Structure and performance of anionic–cationic-organo-montmorillonite in different organic solvents

Abstract

Montmorillonite (Mt) was modified in aqueous solution by cetyltrimethyl ammonium bromide (CTAB) and sodium dodecyl sulfonate (SDS) under ultrasonic conditions at 60 °C. Anionic–cationic-organo-montmorillonite (ACOMt) was characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and contact angle measurement. The results indicated that ACOMt was prepared successfully, and the polar property of ACOMt was higher than that of cationic-organo-montmorillonite (COMt). The results of SEM proved that there were great amounts of loose and thin platelets of ACOMt. In addition, the properties of ACOMt in three organic solvents were investigated using XRD, SEM, optical microscopy and viscometry, etc. The results of the XRD patterns demonstrated that the basal spacing of ACOMt became small because of parts of the surfactants getting out of the interlayer after being dispersed in the organic solvents. Moreover, with the increase in the solvent polarity, the amounts of surfactants getting out of the interlayer increased. The results of optical microscopy revealed that, with the increase of surfactants and the polar property, the dispersibility of powdery samples in the organic solvents improved and ACOMt in a highly polar solvent showed the best dispersibility. However, the results of SEM indicated that the morphology of ACOMt dispersed in three organic solvents was not significantly different. Meanwhile, dispersion of ACOMt in xylene exhibited the best thixotropic properties and apparent viscosity.

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1. Introduction

In past years, the structure, properties and applications of modified clay minerals have been widely investigated, especially the studies of organo-montmorillonite (OMt). OMt is usually modified with organic ionic surfactants in aqueous solution or ethanol solution, mainly including cationic-organo-montmorillonite (COMt),^1–3 anionic–cationic-organo-montmorillonite (ACOMt),^4–7 anionic–nonionic-organo-montmorillonite (ANOMt),⁸ cationic–nonionic-organo-montmorillonite (CNOMt),⁹ etc. Due to its great rheological properties, absorption property and hydrophobic character, which are provided by the intercalation of organic surfactants, OMt is usually applied in the production of coatings,^10,11 nanocomposites,^1,6,12 oil-based drilling fluids¹³ and treatment of contamination,^14–16 etc.

In previous studies, compared with COMt, ACOMt was able to pronouncedly increase the total amount of loaded organic carbon and the basal spacing of clay.^5,17 Chen et al.¹⁷ proved that anionic surfactants cannot intercalate into montmorillonite (Mt) alone, and SDS can intercalate into Mt by interaction with CTAB rather than by ion exchange. Studies showed that Mt modified by CTAB and SDS showed a higher decomposition temperature, derived from the strong attractive interaction between oppositely charged head groups.^17,18

There is no doubt that OMt can swell in non-aqueous solvents and is compatible with organic compounds, because the surface properties of Mt were changed from hydrophilic to hydrophobic and surfactant had intercalated in the interlayer of Mt. Organoclays can exhibit rheological properties while dispersed in organic solvents.¹⁹ However, few studies have been conducted on the structure and properties of ACOMt dispersed in organic solvents.

In this study, aiming to solve the problems mentioned above, the influence of surfactant loadings on the structure of ACOMt, and the structure of ACOMt in organic solvents were studied. In addition, the colloidal and rheological properties of ACOMt/solvents were also investigated.

2. Materials and methods

2.1 Materials

Na-montmorillonite (Na-Mt) was sourced from Inner Mongolia, China. The raw Ca-Mt was purified by sedimentation to remove impurities, and <2 μm fractions were collected and used for the experiments. The cation exchange capacity (CEC) of the Ca-Mt is 94.1 cmol kg⁻¹.

The surfactants are cetyltrimethyl ammonium bromide (CTAB) and sodium dodecyl sulfate (SDS). Their chemical formulas are shown in Table 1. CTAB with a purity of 99% was purchased from Beijing Chemical Works. SDS with a purity of 99% was purchased from the Guangzhou Shan tou Xilong Chemical Plant. As a dispersion medium, the polarity index of three organic solvents is presented in Table 2. All of these solvents have 98% purity.

Table 1 The properties of two kinds of surfactants

Surfactant	Chemical formula	Molar mass (g mol^−1)
CTAB		364.45
SDS		272.38

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Table 2 Polarity index of the solvents used in this study

Solvent	Polarity index (a.u.)
Solvent A (90 wt% 200# solvent oil and 10 wt% xylene)	0.259
Solvent B (xylene)	2.5
Solvent C (80 wt% xylene and 20 wt% butanol)	2.85

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2.2 Organo-montmorillonites synthesis

ACOMt and COMt were prepared in aqueous solution. The procedure for the preparation of ACOMt was as follows. 25 g Na-Mt was dispersed in 500 ml deionized water, at 60 °C, using high speed stirring and ultrasound for 1 h. CTAB was then slowly added, and stirring was maintained over 1.5 h with continuous ultrasound. Finally, SDS was slowly added, and stirring was maintained over 1.5 h with continuous ultrasound. Several concentrations of CTAB/SDS were tested: 1.0 CEC/1.5 CEC, 1.2 CEC/1.5 CEC and 1.8 CEC/1.5 CEC. The OMt prepared with a CTAB concentration of 1.0 CEC and an SDS concentration of 1.5 CEC was marked as 1.0CTAB–1.5SDS–OMt and the others marked in a similar way.

The procedure for the preparation of COMt was identical to that for ACOMt except the stirring needed to be maintained over 3 h with continuous ultrasound after adding CTAB. Several concentrations of CTAB were tested: 1.0 CEC, 1.2 CEC and 1.8 CEC. The OMt prepared with a CTAB concentration of 1.0 CEC was marked as 1.0CTAB–OMt and the others marked in a similar way.

2.3 Characterization

X-ray diffraction (XRD) patterns were recorded using a Rigaku D/max-rA diffractometer. Diffraction patterns were scanned from 1° (or 1.5°) to 25° (2θ) using CuKα radiation (λ = 1.5406 nm) at a step size of 0.02°. Moreover, the scanning speed is 4° min⁻¹.

Contact angles were measured using a contact angle goniometer JC200D produced by Shanghai Zhongchen Digital Technological Co., Ltd., with deionized water as the medium, at room temperature. The reported contact angle was an average value from three measurements.

The surface morphology of montmorillonite was obtained by HITACH SU8010 scanning electron microscopy (SEM). The powdered samples and the samples dispersed in organic solvents, diluted to the appropriate concentration using the corresponding solvent, were coated on a silicon chip.

The model of optical microscope used was an OLYMPUS BX-51, the lens of this microscope was an OLYMPUS DP-73. Optical microscopy was used to observe the morphology of the dispersion of montmorillonite and ACOMt dispersed in organic solvents. The dispersion samples were dropped onto a microscopy slide, and the drop of dispersion was then observed.

The test method used to determine the gel volume was dispersing 3 g modified products in 147 g solvent under continuous stirring for about 10 min at a shear rate of 1500 rpm at 25 °C. Then, the dispersions were moved to a 100 ml graduated cylinder, and the value was obtained after standing for 6 h.

Apparent viscosity of ACOMt in three organic solvents was measured with a viscometer SNB-3 (Shanghai Jingtian Electronic Instrument Co., Ltd, China), using L0 and L1 spindles at a shear rate of 0.1–99.9 (s⁻¹) at 25 °C.

3. Results and discussion

3.1 Characterization of OMt

3.1.1. XRD analysis of OMt. The XRD patterns of Na-Mt, surfactant, COMt and ACOMt are depicted in Fig. 1. The XRD patterns of SDS, CTAB and Na-Mt showed reflections at 3.70 nm, 2.66 nm and 1.27 nm, which indicate the reflection of pristine SDS, CTAB and Na-Mt, respectively. Compared with Na-Mt, the patterns of COMt, including 1.0CTAB–OMt, 1.2CTAB–OMt and 1.8CTAB–OMt, showed reflections at 5.68 (2.23) nm, 5.65 (3.04, 1.96) nm and 3.87 (1.91) nm, respectively, which prove that CTAB successfully intercalated in the interlayer of montmorillonite, and expanded the interlayer space. According to the molecular conformation of CTAB,^20,21 the reflections at 5.68 nm and 5.65 nm indicate that there is approximately a perpendicular paraffin-type bilayer arrangement in the interlayer space of montmorillonite. The reflections at 3.87 nm and 3.04 nm indicate that there is a paraffin-type bilayer arrangement with a smaller tilting angle in the interlayer of montmorillonite. The reflections at 2.23 nm, 1.96 nm (or 1.91 nm) and 1.68–1.74 nm indicate that the arrangements of CTAB are a paraffin-type monolayer, a lateral bilayer arrangement and a lateral monolayer in the interlayer of Mt, respectively.^22,23 In addition, with the increasing amount of CTAB from 1.0 CEC to 1.8 CEC, the reflections exhibited were increasingly more intense and sharp, representing a high order of surfactant arrangement in the interlayer of Mt.

Fig. 1 XRD patterns of SDS, CTAB, Na-Mt, COMt and ACOMt.

As shown in Fig. 1, the patterns of ACOMt, including 1.0CTAB–1.5SDS–OMt, 1.2CTAB–1.5SDS–OMt and 1.8CTAB–1.5SDS–OMt, display reflections at 6.30 (2.53, 1.74) nm, 6.21 (2.62, 1.70) nm and 5.26 (2.59, 1.70) nm, respectively. The results reveal that SDS had successfully intercalated into the interlayer of montmorillonite through the interaction with CTAB which was already present in the interlayer of montmorillonite, and results in a significant increase of basal spacing. The result indicated that ACOMt showed a greater reflection and crystallinity than that of COMt using the same amount of CTAB. It is apparent that the d₀₀₁ reflections of ACOMt gradually decrease, with the increase of surfactants. However, the crystallinity of ACOMt also increases.

3.1.2. SEM of OMt. Scanning electron microscopy (SEM) profiles of Na-Mt and ACOMt are presented in Fig. 2. Compared with Fig. 2a-1 and a-2, the particle size of ACOMt was smaller than that of montmorillonite, and the stack of lamellas was considerably loose and the particles seemed swelled. However, the flakes of ACOMt were more tight and thicker, with the increase of surfactants in the interlayer of montmorillonite. It is conclusive that the larger basal spacing was in accordance with the more loose lamellas as the SEM images demonstrated. The result was in agreement with the patterns of XRD of ACOMt powdery samples. The intercalation of surfactants into the interlayer space of montmorillonite did not only lead to a great increase of the basal spacing, but also resulted in a decrease of the attraction between adjacent lamellas. Finally, more loose, thinner lamellas and a number of well-separated flakes emerged.

Fig. 2 SEM images of Na-Mt and ACOMt samples. (a-1 and a-2) Na-Mt, (b-1 and b-2) 1.0CTAB–1.5SDS–OMt, (c-1 and c-2) 1.2CTAB–1.5SDS–OMt, and (d-1 and d-2) 1.8CTAB–1.5SDS–OMt.

3.1.3. Contact angle of OMt. The result of the contact angle measurement of OMt is shown in Table 3. According to the result of contact angles, we can obtain the surface polarity of OMt. Although surfactant loadings of ACOMt were more than that of COMt, the contact angle of ACOMt was less than or equal to COMt. The result indicates that SDS contributed a high polarity to the surface of the montmorillonites, leading to ACOMt displaying higher hydrophilic characteristics compared to that of COMt. Moreover, with the increase in CTAB loading, angles of COMt initially reduced and finally increased, but angles of ACOMt always increased. The reason might be that the arrangement of CTAB absorbed on the surface of montmorillonite changed from one layer to a multilayer when the amount of CTAB changed from 1.0 CEC to 1.8 CEC.

Table 3 Contact angles of COMt and ACOMt

Sample	d001 (nm)	Contact angle (°)
1.0CTAB–OMt	5.68, 2.23	55
1.2CTAB–OMt	5.65, 3.04, 1.96	44
1.8CTAB–OMt	3.87, 2.53, 1.91	51
1.0CTAB–1.5SDS–OMt	6.30, 2.53, 1.74	23
1.2CTAB–1.5SDS–OMt	6.21, 2.62, 1.70	29
1.8CTAB–1.5SDS–OMt	5.30, 2.59, 1.70	44

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3.2 Structure and properties of ACOMt in three organic solvents

3.2.1. XRD analysis of ACOMt in different organic solvents. The diffraction patterns of Na-Mt and ACOMt in solvent A, solvent B and solvent C are depicted in Fig. 3A–C, respectively. In addition, calcium dodecyl sulfonate (CDS-synthesized),²⁴ synthesized with SDS and CaCl₂, also is depicted in Fig. 3. Solvent A, solvent B and solvent C respectively represent 90 wt% 200# solvent oil mixed with 10 wt% xylene, xylene and 80 wt% xylene mixed with 20 wt% butanol.

Fig. 3 XRD patterns of (a) a-ACOMt, (b) b-ACOMt and (c) c-ACOMt.

Compared with the patterns of the powdery samples, we can observe that the reflections were generally sharper and appeared at bigger angles when these samples were dispersed in organic solvents. The result reveals that surfactants that intercalated into the interlayer of montmorillonite had more regular arrangements than the powdery samples when ACOMt was dispersed in organic solvents. In addition, the basal spacing of samples, modified by anionic and cationic surfactants, dispersed in solvent B and solvent C was lower than that of ACOMt in solvent A. Moreover, ACOMt in solvent B and solvent C exhibited a series of sharper reflections than solvent A. Therefore, it can be concluded that these results derived from part of the surfactants intercalated in the interlayer of montmorillonites getting out of the interlayer. With the increase of solvent polarity, the amount of surfactants getting out of the ACOMt layers increased. We can observe a remarkable reflection at 3.20 nm. Obviously, it was not the reflection of ACOMt, COMt or the surfactants. The reflection at 3.20 nm was in agreement with the reflection of CDS-synthesized, and the reflection was not observed in Fig. 3A, so we certainly suggested that residual Ca²⁺ interacted with dodecyl sulfonate anions becoming a new compound-calcium dodecyl sulfonate when OMt was dispersed in a highly polar medium. Moreover, these results also demonstrated our previous conclusion, the amount of surfactants getting out of the layers was affected by the polarity of solvents.

3.2.2. Optical microscopy analysis of ACOMt in different organic solvents. Optical microscopy graphs of Na-Mt and ACOMt in three different polar solvents are presented in Fig. 4. According to Fig. 4a-1–a-4, we can obtain the dispersibility of samples in solvent A. Particles with the form of flakes greatly aggregated in a-1. Besides, a-2 showed more thin lamellas than a-1, because the plates of ACOMt were bright and the flakes of a-1 were dark. Moreover, with the increase of surfactant loading in the interlayer of montmorillonites, the plates of a-2–a-4 were shown to be thinner and smaller size in the picture. The dispersibility of a-2–a-4 was apparently better than that of a-1, and the products presented a better dispersibility in solvent A, due to a greater amount of surfactant loading.

Fig. 4 The optical microscopy patterns of (a-1) a-Na-Mt-dispersion, (a-2) a-1.0CTAB–1.5SDS–OMt-dispersion, (a-3) a-1.2CTAB–1.5SDS–OMt-dispersion, (a-4) a-1.8CTAB–1.5SDS–OMt-dispersion, (b-1) b-Na-Mt-dispersion, (b-2) b-1.0CTAB–1.5SDS–OMt-dispersion, (b-3) b-1.2CTAB–1.5SDS–OMt-dispersion, (b-4) b-1.8CTAB–1.5SDS–OMt-dispersion, (c-1) c-Na-Mt-dispersion, (c-2) c-1.0CTAB–1.5SDS–OMt-dispersion, (c-3) c-1.2CTAB–1.5SDS–OMt-dispersion and (c-4) c-1.8CTAB–1.5SDS–OMt-dispersion.

The results indicated that all samples of ACOMt in three organic solvents presented the same variation law of dispersibility. With the increase in the polarity of the medium, the plates of ACOMt become thinner and smaller in size. Therefore, we can conclude that the dispersibility of ACOMt improved with the enhanced polarity of organic solvents. Moreover, the dispersibility of ACOMt is not linearly related to the basal spacing and morphology of ACOMt in organic solvents. In addition, as shown in Fig. 4 a-2, a-3, b-2, b-3, b-4, c-2 and c-3, there were some fibrous sheets and with an increase in surfactant loading, the amount of fibrous sheets initially increased and finally decreased. Moreover, medium and highly polar organic solvents produced more sheets that are fibrous. We supposed that was the product of SDS reacting with Ca²⁺. The result coincided with XRD patterns exhibiting a sharp reflection at 3.20 nm.

3.2.3. SEM analysis of ACOMt in different organic solvents. SEM profiles of Na-Mt and ACOMt in three organic solvents were presented in Fig. 5. Na-Mt in organic solvents (a-1–c-1) showed mass agglomerates of montmorillonite platelets. Particles of ACOMt in lamellar form turned out smaller than those of Na-Mt. However, the morphology of ACOMt in organic solvents remained as a stack of flakes and was slightly affected by the surfactant concentration in the interlayer of montmorillonite and the polarity of the solvents. We also observed that the lamellas of ACOMt in solvent A were more loose than those of ACOMt in solvent B and C. This result was also in agreement with the results of XRD of ACOMt in organic solvents.

Fig. 5 SEM images of (a-1) a-Na-Mt-dispersion, (a-2) a-1.0CTAB–1.5SDS-dispersion, (a-3) a-1.2CTAB–1.5SDS-dispersion, (a-4) a-1.8CTAB–1.5SDS-dispersion, (b-1) b-Na-Mt-dispersion, (b-2) b-1.0CTAB–1.5SDS-dispersion, (b-3) b-1.2CTAB–1.5SDS-dispersion, (b-4) b-1.8CTAB–1.5SDS-dispersion, (c-1) c-Na-Mt-dispersion, (c-2) c-1.0CTAB–1.5SDS-dispersion, (c-3) c-1.2CTAB–1.5SDS-dispersion and (c-4) c-1.8CTAB–1.5SDS-dispersion.

3.2.4. Gel volume of organo-montmorillonite in different organic solvents. The gel volumes of OMt dispersed in three different organic solvents are presented in Table 4. The polarity values of the solvents are presented in Table 2. As shown in Table 4, with the increase in the polarity of the organic solvents, from solvent A to solvent C, the gel volumes of COMt increased gradually, and the gel volume of ACOMt initially decreased and finally increased. With the increase of surfactant, the gel volumes of ACOMt decreased in solvent A and solvent B, and the gel volumes of COMt increased. It also can be observed that ACOMt and COMt are well able to disperse in highly polar organic solvents. However, the gel volumes of ACOMt were lower than that of COMt in solvents with a low polarity. Therefore, we can conclude that ACOMt dispersed in highly polar media is greatly uniform. In addition, we know that there is no clear relationship between gel volume and those characterizations.

Table 4 Gel volumes in solvents A, B and C

Sample	Gel volume (ml)
	Solvent A	Solvent B	Solvent C
1.0CTAB–OMt	22	64	100
1.2CTAB–OMt	20	69	100
1.8CTAB–OMt	24	93	100
1.0CTAB–1.5SDS–OMt	23	13	100
1.2CTAB–1.5SDS–OMt	21	11	100
1.8CTAB–1.5SDS–OMt	13	8	100

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3.2.5. Apparent viscosity of organo-montmorillonite in different organic solvents. The apparent viscosity of Na-Mt and OMt is presented in Table 5. According to Table 5, we can conclude that the apparent viscosity of COMt decreased gradually with the increase of CTAB loading. With the increase of organic solvent polarity, the apparent viscosity of COMt increased initially and finally decreased. The results of apparent viscosity of COMt were in agreement with the contact angle data. The thixotropic index of COMt gradually increased, when increasing the amount of surfactant loading. However, the thixotropic index of COMt decreased with an increase in solvent polarity.

Table 5 Apparent viscosity of Na-Mt, COMt and ACOMt dispersed in solvents A, B and C, respectively

Sample	Solvent	Apparent viscosity (mPa s)			Thixotropic index
		60 rpm	30 rpm	6 rpm
1.0CTAB–OMt	Solvent A	1807	3180	5190	2.87
	Solvent B	1116	1819	2510	2.25
	Solvent C	3.7	5.4	15.2	4.11
1.2CTAB–OMt	Solvent A	1253	1973	7867	6.28
	Solvent B	1389	2119	3241	2.33
	Solvent C	3.7	3.8	0	0
1.8CTAB–OMt	Solvent A	631	1031	4388	6.95
	Solvent B	1260	2023	5596	4.44
	Solvent C	2.3	3.5	0	0
1.0CTAB–1.5SDS–OMt	Solvent A	4.4	5.1	0	0
	Solvent B	305	623	2411	7.90
	Solvent C	1.9	33.8	0	0
1.2CTAB–1.5SDS–OMt	Solvent A	12.5	21.1	31.1	2.49
	Solvent B	204	440	1812	8.88
	Solvent C	2.8	5.0	14.0	5.00
1.8CTAB–1.5SDS–OMt	Solvent A	1.9	18.3	0	0
	Solvent B	17.1	32.1	143	8.36
	Solvent C	2.4	2.8	0	0

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In addition, it can be observed that the apparent viscosity and thixotropic index of 1.0CTAB–1.5SDS–OMt in three organic solvents initially increased and finally decreased, with an increase in solvent polarity. 1.2CTAB–1.5SDS–OMt and 1.8CTAB–1.5SDS–OMt had the same variation law of apparent viscosity and thixotropic index. With the increase of CTAB loading, the apparent viscosity and thixotropic indexes of ACOMt in one organic solvent, except solvent B, initially increased and finally decreased, but the apparent viscosity of ACOMt was smaller than that of COMt. The apparent viscosity and thixotropic index do not have a certain relationship with the results of XRD, SEM and optical microscopy. According to the gel volume results, it is obvious that there is an inverse relationship between gel volume and apparent viscosity in different polar organic solvents.

4. Conclusion

In this work, the properties of ACOMt in different organic solvents were characterised using XRD, SEM, optical microscopy and viscometry, etc. The results indicated that the dispersibility of ACOMt in organic solvents was influenced by surfactant concentration and the polarity of the solvents. With an increase in the surfactant concentration and the polarity of the solvents, the dispersibility of ACOMt was significantly improved. The morphology of ACOMt in organic solvents was not likely affected by the surfactant loading and the polarity of solvents, but different from the morphology of powdered ACOMt. With the increase in solvent polarity, gel volumes initially decreased and finally increased, and the apparent viscosity and thixotropic index of ACOMt had a reverse variation. The basal spacing and morphology of the ACOMt powdery samples were consistent. Nevertheless, there is not a linear relationship with the basal spacing and morphology of ACOMt in organic solvents. 1.0CTAB–1.5SDS–OMt has an inverse relationship between gel volume and apparent viscosity of ACOMt in different polar organic solvents. Obviously, 1.2CTAB–1.5SDS–OMt and 1.8CTAB–1.5SDS–OMt have the same variation. Our present study demonstrated that ACOMt dispersed in medium or highly polar organic solvents showed excellent dispersing properties and thixotropic properties.

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