Structures and Wettability Alterations of a Series of Bispyridinium

Article pubs.acs.org/EF

Structures and Wettability Alterations of a Series of Bispyridinium Dibromides Exchanged with Reduced-Charge Montmorillonites Zhongxin Luo, Manglai Gao,* Zheng Gu, and Yage Ye State Key Laboratory of Heavy Oil Processing, College of Science, China University of Petroleum, Beijing 102249, People’s Republic of China S Supporting Information *

ABSTRACT: Capillary rise tests were performed to investigate the influences that the structure of the modifier and layer charge of clay had on wetting properties of organoclays, which were prepared by ion exchange using bispyridinium dibromides (BPs) with different spacer length and the reduced-charge montmorillonites (RCMs). Their structures were examined by Fourier transformed infrared spectroscopy (FTIR), X-ray diffraction (XRD), and nitrogen adsorption−desorption isotherms. The results indicated that BPs had been successfully intercalated into interlayers and lay in the monolayer. The d001 basal spacing of organoRCMs increased with the spacer length of BPs increasing, whereas it decreased gradually as the layer charge of the RCMs was decreased, independent of the type of BPs. Whether the organic modification made the Brunauer−Emmett−Teller (BET) surface area increase or decrease depended upon the size of the organic cations and the layer charge of the clays. The wettability alterations of the organo-RCMs for deionized water and cyclohexane were also compared. Both the spacer length of BPs and the layer charge of RCMs had important effects on the relative wettability of organo-RCMs. The hydrophilicity of organo-RCMs was increased with the spacer length of BPs increasing, namely, in the order as follows: C2-2Py-RCMs < C6-2Py-RCMs < C10-2PyRCMs. In addition, the hydrophobicity was increased with a decrease in the layer charge. The results of this work were supposed to provide some reference information for regulating the wettability of the organo-RCMs by simultaneously controlling the type of modifiers along with the layer charge characteristics, to provide theoretical guidance for the favorable change in reservoir wettability.

1. INTRODUCTION Wettability is defined as the relative ability of a fluid to spread on a solid surface in the presence of another fluid.1 It is a very important property involved in many industrial processes, such as flotation, detergency, deposition, lubrication, and coating, especially in enhanced oil recovery.2−6 Understanding and characterizing the wettability of solid surfaces are thus highly essential. Reservoir wettability plays an important role in displacement efficiency and ultimate oil recovery because it is a crucial factor controlling the location, flow, and distribution of fluids in a reservoir.7−9 Favorable alteration of reservoir wettability will help mobilize residual oil and obtain more oil from reservoirs.9 To improve oil recovery during secondary waterflood by wettability alteration, a surfactant addition was added to a high-salinity injection brine by Mohan et al.10 The results indicated that the oil recovery increases from 62 to 85% by wettability alteration. Thus, the investigation on the wettability of the reservoir is significant for efficient oil recovery. Many different methods have been proposed to measure the wettability of a system. The most commonly used quantitative methods today are contact angle, Amott method, and the U.S. Bureau of Mines (USBM) method.11 The wettability of the solid material surface is generally characterized in terms of the contact angle between solid and liquid interfaces.12 Two traditional methods for measuring the contact angle in powder systems are the Bartell plug method and Washburn capillary rise method.13 The Bartell method is experimentally cumbersome, while the capillary rise technique is extensively used nowadays to measure the contact angles of © 2014 American Chemical Society

small particles and porous materials, whose basic theory is Washburn’s equation.14−16 Montmorillonite is a crucial mineral with numerous favorable properties.17,18 Because it is often present in the oil reservoir, it can be used to mimic reservoir mineral for wettability studies in practice research. The layer charge is perhaps the most significant characteristic of 2:1 layer phyllosilicates.19 Typically, it affects many properties of clay, such as cation-exchange capacity (CEC), cation-retention capacity, adsorption ability, and rheological properties.20−23 According to the Hofmann− Klemen effect,24 montmorillonites with reduced layer charge can be prepared by heating Li+-saturated montmorillonite at a certain temperature. Thus, this series of reduced-charge montmorillonites (RCMs) provide a unique model for probing the effect of the layer charge while keeping the other parameters constant at the same time.25 Organoclays are used in a wide range of applications because of their high CEC, swelling capacity, high surface areas, and consequential strong adsorption capacities.26 The properties of organoclays are highly dependent upon the molecular structure of the organic cation, which is used to modify the clay surface.27 Quaternary ammonium salts (QASs) are commonly used for preparing organoclay because of the versatile functionalization and cost effectiveness.28 In addition, QASs as a potential chemical flooding agent can not only effectively reduce the Received: May 19, 2014 Revised: August 10, 2014 Published: August 11, 2014 6163

dx.doi.org/10.1021/ef5011385 | Energy Fuels 2014, 28, 6163−6171

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extent of clay swelling but also change the wettability of the oil reservoir.29 The length of the alkyl chain and the functional groups of QASs can affect the final properties of the organoclay.28 Bispyridinium dibromides (BPs), a special kind of quaternary ammonium salt with a heteroaromatic ring being a planar and rigid structure, tend to bring new characteristics to the organoclays. In the present work, organo-RCMs were synthesized using BPs with different spacer chain length [Py+(CH2)nPy+·2Br−, where n = 2, 6, and 10] to modify the RCMs through ion exchange. Their structures were characterized by infrared (IR), X-ray diffraction (XRD), and N2 adsorption−desorption isotherms. The wettability alterations for deionized water and cyclohexane were studied on the basis of the capillary rise method and were compared using the lipophilic to hydrophilic ratio (LHR). Therefore, this paper attempts to explore how the spacer chain length of BPs and the layer charge of clays affect the wettability of organo-RCMs, which can be helpful to obtain favorable wettability.

LHR =

LHR =

βγ cos θ η

koηoβw γw k wηw βo γo

(4)

For a similar column packed by the same clay particles, βo and βw could be regarded as identical; thus, eq 4 can be simplified as LHR =

koηo γw k wηw γo 2

(5)

−1

k0 and kw (kPa s ) are the measurement curve slopes of oil and water wetting a clay sample, respectively. Obviously, the LHR value, which reflects the difference of wetting selectivity of a clay powder to oil and water, depends upon not only the penetrating rates of oil and water but also the values of viscosity and surface tension of wetting liquids. This paper applies eq 5 to investigate lipophilic and hydrophilic properties of a series of organo-RCMs to obtain some significant results about the wettability.

3. EXPERIMENTAL SECTION 3.1. Materials. A series of BPs [Py+(CH2)nPy+·2Br−, where n = 2, 6, and 10] were prepared according to the method reported by Almarzoqi et al.32 The synthesis routes, structures, and dimensions of BPs were illustrated in Figure 1. The purity of synthesized BPs had

(1)

where ΔP (kPa) is the pressure increase of air in the porosity of the packed bed, β is a natural parameter that is only based on the powder bed and particle size, γ (mN m−1) is the surface tension of the testing liquid, θ (deg) is the contact angle, η (mPa s) is the viscosity of the penetrating liquid, and t (s) is the time of liquid penetrated in the powder bed. If the packing state and the particle size remain similar, β can be regarded as constant and then the slope of the measurement curve is as follows: k=

(3)

where θo and θw are the contact angles of the oil and water phases, respectively, for one kind of powder. When eq 2 is combined with eq 3, the following expression for LHR can be derived:

2. THEORY The contact angle, which is a parameter involving three phases, is measured at any point along the line where an interface involving two of the phases meets the third phase.13 The Washburn capillary rise method is a standard technique for determining the wettability of powders expressed as a contact angle.30,31 A powder packed into a cylindrical cell with a porous bottom is touched on a liquid, and as the liquid penetrates the powder bed by capillarity, the air in the tube will be compressed and then the air pressure will increase. The relationship of the pressure increment ΔP of air to time t in the powder bed can be expressed as follows:14 ⎛ βγ cos θ ⎞ (ΔP)2 = ⎜ ⎟t η ⎝ ⎠

cos θo cos θw

Figure 1. Synthesis routes, structures, and dimensions of BPs. been confirmed by melting point, element analysis, and highresolution mass spectrometry (HRMS). All of these data had been presented in Table S1 of the Supporting Information. The results indicated that the three BPs were target products and owned a high purity. Montmorillonite (Mt) was purchased from Zhejiang Institute of Geology and Mineral Resources, China. The purity was >97%, and it was used without further purification. Its chemical composition was found to be as follows: 67.16% SiO2, 20.60% Al2O3, 4.11% MgO, 3.21% CaO, 2.20% Fe2O3, 1.47% Na2O, 0.92% K2O, and 0.17% TiO2. Lithium chloride (LiCl), sodium chloride (NaCl), and cyclohexane (analytic grade) were all purchased from Tianjing Guangfu Fine Chemical Research Institute, China. The deionized water was produced by reverse osmosis, passage through two stages of mixed ion-exchange resin bed, followed by a filtration stage of activated carbon. The characteristics of cyclohexane and deionized water are listed in Table 1. 3.2. Sample Preparation. The RCMs were prepared following the procedure proposed by Lim and Jackson.33 Mt was saturated with

(2)

This slope can be experimentally determined for each type of particle packing and can be used to evaluate the wettability. To obtain the parameter β, one totally wetting liquid has to be used (the contact angle in this situation is then assumed to be zero). A strict particle packing control with a reproducible procedure is also essential to ensure that the geometric factor β for this totally wetting liquid is the same as for another liquid. To obviate this problem, a centrifugal packing technique to prepare beds of powder was introduced by Galet et al.,30 which gives reproducible packings and also allows for a certain degree of control of the bed porosity. However, the β value is rather difficult to be determined in any case. Chang et al. defined a LHR concept based on the Washburn equation to compare the wetting differences of powders.14 LHR is defined as 6164


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100 mm), water bath, and automatic driven stage14,34 (the schematic diagram was illustrated in Figure S1 of the Supporting Information). The Washburn glass tube was cylindrical and equipped with a highspeed filter paper at the bottom of the tube to prevent the fall of samples. A pressure sensor was placed on the top of the tube. The values of the contact angle determined by the Washburn method depend upon the bed porosity, and the greater the porosity, the greater the value of the contact angle.30 Therefore, the packing procedure was the most important operation of this method, and great care must be taken when preparing the columns to ensure reproducible results. In this work, 400 ± 0.5 mg of sample was packed in the Washburn tube according to the procedure proposed by Iveson et al.35 The packed Washburn glass tube was manually impacted hundreds of times until the height of the particles was invariable. Deionized water and cyclohexane were used as testing liquid, representing the water and oil phases, respectively. The experiments were performed at 20 ± 0.5 °C. All experiments were performed in duplicate, and the results were given as an average.

Table 1. Characteristics of Cyclohexane and Deionized Water at 20 °C reference liquid

γ (mN m−1)

ρ (×10−3, kg m−3)

η (mPa s)

water cyclohexane

72.8 25.5

0.998 0.778

1.002 0.973

Li+ by washing it 3 times with 1 M LiCl solution at pH 7. Subsequently, excess salt was removed by washing it 4 times with ethanol. The Li+-saturated samples were then placed on a silica slide, dried at 25 °C, and heated at 100, 120, 150, 170, and 200 °C, respectively, in a muffle furnace for 24 h. After cooling in the desiccator, the samples were pulverized to pass through a 200-mesh sieve. Changes in layer charge were monitored by the determination of CEC values using exchange of triethylene tetraamino copper ions ([Cu(Trien)]2+).21,22 The experiment was performed with three repeated trials, and the results were given as an average. The RCMs thus obtained were denoted as Mt-m, where m was the CEC values (mequiv/100 g) of the RCMs. The concentration of C6-2Py onto Mt-99 was determined as follows: 1.0 g of Mt-99 was first dispersed in deionized water, into which a predissolved stoichiometric amount of C6-2Py cations corresponding to 0.2−4.0 CEC of Mt-99 was slowly added. The reaction mixtures were shaken in a water bath oscillator for 3 h at 60 °C to allow for equilibrium to be attained. After centrifugation, the absorbance of C6-2Py in the supernatant was measured via an ultraviolet−visible (UV−vis) spectrophotometer employed at a wavelength of 257.5 nm. The amount of C6-2Py adsorbed onto Mt99 was determined by the difference between the initial and remaining concentration of C6-2Py solution, which was obtained using the following equation:

qe =

C0 − Ce V m

4. RESULTS AND DISCUSSION 4.1. Characterization of RCMs. The preparation temperatures, CEC values, and d001 basal spacing of RCM samples were list in Table 2. From the table, we can see that the CEC Table 2. Preparation Temperatures and Selected Properties of RCMs

(6)

−1

where qe (mmol g ) is the amount of C6-2Py adsorbed onto Mt-99, C0 and Ce (mmol L−1) are the initial and equilibrium liquid-phase concentrations of C6-2Py, respectively, V (L) is the volume of the solution, and m (g) is the weight of the Mt-99 used. All of the experiments were run in duplicate, and experimental errors were found below 3%. Organo-RCMs were prepared by the following procedure: 1.0 g of RCMs was first dispersed in deionized water, into which a predissolved stoichiometric amount of organic cations (Cn-2Py, where n = 2, 6, and 10) corresponding to 1.5 times the CEC of each clay sample was slowly added. The reaction mixtures were shaken in a water bath oscillator for 3 h at 60 °C to allow for equilibrium to be attained. After centrifugation, the products were washed free of bromide anions (tested by AgNO3), dried at 80 °C for 24 h, and then pulverized to pass through a 200-mesh sieve. The organo-RCMs thus obtained were assigned as Cn-2Py-Mt-m, where n = 2, 6, and 10 and m was the CEC values (mequiv/100 g) of the RCMs. They were stored in airtight containers and dried in an oven at 100 °C for 12 h before use. 3.3. Characterization. Fourier transformed infrared spectroscopy (FTIR) spectra were recorded from 4000 to 400 cm−1 using a Nicolet Magna 560 ESP FTIR spectrometer with a resolution of 4 cm−1. The XRD patterns were recorded using a X-ray diffractometer (Shimadzu XRD-6000 powder diffractometer) with Cu Kα radiation (λ = 0.154 06 nm). The 2θ ranged from 1.5° to 10° for the clay samples as the scanning rate of 1°/min was recorded. The basal spacing was calculated according to the 2θ values of the peaks of the XRD patterns using Bragg’s law (λ = 2d sin θ). The surface areas of the samples were measured using an accelerated surface area and porosity analyzer (Micromeritics, ASAP 2420) by the Brunauer−Emmett−Teller (BET) method. 3.4. Wettability. Wettability was measured by the capillary rise method. The experimental setup mainly consists of a JF99A powder contact angle measuring apparatus (Powereach, Shanghai, China), Lenovo personal computer (PC) computer, Washburn glass tube (inner diameter of 6 mm, external diameter of 10 mm, and height of

RCM

preparation temperature (°C)

Mt-99 Mt-80 Mt-68 Mt-56 Mt-38 Mt-18

100 120 140 170 200

CECa (meq g−1)

d001b (nm)

± ± ± ± ± ±

1.22 1.22 1.22 1.12 0.99 0.96

0.99 0.80 0.68 0.56 0.38 0.18

0.02 0.01 0.01 0.02 0.02 0.02

a

CEC = cation-exchange capacity. bd001 = basal spacings of oriented samples.

values gradually decrease with increasing preparation temperature. This is caused by the migration of Li+ from interlayer spaces into the layers, followed by reduction of the layer charge.24 The highest CEC value was confirmed for the untreated sample (Mt-99), which was 0.99 mequiv/g. When Li+-saturated montmorillonite was heated at 100 °C (Mt-80), the CEC value was reduced to 0.80 mequiv/g. They represented the samples of typical smectites with a high layer charge. A greater reduction of CEC was determined for the samples heated at 120 and 140 °C, which led to the lowering of their CEC values to 0.68 and 0.56 mequiv/g, respectively. These samples were probably similar to medium-charge smectites, potentially containing a phase of non-swelling interlayer spaces. The following increase of the temperature to 170 and 200 °C led to the greatest reduction of CEC values, 38 and 18% in comparison to a parent sample, which could be considered as low-charge smectites. The d001 basal spacing of samples Mt-99, Mt-80, and Mt-68 were all equal to 1.22 nm, despite differences in measured layer charge resulting from Li+ fixation. The d001 spacing became smaller staring with Mt-56 (1.12 nm), and this trend continued with a further decrease in layer charge. The basal spacing was shifted to a lower field to 0.96 nm for sample Mt-18. These changes indicated the collapse of the interlayer structure as a consequence of the fixation of Li+, which neutralized the charge of the clay layers.36,37 The formation of non-swelling interlayer spaces was observed for the sample Mt-56, as indicated by the decreased d001 values (1.12 nm). Samples Mt-38 and Mt-18 6165


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contained only the non-swelling phase, and basal spacing values of these samples were similar to the thickness of the silicate layer (0.96 nm).38 4.2. Characterization of the Series of Organo-RCMs. The adsorbed amount of C6-2Py onto Mt-99 at different concentrations was depicted in Figure 2. The correlation

Figure 3. IR spectra of RCMs and organo-RCMs: (a) Mt-99, (b) Mt18, (c) C6-2Py-Mt-99, and (d) C6-2Py-Mt-18.

1051 cm−1 and a disappearance of the peak at 1091 cm−1, which are typical for Li+ fixation in montmorillonite.24 Spectra c and d not only had characteristic bands of clay but also exhibited some new characteristic bands of C6-2Py. The result indicated that C6-2Py had existed at the surface or interlayer space of clays.42−44 The XRD patterns for organo-RCMs were exhibited in Figure 4. From the diffraction 2θ values of the peaks in the patterns, the d001 basal spacings were calculated, which showed that with the cation exchange of the inorganic ion for the BPs, expansion of the RCM layers occurred. This expansion illustrated that BPs has been intercalated into the interlayer of RCMs. Additionally, this figure clearly showed that the basal spacing decreased gradually as the layer charge of the RCMs decreased, independent of the type of BPs. The shape of this 001 reflection became weaker and broader starting with Mt-38, and this trend continued with a further decrease of the layer charge. This indicated the formation of a less ordered structure, which may be related to less ordered phases of cations presented in the interlayer spaces.23 From Figure 4a, we can clearly see that the basal spacing of C2-2Py-RCMs decrease slightly from 1.29 to 1.26 nm with the layer charge decreasing. The interlayer spacings of C2-2PyRCMs, calculated as subtracting the thickness of the silicate layer (0.96 nm)38 from the d001 spacings, were 0.33−0.30 nm. When the molecule dimensions were combined (as shown in Figure 1), the C2-2Py molecule could only lie in the monolayer with the heteroaromatic ring parallel or at an angle to the silicate planes and the two cation heads linked to the siloxane surface in the RCM interlayer. Moreover, in comparison to the basal spacing of RCMs (as shown in Table 2), we can see that the interlayer spacing of Mt-38 and Mt-18 was enlarged again when modified by C2-2Py. Similar trends were observed for C6-2Py-RCMs and C10-2Py-RCMs. Figure 4b showed that the d001 basal spacing of C6-2Py-RCMs varied from 1.38 to 1.27 nm, with an interlayer space height ranging between 0.42 and 0.31 nm. For C10-2Py-RCMs, a unique and well-ordered sequence of the 001 reflection was observed and corresponded to a basal spacing between 1.42 and 1.29 nm, with an interlayer space height ranging between 0.46 and 0.33 nm. When the molecule dimensions were combined (as shown in Figure 1), the arrangement of C6-2Py and C10-2Py in the RCM interlayer

Figure 2. Adsorbed amount of C6-2Py and d001 basal spacings of Mt99 intercalated with different amounts of C6-2Py. The horizontal lines on the right show the CEC values of Mt-99.

between the amounts of the organic cations adsorbed and the CEC values of Mt-99 was also shown. It can be seen that the adsorbed amount increased linearly with an increasing amount of C6-2Py loading up to 0.8 CEC of Mt-99, then increased slightly as the concentration of the modified agent continuous increased, and at last, remained almost constant when the added amount of C6-2Py approached and exceeded 1.4 CEC. However, it was worth noting that adsorbed C6-2Py was only capable of saturating at most 0.9 CEC of Mt-99, even with the added amount of organic cations up to 4.0 CEC. In contrast to the QASs having a long alkyl chain, C6-2Py was adsorbed to Mt-99 only via the cation-exchange process; it could not be adsorbed by the hydrophobic interaction between alkyl chains.39 Therefore, the overall uptake of C6-2Py could not reach more than 1.0 CEC.39,40 The d001 basal spacings obtained from the XRD characterization results were also presented in Figure 2. The graph clearly showed the increase in the basal spacing from 1.22 to 1.39 nm with the concentration of C6-2Py increasing. The basal spacing reached a plateau of 1.38 nm when the initial concentration of C6-2Py was 0.8 CEC, and then it only increased slightly with continuous increasing of the amount of C6-2Py. The similar d001 values at initial C6-2Py inputs of more than 0.8 CEC indicated that [C6-2Py]2+ intercalated in the interlayer of Mt-99 almost attained saturation. From the above analysis, 1.5 CEC of the modified agent can be considered to be a sufficient amount to ensure ion exchange completely, and this amount was chosen to prepare a series of organo-RCMs. The FTIR spectra of RCMs and organo-RCMs are shown in Figure 3. The characteristic bands at 3620−3400 cm−1 were assigned to Al−O−H and H−O−H stretching vibrations (υOH), while the peaks at 1640 and 796 cm−1 were assigned to the O−H bending vibrations (δOH). The peaks at 1038, 521, and 465 cm−1 were attributed to the Si−O stretching and bending vibrations (spectrum a).41 Spectrum b shows a shift of the Si−O stretching band to higher wavenumbers from 1038 to 6166


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Figure 4. XRD patterns of organo-RCMs: (a) C2-2Py-RCMs, (b) C6-2Py-RCMs, and (c) C10-2Py-RCMs.

compact packing in the interlamellar space, causing steric hindrance of the N2 probe.44 For Mt-18, the surface area increased with BP cation substitution of inorganic cations in the sequence C10-2Py < C2-2Py < C6-2Py. This result can be attributed to the “propping” open of the interlamellar space by the exchanged BP cations and, thereby, allowing the N2 probe to access interlamellar surfaces. In addition, the packing density of the BP cations in the interlamellar space was decreased with a decrease in layer charge. All in all, it would appear that the surface areas of the organoRCMs were influenced by both the size of the organic cations and the layer charge of the clays.40 4.3. Wetting Properties of RCMs Modified by a Series of BPs. In the capillary rise experiments, the square of the measured pressure was plotted as a function of time. The linear portion of the wetting curves of RCMs modified by C2-2Py, C6-2Py, or C10-2Py are given in Figures 5, 6, and 7, respectively. The slopes of the lines ΔP2 versus time for the oil and water phases were collected in Figure 8. A good straight line fit was observed in each case. The linear least-squares fits for all wetting rates gave regression coefficients of more than 0.999, confirming the suitability of the experimental method for clay particles and further validating Washburn’s theory. Reproducibility was also good; repeated experiments with the same powder−fluid systems yielded lines with similar slope, and the deviation are less than 0.02 (not shown). Figure 5 indicated that the wetting rates of cyclohexane for C2-2Py-RCMs with different layer charges were similar, while the wetting rates of water were largely changed. In combination with Figure 8, it was known that the k0 values were almost constant, which indicated that the layer charge of clay samples did not have any significant effect on the cyclohexane wettability of C2-2Py-RCMs. The kw values first decreased considerably and then stabilized with a decreasing layer charge. The water wettability for Mt-99 was obviously larger than the other clays with less layer charge. It can be concluded that the water wettability was significantly affected by the layer charge for Mt-99, Mt-80, and Mt-68. However, during the range of low layer charge, because of the few adsorptions of C2-2Py on RCMs, the water wettability changed slightly with the change of the layer charge.

was similar to C2-2Py and they lay in the monolayer and interacted strongly with negatively charged siloxane layers.39 Besides, the d001 spacing of organo-RCMs increased in the order C2-2Py-RCMs < C6-2Py-RCMs < C10-2Py-RCMs, which was in accordance with the order of the increasing spacer length. However, the spacer carbon numbers (n = 2, 6, and 10) had no noticeable impact on d001 spacing of organoRCMs, which might be due to the strong rigidity of the two pyridinium rings.45 The N2 BET surface area determination is a widely accepted method for quantifying the specific surface area of clay.44 The surface area measurements for different RCMs and organoRCMs are given in Table 3. From this table, it was seen that Table 3. BET Surface Area (±Standard Deviation) for RCMs and Organo-RCMs sample Mt-99 C2-2Py-Mt-99 C6-2Py-Mt-99 C10-2Py-Mt-99

BET surface area (m2 g−1)

sample

± ± ± ±

Mt-18 C2-2Py-Mt-18 C6-2Py-Mt-18 C10-2Py-Mt-18

26.39 45.36 22.10 16.46

0.12 0.83 0.08 0.20

BET surface area (m2 g−1) 23.26 26.51 35.92 23.76

± ± ± ±

0.05 0.07 0.32 0.08

Mt-99 had a larger surface area (26.39 m2 g−1) than Mt-18 (23.26 m2 g−1). This was because the reduction of the layer charge was accompanied by complete collapse of the interlayer spaces (according to the XRD results), which may prevent N2 molecules from moving into the interlayer spaces. It is wellknown that replacing inorganic cations of clay to organic cations could make the BET surface area increase or decrease, depending upon the structures and arrangement of the organic cations.39 For Mt-99, C2-2Py modifying led to the increase in the surface area, whereas C6-2Py and C10-2Py modifying led to the decrease in the surface area. The results indicated that smaller C2-2Py ions were isolated from each other when adhering to the interlamellar surfaces of Mt-99, thereby creating large amounts of interlayer surfaces that were accessible to nitrogen molecules.39,40,46 However, as the spacer chain length increased, larger BP cations may be subject to relatively 6167


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Figure 5. Representative capillary rise data for the intake of cyclohexane and water by C2-2Py-RCMs.



for the lowest layer charge clay particle. It was interesting to note that the k0 and kw values of C6-2Py-RCMs were larger than that of C2-2Py-RCMs as a whole. The curves presented in Figure 7 summarized the wettability behavior of the C10-2Py-RCMs. In combination with the k0 and kw values (as shown in Figure 8), it was known that the layer charge of clay has an obvious influence on both wetting

Figures 6 and 8 showed that both the wetting performances of cyclohexane and water for C6-2Py-RCMs were significantly affected by the layer charge of clay samples. The wetting rate of cyclohexane to different clay particles first increased and then decreased with the decrease in the layer charge, but the wetting rate of water (kw value) first decreased considerably, then increased, and at last, decreased again after reaching a minimum 6168


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factors: (1) hydrophilicity of the BPs and (2) interlayer spacing of the organo-RCMs. As the spacer length increased from C22Py to C10-2Py, the hydrophobicity increased accordingly. In addition, the d001 spacing was also gradually increased, with the spacer length increasing according to the XRD results. The BP molecule had an alkyl chain linking two hydrophilic cation heads. The larger basal spacing may cause higher capability of the hydrophilic cation heads of BPs, adsorbing water into the clay interlayer. In other words, the hydrophilicity increased with the d001 basal spacing increasing. The LHR values indicated that C2-2Py-RCMs were the most lipophilic, whereas C10-2PyRCMs were the most hydrophilic. From this result, we may conclude that it was the d001 spacing of the organo-RCMs rather than the hydrophobicity of the organic modifier that played the dominant role in the wettability alteration process. From another side, we can see that the correlation between the LHR values and layer charge was clear. With layer charge decreasing, the LHR values of organo-RCMs increased significantly, namely, the organo-RCMs tend to be more hydrophobic. Organophilization was an ion-exchange process. It should be noted that the adsorption amount of BPs to highcharged clay was larger than that to low-charged clay because of their different CEC values. According to the previous studies,23,47 a high layer charge density of the clay resulted in a smaller distance between the neighboring adsorbed cations, whereas a low layer charge density caused greater distance between cations. For this reason, the amount of siloxane surface exposed was inversely related to the layer charge of the clay.40 Moreover, the XRD results indicated that the BPs only formed monolayers in the interlayer of clays. On the basis of these facts and the XRD and BET characteristic results, the arrangement of BPs in the interlayer of clays with high and low layer charges can be schematically illustrated in Figure 9 (C2-2Py and C102Py are chosen as representatives). Figure 9 clearly showed that the distance of neighboring BPs increased with the layer charge decreasing. Thus, it was reasonable to infer that the organoRCMs with a lower layer charge would possess more exposed siloxane surfaces, which was consisted with trends shown by Huang et al.40 Because the siloxane surface in smectites had a hydrophobic nature,27,48 the more exposed siloxane surfaces, the greater the hydrophobicity of the organo-RCMs. In other words, the wettability of organo-RCMs changed to more hydrophobicity with the decrease in the layer charge. From the

Figure 8. k0, kw, and LHR values of a series of organo-RCMs.

performances of cyclohexane and water for C10-2Py-RCMs. The k0 values first decreased and then increased with the layer charge decreasing. However, the wetting rate of water to different clay particles was more largely changed. The kw values decreased considerably from 1.49 for Mt-99 to 0.82 for Mt-80 and then decreased gradually as the layer charge of the RCMs further decreased. The LHR values of organo-RCMs calculated by eq 5 were presented in Figure 8c. The LHR value not only reflects the difference of wettable selectivity of one organo-clay to apolar or polar liquid but also shows the relative wettabilities of different kinds of organo-clays for oil or water.4 The LHR value conceptually reflects the wettability difference between oil and water phases for the clay particles; namely, the greater the LHR value, the more lipophilic the clay particle and vice versa. From Figure 8c, we can see that the LHR values were all larger than 1 (1.10−3.95), which indicated that RCMs modified by BPs were all lipophilic. The LHR values increased in the order C10-2PyRCMs < C6-2Py-RCMs < C2-2Py-RCMs, independent of the layer charge of clay particles. This indicated that the type of BPs, namely, the spacer length, had great influence on the wettability of organo-RCMs, especially for the low-charged clays. These observations can be explained in terms of two

Figure 9. Schematic illustration of the arrangement of BPs in the interlayer of RCMs. 6169


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above analysis, it was obvious that the relative wettability of organo-RCMs was connected to not only the structure of the organic modifier but also the layer charge of the clay particles.

5. CONCLUSION In this work, we have assessed the relative wettability of a series of organo-RCMs using the capillary rise method. The influence of both the spacer length of BPs and the layer charge of clay on the wetting behavior has been examined. The wetting differences between these organo-RCMs were compared using the LHR concept, which was defined on the basis of Washburn’s equation. The results showed that BP cations were intercalated into the interlamellar spaces of RCMs. They lay in the monolayer and interacted strongly with negatively charged siloxane layers. On one hand, the d001 spacings increased in the order C2-2Py-RCMs < C6-2Py-RCMs < C10-2Py-RCMs, which corresponded to the order of increasing BP molecule size. On the other hand, the d001 spacing was decreased with the layer charge of RCMs decreasing, independent of the type of BPs. The surface areas of the organo-RCMs were influenced by both the size of the organic cations and the layer charge of the clays. Moreover, the relative wettability of organo-RCMs was connected to not only the structure of the organic modifier but also the layer charge of the clay particles. The LHR values showed that the hydrophobicity was increased with a decrease in the layer charge, whereas it was decreased as the spacer length of BPs increased. The results of this work also indicated that simultaneously controlling the type of modifying agent and the layer charge of clay will be helpful for regulating the wetting nature to obtain favorable wettability.

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ASSOCIATED CONTENT

S Supporting Information *

Schematic diagram of the experimental setup for the capillary rise test (Figure S1) and melting point, elemental analysis, and HRMS analysis of the three BPs (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-10-89733680. Fax: +86-10-6972790. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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