2.1. Materials
The two kinds of Ca-MT are obtained from Shandong and Inner Mongolia of China, respectively, and are purified by natural sedimentation method. Their chemical compositions are presented in
Table 1. Their semi-cell structure formulas are (Ca
0.17K
0.02Mg
0.01)
0.38·{(Al
1.63Fe
0.01Mg
0.36)
2.00[(Si
3.98Al
0.02)
4.00O
10](OH)
2} and (Na
0.03Ca
0.29K
0.04Mg
0.02)
0.69·{(Al
1.27Fe
0.04Ti
0.03Mg
0.66)
2.00[(Si
3.94Al
0.06)
4.00O
10](OH)
2}, and their corresponding layer charge density is 0.38 and 0.69, respectively. In order to improve the dispersity of Ca-MT in water, both of them were sodium-modified by Na
2CO
3 of 4.0% (wt %) for 1.0 h at room temperature, dehydrated by centrifugation in the centrifugal machine (TD5A-WS, Jiangsu Jintan Instrument Factory, Jintan, China), dried in the oven (302, Shandong Longkou Xianke Instrument Co., Ltd., Longkou, China) at 80.0 °C for 24.0 h, and then ground to less than 200 mesh by vibrating grinder (XZM-100, Wuhan Prospecting Machinery Factory, Wuhan, China). The prepared Na-MT were denoted as Na-MT1 and Na-MT2, respectively, and their SSA, pore volume (PV), average pore diameter (APD), and cation exchange capacity (CEC) are measured and showed in
Table 2.
Both absolute ethyl alcohol and xylene were purchased from Tianjin Fuyu Fine Chemical Co., Ltd. (Tianjin, China). The four types of alkyl ammonium are dodecyl trimethyl ammonium chloride (DTAC), tetradecyl trimethyl ammonium chloride (TTAC), hexadecyl trimethyl ammonium chloride (HTAC), and octadecyl trimethyl ammonium chloride (OTAC), respectively. The DTAC with 98.0% purity was purchased from Shanghai Shanpu Chemical Co., Ltd. (Shanghai, China). The TTAC with 99.0% purity was purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). The HTAC with 98.0% purity was purchased from Tianjin Zhiyuan Chemical Reagent Co., Ltd. (Tianjin, China). The OTAC with 99.0% purity was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All the alkyl ammonium were directly used without further purification, and their molecular structures are shown in
Figure 1.
2.2. Preparation of Organo-MT
Organo-MT were prepared by dissolving 10.0 g of Na-MT into distilled water at 80.0 °C in a 250.0 mL beaker, then slowly adding the dispersed solution of DTAC, TTAC, HTAC, or OTAC under the same temperature condition. The mass of alkyl ammonium added was equivalent to 1.0 CEC of Na-MT, and the mass ratio of water to dried Na-MT was 20:1. The mixture was stirred for 2.0 h at 80.0 °C, then the prepared organo-MT was collected by centrifugation at 4000 r/min, washed with distilled water until there is no Cl− examined by AgNO3, dried in the drying oven (302, Shandong Longkou Xianke Instrument Co., Ltd., Longkou, China) at 80.0 °C for 48.0 h, and ground to less than 200 mesh by vibrating grinder (XZM-100, Wuhan Prospecting Machinery Factory, Wuhan, China). The obtained samples are denoted as Cn-MT1 or Cn-MT2, where n = 12, 14, 16, or 18, respectively.
2.3. Analytical Methods
Powder XRD patterns of samples were recorded with a D/max-r B diffractometer using Cu Kα radiation at 40.0 KV and 100.0 mA (Rigaku Corporation, Akishima-shi, Japan). The diffraction angle of patterns was collected from 3.0° to 20.0° with a scanning speed of 3.0°/min.
FTIR spectrum of Na-MT and corresponding organo-MT were collected by using the Nicolet Is 50 fourier infrared spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) with the KBr pellet method (1.00 mg of sample homogenized with 200.00 mg KBr). The wavenumber range was 4000–400 cm−1 with resolution of 4.0 cm−1.
TG analysis was performed on TG/SDTA851e thermogravimetric/differential thermal synchronization analyzer (Mettler-Toledo, Zurich, Switzerland). The sample was heated from 30.0 °C to 1000.0 °C at a rate of 20.0 °C/min. The flow of high purity nitrogen was 20.0 mL/min.
Contact angle test was performed on DSA 30 optical contact angle analyzer (Kruss, Hamburg, Germany). For this purpose, a pellet was prepared under 5.0–10.0 MPa pressure using about 0.50 g sample.
MD simulation was performed by Materials Studio 7.0 software (Accelrys, San Diego, CA, USA) to further investigate the conformation of alkyl ammonium within organo-MT interlayer. According to the semi-cell structural formula, 4a × 2b × 1c super-cell models were established for simulation study. Our previous study had proved that when the mass of modifier is equivalent to the 1.0 CEC of Na-MT, alkyl ammonium cation can totally enter into the interlayer of Na-MT [
29]; therefore, according to the calculation result, we, respectively, added six alkyl ammonium cation into C
n-MT1 interlayer and eleven alkyl ammonium cation into C
n-MT2 interlayer to ensure total charge balance of the crystal. Then we added different amounts of H
2O to ensure that the d
001 value of simulation result was consistent with XRD data. The number of alkyl ammonium cation and H
2O added is shown in
Table 3.
The geometry optimization task was performed to relax the structure of organo-MT model under the following conditions: the organo-MT layers were assumed to be rigid, the cell parameters
a,
b,
α,
γ remained unchanged, and
c and
β were variable. The algorithm selected was Smart Minimizer, the long-range electrostatic interaction used was Ewald summation method, and the short-range van der Waals used was atom-based summation method. The vacuum cutoff radius was 12.5 Å, the spline width was 1.0 Å, the buffer width was 0.5 Å, and the number of iteration steps was 5000. The geometry optimization results of organo-MT models are shown in
Table 4. After geometry optimization, molecular dynamics simulation was carried out under NPT ensemble and NVT ensemble, respectively. The simulation temperature was 298.0 K, pressure was 1.0 × 10
−4 GPa, the time step length was 1.0 fs, the numbers of steps were 2 × 10
6, and the total simulation time was 2.0 ns. During the simulation, atomic coordinates and lattice parameters were allowed to change freely, and the output data was used for result analysis.
Before gel apparent viscosity measurement, we weighed 65.80 g xylene solvent in a 50.0 mL beaker, put the beaker at high speed mixer (GFJ-QA, Jiangyin Shuangye Machinery Co., Ltd., Jiangyin, China), submerged solvent dispersion plate to highly 1/3 volume of the beaker, adjusted the speed of high-speed mixer to 500 r/min, added 6.00 g of organo-MT sample under the condition, stirred for 3 min at 1500 r/min, added 95% ethanol solution of 3.8 mL and stirred for 2 min, and then stirred for 5.0 min at 3000 r/min. Then, we removed the beaker after stopping.
We placed the beaker on the digital viscometer (SNB-2, Shanghai Fangrui Instrument Co., Ltd., Shanghai, China) to test the apparent viscosity of organo-MT gel. Using the No. 3 rotor, we adjusted the viscometer until the groove (immersion mark) on the shaft of the rotor just touches the sample. We slowly moved the beaker on the horizontal surface to ensure that the rotor was located in the center of the beaker. We adjusted the measured speed of the viscometer to 6 r/min and read the number of the display screen after rotating it 10 times.