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Insights into the modification for improving the surface property of calcium sulfate whisker: Experimental and DFT simulation study
Applied Surface Science 478 (2019) 594–600
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Insights into the modification for improving the surface property of calcium sulfate whisker: Experimental and DFT simulation study ⁎
T
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Hao Fan, Xingfu Song , Yanxia Xu, Jianguo Yu
National Engineering Research Center for Integrated Utilization of Salt Lake Resources, East China University of Science and Technology, Shanghai 200237, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Calcium sulfate whisker Hydrophobicity Sodium stearate DFT simulation Modification mechanism
In order to improve the compatibility and dispersion of anhydrous calcium sulfate whiskers (ACSW) in polymer composites, the treatment for surface modification was studied with aqueous solution sodium stearate (SS, C17H35COONa) at hot temperature. The surface modification mechanism between stearate and ACSW was carried out by experimental and density functional theory (DFT) simulation. The surface property of modified anhydrous calcium sulfate whiskers (M-ACSW) became hydrophobic by changing the water contact angle with the calcium sulfate whisker (CSW) from 0° to 132°. The TEM and TG results proved the adsorption of stearate on M-ACSW surface, which was further confirmed by FTIR and XPS. The experimental and DFT simulation outcomes for the mechanism were interpreted by taking the reaction of the carboxyl group of stearate with Ca2+ ions to form calcium stearate on the whisker surface. Their long alkyl chains of stearate coated on the surface of whisker made ACSW improve the surface with a hydrophobic property. Thus, here it is provided details of a novel method of modifier selection for CSW and its interaction mechanism with the crystal surface.
1. Introduction Calcium sulfate whisker (CSW) is a potential modified reinforcing agent because it has the characteristics of the perfect structure, integrated shape, specific cross-section and constant size. CSW is usually synthesized from dihydrate calcium sulfate through the hydrothermal process [1–4]. Compared with other inorganic whiskers, CSW has many advantages, such as nontoxicity, good thermal stability, chemical resistance, high strength, low cost and a strong affinity for organic matter, which make it to a green material with high-cost effectiveness. Excellent physical, chemical, and mechanical properties can be obtained with the addition of CSW into materials. Therefore, it can be widely used in the polymer, rubber, papermaking, ceramics and so on [5–8]. It is known that the surface free energy of needle-like whisker is high while in polymer it is low, giving them incompatibility and dispersion for whisker, and consequently poor properties of composites [9,10]. Thus, changes in the surface properties of whisker to improve its compatibility throughout an appropriate surface modification filler played an important role [11–13]. The literature reported different properties after surface modification treatment such as hydrophobiclipophilic through the magnesium oxysulfate whisker [14] or hydrophobic through CSW [15] by the interfacial adhesion between the modified CSW and organic matrix [16,17]. In this case, the
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modification mechanism was mainly attributed to the modifier adsorbed on the surface of the filler, which consisted of physical adsorption [11], chemical adsorption [14,15,18] or both of them [19,20]. Moreover, the compatibility of modified carbon fibers filler into the filler/matrix improved the physical property of polymer composites by enhancing the interfacial adhesion between filler and matrix [18,20]. After a surface modification, the interfacial adhesion and crosslinking degree between the modified CSW and the organic matrix were enriched [21] as well as their mechanical property and uniform distribution of CSW in the matrix [22]. Although plenty of experiments have confirmed that modifiers had a crucial effect on the surface property of ACSW in previous investigations, the interaction mechanism between the modifier and crystal surface at the molecular level is not explained in detail due to the limitation of available methods. With the development of computing capability, computational simulation study becomes an effective technology to assist mechanism research [23–26]. DFT method has explored the interactions between additives and crystals successfully, which gave particular characteristic on the habit of calcium sulfate hemihydrate simulated and a good explanation of their interaction mechanism with different crystal faces [27]. DFT method was a powerful tool for study the interaction between reactants at the atomistic level that might not be accessible by experimental techniques [28]. Nowadays, DFT
Corresponding authors. E-mail addresses: [email protected] (X. Song), [email protected] (J. Yu).
https://doi.org/10.1016/j.apsusc.2019.01.161 Received 6 July 2018; Received in revised form 12 November 2018; Accepted 17 January 2019 Available online 23 January 2019 0169-4332/ © 2019 Published by Elsevier B.V.
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simulation on ACSW is still lacking and it is valuable to deeply understand how modifier affects the surface property of ACSW. In this study, the common hot-water-dissoluble sodium stearate was utilized to modify the ACSW in aqueous solution, and the effects of modifier dosage on hydrophobicity of anhydrate calcium sulfate whisker were studied. The goal of this article was to investigate the surface property of M-ACSW and modification mechanism between modifier and whisker. In order to understand the surface properties, the contact angle, TG and TEM was used. The FTIR and XPS were conducted to explain the modification mechanism which was further revealed molecular level by DFT simulation. The modification process of this research was investigated by experiment and DFT simulation, which provided a novel and meaningful method for modifier selection of CSW.
Fig. 1. (a): Anhydrate calcium sulfate morphology predicted by AE method; (b): simulation box of stearate and (020) crystal face. The white balls are hydrogen atoms, the gray balls are carbon atoms, the red balls are oxygen atoms, the yellow balls are sulfur atoms and the green balls are calcium atoms (the same below).
2. Materials and methods 2.1. Experiment 2.1.1. Materials The main raw materials used in this work were dihydrate calcium sulfate and sodium stearate. The dihydrate calcium sulfate (CaSO4·2H2O) was purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. The sodium stearate was purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals in the experiment were of analytical grade.
2.2. Simulation 2.2.1. Model construction The commercial molecular modeling software package - Materials Studio (MS) - was used to run all calculations with the DMol3 module [29]. The ACSW crystal structure (Fig. S1a) was obtained from G. C. H. Cheng and Zussman [30], with unit cell dimensions of a = 6.991 Å, b = 6.996 Å, c = 6.238 Å. The crystal system was orthorhombic with the space group AMMA. Then, the unit cell was optimized to calculate the pure crystal morphology through the attachment energy (AE) method. Fig. 1a illustrates four dominant faces [(002), (011), (020) and (200)] predicted by AE method. Therefore, these main faces were taken into account in the following simulation study. On the basis of the optimized unit cell structure, the four faces were cleaved to a fractional depth of 3. In order to obtain a large crystal layer, (002), (011), (020) and (200) faces were all extended to 2 × 2 unit cells to form a supercell. The model of stearate was built and conducted with geometry optimization (Fig. S1b). Next, the model of stearate was subsequently docked on the solid layer to form a simulation box (Fig. 1b).
2.1.2. Preparation of calcium sulfate whisker The anhydrate calcium sulfate was prepared in laboratory. The experiments were carried out in a 1 L autoclave. 20 g CaSO4·2H2O was mixed with 500 mL deionized water at room temperature. The slurries were treated under hydrothermal condition (130 °C, 3 h). Then the sample of reaction suspensions were rapidly withdrawn and filtered. The retentate was rinsed with hot calcium sulfate saturated water followed by anhydrous ethanol and dried at 60 °C for 5 h. Finally, ACSW was obtained after calcinating at 650 °C for 1 h in muffle furnace. 2.1.3. Surface modification of anhydrate calcium sulfate whisker A certain content of sodium stearate was dissolved in 200 mL deionized water under the temperature of 85 °C. Then 8 g anhydrate calcium sulfate was added into solution with stirring for 30 min at 80 °C. Lastly, the suspension liquid was filtered and washed with plenty of water to remove excessive sodium stearate and dried at 60 °C for 5 h.
2.2.2. Calculation of property The simulation models of stearate docked on ACSW (020), (011), (020), (200) planes were optimized with DMol3 module. The density functional calculations were conducted to optimize geometries and obtain the properties of energy, electrostatic potentials, and population analysis of the systems. The generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerhof (PBE) of parameterization was used to describe the electronic exchange and correlation effects [27]. The core electrons were treated by the all electron relativistic method and the basis set of double numerical plus polarization (DNP) of version 3.5 was used with the global orbital cutoff distance of 5.5 Å. A vacuum layer with a thickness of 50 Å was set in z-direction to exclude the periodic effect. The electron density distribution was partitioned with the Mulliken method. The adsorption energy of a stearate ion on ACSW surface was calculated from:
2.1.4. Characterization Five drops of deionized water (15 μL) were deposited on products surface to measure the average value of contact angle by using contact angle instrument (JC2000D1, POWEREACH, China). The structures and phase composition were identified by powder X-ray diffractometer (XRD; D8 advanced, Bruker, Germany) using Cu Kα radiation with a scanning rate of 10° min−1 and a scanning 2θ range of 10° to 80°. The morphology of the samples was characterized with the scanning electron microscopy (SEM; Quanta 250, FEI Co., USA) and the high-resolution transmission electron microscopy (TEM; JEM-2100, JEOL, Japan). The thermogravimetric analysis (TG; SDT Q600, TA Co., USA) was performed under identical condition; the measurements were conducted in alumina crucible using about 5 mg of sample with heating rate of 10 °C/min. The Element content analysis of the samples was conducted by elemental analyzer (EA; VARIO BLIII, ELEMENTAR, USA). The functional groups of samples were determined using Fourier Transform Infrared Spectrometer (FTIR; Nicolet 6700, Thermo Fisher, USA), the wave-number range was set from 4000 cm−1 to 400 cm−1. The X-ray photoelectron spectrometer (XPS, Thermo Scientific ESCALAB 250Xi, USA) was employed to examine the surface of calcium sulfate hemihydrate by using an Al Ka X-ray source.
ΔE = Etotal − (Esurface − Emodifier ) where Etotal, Esurface, and Emodifier are the energies of the ACSW surface with adsorbed stearate, isolated ACSW surface and one isolated stearate ion, respectively. According to the equation, a more negative value of ΔE indicates stronger binding between adsorbed molecule and ACSW surface [31,32].
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3. Results and discussion 3.1. Characterization of ACSW and M-ACSW The contact angle (θ), conventionally measured through the liquid, is the angle which quantifies the wettability of a solid surface by a liquid. As shown in Fig. 2, the contact angle increase from 0° to 132° with the modifier dosage increasing from 0 to 4 wt%. In the absence of modifier, the contact angle of ACSW was 0°. The contact angle of MACSW was 97° with the modifier dosage of 2 wt%, which showed that the surface property of ACSW had been changed and became hydrophobic. When the modifier dosage increasing to 4 wt%, the contact angle reached to 132°. Further adding the dosage up to 6 wt%, the contact angle was 136°. It suggested that the surface modification of whisker was completed with the dosage of 4 wt%, and the modified effect will remain unchanged with further adding modifier to 6 wt%. The above results indicated that the property of modified crystal surface was changed and became hydrophobic. However, all of the prepared products consisted of only ACSW crystals without any other crystalline phases (Fig. S2). The morphology of ACSW was smooth and needle-like. This demonstrated that the modification just affected the surface of ACSW, but did not change the crystal structure and morphology. The needle-like whisker has excellent physical and chemical performances, which is very beneficial for the application of M-ACSW. Fig. 3 displays the TG curves of ACSW and M-ACSW in nitrogen atmosphere. The weight loss of ACSW is about 0.15%, which is ascribed to the free water adsorbed on whisker surface. For M-ACSW, two degradation steps can be seen from the curves in Fig. 3b–d. The first step is showed at about 100 °C, which is the process of removing free water. The second step is observed between 300 °C to 500 °C. With the increase of modifier dosage, the weight loss is 1.46%, 3.15% and 4.78%, respectively, which can be attributed to the thermal decomposition of coated organic modifier [33]. Meanwhile, the elemental analysis was also used to measure the carbon content of whisker modified with different modifier dosage. As shown in Table 1, the carbon content of whisker without modification is < 0.3%, which suggests that no carbonaceous material adsorbs on whisker surface. With the increase of modifier dosage, the carbon content of modified whisker changes from 0 to 3.93%. The above results further indicated that modifier can
Fig. 3. TG curves of the products prepared without sodium stearate (a), with different modifier dosage (b, 2 wt%; c, 4 wt%; d, 6 wt%). Table 1 The carbon content of whisker modified with different modifier dosage. Modifier dosage/wt%
0
2
4
6
Carbon content (C%)
< 0.3%
1.47
2.65
3.93
adsorb on whisker surface and the modified effect gradually become better with the increase of modifier dosage. The TEM images of ACSW and M-ACSW are displayed in Fig. 4. From the photographs, it is found that the morphology of ACSW is smooth and integrated. Most electrons were blocked when the electron beam passed through the crystal due to the dense structure of ACSW, thus, only a dark shadow picture (Fig. 4a) was left. While as shown in Fig. 4b, apart from the dark shadow, a light-color part coats on the surface of whisker, which can be the adsorbed modifier. Most electrons were passed through, when the electron beam passed through the coated layer due to the structure of modifier coating layer less dense than whiskers, thus, only a light-color image was left. Meanwhile, the modifier adsorbed uniformly on the surface of M-ACSW. The results demonstrated that the modifier indeed adsorbed on the surface of M-
Fig. 2. The SEM photos of the products prepared without sodium stearate (a), with different modifier dosage (b, 1 wt%; c, 2 wt%; d, 4 wt%; e, 6 wt%) and a plot of contact angle of ACSW versus modifier dosage (f). 596
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Fig. 4. TEM images of (a) ACSW and (b) M-ACSW modified with 4 wt% sodium stearate. (The blue dash lines denote the surface of the whisker; the red arrows denote the surface of the coated layer).
ACSW, which could inhibit the contact of water with whisker and make the surface of whisker become hydrophobic. The XPS spectra of ACSW and M-ACSW are showed in Fig. 5. The curve-fitting analysis of C 1s spectra of ACSW located at 284.6 eV is the standard peak of carbon (Fig. S3). For the Fig. 5a, there is a new signal appearing at 286.08 eV occurred in M-ACSW. This signal can be attributed to methylene (-(CH2)-), owing to the adsorption of stearate. Ca 2p has two peaks located at 347.97 eV and 351.59 eV in calcium sulfate crystal (Fig. S4a) [34]. However, the binding energy of Ca 2p appeared at 347.49 eV and 351.02 eV after sodium stearate modification (Fig. S4b). As shown in Fig. 5b, the binding energy of Ca 2p for M-ACSW decreased by 0.48 eV and 0.57 eV, respectively. It suggested that the electron density of Ca2+ surrounding increased, which can be attributed to the effect of carboxyl of stearate. When stearate reacted with the whisker surface Ca2+, the negatively charged carboxyl group could offer electron, causing the increase of Ca2+ surrounding electron density and the decrease of binding energy. According to the above results, it was inferred that the stearate reacted with Ca2+ on the crystal surface, leading to the adsorption of stearate on M-ACSW surfaces. The long alkyl chains of stearate which stretched out from the crystal surface formed the coated layer. In order to further ascertain the adsorption behavior of the surfactant in the modification process, FTIR spectra were investigated to further identify the presence and the extent of a certain group on the surface of ACSW. Fig. 6 shows the spectra of original ACSW and MACSW with different dosage of sodium stearate [16]. The FTIR spectrum of original ACSW shows the characteristic bands of calcium sulfate at 1153 cm−1, 673 cm−1 and 595 cm−1. It is obvious that FTIR spectrum of M-ACSW shows several similar absorption bands attributed to CaSO4. However, the spectrum of M-ACSW shows several new bands. The bands at 2918 cm−1 and 2850 cm−1 were ascribed to the asymmetric and symmetric vibration of methylene (-CH2-), which was
Fig. 6. FTIR spectra of the products prepared without sodium stearate (a), with different modifier dosage (b, 2 wt%; c, 4 wt%; d, 6 wt%).
contributed by the long alkyl chains of stearate coated on whisker surface. In addition, the bands at 1575 cm−1 and 1383 cm−1 are attributed to the asymmetric and symmetric stretching of carboxylate (-COO−), respectively. With the increase of modifier dosage, the intensity became stronger. The results indicated that the stearate adsorbed on crystal surface and the coated content became more with the increase of modifier. 3.2. DFT study of stearate adsorption on ACSW According to aforementioned results, it was assumed that the modification effect of stearate on ACSW can be attributed to the stearate interaction with the Ca2+ on the crystal surface. In order to
Fig. 5. High-resolution XPS of (a) C 1s peak of M-ACSW and (b) binding energy change of Ca 2p peaks. 597
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and CaSO4, as shown in Fig. 8. The gap energy of HOMO (CH3(CH2)16COO−) and LUMO (CaSO4) was 4.03 eV, which was less than the gap energy (7.71 eV) between HOMO (CaSO4) and LUMO (CH3(CH2)16COO−). It was more likely that CH3(CH2)16COO− (HOMO) donated electrons to CaSO4 (LUMO) rather than CaSO4 (HOMO) to CH3(CH2)16COO− (LUMO). The HOMO of stearate mainly appeared on the carboxyl functional group and the LUMO of CaSO4 mainly appeared on the Ca2+ (Fig. 8), so the reaction between stearate and CaSO4 was likely to occur between carboxyl and Ca2+. The adsorption energy can be used as a qualitative indicator to study the effect of the ions on the crystal. The adsorption energy of stearate adsorbed on the (020) crystal faces was −0.149 eV (Table 2). The negative values of adsorption energy indicated that stearate could adsorb on anyone of the crystal faces, as (020) face was the most stable surface among the four main crystal face. The ionic radius of calcium (Ca2+) is 1.000 Å, and the ionic radius of oxygen (O2−) is 1.400 Å [29]. If the distance of CaeO is less than the sum of radius (2.400 Å), the chemical bond can be formed. From the Fig. 9, the distances of CaeO are 2.307 Å and 2.258 Å, which suggests that the two oxygen of carboxyl interact with the two calcium ions to form the chemical bond. Meanwhile, the charge of Ca 1 decreases from 1.567 e to 1.548 e, Ca 2 from 1.567 e to 1.545 e, which can be attributed to the donated electrons from carboxyl. Because of lacking electron in the outmost electron orbital, the electrons transferred from enriched electron orbital of carboxyl to empty orbital of Ca2+. Then the chemical bond formed when stearate moved near to the surface of ACSW. Therefore, the charge of calcium ions decreased after accepting the electrons, which matched well with the results of XPS.
Fig. 7. The XRD patterns of ACSW prepared by experiment and simulated by MS. (The green vertical lines denote the positions of diffraction peaks in the standard PDF #70-0909 for crystalline anhydrate calcium sulfate).
elucidate the mechanism, the experimental data were evaluated by DFT simulation. Fig. 7 shows the XRD patterns of the ACSW prepared by experiment and simulated by MS. The red curve is the XRD pattern of experimental products and the black curve is the XRD pattern obtained by simulation according to the crystal cell structure. The simulated pattern matched well with that from experiment, which indicated that the crystal cell structure was reasonable. The main peak at 2θ of 25.431° was indexed as the (020) reflections, which was the characteristic peak of anhydrate calcium sulfate. According to the AE method, there were four main crystal surfaces for ACSW. Then, (200), (020), (002) and (011) surfaces of CaSO4 were cleaved to calculate surface energy with the same algorithm as used for all optimizations. For anhydrate calcium sulfate crystal, the surface energy is calculated as:
Es =
Eslab −
3.3. Modification mechanism As a common anionic surfactant, sodium stearate no longer exists as a molecular form in hot water solution, but is dissociated into two parts: an anionic portion (CH3(CH2)16COO−), and a Na+ ion. The anionic part plays a crucial role in determining the surface activity of anionic surfactants. According to the equation of Gibbs free energy of reaction (ΔG), the Gibbs free energy of ion reaction between CH3(CH2)16COO− and Ca2+ is −82.19 kJ/mol, which is obviously smaller than that of the reaction of sulfate with Ca2+ (ΔG2 = −24.58 kJ/mol). It suggested that the chemical reaction as shown in Eq. (1) took place spontaneously on the surface of ACSW. Meanwhile, the solubility product (Ksp) of calcium stearate (3.98 × 10−15, 25 °C) [35] is far less than the Ksp of calcium sulfate (4.93 × 10−5, 25 °C) [36]. It meant that the precipitation of Ca2+ with CH3(CH2)16COO− was more likely to happen than SO42−. When whiskers were added into hot sodium stearate solution, the dissolved CH3(CH2)16COO− moved to the crystal surface under the condition of stirring. Then, the negatively charged carboxyl of CH3(CH2)16COO− reacted with Ca2+ on whisker surface to form the insoluble (CH3(CH2)16COO)2Ca precipitate, which adsorbed on the surface of whiskers. The alkyl chains of calcium stearate coated on the surface of M-ACSW as a film, leading to the hydrophobic whiskers.
Nslab ·E N bulk bulk
2A
where Es is the surface energy; Eslab refers to the total energy of the slab; Ebulk is the energy for CaSO4 unit cell; Nslab is the atom number of the slab; Nbulk is the atom number of the bulk and A is the surface area. The results were 2.20 eV·nm−2 for (200) face, 2.14 eV·nm−2 for (020) face, 2.59 eV·nm−2 for (002) face and 4.51 eV·nm−2 for (011) face, which meant the stability sequence of these surfaces was: (020) > (200) > (002) > (011). It was considered that (020) surface was more easily obtained when CaSO4 crystals were cleaved or in nature, which explained the characteristic peak of anhydrate calcium sulfate correspond to (020) face. Therefore, all calculations were conducted on (020) surface. The reaction site of stearate on (020) surface of CaSO4 was determined by calculating the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of stearate
Fig. 8. HOMO and LUMO of CH3(CH2)16COO− and CaSO4. CH3(CH2)16COO− (a) HOMO and (b) LUMO; CaSO4 (c) HOMO and (d) LUMO. 598
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Table 2 The properties of stearate on ACSW (020) surface calculated by DFT. Crystal surface (020)
Adsorption energy (eV)
Element
Ca-O distance (Å)
Charge of reactant (e)
Charge of product (e)
−0.149
Ca 1 Ca 2
2.307 2.258
1.567 1.567
1.548 1.545
adsorb to the surface and arranged tightly. Therefore, when the modifier dosage higher than 4 wt%, the hydrophobic layer was formed on the entire surface uniformly by the intermolecular entanglement of alkyl chains and the water molecules can't spread out on the surface, leading to the surface property of M-ACSW became hydrophobic.
4. Conclusions In this study, the hydrophobic M-ACSW was successfully synthesized by treating ACSW in sodium stearate aqueous solution at 85 °C. The contact angle of M-ACSW reached up to 132° with 4 wt% modifier dosage, proving that the surface has changed its property. The TEM and TG data showed that the stearate adsorbed on ACSW surface to form a uniform coated film. According to the outcomes of FTIR and XPS experiments, the carboxyl group of stearate reacted with Ca2+ on the whisker surface to form calcium stearate. Moreover, the results of DFT calculation indicated that the electrons transferred from carboxyl of stearate to Ca2+, and the chemical bond was formed between them. Those made the alkyl chains of stearate coated on the crystal surface change the property of ACSW surface becoming hydrophobic. The hydrophobic M-ACSW could be a promising filling material for polymer composites due to its good compatibility with composites. The modification process of this research was investigated by experiment and DFT simulation, which provides a novel and meaningful method for modifier selection of CSW.
Fig. 9. Equilibrium adsorption configurations of stearate on the (020) face.
2CH3(CH2)16COO− + Ca2+ → (CH3(CH2)16COO)2Ca
(1)
SO42− + Ca2+ → CaSO4
(2)
ΔG = −RTlnK
(3)
ΔG1 = −82.19 kJ/mol ΔG2 = −24.58 kJ/mol
Notes
Fig. 10 shows the schematic drawing of the surface modification process of ACSW with SS. The contact angle of whisker without modification was 0°, which suggested that the water molecules can spread out on the surface and the whisker surface was hydrophilic. With the addition of modifier dosage to 2 wt%, the contact angle increased to 98°, duo to the water molecules can't spread out on the surface completely. The local hydrophobic layer was formed as the stearate adsorbed on the whisker surface, which demonstrated that the whisker surface has become hydrophobic. As the dosage increased to 4 wt%, the contact angle reached to maximum and the whisker surface became more hydrophobic. By increasing the modifier dosage, the contact angle was approximate to remain unchanged, while the modifier will further
The authors declare no competing financial interest. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.01.161.
Acknowledgments The authors greatly thank the National Key R&D Program of China (2018YFB0605703), and the Program of Shanghai Academic/ Technology Research Leader (18XD1424600).
Fig. 10. The schematic drawing of the modification process of SS with AH whisker. 599
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Full length article
Insights into the modification for improving the surface property of calcium sulfate whisker: Experimental and DFT simulation study ⁎
T
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Hao Fan, Xingfu Song , Yanxia Xu, Jianguo Yu
National Engineering Research Center for Integrated Utilization of Salt Lake Resources, East China University of Science and Technology, Shanghai 200237, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Calcium sulfate whisker Hydrophobicity Sodium stearate DFT simulation Modification mechanism
In order to improve the compatibility and dispersion of anhydrous calcium sulfate whiskers (ACSW) in polymer composites, the treatment for surface modification was studied with aqueous solution sodium stearate (SS, C17H35COONa) at hot temperature. The surface modification mechanism between stearate and ACSW was carried out by experimental and density functional theory (DFT) simulation. The surface property of modified anhydrous calcium sulfate whiskers (M-ACSW) became hydrophobic by changing the water contact angle with the calcium sulfate whisker (CSW) from 0° to 132°. The TEM and TG results proved the adsorption of stearate on M-ACSW surface, which was further confirmed by FTIR and XPS. The experimental and DFT simulation outcomes for the mechanism were interpreted by taking the reaction of the carboxyl group of stearate with Ca2+ ions to form calcium stearate on the whisker surface. Their long alkyl chains of stearate coated on the surface of whisker made ACSW improve the surface with a hydrophobic property. Thus, here it is provided details of a novel method of modifier selection for CSW and its interaction mechanism with the crystal surface.
1. Introduction Calcium sulfate whisker (CSW) is a potential modified reinforcing agent because it has the characteristics of the perfect structure, integrated shape, specific cross-section and constant size. CSW is usually synthesized from dihydrate calcium sulfate through the hydrothermal process [1–4]. Compared with other inorganic whiskers, CSW has many advantages, such as nontoxicity, good thermal stability, chemical resistance, high strength, low cost and a strong affinity for organic matter, which make it to a green material with high-cost effectiveness. Excellent physical, chemical, and mechanical properties can be obtained with the addition of CSW into materials. Therefore, it can be widely used in the polymer, rubber, papermaking, ceramics and so on [5–8]. It is known that the surface free energy of needle-like whisker is high while in polymer it is low, giving them incompatibility and dispersion for whisker, and consequently poor properties of composites [9,10]. Thus, changes in the surface properties of whisker to improve its compatibility throughout an appropriate surface modification filler played an important role [11–13]. The literature reported different properties after surface modification treatment such as hydrophobiclipophilic through the magnesium oxysulfate whisker [14] or hydrophobic through CSW [15] by the interfacial adhesion between the modified CSW and organic matrix [16,17]. In this case, the
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modification mechanism was mainly attributed to the modifier adsorbed on the surface of the filler, which consisted of physical adsorption [11], chemical adsorption [14,15,18] or both of them [19,20]. Moreover, the compatibility of modified carbon fibers filler into the filler/matrix improved the physical property of polymer composites by enhancing the interfacial adhesion between filler and matrix [18,20]. After a surface modification, the interfacial adhesion and crosslinking degree between the modified CSW and the organic matrix were enriched [21] as well as their mechanical property and uniform distribution of CSW in the matrix [22]. Although plenty of experiments have confirmed that modifiers had a crucial effect on the surface property of ACSW in previous investigations, the interaction mechanism between the modifier and crystal surface at the molecular level is not explained in detail due to the limitation of available methods. With the development of computing capability, computational simulation study becomes an effective technology to assist mechanism research [23–26]. DFT method has explored the interactions between additives and crystals successfully, which gave particular characteristic on the habit of calcium sulfate hemihydrate simulated and a good explanation of their interaction mechanism with different crystal faces [27]. DFT method was a powerful tool for study the interaction between reactants at the atomistic level that might not be accessible by experimental techniques [28]. Nowadays, DFT
Corresponding authors. E-mail addresses: [email protected] (X. Song), [email protected] (J. Yu).
https://doi.org/10.1016/j.apsusc.2019.01.161 Received 6 July 2018; Received in revised form 12 November 2018; Accepted 17 January 2019 Available online 23 January 2019 0169-4332/ © 2019 Published by Elsevier B.V.
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simulation on ACSW is still lacking and it is valuable to deeply understand how modifier affects the surface property of ACSW. In this study, the common hot-water-dissoluble sodium stearate was utilized to modify the ACSW in aqueous solution, and the effects of modifier dosage on hydrophobicity of anhydrate calcium sulfate whisker were studied. The goal of this article was to investigate the surface property of M-ACSW and modification mechanism between modifier and whisker. In order to understand the surface properties, the contact angle, TG and TEM was used. The FTIR and XPS were conducted to explain the modification mechanism which was further revealed molecular level by DFT simulation. The modification process of this research was investigated by experiment and DFT simulation, which provided a novel and meaningful method for modifier selection of CSW.
Fig. 1. (a): Anhydrate calcium sulfate morphology predicted by AE method; (b): simulation box of stearate and (020) crystal face. The white balls are hydrogen atoms, the gray balls are carbon atoms, the red balls are oxygen atoms, the yellow balls are sulfur atoms and the green balls are calcium atoms (the same below).
2. Materials and methods 2.1. Experiment 2.1.1. Materials The main raw materials used in this work were dihydrate calcium sulfate and sodium stearate. The dihydrate calcium sulfate (CaSO4·2H2O) was purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. The sodium stearate was purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals in the experiment were of analytical grade.
2.2. Simulation 2.2.1. Model construction The commercial molecular modeling software package - Materials Studio (MS) - was used to run all calculations with the DMol3 module [29]. The ACSW crystal structure (Fig. S1a) was obtained from G. C. H. Cheng and Zussman [30], with unit cell dimensions of a = 6.991 Å, b = 6.996 Å, c = 6.238 Å. The crystal system was orthorhombic with the space group AMMA. Then, the unit cell was optimized to calculate the pure crystal morphology through the attachment energy (AE) method. Fig. 1a illustrates four dominant faces [(002), (011), (020) and (200)] predicted by AE method. Therefore, these main faces were taken into account in the following simulation study. On the basis of the optimized unit cell structure, the four faces were cleaved to a fractional depth of 3. In order to obtain a large crystal layer, (002), (011), (020) and (200) faces were all extended to 2 × 2 unit cells to form a supercell. The model of stearate was built and conducted with geometry optimization (Fig. S1b). Next, the model of stearate was subsequently docked on the solid layer to form a simulation box (Fig. 1b).
2.1.2. Preparation of calcium sulfate whisker The anhydrate calcium sulfate was prepared in laboratory. The experiments were carried out in a 1 L autoclave. 20 g CaSO4·2H2O was mixed with 500 mL deionized water at room temperature. The slurries were treated under hydrothermal condition (130 °C, 3 h). Then the sample of reaction suspensions were rapidly withdrawn and filtered. The retentate was rinsed with hot calcium sulfate saturated water followed by anhydrous ethanol and dried at 60 °C for 5 h. Finally, ACSW was obtained after calcinating at 650 °C for 1 h in muffle furnace. 2.1.3. Surface modification of anhydrate calcium sulfate whisker A certain content of sodium stearate was dissolved in 200 mL deionized water under the temperature of 85 °C. Then 8 g anhydrate calcium sulfate was added into solution with stirring for 30 min at 80 °C. Lastly, the suspension liquid was filtered and washed with plenty of water to remove excessive sodium stearate and dried at 60 °C for 5 h.
2.2.2. Calculation of property The simulation models of stearate docked on ACSW (020), (011), (020), (200) planes were optimized with DMol3 module. The density functional calculations were conducted to optimize geometries and obtain the properties of energy, electrostatic potentials, and population analysis of the systems. The generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerhof (PBE) of parameterization was used to describe the electronic exchange and correlation effects [27]. The core electrons were treated by the all electron relativistic method and the basis set of double numerical plus polarization (DNP) of version 3.5 was used with the global orbital cutoff distance of 5.5 Å. A vacuum layer with a thickness of 50 Å was set in z-direction to exclude the periodic effect. The electron density distribution was partitioned with the Mulliken method. The adsorption energy of a stearate ion on ACSW surface was calculated from:
2.1.4. Characterization Five drops of deionized water (15 μL) were deposited on products surface to measure the average value of contact angle by using contact angle instrument (JC2000D1, POWEREACH, China). The structures and phase composition were identified by powder X-ray diffractometer (XRD; D8 advanced, Bruker, Germany) using Cu Kα radiation with a scanning rate of 10° min−1 and a scanning 2θ range of 10° to 80°. The morphology of the samples was characterized with the scanning electron microscopy (SEM; Quanta 250, FEI Co., USA) and the high-resolution transmission electron microscopy (TEM; JEM-2100, JEOL, Japan). The thermogravimetric analysis (TG; SDT Q600, TA Co., USA) was performed under identical condition; the measurements were conducted in alumina crucible using about 5 mg of sample with heating rate of 10 °C/min. The Element content analysis of the samples was conducted by elemental analyzer (EA; VARIO BLIII, ELEMENTAR, USA). The functional groups of samples were determined using Fourier Transform Infrared Spectrometer (FTIR; Nicolet 6700, Thermo Fisher, USA), the wave-number range was set from 4000 cm−1 to 400 cm−1. The X-ray photoelectron spectrometer (XPS, Thermo Scientific ESCALAB 250Xi, USA) was employed to examine the surface of calcium sulfate hemihydrate by using an Al Ka X-ray source.
ΔE = Etotal − (Esurface − Emodifier ) where Etotal, Esurface, and Emodifier are the energies of the ACSW surface with adsorbed stearate, isolated ACSW surface and one isolated stearate ion, respectively. According to the equation, a more negative value of ΔE indicates stronger binding between adsorbed molecule and ACSW surface [31,32].
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3. Results and discussion 3.1. Characterization of ACSW and M-ACSW The contact angle (θ), conventionally measured through the liquid, is the angle which quantifies the wettability of a solid surface by a liquid. As shown in Fig. 2, the contact angle increase from 0° to 132° with the modifier dosage increasing from 0 to 4 wt%. In the absence of modifier, the contact angle of ACSW was 0°. The contact angle of MACSW was 97° with the modifier dosage of 2 wt%, which showed that the surface property of ACSW had been changed and became hydrophobic. When the modifier dosage increasing to 4 wt%, the contact angle reached to 132°. Further adding the dosage up to 6 wt%, the contact angle was 136°. It suggested that the surface modification of whisker was completed with the dosage of 4 wt%, and the modified effect will remain unchanged with further adding modifier to 6 wt%. The above results indicated that the property of modified crystal surface was changed and became hydrophobic. However, all of the prepared products consisted of only ACSW crystals without any other crystalline phases (Fig. S2). The morphology of ACSW was smooth and needle-like. This demonstrated that the modification just affected the surface of ACSW, but did not change the crystal structure and morphology. The needle-like whisker has excellent physical and chemical performances, which is very beneficial for the application of M-ACSW. Fig. 3 displays the TG curves of ACSW and M-ACSW in nitrogen atmosphere. The weight loss of ACSW is about 0.15%, which is ascribed to the free water adsorbed on whisker surface. For M-ACSW, two degradation steps can be seen from the curves in Fig. 3b–d. The first step is showed at about 100 °C, which is the process of removing free water. The second step is observed between 300 °C to 500 °C. With the increase of modifier dosage, the weight loss is 1.46%, 3.15% and 4.78%, respectively, which can be attributed to the thermal decomposition of coated organic modifier [33]. Meanwhile, the elemental analysis was also used to measure the carbon content of whisker modified with different modifier dosage. As shown in Table 1, the carbon content of whisker without modification is < 0.3%, which suggests that no carbonaceous material adsorbs on whisker surface. With the increase of modifier dosage, the carbon content of modified whisker changes from 0 to 3.93%. The above results further indicated that modifier can
Fig. 3. TG curves of the products prepared without sodium stearate (a), with different modifier dosage (b, 2 wt%; c, 4 wt%; d, 6 wt%). Table 1 The carbon content of whisker modified with different modifier dosage. Modifier dosage/wt%
0
2
4
6
Carbon content (C%)
< 0.3%
1.47
2.65
3.93
adsorb on whisker surface and the modified effect gradually become better with the increase of modifier dosage. The TEM images of ACSW and M-ACSW are displayed in Fig. 4. From the photographs, it is found that the morphology of ACSW is smooth and integrated. Most electrons were blocked when the electron beam passed through the crystal due to the dense structure of ACSW, thus, only a dark shadow picture (Fig. 4a) was left. While as shown in Fig. 4b, apart from the dark shadow, a light-color part coats on the surface of whisker, which can be the adsorbed modifier. Most electrons were passed through, when the electron beam passed through the coated layer due to the structure of modifier coating layer less dense than whiskers, thus, only a light-color image was left. Meanwhile, the modifier adsorbed uniformly on the surface of M-ACSW. The results demonstrated that the modifier indeed adsorbed on the surface of M-
Fig. 2. The SEM photos of the products prepared without sodium stearate (a), with different modifier dosage (b, 1 wt%; c, 2 wt%; d, 4 wt%; e, 6 wt%) and a plot of contact angle of ACSW versus modifier dosage (f). 596
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Fig. 4. TEM images of (a) ACSW and (b) M-ACSW modified with 4 wt% sodium stearate. (The blue dash lines denote the surface of the whisker; the red arrows denote the surface of the coated layer).
ACSW, which could inhibit the contact of water with whisker and make the surface of whisker become hydrophobic. The XPS spectra of ACSW and M-ACSW are showed in Fig. 5. The curve-fitting analysis of C 1s spectra of ACSW located at 284.6 eV is the standard peak of carbon (Fig. S3). For the Fig. 5a, there is a new signal appearing at 286.08 eV occurred in M-ACSW. This signal can be attributed to methylene (-(CH2)-), owing to the adsorption of stearate. Ca 2p has two peaks located at 347.97 eV and 351.59 eV in calcium sulfate crystal (Fig. S4a) [34]. However, the binding energy of Ca 2p appeared at 347.49 eV and 351.02 eV after sodium stearate modification (Fig. S4b). As shown in Fig. 5b, the binding energy of Ca 2p for M-ACSW decreased by 0.48 eV and 0.57 eV, respectively. It suggested that the electron density of Ca2+ surrounding increased, which can be attributed to the effect of carboxyl of stearate. When stearate reacted with the whisker surface Ca2+, the negatively charged carboxyl group could offer electron, causing the increase of Ca2+ surrounding electron density and the decrease of binding energy. According to the above results, it was inferred that the stearate reacted with Ca2+ on the crystal surface, leading to the adsorption of stearate on M-ACSW surfaces. The long alkyl chains of stearate which stretched out from the crystal surface formed the coated layer. In order to further ascertain the adsorption behavior of the surfactant in the modification process, FTIR spectra were investigated to further identify the presence and the extent of a certain group on the surface of ACSW. Fig. 6 shows the spectra of original ACSW and MACSW with different dosage of sodium stearate [16]. The FTIR spectrum of original ACSW shows the characteristic bands of calcium sulfate at 1153 cm−1, 673 cm−1 and 595 cm−1. It is obvious that FTIR spectrum of M-ACSW shows several similar absorption bands attributed to CaSO4. However, the spectrum of M-ACSW shows several new bands. The bands at 2918 cm−1 and 2850 cm−1 were ascribed to the asymmetric and symmetric vibration of methylene (-CH2-), which was
Fig. 6. FTIR spectra of the products prepared without sodium stearate (a), with different modifier dosage (b, 2 wt%; c, 4 wt%; d, 6 wt%).
contributed by the long alkyl chains of stearate coated on whisker surface. In addition, the bands at 1575 cm−1 and 1383 cm−1 are attributed to the asymmetric and symmetric stretching of carboxylate (-COO−), respectively. With the increase of modifier dosage, the intensity became stronger. The results indicated that the stearate adsorbed on crystal surface and the coated content became more with the increase of modifier. 3.2. DFT study of stearate adsorption on ACSW According to aforementioned results, it was assumed that the modification effect of stearate on ACSW can be attributed to the stearate interaction with the Ca2+ on the crystal surface. In order to
Fig. 5. High-resolution XPS of (a) C 1s peak of M-ACSW and (b) binding energy change of Ca 2p peaks. 597
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and CaSO4, as shown in Fig. 8. The gap energy of HOMO (CH3(CH2)16COO−) and LUMO (CaSO4) was 4.03 eV, which was less than the gap energy (7.71 eV) between HOMO (CaSO4) and LUMO (CH3(CH2)16COO−). It was more likely that CH3(CH2)16COO− (HOMO) donated electrons to CaSO4 (LUMO) rather than CaSO4 (HOMO) to CH3(CH2)16COO− (LUMO). The HOMO of stearate mainly appeared on the carboxyl functional group and the LUMO of CaSO4 mainly appeared on the Ca2+ (Fig. 8), so the reaction between stearate and CaSO4 was likely to occur between carboxyl and Ca2+. The adsorption energy can be used as a qualitative indicator to study the effect of the ions on the crystal. The adsorption energy of stearate adsorbed on the (020) crystal faces was −0.149 eV (Table 2). The negative values of adsorption energy indicated that stearate could adsorb on anyone of the crystal faces, as (020) face was the most stable surface among the four main crystal face. The ionic radius of calcium (Ca2+) is 1.000 Å, and the ionic radius of oxygen (O2−) is 1.400 Å [29]. If the distance of CaeO is less than the sum of radius (2.400 Å), the chemical bond can be formed. From the Fig. 9, the distances of CaeO are 2.307 Å and 2.258 Å, which suggests that the two oxygen of carboxyl interact with the two calcium ions to form the chemical bond. Meanwhile, the charge of Ca 1 decreases from 1.567 e to 1.548 e, Ca 2 from 1.567 e to 1.545 e, which can be attributed to the donated electrons from carboxyl. Because of lacking electron in the outmost electron orbital, the electrons transferred from enriched electron orbital of carboxyl to empty orbital of Ca2+. Then the chemical bond formed when stearate moved near to the surface of ACSW. Therefore, the charge of calcium ions decreased after accepting the electrons, which matched well with the results of XPS.
Fig. 7. The XRD patterns of ACSW prepared by experiment and simulated by MS. (The green vertical lines denote the positions of diffraction peaks in the standard PDF #70-0909 for crystalline anhydrate calcium sulfate).
elucidate the mechanism, the experimental data were evaluated by DFT simulation. Fig. 7 shows the XRD patterns of the ACSW prepared by experiment and simulated by MS. The red curve is the XRD pattern of experimental products and the black curve is the XRD pattern obtained by simulation according to the crystal cell structure. The simulated pattern matched well with that from experiment, which indicated that the crystal cell structure was reasonable. The main peak at 2θ of 25.431° was indexed as the (020) reflections, which was the characteristic peak of anhydrate calcium sulfate. According to the AE method, there were four main crystal surfaces for ACSW. Then, (200), (020), (002) and (011) surfaces of CaSO4 were cleaved to calculate surface energy with the same algorithm as used for all optimizations. For anhydrate calcium sulfate crystal, the surface energy is calculated as:
Es =
Eslab −
3.3. Modification mechanism As a common anionic surfactant, sodium stearate no longer exists as a molecular form in hot water solution, but is dissociated into two parts: an anionic portion (CH3(CH2)16COO−), and a Na+ ion. The anionic part plays a crucial role in determining the surface activity of anionic surfactants. According to the equation of Gibbs free energy of reaction (ΔG), the Gibbs free energy of ion reaction between CH3(CH2)16COO− and Ca2+ is −82.19 kJ/mol, which is obviously smaller than that of the reaction of sulfate with Ca2+ (ΔG2 = −24.58 kJ/mol). It suggested that the chemical reaction as shown in Eq. (1) took place spontaneously on the surface of ACSW. Meanwhile, the solubility product (Ksp) of calcium stearate (3.98 × 10−15, 25 °C) [35] is far less than the Ksp of calcium sulfate (4.93 × 10−5, 25 °C) [36]. It meant that the precipitation of Ca2+ with CH3(CH2)16COO− was more likely to happen than SO42−. When whiskers were added into hot sodium stearate solution, the dissolved CH3(CH2)16COO− moved to the crystal surface under the condition of stirring. Then, the negatively charged carboxyl of CH3(CH2)16COO− reacted with Ca2+ on whisker surface to form the insoluble (CH3(CH2)16COO)2Ca precipitate, which adsorbed on the surface of whiskers. The alkyl chains of calcium stearate coated on the surface of M-ACSW as a film, leading to the hydrophobic whiskers.
Nslab ·E N bulk bulk
2A
where Es is the surface energy; Eslab refers to the total energy of the slab; Ebulk is the energy for CaSO4 unit cell; Nslab is the atom number of the slab; Nbulk is the atom number of the bulk and A is the surface area. The results were 2.20 eV·nm−2 for (200) face, 2.14 eV·nm−2 for (020) face, 2.59 eV·nm−2 for (002) face and 4.51 eV·nm−2 for (011) face, which meant the stability sequence of these surfaces was: (020) > (200) > (002) > (011). It was considered that (020) surface was more easily obtained when CaSO4 crystals were cleaved or in nature, which explained the characteristic peak of anhydrate calcium sulfate correspond to (020) face. Therefore, all calculations were conducted on (020) surface. The reaction site of stearate on (020) surface of CaSO4 was determined by calculating the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of stearate
Fig. 8. HOMO and LUMO of CH3(CH2)16COO− and CaSO4. CH3(CH2)16COO− (a) HOMO and (b) LUMO; CaSO4 (c) HOMO and (d) LUMO. 598
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Table 2 The properties of stearate on ACSW (020) surface calculated by DFT. Crystal surface (020)
Adsorption energy (eV)
Element
Ca-O distance (Å)
Charge of reactant (e)
Charge of product (e)
−0.149
Ca 1 Ca 2
2.307 2.258
1.567 1.567
1.548 1.545
adsorb to the surface and arranged tightly. Therefore, when the modifier dosage higher than 4 wt%, the hydrophobic layer was formed on the entire surface uniformly by the intermolecular entanglement of alkyl chains and the water molecules can't spread out on the surface, leading to the surface property of M-ACSW became hydrophobic.
4. Conclusions In this study, the hydrophobic M-ACSW was successfully synthesized by treating ACSW in sodium stearate aqueous solution at 85 °C. The contact angle of M-ACSW reached up to 132° with 4 wt% modifier dosage, proving that the surface has changed its property. The TEM and TG data showed that the stearate adsorbed on ACSW surface to form a uniform coated film. According to the outcomes of FTIR and XPS experiments, the carboxyl group of stearate reacted with Ca2+ on the whisker surface to form calcium stearate. Moreover, the results of DFT calculation indicated that the electrons transferred from carboxyl of stearate to Ca2+, and the chemical bond was formed between them. Those made the alkyl chains of stearate coated on the crystal surface change the property of ACSW surface becoming hydrophobic. The hydrophobic M-ACSW could be a promising filling material for polymer composites due to its good compatibility with composites. The modification process of this research was investigated by experiment and DFT simulation, which provides a novel and meaningful method for modifier selection of CSW.
Fig. 9. Equilibrium adsorption configurations of stearate on the (020) face.
2CH3(CH2)16COO− + Ca2+ → (CH3(CH2)16COO)2Ca
(1)
SO42− + Ca2+ → CaSO4
(2)
ΔG = −RTlnK
(3)
ΔG1 = −82.19 kJ/mol ΔG2 = −24.58 kJ/mol
Notes
Fig. 10 shows the schematic drawing of the surface modification process of ACSW with SS. The contact angle of whisker without modification was 0°, which suggested that the water molecules can spread out on the surface and the whisker surface was hydrophilic. With the addition of modifier dosage to 2 wt%, the contact angle increased to 98°, duo to the water molecules can't spread out on the surface completely. The local hydrophobic layer was formed as the stearate adsorbed on the whisker surface, which demonstrated that the whisker surface has become hydrophobic. As the dosage increased to 4 wt%, the contact angle reached to maximum and the whisker surface became more hydrophobic. By increasing the modifier dosage, the contact angle was approximate to remain unchanged, while the modifier will further
The authors declare no competing financial interest. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.01.161.
Acknowledgments The authors greatly thank the National Key R&D Program of China (2018YFB0605703), and the Program of Shanghai Academic/ Technology Research Leader (18XD1424600).
Fig. 10. The schematic drawing of the modification process of SS with AH whisker. 599
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