J. Phys. Chem. C 2007, 111, 18307-18316
18307
XPS of Fast-Frozen Hematite Colloids in NaCl Aqueous Solutions: I. Evidence for the Formation of Multiple Layers of Hydrated Sodium and Chloride Ions Induced by the {001} Basal Plane Andrei Shchukarev,† Jean-Franc¸ ois Boily,*,‡ and Andrew R. Felmy‡ Department of Chemistry, Umeå UniVersity, Umeå SE-901 87, Sweden, and Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352 ReceiVed: July 9, 2007; In Final Form: September 10, 2007
The composition of fast-frozen wet pastes of hematite particles of different morphologies equilibrated in NaCl aqueous solutions was investigated by X-ray photoelectron spectroscopy. Two hematite preparations consisted of micrometer-sized platelets with 42% (HEM-1) and 82% (HEM-8) of the surface terminated by the {001} basal plane and a third of spheroids with no recognizable crystal plane (HEM-control). All hematite samples responded to changes in pH (4 and 9) and ionic strength (0, 10, 100 mM), showing that acid/base reactions of surface hydroxyl groups impact the composition of the paste. The HEM-1 and HEM-8 sample exhibited unusually large Na, Cl, and water contents at the highest ionic strength (100 mM) compared to HEM-control and all other minerals studied with this technique previously. The Na 1s and Cl 2p spectra occurred at binding energies typical of hydrated Na+ and Cl- ions and possessed energy-loss features, suggesting a three-dimensional distribution of these ions in the paste. An approximate stochiometric Na/Cl/H2O ratio of 1:1:2 was obtained in all samples in 100 mM NaCl as well as a strong correlation between the these compounds and the fraction of the {001} basal plane present in the hematite particles. The basal plane of hematite is proposed to induce the formation of a hydrated NaCl structure in the fast-frozen pastes, one that is compositionally reminiscent of hydrohalite (NaCl‚2H2O), by stabilizing multiple layers of hydrated Na+ and Cl- ions prior to freezing.
Introduction Electrolyte ion adsorption at metal oxide/water interfaces plays an essential role in the generation of surface charge and is a key feature in the development of electric double-layer models.1 Despite the importance of electrolyte ions in promoting the adsorption of potential-determining ions, the extent of electrolyte adsorption has remained relatively intractable, unlike protons, and has generally been left as an adjustable parameter in thermodynamic models. These models have repeatedly required the presence of additional electrolyte ion surface complexes bound at the outer-Helmholtz plane, in addition to those in the diffuse layer, to predict the ionic strength dependence of proton adsorption isotherms accurately.2,3 Such complexes have at best been rationalized as physisorbed ions retaining at least their first hydration shell and essentially acting as a plane of counterions in a parallel-plate molecular capacitor, decreasing surface potentials down to values typically measured in electrophoresis/electroacoustic experiments at the shear plane and to neutral values in the bulk solvent. An understanding of the hydration state and of the binding modes of electrolyte ions at metal oxide/water interfaces is thereby essential to understand the mechanisms of surface charge generation. Although electrolyte surface complexes are important species in the formulation of thermodynamic adsorption models, little is known about their structures and stabilities. Synchrotron X-ray scattering,4 X-ray reflectivity, and X-ray standing wave5-7 studies of mineral/water interfaces have provided information * Corresponding author. E-mail:
[email protected]. † Umeå University. ‡ Pacific Northwest National Laboratory.
on the structure of the electric double layer (EDL). X-ray photoelectron spectroscopy (XPS) has also been shown to be a promising technique to probe the composition of wet mineral pastes using the fast-frozen/cryogenic technique.8,9 This technique preserves the immediate surrounding of ions in aqueous solutions10,11 and minimizes the evacuation of volatiles to the vacuum of the analysis chamber. In this fashion, XPS measurements can be made on wet pastes to obtain a compositional analysis of a portion of the mineral bulk, the mineral surface, as well as water molecules and ions in the interstices of the paste. Previous XPS analyses of fast-frozen wet pastes of nanoparticles of goethite,12 gibbsite,12 manganite,12,13 and colloidal silica14 have notably revealed strong pH and ionic strength dependences on the presence of sodium and chloride. These changes are moreover highly consistent with thermodynamic models of electrolyte adsorption on mineral surfaces, predicting larger (lower) Na (Cl) content at high (low) pH. The dominant crystal planes of the particles studied thus far contained mixtures of different types of surface hydroxyls (-OH, µ-OH, µ3-OH) whose overall pH-dependent electric charge induces a strong pH dependence on the surface-bound sodium and chloride.12-14 In this study, XPS measurements are carried out on hematite particles exhibiting a strong anisotropic spread of charge between the {012} edge planes and the {001} basal plane (Figure 1 a and b). Although the {012} plane is essentially analogous to particle surfaces studied to date in terms of its distribution of several types of surface hydroxyls of various acidities, the {001} basal plane exhibits a different distribution of sites. This plane is ideally terminated only by neutrally charged µ-OH0 groups that are deemed to be proton-inactive
10.1021/jp075321c CCC: $37.00 © 2007 American Chemical Society Published on Web 11/10/2007
18308 J. Phys. Chem. C, Vol. 111, No. 49, 2007
Figure 1. Scanning electron microscope images of HEM-1 (2 kV), HEM-8 (2 kV), and HEM-control (3 kV).
over a broad range of pH values.15 Recent molecular dynamics studies16,17 proposed that the terminating hydroxyl groups of the basal plane of hematite can induce a local structure of interfacial water molecules. In the presence of electrolyte ions, the surface induces a clear distribution of positive and negative ions arranged in an oscillating pattern up to ∼15 Å from the surface. Recent crystal truncation rod18 and scanning tunneling force microscopy19 studies are also suggesting that the real {001}/water interface of hematite may depart from an idealized classical viewpoint. This surface would exhibit sparsely distributed ferric iron adatoms act as reactive Lewis acid sites and/ or may alternatively be a source of additional Brønsted acidity. The reactivity of this site was also proposed to be highly sensitive to surface defects.20,21 An in situ grazing incidence X-ray absorption fine structure study22 has even reported evidence for stable lead(II) surface complexes on the basal plane of hematite. There is consequently much to learn on the role of the reactivity of the real basal plane of hematite. This study presents evidence that fast-frozen pastes containing particles that exhibit an important proportion of the basal plane can accumulate several layers of hydrated Na+ and Cl- ions with a stoichiometric proportion of water molecules akin to the lowtemperature phase hydrohalite (NaCl‚2H2O). Experimental Methods Hematite Synthesis. Three hematite preparations were used for this study (Figure 1). Two preparations consisted of tabular hexagonal hematite particles of average diameters of about 1 (HEM-1) and 8 µm (HEM-8) and that characterized by the presence of the predominant {001} basal plane. A third
Shchukarev et al. preparation (HEM-control) consisted of subspherical hematite particles of 0.1 µm diameter with no characteristic crystallographic plane. This latter preparation was used as a control to study the role of the {001} basal plane in the HEM-1 and HEM-8 preparations. All aqueous solutions and suspensions were stored in polyethylene bottles. The HEM-1 particles were synthesized according to a modified version of the hydrothermal gel-sol technique of Sugimoto et al.23 An amorphous iron hydroxide suspension was first synthesized at 90 °C by a slow dropwise addition of a NaOH (Aldrich) solution (250 mL of 2.5 mol kg-1) to a solution of FeCl3 (Aldrich) (250 mL at 0.5 mol kg-1) stirred continuously with an overhead polyethylene propeller. The slow addition of NaOH and the high temperature of synthesis ensured the formation of smaller precursor iron (hydr)oxide particles. The resulting suspension was thereafter aged overnight at 100 °C, resulting in a suspension dominated by akagane´ite particles that contained hematite seeds. This suspension was washed twice in 0.1 mol kg-1 NaNO3 (Aldrich) by centrifugation and twice more with doubly deionized distilled water. The solid was then redispersed in an aqueous solution containing 7.5 mol kg-1 NaOH and 2.0 mol kg-1 NaCl (Aldrich) and stored for a 7-day period at 80 °C without stirring or shaking. The resulting hematite suspension was washed twice in doubly distilled dionised water and subsequently dialyzed over a 7-day period after which the conductivity of the supernatant was comparable to that of doubly deionized water. This stock suspension of low salt concentration was then centrifuged at slow speeds to remove residual fines, if any, and was resuspended in water. The resulting suspension was used to prepare stock suspensions in 10 mM and 100 mM NaCl (Aldrich, dried at 220° for 2 h), which were thereafter stored in translucent polyethylene bottles. The HEM-8 particles were synthesized by hydrothermal conversion of acicular goethite nanoparticles at 180 °C.24 The synthesis was carried out in a in a Teflon-coated titanium bomb22 for 72 h. The starting material consisted of 97 m2/g goethite particles that were synthesized using a method described previously25 and stored in polyethylene bottles. The resulting hematite suspension was washed with distilled deionized water by repeated sessions of centrifugation until the conductivity of the supernatant was comparable to that of doubly deioinized water. Finally, the spherical hematite nanoparticles were synthesized by the forced hydrolysis of a FeCl3 solution at 98 °C for 10 days.24 The suspension was dialyzed as described previously, and the resulting suspension was stored in a polyethylene bottle. Aliquots of the electrolyte-free suspension were dried at 80 °C, ground with a mortar and pestle, and used for particle characterization, including powder X-ray diffraction, scanning electron microscope imaging, and N2(g) adsorption isotherms (Micromeritics). Last, Fourier transform infrared (FTIR) spectra of the hematites were carried out with a Bruker IFS 66/S spectrometer in attenuated total reflectance mode, ATR (singlebounce diamond cell, DurasamplIR). Zeta potential measurements were carried out with the AcoustoSizer II (Colloidal Dynamics, Inc.). XPS with Fast-Freezing. All XPS spectra were recorded with Kratos Axis Ultra electron spectrometer equipped with a delay line detector. A monochromated Al KR source operated at 150 W, a hybrid lens system with a magnetic lens, providing an analysis area of 0.3 × 0.7 mm, and a charge neutralizer were used for the measurements. The binding energy (BE) scale was
XPS of Fast-Frozen Hematite Colloids referenced to the C 1s line of aliphatic carbon, set at 285.0 eV. Processing of the spectra was accomplished with the Kratos software. Hematite suspensions were equilibrated for 24 h in a humidified Ar(g) atmosphere at pH 4 and 9 at ionic strengths of 0, 10, and 100 mM NaCl in clear polyethylene test tubes. One HEM-1 suspension was also prepared in 100 mM CsCl (Merck, p.a.) at pH 9. The wet pastes of these equilibrated hematite suspensions were obtained by centrifugation at 4000 rpm for 15 min. The supernatant was decanted from the test tube just before introducing the wet paste into spectrometer. The paste was then applied onto the holder at room temperature with a spatula and transferred to the precooled (-170 °C) claw of the sample transfer rod of the introduction chamber of the spectrometer. Two liquid samples were also studied by pipetting 5 µL of 100 mM NaCl and CsCl solutions on the cold sample holder. To minimize contamination from the atmosphere, we first kept the introduction chamber to 4-5 × 10-5 Pa for a period of at least 20 min. It was then vented with dry N2(g) at a pressure larger than 1 bar and opened under this condition to introduce the sample to the cold claw of the sample transfer rod. The introduction chamber was thereafter locked. This procedure never exceeded 1 min. The pastes then froze within the first 10-14 s of contact with the claw, during which the lustrous surface consistently changed to a homogeneous matte finish. The liquid samples froze, however, as solid spheres, as notably shown in Shchukarev.8 All samples were allowed to cool for a total period of 45 s under the same dry N2(g) atmosphere. During this time, a portion of the water evaporated from the sample surface to the cold metallic parts of the introduction chamber, appearing as a thin layer of frost. Analyses of the fast-frozen aqueous solutions in fact revealed losses up to about 75 atomic % of water. Losses from the wet mineral pastes are, however, deemed to be less substantial because water is also chemi/physisorbed to the mineral surfaces. These losses are nonetheless accepted as a consequence of the technique, preserving, to the least, the first hydration environment of the ions and the mineral surface, and intensifying the signals of these components over that of water. In fact, water loss from the sample is more desired than any adventitious contamination from the atmosphere that would simply yield a thick layer of frost on the sample surface. After this 45 s exposure to the dry N2(g) atmosphere, the introduction chamber was pumped to 4-5 × 10-5 Pa within 15-20 min to remove the nitrogen and any adventitious volatiles. Once this vacuum pressure was maintained, the sample was transferred onto the precooled (-170 °C) manipulator of the analysis chamber. The XPS spectra of the frozen sample were then collected. Control experiments showed that the samples retained their hydration level for several hours in the atmosphere of the analysis chamber, enabling repeated analyses for reproducibility. After the completion of the measurements, the sample was warmed overnight to room temperature in the pumped atmosphere of the analysis chamber to evaporate volatile species (e.g., water, hydrochloric acid). The XPS spectra of the room-temperature samples were taken the following day under the same conditions. Results and Discussion Hematite Characterization. The hematite preparations, whose salient physical attributes are reported in Table 1, were confirmed to be solely composed of crystalline hematite by powder X-ray diffraction. ATR-FTIR spectra of hematite thinfilm revealed features characteristic of hematite24 and the
J. Phys. Chem. C, Vol. 111, No. 49, 2007 18309 TABLE 1: Physical Attributes of the Synthetic Hematites material
diameter (µm)
specific surface area (m2/g)
basal/edge
HEM-1 HEM-8 HEM-control
1 8 0.1
1.8 0.2 13
0.45:0.58 0.82:0.18 n.a./0
absence of vibrational modes (e.g., OH stretch, Fe-O-H bends) characteristic of adventitious ferric iron (hydr)oxide precipitates. The SEM images, shown in Figure 1, also revealed well-defined hematite particles of relatively small variations in size. The HEM-1 particles are tabular and are, on average, roughly 1 µm wide and 0.3 µm thick. The flat surfaces of these particles were moreover confirmed to be the {001} basal plane by oriented particulate monolayer-XRD by Sugimoto et al.23 The larger HEM-8 particles are about 8 µm wide and 1 µm thick. Finally, the HEM-control preparation consists of spherical particles of 0.1 µm in diameter with no identifiable crystal planes. The dominant crystal planes of the HEM-1 particles are the basal {001} and {012} edge planes, as demonstrated previously.23 Considering the particle morphology depicted in Figure 1, the surface area of the two {001} faces on one particle is of 1.35 µm2 and of the six {012} faces of 1.84 µm2, yielding a {001}/{012} ratio of 0.42:0.58. The total surface hydroxyl density is therefore evaluated to be 14.2 sites/nm2 (13.7 sites/ nm2 of µ-OH on the {001} and 7.3 sites/nm2 of -OH and µ3-OH on the {012} plane26), yielding an average surface OH/ Fe ratio of 0.5. In the case of the HEM-8 preparation, the edge/ face ratio is estimated to be 0.82:0.18, using the same line of reasoning as in the HEM-1 preparation. Although the identity of the edge plane could not be confirmed, we assume that the {012} plane yields a total surface hydroxyl density of 13.9 sites/ nm2. The specific surface was also estimated from the particle morphologies. The average volume of a HEM-1 particle is estimated to be 3.2 µm3, therefore yielding a specific surface area of 1.8 m2/g (specific density of hematite is 5.24 g/cm3). This value is in good agreement with the quantitative analyses of the N2(g) adsorption isotherm, yielding a B.E.T. surface area of 2.1 ( 0.1 m2/g associated with negligible quantities of mesoand macroporosity. Similarly, the volume of the HEM-8 particle is 77.3 µm3, with a resulting specific surface area of 0.46 m2/g. This value is also similar to the B.E.T. values. Finally, the specific surface area of the HEM-control particle is 13 m2/g. The isoelectric point of spherical hematite colloids, such as HEM-control, has been investigated extensively, with values in the approximate 8.5-9.5 range.27,28 Important differences are, however, expected in HEM-1 and HEM-8 particles because of the predominance of the {001} basal plane. Attempts at measuring the isoelectric points of HEM-1 and HEM-8 were made first by laser-doppler electrophoresis but could not be pursued because the particles sediment too readily. Attempts at measuring the dynamic mobility with the electroacoustic method yielded highly variable values (4.7-7.5) because of the instability of the suspension and to film deposition on the electrodes of the AcoustoSizer II. Renewed efforts are currently in progress to minimize these shortcomings. It should, however, be stressed that particles with important anisotropic charge distributions tend to exhibit unusual electrophoretic/dynamic mobilities with, for example, neutral values when particles are charged and finite mobilities when the particles are neutrally charged.29-31 It is consequently unsure whether dynamic mobility analyses of colloidal suspensions of HEM-1 should retrieve a pertinent isoelectric point. The experiments conducted at pH 9 are therefore considered to be under conditions where the hematite
18310 J. Phys. Chem. C, Vol. 111, No. 49, 2007
Shchukarev et al. TABLE 2: XPS Results of Fast-Frozen Hematite Pastes at pH 9 in Zero Ionic Strength (0 mM NaCl) HEM-1
HEM-8
HEM-control
line
BEa
ACb
BEa
ACb
BEa
ACb
Na 1s Cl 2p3/2 O 1s: O OH H2O Fe 2p3/2c Na/Cl Na/Fe Cl/Fe
1071.00 198.20 529.70 530.90 532.70 710.70 5.54 0.0675 0.01
2.16 0.39 35.42 9.16 12.68 32.00 n/a 0.0540 n/a
1071.10 n/a 529.60 530.80 532.60 710.60 n/a n/a n/a
1.40 bdl 32.12 9.60 11.14 25.94
n/a n/a 529.48 530.72 532.46 710.57
Bdl bdl 36.04 10.29 8.62 32.51
a Binding energy (eV). b Atomic concentration (atomic %). c Binding energy of main component.
TABLE 3: Water Content Measured in the Fast-Frozen Mode material goethite manganite gibbsite HEM-control HEM-1
HEM-8
Figure 2. Staggered wide spectra of fast-frozen wet pastes of HEM1, HEM-8, and HEM-control at pH 9 at (a) 0 mM NaCl, (b) 10 mM NaCl, and (c) 100 mM NaCl. The ordinate axis is in counts per minute (CPS) and is offset by constant values to facilitate comparison.
surfaces exhibit little net charge while those at pH 4 are where the hematite surface is positively charged. The charge on the {001} basal plane is, however, deemed to be intrinsically neutrally charged under those conditions.15
I (mM NaCl)
H2O (O 1s)/ atomic %
reference
10 100 100 10 100 0 100 0 10 100 100 (CsCl) 0 10 100
6.5-9.8 9.8-16.5 8.4-12.4 5.5-9.8 5.5-9.8 8.62 13.81-14.66 12.7 9.5-10.0 24.7-25.4 9.8 11.14 8.35-10.52 31.38-32.09
12 12-13 12 12 12 this study this study this study this study this study this study this study this study this study
XPS of Fast-Frozen Hematite Wet Pastes. SurVey Spectra. The survey spectra of the fast-frozen dialyzed hematite pastes at pH 9 are shown in Figure 2a. The results of the curve-fitting procedures of the high-resolution scans are shown in Table 2. The Fe 2p spectra are typical for R-Fe2O330 and the (O + OH)/ Fe ratios are close to the expected stochiometric value. The HEM-1 sample contains excess Na (Na 1s, Na KLL lines) and Cl (Cl 2p and Cl 2s lines) that were resilient to the dialysis procedures. The HEM-8 sample contains, however, only excess Na, while the Na and Cl levels in the HEM-control sample were below detection limit. Water Content in Fast-Frozen Pastes at 0 mM NaCl. Water accounts for 8.62-12.68 atomic % in the three samples at 0 mM NaCl (Table 2), a result that lies well within the typical range of values obtained in mineral pastes studied previously at various ionic strengths (Table 3), ranging from 5.5 to 16.5 atomic %. No correlation has been found between the water content, ionic strength or the size, aspect ratio, or nature of the particles studied thus far in our laboratory. The H2O/Fe ratios of 0.27-0.43 for the hematite particles under study indicate that the water and iron contents of the pastes are of the same order of magnitude. However, because of the inherent depth of analysis of the XPS technique, a portion of the measured Fe arises from the hematite bulk. To provide a first-order estimate of the true H2O/Fesurface ratio in the fastfrozen hematite wet pastes, we consider the following arguments. Assuming that the measured OH (Table 2) solely consists of surface OH groups, there should be twice as many Fe groups, as was discussed in the previous section. Surface Fe atoms should consequently account for 42% (2 × 9.16/32.00) of the measured Fe content in HEM-1 and 74% in HEM-8. The adjusted H2O/Fesurface ratios of 0.42-0.94 imply that there is
XPS of Fast-Frozen Hematite Colloids
J. Phys. Chem. C, Vol. 111, No. 49, 2007 18311
TABLE 4: XPS Results of Fast-Frozen Hematite Pastes at 10 mM NaCl HEM-1 line
HEM-8
pH 4
pH 9
pH 9
BEa
ACb
BEa
ACb
BEa
ACb
1071.30
2.15
1071.40
3.51
1071.20
1.04
Cl 2p3/2
198.30
1.87
198.40
1.66
O 1s: O OH OH H2O Fe 2p3/2c Na/Cl Na/Fe Cl/Fe H2O/Fe H2O/(Na + Cl)
529.90 531.10
36.28 6.42
529.8 530.9
32.40 7.72
532.90 10.02 710.80 26.26 1.15 0.08 0.07 1.71 2.49
532.9 710.8
8.98 26.72
Na 1s
a
HEM-control
pH 4
198.50 0.73 200.00 0.07 529.70 29.02 530.90 4.10 531.80 2.84 532.80 10.52 710.57 14.93 1.30 0.07 0.05 6.46 5.72
2.11 0.13 0.06 1.89 1.72
pH 9
BEa
ACb
1071.40 2.67 1072.80 0.22 198.50 1.06 199.60 0.11 529.70 30.07 530.70 6.08 531.60 3.67 532.90 8.35 710.60 21.69 2.47 0.13 0.05 6.78 2.06
BEa
ACb
1070.70
0.21
197.90
0.13
529.50 530.60
31.09 12.59
532.50 10.12 710.60 28.81 1.62 0.007 0.005 0.35 29.76
Binding energy (eV). b Atomic concentration (atomic %). c Binding energy of main component.
TABLE 5: XPS Results of Fast-Frozen Hematite Pastes at 100 mM NaCl HEM-1 line
pH 4 a
Na 1s Cl 2p3/2 O 1s: O OH OH H2O Fe 2p3/2c Na/Cl Na/Fe Cl/Fe H2O/Fe H2O/(Na + Cl) a
HEM-8 pH 9
BE
ACb
BEa
ACb
1071.4 1072.8 198.5
6.74 0.51 9.76
529.9 531.1
17.82 5.22
1071.4 1072.6 198.7 199.7 529.8 530.9
10.56 1.44 9.5 1.69 17.06 3.42
533.2 25.44 710.8 14.38 0.74 0.50 0.68 1.71 1.50
HEM-control
pH 4
533.1 24.72 710.8 13.09 1.07 0.92 0.85 1.89 1.07
BEa 1071.4 1072.5 198.7 199.8 529.8 530.5 531.5 533.3 710.57 0.88 3.04 3.44 6.46 1.00
pH 9 ACb 12.3 2.47 14.1 2.61 6.42 1.62 1.45 31.38 4.86
BEa 1071.2 1072.3 198.6 200.1 529.5 530.4 531.3 533.2 710.50 1.05 2.70 2.58 6.78 1.26
pH 4
pH 9
ACb
BEa
ACb
BEa
ACb
11.48 1.31 11.39 0.81 7.05 2.26 1.68 32.09 4.73
1071.13
1.36
1070.65
2.59
198.18 199.8 529.74 530.88
0.97 0.53 35.48 11.36
198.28 199.83 529.62 530.85
0.74 0.43 30.72 11.16
532.07 13.81 710.79 34.1 1.40 0.04 0.04 0.40 4.83
532.73 14.66 710.56 26.12 2.21 0.10 0.04 0.56 3.90
Binding energy (eV). b Atomic concentration (atomic %). c Binding energy of main component.
no more than roughly one water molecule per surface Fe. This value should, however, also be adjusted further to take into account the loss of water from the sample during the time under which it was exposed to the nitrogen atmosphere of the introduction chamber. Results from our laboratory show that fast-frozen drops of pure aqueous electrolyte aqueous solutions can loose up to 75 atomic % during the cooling procedure in the N2(g) atmosphere of the introduction chamber. These losses are, however, expected to be diminished in the mineral pastes because of the competing action of the chemi-/physisorption reactions between water and the mineral surface. Correcting the H2O/Fesurface ratios of 0.42-0.94 ratios to an extreme uppercase loss of 75 atomic % of water nonetheless yields ratios of 1.683.76. Recalling once more that there are on average two surface Fe atoms per surface OH the hematite surface could be covered by up to seven/eight (∼3.76 × 2) layers of water molecules. This line of reasoning obviously consists of a rough estimate, but it does show that the fast-frozen pastes contain only minor residual supernatant water molecules. The H2O/(Na + Cl) ratios in the presence of a background electrolyte (Tables 4 and 5), which are no larger than 5.72, are moreover considerably smaller than the values expected for aqueous solutions (>70), once more showing that the pastes contain little residual supernatant water molecules. Moreover, differences in particle size have little effect on the water content. The pastes can therefore be envisioned as a network of compact hematite particles in contact with closely linked chemisorbed and physisorbed adsorbed
water molecules. The compactness of the paste therefore implies that the composition of the water is controlled by the reactions involving the hematite surface, as was notably demonstrated in previous XPS studies of fast-frozen pastes of mineral particles.8,9,12-14 Variations in the ionic strength and pH of the hematite suspensions are used to induce changes in the compositions of the centrifuged pastes and to consequently provide clues on the reactivity of the mineral surface, just as it has been done in previous studies on other minerals.8,9,12-14 Ionic Strength Effects on Sodium and Chloride. The survey spectra of the hematite particles equilibrated in 10 and 100 mM NaCl at pH 9 are shown in Figure 2b and c. In all cases studied, the presence of NaCl intensified the Na 1s and Cl 2p lines, relative to the Fe 2p and O 1s lines. Important differences, however, arise between the HEM-control sample and the HEM-1 and HEM-8 samples, namely those that exhibit a prominent {001} basal plane. At 10 mM NaCl (Table 4), the Na/Fe and Cl/Fe ratios of the HEM-1 and HEM-8 samples lie in the 0.05-0.13 range. Those of the HEM-control sample are 1 order of magnitude lower, with values in the 0.005-0.007 range, already suggesting a difference in the affinity of this particle for sodium and chloride. The effects of pH on these measurements, which will be addressed more thoroughly in a forthcoming communication, also confirm that Na/Cl ratios are greater at pH 9 than at pH 4 due to differences in surface charge.
18312 J. Phys. Chem. C, Vol. 111, No. 49, 2007
Figure 3. Close-up of the wide spectra of Figure 2 in the (a) Na 1s and (b) Cl 2p regions. The high-energy regions of HEM-1 and HEM-8 at only 100 mM NaCl reveal energy-loss features. The ordinate axis is in CPS and each spectra are offset by a constant value to facilitate comparison.
The experiments in 100 mM NaCl (Table 5) reveal important differences in the Na 1s and Cl 2p spectra of the HEM-1 and HEM-8 samples, compared to HEM-control. At this ionic strength, the Na 1s and Cl 2p lines become important features of the wide spectra of HEM-1 and HEM-8 (Figure 2b and c). The resulting Na/Fe and Cl/Fe ratios (0.50-0.97) of HEM-1 and HEM-8 (2.58-3.44) are considerably larger than those of HEM-control (0.04-0.10) and of the typical values for a range of other minerals.8,9,12-14 Correcting these ratios for the proportion of surface Fe and the OH/Fesurface ratio of 2, we obtain Na/Fesurface and Cl/Fesurface values of 2.38-4.62 for HEM-1, 6.97-9.30 for HEM-8, and 0.12-0.32 for HEM-control. The considerably large values for HEM-1 and HEM-8 are atypical for physisorbed ions and already suggest multiple layers of Na and Cl ions. The large Na/Fe and Cl/Fe ratios for the HEM-1 and HEM-8 surface, compared to HEM-control, therefore represent a first important clue that particles with prominent {001} basal planes induce an unusual preferential accumulation of Na and Cl in the pastes. The wide spectra of the 100 mM NaCl pastes (Figure 2b and c) reveal additional clues to the nature of this strong accumulation of salts. Both Na 1s and Cl 2p lines of the HEM-1 and HEM-8 samples (Figure 2c) exhibit important energy-loss31
Shchukarev et al.
Figure 4. Na 1s (a) and Cl 2p (b) spectra fast-frozen pastes of HEM-1 at pH 9 at 0, 10, and 100 mM NaCl. The spectra of a fast-frozen 100 mM NaCl solution and its corresponding evaporated products at room temperature are also shown for comparison. The spectra show that Na and Cl are ions in the fast-frozen pastes. The ions of the pastes form a NaCl solid when they are warmed to room temperature (cf. Figure 7). The ordinate is expressed as ∆CPS, i.e., CPS offset to a zero value at 1077 eV in a and 204 eV in b to facilitate comparison of the different spectra.
features, which are also shown in Figure 3 (
