pubs.acs.org/Langmuir © 2010 American Chemical Society
Hydrated Cation Speciation at the Muscovite (001)-Water Interface Sang Soo Lee,*,† Paul Fenter,† Changyong Park,†,§ Neil C. Sturchio,‡ and Kathryn L. Nagy‡ †
Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States, and ‡Department of Earth and Environmental Sciences, University of Illinois at Chicago, 845 West Taylor Street, MC-186, Chicago, Illinois 60607, United States. § Present address: HPCAT, Geophysical Laboratory, Carnegie Institution of Washington, Argonne, Illinois 60439, United States. Received August 17, 2010. Revised Manuscript Received September 30, 2010 Charged materials in aqueous systems interact according to their interfacial properties, typically described by the electrical double layer (EDL). Distributions of divalent metal cations at the muscovite (001)-solution interface observed using resonant anomalous X-ray reflectivity demonstrate an unexpected complexity with respect to the EDL structure. Three forms of adsorbed cations can coexist: the classical inner-sphere and outer-sphere complexes and a third “extended” outer-sphere complex located farther from the surface. Their relative proportions are controlled by the energy balance among cation hydration, interface hydration, and electrostatic attraction. Systematic trends in coverage and position establish the defining role of cation hydration in stabilizing the multiple coexisting species.
Introduction The interaction of solvated ions with charged surfaces is crucial to many natural and technological phenomena. For example, adsorption at mineral-water interfaces effectively controls the transport of elements in earth’s near-surface environment, and a robust characterization of this process is essential for assessing societal risks associated with the mobility of toxic metal contaminants in aquifers and surface water.1 Similarly, the adsorption of counterions is central to controlling the structure and behavior of charged biological materials (e.g., membranes and polyelectrolytes such as DNA).2 Furthermore, the efficacy of energy storage technologies, such as electrochemical capacitors, depends on the ion adsorption mechanism, and significant improvements in capacity are promoted by adsorption in nanoporous materials.3 The basic ideas describing adsorption (particularly electrostatic adsorption including the formation of a condensed Stern layer and a diffuse ion profile) are conceptually simple and were expressed nearly a century ago.4-6 These classical models laid the foundations of modern electrical double layer (EDL) theories. Recent extensions, including ion condensation theory, take into account deviations from the Poisson-Boltzmann (PB) model caused by ion-ion correlations7 and other factors, such as hard-core *Corresponding author. Phone: (630) 252-6679. Fax: (630) 252-9570. E-mail:
[email protected]. (1) Brown, G. E., Jr.; Henrich, V. E.; Casey, W. H.; Clark, D. L.; Eggleston, C.; Felmy, A.; Goodman, D. W.; Gr€atzel, M.; Maciel, G.; McCarthy, M. I.; Nealson, K. H.; Sverjensky, D. A.; Toney, M. F.; Zachara, J. M. Metal oxide surfaces and their interactions with aqueous solutions and microbial organisms. Chem. Rev. 1999, 99, 77–174. (2) de la Cruz, M. O. Electrostatic control of self-organization: the role of charge gradients in heterogeneous media. Soft Matter 2008, 4, 1735–1739. (3) Chmiola, J.; Yushin, G.; Gogotsi, Y.; Portet, C.; Simon, P.; Taberna, P. L. Anomalous increase in carbon capacitance at pore sizes less than 1 nanometer. Science 2006, 313, 1760–1763. (4) Gouy, M. Sur la constitution de la charge electrique a la surface d’un electrolyte. J. Phys. Theor. Appl. 1910, 9, 457–468. (5) Chapman, D. L. A contribution to the theory of electrocapillarity. Philos. Mag. 1913, 25, 475–481. (6) Stern, O. Zur Theorie Der Elektrolytischen Doppelschicht (the theory of the electrolytic double-layer). Z. Electrochem. 1924, 30, 508–516. (7) Shklovskii, B. I. Screening of a macroion by multivalent ions: correlationinduced inversion of charge. Phys. Rev. E 1999, 60, 5802–5811.
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correlations and surface-specific interactions.8,9 These theoretical approaches often simplify important chemical details, such as the effects of ion hydration, which are normally incorporated through semiempirical considerations of changes in the effective ion size (e.g., hydrated vs dehydrated) and the presence of interfacial dielectric layers.10-12 Nevertheless, it has long been recognized that ion hydration may strongly influence the ion adsorption speciation and adsorption strength.13,14 Various experimental studies have confirmed the important role of water, identifying two distinct types of adsorbed species, inner-sphere (IS) and outersphere (OS) complexes, depending on the hydrated state of an ion.15,16 Both computational16,17 and experimental studies18-20 have demonstrated that IS and OS species can coexist at charged (8) Solis, F. J.; de la Cruz, M. O. Attractive interactions between rodlike polyelectrolytes: polarization, crystallization, and packing. Phys. Rev. E 1999, 60, 4496–4499. (9) Travesset, A.; Vangaveti, S. Electrostatic correlations at the Stern layer: physics or chemistry? J. Chem. Phys. 2009, 131, 185102-1–11. (10) Burak, Y.; Andelman, D. Hydration interactions: aqueous solvent effects in electric double layers. Phys. Rev. E 2000, 62, 5296–5312. (11) Guerrero-Garcı´ a, G. I.; Gonzalez-Tovar, E.; de la Cruz, M. O. Effects of the ionic size-asymmetry around a charged nanoparticle: unequal charge neutralization and electrostatic screening. Soft Matter 2010, 6, 2056–2065. (12) Guerrero-Garcı´ a, G. I.; Gonzalez-Tovar, E.; Chavez-Paez, M.; LozadaCassou, M. Overcharging and charge reversal in the electrical double layer around the point of zero charge. J. Chem. Phys. 2010, 132, 054903-1–054903-19. (13) James, R. O.; Healy, T. W. Adsorption of hydrolyzable metal ions at the oxide-water interface. J. Colloid Interface Sci. 1972, 40, 65–81. (14) Sverjensky, D. A. Physical surface-complexation models for sorption at the mineral-water interface. Nature 1993, 364, 776–780. (15) Stumm, W. Chemistry of the Solid-Water Interface: Processes at the Mineral-Water and Particle-Water Interface in Natural Systems. John Wiley & Sons: New York, 1992; p 428. (16) Sposito, G.; Skipper, N. T.; Sutton, R.; Park, S.-H.; Soper, A. K.; Greathouse, J. A. Surface geochemistry of the clay minerals. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 3358–3364. (17) Chang, F.-R. C.; Skipper, N. T.; Sposito, G. Monte Carlo and molecular dynamics simulations of electrical double-layer structure in potassium-montmorillonite hydrates. Langmuir 1998, 14, 1201–1207. (18) Park, C.; Fenter, P. A.; Nagy, K. L.; Sturchio, N. C. Hydration and distribution of ions at the mica-water interface. Phys. Rev. Lett. 2006, 97, 0161011–016101-4. (19) Catalano, J. G.; Park, C.; Fenter, P.; Zhang, Z. Simultaneous inner- and outer-sphere arsenate adsorption on corundum and hematite. Geochim. Cosmochim. Acta 2008, 72, 1986–2004. (20) Lee, S. S.; Nagy, K. L.; Park, C.; Fenter, P. Enhanced uptake and modified distribution of mercury(II) by fulvic acid on the muscovite (001) surface. Environ. Sci. Technol. 2009, 43, 5295–5300.
Published on Web 10/08/2010
DOI: 10.1021/la1032866
16647
16648 DOI: 10.1021/la1032866
Sr(NO3)2
1 10-2
pH solutions
3.7
width (A˚) occupancy (atom/AUC) height (A˚)
occupancy (atom/AUC)
width (A˚)
height (A˚)
extended outer sphere adsorbed outer sphere
width (A˚) occupancy (atom/AUC) height (A˚) χ (R factor)a concentration (mol/kg)
(21) Israelachvili, J. N.; Wennerstr€om, H. Role of hydration and water structure in biological and colloidal interactions. Nature 1996, 379, 219–225. (22) Roivetti, C.; Guthhold, M.; Bustamante, C. Scanning force microscopy of DNA deposited onto mica: equilibration versus kinetic trapping studied by statistical polymer chain analysis. J. Mol. Biol. 1996, 264, 919–932. (23) Yang, H.; Kuperman, A.; Coombs, N.; Mamiche-Afara, S.; Ozin, G. A. Synthesis of oriented films of mesoporous silica on mica. Nature 1996, 379, 703–705. (24) Oelkers, E. H.; Schott, J.; Gauthier, J.-M.; Herrero-Roncal, T. An experimental study of the dissolution mechanism and rates of muscovite. Geochim. Cosmochim. Acta 2008, 72, 4948–4961. (25) Martell, A. F.; Smith, R. M. Critical Stability Constants: Other Organic Ligands; Plenum Press: New York, 1976; Vol. 3, p 495. (26) Cheng, L.; Fenter, P.; Nagy, K. L.; Schlegel, M. L.; Sturchio, N. C. Molecular-scale density oscillations in water adjacent to a mica surface. Phys. Rev. Lett. 2001, 87, 156103-1–156103-4. (27) Fenter, P. A. X-ray Reflectivity as a Probe of Mineral-Fluid Interfaces: A User Guide. In Applications of Synchrotron Radiation in Low-Temperature Geochemistry and Environmental Sciences; Fenter, P. A., Rivers, M. L., Sturchio, N. C., Sutton, S. R., Eds.; Mineralogical Society of America: Washington DC, 2002; Vol. 49, pp 149-220.
inner sphere
by dissolving high-purity (g99.99%) metal nitrate salts (Aldrich Chemical Co.) in deionized water (Table 1). The concentrations of metal ions (1 10-3 or 1 10-2 mol/kg; molality) were chosen to maximize ion sorption and enhance changes in interfacial structure. These high metal concentrations would minimize the competitive adsorption of other cations (e.g., Kþ and Al3þ) derived from the dissolution of muscovite, whose maximum concentrations are estimated to be ∼10-7 mol/kg at pH 2.0 on the basis of measured dissolution rates of muscovite at acidic pH and the time required to perform these experiments.24 Under the given experimental conditions, we did not observe explicit evidence for muscovite dissolution, such as the reduction of adsorbed ion coverage with increasing time, during the XR and RAXR measurements (described in the next section). Mercury- and lead-containing solutions were prepared at acidic pH (2.0 for Hg and 2.0 and 3.7 for Pb) to prevent the formation of hydrolysis products.25 Additional sets of Sr- and Zncontaining solutions were prepared at pH 3.7, and the results were compared with those at pH 5.5. The solution pH was adjusted with a small volume of 0.1 N HNO3 or NaOH as required. Gem-quality single crystals of muscovite (from Asheville Schoonmaker Mica Company; 25 mm 25 mm 0.2 mm) were cleaved to expose a fresh (001) cleavage surface and immediately immersed in a 50 mL centrifuge tube containing an experimental solution. Each crystal was allowed to equilibrate in solution for more than 1 h and was then transferred to a thin-film sample cell26,27 filled with the same solution for the X-ray reflectivity measurements.
2
Materials and Methods Sample Preparation. Experimental solutions were prepared
Table 1. Distribution of Heavy Metal Cations Adsorbed on the Muscovite (001) Surface Derived from the Best-Fit Models of RAXR Datac
interfaces. However, fundamental control of this interfacial process has yet to be investigated systematically. In this article, new experiments were designed to observe variations in the atomic-scale distributions of adsorbed species as a function of cation properties and therefore to identify factors controlling the relative proportions of the various adsorption states. A set of divalent cations (Cu2þ, Zn2þ, Sr2þ, Hg2þ, and Pb2þ) were selected with a wide range of sizes and hydration enthalpies. The distributions of these cations were observed using in situ specular X-ray reflectivity (XR) and resonant anomalous X-ray reflectivity (RAXR) at the muscovite (001)-aqueous solution interface with a˚ngstr€om-scale resolution (Figures S1 and S2 in the Supporting Information (SI)]. The ab plane of muscovite cleaves easily to expose a large atomically flat surface with a permanent negative charge (∼1e- per unit cell area, AUC = 46.72 A˚2). The (001) surface has been used extensively as a substrate in a wide range of studies such as biomolecule adsorption and film growth21-23 and represents the dominant reactive surfaces of phyllosilicates in nature including those of many clay minerals. Therefore, understanding its adsorption reactivity also provides fundamental insight into metal contaminant transport behavior in soils and groundwater environments.
1.23 1.38(f) 0.00(1) 0.29(f) 4.57(5) 0.18(3) 0.54(11) 7.89(64) 0.15(4) 2.38(57) 0.68(10) (0.006) 1 10-2 Sr(NO3)2 5.518,46 1.52 1.38(7) 0.26(2) 0.62(9) 4.58(7) 0.31(2) 0.84(10) 1.18(6) (0.009) -2 Pb(NO3)2 2.0 1.20 1.93(5) 0.12(2) 0.28(13) 3.61(40) 0.09(3) 1.40(40) 8.34(85) 0.06(3) 1.88(81) 0.58(9) 1 10 (0.012) Pb(NO3)2 3.7 1.54 1.90(3) 0.29(4) 0.27(11) 4.39(42) 0.19(5) 1.60(43) 9.58(61) 0.05(1) 2.00(f) 1.11(13) 1 10-2 (0.012) -3 20 1 10 Hg(NO3)2 2.0 1.15 0.62(5) 0.06(1) 0.13(f) 3.58(4) 0.14(1) 0.68(6) 9.57(36) 0.06(2) 2.26(55) 0.55(4) (0.005) 1 10-2 Zn(NO3)2 3.7 1.23 1.28(72) 0.01(1) 0.29(f) 3.90(4) 0.20(4) 0.40(10) 5.37(75) 0.14(5) 1.77(38) 0.72(13) (0.007) -3 Zn(NO3)2 5.5 1.46 1.28(f) 0.00(0) 0.29(f) 3.88(1) 0.27(2) 0.32(4) 6.11(40) 0.11(2) 1.81(28) 0.78(5) 1 10 (0.005) 1 10-2 Cu(NO3)2 3.7 1.32 -0.14(19) 0.03(1) 0.29(f) 3.97(4) 0.28(4) 0.92(11) 7.69(106) 0.19(7) 3.45(88) 1.04(16) (0.007) a 2 2 2 χ = [Σk(Ik - Icalc,k) /σk ]/(N - Np) and R factor (= Σk|(Ik - Icalc,k)/Ik|/N), where N and Np are the numbers of data points and parameters used in the model fit, respectively, Ik and Icalc,k are the measured and calculated reflected intensities, respectively, and σk is the uncertainty in the kth data point. b Monolayer coverage: Σ(occupancy (ion charge = 2))/[muscovite surface charge = 0.966e-/AUC45]. This estimated coverage can be less accurate when an extended outer-sphere (OSext) complex, whose coverage is more difficult to quantify by RAXR data, is a prevalent species. This value does not include any diffuse layer ions that are invisible to these measurements. c The numbers in parentheses indicate 1σ uncertainties in the last digits of the fitting parameters. f indicates a parameter that is fixed during fitting.
Lee et al. total ion coverage (ML)b
Letter
Langmuir 2010, 26(22), 16647–16651
Lee et al.
Letter lattice, the interfacial region including the two top unit-cell layers of the relaxed muscovite surface and surface-adsorbed species such as metals and water molecules, and bulk water (Table S1). The structure factor of a given sublayer is defined as Fi ¼ Σj cj fj ðqÞ expðiqzj Þ exp½ -
ðquj Þ2 2
ð1Þ
where fj(q) is the atomic scattering factor and cj, zj, and uj are the occupancy, position, and distribution width of the jth atom. The structure of bulk water was expressed by a layered-water model.26,30 The RAXR data were fit using a model consisting of three Gaussian peaks whose initial distributions were guided by the semiquantitative profile derived from model-independent analysis28 (details in SI). The partial resonant structure factor from the model at qo is expressed as FR ðqo Þ ¼ ð f 0 ðEÞ þ if 00 ðEÞÞΣj cj expðiqo zj Þ exp½ -
Figure 1. Relationship among measured heights of IS, OSads, and OSext complexes of divalent metal cations adsorbed at the muscovite (001)-water interface vs the effective ionic radius. The IS, OSads, and OSext complexes are identified by symbol color (green, red, and blue, respectively) and were determined at pH 2.0 (triangles), 3.7 (open circles), and 5.5 (diamonds). See Figure 2 for a schematic illustration of the adsorption configurations of these three species. Also included are data from previous studies using XR (i.e., H2O,26 Kþ and Csþ,30 and Ba2þ,45 open squares) and RAXR (i.e., Rbþ, Sr2þ,18,46 and Hg2þ20). Effective ionic radii are those for the cations in oxide minerals47 with the same coordination numbers as their dominant aqueous species.34 The radius of H2O was assumed to be that of oxygen (1.38 A˚).47 Height uncertainties are given by vertical error bars, and rms distribution widths are indicated by ellipsoidal tails. See Figure S3 in SI for the actual electron-density profiles derived from the best-fit models. The heights of the IS complexes are compared to heights calculated for ions adsorbed in the ditrigonal cavity of the muscovite surface (green curve). The heights of OSads complexes are compared to those for the nearest approach of a hydrated ion in the presence and absence of a water molecule in the ditrigonal cavity (short dashed and dotted-dashed magenta lines, respectively).
Data Measurements. Detailed descriptions of the XR and RAXR techniques and measurements are given elsewhere.27,28 The reflectivity data were measured at the 6-ID-B (MU-CAT) and 33ID-D (UNI-XOR) beamlines of the Advanced Photon Source. The incident beam defined by slits [0.05-0.4 mm (vertical) 0.5-2 mm (horizontal)] with a typical flux of 1011-1012 photons/s was reflected from the sample, and the reflected beam was collected using an X-ray CCD detector.29 Specular XR was measured at a fixed photon energy (E) as a function of momentum transfer q (Figure S1), whereas RAXR was measured by tuning E around the absorption energy (Eo) of a specific element at a series of fixed qo values sampled at an approximately regular spacing (Figure S2). One full set of XR and RAXR data was usually collected within 5-8 h. The stability of the experimental system was monitored by periodically measuring the reflectivity at reference q, and all reported data showed less than a 1σ intensity variation of
