploring the free energy relationships discussed above. Consider Sr2+ ions adsorbing to the charged fused silica/water interface. Initially, the interfacial potential results from the negative charge carried by the silica surface at pH 7. As the metal cations adsorb to the surface, the magnitude of the interfacial potential decreases, which decreases the SHG intensity. The decrease in the SHG signal intensity as a result of metal cation adsorption is then used to quantify the thermodynamics of the adsorption process in the following way: first, the Gouy-Chapman equation (eq 3) was substituted for the interfacial potential in the χ(3) expression (eq 6). Then, the following metal analyte dependent expression was used for the surface charge density (σ) in the Gouy-Chapman representation of the interfacial potential:
(
σ ) σ0 + σm
K*ads[M] 1 + K*ads[M]
)
(7)
Here, the overall surface charge density was modeled as the sum of the initial surface charge density of the fused silica at pH 7 (σ0) and the maximum charge density established by the adsorbed metal cations at monolayer coverage (σm).54 To express the adsorbed metal surface charge density at any given bulk metal concentration, σm was scaled by the relative surface coverage at that bulk concentration. This was achieved by multiplying σm by the Langmuir equation, where K*ads is the observed binding constant for adsorption and [M] is the bulk concentration of metal ions in the solution. By combining eqs 5, 6, and 7, the χ(3) SHG response from the system was expressed as a function of bulk metal ion concentration. Equation 8 was then used to fit the measured SHG adsorption isotherms to obtain binding constants (K*ads) and maximum Sr2+ surface charge densities (σm). From these fit parameters, the adsorption free energies and Sr2+ adsorbate number densities were calculated.49-51,54
√ISHG ) ESHG ) A + B' sinh
-1
[(
(
σ0 + σm
K*ads[M] 1 + K*ads[M]
30.2M-1/2m2C-2
√Celec
]
))
×
of dilute NaOH (Sigma-Aldrich, 99.99%) and HCl (E.M.D., ACS grade) solutions. The aqueous Sr2+ source was SrCl2 · 6H2O (Sigma-Aldrich, 99%), and the background electrolytes were prepared using NaCl (VWR, 99%), NaBr (Sigma-Aldrich, 99+%), and NaI (Sigma-Aldrich, 99+%). Each background electrolyte concentration was verified using a conductivity meter (Fisher Traceable Conductivity) and TDS meter (Fisher Scientific). B. Laser/Flow System. A detailed description and schematic of our laser setup can be found in our previously published work.50,51,60-64 The SHG experiments used fundamental light produced by a regeneratively amplified Ti:sapphire laser (Hurricane, Spectra Physics, 120 fs pulse) pumping an optical parametric amplifier (OPA-CF, Spectra Physics, 1 kHz repetition rate) tuned to a frequency of ω ) 600 ( 5 nm. The laser beam was directed through a variable density filter, where the power was attenuated to 0.4 µJ. After the density filter, the beam was passed through a half-wave plate set for p-polarized light and focused onto the silica/water interface at an angle just below that of total internal reflection. The reflected beam from the interface was then directed through a UV-grade Schott filter to remove the fundamental, 600 nm light (ω), allowing the 300 nm second harmonic beam (2ω) to be isolated, and sent into a monochromator set at 2ω. The second harmonic output from the monochromator was directed into a PMT, and then quantified through use of a gated, single photon counting system. Before each experiment, the expected SHG quadratic power dependence and the wavelength bandwidth of the SHG signal were verified to ensure no optical damage of the sample was occurring. All SHG measurements were performed under flow conditions using the pump system we have established for χ(3) studies of metal interactions at buried solid/water interfaces.50,51,60-62 The setup utilizes a custom-built Teflon flow cell designed to place the flat surface of a hemispherical lens in contact with the aqueous solution contained within the cell. The lens was clamped atop the flow cell lined with a Viton O-ring to ensure a leak proof seal under flow conditions. Flow through the cell was controlled via adjustable peristaltic pumps pulling from two separate reservoirs. One reservoir contained the background electrolyte solution, and the other contained the same background electrolyte solution plus the desired concentration of Sr2+. Using a flow meter, the flow was monitored and maintained at ∼1 mL/s for all experiments. To determine the bulk strontium concentration of each solution, aliquots were collected from the output flow and analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Varian). Each adsorption isotherm was recorded in either duplicate or triplicate.
(8)
III. Experimental Section A. Sample and Solution Preparation and Materials. The adsorption substrates used in this study were hemispherical fused silica lenses (ISP Optics). Prior to each experiment, the lens was treated for 1 h with the commercial glass cleaner NoChromix (Godax Laboratories). The lens was then rinsed thoroughly with Millipore water (18.2 MΩ), followed by 6 min of sonication in methanol. Next, the lens was place in an oven at 100 °C for 15 min to dry, and finally plasma cleaned (Harrick Plasma, Ithaca, NY) on high for 30 s. Following the cleaning procedure, each lens was stored under Millipore water until further use. All solutions used in this study were prepared in Millipore water and maintained at pH 7.0 ( 0.2 using 10 mM), the slope from the free energy vs potential plot, and therefore ∆z, is zero. We attribute this change in slope from that obtained
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Figure 3. Proposed Coulombic adsorption pathways for Sr2+ to silica. At screening electrolyte concentrations of 10 mM and lower, pathways A and B occur, resulting in an observed average ∆z of +1.5. Above 10 mM screening electrolyte concentration, Sr adsorption occurs, as depicted in pathway C for an observed ∆z of zero.
for the lower electrolyte concentrations to a change in the charge state of the Sr adsorbate. At high electrolyte concentrations, a water molecule in the Sr2+ inner or outer hydration sphere may be replaced by a counteranion. Alternatively, a counteranion may simply coordinate to the inner or outer hydration sphere. These scenarios would result in a reduction of the overall charge on the strontium adsorbate, which may be called a contact ion or simply ion pair, from +2 to +1. This situation is analogous to what has been reported for chloride-coordinated, or complexed, rare earth ions.66 Previous charge screening experiments performed by our group showed that the fused silica interface is near saturation with Na+ above 10 mM bulk NaCl concentration.50 On the basis of this evidence, we propose scenario C in Figure 3 to reconcile a ∆z of zero for background electrolyte concentrations greater than 10 mM. In this scenario, the singly charged species [Sr2+(H2O)nX-]+, where X- ) Cl- or I- and n is the number of water molecules in the hydration sphere, displaces a bound hydrated Na+ as it adsorbs to the negatively charged surface site, resulting in the observed ∆z of zero. A powerful capability of the Eisenthal χ(3) technique is the quantification of the surface charge density of adsorbed metal ions (σm) at saturation coverage (see eq 8).49-51,67 Table 1 lists the maximum Sr2+ surface charge density from the fit of each of the SHG adsorption isotherms performed. The results show that the maximum surface charge density adsorbed to the fused silica/water interface ranges from +0.010(2) to +0.017(4) C/m2 for NaCl as the screening electrolyte and +0.005(2) to +0.014(2) C/m2 for the case of NaI. Considering the initial surface charge density on the fused silica at pH 7 of -0.013 C/m2, it is evident that Sr2+ adsorption occurs to the extent of Coulombic saturation, with negligible overcharging of the surface. From the surface charge density data, the absolute number density of Sr adsorbates at saturation coverage was calculated, and these values are listed in Table 1. For low screening electrolyte concentrations (1-10 mM), where we assumed the adsorbed species to be the divalent hydrated Sr2+, the average adsorbate number density was calculated to be 3.5 × 1012 Sr2+ ions/cm2 for NaCl and 3.2 × 1012 Sr2+ ions/cm2 for NaI. Note that the uncertainties in these values are given in Table 1. In the case of high electrolyte concentrations (>10 mM), we
Malin et al. assumed the adsorbate to be the univalent [Sr2+(H2O)nX-]+, resulting in average adsorbate number densities of 10 × 1012 and 4 × 1012 ions/cm2 for NaCl and NaI, respectively. With reference to the 1014 geometric surface sites typical for mineral oxides, Sr2+ adsorbates occupy ∼4% of the available surface sites in the low electrolyte regime and up to ∼10% of the surface sites for the univalent Sr ion pair at high electrolyte concentrations. Duval et al. performed X-ray photoelectron spectroscopy (XPS) measurements on quartz as a function of exposed aqueous phase pH, and determined that slightly more than 15% of the quartz surface sites are in a charged, deprotonated state (>SiO-) at pH 7.68 This X-ray study also revealed that >75% of the surface sites on quartz remain neutral in the form of >SiOH groups throughout the pH range 0-10. On the basis of this evidence, and the surface coverage data collected in this work, we conclude that Sr adsorption at the fused silica/water interface occurs at negatively charged >SiO- sites, with neutral >SiOH groups being spectator sites for Sr binding. As a result, all of our proposed adsorption pathways involve the surface silanol group in its deprotonated state. Note that, if strontium binding to the fused silica/water interface occurred predominantly at neutral SiOH groups, then it could be suggested that the two relevant reactions at low ionic strengths are SiOH + Sr2+ f SiOH · · · Sr2+, for which ∆z ) 2.0, and SiOH + Sr2+ + H2O f SiOH · · · Sr(OH)+ + H+, for which ∆z ) 1.0. At the highest ionic strengths, the relevant reaction could be SiOH · · · Na+ + Sr2+ + H2O f SiOH · · · Sr(OH)+ + H+ + Na+, for which ∆z ) 0.0. However, on the basis of the observation that strontium binding is dominated by the charged SiO- groups and not the neutral SiOH groups, we suggest the pathways depicted in Figure 3 to be operative. D. Electrolyte Anion Effects on Sr Adsorption: Chloride, Bromide, and Iodide. The chemical nature of the alkali halide used as the screening electrolyte impacts both the observed adsorption free energy as well as the surface coverage associated with Sr2+ binding at the fused silica/water interface. Figure 2 clearly shows that changing the screening electrolyte anion from Cl- to I- leads to Sr adsorption free energies that are on average 3 kJ/mol more favorable for I- vs Cl-. This finding can be reconciled as follows: The interfacial electric double layer formed at a negatively charged fused silica/water interface consists of a layer of electrolyte cations (Na+) located adjacent to the silica surface, with the electrolyte anions further way from the interface. More polarizable anions such as I- are associated with induced dipole moments that are larger than those associated with less polarizable anions such as Cl-.69 Upon Sr adsorption, the induced dipole adds an increased favorable interaction to the overall adsorption free energy. On the basis of the offset between the parallel lines for NaCl and NaI on the free energy versus potential plot, the anion polarizability effect was quantified as -3 kJ/mol when switching the electrolyte anion to I- from Cl-. Note that the parallel nature of the NaCl and NaI data indicates that the free energy contributions resulting from purely Coulombic interactions, i.e., ∆Gelectrostatic, are the same regardless of the chemical nature of the halide used in the 1:1 screening electrolyte. Figure 4 shows that the observed adsorption free energy does indeed become more favorable with increasing electrolyte anion polarizability. Attempts were made to complete the halide series by measuring isotherms using NaF as the screening electrolyte; however, the observed Sr2+ adsorption was so slight that an isotherm could not be fit and quantified. From the linear relationship between the free adsorption energy and the inverse
Free Energy Relationships in the Electric Double Layer
Figure 4. Observed free energy of adsorption of strontium to fused silica/water interfaces at pH 7 and 4 mM electrolyte concentration for NaCl, NaBr, and NaI as a function of inverse halide polarizability.
halogen polarizability, we conclude that the anion polarizability effects are important and that they contribute about 10% to the ∆Gchemical term. Furthermore, the nature of the electrolyte anion was found to lead to differences in the average Sr adsorbate number density in the high electrolyte concentration regime (>10 mM). The adsorbed number density was twice as large for NaCl compared to NaI. This difference was attributed to the larger ionic radius of I- over Cl-, in that the [Sr2+(H2O)nI-]+ species takes up more space on the surface than the [Sr2+(H2O)nCl-]+ species. The fact that a dependence of adsorbate surface coverage on the electrolyte anion was only observed at high electrolyte concentrations serves as further evidence that the proposed ion pair or contact ion pair adsorption pathway in Figure 3C is likely. The fact that the mean activities of our strontium solutions can be as low as 0.87 for a 5 mM SrCl2 solution is consistent with ion pairing at the interface, where the activities may be even lower due to local field effects. V. Conclusions In this work, we investigated the interaction of divalent Sr ions at the fused silica/water interface using second harmonic generation (SHG). Specifically, we used the Eisenthal χ(3) technique in real time and under flow conditions to quantify the binding constants, the free adsorption energies, and the number density of adsorbed Sr2+ ions at neutral pH and as a function of screening electrolyte concentration. These studies provide the first direct experimental investigation into the widely used additive adsorption free energy expression, in which the observed free energy is modeled as the sum of an electrostatic free energy and an intrinsic chemical free energy. Our results show that the measured adsorption free energy for Sr2+ at the fused silica/water interface depends linearly on the interfacial potential at screening electrolyte concentrations of 10 mM and lower. However, the observed adsorption free energy becomes independent of the interfacial potential when the screening electrolyte concentration exceeds 10 mM. This change in the free energy/potential relationship signifies a change in the type of interaction that Sr2+ undergoes with the fused silica/water interface. From our adsorption data, we determined the change in the charge state of the surface species upon Sr2+ adsorption. The adsorption isotherm data indicate a change in strontium speciation from an overall Sr adsorate charge of +2 to +1 under high screening electrolyte conditions (>10 mM). We attribute this decrease in charge to the exchange of an
J. Phys. Chem. C, Vol. 113, No. 41, 2009 17801 electrolyte anion for a water molecule in the Sr2+ hydration sphere, or the addition of an electrolyte anion to the Sr2+ hydration sphere, leading to the formation of [Sr2+(H2O)nX-]+ as the species relevant for adsorption. Strontium adsorption isotherms recorded for a series of alkali halide electrolytes reveal that the strontium adsorption free energy becomes more favorable by 3 kJ/mol when NaI instead of NaCl is used as the electrolyte, and the free energy depends linearly on the inverse anion polarizability. Our fundamental investigations into Sr2+ adsorption at the silica/water interface serve as benchmarks for alkali earth surface complexation models and highlight screening electrolyte contributions that should be considered to accurately predict the interaction of metal cations with interfaces, including 90Sr mobility in soils and groundwater. Our studies also show that the separation of the observed free energies of adsorption into an electrostatic and a chemical term is only correct under conditions where speciation does not change. Using strontium, which exhibits straightforward bulk speciation based on bulk thermodynamics, as an example, we report a factor of 2 variance between theoretical and experimental chemical adsorption free energies (∆Gchem) under conditions of high screening electrolyte concentrations, highlighting the need for and the utility of molecular-level experimental work. These differences in interfacial speciation lead to large differences in the absolute strontium surface coverages. Our findings are likely to be relevant for other divalent alkali earth cations that are associated with a single speciation state in solution at circumneutral pH, such as calcium, barium, and magnesium. We therefore expect our findings to be directly applicable to biological systems as well as battery and related materials applications where fluid/solid interfaces are important. As mentioned in the Introduction, the surface speciation changes will also be important in atmospheric aerosol processing, as well as metal pollutant and surfactant interactions with fluid/solid interfaces. Clearly, changes in the charge state and the chemical nature of aqueous adsorbates that are not self-evident but that nevertheless occur as a function of background electrolyte concentration should be considered when evaluating experimental adsorption data for use in geochemical modeling, pollutant mobility simulations, aerosol microphysics, biosensor design, and energy systems design. Acknowledgment. This work was supported by the Director, Chemical Sciences, Geosciences and Biosciences Division, of the U.S. Department of Energy under Grant No. DE-FG0206ER15787, the National Science Foundation Experimental Physical Chemistry program under Grant No. CHE-0348873, and the NSF Nanoscale Science and Engineering Center (NSEC). F.M.G. is an Alfred P. Sloan Fellow. References and Notes (1) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; John Wiley & Sons: New York, 1990. (2) Stumm, W.; Morgan, J. J. Aquatic Chemistry, Chemical Equilibria and Rates in Natural Waters, 3rd ed.; John Wiley & Sons: New York, 1996. (3) Somorjai, G. A. Chemistry in Two Dimensions; Cornell University Press: Ithaca, NY, 1981. (4) Mu¨ller, B. ChemEQL, 3.0 ed.; Limnological Research Center, Swiss Federal Institute of Aquatic Science and Technology: Kastanienbaum, Switzerland, 2005. (5) Bhattacharyya, K.; Sitzmann, E. V.; Eisenthal, K. B. J. Chem. Phys. 1987, 87, 1442. (6) Xiao, X. D.; Vogel, V.; Shen, Y. R. Chem. Phys. Lett. 1989, 163, 555. (7) Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370.
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