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Applied Surface Science 425 (2017) 996–1005

Contents lists available at ScienceDirect

Applied Surface Science

journa l h om epa ge: www.elsev ier .com/ locate /apsusc

ull Length Article

ffect of pH on the adsorption of dodecylamine on montmorillonite:nsights from experiments and molecular dynamics simulations

henliang Penga, Fanfei Minb,∗, Lingyun Liub

Institute of Engineering and Research, Jiangxi University of Science and Technology, Ganzhou 341000, ChinaDepartment of Materials Science and Engineering, Anhui University of Science and Technology, Huainan 232001, China

r t i c l e i n f o

rticle history:eceived 11 April 2017eceived in revised form 12 June 2017ccepted 11 July 2017vailable online 15 July 2017

a b s t r a c t

The hydrophobic aggregation in cationic surfactant suspension is an effective method to enhance thedewatering of clay-rich tailing. The solution pH can affect the adsorption behavior of cationic surfac-tant on clay mineral. The effect of pH on the adsorption of dodecylamine (DDA) on montmorillonite wasinvestigated by the sedimentation test and the characterization of flocs images, contact angle, adsorptionquantity, and fourier transform infrared (FTIR) spectroscopy, as well as molecular dynamics (MD) simu-

eywords:ontmorilloniteodecylaminedsorptionolecular dynamic

lation. It was found that DDA ions were adsorbed on montmorillonite basal surfaces mainly by physicaladsorption, including the electrostatic attraction and hydrogen bonding. A certain number of neutralDDA molecules can favor the adsorption of DDA. At pH around 8, the effect of hydrophobic modificationwas the best because DDA molecules and ions form compact and well-organized monolayer. The MDsimulation results were in good agreement with that of contact angle, adsorption quantity and FTIR.

© 2017 Elsevier B.V. All rights reserved.

. Introduction

Clay minerals are layered silicate minerals, including kaolin-te, montmorillonite and illite, which are widely used in ceramics,lastics, coatings, paper, rubber and cosmetics and other industrialroducts [1–3]. Recently, it becomes a growing problem to dewa-er the clay-rich tailing in mineral processing and hydrometallurgy4,5]. The suspensions of clay-rich tailings are highly dispersed toettle very slowly due to the presence of clay hydration [6,7]. Inhis case, hydrophobic aggregation would be an effective methodo promote the dewatering of clay-rich tailing by adding hydropho-ic modifier instead of electrolyte coagulant and macromoleculeocculant [8]. In the method, hydrophilic clay surface becomeydrophobic after adsorbing hydrophobic modifiers to form large-ize aggregations under hydrophobic attraction and kinetic energynput, promoting the sedimentation of clay particles under gravity9,10]. Usually, long-chain alkylamines, which are popular collec-ors in nonsulfide floatation [11,12], can be used as the hydrophobic

odifier in the process of hydrophobic aggregation. For exam-

le, Zhang et al. [13] investigated the hydrophobic aggregationf kaolinite in dodecylamine surfactant suspension and found theggregation model at different solution pH. In the hydrophobic

∗ Corresponding author.E-mail address: ffmin@aust.edu.cn (F. Min).

ttp://dx.doi.org/10.1016/j.apsusc.2017.07.085169-4332/© 2017 Elsevier B.V. All rights reserved.

aggregation of clay-rich tailing and the flotation of nonsulfidic min-eral flotation, it is very important to understand the adsorptionmechanism of long-chain alkylamines at the mineral/aqueous.

Previous experimental studies [14,15] showed that in addi-tion to the type and concentration of long-chain alkylamines, thesolution pH had an important effect on the adsorption behav-ior. Long-chain alkylamines almost existed in the cationic formunder the acidic condition and could be physically adsorbed atthe negatively charged silicate surface mainly through electrostaticinteraction 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 pHaround 10 [16,17]. The co-adsorption of neutral molecular aminescould enhance the adsorption of amine ions [17,18]. In recent years,the molecular dynamics (MD) simulation has become an effectivemeans to explore the complex surface reaction and structure atthe molecular level, avoiding the limitation of the resolution of theexperimental analysis and the interference to the target system[19]. Tang et al. [20] used MD simulations to study the adsorption ofDDA on iron surfaces in aqueous solution, and found that in acidicsolution DDA can be adsorbed on the iron surface in both of themolecular form and ionic form, but they just considered one DDAmolecule or ion per model and did not determine the effect of the

pH values. Other MD simulations [21,22] have been carried out todescribe the adsorption of DDA on silica and muscovite surface atdifferent pH, and found that the state of adsorbed DDA at the inter-

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ace varies significantly at different pH values, which was provedy the vibrational spectroscopy results.

In the work, we focused on the adsorption of DDA at theontmorillonite/water interface as a function of pH, because mont-orillonite has more detrimental effect on the sedimentation and

ltration of tailing water than other clay minerals. Montmorillonites 2:1 type of layered silicate mineral with an alumina octahedralheet sandwiched by two silica tetrahedron sheets. Each layer istrongly negatively charged (0.4–1.2 e per unit cell) because of thesomorphous substitution of aluminum by magnesium in aluminactahedral sheet [23]. The charge in the crystal is compensated byodium or calcium ions in the interlayer. At present, many stud-es are limited to the intercalation of alkylamine ions into thenterlayer spaces of montmorillonite to synthesize organoclay atigh temperature or under ultrasonic processing [24–26] while

ittle studies involves the adsorption behavior of alkylamines atontmorillonte/water interface in aqueous solution at room tem-

erature, which is important in the hydrophobic aggregation oflay-rich tailings. In the present study, sedimentation test followedy the characterization of flocs images, contact angle and adsorp-ion quantity, and FTIR were undertaken to study the adsorptionehavior of DDA on montmorillonite. MD simulations were usedo study the structure and dynamics of the adsorbed DDA layer at

ontmorillonte/water interface.

. Materials and methods

.1. Materials

The montmorilloite were purified from bentonite raw ores innji, Zhejiang, China using the natural settling method reported in

iterature[27]. Then the exchangeable cations of purified montmo-illonite were replaced by Na+ ions, which was followed by washingsing de-ionized water. The X-ray diffraction (XRD) diagram andcanning electron microscope (SEM) photo of the purified mont-orillonite sample (Fig. 1) showed that the purified samples only

ontained montmorillonite phase which were lamellar in shapeith a diameter of around 2 �m.

The dodecylamnie with 99% purity was purchased frominopharm Chemical Reagent Co.,Ltd (in China). DDA solution wasrepared by adding equimolar acetic acid. The solution pH wasdjusted by HCl or NaOH solution. De-ionized water with theesidual conductivity less than 1 �S/cm was used throughout thexperiments.

.2. Sedimentation test and characterization

The sedimentation test was carried out as follows: (1) weighted certain number of montmorillonite samples and immersed themn less than 100 mL de-ionized water for 12 h, and then stirred for0 min; (2) added a certain amount of DDA and de-ionized water torepare 100 mL 0.6 wt.% montmorillonite suspension; (3) adjustedhe pH to the required value by HCl or NaOH solutions, and stirredt the 200 r/min speed for 10 min; (4) transferred the montmoril-onite suspension to a 100 mL graduated cylinder, and shaken upnd down five times to start the sedimentation test which lastedor 10 min; (5) after the sedimentation, extracted the top 50 mLuspension by siphon method to measure the light transmittancesing UV2600 Ultraviolet Spectrophotometry (Shimadzu, Japan).

The montmorillonite samples in the bottom 50 mL suspensionere dried bellow 80 ◦C. Then the dried samples were ground to

ass through a 450 �m sieve to use for the measurement of con-act angle and FTIR analysis. In the measurement of contact angle,he samples were pressed at 20 MPa pressure for 2 min to obtainound sheets with thickness of about 2 mm to measure the con-

ence 425 (2017) 996–1005 997

tact angle in a C20 automatic contact angle meter (Kino, USA). Eachmeasurement was repeated three times at different sites of the sur-face. In FTIR analysis, the sample was mixed with KBr, then groundand pressed to small pelletized discs for FTIR analyses which wereconducted in the range of 400–4000 cm−1 by a Nicolet-380 FTIRinstrument (Thermo Fisher, USA).

The zeta potential of montmorillonite in DDA solutions wasmeasured by ZetaProbe (Colloidal Dynamics, USA). The montmo-rillonite suspensions at the solid concentration of 0.6 wt% wereprepared in pure water and 7 × 10−4mol/L DDA solutions. The solu-tion 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 andfrom 7 to 2 in the model of automatic titration.

In the measurement of adsorption quantity, the suspension afterthe sedimentation test was centrifuged at 3000 r/min for 30 min.Then the centrifugal liquid was used to measure the final concen-tration of residual DDA in the solution based on the calibrationcurve using UV2600 Ultraviolet Spectrophotometry. The adsorp-tion quantity of DDA on montmorillonite particles was calculatedby the following expression:

A = V(C − C0)m

(1)

where A was the adsorption quantity, mol/g; C0 and C were theinitial and final concentrations of DDA, respectively, mol/L; V wasthe solution volume, L; m was the weight of the particles, g.

2.3. Molecular dynamics (MD) simulations

The montmorillonite model had only Mg2+ → Al3+ substi-tution in the octahedral layer. The structural formula wasNa0.333[Si4O8][Al1.667Mg0.333O2(OH)2]. Each cell contained 0.666 enegative charge, which was common for most montmorillonite. A(5 × 3 × 2) periodic repeating supercell was constructed, containingtwo montmorillonite sheets. To ensure that the surfactant struc-tures had sufficient space to assemble at the motmorillonite/waterinterface and prevented them from migrating to the gas/waterinterface, the aqueous solution thickness was set to about 5 nm.About 1000 water molecules were required to construct the bulksolution. To yield an overall neutral system, a certain number ofNa+ or Cl− ions were added to the model. The vacuum layer withthe thickness of 80 nm was added on the aqueous solution layer toavoid the interference from the periodic image layer in the z axisdirection. Thus, the supercell size became 26 × 27 × 150 Å3.

According to the previous DFT and MD results [28–30], thecentre of the surface six oxygen ring (SOR) above the lattice sub-stitution in the motmorillonite was the best adsorption site forinorganic cations, e.g., Na+ and Ca2+, and head-groups of cationicsurfactants. The Na+ of sodium montmorillonite in aqueous solu-tion would move away from the surface to the bulk solutions toform outer-sphere coordination complexes. Therefore, in the ini-tial model, it was reasonable to arrange the head-groups of cationicsurfactants to orient toward the SOR of montmorillonite with thealky chain toward the bulk solution, as shown in Fig. 2. Similarmethods to construct the initial model were also used in the sim-ulation studies on the adsorption of alkylamine on mica surface[31–33]. Because the size of head group of an alkylamine moleculeor ion approximated to that of SOR, the coverage of one alkylaminemolecule 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 addi-tion, as a kind of hydrolyzable surfactant, DDA mainly presents in

acidic solution in the ionic form or in alkaline solution in molec-ular form depending on the solution pH [34,35]. For convenience,the component of DDA in aqueous solution was represented byI:M, where I and M were the number of DDA ions and molecules,

998 C. Peng et al. / Applied Surface Science 425 (2017) 996–1005

Fig. 1. The XRD diagram (a) and SEM photo (b) of the purified montmorillonite.

DDA a

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Fig. 2. The initial configuration for

espectively. At the DDA concentration of 7 × 10−4 mol/L, the criti-al pH for DDA to form molecular precipitation is 9.1. When pH isround 12, DDA species are completely DDA molecules; when pH isellow 4, species are completely DDA ions. Therefore, at 1 ML cov-rage, pH = 12, 10, 8, 6 and 4, would correspond to I:M = 0:30, 5:25,0:20, 20:10 and 30:0, respectively. Similar methods appeared inhe literatures [21,22].

MD simulations were performed in the Forcite plus module ofaterial studios 8.0 software. The force field used in the calcula-

ion was PCFF INTERFACE developed by Heinz et al. [36], in which

NTERFACE force field was integrated in harmonic PCFF force fieldsor biomolecules, solvents, and polymers. The PCFF INTERFACEorce field enabled atomistic simulations of nanostructures at the

t montmorillonite/water interface.

1–100 nm scale in high accuracy and served as a uniform simulationplatform for inorganic, organic, and biomolecular compounds. Aflexible SPC model [37] was used for water molecules. The Mullikenatomic partial charges of DDA ions and molecule were distributedafter the geometry optimization at the PW91/DNP level in theDmol3 module, as shown in Fig. 3.

In the simulation, firstly, geometry optimization for the systemwas performed using conjugate gradient algorithm to reduce theresidual force of each atom. The convergence achieved once themaximum force on any one atom was less than 100 kJ mol−1 nm−1.

Subsequently, a 100 ps pre-equilibrium simulation was run inthe NPT ensemble. Berendsen thermostat and Berendsen baro-stat which offered swift equilibration of the system, were selectedwith the temperature coupling constant of 0.1 ps and the pressure-

C. Peng et al. / Applied Surface Science 425 (2017) 996–1005 999

Fig. 3. The optimized structures labeled atomic partial charge of DDA molecule (a) and ion (b) at the PW91/DNP level.

e susp

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Fig. 4. The light transmittance of the supernatant of montmorillonit

oupling constant of 1 ps, respectively. Finally, equilibration wasollowed by a 2 ns production run in the NVT ensemble where Nosehermostat was used. The last 0.5 ns trajectory was used for analysisf relevant properties. All dynamic simulations were run in step of

fs and at a pressure of 1 bar and a temperature of 300 K. The long-ange electrostatic interaction and van der Waals interaction werealculated using the Ewald and Atom based methods, respectively,ith a cutoff radius of 1.25 nm.

. Results and discussion

.1. Sedimentation test

The effect of sedimentation can be evaluated by the light trans-ittance of supernatant after the sedimentation test. The larger the

ight transmittance is, the better the effect of sedimentation is. TheDA concentration and solution pH dependence of the light trans-ittance of the supernatant of montmorillonite suspensions was

howed in Fig. 4. From Fig. 4a, the light transmittance in pure wateras 12.9% under weak alkaline condition (pH = 8). When the DDA

t the concentration of 1 × 10−4 mol/L was added in the suspension,he light transmittance was 14.3% and close to that in pure water,ut when the concentration of DDA increased from 1 × 10−4 mol/Lo 7 × 10−4 mol/L, the light transmittance approximately linearlyncreases from 14.3% to 65.3%, then changed little when the con-entration of DDA continued to increase to 1.5 × 10−3 mol/L.

From Fig. 4b, the light transmittance of the supernatant in theDA 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.nder the condition, hydrophobic modifier DDA did not promote

he sedimentation of montmorillonite particles. One reason shoulde that at the DDA concentration of 7 × 10−4 mol/L, the critical pHor DDA to form molecular precipitation was pH 9.1. When pH wasbove 9.1, DDA molecules started to form precipitates in the solu-

ensions as a function of DDA concentration (a) and solution pH (b).

tion, losing chemical activity. When the pH decreased to 9, thelight transmittance of supernatant added DDA was higher thanthat in the absence of DDA, indicating that DDA has begun to func-tion. As pH decreased from 9 to7, the light transmittance increasedsharply. In this pH range, the ratio of DDA ion to molecule in solu-tion became much larger, suggesting that these DDA ions wereadsorbed on montmorillonite surfaces by electrostatic attraction,not only decreasing the surface charge, but also rendering the sur-face good hydrophobicity[13,38,39]. In this case, the electrostaticrepulsion between the negatively charged surface weakened whilethe hydrophobic attractive strengthened so that the fine particlescould aggregate each other to form big flocs. The flocs could easilysettle under the gravity, resulting in the increase of the light trans-mittance of supernatant. When pH continued to decrease from 7 to4, the transmittance did not change significantly because most ofthe fine particles had settled after the sedimentation test.

3.2. Flocs images

The in-situ morphological characteristics of montmorilloniteflocs 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 montmorilloniteflocs images at different pH with and without 7 × 10−4 mol/L DDA.It was observed that the individual montmorillonite particles werevery small, that is, in the micron range. When the solution pH wasabove 10, the flocs did not appear (not showed in Fig. 5) becausemontmorillonite particles were highly dispersive in the solution.With the decreasing of pH and adsorption of DDA, the montmo-rillonite flocs began to appear at pH 8, but they were relatively

small and loosely linked together with each other (Fig. 5a). As pHdecreased to 6, the flocs became larger and more compact, andreached the millimeter level (Fig. 5b). At the very low pH of 4, theflocs were less compact than that at pH 6 (Fig. 5c).

1000 C. Peng et al. / Applied Surface Science 425 (2017) 996–1005

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.

antity

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Fig. 6. The contact angle of montmorillonite surface and adsorption qu

.3. Contact angle and adsorption quantity

The values of contact angle can reflect the degree of hydropho-icity of mineral surface. The larger the contact angle is, the strongerhe hydrophobicity of the surface is. In fact, the hydrophobicityf the surface is attributed to the adsorption quantity and aggre-ation structures of surfactant on the surface. Fig. 6 showed thelots of the contact angle of montmorillonite and adsorption quan-ity of DDA against the solution pH at the DDA concentration of

× 10−4 mol/L. When more DDA were adsorbed on the surface and

he adsorbed DDA layer was more compact and well-organized, theurface would become more hydrophobic. It can be seen that theontact angle of montmorillonite increased when the pH decreasedrom 10 to 8, and reached a maximum around pH 8, and then

of DDA as a function of pH at the DDA concentration of 7 × 10−4 mol/L.

decreased with the pH decreasing from 8 to 4. Likewise, the adsorp-tion quantity had the similar behavior against the solution pH. Theresults showed that at pH around 8, the montmorilloite surfaceswere most hydrophobic. These behaviors would be explained bythe following MD simulation results at the molecular level.

3.4. FTIR analysis

FTIR spectroscopy can be performed to clarify the adsorptionmechanism of reagent on the mineral surfaces. The FTIR spec-

tra of montmorillonite conditioned with 7 × 10−4mol/L DDA inthe pH range of 10–4 were showed in Fig. 7. The broader peaknear ∼3430 cm−1 was assigned to the stretching modes of NH2 ofDDA and hydroxyl of water (OHw). Two new peaks at 2930 and

C. Peng et al. / Applied Surface Sci

Fig. 7. FTIR spectra of motmorillonite as a function of pH at the DDA concentrationof 7 × 10−4 mol/L.

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ig. 8. The zeta potential of montmorillonite as a function of pH in pure water andn DDA solutions at the concentration of 7 × 10−4 mol/L.

850 cm−1 were assigned to CH stretching group. The new peakt 2370 cm−1 with a shoulder at 2330 cm−1 belonged to the CNtretching group [40]. And the peak at ∼1630 cm−1represented theending modes of NH of DDA and OHw of water [41]. Though thepectrum of montmorillonite treated by DDA presented character-stic absorption peaks of DDA, no band shift occurred, implying thatDA were adsorbed on montmorillonite mainly by physical adsorp-

ion. The decreasing transmittances in the NH2 and NH stretchingegion spectra of DDA from pH 8 to pH 4 indicated the decreaseddsorption of DDA, which was in accord with the results of contactngle and adsorption quantity.

.5. The model of hydrophobic aggregation

As a kind of 2:1 type phyllosilicate, montmorillonite has basalurface and edge surfaces with different charge properties. Theasal surface of montmorillonite, e.g., (001) face, has the permanentegative charges due to isomorphous substitution of Mg2+ → Al3+

ithin alumina octahedral sheet. Fig. 8 showed the zeta potentialf montmorillonite as a function of pH in pure water and in DDAolutions at the concentration of 7 × 10−4mol/L. In the whole pHange of 2–12, the zeta potential of montmorillonite was negative,

ut the value decreased from −43.2 mV to −26.3 mV with the pHecreasing from 12 to 2. The reason should be attributed to theign and density of the charges at the edge surface such as (010)nd (110) faces which depend on the pH of the solution.

ence 425 (2017) 996–1005 1001

Generally, the pH in the neutral and alkaline solution is usu-ally above the PZC of edge surface of montmorillonite becausethe point of zero charge (PZC) of the edge surface is not a cer-tain value, generally between pH 4 and 8 according to differentexperimental methods, such as slightly above pH 5 by rheologicalmeasurement [42], pH 6.1 by titration experiment [43] and aroundpH 7 by electrokinetic measurement [44]. In the case, both the basaland edge surfaces of montmorillontie are negatively charged. If thecationic surfactants DDA are added in the suspension, DDA wouldbe adsorbed on the surface by electrostatic attraction [45–47]. Asshown in Fig. 8, the adsorption of DDA decreased the zeta poten-tial of montmorillonite at large-scale pH except at pH above 11.In addition, the hydrophobic carbon chain of DDA could associ-ated with each other by the hydrophobic attractive [48], resultingin 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 edgesurface, the edge surfaces would be positively charged while thebasal surfaces are still negatively charged. Then the basal and edgesurfaces of montmorillonite would attract each other to produceface-to-edge T-type or card-house type aggregation. In addition,the basal surface had a special affinity with DDA ions to renderthe surface hydrophobicity, promoting the aggregation of particles(Fig. 9b). Therefore, it was concluded that under the combinationof face-to-edge electrostatic attraction and hydrophobic attraction,the face-to-edge flocs produced at acidic pH would be larger thanthose face-to-face flocs at neutral and slightly alkaline pH, result-ing in better sedimentation effect. The conclusion was in goodagreement with the results of light transmittance of supernatantand flocs images, and could explain why the effect of sedimenta-tion became better when the hydrophobicity of montmorillonitedecreased with pH decreasing from 8 to 4.

3.6. MD simulation results

Because DDA is a kind of hydrolyzable surfactant, its speciesin solution change with the solution pH. Fig. 10 showed the equi-librium configurations of the DDA molecules or ions adsorbed atthe montmorillonite/water interface at different pH. When pH was12 (Fig. 10a), DDA existed completely in the form of molecules(I:M = 0:30), and self-agglomeration of DDA molecules occurred ata distance from the montmorillonite basal surface. DDA moleculeshad no hydrophobic modification effect on the montmorillonite sothat the montmorillonite suspensions was highly dispersed.

When pH decreased to 10, that is, I:M = 5:25 (Fig. 10b), five DDAions were adsorbed on the surface with their polar head groupstoward the surface to form the inner layer while 25 DDA moleculeswere bound to the inner layer through the hydrophobic associa-tion among their non-polar carbon chains with some of polar headgroups toward the bulk solution. The adsorbed structure was sim-ilar to an irregular bilayer. At this pH, a certain extent effect ofhydrophobic modification started to appear. However, because alarge number of polar head groups in the adsorbed monolayerwere oriented toward the solution, it was disadvantageous forthe hydrophobic modification of montmorillontie. Under the cir-cumstances, the strong electrostatic repulsion between particlesdominated the high stability of the montmorillonite suspensionbecause of the large negative charge of edge surface, which agreedwith the very low light transmittance (about 0.47%) of the super-natant (Fig. 4).

When pH decreased to 8 (I:M = 10:20) (Fig. 10c), it was obvi-ously observed that all the DDA molecules and ions were adsorbedon the surface through their polar head groups to form monolayer,

namely, hemi-micelle. The adsorption layer was very compactand the surfactant packed in parallel through the hydrophobicassociation between the hydrophobic carbon chains so that thehydrophobic modification effect was very significant, which was

1002 C. Peng et al. / Applied Surface Science 425 (2017) 996–1005

Fig. 9. The model of the interaction of DDA with montmorillonite at different pH [49].

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ig. 10. The equilibrium structures of DDA on montmorillonite (001) surfaces at d:M = 20:10 (d); pH = 4, I:M = 30:0 (e).

n good agreement with the maximum adsorption quantity andontact angle at pH 8 (Fig. 6). A similar behavior was reported byutland et al. [17]. They investigated the adsorption of DDA on micay surface force apparatus (SFA), and found that at pH around 8,

dsorption of neutral DDA molecules became important to renderhe monolayer more compact with the thickness close to the lengthf an extended molecule, and strong attractive force was observedhen two hydrophobic surfaces contacted.

t pH. pH = 12, I:M = 0:30 (a); pH = 10, I:M = 5:25 (b); pH = 8, I:M = 10:20 (c); pH = 6,

When the solution pH were lower, such as 6 and 4, correspond-ing to I:M of 20:10 and 30:0 (Fig. 10d and e), respectively, the formof ions predominated in the DDA species. The DDA adsorption layerwas still compact, but some DDA ions moved away from the inner

to upside of adsorbed layer with their head groups oriented to thesolution. Herder[50] found the similar behavior in studying theinteractions between mica surfaces in DDA solutions at pH valuesbetween 5 and 6. In the case, the structure of the adsorbed layer was

C. Peng et al. / Applied Surface Science 425 (2017) 996–1005 1003

Fig. 11. The close-up snapshot of adsorbed DDA ions at the interface. (The black dash line denoted hydrogen bond).

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ig. 12. The concentration profile of N atoms of DDA head groups perpendicular toontmorillonite (001) surface at the pH 12, 8 and 4, respectively.

ot that of a perfect monolayer but contains imperfections[51,52].herefore, the effect of hydrophobic modification decreased as pHecreased from 8 to 4, which is consistent with the foregoing resultsf contact angle and adsorption quantity.

The close-up snapshots of the DDA ions at the interface at pH were showed in Fig. 11. It was found that at the interface theolar head groups of DDA not only interacted with not only the sur-ace oxygen atoms, but also water molecules through the formationf hydrogen bonds between the hydrogen atoms (Hn) of the headroup and surface oxygen atoms (Os) or water oxygen atoms (Ow). Aompetitive adsorption existed between the water molecules andDA toward the surface. In Fig. 9a, the head group of DDA only

nteracted with the surface, forming three Hn· · ·Os hydrogen bondsnd non Hn· · ·Ow hydrogen bond. However, one, two and threen· · ·Ow hydrogen bonds were formed in Fig. 10b–d, respectively.he increase of number of Hn· · ·Ow hydrogen bond indicated thathe interaction between head group of DDA and water strength-ned, and in turn the interaction between head group and surfaceeakened so that the distance between the head group and the

urface increased. The hydrogen bonding between the polar headroup and the water molecule resulted in DDA no longer being per-endicular to the surface but being tilted. The similar behavior wasound in the study of DDA on mica (001) surface by Xu et al. [31]

The concentration profile of N atoms of DDA in the normalirection of montmorillonite surface at different pH was showed

n Fig. 12. It could be found that at pH 12, several concentration

eaks of N atoms were concentrated in a distance range of 7–40 Årom the surface, indicating that the DDA were far from the surfacend formed self-agglomeration, bringing no hydrophobic modifica-

Fig. 13. The concentration profile of water Ow atoms perpendicular to montmoril-lonite (001) surface at the pH 12, 8 and 4, respectively.

tion effect to montmorillonite. At pH 8, the concentration peaks ofN atoms were concentrated in the narrow distance range of 0–7 Åfrom the surface, indicating that all the polar head groups were ori-ented towards the surface of the montmorillonite rather than thenon-polar carbon chain. This case can produce the good hydropho-bic modification effect. At pH 4, although the concentration peaksof N atoms are mainly concentrated in the range of 0–12 Å fromthe surface, there were still obvious distributions in the range of22–30 Å. It suggested that some polar head groups at the interfacetransferred from the initial adsorption position at the interface tothe top of the adsorption monolayer, weakening the hydrophobicmodification effect of the surface to a certain extent.

The concentration distribution of water molecules perpendic-ular to the interface can indirectly reflect the effect of pH on thecompetitive adsorption between waters and DDA ions or moleculesat the interface, as shown in Fig. 13. It was observed that the con-centration of Ow atoms in the distance range of 15–35 Å was muchsmaller than that in other places at pH 12, implying the formationof self-aggregation of DDA. When pH was 8, the Ow concentrationat the distance from 3 to 18 Å was zero, indicating that all theseplaces were occupied by the DDA instead of water molecules. WhenpH decreased to 4, the Ow concentration at the distance of 3–18 Åobviously increased, indicating that some of the water moleculeswere occupied in this region. The reason was that some of the DDAstructures moved outside the adsorption layer so that some water

Ow concentration peak in the distance range of 0–5 Å appeared likethe N concentration peak (Fig. 12) because the water moleculeswere polarized by the surface or polar head of DDA, showing that

1004 C. Peng et al. / Applied Surface Sci

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�� 26.5% 4.7% 22.6%

ompetitive adsorption existed between the water molecules andDA toward the surface.

When the surfactant DDA aggregated at the montmoril-onite/water interface, the alkyl carbon chain will be distorted,eviating from the initial linear structure. The degree of twist ofhe alkyl carbon chain can be characterized by the distribution ofihedral angle of the carbon atoms. The percentage of the dihedralngle in the range of [–120◦,120◦] was defined as the torsion devi-tion 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

he degree of order of DDA was. It could be found that �� was low-st (4.7%) at pH 8, implying that the DDA were well-organized andn favor of the hydrophobic modification. However, at pH 12 and 4,

� increased to 26.5% and 22.6%, respectively, suggesting that theegree of order decreased and formed self-aggregation. The resultsgreed with the equilibrium structures and concentration profilef atoms.

. Conclusion

Effect of pH on the adsorption of DDA on montmorillonite sur-ace in aqueous solution was studied by the experimental and MDimulation methods. The experimental results showed that theffect of hydrophobic modification for montmorillonite increaseds the pH decreased from 10 to 8, reaching a maximum aroundH 8, and then decreased with the pH decreasing from 8 to 4.hough the hydrophobicity of montrmorillonite decrease as pHecreased from 8 to 4, the effect of sedimentation increased dueo the formation of big face-to-edge flocs under low pH condi-ions. When pH was below 7, the effect of sedimentation did nothange significantly. MD simulation results showed that when theositively charged DDA ions were adsorbed onto the negativelyharged montmorillonite surfaces, the initial stage of the adsorp-ion was driven by electrostatic forces and hydrogen bonding, andhe later stage by hydrophobic association between the alkyl carbonhains. A competitive adsorption existed between water moleculesnd DDA toward the montmorillonite surface. A certain numbers ofeutral DDA molecules could favor the adsorption of DDA, render-

ng the monolayer more compact. The structure and compositionf the adsorbed layer was different by decreasing the pH from 12 to. At pH 8, DDA molecules and ions were adsorbed on the surfacehrough their polar head groups to form monolayer, namely, hemi-

icelle. The adsorption layer was very compact and the surfactantacked in parallel, so that the hydrophobic modification effect washe best.

cknowledgements

This work was supported by the Natural Science Foundation ofhina [grant number 51474011]; the Post-doctoral Science Foun-ation of China [grant number 2014M561810]; the Anhui Province

nternational Cooperation Project [grant number 1303063011];nd the Anhui Provincial Natural Science Foundation of Chinagrant number 1508085QE90].

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