Crude Oil Adsorbates on Calcite and Quartz Surfaces Investigated by

Article pubs.acs.org/JPCC

Crude Oil Adsorbates on Calcite and Quartz Surfaces Investigated by NMR Spectroscopy Henrik N. Sørgård,* Christian Totland, Willy Nerdal, and John G. Seland Department of Chemistry, University of Bergen, Allegaten 41, N-5007 Bergen, Norway ABSTRACT: The complexity of crude oils may contribute to alteration of petroleum reservoir wetting states in ways that cannot be mimicked by model oils. Subsequent to a wet aging procedure, the molecular arrangements of crude oil adsorbates and water on calcite and quartz surfaces were investigated by various 1H NMR techniques. Solid state magic angle spinning (MAS) magnitude mode, 1 H−1H double-quantum single-quantum (DQSQ), and 1H single pulse NMR spectra were used to directly observe molecules on both surfaces. T1 and T2 relaxation time measurements were used to determine molecular surface arrangement. Liquid state 1H NMR was used to investigate possible differences in the crude oil caused by the aging procedure. 1H DQSQ spectra were found most valuable in assignment of the quartz surface where the resonances from the entire adsorbate were relatively well resolved. In addition to adsorbate molecular positioning, this study also provides quantitative measurements of water present at both surfaces subsequent to wet crude oil aging. Acid adsorption was determined to be the main mechanism responsible for alteration of wetting state on both the calcite and the quartz surface. NMR experiments show that organic acids adsorb directly onto calcite through electrostatic attraction while the same acids are indirectly adsorbed onto quartz through solubilization in the water phase present at the surface. A series of 1H single pulse NMR experiments at temperatures ranging from 293 to 233 K revealed that the quartz surface possesses two different water environments while the calcite surface has only one. the pH scale. Below the isoelectric point, −Ca+ will predominate at the surface and thus generate a positive surface charge. An excess of −CO3− will generate a negative surface for pH values above the isoelectric point.11,12 The calcite surface is thus positively charged at neutral pH (see Figure 1A).

1. INTRODUCTION It is well documented that adsorption of polar components (acids, bases, and asphaltenes) from crude oil onto reservoir rock surfaces is an important mechanism for the naturally occurring change in wetting properties from water-wet to more oil-wet, thus increasing the retention of crude oil in reservoir rock pores.1−6 The term wettability alteration of petroleum reservoirs usually refers to the process of restoring the presumed originally water-wet nature of the reservoir surface.7 On the other hand, the process of changing the wettability of naturally water-wet surfaces toward a more oil-wet wetting state is usually referred to as aging.8,9 It is generally accepted that water-wet to neutral-wet reservoirs provide a more efficient sweep than oil-wet ones during water flooding.7,10 Synthetically achieving wettability alteration through enhanced oil recovery (EOR) methods obviously requires a detailed understanding of the mechanisms responsible for said process. However, it also requires insight into the naturally occurring process of altering the originally water-wet reservoir rock toward more oil-wet. This natural phenomenon begins when oil migrates into the reservoir pore network and replaces parts of the water fraction, a process known as primary drainage.3 During aging of a water-impregnated surface by a multicomponent liquid, the adsorbate will depend on the surface charge.3,11,12 When calcium carbonate surfaces are immersed in water, hydrolysis reactions involving −Ca+ and −CO3− can generate a surface charge. The measured isoelectric point of calcite varies; however, most studies agree that it is above 7 on © XXXX American Chemical Society

Figure 1. Surface of calcite crystals (A) and quartz crystals (B) at neutral pH.

Upon immersion of silicate surfaces (e.g., quartz) in water, dissociation of surface hydroxyls generates a surface charge which is positive for pH values below the isoelectric point and negative for pH values above the isoelectric point.11 The isoelectric point of quartz for low ionic strength solutions has been found to be between 1.5 and 3.7 on the pH scale,11,12 meaning that the quartz surface is negatively charged at neutral pH (see Figure 1B). Adsorption of different acids from acid/decane model oils onto calcite surfaces of varying specific surface areas has been researched by several authors.9,13−16 Wu et al. impregnated Received: July 19, 2017 Revised: August 30, 2017 Published: August 31, 2017 A

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The Journal of Physical Chemistry C calcite surfaces with various naphtenic acids from naphthenic acids/decane model oils and investigated changes in wetting properties as a function of both molecular structure and amount adsorbed by means of contact angle measurements and a flotation test. They found that alteration of wetting properties is controlled mainly by the molecular structure of the adsorbate acid and not by the amount adsorbed.13 Gomari et al. investigated amount of stearic acid adsorbed onto dry and wet calcite from a stearic acid/decane model oil using a high-resolution thermogravimetric analyzer (TGA). They found that wet aging resulted in an adsorption nearly twice as efficient as for the dry aging.16 Similar results were found through Fourier transform infrared (FTIR) by Jarrahian et al.9 However, in order to further understand petroleum reservoir processes, it is necessary to obtain a more detailed understanding of more realistic systems. The complex composition of crude oil can make contributions to an aging process that cannot be mimicked by a model oil. This study utilizes state-of-the-art liquid and solid state NMR spectroscopy to investigate the molecular arrangement of organic acids from a high acid crude oil adsorbed onto calcite and quartz surfaces as the result of a wet crude oil aging process. These minerals are among the most commonly encountered in petroleum reservoirs. It is generally accepted that only a third of the petroleum in known reservoirs is economically recoverable with established technology.17 Understanding these aging processes on a molecular level will aid in designing more effective methods for enhanced oil recovery.

2. MATERIALS AND METHODS 2.1. Materials. Precipitated calcium carbonate crystals from the BioUltra series (≥99.0%), produced by Sigma-Aldrich, were used as the calcite model. The scanning electron microscopy (SEM) image in Figure 2A shows a heterogeneous particle size with a diameter range of approximately 1−25 μm. The calcium carbonate powder has a specific surface area of 0.99 m2 g−1. NC4X quartz crystals, produced by The Quartz Corp, were a generous gift from Statoil ASA. The SEM image in Figure 2B shows that the crushed crystals have a heterogeneous particle size with a diameter range of approximately 50−260 μm. All SEM images were recoded with a secondary electron detector on a Zeiss Supra 55VP scanning electron microscope. The quartz sand have a specific surface area of 0.089 m2 g−1. Specific surface areas were found by N2 adsorption at 77 K with a pretreatment temperature of 423 K.18 The crude oil sample used for aging of both surfaces originates from a sandstone reservoir (SiO2) on the Norwegian continental shelf (Grane field). The crude oil density is 0.93 g/ cm3, and the API gravity is 20° API. It is characterized as a high acid crude (total acid number >0.5 mg KOH g−1 oil)19 with a total acid number (TAN) of 2.0 mg KOH g−1. The crude oil was not kept under a nitrogen blanket and must therefore be considered as exposed to oxidation. 2.2. Sample Preparation. In a previous adsorption study, by Totland and Lewis,15 it was shown that leaching of surface active species from plastic containers significantly reduces adsorption of stearic acid from a decane/stearic acid model oil onto calcite particles due to competitive adsorption. Consequently, all samples in this study were prepared exclusively in glass labware. Prior to the aging procedure, both the quartz and calcite powders were placed in a desiccator together with a saturated

Figure 2. Scanning electron microscopy image of calcium carbonate crystals at magnification = 5K (A) and quartz crystals at magnification = 600 (B).

K2SO4 solution for an excess of 10 days. The saturated K2SO4 solution provides a humidity of 97%, which produces a thin film of water on the surface of the powder.20 This surface pretreatment has previously shown to increase the efficiency of fatty acid adsorption from model oils on calcite surface by nearly 100%.16 It also simulates the wetted surface of a fully water saturated reservoir rock prior to primary drainage. The aging of both quartz and calcite was achieved by the following procedure: Approximately 1 g of pretreated powder was placed in a sufficient amount of crude oil to saturate the surface with adsorbed species. The powder was left in the crude oil at 80 °C with constant stirring for 24 h to ensure equilibrium conditions.16,21 The suspension was then centrifuged, and supernatant crude oil was removed and investigated using liquid state single pulse 1 H NMR. The results from this investigation were compared to identical NMR experiments on a reference crude oil sample to investigate if any observable changes occurred due to adsorption. The excess crude oil was cleared from the powder surface by dispersing it three consecutive times in decane. The powder was then separated from the precipitated asphaltene fraction by dispersing it five consecutive times in decaline.8 Finally, the powder was dispersed once in heptane, centrifuged, and left to dry in a fume hood overnight. The dry powder was packed in a 4 mm MAS rotor and investigated using solid state 1H NMR. The calcite sample consisted of 162 mg of dry powder, which B

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The Journal of Physical Chemistry C equals a surface area of 0.16 m2. The quartz sample consisted of 149 mg of dry powder, which equals a surface area of 0.013 m2. The amount of water present on the surfaces post aging was determined by comparing the integral of the water signal from both samples to the integral of a reference water sample. The reference sample consisted of a capillary tube filled with a known amount of water. The capillary tube was placed in a 4 mm MAS rotor and examined without spinning the sample (stationary sample). Because of ambiguous spin−lattice (T1) relaxation time measurements on the water present at the calcite surface, two additional samples were prepared in the same manner as described above to allow for spin−spin (T2) relaxation measurements of adsorbed species on both surfaces. These two samples are denoted as the second calcite and quartz sample. 2.3. NMR. Solid state NMR specra were recorded on a Bruker Biospin 500 WB Ultrashield spectrometer with a solidstate probe head for 4 mm MAS rotors. Single pulse 1H NMR spectra were recorded over 1000 scans using Bruker‘s onepulse pulse sequence with relaxation delay = 2.5 s, flip angle = 90°, receiver gain = 16, acquisition time = 0.2 s, spectral width = 30 ppm, and transmitter offset frequency = 2 ppm. Integrals of overlapping peaks in the 1H single pulse NMR spectra were found from the Fit Solids NMR Models (sola) function available in Topspin version 3.5. Gaussian and Lorentzian line shapes were used in the fitting. Magnitude mode double-quantum/single-quantum (DQSQ) correlation spectra were recorded over 256 scans using Bruker’s dqsqf pulse sequence with F1 spectral width = 30 ppm, F2 spectral width = 15 ppm, receiver gain = 101, transmitter offset frequency (F1 and F2) = 2 ppm, and acquisition mode = DQD. Proton spin−lattice (T1) relaxation times were obtained from an inversion recovery experiment with 16 scans, F1 spectral width = 10 ppm, F2 spectral width = 15 ppm, and relaxation delays logarithmically distributed from 1 ms to 15 s. All MAS NMR experiments were conducted with a MAS rate of 10 kHz and at a temperature of 298 K. In addition to the MAS experiments, a static sample freeze study was performed on the second calcite and quartz samples accompanied by T2 measurements for each temperature drop. The samples were left to acclimate themselves for 20 min for each experimental temperature drop. T2 measurements were conducted with a Carr−Purcell−Meiboom−Gill (CPMG) sequence that averages the signal over the entire sample with 16 scans, echo time = 100 μs, and the results were fitted to a three-component model using MATLAB. Liquid state NMR spectra were recorded on a Bruker Biospin 850 SB Ultrashield spectrometer with a liquid state cryo probe head. Single pulse 1H experiments were conducted with a 7° flip angle and a relaxation delay of 10 s. All liquid state experiments were conducted at a temperature of 298 K. All crude oil samples were diluted 1:1 with deuterated chloroform (>99.8%) to obtain a deuterium lock and to dilute the sample to an appropriate concentration without introducing additional proton resonances.

Figure 3. Liquid state 1H NMR spectra of the aliphatic region of the crude oil reference sample (top), crude oil supernatant after quartz aging (middle), and crude oil supernatant after calcite aging (bottom). Emphasis on the signal just below 0.17 ppm.

supernatant samples. The intensity decrease in this resonance, as a result of both aging procedures, suggests that compounds containing strongly shielded protons have adsorbed onto both surfaces. There were no other observable differences in the liquid state 1H NMR spectra between the reference sample and the two supernatant samples. Hence, in order to evaluate the adsorbed species, it is necessary to use solid state NMR to directly observe the adsorbates on the two surfaces. 3.2. Solid State NMR. Figure 4 shows the solid state 1H MAS spectra of the calcite surface (Figure 4A) and the quartz

Figure 4. 1H NMR MAS spectra of crude oil adsorbate on calcite surface (A) and quartz surface (B). The quartz spectra are enhanced by a factor of 12.3 to account for the difference in surface area among the two samples.

surface (Figure 4B) when scaled according to an equal surface area. Since a fully packed 4 mm MAS rotor corresponds to 0.16 m2 of calcite surface and 0.013 m2 of quartz surface, the quartz spectra have been enlarged 12.3-fold. Table 1 shows that the amount of water (δ = 4.70 ppm) present on the surface post aging was 1.06 × 10−3 and 3.75 × 10−3 g/m2 for the calcite and quartz surface, respectively. This was calculated by comparing the integral of the water peak (δ = 4.70 ppm) on both surfaces with the water integral of a reference sample containing a known amount of water. Thus, the water on the calcite surface equals 28% of the water on the Table 1. Amount of Water Present on the Calcite and Quartz Surface Postaging per m2

3. RESULTS AND DISCUSSION 3.1. Liquid State NMR. The crude oil was investigated using liquid state 1H NMR before and after the aging procedure to identify observable intensity changes due to adsorption. Figure 3 shows changes in intensity, for the peak located just below 0.17 ppm, from the reference sample to the two C

sample

amount of water [g/m2]

calcite quartz

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pH,11,12 the crude oil adsorbate is thought to consist of carboxylic acid derivatives. The negatively charged head groups of carboxylic acids are known to bind to a positively charged surface through an electrostatic adsorption mechanism.15 Methylene groups close to a carboxylic acid carbonyl will experience an electron-withdrawing effect which will push their chemical shift toward higher ppm values than for the unaffected methylene groups. This is seen in Figure 5 and Table 2 where peak d (δ = 1.62) is slightly downfield of the main methylene peak (δ = 1.32). Peak d therefore represents protons on the βcarbon with respect to the carbonyl. Through electrostatic adsorption, an acid headgroup is rigidly bound to the surface (immobile end) and the alkyl tail will point away from the surface (mobile end). The spin−lattice (T1) relaxation time was determined to be 3.35 and 3.37 s for the methylene and methyl species on the calcite surface, respectively (Table 2). These relatively long T1 relaxation times suggest a mobile environment for the alkyl chain with low surface relaxation. The immobile end of an acid molecule bound to a positively charged surface through electrostatic adsorption will most likely consist of the COOH group and the α-carbon methylene group. Signals from low mobility protons on the α-carbon with respect to the carbonyl will be greatly broadened, and if adjacent to other intense signals, it may not appear in the solid state 1H NMR spectra due to overlapping resonances. Signals from α-protons should appear at approximately 2.2 ppm.15 However, Figure 5 shows that the chemical shift labeled peak e is adjacent to very intense methylene signals (peak c) and is not visible. Hence, the immobilization of the αprotons demonstrates a “head-on” arrangement due to electrostatic adsorption. Peak f appears at 3.91 ppm, and this chemical shift indicates that it originates from protons in oxygen adjacent methyl or methylene groups22 most likely on an alkyl chain branch. The integral of the water signal (peak g) is 0.85 relative to peak b, suggesting that water is present as a minority species relative to the crude oil adsorbate. The T1 relaxation time recorded for the water was 1.37 s, which is significantly longer than for the water on the quartz surface (88 ms). Spin−lattice relaxation is most efficient when the correlation time τc is such that the molecular tumbling rate equals the Larmor frequency. Therefore, the T1 relaxation time will increase as the correlation time τc both increases and decreases, resulting in a U-plot.23,24 A T1 value of 1.37 is therefore not sufficient information to make conclusions regarding the calcite water environment.

quartz surface post aging. Both surfaces are discussed individually below, starting with calcite. 3.2.1. Calcite Surface. Figure 3 shows that the peak just below 0.17 ppm in the liquid state 1H NMR spectrum of the supernatant from the calcite surface aging process was less intense than the corresponding signal in the reference crude oil sample. The low ppm value of this signal indicates protons in methyl groups adjacent to strongly electron donating atoms. The crude oil originates from a sand stone reservoir (SiO2), and peak a (δ = 0.17) in Figure 5 is therefore thought to originate

Figure 5. 1H NMR MAS spectra (10 kHz) of crude oil adsorbate on calcite surface in full view (top) and enlarged view (bottom).

from protons in silicon adjacent methyl groups present in adsorbed silicon derivatives. Peak a is barely observable in Figure 5 due to the low surface concentration of this species and high presence of other adsorbates. The aging procedure has covered the surface of the calcite crystals with compounds containing alkyl chains. This is evident from Figure 5 where the chemical shifts of peak b (δ = 0.92 ppm) and c (δ = 1.32 ppm) suggest that they originate from protons in methyl and methylene groups, respectively. Table 2 shows that the integral for the methylene peak (peak c) is 1.62 relative to the integral of the methyl peak (peak b) on the calcite surface, which suggests a short alkyl chain. As the Grane crude oil sample is considered to be a high acid crude and the calcite surface is positively charged at neutral

Table 2. Identified Species, 1H Solid State NMR Chemical Shifts (δ), Spin−Lattice Relaxation Times (T1), and Integrals Relative to Peak b (∫ ) for Peaks a−j on Calcite and Quartz Surfaces

a

peak

species

δa [ppm]

T1a,c [s]

∫ a (±5%)

a b c d e f g′ g h i j

Si-methyl methyl methylene β-protons α-protons

0.92 1.32 1.62

3.37 3.35

1.00 1.62 0.120

3.91 water aromatic aromatic aromatic

4.70 7.36 7.64 8.53

1.37

0.850

δb [ppm]

T1b,c [s]

∫ b (±5%)

0.17 0.96 1.36 1.69 2.21 3.94 4.24 4.70 7.46 7.68 8.76

1.50 0.382 0.356

0.810 1.00 1.60 0.570 0.350 0.110

0.103 0.163 0.088

39.9 0.0799 0.152 0.0343

Adsorbate on calcite. bAdsorbate on quartz. cT1 at 298 K. D

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Previous studies have investigated the effect of crude oil TAN on the wettability of carbonate.26,27 They found correlations between TAN and wetting states, which indicates that acid adsorption is the primary mechanism responsible for altering the wetting state of carbonates. The solid state spectrum shown in Figure 5 confirms this by demonstrating bound acid on the surface. It is however, interesting to note the small peaks around 8 ppm (peaks h−j). These coincide with resonances expected for asphaltenes.28 However, Austad and Standnes26 found no direct effect of crude oil asphaltene content on wetting states for carbonates. An early study29 suggested that basic nitrogeneous surfactants affect the wetting state of limestone. However, such surfactants would produce a resonance at ppm values between peaks e and f in Figure 5, which is not observed here. The 1H−1H DQSQ spectrum presented in Figure 7 shows a significant short-range correlation between peak b (methyl) and

The spin−spin (T2) relaxation time should theoretically not experience a local minimum where the tumbling rate equals the Larmor frequency. Spin−spin relaxation time decreases with an increase in correlation time τc.23,24 The T2 measurements performed on the second calcite sample was accompanied by a freeze study on static samples. Only liquid water will appear in the NMR spectra as ice has a very short T 2 value (approximately 5 μs).25 Figure 6A shows almost identical

Figure 6. 1H NMR spectra (static sample) of crude oil adsorbate on the surface of the second calcite sample at decreasing temperatures (A). NMR signal development with decreasing temperatures for component 1 (water), component 2 (acid headgroup adjacent species), and component 3 (aliphatic tail species).

Figure 7. 1H−1H DQSQ MAS spectrum (10 kHz) of crude oil adsorbate on the calcite surface. F1 = 1H double-quantum frequency. F2 = 1H single-quantum frequency. Each 1H−1H coupling generates signals symmetrically disposed with respect to the diagonal (indicated here with a horizontal red line).30

spectra from 293 to 233 K, suggesting no frozen water. A threecomponent model fit best described the spin-echo (CPMG) results. The T2 relaxation times for components 1, 2, and 3 were determined to be approximately 1 ms (signal intensity ≈26 [arb unit]), 45 ms (signal intensity ≈7.5 [arb unit]), and 1500 ms (signal intensity ≈ 1 [arb unit]), respectively (see Figure 6B and Table 3). Component 1 represents the water, and the short T2 relaxation time (1 ms) and the T1 relaxation time of 1.37 s suggest that the water on the calcite surface experiences an ordered (long τc) environment.23,24 From Figure 6B, it is evident that the individual components contributions to the total NMR signal remain constant through the entire range of temperatures.

peak c (methylene), which further supports the assignment of peaks b and c. The high intensity of the short-range correlation between peaks b and c puts the rest of possible correlations that may exist in Figure 7 into the overlapping base level of peaks b and c. 3.2.2. Quartz Surface. The quartz surface is negatively charged at neutral pH, and one would expect the crude oil adsorbate to consist of organic nitrogenous bases.11,12 Instead, Figure 8 shows a very similar adsorption pattern to the one seen for the positively charged calcite surface (Figure 5). However, it is also evident from Figure 8 that the aging procedure has resulted in a significantly lower crude oil

Table 3. Temperature (T), Spin−Spin Relaxation Time (T2), Signal Fraction (F), and Signal Intensity (I) for Component 1 (Surface Water), 2 (Acid Headgroup Adjacent Species), and 3 (Aliphatic Tail Species) Resulting from a Three-Component Model Fitting Analysis of the CPMG Results from the Second Calcite Sample T [K]

T2(1) [ms]

F(1)

I(1)

T2(2) [ms]

F(2)

I(2)

T2(3) [ms]

F(3)

I(3)

I(Tot)

293 273 263 253 243 233

2 1 2 2 1 2

0.74 0.74 0.75 0.75 0.78 0.77

27 26 25 26 28 26

52 43 46 46 38 37

0.22 0.22 0.22 0.22 0.20 0.21

8 8 8 8 7 7

2791 2367 1122 1448 1495 3850

0.04 0.04 0.03 0.03 0.02 0.02

2 1 1 1 1 1

37 35 34 35 36 34

E

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charged surfaces. Peaks h−j appear around 8 ppm and represent aromatic resonances possibly from adsorbed asphaltenes.28 The fact that the resonance from the α-protons (peak e) appears as a relatively narrow peak on the quartz surface (Figure 8) suggests that the alkyl chains are not bound to the surface in a specific manner, as seen for the calcite surface. Since there will not be a significant electrostatic attraction between the negatively charged quartz surface and the negatively charged carboxylic acid headgroup, a possible explanation could be that the adsorbed species can be found in the water film coating the particles. This would also explain the low abundance of alkyl species on the surface compared to the calcite surface. The found T1 relaxation time of the water protons is 88 ms (Table 2), indicating that the water signal originates from an environment in rapid exchange with water at the quartz surface. The recorded T1 relaxation times for the methylene and methyl protons at the quartz surface are approximately 1/10 of the values from the corresponding species measured on the calcite surface (Table 2). Short T1 relaxation times demonstrate surface access for the alkyl chains which indicates that the alkyl chain is not pointing away from the surface, but rather occupies an arbitrary position with respect to the surface. Without a significant electrostatic attraction, the acid species abundance at the quartz surface is low. The acid species will therefore be more scattered at the quartz surface compared to at the calcite surface where they will be neatly packed in a “head-on” arrangement. Also the T2 measurements performed on the second quartz sample were accompanied by a freeze study. Figure 9A shows a significant drop in the water signal from 253 to 243 K,

Figure 8. 1H MAS NMR spectra (10 kHz) of crude oil adsorbate on quartz surface. Full view (top) and enlarged view (bottom).

adsorbate/water ratio on the quartz surface compared to the calcite surface seen in Figure 5. In Figure 8, the peak at 0.17 ppm (peak a) most likely originates from strongly shielded protons in Si-bound methyl groups. As shown in Figure 3, strongly shielded protons have adsorbed onto both the calcite and the quartz surfaces during crude oil aging. Thus, peak a is not assumed to originate from a reaction between the Si sites on the quartz surface and alkyl chains in the crude oil. It most likely originates from an adsorbed silicon derivative, originally present in the crude oil. The crude oil is from a sandstone reservoir (SiO2) and most likely contains Si derivatives. For the quartz surface (Figure 8), the integral of resonance a relative to resonance b is significantly higher than for the calcite surface (Figure 5). This suggests that peak a is independent of peak b and thus also of the alkyl chain. This is supported by the fact that the recorded spin−lattice relaxation time (T1) for peak a (1.50 s) is greater than the most mobile alkyl chain protons (0.382 s) by a factor of more than 4 (Table 2). The resonances at 0.96 ppm (peak b) and 1.36 ppm (peak c) originate from methyl and methylene protons, respectively. Table 2 shows that the integral of the methylene resonance (peak c) is 1.60 relative to the integral of the methyl resonance (peak b) on the quartz surface. Thus, the methylene/methyl ratio of the crude oil adsorbate on the quartz surface is the same as for the calcite surface. Equal methylene/methyl ratio indicates that both surfaces are impregnated with the same alkyl chain and thus the same carboxylic acid species. Peak d at 1.69 ppm appears slightly downfield of peak c, and these two signals mutually overlap, suggesting that peak d originates from methylene protons close enough to the carbonyl to experience an electron-withdrawing effect. Such an effect may be observable for protons on the β-carbon with respect to the carbonyl. Resonance e appears at 2.21 ppm, a chemical shift typical for protons on the α-carbon with respect to the carbonyl in carboxylic acids.15 Figure 8 additionally shows peaks f−j. Peak f appears at 3.94 ppm, which is consistent with protons on an oxygen-adjacent carbon;22 however, this is just conjecture. Peak g is the water signal, and it originates from the pretreatment of the surface in a desiccator with a saturated K2SO4 solution. The integral of peak g is 39.9 relative to the integral of the methyl peak (peak b). This is due to the less effective acid adsorption on negatively

Figure 9. 1H NMR spectra (static sample) of crude oil adsorbate on the surface of the second quartz sample at decreasing temperatures (A). NMR signal development with decreasing temperatures for component 1 (surface water), component 2 (nonsurface water), and component 3 (acid species). F

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Table 4. Temperature (T), Spin−Spin Relaxation Time (T2), Signal Fraction (F), and Signal Intensity (I) for Component 1 (Surface Water), 2 (Non Surface Water), and 3 (Acid Species) Resulting from a Three-Component Model Fitting Analysis of the CPMG Results from the Second Quartz Sample T [K]

T2(1) [ms]

F(1)

I(1)

T2(2) [ms]

F(2)

I(2)

T2(3) [ms]

F(3)

I(3)

I(Tot)

293 273 263 253 243 233

12 16 12 8 5 5

0.14 0.17 0.10 0.08 0.44 0.39

140 192 133 95 115 97

59 52 59 56 74 79

0.78 0.76 0.81 0.83 0.27 0.3

797 864 1024 1051 71 73

369 423 394 377 313 332

0.08 0.07 0.09 0.09 0.29 0.31

80 84 108 119 73 76

1019 1145 1262 1260 266 253

suggesting freezing of water. A three-component model fit best described the CPMG results. The T2 relaxation times for component 1, 2, and 3 was determined to be approximately 14 ms (signal intensity ≈140 [arb unit]), 55 ms (signal intensity ≈930 [arb unit]), and 350 ms (signal intensity ≈100 [arb unit]), respectively, before freezing (see Table 4). From Figure 9B, it is evident that the contribution from component 2 to the total NMR signal significantly drops from 253 to 243 K. Component 2 therefore represents nonsurface water available for freezing. Figure 9A shows that water is the dominant species on the second quartz sample. The short T2 value of approximately 10 ms (Table 4) found for component 1 suggests an ordered water environment (long τ c ) such as near the surface. 23,24 Component 3 has a T2 value of approximately 360 ms and therefore represents the indirectly adsorbed acid species. In Figure 10, the DQSQ spectrum shows a carboxylic acid pattern. Short-range correlations appear between methyl and

4. CONCLUSION Solid state MAS NMR spectroscopy was found to be a useful analytical tool for directly observing molecules adsorbed onto surfaces. This study yielded the following observations regarding wet crude oil aging: 1. Acid adsorption is despite minor contributions from asphaltenes, the main driving force in wetting state alteration for both the calcite and quartz surface. For the calcite surface the acid species are electrostatically adsorbed and exhibit a “head-on” arrangement, while for the quartz surface the acids are present in the water film and exhibit no specific arrangement in reference to the surface. 2. Post wet crude oil aging, water remains on both surfaces. The amount of water left on the calcite surface corresponds to 28% of the amount of water left on the quartz surface. The quartz surface exhibits two water environments where the outer is subject to freezing at temperatures below 243 K. The calcite surface exhibits only one water environment, which is located at the surface and remains in liquid form for temperatures down to 233 K. 3. For DQSQ coherence spectra to be a valuable tool in assignment of adsorbates, the resonances from the entire molecule must be resolved. This is not the case for electrostatically adsorbed acids where immobilization greatly broadens the α-proton resonance.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.N.S.). ORCID

Henrik N. Sørgård: 0000-0003-4772-0614 Christian Totland: 0000-0002-1457-9012 Figure 10. 1H−1H DQSQ MAS spectrum (10 kHz) of crude oil adsorbate on quartz surface. F1 = 1H double-quantum frequency. F2 = 1 H single-quantum frequency. Each 1H−1H coupling generates signals symmetrically disposed with respect to the diagonal (indicated here with horizontal red lines).30

Notes

methylene groups (b and c, respectively), between methylene groups and β-protons (c and d, respectively), and between βprotons and α-protons (d and e, respectively). In addition to the main alkyl chain, the DQSQ spectrum suggests possible branching on the β-carbon (peak d) as there is a short-range correlation between peaks d and g′. Peak g′ is fully overlapped by the water signal in the solid state 1H NMR spectrum (Figure 8).

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge The Research Council of Norway for support through the Norwegian NMR Platform, NNP (226244/F50), and Statoil ASA for funding the project “Wetting in porous media” through AKADEMIA. REFERENCES

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DOI: 10.1021/acs.jpcc.7b07125 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.7b07125 J. Phys. Chem. C XXXX, XXX, XXX−XXX