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Effect of pH on the adsorption of dodecylamine on montmorillonite: Insights from experiments and molecular dynamics simulations
Applied Surface Science 425 (2017) 996–1005
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Full Length Article
Effect of pH on the adsorption of dodecylamine on montmorillonite: Insights from experiments and molecular dynamics simulations Chenliang Peng a , Fanfei Min b,∗ , Lingyun Liu b a b
Institute of Engineering and Research, Jiangxi University of Science and Technology, Ganzhou 341000, China Department of Materials Science and Engineering, Anhui University of Science and Technology, Huainan 232001, China
a r t i c l e
i n f o
Article history: Received 11 April 2017 Received in revised form 12 June 2017 Accepted 11 July 2017 Available online 15 July 2017 Keywords: Montmorillonite Dodecylamine Adsorption Molecular dynamic
a b s t r a c t The hydrophobic aggregation in cationic surfactant suspension is an effective method to enhance the dewatering of clay-rich tailing. The solution pH can affect the adsorption behavior of cationic surfactant on clay mineral. The effect of pH on the adsorption of dodecylamine (DDA) on montmorillonite was investigated by the sedimentation test and the characterization of flocs images, contact angle, adsorption quantity, and fourier transform infrared (FTIR) spectroscopy, as well as molecular dynamics (MD) simulation. It was found that DDA ions were adsorbed on montmorillonite basal surfaces mainly by physical adsorption, including the electrostatic attraction and hydrogen bonding. A certain number of neutral DDA molecules can favor the adsorption of DDA. At pH around 8, the effect of hydrophobic modification was the best because DDA molecules and ions form compact and well-organized monolayer. The MD simulation results were in good agreement with that of contact angle, adsorption quantity and FTIR. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Clay minerals are layered silicate minerals, including kaolinite, montmorillonite and illite, which are widely used in ceramics, plastics, coatings, paper, rubber and cosmetics and other industrial products [1–3]. Recently, it becomes a growing problem to dewater the clay-rich tailing in mineral processing and hydrometallurgy [4,5]. The suspensions of clay-rich tailings are highly dispersed to settle very slowly due to the presence of clay hydration [6,7]. In this case, hydrophobic aggregation would be an effective method to promote the dewatering of clay-rich tailing by adding hydrophobic modifier instead of electrolyte coagulant and macromolecule flocculant [8]. In the method, hydrophilic clay surface become hydrophobic after adsorbing hydrophobic modifiers to form largesize aggregations under hydrophobic attraction and kinetic energy input, promoting the sedimentation of clay particles under gravity [9,10]. Usually, long-chain alkylamines, which are popular collectors in nonsulfide floatation [11,12], can be used as the hydrophobic modifier in the process of hydrophobic aggregation. For example, Zhang et al. [13] investigated the hydrophobic aggregation of kaolinite in dodecylamine surfactant suspension and found the aggregation model at different solution pH. In the hydrophobic
∗ Corresponding author. E-mail address: [email protected] (F. Min). http://dx.doi.org/10.1016/j.apsusc.2017.07.085 0169-4332/© 2017 Elsevier B.V. All rights reserved.
aggregation of clay-rich tailing and the flotation of nonsulfidic mineral flotation, it is very important to understand the adsorption mechanism of long-chain alkylamines at the mineral/aqueous. Previous experimental studies [14,15] showed that in addition to the type and concentration of long-chain alkylamines, the solution pH had an important effect on the adsorption behavior. Long-chain alkylamines almost existed in the cationic form under the acidic condition and could be physically adsorbed at the negatively charged silicate surface mainly through electrostatic interaction when below the critical micelle concentration (CMC). At low pH, limited adsorption occurred; with the increasing of pH, adsorption of ions increased and even a bilayer could formed at pH around 10 [16,17]. The co-adsorption of neutral molecular amines could enhance the adsorption of amine ions [17,18]. In recent years, the molecular dynamics (MD) simulation has become an effective means to explore the complex surface reaction and structure at the molecular level, avoiding the limitation of the resolution of the experimental analysis and the interference to the target system [19]. Tang et al. [20] used MD simulations to study the adsorption of DDA on iron surfaces in aqueous solution, and found that in acidic solution DDA can be adsorbed on the iron surface in both of the molecular form and ionic form, but they just considered one DDA molecule or ion per model and did not determine the effect of the pH values. Other MD simulations [21,22] have been carried out to describe the adsorption of DDA on silica and muscovite surface at different pH, and found that the state of adsorbed DDA at the inter-
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face varies significantly at different pH values, which was proved by the vibrational spectroscopy results. In the work, we focused on the adsorption of DDA at the montmorillonite/water interface as a function of pH, because montmorillonite has more detrimental effect on the sedimentation and filtration of tailing water than other clay minerals. Montmorillonite is 2:1 type of layered silicate mineral with an alumina octahedral sheet sandwiched by two silica tetrahedron sheets. Each layer is strongly negatively charged (0.4–1.2 e per unit cell) because of the isomorphous substitution of aluminum by magnesium in alumina octahedral sheet [23]. The charge in the crystal is compensated by sodium or calcium ions in the interlayer. At present, many studies are limited to the intercalation of alkylamine ions into the interlayer spaces of montmorillonite to synthesize organoclay at high temperature or under ultrasonic processing [24–26] while little studies involves the adsorption behavior of alkylamines at montmorillonte/water interface in aqueous solution at room temperature, which is important in the hydrophobic aggregation of clay-rich tailings. In the present study, sedimentation test followed by the characterization of flocs images, contact angle and adsorption quantity, and FTIR were undertaken to study the adsorption behavior of DDA on montmorillonite. MD simulations were used to study the structure and dynamics of the adsorbed DDA layer at montmorillonte/water interface. 2. Materials and methods 2.1. Materials The montmorilloite were purified from bentonite raw ores in Anji, Zhejiang, China using the natural settling method reported in literature[27]. Then the exchangeable cations of purified montmorillonite were replaced by Na+ ions, which was followed by washing using de-ionized water. The X-ray diffraction (XRD) diagram and Scanning electron microscope (SEM) photo of the purified montmorillonite sample (Fig. 1) showed that the purified samples only contained montmorillonite phase which were lamellar in shape with a diameter of around 2 m. The dodecylamnie with 99% purity was purchased from Sinopharm Chemical Reagent Co.,Ltd (in China). DDA solution was prepared by adding equimolar acetic acid. The solution pH was adjusted by HCl or NaOH solution. De-ionized water with the residual conductivity less than 1 S/cm was used throughout the experiments. 2.2. Sedimentation test and characterization The sedimentation test was carried out as follows: (1) weighted a certain number of montmorillonite samples and immersed them in less than 100 mL de-ionized water for 12 h, and then stirred for 30 min; (2) added a certain amount of DDA and de-ionized water to prepare 100 mL 0.6 wt.% montmorillonite suspension; (3) adjusted the pH to the required value by HCl or NaOH solutions, and stirred at the 200 r/min speed for 10 min; (4) transferred the montmorillonite suspension to a 100 mL graduated cylinder, and shaken up and down five times to start the sedimentation test which lasted for 10 min; (5) after the sedimentation, extracted the top 50 mL suspension by siphon method to measure the light transmittance using UV2600 Ultraviolet Spectrophotometry (Shimadzu, Japan). The montmorillonite samples in the bottom 50 mL suspension were dried bellow 80 ◦ C. Then the dried samples were ground to pass through a 450 m sieve to use for the measurement of contact angle and FTIR analysis. In the measurement of contact angle, the samples were pressed at 20 MPa pressure for 2 min to obtain round sheets with thickness of about 2 mm to measure the con-
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tact angle in a C20 automatic contact angle meter (Kino, USA). Each measurement was repeated three times at different sites of the surface. In FTIR analysis, the sample was mixed with KBr, then ground and pressed to small pelletized discs for FTIR analyses which were conducted in the range of 400–4000 cm−1 by a Nicolet-380 FTIR instrument (Thermo Fisher, USA). The zeta potential of montmorillonite in DDA solutions was measured by ZetaProbe (Colloidal Dynamics, USA). The montmorillonite suspensions at the solid concentration of 0.6 wt% were prepared in pure water and 7 × 10−4 mol/L DDA solutions. The solution pH was adjusted using 1 mol/L HCl solution and NaOH solution. The values of zeta potential were obtained at pH from 7 to 12 and from 7 to 2 in the model of automatic titration. In the measurement of adsorption quantity, the suspension after the sedimentation test was centrifuged at 3000 r/min for 30 min. Then the centrifugal liquid was used to measure the final concentration of residual DDA in the solution based on the calibration curve using UV2600 Ultraviolet Spectrophotometry. The adsorption quantity of DDA on montmorillonite particles was calculated by the following expression: A=
V (C − C0 ) m
(1)
where A was the adsorption quantity, mol/g; C0 and C were the initial and final concentrations of DDA, respectively, mol/L; V was the solution volume, L; m was the weight of the particles, g. 2.3. Molecular dynamics (MD) simulations The montmorillonite model had only Mg2+ → Al3+ substitution in the octahedral layer. The structural formula was Na0.333 [Si4 O8 ][Al1.667 Mg0.333 O2 (OH)2 ]. Each cell contained 0.666 e negative charge, which was common for most montmorillonite. A (5 × 3 × 2) periodic repeating supercell was constructed, containing two montmorillonite sheets. To ensure that the surfactant structures had sufficient space to assemble at the motmorillonite/water interface and prevented them from migrating to the gas/water interface, the aqueous solution thickness was set to about 5 nm. About 1000 water molecules were required to construct the bulk solution. To yield an overall neutral system, a certain number of Na+ or Cl− ions were added to the model. The vacuum layer with the thickness of 80 nm was added on the aqueous solution layer to avoid the interference from the periodic image layer in the z axis direction. Thus, the supercell size became 26 × 27 × 150 Å3 . According to the previous DFT and MD results [28–30], the centre of the surface six oxygen ring (SOR) above the lattice substitution in the motmorillonite was the best adsorption site for inorganic cations, e.g., Na+ and Ca2+ , and head-groups of cationic surfactants. The Na+ of sodium montmorillonite in aqueous solution would move away from the surface to the bulk solutions to form outer-sphere coordination complexes. Therefore, in the initial model, it was reasonable to arrange the head-groups of cationic surfactants to orient toward the SOR of montmorillonite with the alky chain toward the bulk solution, as shown in Fig. 2. Similar methods to construct the initial model were also used in the simulation studies on the adsorption of alkylamine on mica surface [31–33]. Because the size of head group of an alkylamine molecule or ion approximated to that of SOR, the coverage of one alkylamine molecule or ion per SOR can be considered as dense monolayer (ML). In our model, the (001) basal surface contained 30 SOR rings, so 1 ML coverage meant 30 adsorbed alkylamine structures. In addition, as a kind of hydrolyzable surfactant, DDA mainly presents in acidic solution in the ionic form or in alkaline solution in molecular form depending on the solution pH [34,35]. For convenience, the component of DDA in aqueous solution was represented by I:M, where I and M were the number of DDA ions and molecules,
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Fig. 1. The XRD diagram (a) and SEM photo (b) of the purified montmorillonite.
Fig. 2. The initial configuration for DDA at montmorillonite/water interface.
respectively. At the DDA concentration of 7 × 10−4 mol/L, the critical pH for DDA to form molecular precipitation is 9.1. When pH is around 12, DDA species are completely DDA molecules; when pH is bellow 4, species are completely DDA ions. Therefore, at 1 ML coverage, pH = 12, 10, 8, 6 and 4, would correspond to I:M = 0:30, 5:25, 10:20, 20:10 and 30:0, respectively. Similar methods appeared in the literatures [21,22]. MD simulations were performed in the Forcite plus module of Material studios 8.0 software. The force field used in the calculation was PCFF INTERFACE developed by Heinz et al. [36], in which INTERFACE force field was integrated in harmonic PCFF force fields for biomolecules, solvents, and polymers. The PCFF INTERFACE force field enabled atomistic simulations of nanostructures at the
1–100 nm scale in high accuracy and served as a uniform simulation platform for inorganic, organic, and biomolecular compounds. A flexible SPC model [37] was used for water molecules. The Mulliken atomic partial charges of DDA ions and molecule were distributed after the geometry optimization at the PW91/DNP level in the Dmol3 module, as shown in Fig. 3. In the simulation, firstly, geometry optimization for the system was performed using conjugate gradient algorithm to reduce the residual force of each atom. The convergence achieved once the maximum force on any one atom was less than 100 kJ mol−1 nm−1 . Subsequently, a 100 ps pre-equilibrium simulation was run in the NPT ensemble. Berendsen thermostat and Berendsen barostat which offered swift equilibration of the system, were selected with the temperature coupling constant of 0.1 ps and the pressure-
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Fig. 3. The optimized structures labeled atomic partial charge of DDA molecule (a) and ion (b) at the PW91/DNP level.
Fig. 4. The light transmittance of the supernatant of montmorillonite suspensions as a function of DDA concentration (a) and solution pH (b).
coupling constant of 1 ps, respectively. Finally, equilibration was followed by a 2 ns production run in the NVT ensemble where Nose thermostat was used. The last 0.5 ns trajectory was used for analysis of relevant properties. All dynamic simulations were run in step of 1 fs and at a pressure of 1 bar and a temperature of 300 K. The longrange electrostatic interaction and van der Waals interaction were calculated using the Ewald and Atom based methods, respectively, with a cutoff radius of 1.25 nm.
3. Results and discussion 3.1. Sedimentation test The effect of sedimentation can be evaluated by the light transmittance of supernatant after the sedimentation test. The larger the light transmittance is, the better the effect of sedimentation is. The DDA concentration and solution pH dependence of the light transmittance of the supernatant of montmorillonite suspensions was showed in Fig. 4. From Fig. 4a, the light transmittance in pure water was 12.9% under weak alkaline condition (pH = 8). When the DDA at the concentration of 1 × 10−4 mol/L was added in the suspension, the light transmittance was 14.3% and close to that in pure water, but when the concentration of DDA increased from 1 × 10−4 mol/L to 7 × 10−4 mol/L, the light transmittance approximately linearly increases from 14.3% to 65.3%, then changed little when the concentration of DDA continued to increase to 1.5 × 10−3 mol/L. From Fig. 4b, the light transmittance of the supernatant in the DDA solutions at the concentration of 7 × 10−4 mol/L was very small (about 2%) and close to that without DDA at pH from 12 to 10. Under the condition, hydrophobic modifier DDA did not promote the sedimentation of montmorillonite particles. One reason should be that at the DDA concentration of 7 × 10−4 mol/L, the critical pH for DDA to form molecular precipitation was pH 9.1. When pH was above 9.1, DDA molecules started to form precipitates in the solu-
tion, losing chemical activity. When the pH decreased to 9, the light transmittance of supernatant added DDA was higher than that in the absence of DDA, indicating that DDA has begun to function. As pH decreased from 9 to7, the light transmittance increased sharply. In this pH range, the ratio of DDA ion to molecule in solution became much larger, suggesting that these DDA ions were adsorbed on montmorillonite surfaces by electrostatic attraction, not only decreasing the surface charge, but also rendering the surface good hydrophobicity[13,38,39]. In this case, the electrostatic repulsion between the negatively charged surface weakened while the hydrophobic attractive strengthened so that the fine particles could aggregate each other to form big flocs. The flocs could easily settle under the gravity, resulting in the increase of the light transmittance of supernatant. When pH continued to decrease from 7 to 4, the transmittance did not change significantly because most of the fine particles had settled after the sedimentation test.
3.2. Flocs images The in-situ morphological characteristics of montmorillonite flocs could be observed by monocular variable optical microscope. The original floc structures would not be destroyed by other factors, such as drying and squeezing. Fig. 5 showed the montmorillonite flocs images at different pH with and without 7 × 10−4 mol/L DDA. It was observed that the individual montmorillonite particles were very small, that is, in the micron range. When the solution pH was above 10, the flocs did not appear (not showed in Fig. 5) because montmorillonite particles were highly dispersive in the solution. With the decreasing of pH and adsorption of DDA, the montmorillonite flocs began to appear at pH 8, but they were relatively small and loosely linked together with each other (Fig. 5a). As pH decreased to 6, the flocs became larger and more compact, and reached the millimeter level (Fig. 5b). At the very low pH of 4, the flocs were less compact than that at pH 6 (Fig. 5c).
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Fig. 5. The images of montmorillonite flocs at the DDA concentration of 7 × 10−4 mol/L at pH 8 (a), 6 (b), and 4 (c), respectively.
Fig. 6. The contact angle of montmorillonite surface and adsorption quantity of DDA as a function of pH at the DDA concentration of 7 × 10−4 mol/L.
3.3. Contact angle and adsorption quantity The values of contact angle can reflect the degree of hydrophobicity of mineral surface. The larger the contact angle is, the stronger the hydrophobicity of the surface is. In fact, the hydrophobicity of the surface is attributed to the adsorption quantity and aggregation structures of surfactant on the surface. Fig. 6 showed the plots of the contact angle of montmorillonite and adsorption quantity of DDA against the solution pH at the DDA concentration of 7 × 10−4 mol/L. When more DDA were adsorbed on the surface and the adsorbed DDA layer was more compact and well-organized, the surface would become more hydrophobic. It can be seen that the contact angle of montmorillonite increased when the pH decreased from 10 to 8, and reached a maximum around pH 8, and then
decreased with the pH decreasing from 8 to 4. Likewise, the adsorption quantity had the similar behavior against the solution pH. The results showed that at pH around 8, the montmorilloite surfaces were most hydrophobic. These behaviors would be explained by the following MD simulation results at the molecular level. 3.4. FTIR analysis FTIR spectroscopy can be performed to clarify the adsorption mechanism of reagent on the mineral surfaces. The FTIR spectra of montmorillonite conditioned with 7 × 10−4 mol/L DDA in the pH range of 10–4 were showed in Fig. 7. The broader peak near ∼3430 cm−1 was assigned to the stretching modes of NH2 of DDA and hydroxyl of water (OHw ). Two new peaks at 2930 and
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Fig. 7. FTIR spectra of motmorillonite as a function of pH at the DDA concentration of 7 × 10−4 mol/L.
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Generally, the pH in the neutral and alkaline solution is usually above the PZC of edge surface of montmorillonite because the point of zero charge (PZC) of the edge surface is not a certain value, generally between pH 4 and 8 according to different experimental methods, such as slightly above pH 5 by rheological measurement [42], pH 6.1 by titration experiment [43] and around pH 7 by electrokinetic measurement [44]. In the case, both the basal and edge surfaces of montmorillontie are negatively charged. If the cationic surfactants DDA are added in the suspension, DDA would be adsorbed on the surface by electrostatic attraction [45–47]. As shown in Fig. 8, the adsorption of DDA decreased the zeta potential of montmorillonite at large-scale pH except at pH above 11. In addition, the hydrophobic carbon chain of DDA could associated with each other by the hydrophobic attractive [48], resulting in the formation of flocs by face-to-face or parallel aggregation (Fig. 9a). In the acidic solution, when the pH is below the PZC of edge surface, the edge surfaces would be positively charged while the basal surfaces are still negatively charged. Then the basal and edge surfaces of montmorillonite would attract each other to produce face-to-edge T-type or card-house type aggregation. In addition, the basal surface had a special affinity with DDA ions to render the surface hydrophobicity, promoting the aggregation of particles (Fig. 9b). Therefore, it was concluded that under the combination of face-to-edge electrostatic attraction and hydrophobic attraction, the face-to-edge flocs produced at acidic pH would be larger than those face-to-face flocs at neutral and slightly alkaline pH, resulting in better sedimentation effect. The conclusion was in good agreement with the results of light transmittance of supernatant and flocs images, and could explain why the effect of sedimentation became better when the hydrophobicity of montmorillonite decreased with pH decreasing from 8 to 4. 3.6. MD simulation results
Fig. 8. The zeta potential of montmorillonite as a function of pH in pure water and in DDA solutions at the concentration of 7 × 10−4 mol/L.
2850 cm−1 were assigned to CH stretching group. The new peak at 2370 cm−1 with a shoulder at 2330 cm−1 belonged to the CN stretching group [40]. And the peak at ∼1630 cm−1 represented the bending modes of NH of DDA and OHw of water [41]. Though the spectrum of montmorillonite treated by DDA presented characteristic absorption peaks of DDA, no band shift occurred, implying that DDA were adsorbed on montmorillonite mainly by physical adsorption. The decreasing transmittances in the NH2 and NH stretching region spectra of DDA from pH 8 to pH 4 indicated the decreased adsorption of DDA, which was in accord with the results of contact angle and adsorption quantity. 3.5. The model of hydrophobic aggregation As a kind of 2:1 type phyllosilicate, montmorillonite has basal surface and edge surfaces with different charge properties. The basal surface of montmorillonite, e.g., (001) face, has the permanent negative charges due to isomorphous substitution of Mg2+ → Al3+ within alumina octahedral sheet. Fig. 8 showed the zeta potential of montmorillonite as a function of pH in pure water and in DDA solutions at the concentration of 7 × 10−4 mol/L. In the whole pH range of 2–12, the zeta potential of montmorillonite was negative, but the value decreased from −43.2 mV to −26.3 mV with the pH decreasing from 12 to 2. The reason should be attributed to the sign and density of the charges at the edge surface such as (010) and (110) faces which depend on the pH of the solution.
Because DDA is a kind of hydrolyzable surfactant, its species in solution change with the solution pH. Fig. 10 showed the equilibrium configurations of the DDA molecules or ions adsorbed at the montmorillonite/water interface at different pH. When pH was 12 (Fig. 10a), DDA existed completely in the form of molecules (I:M = 0:30), and self-agglomeration of DDA molecules occurred at a distance from the montmorillonite basal surface. DDA molecules had no hydrophobic modification effect on the montmorillonite so that the montmorillonite suspensions was highly dispersed. When pH decreased to 10, that is, I:M = 5:25 (Fig. 10b), five DDA ions were adsorbed on the surface with their polar head groups toward the surface to form the inner layer while 25 DDA molecules were bound to the inner layer through the hydrophobic association among their non-polar carbon chains with some of polar head groups toward the bulk solution. The adsorbed structure was similar to an irregular bilayer. At this pH, a certain extent effect of hydrophobic modification started to appear. However, because a large number of polar head groups in the adsorbed monolayer were oriented toward the solution, it was disadvantageous for the hydrophobic modification of montmorillontie. Under the circumstances, the strong electrostatic repulsion between particles dominated the high stability of the montmorillonite suspension because of the large negative charge of edge surface, which agreed with the very low light transmittance (about 0.47%) of the supernatant (Fig. 4). When pH decreased to 8 (I:M = 10:20) (Fig. 10c), it was obviously observed that all the DDA molecules and ions were adsorbed on the surface through their polar head groups to form monolayer, namely, hemi-micelle. The adsorption layer was very compact and the surfactant packed in parallel through the hydrophobic association between the hydrophobic carbon chains so that the hydrophobic modification effect was very significant, which was
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Fig. 9. The model of the interaction of DDA with montmorillonite at different pH [49].
Fig. 10. The equilibrium structures of DDA on montmorillonite (001) surfaces at different pH. pH = 12, I:M = 0:30 (a); pH = 10, I:M = 5:25 (b); pH = 8, I:M = 10:20 (c); pH = 6, I:M = 20:10 (d); pH = 4, I:M = 30:0 (e).
in good agreement with the maximum adsorption quantity and contact angle at pH 8 (Fig. 6). A similar behavior was reported by Rutland et al. [17]. They investigated the adsorption of DDA on mica by surface force apparatus (SFA), and found that at pH around 8, adsorption of neutral DDA molecules became important to render the monolayer more compact with the thickness close to the length of an extended molecule, and strong attractive force was observed when two hydrophobic surfaces contacted.
When the solution pH were lower, such as 6 and 4, corresponding to I:M of 20:10 and 30:0 (Fig. 10d and e), respectively, the form of ions predominated in the DDA species. The DDA adsorption layer was still compact, but some DDA ions moved away from the inner to upside of adsorbed layer with their head groups oriented to the solution. Herder[50] found the similar behavior in studying the interactions between mica surfaces in DDA solutions at pH values between 5 and 6. In the case, the structure of the adsorbed layer was
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Fig. 11. The close-up snapshot of adsorbed DDA ions at the interface. (The black dash line denoted hydrogen bond).
Fig. 12. The concentration profile of N atoms of DDA head groups perpendicular to montmorillonite (001) surface at the pH 12, 8 and 4, respectively.
not that of a perfect monolayer but contains imperfections[51,52]. Therefore, the effect of hydrophobic modification decreased as pH decreased from 8 to 4, which is consistent with the foregoing results of contact angle and adsorption quantity. The close-up snapshots of the DDA ions at the interface at pH 8 were showed in Fig. 11. It was found that at the interface the polar head groups of DDA not only interacted with not only the surface oxygen atoms, but also water molecules through the formation of hydrogen bonds between the hydrogen atoms (Hn ) of the head group and surface oxygen atoms (Os ) or water oxygen atoms (Ow ). A competitive adsorption existed between the water molecules and DDA toward the surface. In Fig. 9a, the head group of DDA only interacted with the surface, forming three Hn · · ·Os hydrogen bonds and non Hn · · ·Ow hydrogen bond. However, one, two and three Hn · · ·Ow hydrogen bonds were formed in Fig. 10b–d, respectively. The increase of number of Hn · · ·Ow hydrogen bond indicated that the interaction between head group of DDA and water strengthened, and in turn the interaction between head group and surface weakened so that the distance between the head group and the surface increased. The hydrogen bonding between the polar head group and the water molecule resulted in DDA no longer being perpendicular to the surface but being tilted. The similar behavior was found in the study of DDA on mica (001) surface by Xu et al. [31] The concentration profile of N atoms of DDA in the normal direction of montmorillonite surface at different pH was showed in Fig. 12. It could be found that at pH 12, several concentration peaks of N atoms were concentrated in a distance range of 7–40 Å from the surface, indicating that the DDA were far from the surface and formed self-agglomeration, bringing no hydrophobic modifica-
Fig. 13. The concentration profile of water Ow atoms perpendicular to montmorillonite (001) surface at the pH 12, 8 and 4, respectively.
tion effect to montmorillonite. At pH 8, the concentration peaks of N atoms were concentrated in the narrow distance range of 0–7 Å from the surface, indicating that all the polar head groups were oriented towards the surface of the montmorillonite rather than the non-polar carbon chain. This case can produce the good hydrophobic modification effect. At pH 4, although the concentration peaks of N atoms are mainly concentrated in the range of 0–12 Å from the surface, there were still obvious distributions in the range of 22–30 Å. It suggested that some polar head groups at the interface transferred from the initial adsorption position at the interface to the top of the adsorption monolayer, weakening the hydrophobic modification effect of the surface to a certain extent. The concentration distribution of water molecules perpendicular to the interface can indirectly reflect the effect of pH on the competitive adsorption between waters and DDA ions or molecules at the interface, as shown in Fig. 13. It was observed that the concentration of Ow atoms in the distance range of 15–35 Å was much smaller than that in other places at pH 12, implying the formation of self-aggregation of DDA. When pH was 8, the Ow concentration at the distance from 3 to 18 Å was zero, indicating that all these places were occupied by the DDA instead of water molecules. When pH decreased to 4, the Ow concentration at the distance of 3–18 Å obviously increased, indicating that some of the water molecules were occupied in this region. The reason was that some of the DDA structures moved outside the adsorption layer so that some water could move in the place. Additionally, under all pH conditions, the Ow concentration peak in the distance range of 0–5 Å appeared like the N concentration peak (Fig. 12) because the water molecules were polarized by the surface or polar head of DDA, showing that
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Table 1 The torsion deviation of dihedral angle . pH
12
8
4
26.5%
4.7%
22.6%
competitive adsorption existed between the water molecules and DDA toward the surface. When the surfactant DDA aggregated at the montmorillonite/water interface, the alkyl carbon chain will be distorted, deviating from the initial linear structure. The degree of twist of the alkyl carbon chain can be characterized by the distribution of dihedral angle of the carbon atoms. The percentage of the dihedral angle in the range of [–120◦ ,120◦ ] was defined as the torsion deviation of dihedral angle t [31], as shown in Table 1. The greater the was, the stronger the degree of twist of DDA was and the lower the degree of order of DDA was. It could be found that was lowest (4.7%) at pH 8, implying that the DDA were well-organized and in favor of the hydrophobic modification. However, at pH 12 and 4, increased to 26.5% and 22.6%, respectively, suggesting that the degree of order decreased and formed self-aggregation. The results agreed with the equilibrium structures and concentration profile of atoms. 4. Conclusion Effect of pH on the adsorption of DDA on montmorillonite surface in aqueous solution was studied by the experimental and MD simulation methods. The experimental results showed that the effect of hydrophobic modification for montmorillonite increased as the pH decreased from 10 to 8, reaching a maximum around pH 8, and then decreased with the pH decreasing from 8 to 4. Though the hydrophobicity of montrmorillonite decrease as pH decreased from 8 to 4, the effect of sedimentation increased due to the formation of big face-to-edge flocs under low pH conditions. When pH was below 7, the effect of sedimentation did not change significantly. MD simulation results showed that when the positively charged DDA ions were adsorbed onto the negatively charged montmorillonite surfaces, the initial stage of the adsorption was driven by electrostatic forces and hydrogen bonding, and the later stage by hydrophobic association between the alkyl carbon chains. A competitive adsorption existed between water molecules and DDA toward the montmorillonite surface. A certain numbers of neutral DDA molecules could favor the adsorption of DDA, rendering the monolayer more compact. The structure and composition of the adsorbed layer was different by decreasing the pH from 12 to 4. At pH 8, DDA molecules and ions were adsorbed on the surface through their polar head groups to form monolayer, namely, hemimicelle. The adsorption layer was very compact and the surfactant packed in parallel, so that the hydrophobic modification effect was the best. Acknowledgements This work was supported by the Natural Science Foundation of China [grant number 51474011]; the Post-doctoral Science Foundation of China [grant number 2014M561810]; the Anhui Province International Cooperation Project [grant number 1303063011]; and the Anhui Provincial Natural Science Foundation of China [grant number 1508085QE90]. References [1] F. Uddin, Clays, Nanoclays, and montmorillonite minerals, Metal. Mater. Trans. A 39 (2008) 2804–2814.
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Full Length Article
Effect of pH on the adsorption of dodecylamine on montmorillonite: Insights from experiments and molecular dynamics simulations Chenliang Peng a , Fanfei Min b,∗ , Lingyun Liu b a b
Institute of Engineering and Research, Jiangxi University of Science and Technology, Ganzhou 341000, China Department of Materials Science and Engineering, Anhui University of Science and Technology, Huainan 232001, China
a r t i c l e
i n f o
Article history: Received 11 April 2017 Received in revised form 12 June 2017 Accepted 11 July 2017 Available online 15 July 2017 Keywords: Montmorillonite Dodecylamine Adsorption Molecular dynamic
a b s t r a c t The hydrophobic aggregation in cationic surfactant suspension is an effective method to enhance the dewatering of clay-rich tailing. The solution pH can affect the adsorption behavior of cationic surfactant on clay mineral. The effect of pH on the adsorption of dodecylamine (DDA) on montmorillonite was investigated by the sedimentation test and the characterization of flocs images, contact angle, adsorption quantity, and fourier transform infrared (FTIR) spectroscopy, as well as molecular dynamics (MD) simulation. It was found that DDA ions were adsorbed on montmorillonite basal surfaces mainly by physical adsorption, including the electrostatic attraction and hydrogen bonding. A certain number of neutral DDA molecules can favor the adsorption of DDA. At pH around 8, the effect of hydrophobic modification was the best because DDA molecules and ions form compact and well-organized monolayer. The MD simulation results were in good agreement with that of contact angle, adsorption quantity and FTIR. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Clay minerals are layered silicate minerals, including kaolinite, montmorillonite and illite, which are widely used in ceramics, plastics, coatings, paper, rubber and cosmetics and other industrial products [1–3]. Recently, it becomes a growing problem to dewater the clay-rich tailing in mineral processing and hydrometallurgy [4,5]. The suspensions of clay-rich tailings are highly dispersed to settle very slowly due to the presence of clay hydration [6,7]. In this case, hydrophobic aggregation would be an effective method to promote the dewatering of clay-rich tailing by adding hydrophobic modifier instead of electrolyte coagulant and macromolecule flocculant [8]. In the method, hydrophilic clay surface become hydrophobic after adsorbing hydrophobic modifiers to form largesize aggregations under hydrophobic attraction and kinetic energy input, promoting the sedimentation of clay particles under gravity [9,10]. Usually, long-chain alkylamines, which are popular collectors in nonsulfide floatation [11,12], can be used as the hydrophobic modifier in the process of hydrophobic aggregation. For example, Zhang et al. [13] investigated the hydrophobic aggregation of kaolinite in dodecylamine surfactant suspension and found the aggregation model at different solution pH. In the hydrophobic
∗ Corresponding author. E-mail address: [email protected] (F. Min). http://dx.doi.org/10.1016/j.apsusc.2017.07.085 0169-4332/© 2017 Elsevier B.V. All rights reserved.
aggregation of clay-rich tailing and the flotation of nonsulfidic mineral flotation, it is very important to understand the adsorption mechanism of long-chain alkylamines at the mineral/aqueous. Previous experimental studies [14,15] showed that in addition to the type and concentration of long-chain alkylamines, the solution pH had an important effect on the adsorption behavior. Long-chain alkylamines almost existed in the cationic form under the acidic condition and could be physically adsorbed at the negatively charged silicate surface mainly through electrostatic interaction when below the critical micelle concentration (CMC). At low pH, limited adsorption occurred; with the increasing of pH, adsorption of ions increased and even a bilayer could formed at pH around 10 [16,17]. The co-adsorption of neutral molecular amines could enhance the adsorption of amine ions [17,18]. In recent years, the molecular dynamics (MD) simulation has become an effective means to explore the complex surface reaction and structure at the molecular level, avoiding the limitation of the resolution of the experimental analysis and the interference to the target system [19]. Tang et al. [20] used MD simulations to study the adsorption of DDA on iron surfaces in aqueous solution, and found that in acidic solution DDA can be adsorbed on the iron surface in both of the molecular form and ionic form, but they just considered one DDA molecule or ion per model and did not determine the effect of the pH values. Other MD simulations [21,22] have been carried out to describe the adsorption of DDA on silica and muscovite surface at different pH, and found that the state of adsorbed DDA at the inter-
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face varies significantly at different pH values, which was proved by the vibrational spectroscopy results. In the work, we focused on the adsorption of DDA at the montmorillonite/water interface as a function of pH, because montmorillonite has more detrimental effect on the sedimentation and filtration of tailing water than other clay minerals. Montmorillonite is 2:1 type of layered silicate mineral with an alumina octahedral sheet sandwiched by two silica tetrahedron sheets. Each layer is strongly negatively charged (0.4–1.2 e per unit cell) because of the isomorphous substitution of aluminum by magnesium in alumina octahedral sheet [23]. The charge in the crystal is compensated by sodium or calcium ions in the interlayer. At present, many studies are limited to the intercalation of alkylamine ions into the interlayer spaces of montmorillonite to synthesize organoclay at high temperature or under ultrasonic processing [24–26] while little studies involves the adsorption behavior of alkylamines at montmorillonte/water interface in aqueous solution at room temperature, which is important in the hydrophobic aggregation of clay-rich tailings. In the present study, sedimentation test followed by the characterization of flocs images, contact angle and adsorption quantity, and FTIR were undertaken to study the adsorption behavior of DDA on montmorillonite. MD simulations were used to study the structure and dynamics of the adsorbed DDA layer at montmorillonte/water interface. 2. Materials and methods 2.1. Materials The montmorilloite were purified from bentonite raw ores in Anji, Zhejiang, China using the natural settling method reported in literature[27]. Then the exchangeable cations of purified montmorillonite were replaced by Na+ ions, which was followed by washing using de-ionized water. The X-ray diffraction (XRD) diagram and Scanning electron microscope (SEM) photo of the purified montmorillonite sample (Fig. 1) showed that the purified samples only contained montmorillonite phase which were lamellar in shape with a diameter of around 2 m. The dodecylamnie with 99% purity was purchased from Sinopharm Chemical Reagent Co.,Ltd (in China). DDA solution was prepared by adding equimolar acetic acid. The solution pH was adjusted by HCl or NaOH solution. De-ionized water with the residual conductivity less than 1 S/cm was used throughout the experiments. 2.2. Sedimentation test and characterization The sedimentation test was carried out as follows: (1) weighted a certain number of montmorillonite samples and immersed them in less than 100 mL de-ionized water for 12 h, and then stirred for 30 min; (2) added a certain amount of DDA and de-ionized water to prepare 100 mL 0.6 wt.% montmorillonite suspension; (3) adjusted the pH to the required value by HCl or NaOH solutions, and stirred at the 200 r/min speed for 10 min; (4) transferred the montmorillonite suspension to a 100 mL graduated cylinder, and shaken up and down five times to start the sedimentation test which lasted for 10 min; (5) after the sedimentation, extracted the top 50 mL suspension by siphon method to measure the light transmittance using UV2600 Ultraviolet Spectrophotometry (Shimadzu, Japan). The montmorillonite samples in the bottom 50 mL suspension were dried bellow 80 ◦ C. Then the dried samples were ground to pass through a 450 m sieve to use for the measurement of contact angle and FTIR analysis. In the measurement of contact angle, the samples were pressed at 20 MPa pressure for 2 min to obtain round sheets with thickness of about 2 mm to measure the con-
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tact angle in a C20 automatic contact angle meter (Kino, USA). Each measurement was repeated three times at different sites of the surface. In FTIR analysis, the sample was mixed with KBr, then ground and pressed to small pelletized discs for FTIR analyses which were conducted in the range of 400–4000 cm−1 by a Nicolet-380 FTIR instrument (Thermo Fisher, USA). The zeta potential of montmorillonite in DDA solutions was measured by ZetaProbe (Colloidal Dynamics, USA). The montmorillonite suspensions at the solid concentration of 0.6 wt% were prepared in pure water and 7 × 10−4 mol/L DDA solutions. The solution pH was adjusted using 1 mol/L HCl solution and NaOH solution. The values of zeta potential were obtained at pH from 7 to 12 and from 7 to 2 in the model of automatic titration. In the measurement of adsorption quantity, the suspension after the sedimentation test was centrifuged at 3000 r/min for 30 min. Then the centrifugal liquid was used to measure the final concentration of residual DDA in the solution based on the calibration curve using UV2600 Ultraviolet Spectrophotometry. The adsorption quantity of DDA on montmorillonite particles was calculated by the following expression: A=
V (C − C0 ) m
(1)
where A was the adsorption quantity, mol/g; C0 and C were the initial and final concentrations of DDA, respectively, mol/L; V was the solution volume, L; m was the weight of the particles, g. 2.3. Molecular dynamics (MD) simulations The montmorillonite model had only Mg2+ → Al3+ substitution in the octahedral layer. The structural formula was Na0.333 [Si4 O8 ][Al1.667 Mg0.333 O2 (OH)2 ]. Each cell contained 0.666 e negative charge, which was common for most montmorillonite. A (5 × 3 × 2) periodic repeating supercell was constructed, containing two montmorillonite sheets. To ensure that the surfactant structures had sufficient space to assemble at the motmorillonite/water interface and prevented them from migrating to the gas/water interface, the aqueous solution thickness was set to about 5 nm. About 1000 water molecules were required to construct the bulk solution. To yield an overall neutral system, a certain number of Na+ or Cl− ions were added to the model. The vacuum layer with the thickness of 80 nm was added on the aqueous solution layer to avoid the interference from the periodic image layer in the z axis direction. Thus, the supercell size became 26 × 27 × 150 Å3 . According to the previous DFT and MD results [28–30], the centre of the surface six oxygen ring (SOR) above the lattice substitution in the motmorillonite was the best adsorption site for inorganic cations, e.g., Na+ and Ca2+ , and head-groups of cationic surfactants. The Na+ of sodium montmorillonite in aqueous solution would move away from the surface to the bulk solutions to form outer-sphere coordination complexes. Therefore, in the initial model, it was reasonable to arrange the head-groups of cationic surfactants to orient toward the SOR of montmorillonite with the alky chain toward the bulk solution, as shown in Fig. 2. Similar methods to construct the initial model were also used in the simulation studies on the adsorption of alkylamine on mica surface [31–33]. Because the size of head group of an alkylamine molecule or ion approximated to that of SOR, the coverage of one alkylamine molecule or ion per SOR can be considered as dense monolayer (ML). In our model, the (001) basal surface contained 30 SOR rings, so 1 ML coverage meant 30 adsorbed alkylamine structures. In addition, as a kind of hydrolyzable surfactant, DDA mainly presents in acidic solution in the ionic form or in alkaline solution in molecular form depending on the solution pH [34,35]. For convenience, the component of DDA in aqueous solution was represented by I:M, where I and M were the number of DDA ions and molecules,
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Fig. 1. The XRD diagram (a) and SEM photo (b) of the purified montmorillonite.
Fig. 2. The initial configuration for DDA at montmorillonite/water interface.
respectively. At the DDA concentration of 7 × 10−4 mol/L, the critical pH for DDA to form molecular precipitation is 9.1. When pH is around 12, DDA species are completely DDA molecules; when pH is bellow 4, species are completely DDA ions. Therefore, at 1 ML coverage, pH = 12, 10, 8, 6 and 4, would correspond to I:M = 0:30, 5:25, 10:20, 20:10 and 30:0, respectively. Similar methods appeared in the literatures [21,22]. MD simulations were performed in the Forcite plus module of Material studios 8.0 software. The force field used in the calculation was PCFF INTERFACE developed by Heinz et al. [36], in which INTERFACE force field was integrated in harmonic PCFF force fields for biomolecules, solvents, and polymers. The PCFF INTERFACE force field enabled atomistic simulations of nanostructures at the
1–100 nm scale in high accuracy and served as a uniform simulation platform for inorganic, organic, and biomolecular compounds. A flexible SPC model [37] was used for water molecules. The Mulliken atomic partial charges of DDA ions and molecule were distributed after the geometry optimization at the PW91/DNP level in the Dmol3 module, as shown in Fig. 3. In the simulation, firstly, geometry optimization for the system was performed using conjugate gradient algorithm to reduce the residual force of each atom. The convergence achieved once the maximum force on any one atom was less than 100 kJ mol−1 nm−1 . Subsequently, a 100 ps pre-equilibrium simulation was run in the NPT ensemble. Berendsen thermostat and Berendsen barostat which offered swift equilibration of the system, were selected with the temperature coupling constant of 0.1 ps and the pressure-
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Fig. 3. The optimized structures labeled atomic partial charge of DDA molecule (a) and ion (b) at the PW91/DNP level.
Fig. 4. The light transmittance of the supernatant of montmorillonite suspensions as a function of DDA concentration (a) and solution pH (b).
coupling constant of 1 ps, respectively. Finally, equilibration was followed by a 2 ns production run in the NVT ensemble where Nose thermostat was used. The last 0.5 ns trajectory was used for analysis of relevant properties. All dynamic simulations were run in step of 1 fs and at a pressure of 1 bar and a temperature of 300 K. The longrange electrostatic interaction and van der Waals interaction were calculated using the Ewald and Atom based methods, respectively, with a cutoff radius of 1.25 nm.
3. Results and discussion 3.1. Sedimentation test The effect of sedimentation can be evaluated by the light transmittance of supernatant after the sedimentation test. The larger the light transmittance is, the better the effect of sedimentation is. The DDA concentration and solution pH dependence of the light transmittance of the supernatant of montmorillonite suspensions was showed in Fig. 4. From Fig. 4a, the light transmittance in pure water was 12.9% under weak alkaline condition (pH = 8). When the DDA at the concentration of 1 × 10−4 mol/L was added in the suspension, the light transmittance was 14.3% and close to that in pure water, but when the concentration of DDA increased from 1 × 10−4 mol/L to 7 × 10−4 mol/L, the light transmittance approximately linearly increases from 14.3% to 65.3%, then changed little when the concentration of DDA continued to increase to 1.5 × 10−3 mol/L. From Fig. 4b, the light transmittance of the supernatant in the DDA solutions at the concentration of 7 × 10−4 mol/L was very small (about 2%) and close to that without DDA at pH from 12 to 10. Under the condition, hydrophobic modifier DDA did not promote the sedimentation of montmorillonite particles. One reason should be that at the DDA concentration of 7 × 10−4 mol/L, the critical pH for DDA to form molecular precipitation was pH 9.1. When pH was above 9.1, DDA molecules started to form precipitates in the solu-
tion, losing chemical activity. When the pH decreased to 9, the light transmittance of supernatant added DDA was higher than that in the absence of DDA, indicating that DDA has begun to function. As pH decreased from 9 to7, the light transmittance increased sharply. In this pH range, the ratio of DDA ion to molecule in solution became much larger, suggesting that these DDA ions were adsorbed on montmorillonite surfaces by electrostatic attraction, not only decreasing the surface charge, but also rendering the surface good hydrophobicity[13,38,39]. In this case, the electrostatic repulsion between the negatively charged surface weakened while the hydrophobic attractive strengthened so that the fine particles could aggregate each other to form big flocs. The flocs could easily settle under the gravity, resulting in the increase of the light transmittance of supernatant. When pH continued to decrease from 7 to 4, the transmittance did not change significantly because most of the fine particles had settled after the sedimentation test.
3.2. Flocs images The in-situ morphological characteristics of montmorillonite flocs could be observed by monocular variable optical microscope. The original floc structures would not be destroyed by other factors, such as drying and squeezing. Fig. 5 showed the montmorillonite flocs images at different pH with and without 7 × 10−4 mol/L DDA. It was observed that the individual montmorillonite particles were very small, that is, in the micron range. When the solution pH was above 10, the flocs did not appear (not showed in Fig. 5) because montmorillonite particles were highly dispersive in the solution. With the decreasing of pH and adsorption of DDA, the montmorillonite flocs began to appear at pH 8, but they were relatively small and loosely linked together with each other (Fig. 5a). As pH decreased to 6, the flocs became larger and more compact, and reached the millimeter level (Fig. 5b). At the very low pH of 4, the flocs were less compact than that at pH 6 (Fig. 5c).
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Fig. 5. The images of montmorillonite flocs at the DDA concentration of 7 × 10−4 mol/L at pH 8 (a), 6 (b), and 4 (c), respectively.
Fig. 6. The contact angle of montmorillonite surface and adsorption quantity of DDA as a function of pH at the DDA concentration of 7 × 10−4 mol/L.
3.3. Contact angle and adsorption quantity The values of contact angle can reflect the degree of hydrophobicity of mineral surface. The larger the contact angle is, the stronger the hydrophobicity of the surface is. In fact, the hydrophobicity of the surface is attributed to the adsorption quantity and aggregation structures of surfactant on the surface. Fig. 6 showed the plots of the contact angle of montmorillonite and adsorption quantity of DDA against the solution pH at the DDA concentration of 7 × 10−4 mol/L. When more DDA were adsorbed on the surface and the adsorbed DDA layer was more compact and well-organized, the surface would become more hydrophobic. It can be seen that the contact angle of montmorillonite increased when the pH decreased from 10 to 8, and reached a maximum around pH 8, and then
decreased with the pH decreasing from 8 to 4. Likewise, the adsorption quantity had the similar behavior against the solution pH. The results showed that at pH around 8, the montmorilloite surfaces were most hydrophobic. These behaviors would be explained by the following MD simulation results at the molecular level. 3.4. FTIR analysis FTIR spectroscopy can be performed to clarify the adsorption mechanism of reagent on the mineral surfaces. The FTIR spectra of montmorillonite conditioned with 7 × 10−4 mol/L DDA in the pH range of 10–4 were showed in Fig. 7. The broader peak near ∼3430 cm−1 was assigned to the stretching modes of NH2 of DDA and hydroxyl of water (OHw ). Two new peaks at 2930 and
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Fig. 7. FTIR spectra of motmorillonite as a function of pH at the DDA concentration of 7 × 10−4 mol/L.
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Generally, the pH in the neutral and alkaline solution is usually above the PZC of edge surface of montmorillonite because the point of zero charge (PZC) of the edge surface is not a certain value, generally between pH 4 and 8 according to different experimental methods, such as slightly above pH 5 by rheological measurement [42], pH 6.1 by titration experiment [43] and around pH 7 by electrokinetic measurement [44]. In the case, both the basal and edge surfaces of montmorillontie are negatively charged. If the cationic surfactants DDA are added in the suspension, DDA would be adsorbed on the surface by electrostatic attraction [45–47]. As shown in Fig. 8, the adsorption of DDA decreased the zeta potential of montmorillonite at large-scale pH except at pH above 11. In addition, the hydrophobic carbon chain of DDA could associated with each other by the hydrophobic attractive [48], resulting in the formation of flocs by face-to-face or parallel aggregation (Fig. 9a). In the acidic solution, when the pH is below the PZC of edge surface, the edge surfaces would be positively charged while the basal surfaces are still negatively charged. Then the basal and edge surfaces of montmorillonite would attract each other to produce face-to-edge T-type or card-house type aggregation. In addition, the basal surface had a special affinity with DDA ions to render the surface hydrophobicity, promoting the aggregation of particles (Fig. 9b). Therefore, it was concluded that under the combination of face-to-edge electrostatic attraction and hydrophobic attraction, the face-to-edge flocs produced at acidic pH would be larger than those face-to-face flocs at neutral and slightly alkaline pH, resulting in better sedimentation effect. The conclusion was in good agreement with the results of light transmittance of supernatant and flocs images, and could explain why the effect of sedimentation became better when the hydrophobicity of montmorillonite decreased with pH decreasing from 8 to 4. 3.6. MD simulation results
Fig. 8. The zeta potential of montmorillonite as a function of pH in pure water and in DDA solutions at the concentration of 7 × 10−4 mol/L.
2850 cm−1 were assigned to CH stretching group. The new peak at 2370 cm−1 with a shoulder at 2330 cm−1 belonged to the CN stretching group [40]. And the peak at ∼1630 cm−1 represented the bending modes of NH of DDA and OHw of water [41]. Though the spectrum of montmorillonite treated by DDA presented characteristic absorption peaks of DDA, no band shift occurred, implying that DDA were adsorbed on montmorillonite mainly by physical adsorption. The decreasing transmittances in the NH2 and NH stretching region spectra of DDA from pH 8 to pH 4 indicated the decreased adsorption of DDA, which was in accord with the results of contact angle and adsorption quantity. 3.5. The model of hydrophobic aggregation As a kind of 2:1 type phyllosilicate, montmorillonite has basal surface and edge surfaces with different charge properties. The basal surface of montmorillonite, e.g., (001) face, has the permanent negative charges due to isomorphous substitution of Mg2+ → Al3+ within alumina octahedral sheet. Fig. 8 showed the zeta potential of montmorillonite as a function of pH in pure water and in DDA solutions at the concentration of 7 × 10−4 mol/L. In the whole pH range of 2–12, the zeta potential of montmorillonite was negative, but the value decreased from −43.2 mV to −26.3 mV with the pH decreasing from 12 to 2. The reason should be attributed to the sign and density of the charges at the edge surface such as (010) and (110) faces which depend on the pH of the solution.
Because DDA is a kind of hydrolyzable surfactant, its species in solution change with the solution pH. Fig. 10 showed the equilibrium configurations of the DDA molecules or ions adsorbed at the montmorillonite/water interface at different pH. When pH was 12 (Fig. 10a), DDA existed completely in the form of molecules (I:M = 0:30), and self-agglomeration of DDA molecules occurred at a distance from the montmorillonite basal surface. DDA molecules had no hydrophobic modification effect on the montmorillonite so that the montmorillonite suspensions was highly dispersed. When pH decreased to 10, that is, I:M = 5:25 (Fig. 10b), five DDA ions were adsorbed on the surface with their polar head groups toward the surface to form the inner layer while 25 DDA molecules were bound to the inner layer through the hydrophobic association among their non-polar carbon chains with some of polar head groups toward the bulk solution. The adsorbed structure was similar to an irregular bilayer. At this pH, a certain extent effect of hydrophobic modification started to appear. However, because a large number of polar head groups in the adsorbed monolayer were oriented toward the solution, it was disadvantageous for the hydrophobic modification of montmorillontie. Under the circumstances, the strong electrostatic repulsion between particles dominated the high stability of the montmorillonite suspension because of the large negative charge of edge surface, which agreed with the very low light transmittance (about 0.47%) of the supernatant (Fig. 4). When pH decreased to 8 (I:M = 10:20) (Fig. 10c), it was obviously observed that all the DDA molecules and ions were adsorbed on the surface through their polar head groups to form monolayer, namely, hemi-micelle. The adsorption layer was very compact and the surfactant packed in parallel through the hydrophobic association between the hydrophobic carbon chains so that the hydrophobic modification effect was very significant, which was
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Fig. 9. The model of the interaction of DDA with montmorillonite at different pH [49].
Fig. 10. The equilibrium structures of DDA on montmorillonite (001) surfaces at different pH. pH = 12, I:M = 0:30 (a); pH = 10, I:M = 5:25 (b); pH = 8, I:M = 10:20 (c); pH = 6, I:M = 20:10 (d); pH = 4, I:M = 30:0 (e).
in good agreement with the maximum adsorption quantity and contact angle at pH 8 (Fig. 6). A similar behavior was reported by Rutland et al. [17]. They investigated the adsorption of DDA on mica by surface force apparatus (SFA), and found that at pH around 8, adsorption of neutral DDA molecules became important to render the monolayer more compact with the thickness close to the length of an extended molecule, and strong attractive force was observed when two hydrophobic surfaces contacted.
When the solution pH were lower, such as 6 and 4, corresponding to I:M of 20:10 and 30:0 (Fig. 10d and e), respectively, the form of ions predominated in the DDA species. The DDA adsorption layer was still compact, but some DDA ions moved away from the inner to upside of adsorbed layer with their head groups oriented to the solution. Herder[50] found the similar behavior in studying the interactions between mica surfaces in DDA solutions at pH values between 5 and 6. In the case, the structure of the adsorbed layer was
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Fig. 11. The close-up snapshot of adsorbed DDA ions at the interface. (The black dash line denoted hydrogen bond).
Fig. 12. The concentration profile of N atoms of DDA head groups perpendicular to montmorillonite (001) surface at the pH 12, 8 and 4, respectively.
not that of a perfect monolayer but contains imperfections[51,52]. Therefore, the effect of hydrophobic modification decreased as pH decreased from 8 to 4, which is consistent with the foregoing results of contact angle and adsorption quantity. The close-up snapshots of the DDA ions at the interface at pH 8 were showed in Fig. 11. It was found that at the interface the polar head groups of DDA not only interacted with not only the surface oxygen atoms, but also water molecules through the formation of hydrogen bonds between the hydrogen atoms (Hn ) of the head group and surface oxygen atoms (Os ) or water oxygen atoms (Ow ). A competitive adsorption existed between the water molecules and DDA toward the surface. In Fig. 9a, the head group of DDA only interacted with the surface, forming three Hn · · ·Os hydrogen bonds and non Hn · · ·Ow hydrogen bond. However, one, two and three Hn · · ·Ow hydrogen bonds were formed in Fig. 10b–d, respectively. The increase of number of Hn · · ·Ow hydrogen bond indicated that the interaction between head group of DDA and water strengthened, and in turn the interaction between head group and surface weakened so that the distance between the head group and the surface increased. The hydrogen bonding between the polar head group and the water molecule resulted in DDA no longer being perpendicular to the surface but being tilted. The similar behavior was found in the study of DDA on mica (001) surface by Xu et al. [31] The concentration profile of N atoms of DDA in the normal direction of montmorillonite surface at different pH was showed in Fig. 12. It could be found that at pH 12, several concentration peaks of N atoms were concentrated in a distance range of 7–40 Å from the surface, indicating that the DDA were far from the surface and formed self-agglomeration, bringing no hydrophobic modifica-
Fig. 13. The concentration profile of water Ow atoms perpendicular to montmorillonite (001) surface at the pH 12, 8 and 4, respectively.
tion effect to montmorillonite. At pH 8, the concentration peaks of N atoms were concentrated in the narrow distance range of 0–7 Å from the surface, indicating that all the polar head groups were oriented towards the surface of the montmorillonite rather than the non-polar carbon chain. This case can produce the good hydrophobic modification effect. At pH 4, although the concentration peaks of N atoms are mainly concentrated in the range of 0–12 Å from the surface, there were still obvious distributions in the range of 22–30 Å. It suggested that some polar head groups at the interface transferred from the initial adsorption position at the interface to the top of the adsorption monolayer, weakening the hydrophobic modification effect of the surface to a certain extent. The concentration distribution of water molecules perpendicular to the interface can indirectly reflect the effect of pH on the competitive adsorption between waters and DDA ions or molecules at the interface, as shown in Fig. 13. It was observed that the concentration of Ow atoms in the distance range of 15–35 Å was much smaller than that in other places at pH 12, implying the formation of self-aggregation of DDA. When pH was 8, the Ow concentration at the distance from 3 to 18 Å was zero, indicating that all these places were occupied by the DDA instead of water molecules. When pH decreased to 4, the Ow concentration at the distance of 3–18 Å obviously increased, indicating that some of the water molecules were occupied in this region. The reason was that some of the DDA structures moved outside the adsorption layer so that some water could move in the place. Additionally, under all pH conditions, the Ow concentration peak in the distance range of 0–5 Å appeared like the N concentration peak (Fig. 12) because the water molecules were polarized by the surface or polar head of DDA, showing that
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Table 1 The torsion deviation of dihedral angle . pH
12
8
4
26.5%
4.7%
22.6%
competitive adsorption existed between the water molecules and DDA toward the surface. When the surfactant DDA aggregated at the montmorillonite/water interface, the alkyl carbon chain will be distorted, deviating from the initial linear structure. The degree of twist of the alkyl carbon chain can be characterized by the distribution of dihedral angle of the carbon atoms. The percentage of the dihedral angle in the range of [–120◦ ,120◦ ] was defined as the torsion deviation of dihedral angle t [31], as shown in Table 1. The greater the was, the stronger the degree of twist of DDA was and the lower the degree of order of DDA was. It could be found that was lowest (4.7%) at pH 8, implying that the DDA were well-organized and in favor of the hydrophobic modification. However, at pH 12 and 4, increased to 26.5% and 22.6%, respectively, suggesting that the degree of order decreased and formed self-aggregation. The results agreed with the equilibrium structures and concentration profile of atoms. 4. Conclusion Effect of pH on the adsorption of DDA on montmorillonite surface in aqueous solution was studied by the experimental and MD simulation methods. The experimental results showed that the effect of hydrophobic modification for montmorillonite increased as the pH decreased from 10 to 8, reaching a maximum around pH 8, and then decreased with the pH decreasing from 8 to 4. Though the hydrophobicity of montrmorillonite decrease as pH decreased from 8 to 4, the effect of sedimentation increased due to the formation of big face-to-edge flocs under low pH conditions. When pH was below 7, the effect of sedimentation did not change significantly. MD simulation results showed that when the positively charged DDA ions were adsorbed onto the negatively charged montmorillonite surfaces, the initial stage of the adsorption was driven by electrostatic forces and hydrogen bonding, and the later stage by hydrophobic association between the alkyl carbon chains. A competitive adsorption existed between water molecules and DDA toward the montmorillonite surface. A certain numbers of neutral DDA molecules could favor the adsorption of DDA, rendering the monolayer more compact. The structure and composition of the adsorbed layer was different by decreasing the pH from 12 to 4. At pH 8, DDA molecules and ions were adsorbed on the surface through their polar head groups to form monolayer, namely, hemimicelle. The adsorption layer was very compact and the surfactant packed in parallel, so that the hydrophobic modification effect was the best. Acknowledgements This work was supported by the Natural Science Foundation of China [grant number 51474011]; the Post-doctoral Science Foundation of China [grant number 2014M561810]; the Anhui Province International Cooperation Project [grant number 1303063011]; and the Anhui Provincial Natural Science Foundation of China [grant number 1508085QE90]. References [1] F. Uddin, Clays, Nanoclays, and montmorillonite minerals, Metal. Mater. Trans. A 39 (2008) 2804–2814.
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