A comparative study performance of cationic organic montmorillonite prepared by different methods

2.1 Mt

Mt was purchased from Linan, Zhejiang province, China. The Mt was ground in an agate mortar and passed through a 200-mesh sieve, without purification. Being analyzed by XRD, basal spacing of the sample was 1.50 nm (Figure 1). The CEC of the Mt, measured by the methylene blue method, was 106.4 mmol/100 g. The cationic surfactant was cetyltrimethyl ammonium bromide (CTAB), purchased from Shantou Xilong Co., Ltd., China.

Figure 1:

XRD patterns of montmorillonite.

2.2 Preparation of the cationic OMts samples

The cationic OMts were prepared through the dry method by adding 10 g of Mt and CTAB into the ball mill and milling for 1 h. The amounts of surfactants were equivalent to 1.0 and 1.5 times the CEC of Mt. The products were ground in an agate mortar and passed through a 200-mesh sieve.

The cationic OMts were prepared through the semi-dry method by adding 10 g of Mt, CTAB, and 1 g water into the ball mill and milling for 1 h. The amount of surfactants and the procedure were the same to OMts prepared by dry method.

The cationic OMts prepared through the wet method were according to a previous study [21]. The amounts of surfactants used were the same as those in the dry method. The products were centrifuged and washed two times with deionized water, dried at 60°C, ground in an agate mortar, and passed through a 200-mesh sieve.

Modified by different methods (dry, semi-dry, and wet methods) under different amount of surfactants (1.0 and 1.5 CEC), samples were named as OMt (1.0)-D and OMt (1.5)-D, OMt (1.0)-S and OMt (1.5)-S, and OMt (1.0)-W and OMt (1.5)-W.

2.3 Characterization of the cationic OMts samples

XRD analysis was performed on a D/max-rA 12 kW diffraction (Rikagu, Japan) at 40 kV and 100 mA, using a Cu tube (Cu-Ka radiation, l=0.154 nm). Scans were recorded between 2° and 25° (2θ) with a step size of 0.02°, at a scanning rate of 4°/min.

Static contact angle measurements of OMts were made by ALB-XY-50-3JL (Zhongchen Co., Ltd., Shanghai, China). The sessile drop method was used for the wettability measurement of the OMts to determine the samples’ hydrophilicity or hydrophobicity. A video camera equipped with a homemade image analysis device allowed the determination of contact angle between deionized water and the surface of OMts.

Dispersibility measurement of OMts was determined according to GB27798-2011. First, 3-g samples were added into 80-g mixture of petroleum ether and xylene (petroleum ether:xylene=9:1), the colloid was stirred at 1500 r/min for 5 min, and then 67-g mixture of petroleum ether and xylene was added into the colloid and stirred for another 5 min. The colloid was transferred to a 100-ml graduated cylinderto mark 100 ml, stewing for 6 h.

Thixotropy of OMts was tested by RotoVisco1, by ThermoHAAKE RV1 rotary viscosimeter from Thermo Electron (Karlsruhe) GmbH, Germany, according to GB27798-2011. The difference of two thixotropic curve areas was the ring size, which measures thixotropy of OMts.

The TG-DTA analysis was conducted on Shimadzu (Japan) DTA-TG simultaneously measuring device DTG-60 under air atmosphere, starting at 30°C. A heating rate of 5°C/min was applied until the temperature reached 900°C.

3.1 XRD analysis

Figure 2A shows XRD patterns of OMt (1.0)-D, OMt (1.0)-S, and OMt (1.0)-W. The typical XRD pattern of Ca-Mt (d₀₀₁=1.50 nm, Figure 1) disappears, and a new group of reflections occurs in Figure 2A. d₀₀₁ Values of samples are d_001A=4.09 nm, d_001B=4.12 nm, and d_001C=3.91 nm, respectively. The d₀₀₁ diffraction intensity of A and B curves are relatively higher than that of d₀₀₂, while the intensity of d_002C value is higher than that of d_001C (Figure 2A). Under low surfactants addition, OMt (1.0)-W has smaller basal spacing than that of OMt (1.0)-D and OMt (1.0)-S. The results show that the degrees of the modification are influenced by different methods. In the wet modification method, a large part of surfactants dissolve in the water and dilute so part of them cannot be used to modify Mt, which led to insufficient modification, while for surfactants used in dry and semi-dry methods, under the mechanical force modification, none of surfactants are diluted, so surfactants equivalent to 1.0 CEC are sufficient to modify the Mt.

Figure 2:

XRD patterns of cationic OMts.

(A) XRD patterns of low surfactants ratio organically modified montmorillonite [a: OMt (1.0)-D, b: OMt (1.0)-S, c: OMt (1.0)-W]. (B) XRD patterns of high surfactants ratio organically modified montmorillonite [a: OMt (1.5)-D, b: OMt (1.5)-S, c: OMt (1.5)-W].

Figure 2B shows XRD patterns of OMt (1.5)-D, OMt (1.5)-S, and OMt (1.5)-W. When comparing Figure 2A and B, the effect of intercalation of OMt (1.0)-S and OMt (1.0)-D has changed slightly under different amount of surfactants; their basal spacings remain at d_001A=4.11 nm and d_001B=4.09 nm. Comparing basal reflection of OMt (1.0)-W with that of OMt (1.5)-W in Figure 2A and B, more regular diffraction curves are presented in Figure 2B. The reflection of d_001C in Figure 2B has a relatively high intensity, accompanying with regular d₀₀₂ and d₀₀₃ diffraction peaks, and indicates that the OMts are modified efficiently by wet methods. In dry and semi-dry methods, surfactants beyond 1.0 CEC can hardly change the basal spacing because surfactants equivalent to 1.0 CEC have already modified Mt sufficiently, extra surfactants can only be attached to the surface of OMts. With surfactants equivalent to 1.5 CEC in the wet method, sufficient surfactants can be used to intercalate the Mt, so the basal spacing of OMt (1.5)-W is larger than that of OMt (1.0)-W. The results also verify the conclusion before that under low surfactants ratio, a large part of surfactants dissolve in the water and cannot be used to modify Mt.

As experiments described above, modification methods have an impact on the modification effect of cationic OMts. With surfactants equivalent to 1.0 CEC, Mt is modified sufficiently through dry and semi-dry methods but not with wet method. With a higher amount of surfactants used, Mt can be modified sufficiently by all three methods. For OMt (1.5)-W, basal spacing reaches 3.98 nm and the intensity of d₀₀₁ reflection is higher than that of OMt (1.5)-D and OMt (1.5)-S, indicating that its modification effect is improved, while basal spacings of OMt (1.5)-S and OMt (1.5)-D have not changed significantly. It could be concluded that with more amount of surfactants, OMts prepared by wet method have similar interlayer space structures as OMts obtained by dry and semi-dry methods, while the same d₀₀₁ value of OMts can be prepared with less dose of surfactants by dry and semi-dry methods.

3.2 TG-DTA analysis

The TG and DTA analysis results of OMts prepared by dry, semi-dry, and wet methods, respectively, are illustrated in Figure 3A and B.

Figure 3:

TG and DTA diagrams of OMts.

(A) TG diagram of OMI (B) DT diagram of OMI. OMt (1.0)-W: montmorillonite modified by wet method with 1.0 CEC of cationic surfactants. OMt (1.0)-S: montmorillonite modified by semi-dry method with 1.0 CEC of cationic surfactants. OMt (1.0)-D: montmorillonite modified by dry method with 1.0 CEC of cationic surfactants. OMt (1.5)-W: montmorillonite modified by wet method with 1.5 CEC of cationic surfactants. OMt (1.5)-S: montmorillonite modified by semi-dry method with 1.5 CEC of cationic surfactants. OMt (1.5)-D: montmorillonite modified by dry method with 1.5 CEC of cationic surfactants.

Mass loss of the samples takes place in two steps, as shown in Figure 3A. In the first part of decomposition of OMts, mass loss below 205°C is related to the small organic molecule matter and water molecules adsorbed on the OMts particles [22]. The different mass loss percentage of OMts before 205°C reveals the different decomposition of residing small organic matter and water molecules on the surface of OMts. OMts prepared through wet method has the least mass loss percentage than OMts prepared by other methods regardless of the amount of organic modifier used during the modification process, while OMts prepared by dry method has the most mass loss. In TG curves of three OMts samples, the mass losses are insignificant between 100°C and 170°C, while in Figure 3B, there is no evident endothermic peaks appearing between 100°C and 170°C, indicating that there are hardly water molecules in the interlayer of OMts due to organic modification, which verifies the efficient organic modification [23], [24], [25].

The second-stage mass loss of three OMts samples in TG curves starts at approximately 170°C and ends at roughly 700°C. Starting temperatures of mass losses for OMt (1.0)-D, OMt (1.0)-S, and OMt (1.0)-W are 180°C, 187°C, and 205°C; mass losses end at 682°C, 684°C, and 687°C, respectively. Starting temperatures of mass losses for OMt (1.5)-D, OMt (1.5)-S, and OMt (1.5)-W are 171°C, 174°C, and 194°C; mass losses end at 680°C, 683°C, and 692°C, respectively. The more amount of surfactants remaining on the surface of OMts, the earlier the decomposition temperature of organic molecules in the OMts. Obviously, due to less organic molecules on the surface of OMts prepared by wet method, surfactants in the interlayer of OMt-W decompose at higher temperature. Furthermore, DTA results of different OMts in Figure 3B verify that the thermal stability of OMt-W is better than that of OMt-D and OMt-S; thermal stability of OMt-S is better than that of OMt-D. The results show that order of surfactants in the interlayer of OMt-W and structure regularity of OMt-W have a positive effect on the thermal stability of surfactants and the OMts. The results show that OMts-W has the best thermal stability among OMts prepared by dry, semi-dry, and wet methods.

3.3 Contact angle analysis

Figure 4 demonstrates contact angles of OMts modified through different methods under different amount of surfactants. Contact angles of OMt (1.0)-D, OMt (1.0)-S, and OMt (1.0)-W are 43.5±0.5°, 47.5±0.5°, and 49.0±0.5°, respectively. Among the results, OMt (1.0)-S and OMt (1.0)-W have similar contact angles. From the data above, during the process of modification, the more water used, the better coverage of surfactants on the surface of OMts, the better hydrophobicity of OMts achieved.

Figure 4:

Contact angles of cationic OMts.

(A) Contact angles of OMt (1.0)-D. (B) Contact angles of OMt (1.0)-S. (C) Contact angles of OMt (1.0)-W. (D) Contact angles of OMt (1.5)-D. (E) Contact angles of OMt (1.5)-S. (F) Contact angles of OMt (1.5)-W.

Dispersity of surfactants during the modification process has a positive relationship with the coverage on the surface of OMts. As there is no dispersion medium in the dry method, the dispersion of surfactants on the surface of OMts obtained by dry method is worse than that of the other two methods, which leads to poor coverage and effect of modifications. For OMts prepared through semi-dry method, the small amount of water added as medium can help surfactants disperse more evenly on the surface of OMts, which leads to better hydrophobicity. For OMts prepared by wet method, over 90% water molecules in the modification process enhance the dispersity of surfactants on the surface of OMts. With the most even dispersity of surfactants on the surface of OMts, OMt (1.0)-W has the best hydrophobicity of all the three samples.

Contact angle of OMt (1.5)-D, OMt (1.5)-S, and OMt (1.5)-W are 43.5±0.5°, 46.0±0.5°, and 52.0±0.5°, respectively, which reveals that the hydrophobicity of OMts is OMt (1.5)-W>OMt (1.5)-S>OMt (1.5)-D. The comparison between Tables 1 and 2 shows that the contact angle of OMt (1.5)-W has changed significantly compared with OMt (1.0)-W, while the contact angle of OMt (1.5)-D and OMt (1.5)-S has little change compared with OMt (1.0)-D and OMt (1.0)-S. Obviously, increased lipophilicity indicates that in wet method modification, sufficient surfactants in liquid phase cover evenly on the surface of Mt and lead to good hydrophobility in the wet method, while for the preparation of OMts by dry and semi-dry methods, over-used surfactants can hardly change the surfactants coverage of the surface of OMts.

3.4 Dispersibility analysis

Table 1 illustrates d₀₀₁ values and dispersibility of OMt (1.0)-D, OMt (1.0)-S, and OMt (1.0)-W.

The expanding volume of samples in a graduated cylinder is 22.0 ml, 39.5 ml, and 39.0 ml, respectively. Dispersibility of OMt (1.0)-S and OMt (1.0)-W has no significant differences, which distributes around 40 ml and corresponds to the results in Table 1. The lipophilicity of OMt (1.0)-S and OMt (1.0)-W is better than that of OMt (1.0)-D, which also verifies the previous conclusion that the surfactants are unevenly distributed on the surface of OMts modified by the dry method.

Table 2 shows d₀₀₁ values and dispersibility of OMt (1.5)-D, OMt (1.5)-S, and OMt (1.5)-W.

The expanding volumes of samples in a graduated cylinder are 30.0 ml, 39.5 ml, and 67.0 ml, respectively. The calibration of OMt (1.5)-W is about 67 ml, significantly higher than that of OMts obtained by the other two methods. The results are consistent with results in Table 2 that surfactants uniformly distributed on the surface of Mt in the wet method lead to good hydrophobicity.

In this part of the experiment, the dispersibility of OMts in petroleum ether and xylene solution is OMts-W>OMts-S>OMts-D. Thus, to a very real extent, modified methods affect the dispersion properties of the OMts.

3.5 Thixotropy analysis

Figure 5 shows thixotropic loop areas of OMts modified through different methods under different amounts of surfactants. Thixotropy as a crucial property illustrates the rheological property of the solution [26], [27]. From results in Figure 5, there are differences in the thixotropic loop between samples. Thixotropic loop areas of OMt (1.0)-D, OMt (1.0)-S, and OMt (1.0)-W in (A), (B), and (C) are 8.0 Pa/s, 19.3 Pa/s, and 21.6 Pa/s, respectively. Obviously, OMt (1.0)-D has a smaller thixotropic loop and worse thixotropy than that of OMt (1.0)-S and OMt (1.0)-W. OMt (1.0)-S and OMt (1.0)-W have little difference in the thixotropic loop area at roughly 20 Pa/s. Results show that OMt (1.0)-S and OMt (1.0)-W have better thixotropy than that of OMt (1.0)-D. And thixotropy of OMt (1.0)-W is slightly better than that of OMt (1.0)-S. Thixotropic loop areas of the OMt (1.5)-D, OMt (1.5)-S, and OMt (1.5)-W are 8.1 Pa/s, 22.6 Pa/s, and 28.7 Pa/s, respectively. Both OMt (1.5)-S and OMt (1.5)-W have an increase in thixotropy, while OMt (1.5)-W has increased more than the other OMts. The increase of surfactants concentration will lead to the increased interchain interaction and more ordered structures [28], [29]. OMts with well-ordered structure surfactants can disperse homogeneously in organic solvent and quickly form “house-of-cards” aggregate structure in the solvent [17], which shows better thixotropic property.

Figure 5:

Thixotropic loop diagrams of cationic OMts.

(A) OMt (1.0)-D thixotropic loop diagrams. (B) OMt (1.0)-S thixotropic loop diagrams. (C) OMt (1.0)-W thixotropic loop diagrams. (D) OMt (1.5)-D thixotropic loop diagrams. (E) OMt (1.5)-S thixotropic loop diagrams. (F) OMt (1.5)-W thixotropic loop diagrams.

Thixotropy results show that the organic modification methods of Mt have an obvious impact on its thixotropy; order of thixotropy is OMts-W>OMts-S>OMts-D, while the differences between OMts-S and OMts-W are relatively small with less amount of surfactants used.