Calcite Wettability in the Presence of Dissolved Mg2+ and SO42

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Calcite Wettability in the Presence of Dissolved Mg2+ and SO42− J. Generosi,* M. Ceccato, M. P. Andersson, T. Hassenkam, S. Dobberschütz, N. Bovet, and S. L. S. Stipp Nano-Science Center, Department of Chemistry, University of Copenhagen, Copenhagen 2100, Denmark ABSTRACT: The wettability of mineral surfaces controls a range of phenomena in natural and industrial processes. In reservoirs, rock wettability determines the effectiveness of oil production; thus, modification of mineral surface properties can lead to enhanced oil recovery. Recent work reports that potential determining ions in seawater, Mg2+, Ca2+, and SO42−, are responsible for altering the wettability of calcite surfaces. In favorable conditions, e.g., elevated temperature, calcium at the calcite surface can be replaced by magnesium, making organic molecules bind more weakly and water molecules bind more strongly, rendering the surface more hydrophilic. We used atomic force microscopy in chemical force mapping mode to probe the adhesion forces between a hydrophobic CH3-terminated AFM tip and a freshly cleaved calcite {10.4} surface to investigate wettability change in the presence of Mg2+ and SO42− at 75 and 80 °C. We made submicrometer scale maps of adhesion force and contact angle and demonstrated that the adhesion force between the hydrophobic tip and calcite decreases when both Mg and SO4 are present. Surface analysis with X-ray photoelectron spectroscopy showed Mg associated with calcite even after rinsing with CaCO3-saturated deionized water, suggesting sorption on or in calcite. When the calcite-saturated solution of MgSO4 was replaced by calcite-saturated NaCl at the same ionic strength, adhesion force increased again, indicating that the effect is reversible and suggesting Mg replacement by Ca. Experiments with solutions of Na2SO4 and MgCl2 suggest that Mg2+ uptake is favored when SO42− is also present.

1. INTRODUCTION Elucidating the mechanisms that control the wettability of mineral surfaces is an important part of understanding a wide range of phenomena such as biomineralization, diagenesis, colloid aggregation, contaminant remediation, and enhanced oil recovery. In oil reservoirs, pore surface wettability is influenced by the properties of the bulk solid and by the chemical composition of the pore fluids, including the formation water and the crude oil, and it is one of the main factors that determines the effectiveness of oil recovery.1 Several studies on reservoir wettability have demonstrated how changing the composition of the water in contact with the solid can affect the mineral surface properties and enhance oil recovery. For example, injecting water with decreased salinity increases the amount of oil produced from sandstone.2−4 Austad and colleagues studied carbonate samples5,6 and reported that salinity and the composition of the injection water determine wettability in chalk.7 Several hypotheses have been proposed,8−13 but a complete explanation of the mechanism behind this effect has not yet been presented. Zhang et al.8 suggested that the seawater potential determining ions, Mg2+, Ca2+, and SO42−, make chalk surfaces more water wet, particularly at temperatures above 70 °C. Their hypothesis is that magnesium displaces calcium, which serves as an anchor for carboxylate, triggering wettability change. Alipour Tabrizy et al.10 proposed that when Mg2+ ions are introduced, the calcite surface is modified as a result of a possible exchange or precipitation, resulting in a less hydrophobic surface, where the adsorption of organic components decreases. Madland et al.11 suggested that Mg substitution for Ca is insufficient to explain calcium production and magnesium retention in chalk core flooding experiments. In a recent paper, Andersen et al.12 developed a geochemical model to interpret their experimental © XXXX American Chemical Society

data and proposed the combined effect of ion exchange and dissolution or precipitation to explain chalk surface properties, their modification, and enhanced oil recovery. However, evidence for the exchange process is still not clear. For this reason, Sakuma et al.14 recently investigated the interaction of calcite with ions in solution using a theoretical approach. Density functional theory (DFT) calculations were used to explore wettability changes on calcite when Mg2+ and SO42− replaced Ca2+ and CO32− both in and on the surface. They demonstrated that exchange of calcium by magnesium makes the surface more hydrophilic, and the effect is even stronger when Mg2+ and SO42− are both present. Similar conclusions were drawn by Abdallah and Gmira13 from their study of calcite surface wettability alterations from 90 °C by varying Mg2+, Ca2+, and SO42− concentrations. Static contact angle measurements showed that calcite aged in crude oil is more able to reverse its wetting properties. When Mg2+ and SO42− are combined, the surface becomes more water wet. On the basis of the theoretical work of Sakuma et al.14 and the macroscopic core plug experiments by Zhang et al.,8 we set out to experimentally investigate the wettability properties of calcite in the presence of magnesium and sulfate at the molecular scale. We used atomic force microscopy (AFM) in chemical force mapping (CFM) mode and recorded adhesion force at nanometer resolution on freshly cleaved calcite surfaces in the presence of solutions containing Mg2+ and SO42− at elevated temperature (75 and 80 °C). Calcite is the main component of carbonate rocks, in many cases contributing >95% to limestone and chalk. It is one of the Received: August 12, 2016 Revised: November 14, 2016 Published: November 22, 2016 A

DOI: 10.1021/acs.energyfuels.6b02029 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Composition of Solutions Used for Calcite Wettability Studiesa mass concentration (mg/L)

a

solution

composition

concn (M)

Na+

Cl−

Mg2+

SO42−

A B C D

NaCl NaCl/MgSO4 NaCl/MgCl2 NaCl/Na2SO4

0.500 0.320/0.045 0.365/0.045 0.428/0.024

11495 7357 8391 10943

17727 11345 16131 15174

1094 1094

4323 2305

ionic strength (M) 0.503 0.503 0.503 0.503

All solutions were prepared from compounds of reagent grade or better, ultrapure deionized water, and were saturated with CaCO3 (ACS reagent).

Figure 1. (a) A typical force−distance curve collected from a freshly cleaved calcite surface using a tip functionalized with −CH3. The tip approaches (red) and is pushed onto the surface with its preset maximum force (500 pN). This provides a pixel for a map showing the height of the features on the surface, such as the gray scale image (b, top), where light gray represents areas that are higher. The adhesion force (200 pN) is determined from the tip deflection during retraction (blue curve). This provides a pixel for the adhesion map at the same (x,y) location (b, bottom). The images cover an area of 3 μm × 3 μm.

that we acquired in the various solutions and to determine changes in wetting properties on the calcite surface that were triggered by the presence of the potential determining ions Mg2+ and SO42−.

most studied minerals because of its widespread presence in geological systems and its extensive use in industry, for such products as paper, cement, pharmaceuticals, paint, pigments, and plastic. Pure, clean calcite is water wet by nature, and it is convenient to use in surface science studies because it can be easily cleaved to produce a smooth, homogeneous surface that is often flat over many micrometers. Atomic force microscopy has frequently been used as a high-resolution imaging tool to analyze calcite structure,15,16 growth and dissolution,17,18 and element incorporation.19−22 In addition to topographic characterization, AFM can provide information about molecular scale processes on a surface because it can probe interactions between molecules on a tip and a sample. In chemical force mapping (CFM) mode, the tip can be functionalized using molecules with specific functional groups, and the adhesion force that results between the tip and the sample can be mapped with resolution of a few tens of nanometers. Hassenkam and colleagues have applied CFM to map wettability and elasticity on chalk pore surfaces and to investigate the properties of surfaces from single, pure crystals and reservoir sandstone in high and low salinity water.23−25 The purpose of our study was to track changes in calcite wettability when Mg2+ and SO42− were added to the solution in contact. For this purpose, we measured interaction forces between a hydrophobic -CH3 terminated tip, which we used to simulate a simple aliphatic hydrocarbon as it interacted with a freshly cleaved calcite {10.4} surface in the presence of calcite equilibrated solutions of NaCl, MgSO4, MgCl2, and Na2SO4 at 75 and 80 °C. This allowed us to compare adhesion force maps

2. EXPERIMENTAL DETAILS 2.1. Samples and Solutions. Calcite single crystals (Iceland spar; Ward’s Science, Chihuahua, Mexico) were cleaved from a cleaned, optical quality sample and glued with a two component epoxy (Dana Lim, Denmark) to cleaned, circular glass slides that were compatible with the temperature-controlled, closed fluid cell (BioHeater; Asylum Research, Santa Barbara, USA). The glue was cured at 120 °C for 30 min on a hot plate. Before the experiment, the glass slide with the glued sample was washed in ultrapure deionized water (Milli-Q; conductivity < 0.1 μS/cm) and ozone cleaned (BioForce Nanosciences, USA) for 20 min to remove residual organic contaminants. The upper crystal surface was then cleaved using a scalpel blade, and the calcite dust generated during cleavage was mechanically removed with a jet of pure nitrogen. The sample was fixed on the sample stage, and solutions were promptly injected into the sealed fluid cell through inlet tubing. The aim was to minimize the time that the fresh calcite surface was exposed to air, thus minimizing adventitious carbon contamination26 and the spontaneous recrystallization that takes place in response to the humidity in air.27 Using single crystal Iceland spar was preferable to working with grains of synthetic calcite, which cannot be cleaved and thus are covered with organic compounds from the solution in which they were grown. We took a number of precautions to minimize contamination from the glue during its exposure to the solution. We tested several glues and curing procedures. With the aid of XPS, we were able to analyze the calcite surface and look for contaminants such as N, an element B


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Energy & Fuels typically found in epoxy. We decreased the amount of glue we used to an extremely thin streak, spread only under the calcite particle, and cured the glue at 120 °C, which is considerably higher than the temperature at which we performed the experiments (75−80 °C). Before cleaving the calcite, we rinsed the sample in ultrapure deionized water, mechanically removed the remaining droplet with a jet of nitrogen, and cleaned it with ozone. Four solutions were prepared by dissolving sodium chloride (SigmaAldrich; MW = 58.44, ACS reagent, purity ≥ 99%), magnesium sulfate heptahydrate (Sigma-Aldrich; MW = 246.48, ACS reagent, purity 98+ %), magnesium chloride hexahydrate (Merck; MW = 203.30, ACS reagent, purity ≥ 99%), and sodium sulfate (Sigma-Aldrich; MW = 142.04, ACS reagent, purity ≥ 99%) in ultrapure deionized water that had been saturated with calcium carbonate (Sigma-Aldrich; MW = 100.09, ACS reagent, purity ≥ 99.95). Specific concentrations are reported in Table 1. The CaCO3 saturated solution was stirred and equilibrated with the CO2 in air at room temperature for at least 2 days before adding the salts. We used calcite saturated solution in the experiments to minimize dissolution of the surfaces. Dynamic equilibrium would result in some dissolution and precipitation, but we assume the extent to be small and its effect on surface behavior negligible. Concentrations of Mg2+ and SO42− were chosen to mimic seawater, whereas the concentration of NaCl was used to keep ionic strength constant (0.503 M) for all four solutions. The difference between concentration and ionic strength is attributed to the dissolved CaCO3. The ionic strength was estimated using the geochemical speciation code PHREEQC28 using the Pitzer database.29 pH was measured using a Metrohm NTS/3 M KCl glass combination electrode. The pH meter (Metrohm, Switzerland) was calibrated against NBS buffers (4, 7, and 9). For all four solutions, pH was always in the range of 8.10− 8.45. 2.2. Atomic Force Microscopy (AFM). AFM provides information about physical features on the surface of a solid at resolution in the submicrometer range. We made calcite wettability measurements using the chemical force mapping (CFM) mode of an MFP-3D AFM from Asylum Research (Santa Barbara, USA). CFM has previously proven to be a useful tool that allows estimation of wettability properties at the nanometer scale, and the adhesion force measurements can be directly correlated with the wettability properties on a macroscopic scale.30 We collected force curves to determine the interaction force between a hydrophobic tip and the calcite surface. Figure 1a shows an example force curve, where the adhesion force is plotted against the distance between the tip and surface. During approach (red curve), the tip is mechanically brought close to the sample until it touches the surface. It is pushed into the surface with a specific force (trigger force, shown as maximum force; in this case, 500 pN) for a set amount of time (dwell time; for these experiments, 100 ms). As the tip is mechanically retracted (blue curve), adhesion forces cause the tip to be held at the surface until separation from the sample pulls it free. The deflection is related to the adhesion force, which in this case is 200 pN. The tip then continues to retract to a fixed distance from the surface (in this case, 1 μm). This sequence provides data for 1 pixel on a topographic map, such as the gray tone image in Figure 1b, and on an adhesion force map, such as the image in tones of blue, black, and red. At the retract distance, the tip is free of the forces from the surface and it can approach again at the next (x,y) location to make the next measurement, providing the next pixel. The tip rasters over the surface, making a matrix of force curves from which the data are extracted to make the maps. The map resolution, i.e., the number of force curves collected and the size of the area scanned, can be varied according to the requirements of the experiment. Data for the height of surface features is obtained simultaneously with data for adhesion force, so information about the topographical features of the analyzed area are also available. An example is shown in Figure 1b. In our experiments, each map was composed of 40 × 40 (in some cases 30 × 30) force curves from a freshly cleaved calcite area of 3 μm × 3 μm.

The functionalized tips required at least 24 h for preparation. Freshly cleaned, gold-coated microcantilevers (BioLever Olympus, Japan), with 37 kHz resonant frequency and 30 pN/nm average spring constant, were treated during several hours of exposure to a 3.3 mM hexadodecane solution to allow the formation of a CH3-terminated tip. This produced a hydrophobic probe that mimicked a tiny oil droplet. The molecules have a thiol functional group on one end that bonds strongly to the gold layer on the AFM tip, forming a self-assembled monolayer that is robust. Typically, there is a carbon chain of approximately 16 carbon atoms long before the methyl termination. In general, we assume that the functionalizing molecules form an even and smooth layer and that the tip is spherical with an estimated average contact area of 365 nm2.23 All measurements conducted at 75 or 80 °C were made in the BioHeater closed fluid cell to ensure a controlled fluid exchange, clean environment, and uniform heating. At the beginning of each experiment, the sample, tip holder, and other parts of the fluid cell were ozone cleaned for 20 min to remove organic contaminants. Then, a sample of freshly cleaved calcite and the microcantilever were mounted, and the fluid cell was assembled and sealed. Finally, ∼4 mL of solution A, NaCl, at room temperature was filtered (cellulose acetate syringe filter, pore size 0.22 μm; Frisenette, Denmark) and slowly injected from a syringe through inlet tubing. The aim of this method was to fill the entire cell, avoiding the formation of air bubbles. An outlet tube allowed air and excess liquid to be expelled. After checking for leaks, we mounted the fluid cell into the AFM. The cantilever was calibrated and the spring constant was estimated; then, two or three adhesion force maps were acquired at room temperature for use in choosing a good area where the features on the surface were within the range of normal AFM scanning to allow the cantilever to stabilize in the solution and to be sure that all was working properly. Then, the solution target temperature was set to 75 °C (or 80 °C), and after ∼20 min, we could start the force map acquisition at elevated temperature. In half of the experiments, when the solution temperature reached 50−60 °C, we injected new, calcitesaturated sodium chloride solution that had been preheated to 70 °C on an external hot plate. It was in a glass bottle open to atmosphere to allow CO2 to equilibrate with the atmosphere. We found this method allowed for quicker cantilever stabilization than injecting the solution at room temperature and heating it in the fluid cell. After data acquisition in NaCl at 75 °C (or 80 °C), we slowly injected 15−20 mL of one of the preheated solutions, B, C, or D (i.e., MgSO4, MgCl2, or Na2SO4, respectively; Table 1) into the flow through system. This amount was enough to allow a complete exchange of the solution previously present in the cell, which is ∼2 mL. We waited ∼20 min to let the solution reach the desired temperature and to allow the cantilever to equilibrate, and then we collected a new set of adhesion force maps. Data acquisition in solution A and the replacement solution, namely solution B (or C or D), was considered as one full cycle. After the first cycle (cycle 1), we began a new cycle by exchanging the existing solution with 15−20 mL of fresh NaCl solution. During each experiment, the fluid cell remained closed. We always aimed to acquire the force maps on the same 3 μm × 3 μm calcite area. Drift is a known and unavoidable artifact in AFM imaging, especially at elevated temperature. To make it possible to account for drift, we collected on average 3 to 4 force maps during exposure to each solution, and we inserted the NaCl solution followed by a solution containing MgSO4, MgCl2, or Na2SO4 two or three times (2 to 3 cycles). In this way, the calcite surface was exposed to solution A and one other solution through at least two rounds. The aim was to also exclude eventual tip contamination. 2.3. X-ray Photoelectron Spectroscopy. XPS provides chemical information from the top 10 nm of a solid surface. Our analysis was performed on a Kratos Axis UltraDLD instrument operated with a monochromatic Al Kα X-ray source (hν = 1486.6 eV) at a power of 150 W. Survey scans were acquired in the 0 to 1355 eV range at a pass energy of 160 eV and step size of 0.5 eV. High-resolution scans of the C 1s region (280−295 eV) were acquired at a pass energy of 10 eV and step size of 0.1 eV. The pressure in the main chamber during the C


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Energy & Fuels Table 2. Average Adhesion Force for Each Map for All of the Experiments Made with NaCl and MgSO4a experiment

cycle 1

3

4

5

6

7

T (°C)

75

75

75

75

75

80

80

339

302

273

190

256

177

168

89 54 65

225 499 379

983 1087

4562 3225 3446 2680

95 91 149 134

87 118 98 44 43

69 ± 15

368 ± 112

1035 ± 52

3478 ± 685

117 ± 25

73 ± 30

60 49 60 55 60 50 62

220 109 78 92 61 104 83 113 76 90 100 91

546 610 589

1123 964 382 67 45

569 517 491 580 507

122 98 102 112 134

28 36 15 16

57 ± 5

101 ± 38

582 ± 27

516 ± 450

533 ± 35

114 ± 13

24 ± 9

123 83 90 77 94

120 94 122 176

1155 1238 1369

105 170 190 182

1243 1544 1563 1480

159 130 137 175 196

94 100 91 45

93 ± 16

128 ± 30

1254 ± 88

162 ± 33

1457 ± 128

159 ± 24

82 ± 22

89 110 97 101 94 97 104 108

130 125 128 124 118

196 63 60

101 124 65

306 346 305 205

82 95 141 140 110

100 ± 7

125 ± 4

106 ± 63

97 ± 24

290 ± 52

114 ± 24

544 617 508 497 500

127 88 116

248 212 200 297 275 271

533 ± 45

110 ± 16

250 ± 35

Fadh(A) (pN)

Fadh(B) (pN)

± SD (pN) Fadh(A) (pN)

± SD (pN) Fadh(B) (pN)

± SD (pN) cycle 3

2

duration (min)

± SD (pN)

cycle 2

1

Fadh(A) (pN)

± SD (pN) Fadh(B) (pN)

494 403 351 317 301 314 314 D


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Energy & Fuels Table 2. continued experiment

cycle 4

1

2

3

4

5

6

7

T (°C)

75

75

75

75

75

80

80

duration (min)

339

302

273

190

256

177

168

± SD (pN)

356 ± 65

Fadh(A) (pN)

± SD (pN)

415 361 316 364 ± 40

a

Fadh(i) represents the adhesion force, where i represents solution A of 0.5 M NaCl or solution B of 0.045 M MgSO4 in 0.32 M NaCl; represents the average adhesion force over all of the images from each set under the same conditions.

Figure 2. Set of force maps collected during experiment 7. Maps are composed of 30 × 30 single force curves obtained over the same (3 μm)2 area. Force maps (a−e) were acquired one after the other in solution A (0.5 M NaCl). Then, the fluid was exchanged to solution B (0.045 M MgSO4 in 0.320 M NaCl), and maps (f−i) were collected consecutively. Finally, the solution was replaced with solution A, and the last maps (j−m) were obtained. Blue indicates low adhesion force; black, red, and pink indicate increasingly higher force. The average adhesion calculated over the whole map is , where (i) is solution A or B, is noted in the bottom left corner. The scale bar at the bottom right is 500 nm. analysis was in the 10−9 Torr range. The generated XPS data were processed using the CasaXPS software. Atomic surface concentrations were determined by fitting the core level spectra using Gaussian− Lorentzian line shapes and a Shirley background correction. Each reported value represents the average of at least two measurements. The absolute energy scale was calibrated to the carbonate C 1s binding energy of 290.1 eV.26 The systematic error was estimated to be on the order of 5−10%. The measurements were done using a spot size of 0.21 mm2. XPS measurements were made on calcite crystals after the AFM experiments. Samples were dismounted from the fluid cell, and the residual solution was removed mechanically by sweeping the fluid from the surface with a jet of nitrogen. A few samples were further treated in an ultrasonic bath (Branson 2510, USA) to remove residual salts and adsorbed ions. The samples were sonicated 3 times for 30 s in CaCO3saturated solution that had been preheated to the temperature at which the AFM experiment had been performed. We assumed that sonication removes only the ions that are weakly associated with the surface. To verify this, we took samples from a series of solutions that

contained different sorbed concentrations of Mg, and we sonicated each sample sequentially 4 times (30 s each). Then, we used XPS to analyze them after each sonication. The Mg/Ca ratio did not change (within standard error) between the first and subsequent samples. For these and all of the other XPS analyses, measurements were carried out within a day of the AFM experiments to minimize recrystallization and burial of the surface sorbed ions, as has been observed previously.31,32

3. RESULTS AND DISCUSSION 3.1. Calcite Wettability Change in the Presence of Mg2+ and SO42−. We performed seven experiments with the solution containing Mg2+ and SO42−, five at 75 °C, and two at 80 °C. The adhesion data for all experiments with solutions A, 0.5 M NaCl, and B, 0.045 M MgSO4 in 0.32 M NaCl, are presented in Table 2. All of the experiments were carried out following this same procedure. We varied the number of E


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Figure 3. (a−d) Maps (40 × 40 pixels) from experiment 3. We collected data from a 3 μm × 3 μm area of freshly cleaved calcite during exposure to solution A (NaCl) and solution B (MgSO4) at 75 °C (composition presented in Table 1). Each pixel on each image represents the adhesion force extracted from a force curve collected at that specific point on the surface (such as Figure 1). Blue indicates low adhesion force, and black, red, and pink indicate increasingly higher force. The average adhesion over the whole map, , where (i) is solution A or B, is noted in the bottom left corner. Scale bar at the bottom right is 500 nm. (e) Average adhesion force, , from all of the maps from experiment 3 plotted against acquisition time. During the time intervals 24−44, 67−89, and 112−136 min, the solutions were exchanged, from A to B, from B to A, and from A to B, respectively.

The decrease in adhesion when the NaCl solution was replaced by one containing MgSO4 is consistent with predictions by Sakuma and colleagues.14 Using density functional theory, they showed that Mg2+ incorporation into the calcite {10.4} surface enhances water adsorption and inhibits attachment of organic compounds. They used the adsorption energy that they calculated to estimate the change in surface wettability in terms of contact angle. In a mixed wet system, where the initial contact angle was 90° if only 1% of the Ca2+ ions were substituted by Mg2+, the predicted contact angle decreased by ∼5% (i.e., from 90° to 86°), but if 10% were substituted, the contact angle decreased by 47% (i.e., from 90° to 48°). If the initial surface was completely oil wet calcite, substitution of 1% Ca2+ by Mg2+ changed the contact angle by 12% from 180° to 159° and, for 10% substitution, by 40% to 109°. These predictions can be compared directly with our chemical force mapping measurements because the adhesion work for each pixel can be estimated from the force curves. We determined contact angle using the method described by Hassenkam et al.,23 which is based on the Young−Dupré equation. Thus, for each force map, we can estimate contact angle as if a water droplet were sitting on the 3 × 3 μm2 area. To allow comparison with the modeling data of Sakuma et al.,14 we considered data from experiments 2−4, which had estimated average contact angles >75° in NaCl (solution A), i.e., mixed wet and oil wet surfaces. These are reported in Table 3. The overall average decrease in contact angle was 48% when MgSO4 was added. This fits with the results from the DFT modeling of Sakuma and colleagues14 and is also consistent with the results of the core plug experiments with Ca2+, Mg2+, and SO42− at T > 70 °C reported by Zhang and co-workers8 and the work of Alipuor Tabrizy et al.10 For each experiment, all images were collected from the same site on the surface with the same tip, and for each new experiment, we used a fresh sample and a new tip. The samples, cleaved from optical quality Iceland spar crystals, all came from

consecutive maps that were collected in the same solution and the number of cycles in each experiment. Figure 2 illustrates a set of force maps from experiment 7. We first collected maps (a−e) in solution A; then, we exchanged it with solution B and acquired maps (f−i). We then replaced solution B with A and collected maps (j−m). Every map is constructed from data from 30 × 30 single force curves over a (3 μm)2 area. We did all that was possible to keep the scanning on the same area, though we cannot exclude the possibility of minor drift, which results from slight hysteretic behavior of the piezoelectric scanner in the AFM. This slight drift can introduce some uncertainty into the calculation of average adhesion, but in most cases, this is of minor consequence. In Figure 2, low adhesion force is shown in blue, higher adhesion force in black and red, and highest in pink. For each adhesion map, we determined the average adhesion, Fadh(i), where i represents the solution, to assess the average change in adhesion force. In Figure 2, Fadh(i) is indicated at the bottom left corner of each map. These values show that the average adhesion force in this series of images is consistently higher in solution A and lower in solution B, which contains magnesium. Figure 3 shows the results from experiment 3. The force maps (a−d) were collected through two cycles in a solution of NaCl (a and c) and NaCl + MgSO4 (b and d). The data plotted in Figure 3e show the average adhesion force, Fadh(i), from each map plotted with acquisition time, i.e., time elapsed from the beginning of the experiment. The plot shows the same trend that we observed in the majority of the experiments. When the calcite-saturated solution containing Mg2+ and SO42− was injected, adhesion force dropped significantly. When the control, solution A, saturated with calcite and containing Na and Cl at the same ionic strength as solution B was reinjected, the adhesion force increased, indicating that the sorption causing the change in adhesion is reversible. The adhesion force over the area of the maps is rather homogeneous, but some features, such as in Figure 3c and d, show higher adhesion force than the background. F


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Table 4. Change in Average Adhesion Force, Δ, for Experiments in which Solutions of NaCl (A) and NaCl + MgSO4 (B) Were Exchangeda

Table 3. Average Contact Angles in NaCl (A), NaCl + MgSO4 (B), and Relative Contact Angle Change in %a exp.

Δ(CA)

2 3 4

109° 180° 102°

61° 71° 60°

44% 61% 41%

experiment cycle 1

represents the average contact angle for the 3 μm × 3 μm calcite surface in 0.5 M NaCl; represents the average contact angle in 0.320 M NaCl + 0.045 M MgSO4, and Δ(CA) represents the percent change of the average contact angle in solution B with respect to the average contact angle in solution A. a

cycle 2 cycle 3

A B A B A B

to to to to to to

B A B A B A

1

2

3

4

12 36 −7 433 177 8

267 27 3 −15

453 672 1148

2962 −354 65

5

6

7

924 1167

3 45 45

49 58

a Δ was determined using equation 1: Δ = − .

different pieces of the same batch, but they cannot be guaranteed to have precisely the same bulk or surface composition. Natural samples vary considerably in terms of impurities, i.e., cation and anion substitution and fluid inclusions, which make each surface slightly different33,34 such that the surface properties and innate wetting behavior of each cleaved face would be slightly different. Although each tip was prepared in the same way, there are small differences in the size and shape, making the characteristics of the functionalized surface slightly different. The adhesion force measured results as a combination of the characteristics of the surface and the tip. Keeping the same tip and sample throughout several cycles of solution exchange allows us to compare the adhesion measurements within that experiment because it is the change in adhesion that is important, not the absolute adhesion. Because we used a fresh sample and tip for each experiment, we cannot directly compare the initial adhesion between experiments, but we can compare the changes in adhesion between different experiments. Another variable that is impossible to control is the amount of adventitious carbon in the system. Adventitious carbon is organic material that comes spontaneously from the air and the solutions in contact with the surface.26,35 It is unavoidable. XPS confirms its presence. It is likely that this organic material makes the calcite surface less hydrophilic than it would be in an absolutely pure system, but again, we are not interested in the absolute adhesion but rather its change when we exchange the solutions. Thus, the presence of adventitious carbon does not adversely affect the results because its concentration is relatively constant, and although it affects absolute adhesion, it is the change in adhesion associated with changing solution composition that matters. We calculated the average adhesion, , for all of the maps for each set of conditions. Then, to compare the extent of the effect when the solutions were exchanged, we determined the change in average adhesion Δ = −

Figure 4. Change in average adhesion force, Δ, when solution A (calcite-saturated 0.5 M NaCl) was replaced with solution B (calcitesaturated 0.32 M NaCl + 0.045 M MgSO4) for the seven experiments. Δ is reported in Table 4. Positive Δ means the the surface became more hydrophilic in the solution containing MgSO4. Experiments 1−5 were conducted at 75 °C and 6 and 7 at 80 °C.

If we look closer at the three cases with the reverse trend, we see dramatic local variation. For the extreme case (experiment 4, negative value of −354), where the average adhesion force in NaCl + MgSO4 (end of cycle 1) is higher than the average adhesion force in NaCl (beginning of cycle 2), it is only the values averaged for each solution that provide the opposite response. If we compare the last adhesion force measured in solution B (NaCl + MgSO4; 45 pN) with the force measured after replacement with solution A (NaCl; 105 pN), adhesion was lower in the NaCl + MgSO4 solution. The change of adhesion in the presence of MgSO4 was gradual over time, resulting in average adhesion force of 516 pN, which was much larger than the last value of 45 pN. In this case, the force mapping happened to be on a site where the initial adhesion was very high. It is possible that it takes longer for the Mg to enhance hydrophilicity in such a case. 3.2. Individual Contributions of Mg2+ and SO42− on Calcite Wettability. To test the roles of Mg2+ and SO42−

(1)

The Δ values are reported in Table 4 and plotted in Figure 4. For exchange of solution A with solution B, 20 of the adhesion change values were positive, meaning that MgSO4 solution rendered the surface more hydrophilic. Two were quite close to 0, and three had negative Δ, meaning that average adhesion increased when solution B replaced solution A. Thus, the majority of the experiments showed that the hydrophobic tip, which serves as a model for a tiny oil droplet, is less attracted to freshly cleaved calcite when the solution contains Mg2+ and SO42− than when only Na+ and Cl− are present. We see this effect both at 75 and 80 °C. G


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Energy & Fuels Table 5. Average Adhesion Force for Experiments Performed with NaCl, MgCl2, and NaSO4a

cycle 1

exp.

8

9

experiment

10

11

T (°C)

75

80

T (°C)

75

75

duration(min)

204

185

323 157 116

261 225 239 238 205 221 203

199 ± 90

227 ± 19

82 208 247 207

175 221 311 201

186 ± 62

227 ± 51

82 89 82

933 1017 1003 999 1078 1058

84 ± 3

1015 ± 46

Fadh(A) (pN)

± SD (pN) Fadh(C) (pN)

± SD (pN) cycle 2

Fadh(A) (pN)

± SD (pN) Fadh(C) (pN)

± SD (pN)

165 158 138 245 192 152 175 ± 35

duration(min) Fadh(A) (pN)

± SD (pN) Fadh(D) (pN)

± SD (pN) Fadh(A) (pN)

± SD (pN) Fadh(D) (pN)

± SD (pN)

167

272

2795 3085 3179

156 173 172

3020 ± 163

167 ± 8

1827 1609 1440 1442

188 194 188 106 72

1580 ± 159

150 ± 51

1379

165 112 112

1379 ± 0

130 ± 25 87 90 92

89 ± 2

a

Fadh(i) represents the average adhesion force for each map, where i represents solution A, 0.5 M NaCl, solution C, 0.045 M MgCl2 in 0.365 M NaCl, or solution D, 0.024 M Na2SO4 in 0.428 M NaCl; represents the average adhesion force over all of the images from each set of the same conditions.

Figure 5. Change in average adhesion when solution A (calcite-saturated 0.5 M NaCl) is exchanged with (a) solution C (calcite-saturated 0.365 M NaCl + 0.045 M MgCl2) and (b) solution D (calcite-saturated 0.428 M NaCl + 0.024 M Na2SO4). Experiments 8, 10, and 11 were performed at 75 °C and experiment 9 at 80 °C.

H


Article

Energy & Fuels Table 6. X-ray Photoemission Spectroscopy Surface Composition of Calcite Exposed to MgSO4a untreated (exp. 3) sonicated (exp. 6)

O 1s

C 1s

Cl 2p

Ca 2p

Na 1s

S 2p

Mg 2s

Adv C

34.4 48.4

33.2 31.6

5.1 0.3

8.9 18.7

1.7 0

4.2 0

4.4 0.8

60.8 35.5

a

Values are expressed in atom % for two samples where solutions A and B were exchanged through two cycles. Adv C is adventitious carbon, and the value is the % of the total carbon. Uncertainty is generally 10−15%.

mechanically by a jet of nitrogen gas, we could see clear evidence of Na and Cl, which are not expected to adsorb. This suggested that our solution removal method was not completely effective, which meant that we could not prove that Mg and SO4 had adsorbed, as they could be present as a result of precipitation of some remaining solution. Therefore, the rest of the samples were for 3 short cycles in an ultrasonic bath with calcite-saturated, deionized water. For these samples, Na was undetectable, and Cl was present only as a trace. Clearly, some of the sorbed Mg and SO4 had also been removed by the treatment, but the presence of some Mg provided evidence that it had been sorbed on and/or in the calcite. Surface concentrations for two samples, representative of the two sample preparation methods, are presented in Table 6. In both samples, the C 1s peak intensity is considerably higher than would be expected for a sample of calcite cleaved in air and quickly analyzed afterward. The carbon concentration was determined by summing the area under all of the peaks in the C 1s region, those that represent CO3 from the sample, as well as those from organic compounds on the surface. The fraction of adventitious carbon, Adv C, that was present as C− C, C−H, and C−O bonds, i.e., not CO3 from the calcite substrate, varied from sample to sample, ranging from ∼55 to 75%. This carbon contamination is unavoidable in experiments where surfaces are exposed to air or solution. We used chemicals that were reagent grade or better and ultrapure deionized water, and we were very careful with the cleanliness of our equipment and handling procedures, yet there was still a significant mass of organic material associated with the pure crystal surfaces. This has been observed previously26 and has two implications: (i) In experiments where such pains were not taken, one could expect even more contamination, casting suspicion on whether contact angle measurements in general represent true mineral surfaces. Rather, they more likely represent the effects of both the mineral surface and the organic material that mineral is generally capable of accumulating. (ii) Inhomogeneous distribution of adsorbed carbon compounds over a sample surface and over time would result in local and temporal differences in adhesion properties, thus accounting for some of the fluctuations we observed in adhesion force measurements (Table 2 and Figure 4). The decrease in adventitious carbon associated with the treated sample is logical. In treated samples, the adsorbed ions, precipitated salts, and adventitious carbon that the sample collected during the AFM experiments would be released by ultrasonication.

individually, we performed four additional experiments, where the adhesion force between a -CH3-terminated tip and a freshly cleaved calcite surface was measured in the presence of solution C, which was calcite-saturated NaCl + MgCl2, or solution D, which was calcite-saturated NaCl + Na2SO4, all at the same ionic strength. The results are presented in Table 5 and Figure 5. Figure 5 shows the change in average adhesion force, ΔFadh(i), when solution A, NaCl, is exchanged with solution C, MgCl2 (plot a), or D, NaSO4 (plot b). When only Mg2+ was added at 75 °C (experiment 8), the surface did not become more hydrophilic, i.e., Δ was negative (Figure 5a), but in experiment 9 performed at 80 °C, it did become more hydrophilic (positive Δ), especially during the second cycle. It is unlikely that the difference in behavior is a result of the slight difference in temperature. It is much more likely that it results from the natural local variation in surface composition for the Iceland spar calcite crystal that we used for the experiments. Regardless, the overall trend suggests that Mg2+ alone might have an effect on wettability and that the uptake of Mg2+ could occur at higher temperature when SO42− is absent. Zhang et al.8 concluded similarly, stating that sulfate helps magnesium dehydrate, facilitating its uptake at 70 °C. Abdallah and Gmira13 also observed that calcite wettability increases when Mg2+and SO42− are both present. Our results can be explained using the predictions of Sakuma et al.,14 which showed that magnesium has a stronger effect on adhesion when it is present as the MgSO4 ion pair. Figure 5b shows the change in average adhesion force for experiments 10 and 11 in solution D, Na2SO4, at 75 °C. Adhesion force decreased each time a new solution was injected, including the control NaCl; thus, we conclude that SO42− alone has no effect. Such a general decrease in adhesion as a function of time has been observed previously. There are too few experiments made with only Mg2+ and SO42− to draw definitive conclusions, but the trends do not contradict the modeling results presented by Sakuma et al.,14 namely, that Mg2+ renders the surface more water wet but with less effect than when both Mg2+ and SO42− are present. Table 5 shows fluctuations in average adhesion force from one image to the next when the solution remains the same, but the difference in adhesion force when the solution is changed is systematic and considerable in comparison. To verify that the time of exposure of the sample to the solution and to scanning were not responsible for the observed adhesion differences, we performed a control experiment in which we collected a long series of consecutive force maps in 0.5 M NaCl only at 80 °C for 2 h. The adhesion force was stable throughout. Extended scanning and extended exposure to solution did not make the surface more hydrophilic. 3.3. X-ray Photoelectron Spectroscopy Evidence for Magnesium on Calcite. To confirm that Mg and SO4 were sorbed on or in the top 10 nm of the solid, we analyzed all of the samples after the AFM experiments using XPS. For some samples, where we removed the remaining droplet of solution

4. CONCLUSIONS The adhesion force between a freshly cleaved calcite surface and a hydrophobic -CH3-terminated AFM tip decreases when pure NaCl solution (0.5 M) is replaced with a solution of the same ionic strength that contains MgSO4 at 75 and 80 °C. The effect is reversible when the solution is replaced by NaCl. Local contact angle, calculated from work of adhesion using the I


Article

Energy & Fuels

(12) Andersen, P. Ø.; Evje, S.; Madland, M. V.; Hiorth, A. Chem. Eng. Sci. 2012, 84, 218−241. (13) Abdallah, W.; Gmira, A. Energy Fuels 2014, 28, 1652−1663. (14) Sakuma, H.; Andersson, M. P.; Bechgaard, K.; Stipp, S. L. S. J. Phys. Chem. C 2014, 118, 3078−3087. (15) Stipp, S. L. S.; Eggleston, C. M.; Nielsen, B. S. Geochim. Cosmochim. Acta 1994, 58, 3023−3033. (16) Stipp, S. L. S. Geochim. Cosmochim. Acta 1999, 63, 3121−3131. (17) Perdikouri, C.; Putnis, C. V.; Kasioptas, A.; Putnis, A. Cryst. Growth Des. 2009, 9, 4344−4350. (18) Jordan, G.; Rammensee, W. Geochim. Cosmochim. Acta 1998, 62, 941−947. (19) Turner, B. D.; Binning, P.; Stipp, S. L. S. Environ. Sci. Technol. 2005, 39, 9561−9568. (20) Astilleros, J. M.; Fernandez-Diaz, L.; Putnis, A. Chem. Geol. 2010, 271, 52−58. (21) Davis, K. J.; Dove, P. M.; Wasylenki, L. E.; De Yoreo, J. Am. Mineral. 2004, 89, 714−720. (22) Stipp, S. L. S.; Lakshtanov, L. Z.; Jensen, J. T.; Baker, J. A. J. Contam. Hydrol. 2003, 61, 33−43. (23) Hassenkam, T.; Skovbjerg, L. L.; Stipp, S. L. S. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 6071−6076. (24) Hassenkam, T.; Mitchell, A. C.; Pedersen, C. S.; Skovbjerg, L. L.; Bovet, N.; Stipp, S. L. S. Colloids Surf., A 2012, 403, 79−86. (25) Hassenkam, T.; Pedersen, C. S.; Dalby, K.; Austad, T.; Stipp, S. L. S. Colloids Surf., A 2011, 390, 179−188. (26) Stipp, S. L.; Hochella, M. F. Geochim. Cosmochim. Acta 1991, 55, 1723−1736. (27) Stipp, S. L. S.; Gutmannsbauer, W.; Lehmann, T. Am. Mineral. 1996, 81, 1−8. (28) Parkhurst, D. L. P.; Appelo, C. A. J. Description of input and examples for PHREEQC, version 3. A computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations; U.S. Geological Survey: Virginia, USA, 2013; pp 1−497. (29) Plummer, L. N.; Parkhurst, D. L.; Fleming, G. W.; Dunkle, S. A. A computer program incorporating Pitzer’s equations for calculation of geochemical reactions in brines; U.S. Geological Survey: Virginia, USA, 1988; pp 1−37. (30) Hilner, E.; Andersson, M. P.; Hassenkam, T.; Matthiesen, J.; Salino, P. A.; Stipp, S. L. S. Sci. Rep. 2015, 5, 9933. (31) Stipp, S. L.; Hochella, M. F.; Parks, G. A.; Leckie, J. O. Geochim. Cosmochim. Acta 1992, 56, 1941−1954. (32) Hoffmann, U.; Stipp, S. L. S. Geochim. Cosmochim. Acta 2001, 65, 4131−4139. (33) Stipp, S. L. S.; Konnerup-Madsen, J.; Franzreb, K.; Kulik, A.; Mathieu, H. J. Nature 1998, 396, 356−359. (34) Harstad, A. O.; Stipp, S. L. S. Geochim. Cosmochim. Acta 2007, 71, 56−70. (35) Matthiesen, J.; Bovet, N.; Hilner, E.; Andersson, M. P.; Schmidt, D. A.; Webb, K. J.; Dalby, K. N.; Hassenkam, T.; Crouch, J.; Collins, I. R.; Stipp, S. L. S. Energy Fuels 2014, 28, 4849−4858.

Young−Dupré equation, shows a change in the average contact angle of 48%. These results agree with those of Sakuma et al.14 who used density functional theory to predict that, when Mg2+ is incorporated into calcite, organic molecules bind more weakly and the surface becomes more hydrophilic. Although there are not enough data to make definitive conclusions, the results from the experiments with solutions of MgCl2 and Na2SO4 are not contradictory to the predictions from molecular modeling,14 i.e., that SO4 enhances the change in wettability when Mg sorbs on calcite. Surface composition analysis using XPS shows considerable adventitious carbon that comes from air and solutions in contact with the surface even when pains are taken to minimize contamination. This organic material has an effect on the wettability of the surface and can help explain local and temporal differences in adhesion force at submicrometer scale. In these experiments, this adventitious material did not prevent wettability change when Mg and SO4 are present. However, there are a great many types of organic compounds, so it is possible that some organic material would indeed influence wettability.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

J. Generosi: 0000-0002-4683-5615 M. P. Andersson: 0000-0002-4921-1461 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank the NanoGeoScience group members, especially J. D. Rodriguez, for helpful discussions and K. Dalby for SEM/EDXS measurements and we are grateful to Søren Frank and Kristian Mogensen from Maersk Oil and Gas A/S for their stimulating and encouraging role during the first three years of the project. This work was carried out as part of the W-EOR Project, funded by Maersk Oil Research and Technology Centre.

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REFERENCES

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Calcite Wettability in the Presence of Dissolved Mg2+ and SO42

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