Adsorption of Crude Oil on Na+-Montmorillonite

Energy & Fuels 2005, 19, 1417-1424

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Adsorption of Crude Oil on Na+-Montmorillonite† A. Cosultchi,*,‡ I. Cordova,§ M. A. Valenzuela,§ D. R. Acosta,| P. Bosch,¶ and V. H. Lara⊥ Instituto Mexicano del Petro´ leo, 152 Eje Central “L. Ca´ rdenas”, 07730 Me´ xico D. F., Mexico, Laboratorio de Cata´ lisis y Materiales, ESIQIE-IPN, 07738 Me´ xico D. F., Mexico, Instituto de Fı´sica, UNAM, A. P. 20-364, 01000, Me´ xico D. F., Instituto de Investigaciones en Materiales, UNAM, 04510 Me´ xico D. F., Mexico, and UAM-Iztapalapa, Michoaca´ n y La Purı´sima, 09340 Me´ xico D. F., Me´ xico Received July 20, 2004. Revised Manuscript Received April 8, 2005

The interaction between a natural Na+-montmorillonite and a crude oil sample containing 2.5 wt % of asphaltene and 6.4 wt % of polar species is studied as a function of time. The microscopic and the macroscopic mechanisms of diffusion are examined and discussed. At a microscopic scale, diffusion of crude oil within the clay interlayer space is a slow and a continuous process with the formation of O/W microemulsions within the d(001) space, which involves those crude oil polar molecules with tensoactive properties, interlayer water molecules, and the clay Na+ interlayer cations. At a macroscopic scale, the diffusion of the organic species is also a slow but stepped process. At the beginning, such a process is controlled by a concentration gradient (the Fick law) of crude oil species which diffuses into clay pellet, and it is followed by a viscous flow-type diffusion, when the compacted clay mineral pellet is surrounded by a thick O/W emulsion of crude oil, clay colloids, and water expulsed from the interlayer space.

Introduction Generally, when a reservoir contains clay minerals, and especially those with swelling properties, formation damage may occur around the well bore as a consequence of improper stimulation operations. Clay osmotic swelling has been observed by petrographic analyses: in most reservoir rocks, a thin layer of clay coats the quartz grains along the rock microfractures.1 This process is also related to the interaction of colloidal clay suspensions with crude oil and especially with asphaltene and resins, the heaviest and nonvolatile petroleum fractions.2,3 Clay minerals, such as montmorillonite, have been found by XPS mixed in the asphaltenic deposit formed on the surface of tubing steel.4 Clay minerals are materials with a layered structure; they contain mostly aluminum, silicon, and small amounts of other elements such as iron, sodium, magnesium, calcium, potassium, and so forth.5,6 Smectite is a 2:1 layer type clay which consists of two silicon tetrahedral sheets and one aluminum octahedral sheet; the tetrahedral sheets are interconnected with the octa† Presented at the 5th International Conference on Petroleum Phase Behavior and Fouling. * To whom correspondence should be addressed. E-mail: [email protected]. ‡ Instituto Mexicano del Petro ´ leo. § ESIQIE-IPN. | Instituto de Fı´sica, UNAM. ¶ Instituto de Investigaciones en Materiales, UNAM. ⊥ UAM-Iztapalapa. (1) Hill, H. J.; Milburn, J. D. SPE Repr. Ser. 2003, 55, 31-38. (2) Kaminsky, R.; Radke,C. J. SPE J. 1997, 2, 485-493. (Paper No. 39087.) (3) Freer, E. M.; Sitova, T.; Radke, C. J. J. Pet. Sci. Eng. 2003, 39, 137-158. (4) Cosultchi, A.; Rossbach, P.; Hernandez-Caldero´n, I. Surf. Interface Anal. 2003, 35 (3), 329.

hedral sheet by shearing their apical oxygen atoms. Layers normally have negative charge because of the substitution of cations either in tetrahedral or in octahedral sites; this charge must be balanced by the presence of extra positive charge, the interlayer cations, situated between the layers. In addition, there is only one type of structural hydroxyl in the 2:1 structure, the one situated in the plane of oxygen atoms shared by adjacent tetrahedral and octahedral sheets. With a predominantly octahedral charge, Wyoming montmorillonite, Na0.33(Al1.67Si3.67)O10(OH)2‚nH2O, contains Na as interlayer cations and a variable amount of water coordinating the interlayer cations; the interlaminary cations can be surrounded by one, two, or more strata of H2O molecules. Water found in the interlayer space, which is a small part of the liquid-solid system, depends on the relative humidity and the type of interlayer cation; moreover, montmorillonite presents a large contact area. Petroleum contains mostly hydrocarbons such as paraffin and aromatic molecules and a smaller fraction of polar compounds, which in addition to carbon and hydrogen atoms have either oxygen, nitrogen, or sulfur atoms in their molecular composition. Polar compounds, especially those containing nitrogen, present a high affinity toward clay mineral surface, as shown by geochemical studies.1,7 Asphaltene, formed within the reservoir as a consequence of variation of thermodynamic conditions and (5) Crystal Structure of Clay Minerals and their X-ray Identification; Brindley, G. W., Brown, G., Eds.; Mineralogical Society: London, 1980. (6) Giese, R. F.; van Oss, C. J., Eds. Colloid and surface properties of clays and related minerals; Surfactant Science Series; Vol. 105; Marcel Dekker: New York, 2002. (7) Li, M.; Larter, S. R.; Frolow, Y. B. J. High Resolut. Chromatogr. 1994, 17, 230-236.

10.1021/ef049825a CCC: $30.25 © 2005 American Chemical Society Published on Web 05/18/2005

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which has a different composition compared to that precipitated in laboratory, is supposed to be responsible for the natural wettability alteration of the formation occurring during petroleum exploitation.3,8,9 On the contrary, the asphaltenes prepared in laboratory cannot penetrate into the clay interlayers, and the adsorption of such fraction only occurs on the edges of the clay sheets or on their external surfaces.10 Other authors have shown that the diffusion of most polar compounds through a thin aqueous film and toward the mineral surface is possible, but it is very slow process;1,3 however, under geological conditions, such diffusion is supposed to be no longer a restrictive step.7 The retention of those organic molecules may alter some surface properties of the mineral matrix such as wettability.11-16 Thus, the organic uptake by clay minerals depends on the nature of the alumino-silicate surfaces, the type of organic molecules, and the thermodynamic conditions. The interlayer water molecules are involved in a complex arrangement of hydrogen bonds either with adjacent water molecules or with the oxygen atoms of the silicate surface; water molecules also coordinate to interlayer cations.5,17,18 Organic molecules may attach to the clay surface sites and also interact with the interlayer water molecules. The wide variety of possible interactions between the clay surface, water, or organic functionalities may also affect the clay structure and integrity.17-21 The purpose of this paper is to evaluate how crude oil organic molecules affect the montorillonite clay structure. The following questions arise: Are crude oil compounds intercalated within the clay interlayer space? Do such organic species affect the clay structure and the bonds of interlayer water with the siloxane surface? How important is this structural modification? How strong are the interactions formed between organic species, water, and clay? To answer these questions, impregnation experiments were conducted at room temperature and atmospheric pressure, using a nonmodified Na+-montmorillonite clay and a centrifuged crude oil. The two species were kept in contact for 216 h and observations were carried out on extracted samples at fixed intervals. The structural changes of the clay matrix impregnated with crude oil (8) Crocker, M. E.; Marchin, L. M. JPT, J. Pet. Technol. 1988, (April), 470-474. (9) Piro, G.; Canonico, L. B.; Galbarigi, G. SPE Prod. Facil. 1996, (August), 156-160. (10) Pernyeszi, T.; Paztko, A.; Berkesi, O.; Dekany. I. Colloids Surf., A: Phys. Eng. Asp. 1998, 137, 373. (11) Mercier, F.; Toulhoat, N.; Potoccek, V.; Trocellier, P. Nucl. Instrum. Methods Phys. Res., Sect. B 1999, 152 (1), 122-128. (12) Pillon, P.; Monrozier, L. J.; Gonzalez, C.; Saliot, A. Org. Geochem. 1985, 10 (4-6), 711-716. (13) Johnston, C. T.; Fernandes de Oliveira, M.; Teppen, B. J.; Sheng, G.; Boyd S. A. Environ. Sci. Technol. 2001, 35, 4767-4772. (14) Lo, I. M. C.; Yang, X. Environ. Sci. Technol. 2001, 35, 620625. (15) Barrer, R. M. Zeolite and Clay Minerals as Sorbents and Molecular Sieves; Academic Press: London, 1978. (16) Hundal, L. S.; Thomson, M. L.; Laird, D. A.; Carmo, A. M. Environ. Sci. Technol. 2001, 35, 3456-3461. (17) Yan, L.; Roth, C. B.; Low, P. F. Langmuir 1996, 12, 4421-4429. (18) Yan, L.; Stucki, J. W. Langmuir 1999, 15, 4648-4657. (19) Sposito, G.; Prost, R. Chem. Rev. 1982, 82 (6), 554. (20) Madejova, J. Vib. Spectrosc. 2003, 31, 1. (21) Johnston, C. T.; Premachandra, G. S. Langmuir 2001, 17 (12), 3712.

Cosultchi et al. Table 1. HPLC Analysis of CH-3 Crude Oil

hydrocarbon type

as aromatic (wt %)

as alkyl chain (wt %)

total (wt %)

relative to crude oil (wt %)

saturate one-ring aromatics two-ring aromatics three-ring aromatics four-ring aromatics polar compounds

1.9 3.1 1.7 1.5 3.5

28.1 20.3 8.3 6.3 9.4 15.9

28.1 22.2 11.2 8.0 11.1 19.4

9.27 7.33 3.70 2.64 3.66 6.40

11.7

88.3

partial wt %

100

total wt % recovered

33.00

were followed by X-ray diffraction, thermal analysis, infrared spectroscopy, and high-resolution transmission electron microscopy techniques. Experimental Procedures Materials. A Na+-montmorillonite Wyoming-type clay mineral from Reade Advanced Materials was used as received as the starting host for the organic intercalation. The crude oil sample, collected from a Mexican petroleum well, was centrifuged at 2000 rpm for 30 min to eliminate the naturally suspended solids (sediment) prior to its use. Sample Preparation and Characterization Techniques. The elemental composition (carbon, hydrogen, and nitrogen) of a Mexican crude oil (CH-3) was obtained in an Elemental Vario EL and in a Leco SC-444 for sulfur content. Oxygen content was obtained by means of 100% mass balance. Paraffin, aromatic, and polar fractions of the soluble part of the crude oil sample were determined with a high-performance liquid chromatography (HPLC) Hewlett-Packard 1100 with diode detector, Table 1. The montmorillonite clay swelling in the presence of crude oil was evaluated following two experiments: (a) a linear swelling test, which measures the volume variation when a compacted clay sample is surrounded by the tested fluid at room temperature, and (b) an impregnation experiment, where the in situ variation of the clay interplanar distance d(001) was measured by X-ray diffraction (XRD). For the linear swelling test, a pellet of 1 g (20-mm high and 20-mm diameter) was prepared and contacted with the crude oil at room temperature for 100 hour. The vertical displacement was measured until no volume variation was observed. The linear swelling test (OFI Testing Equipment, Inc.) normally is used for drilling mud formula evaluation. The X-ray diffraction patterns of the impregnated clay sample were obtained with a D500 diffractometer (from Siemens) coupled to an X-ray tube with copper anode. A diffracted beam monochromator selected the Cu KR radiation. A sample of montmorillonite of approximately 0.5 g was put in contact with an excess of crude oil and was maintained with the organic phase in sealed amber bottles during the experiment time. Small portions of the sample were extracted from the recipient each 24 h and measured by XRD. Thermal gravimetric analysis (TGA) was used to quantify the water content of the natural clay and the absorbed amount of organic material after contact with crude oil. The sample was heated from room temperature up to 750 °C at a rate of 10 °C/min. The phase transitions of the clay and clay-organic complex formed by impregnation were obtained by differential thermal analysis (DTA). For TGA, a Perkin-Elmer TGA-7 was used according to the ASTM E-1131 standard, while DTA was performed in a Perkin-Elmer DTA-1700 apparatus. For the clay impregnated sample, the thermal desorption process was followed by FTIR, and the spectra were collected from room temperature to 340 °C. The FTIR spectra in transmission mode of all samples were registered with a Nicolet Nexus 470 spectrometer, after 32 scans at 4 cm-1

Adsorption of Crude Oil on Na+-Montmorillonite

Energy & Fuels, Vol. 19, No. 4, 2005 1419

resolution. All samples were prepared using KBr pellets with a sample concentration of about 1 wt %. For the untreated clay sample, the thermal desorption process was followed in the DRIFT chamber; the spectra were collected at different temperatures from room temperature to 120 °C. The DRIFT spectra were obtained using the same spectrometer equipped with an integration sphere Spectra Tech Collector II. The radiation that reflects from an absorbing material is composed by surface-reflected and bulk re-emitted components, which when summed up give the diffuse reflectance of the sample. The crystalline parameters of the samples were obtained by transmission electron microscopy in high-resolution electron microscopy (HREM) mode. The powder samples were ground softly in an agate mortar and were dispersed in isopropylic alcohol in an ultrasonic bath for several minutes. Some drops from a fine pipet were deposited on a 200 mesh copper grid covered with a holey carbon film. The samples, once mounted in the electron microscope, were keep overnight in highvacuum conditions within the microscope column. The observations were carried out in a JEOL FEG 2010 electron microscope working in high-resolution mode. The micrographs presented in this work were recorded with a digital camera system attached to the microscope, and the measurements were made with the assistance of a commercial computing program directly on the details of the lattice array configuration. Diffraction patterns obtained from the Fourier transforms of the corresponding HREM images also were used for the measurements of the lattice parameters.

Results and Discussion The CH-3 crude oil used in this study has 32.9 API° and a molecular weight of 1800. The corresponding H/C atomic ratio is 1.82. The sample contains 2.5 wt % of asphaltene separated with n-pentane. The HPLC results indicate the following composition: 9.27 wt % of saturates, 17.33 wt % of aromatic compounds with 1-4 rings in their structures, 6.4 wt % of polar compounds, and 67 wt % of heavier material not detected by this technique. Most aromatic rings have alkyl side chains, and most polar compounds are linear. Acidity, determined as milligram of KOH per gram of crude oil, is 0.10. Linear Swelling Test Results. Figure 1 shows the linear swelling test results. The adsorption process occurs in two steps: a strong uptake of organic material occurs within the first 2 h of contact, and it is followed by a sort plateau. Then follows a slow uptake period until, after 50 h of contact, a plateau is reached and the adsorption process stops. The observation of the pellet at the end of the experiment indicates that only a small volume of the pellet was impregnated with crude oil, the core of the pellet is observed free of crude oil, while the borders crumbled. Hence, it is evident that only the outward part of the pellet was accessible to the fluid. An equation such as y ) A1 - A2 × e-kt is one that better describes any of the two parts of the linear swelling curve. For the I-section of the swelling curve,

yI )1.20 - 1.188e-0.783t (0 < t < 24 h)

(1)

For the II-section of the swelling curve,

yII ) 1.50 - 185.0e-0.268t (24 < t < 100 h)

(2)

Figure 1. Na-montmorillonite uptake of crude oil as volume variation measured following a linear swelling test, following the contact time. The two parts of the swelling curve are separated by a dotted line.

where y is the linear swelling in % of volume and t is the time of contact. XRD Results. The XRD diffractograms measured each 24 h (Figure 2a) show the evolution of the (001) interplanar distance; the displacement toward lower 2θ positions indicates the increase of the d(001) interlayer space, as shown in Figure 2b. The process does not stop after 96 h. If the diffractograms corresponding to 192 and 216 h are examined, it can be seen that the peak fades out while the baseline is modified; thus, the clay layers are no more ordered, which means that the clay is delaminated. For the d(001) spacing variation, the equation that fits the experimental data could be described by an exponential-growth equation: y ) A1 + A2 × ex/k as follows:

yd(001) (nm) ) 124.8 + 0.0434et/21.73

(3)

where y is the d(001)-spacing and t the time of contact. Thus, if a linear test is applied to evaluate the impregnation of a clay pellet with crude oil, a macroscopic adsorption mechanism is obtained, while if X-ray diffraction is used to evaluate the variation of the clay interlayer space, a molecular adsorption mechanism is obtained. Thermal Analysis Results. TGA and DTA results are presented in Table 2. When heated in air, from room temperature up to 1000 °C, the untreated montmorillonite sample shows three endothermic reactions followed by an exothermic reaction, as expected. However, more than three endothermic events were registered when the sample is impregnated with crude oil and aged for 216 h. When this type of clay contains less than 50 wt % of water, a unique endothermal event is registered between 70 and 250 °C; however, when more water is

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Figure 2. (a) Time sequence X-ray diffraction profiles for crude oil uptake on montmorillonite clay. (b) Evolution of d(001) spacing during the impregnation process up to 100 h of contact.

adsorbed by the clay, this peak splits and another peak could be observed at a lower temperature.22 However, in the present cases, three extra thermal events appear at a higher temperature (at 220, 361, and 414 °C) as shown in Table 2, associated, then, to the adsorbed organic species. When clay is impregnated with crude oil, it is worth to note the absence of an exothermic event; this means that the clay platelets are protected against decomposition by an organic layer that provides them thermal stability up to 1000 °C.

The TGA results indicate that, when the clay is impregnated with crude oil, the weight loss is a multistep process instead of the four events observed in the untreated sample thermogram. The total amount of mass loss is higher for the clay sample impregnated with crude oil than for the untreated sample: seven in the first case versus four in the last one. For the untreated clay, the amount of water loss within the range 26-99 °C, the dehydration event, is 7.2 wt % and corresponds, approximately, to a single layer of water molecules in the clay interlaminar space. After clay is impregnated with crude oil, the amount of material eliminated as the first thermal event is only 4.2 wt %; however, a second weight-loss thermal event of similar amount, 4.81 wt %, is observed within a higher temperature interval (90-155 °C). DRIFTS Results. The position of the hydroxyl vibration bands corresponds to the IR characteristic patterns of clay minerals as shown by Farmer;23 thus, the positions and intensities of the bands observed at 3620 and 3445 cm-1 correspond to Na+-montmorillonite clay. As mentioned in the Introduction, the clay sample for the experiments was used as received without any previous treatment; thus, any shift or intensity variation of the bands could only be attributed to the interaction of the organic species with the clay surface. For untreated Na-montmorillonite, Figure 3 shows the DRIFT spectra region (3800-2800 cm-1) of the ν1 hydroxyl bands and the region (2000-1500 cm-1) of the ν2 (OH) vibration bands, from room temperature up to 120 °C. As the ν1 hydroxyl bands at 3445 and 3220 cm-1 are wide, it is difficult to follow the decrease of their intensities; thus, the dehydration thermal process can be studied following the variation of the intensity of the band at 1635 cm-1 corresponding to ν2 (H-O-H) bending vibrations of free water.

(22) Hendix, S. B.; Nelson, R. A.; Alexander, L. T. J. Am. Chem. Soc. 1940, 62, 1457-1463.

(23) Farmer, V. C. Mineralogical Society. Monograph 4. The Infrared Spectra of Minerals; Mineralogical Society: London, 1974.

Table 2. DTA and TGA Results for Untreated and Impregnated Montmorillonite Sample Untreated Montmorillonite Clay

Montmorillonite Clay Impregnated with Crude Oil DTA Resultsa

temperature interval (°C)

peak position

temperature interval (°C)

peak position

70-170

79.4

75-150 220-330 340-395 395-470 470-520 560-710

80.7 220.5 361.4 414.3 473.7 600.7

490-550 550-730 900-980*

503.4 623.4 913.3*

TGA results temperature interval(°C)

weight loss (wt %)

temperature interval(°C)

weight loss (wt %)

26-99 99-250

7.2 0.8

250-520

0.74

520-730

3.6

25-90 90-155 155-338 338-435 435-570 570-705 705-750

4.20 4.81 13.12 2.17 3.29 3.11 0.54

total weight lost:

12.4

total weight lost:

31.3

a

All transitions are endothermal except for that marked with an asterisk (*), which is exothermal.


Figure 3. DRIFT spectra of untreated Na-montmorillonite following a desorption process from room temperature to 120 °C. The following regions are shown: (a) 3800-2600 cm-1 and (b) 2000-1500 cm-1.

Two bands are observed in the range 1700-1600 cm-1: one at 1635 cm-1 and another at 1698 cm-1; the first one corresponds to the H-O-H bending vibrations, the ν2 mode, and differs from the vibration corresponding to bulk water at 1643.5 cm-1. The second band, observed at 1698 cm-1, may be related to water coordinated to the interlayer cations, as Farmer reported for similar compounds.23 As observed in Figure 3b, the intensity of the band at 1635 cm-1 diminishes as the sample is heated to 120 °C; however, as expected, it is not fully eliminated.18 FTIR Results. Figure 4 shows the sequence of IR spectra registered in transmission mode for the impregnated clay sample also following a thermal desorption process from room temperature up to 340 °C. Figure 4a shows the region from 3800 to 2800 cm-1 which includes the stretching vibrations of hydroxyl as well as those of methyl and methylene groups. Figure 4b shows the region from 1800 to 1500 cm-1, which contains the bending vibrations of H-O-H and those methyl and methylene groups. Consequently, the nature of the material eliminated could be identified and correlated to the weight-loss TGA events occurring within the following intervals: 25-90, 90-155, and 155-338 °C. At room temperature, the intensity of the OH band at 3445 cm-1 is lower compared to the same band in the untreated Na-montmorillonite spectra, although no shift was evident. The strength of hydroxyl bonds between adjacent water molecules reduces if organic molecules are interfering. The H-O-H bending band, ν2, used to evaluate the dehydration of the sample, is observed as an intense and wide band at room temperature, but the intensity diminishes after 80 °C. Hence, the sample apparently dehydrates faster than the untreated clay. The reduction of the intensity of the OH vibration bands at 3445 and 1635 cm-1 is, then, related to the first TGA peak corresponding to clay dehydration from 25 to 90 °C.


The bands corresponding to aliphatic or aromatic compounds at 2923 and 2853 cm-1 show an abrupt reduction of their intensity at 340 °C. This confirms the hydrocarbon combustion and may, then, be related to the third TGA event of mass loss of 13.12 wt %, from 155 to 338 °C. Figure 5 shows the region of the transmission FTIR spectra corresponding to Si-O stretching vibration bands (1200-950 cm-1) and Si-O bending modes (550-450 cm-1). The strong and wide band centered at approximately 1030 cm-1 corresponds to Si-O stretching vibrations. The band consists of four overlapping components: peak II at approximately 1080 cm-1 corresponds to out-of-plane ν(Si-O) vibration, peak I (1100-1150 cm-1) is the in-plane longitudinal mode, and peaks III and IV (940-1065 cm-1) correspond to the in-plane ν(Si-O) vibrations. Si-O stretching vibrations of the montmorillonite layers are coupled to the ν2 (H-O-H) bending vibrations of the interlayer space.18 However, it is well-known that the four Si-O bands are not all sensitive to hydration-dehydration events.20,21 In the presence of water, the bands at 1021 and 1046 cm-1 are not shifted from their original positions when crude oil molecules are adsorbed on clay. Instead, the bands at 1075 and 1117 cm-1, corresponding to clay lattice, are usually perturbed in the presence of water,18-22 as well as in the presence of crude oil. Shifting or broadening of any infrared vibrational absorption bands is due to perturbations of the electronic structure in the vicinity of the vibrating atoms. The band located at 1075 cm-1 corresponds to the Si-O out-of-plane vibration; the intensity and position of this band within the interval 1084-1075 cm-1 are related to Si-O bond orientation in the presence of water molecules; especially, the shift of this band to a higher frequency has been related to the amorphization of silica.20,21 When crude oil is present, this band shifts from its original position at 1078 cm-1 to 1088 cm-1. In addition, the shifting of the 1117 cm-1 band to higher frequencies is observed as a consequence of the thermal process shown in Figure 5 and could be related to clay dehydration process. Transmission Electron Microscopy Results. The study was completed with observations of the local morphology and crystalline configuration using highresolution transmission electron microscopy HREM. Figure 6 shows the micrographs of the untreated Na-montmorillonite and the same sample after being in contact with crude oil for 216 h and reveals the ordered configuration. In Figure 6a, which shows the untreated sample, only the 110 planes are observed. Figure 6b shows the HREM micrographs of Na+montmorillonite after contact with crude oil. The sample grains present regular crystalline configuration surrounded by amorphous material. This type of configuration, crystalline material surrounded by amorphous material, is commonly found along this sample. A closer view of the crystalline phase, observed in the right lower part of the image, shows variations of the (001) interplanar distance, from 1.174 to 1.192 nm, as well as the bending of the crystalline planes, especially close to the contact with the amorphous phase. The layer of the

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Figure 4. FTIR spectra of untreated and impregnated montmorillonite following a desorption process from room temperature to 340 °C. The following regions are shown: (a) 3800-2600 cm-1 and (b) 2000-1300 cm-1.

Figure 5. FTIR spectra of montmorillonite impregnated with crude oil and following a desorption process, from room temperature to 340 °C. The region 1200-500 cm-1 corresponds to Si-O vibration bands.

amorphous material observed on the edge of the clay particle, after contact with crude oil, is about 4-nm thin. Discussion The untreated clay sample used as host for the crude oil corresponds to Na+-montmorillonite clay with a

single water layer within the d(001) space as indicated elsewhere.23,24 Our infrared observations confirmed such identity and agree that a single interstitial water layer determined the obtained d(001) spacing. Most studies considered that the adsorption of crude oil or asphaltene-like species on mineral surfaces is inhibited by the surface aqueous film25 while others point toward a slow diffusion process through this layer.1,2 Moreover, adsorption of crude oil on clay only could occur on the external surfaces10 or as a slow diffusion process followed by adsorption on clay surface of either external or interlayer space.2,3 Such adsorption is also mentioned as a dehydration process26 which involves crude oil polar compounds. Our experiments have shown that when this mineral is put in contact with crude oil, hydrocarbon molecules contend with the already existing water to occupy the interlayer space. Impregnation of montmorillonite with crude oil is, indeed, a slow process. The diffusion of the organic species was carried out at two levels: (a) at a microscopic scale studying the d(001) interlayer space, the infrared vibrations, and the microscopic crystalline configuration and (b) at macroscopic scale through the linear swelling test and thermal data evaluations. (a) The Microscopic Mechanism. Equation 3 describes the uptake of crude oil by montmorillonite up to 100 hours following the evolution of d(001) spacing as a function of time. The variation of the interlayer space is continuous, showing that the process is not selective. Simulation studies already demonstrate that small molecules such as ethane are entering the interlayer space and displaces some of the interlayer water molecules; this interchange increases at the burial conditions.27 This means that the small molecules first enter within the interlayer space and displace part of the (24) Cosultchi, A.; Bosch, P.; Lara, V. H. Colloids Surf., A: Phys. Eng Asp. 2004, 243, 53-61. (25) Kokal, S.; Tang, T.; Schramm, L.; Sayegh, S. Colloids Surf., A: Phys. Eng. Asp. 1995, 94, 253. (26) Magara, K. AAPG Bull. 1975, 59 (2), 292.



Figure 6. HREM micrographs of (a) untreated montmorillonite (only the 110 plane is shown) and (b) the same sample after crude oil contact (the 001 planes are shown).

water molecules already there. These molecules are retained and then the largest ones may be intercalated. If not, the (001) peak in Figure 2a should have shown as double or triple or the shape of the curve d(001) versus time in Figure 2b should have been stepped. Furthermore, as more organic species or larger molecules are incorporated into the interlayer space, the sheets of the clay are separated. The process is apparently slow at the beginning but accelerates at the end. If t, the time of contact, is small, the d(001) calculated from eq 3 tends to be 124.8 nm (12.5 Å), but if t is large enough, d(001) tends to ∞; indeed, the process does not reach a plateau. These remarks agree with the electron microscopy observations of a clay particle aged in the presence of crude oil. Thus, the (001) interplanar distances observed in Figure 6b and the bending of the clay planes indicate a distortion of the clay crystal structure because of the diffusion of the organic species; although, we cannot exclude a marginal effect of the microscope-chamber vacuum on the border structure of the clay crystal. Most probably, the bending of the clay sheets may be attributed to the progressive diffusion of large molecules deep into the clay interlayer space. In a rather similar experiment, Rouquerol28 adsorbed a nonionic surfactant (nonyl-phenol-oxy-ethylene, with 9-10 ethoxy groups) on kaolin in the presence of 1% of NaCl at 40 °C. In this case, only one step was observed in the normal L-shaped adsorption isotherm and was attributed to the displacement of water by the surfactant. Instead, for the same experiment, two steps were registered by microcalorimetry; this second step was attributed to the partial opening and hydration of the kaolin sheets under the action of surfactant and salt. In this trend, minerals with oxygen atoms on the surface, as silicate minerals, are prone to interact with compounds containing polar groups as amine, carbonyl, or carboxyl by means of hydrogen or water bridging in the presence of cations.5 Crude oil contains a small amount of polar molecules (6.4 wt %, most of them with (27) Odriozola, G.; Aguilar, J. F.; Lo´pez-Lemus, J. J. Chem. Phys. 2004, 121 (9), 4266. (28) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by powders and porous solids. Principles, methodology and applications; Academic Press: London, 1999; p 161.

straight chains) which could be natural tensoactive substances with high affinity toward water. Hence, the formation of microemulsions at a nanometric scale is favored either within the interlayer space or out of this space. In addition, the Na+ cations of the montmorillonite interlayer may participate in the stabilization of such microemulsion.29,30 An O/W microemulsion within the interlayer space contributes to the weakening of the water bonds with the siloxane surface. This explains why the OH vibrations are weaker when crude oil is present, as the infrared results show in Figure 4 compared to Figure 3. In addition, the intensities of two of the Si-O stretching vibration bands, at 1075 and 1117 cm-1, indicate that one of the following events could happen: (a) elimination of part of the water molecules or clay dehydration or (b) the interaction of water with the siloxane surface is wakened as new interactions are formed. Through such a mechanism, the beginning of dehydration of clay may be understood. Last but not least, one must be aware of the possible dissolution of the clay and especially the edge of the particles suggested by the shape of the baseline of the XRD patterns at 192 and 216 h, shown in Figure 2a. Briefly, for the microscopic process, miscibility in water of specific polar components of crude oil and the properties of the clay surface are the key factors. (b) The Macroscopic Mechanism. At a macroscopic level, the structure of clay is no more a determinant factor; the process, thus, is reduced to the diffusion of oil into a thick and porous aluminosilicate pellet. The corresponding behavior is described by the stepped curve obtained from the linear swelling test and is presented in Figure 1. Two equations represent such a curve: eq 1 and eq 2. Up to 100 hours of contact, the crude oil diffusion, then, follows two steps. Initially, a fast adsorption step is observed which, after 1.2 wt % of the material was adsorbed, stabilizes to a plateau. After a few hours comes a second diffusion step which enhances the (29) Chattopadhyay, A. K.; Gaicha, L.; Oh, S. G.; Shah, D. O. J. Phys. Chem. 1992, 96, 6509-6513. (30) Palla, B. J.; Shah, D. O. J. Colloids Interface Sci. 2002, 256, 143-152.

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adsorption 0.3 wt % more and also stabilizes to a second plateau. Following the remarks previously presented, most probably the first part of the curve may be attributed to a fast intercalation of organic molecules within the clay. Such intercalation stops during 20 h when a small montmorillonite fraction is delaminated and forms a viscous mixture around the pellet. As a consequence, nonimpregnated surfaces are exposed, diffusion starts again, and the organic molecules follow the same process and saturate the new areas until the mixture of crude oil, water, and clay particles becomes thicker and surrounds the clay pellet and the organic diffusion stops once more. This hypothesis is confirmed by the TGA results in Table 2. When the clay sample is aged with crude oil, a lower amount of water (4.2 wt %) is eliminated within the temperature interval corresponding to clay dehydration, compared to the untreated sample (7.2 wt %). This amount may correspond to the water molecules implicated in the O/W emulsion. The different mechanisms by which diffusion of fluid through a porous media occurs are as follows: (a) molecular or ordinary diffusion in which the molecules of a mixture move relative to each other under the influence of concentration gradient (Fick law), temperature gradient (thermal diffusion), or external forces (forced diffusion); (b) viscous flow in which the collision between molecules dominate over the collision between molecules and pores walls; (c) Knudsen diffusion in which collision between molecules could be ignored compared to the collision of molecules with the wall of the pores; and (d) surface flow in which the molecules move along the surface in an adsorbed layer. Hence, initially, the diffusion of organic molecules into clay is molecular, as molecules move relative to each other under the influence of a concentration gradient (Fick law). Then, the flow becomes viscous, as clay colloids are integrated to the O/W emulsion, and the collisions among molecules become dominant. Consequently, the process stops when the second mechanism

Cosultchi et al.

predominates over the first one and the composition of the surrounded crude oil is altered by the presence of clay colloids. However, the mean size of the interlayer space, of about 12 Å, suggests that Knudsen diffusion cannot be neglected but can only be related to the organic molecules of high shapes. As was previously indicated,31 the n-heptane asphaltene of this crude oil presents a fiberlike shape with radii between 20 and 75 Å. This confirms the difficulty of these molecules to diffuse into the clay interlayer space, and probably they form the amorphous layer observed in the HREM micrograph in Figure 6b. Conclusions Naturally, a clay mineral stratum is considered a permeability barrier within the formation which prevents oil phase to be contaminated with water. However, examination of the microscopic and macroscopic mechanisms of crude oil diffusion into Na+-montmorillonite indicates that it is a slow and continuous process. In this process, interaction of water molecules with such polar compounds is one of the key factor. Thus, the clay interlayer space may only be occupied by small molecules and specially those with tensoactive properties before the structure delaminates. Neither asphaltene nor other high-molecular-weight species can penetrate into the clay d(001) spacing10 because of their molecular volume. In addition, the macroscopic process of diffusion is slow by the viscous O/W microemulsions of crude oil, water, and clay colloids formed outward from the clay particles. Acknowledgment. This research was supported by Instituto Mexicano del Petro´leo (through Grant No. D.00072). We also thank Youri Douda for the helpful discussions. EF049825A (31) Cosultchi, A.; Bosch, P.; Lara, V. H. Colloid Polym. Sci. 2003, 281, 325-330.

Adsorption of Crude Oil on Na+-Montmorillonite

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