Interpretation of Hydrophobization Behavior of Dodecylamine on Muscovite and Talc Surface Through Dynamic Wettability and AFM Analysis

Article Interpretation of Hydrophobization Behavior of Dodecylamine on Muscovite and Talc Surface through Dynamic Wettability and AFM Analysis

Hao Jiang 1,2,*, Ya Gao 1, Sultan Ahmed Khoso 1, Wanying Ji 1 and Yuehua Hu 1,2 1 School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China; [email protected] (Y.G.); [email protected] (S.A.K.); [email protected] (W.J.); [email protected] (Y.H.) 2 Key Laboratory of Hunan Province for Clean and Efﬁcient Utilization of Strategic Calcium-Containing Mineral Resources, Central South University, Changsha 410083, China * Correspondence: [email protected]; Tel.: +86-731-8883-0545

Received: 31 July 2018; Accepted: 4 September 2018; Published: 6 September 2018

Abstract: In this study, a new approach, “dynamic wettability”, and atomic force microscopy (AFM) imaging analysis techniques were successfully used to characterize the hydrophobization mechanism of the collector dodecylamine (DDA) on muscovite and talc surfaces. The attachment of bubbles to the minerals was studied through the dynamic contact angle to gain a detailed understanding of the hydrophobization mechanism of DDA on a muscovite and talc surface. AFM imaging and interaction forces were performed to explain the DDA adsorption mechanism on both minerals. Finally, ﬂotation tests were performed to verify the effectiveness of these techniques. After treatments with DDA, the contact angles became much larger compared to initial angles, particularly for muscovite, and the attachment of bubbles on the talc surface was much easier than muscovite due to its natural hydrophobicity. From AFM imaging, both the muscovite and talc showed a similar tendency; the higher the DDA concentration, the more the adsorbed amount. However, the adsorbed amount of DDA on talc surface was obviously more than that on muscovite. As far as interaction forces are concerned, the maximum attractions occurred at certain different concentrations respectively for muscovite and talc and agreed well with the AFM-imaging results. Moreover, results obtained from ﬂotation tests were promising and quite in agreement with the phenomenon of these techniques.

Keywords: hydrophobization; dodecylamine; characterization technique; muscovite; talc; AFM

1. Introduction Phyllosilicates, after quartz, are most versatile mineral material mined from Earth’s crust for daily and industrial needs. Phyllosilicates usually consist of stacked tetrahedral and octahedral sheets that further constitute Si-O tetrahedral layers by sharing three oxygen atoms and Al-O or Mg-O octahedral layers. Talc and muscovite (with the composition Mg3(Si2O5)2(OH)2 and KAl2(AlSi3O10)(OH)2), typical hydrophobic and hydrophilic phyllosilicates, both have a 2:1 structure, where one octahedral layer is sandwiched between two tetrahedral layers. The subtle difference is their metal cations, which for talc is an Mg-O octahedron to be held between two tetrahedrons [1–3], but Al-O for muscovite. In the structure of muscovite, the tetrahedral layer is usually negatively charged as a result of the ionic isomorphic substitution of about one-fourth of Si4+ by Al3+. These permanent negative charges are neutralized by the interlayer K+, serving as a bridge between two sheets [4]. In talc structure, this substitution can also occur where Si4+ and Mg2+ are replaced by Al3+ and Ca2+ severally [5,6]. As a signiﬁcant class of valuable industrial materials, the colloidal interaction forces of mineral particles between basal planes, edge planes, or basal planes and edge planes in muscovite

Minerals 2018, 8, 391; doi:10.3390/min8090391 www.mdpi.com/journal/minerals Minerals 2018, 8, 391 2 of 16

flotation can be evaluated by the classical Derjaguin–Landau–Verwey–Overbeek (DLVO) theory [7]. It consists of two fundamental colloidal forces, the attractive Van der Waals force and the repulsive electrostatic double-layer force, respectively. The former can be determined by the Hamaker constant and the distance between the two particles that doesn’t substantially change when changing other conditions. Meanwhile, the latter is strongly influenced by many situations such as the type of solutions, its concentrations, and pH. The measurements of colloid forces between two solid surfaces have been reported widely in previous studies [8–11]. Lujie Yan et al. [12] focused deeply on the research of the interaction of divalent cations with basal planes and edge surfaces of muscovite and talc by atomic force microscopy (AFM). They got the values of Stern potential of basal and edge planes via fitting the measured force profiles with the DLVO theory and found that on the basal planes of muscovite, the fitted Stern potential became less negative with an increase of Ca2+ or Mg2+ concentration. Vishal Gupta and Jan D. Miller [8] measured the interaction forces between basal planes of ordered kaolinite particles on substrates that revealed that at pH > 4, the silica tetrahedral face of kaolinite is negatively charged. For the alumina octahedral face, it is nevertheless charged positively and negatively at pH < 6 and pH > 8, respectively. In our previous study [13], the interaction forces in electrolyte solutions (KCl) between a silica colloidal sphere and a muscovite substrate were measured and fitted with classical DLVO theory at different pH values, concentrations, and effects of time. The conclusion showed that at the lower KCl concentration or the higher pH, what dominated in muscovite–silica system was mainly the electrostatic double-layer force. But, in the opposite condition (the higher electrolyte concentration or lower pH), it was the van der Waals force that dominated the non-contact force at a closer distance. The wettability of the surface often plays an important role in a variety of research fields such as filtration [14,15] and dewatering [16]. Especially in the froth flotation of minerals [17], it is the precondition for getting better results in flotation separation where the wettability on mineral surfaces is usually characterized by contact-angle measurements. Nowadays, a substantial amount of investigations have been carried out on the hydrophobization and the reagent adsorption of minerals [18–21]. Muscovite, dodecylamine hydrochloride, and sodium oleate were combined to increase the collector-adsorbed amount on its surface, thereby achieving higher recovery [22]. In the flotation of spodumene from feldspar, the most adsorbed amount of collector appeared at the molar ratio of 1:9 for dodecyl ammonium chloride and sodium oleate, where the highest selectivity was reported. However, there is hardly any work aimed at measuring the varying hydrophobicity of minerals by different ways. In this study, a new approach was employed to measure the change of wettability on a hydrophilic muscovite surface and hydrophobic talc surface before and after treatment with dodecylamine (DDA). The attachment of bubbles to the minerals was studied through the dynamic contact angle to gain a detailed understanding of the hydrophobization mechanism of DDA on the muscovite and talc surface. Furthermore, AFM was used to observe the adsorption mechanism of DDA on a mineral surface, and the interaction forces between the basal planes of muscovite and talc in electrolyte and collector solutions under different conditions were also carried out. The classical DLVO theory was fitted to the experimental interaction force profiles.

2. Materials and Methods

2.1. Samples and Chemicals The samples of muscovite and talc required for this study were received from Lingshou, Hebei province, China, and Kuandian, Liaoning province, China, respectively. Lump samples were handpicked, crushed, and ground in a laboratory porcelain mill to obtain a maximum amount with a size range of 45–75 µm. With regard to AFM and dynamic wettability analysis, samples were freshly cleaved into sheets as their perfect cleavages along basal planes. Chemical composition and X-ray diffractometry (XRD) were carried out to examine the purity of these two samples. Using Minerals 2018, 8, 391 3 of 16 Minerals 2018, 8, x 3 of 16

X-rayfluorescence fluorescence (XRF) and (XRF) XRD and analysis, XRD analysis, the chemical the chemical and mineralogical and mineralogical composition composition of both minerals of both mineralswere determined were determined and results and (see results Table (see l and Table Figure1 and 1) Figure indicated1) indicated their purity their purityto be about to be about90%, which 90%, whichwas good was enough good enough to utilized to utilized these minerals these minerals in this inwork. this work. DDA, usedused asas a a collector collector for for flotation flotation tests, tests, was was purchased purchased from from Sinopharm Sinopharm Chemical Chemical Reagent Reagent Co., Ltd.Co., (Shanghai,Ltd. (Shanghai, China). China). Freshly Freshly prepared prepared KCl solution KClwas solution used aswas electrolyte used as in electrolyte force measurements in force bymeasurements AFM. HCl and by NaOH,AFM. HCl from and Damao NaOH, Chemical from Dama Reagento Chemical Factory China,Reagent were Factory used China, to control were the used pH. Allto control the reagents the pH. were All analytical-grade. the reagents were Different analyt dropsical-grade. of HCl Different (3%, volume drops concentration) of HCl (3%, or volume NaOH (5%,concentration) mass concentration) or NaOH were(5%, mass added concentration) to control the were pH to added the desired to cont value.rol the It pH is worth to the notingdesired that value. pH regulatorsIt is worth were noting added that through pH regulators glass dropper were ad bottlesded through to eliminate glass the dropper influence bottles of the to volume eliminate change the ofinfluence slurry. Ultrapureof the volume water change was used of slurry. throughout Ultrapure the study.water was used throughout the study.

Table 1.1. Chemical composition of mineralmineral samplessamples (mass(mass fraction,fraction, %).%).

1 Sample Al2O3 SiO2 Fe2O3 TiO2 CaO MgO K2O Na2O H2O 1LOI Sample Al2O3 SiO2 Fe2O3 TiO2 CaO MgO K2O Na2OH2O LOI Muscovite 37.46 44.55 2.30 0.74 0.19 0.97 6.53 2.24 2.33 2.69 Muscovite 37.46 44.55 2.30 0.74 0.19 0.97 6.53 2.24 2.33 2.69 Talc Talc0.84 0.84 60.61 60.61 0.71 0.71 - --- 29.07 -- -- 6.656.65 2.12 2.12 1 Loss1 Loss on on ignition; ignition; --Not Not detected. detected.

V 100000 2500 V (a) V—Muscovite T O—Quartz (b) T—Talc 2000 80000

1500 60000 T

1000 V 40000

Intensity (counts) Intensity V Intensity (counts) 500 V O V 20000 V V T V V V V V V V O V O V VV V V 0 TTT T 0 0 10203040506070 2-theta-scale (°) 0 1020304050607080 2-theta-scale (°) Figure 1. X-ray diffraction (XRD) patterns of (a) muscovitemuscovite andand ((b)) talc.talc.

2.2. Methods 2.2. Methods 2.2.1. Dynamic Wettability Tests 2.2.1. Dynamic Wettability Tests Wettability tests were performed by a dynamic contact angle that was determined through Wettability tests were performed by a dynamic contact angle that was determined through the the pendant-drop method by a video contact angle-measuring instrument (JY-82C, Chengde Dingsheng pendant-drop method by a video contact angle-measuring instrument (JY-82C, Chengde Dingsheng Tester Co., Ltd., Chengde, China). Before the test, the position of the needle tip of the syringe above Tester Co., Ltd., Chengde, China). Before the test, the position of the needle tip of the syringe above the sample surface was accurately adjusted and the volume of every droplet was controlled at 3 µL the sample surface was accurately adjusted and the volume of every droplet was controlled at 3 μL by a constant speed injector. The method used here was quite different from the conventional static by a constant speed injector. The method used here was quite different from the conventional static method; the experimental droplets used for the dynamic contact angle were collectors (DDA) instead method; the experimental droplets used for the dynamic contact angle were collectors (DDA) instead of water. The freshly cleaved muscovite and talc sheets were placed on the stage and collector droplets of water. The freshly cleaved muscovite and talc sheets were placed on the stage and collector were dropped on the mineral surface to observe their changes within 6 min. The images of the droplets droplets were dropped on the mineral surface to observe their changes within 6 min. The images of that settled on the sample surface were created with a digital camera, and magniﬁed by an optical lens. the droplets that settled on the sample surface were created with a digital camera, and magnified by Besides that, the adhesion ability of the bubbles on the surface was also studied. Mineral sheets that an optical lens. Besides that, the adhesion ability of the bubbles on the surface was also studied. had been immersed in the DDA solution for 30 min were placed in a container with ultrapure water. Mineral sheets that had been immersed in the DDA solution for 30 min were placed in a container The bubble that was produced by an injector approached and then adhered to the mineral surface. with ultrapure water. The bubble that was produced by an injector approached and then adhered to Finally, the angle between the bubble and the surface was measured. The procedure was repeated at the mineral surface. Finally, the angle between the bubble and the surface was measured. The least 3 times under the same conditions, and the average was calculated as the ﬁnal result. procedure was repeated at least 3 times under the same conditions, and the average was calculated as the final result.

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2.2.2. Tapping ModeMode AFMAFM ImagingImaging Adsorption surfacesurface AFM AFM imaging imaging (in situ)(in throughsitu) through tapping tapping mode was mode performed was performed by a Multimode by a SPMMultimode with Nanoscope SPM with VNanoscope (Bruker, Billerica, V (Bruker, MA, Billerica, USA). A MA, silicon USA). probe A silicon (RTESP-300 probe type, (RTESP-300 Bruker) type, with aBruker) typical with spring a typical constant spring of 0.2 constant N/m and of a0.2 resonant N/m and scanning a resonant frequency scanning of aroundfrequency 2 Hz of (Bruker)around 2 was Hz used(Bruker) in this was measurement. used in this measurement. Tapping mode Tapping was employed mode was to employed collect the to images collect using the images a phosphorus using a (n)-dopedphosphorus silicon (n)-doped cantilever silicon probe cantilever and toprobe obtain and some to obtain larger-scale some larger-scale images (1.5 imagesµm × (1.51.5 μmµm) × 1.5 of theμm) substrates of the substrates and adsorption and morphology.adsorption morpholo All imagesgy. were All ﬂattenedimages bywere polynomial flattened functions by polynomial for ease offunctions comparison for ease and of to comparison eliminate the and effects to eliminate of image the bending effects of and image distortion bending due and to thedistortion movement due ofto the piezoelectricmovement of scanning the piezoelectric tube. At ﬁrst, scanning the lamellar tube. piecesAt first, of the sample lamellar substrates, pieces which of sample were substrates, glued onto awhich magnetic were plate, glued were onto immersed a magnetic in DDA plate, solutions were with immersed a certain concentrationin DDA solutions for 30 min.with After a certain being takenconcentration out, the samplefor 30 min. surface After was be gentlying taken rinsed out, with the sample ultrapure surface water was to remove gently therinsed remaining with ultrapure reagent onwater the to surface remove and the blow-dried remaining withreagent high-purity on the surf nitrogenace and forblow-dried adsorption with morphology high-purity tests. nitrogen All thefor ◦ AFMadsorption observations morphology were conductedtests. All the in AFM air at observations 20 C. were conducted in air at 20 °C.

2.2.3. AFM Force Measurements Interaction force force measurements measurements were were carried carried out out in in KCl KCl and and collector collector solutions, solutions, respectively, respectively, at atdifferent different concentrations concentrations and and pH pHvalues. values. A tipless A tipless silicon silicon probe probe with with NP-O10 NP-O10 type type(Bruker) (Bruker) was used was usedthrough through contact contact mode. mode.Before the Before measurements, the measurements, the magnetic the magneticplate with plate freshly with cleaved freshly muscovite cleaved muscoviteand talc sheets and was talc fixed sheets on was the fixedpiezo ontranslation the piezo stage. translation Sample stage.sheets Samplewith lamellar sheets pieces with lamellarof about µ pieces20 μm ofin aboutlength 20 werem picked in length by werea light picked microscope by a light and microscope glued with anda two-component glued with a two-component epoxy onto the epoxyapex of onto the cantilever. the apex of Then, the cantilever. the prepared Then, mineral the prepared probes were mineral placed probes in a were vacuum placed desiccator in a vacuum for at desiccatorleast 24 h, and for at the least micrographs 24 h, and the of the micrographs muscovite of pr theobe muscovite and the talc probe probe and are the shown talc probe in the are Figure shown 2. inIt was the Figuremounted2. Itin was the mountedliquid cell in where the liquid solutions cell were where injected solutions slowly. were About injected 1 h slowly. after the About solutions 1 h afterwere thestable, solutions the interaction were stable, force the interaction measurements force measurements were performed. were When performed. the mineral When the substrate mineral substrateapproaches/departs approaches/departs from the mineral from the probe mineral in the probe vertical in the (Z) vertical direction, (Z) direction, the force thebetween force betweenthe two thebasal two planes basal of planes the sample of the causes sample the causes cantilever the cantileverspring to deflect spring upward to deflect or upwarddownward, or downward, depending dependingon the nature on of the the nature force of between the force sample between surfaces sample [23]. surfaces Some [ 23litera]. Sometures literatures can be referred can be to referred for more to fordetails more about details force about measurements force measurements [24–27]. [For24– 27each]. For test each condition, test condition, many different many different locations locations on the onmineral the mineral substrate substrate were measured were measured to ensure to ensure a repr aoducible reproducible force profile. force profile. All the All experiments the experiments were ◦ ψ wereoperated operated at room at roomtemperature temperature (about (about20 °C) 20in a C)class-100 in a class-100 clean room. clean Regarding room. Regarding to the ψ potential to the potentialof minerals, of minerals,a Coulter Delsa a Coulter 440sx Delsa Zeta 440sx potential Zeta an potentialalyzer (Beckman analyzer Coulter, (Beckman Inc., Coulter, Chaska, Inc., MN, Chaska, USA) MN,equipped USA) with equipped a rectangular with a rectangular electrophoresis electrophoresis cell was employed cell was employed to determine to determine their values. their Mineral values. µ Mineralsamples were samples ground were to ground–2 μm and to the –2 suspensionm and the was suspension prepared wasby adding prepared 40 mg by of adding mineral 40 samples mg of mineral samples to 80 mL desired solutions containing 1 mmol/L KNO as supporting electrolytes. to 80 mL desired solutions containing 1 mmol/L KNO3 as supporting3 electrolytes. Then, the Then,suspension the suspension was conditioned was conditioned by magnetic by stirring magnetic for stirring 6 min to for ensure 6 min adequate to ensure dispersion adequate dispersionof the fine ofmineral the fine particles. mineral After particles. settling After for 10 settling min, the for supe 10rnatant min, the of supernatantthe dilute fine of particle the dilute suspension fine particle was suspensiontaken for potential was taken measurements. for potential measurements.

(a) (b)

Figure 2. Micrograph of prepared (a)) muscovitemuscovite andand ((b)) talctalc probes.probes.

Minerals 2018, 8, 391 5 of 16

2.2.4. Microflotation Tests Microflotation experiments were performed in a 40 mL flotation cell with a fixed impeller speed of 1500 r/min. Two grams of samples with 35 mL of ultrapure water were used in each test. The pulp was first conditioned for 1 min to uniformly disperse the particles. Then, HCl or NaOH was added to adjust the pH to desired values. After that, the pulp was conditioned with collectors (DDA) for 3 min. The flotation was conducted for 3 min. The froth products and tails were weighed after filtration and drying. The recovery was calculated based on the mass of products. The flotation tests were performed 3 times with the same procedure and conditions and the average was reported as a final value.

3. Results and Discussion

3.1. Dynamic Wettability Analysis Differing from traditional static methods, how the collectors spread on the mineral surface was visually displayed by dynamic contact angle to explain the hydrophobization mechanism. The contact angle of muscovite, as a natural hydrophilic mineral, is only about 7◦. In the mineral flotation process, we usually add some specific reagents to expand the differences in wettability among minerals to achieve the aim of flotation separation. The hydrophobicity changes of muscovite surface by the DDA collector are shown in Figure3. At a lower DDA concentration (1.0 × 10−5 mol/L), when the DDA solution has just dropped on the muscovite surface for a period of time (shown in Figure3(a1)–(a3)), the formed contact angle between the droplet and the muscovite surface was almost equivalent to that in pure water, which appeared to still be hydrophilic at this time. After the passing of some time, there was an interaction between the collector and muscovite indicating from Figure3(a4) that the wettability varied from superhydrophilic to less hydrophilic. As time continued to pass, the contact angle continued to increase, and then ultimately got stable at 27.84◦ after 6 min (shown in Figure3(a5)). When the DDA concentration increased to 5.0 × 10−5 mol/L, as shown in Figure3(b1)–(b5), not only was the time required for DDA to interact with the muscovite surface shortened from 133 s in Figure3(a1)–(a5) to 60 s, but the formed contact angle also became a little bigger (29. 45 ◦). At 1.0 × 10−4 mol/L (see Figure3(c1)–(c5)), the whole process can be observed where the muscovite surface turned from extremely hydrophilic to hydrophobic. It took only 10 s for the muscovite surface to interact, resulting in an instantly big contact angle. The longer time went on, the bigger the angle was, and the stronger the hydrophobicity became, finally achieving a steady state. At this point, the contact angle reached 89◦, indicating that muscovite had been quite hydrophobic. The interaction times were shortened to just 2 s and 1 s, respectively, at 5.0 × 10−4 mol/L and 1.0 × 10−3 mol/L of DDA concentration. The final contact angles were, respectively, 91.61◦ and 93.95◦, which illustrated that the muscovite surface had changed successfully from natural hydrophilicity to exceeding hydrophobicity. Generally, the collector DDA can change the wettability of muscovite, making the muscovite surface from hydrophilic to hydrophobic, and the higher the DDA concentration, the shorter the time required for interaction with the muscovite surface and the stronger the hydrophobicity of the muscovite surface is. Minerals 2018, 8, 391 6 of 16 Minerals 2018, 8, x 6 of 16

FigureFigure 3. 3. Dynamic contactcontact angles angles of of muscovite muscovite surface surface in a dodecylaminein a dodecylamine (DDA) (DDA) solution solution with different with differentconcentrations concentrations at natural at pH natural = 6 ± 0.5.pH (=a )6 1.0 ± 0.5.× 10 (a−)5 1.0mol/L; × 10− (5b mol/L;) 5.0 × (10b)− 5.05 mol/L; × 10−5 (mol/L;c) 1.0 × (c10) 1.0−4 mol/L;× 10−4 − − mol/L;(d) 5.0 (×d)10 5.04 ×mol/L; 10−4 mol/L; (e) 1.0 (e×) 1.010 × 310mol/L.−3 mol/L.

DifferingDiffering from from hydrophilic hydrophilic muscovite, muscovite, talc talc is is known known for for its its natural natural hydrophobicity. hydrophobicity. In In natural natural ◦ conditions,conditions, the contact angleangle of of talc talc was was measured measured at aboutat about 60 60°,, which which was was in accordance in accordance with with the value the valuein Reference in Reference [28]. [28]. However, However, the the effects effects of of DDA DDA and and its its different different concentrations concentrations onon the the dynamic dynamic −5 contactcontact angles angles of of talc talc are are shown shown in Figure in Figure 4. At4 .the At lowest the lowest concentration concentration of 1.0 × of10− 1.05 mol/L× 10 (see Figuremol/L 4(a1)–(a5)),(see Figure 4as(a1)–(a5)), soon as asthe soon collector as the droplet collector fell droplet on surface, fell on surface,the reagent the reagent reacted reacted with the with talc. the The talc. formedThe formed contact contact angle angle was waseven even slightly slightly larger larger than than that that of pure of pure water water on on the the talc talc surface. surface. However, However, therethere was was a minorminor decreasedecrease in in the the contact contact angle angle after after a while, a while, which which could could be explained be explained due todue the to effect the ◦ ◦ effectof gravity of gravity or evaporation. or evaporation. After some After time, some the time, formed the angleformed was angle noted was at 69.58 noted, aboutat 69.58°, 9 higher about than 9° −5 higherin natural than conditions. in natural Increasingconditions. the Incr concentrationeasing the concentration up to 5.0 × 10 up tomol/L, 5.0 × 10 as−5 seen mol/L, in Figureas seen4(b1)–(b5)), in Figure 4(b1)–(b5)),it still took it quite still atook few quite seconds a few to undergoseconds to changes, undergo and changes, the ultimate and the angle ultimate was higher angle was than higher that of thanthe lowestthat of concentrationthe lowest concentration but the increase but the was increa not veryse was obvious. not very Substantial obvious. Substantial increment inincrement the contact in −4 −4 theangles, contact with angles, increasing with increasing concentration concentration from 1.0 × from10 1.0mol/L × 10−4 tomol/L 5.0 × to10 5.0 ×mol/L 10−4 mol/L suggested suggested that thatthe wettabilitythe wettability of talc of talc evidently evidently changed. changed. However, However, under under the the same same concentrations, concentrations, the the amount amount of ofincrease increase was was not sonot good so good as compared as compared to muscovite, to mu revealingscovite, revealing the signiﬁcance the significance of surface wettability of surface in −3 wettabilitythe formation in the of contactformation angles. of contact As seen angles. in Figure As 4seen(e1)–(e5), in Figure at 1.0 4(e1)–(e5),× 10 mol/L, at 1.0 × the 10 contact−3 mol/L, angle the ◦ contactformed angle by the formed DDA droplet by the onDDA the talcdroplet surface on the had talc exceeded surface 90 had, making exceeded the talc90°, surfacemaking extremely the talc surfacehydrophobic. extremely Thus, hydrophobic. results suggest Thus, that results the highersuggest DDA that concentrationthe higher DDA can concentration make the contact can make angle thelarger, contact but comparedangle larger, with but the muscovite,compared thewith amount the mu ofscovite, increase the for amount talc is somewhat of increase smaller. for talc is somewhat smaller.

Minerals 2018, 8, 391 7 of 16 Minerals 2018, 8, x 7 of 16

Figure 4. Dynamic contact angles of talc surface in DDA solution with different concentrations at pH = 6 ±± 0.5.0.5. (a ()a 1.0) 1.0 × ×10−105 mol/L;−5 mol/L; (b) 5.0 (b )× 5.0 10−×5 mol/L;10−5 mol/L; (c) 1.0 × ( c10) 1.0−4 mol/L;× 10− (4dmol/L;) 5.0 × 10 (d−4) mol/L; 5.0 × 10(e−) 1.04 mol/L; × 10−3 (mol/L.e) 1.0 × 10−3 mol/L.

Apart from dynamic dynamic contact-angle contact-angle tests, tests, the the ability ability of ofbubble bubble adhesion adhesion for formineral mineral surface surface can canfurther further characterize characterize its wettability. its wettability. Figure 5 Figureclearly5 interpretsclearly interprets the adhesion the adhesionprocess of processminerals of in mineralsultrapure inwater ultrapure before waterand after before samples and were after treat samplesed by were collector. treated For bymuscovite collector. (see For Figure muscovite 5(a1)– (see(a5)), Figure when5 (a1)–(a5)),the bubble whenapproached the bubble its surface approached and th itsen surfaceleft, there and was then no left, sign there of adherence was no sign to the of adherencemuscovite tosurface, the muscovite indicating surface, that the indicating freshly thatcleaved the freshly muscovite cleaved was muscovite too hydrophilic was too to hydrophilic stick to a tobubble. stick After to a bubble. the muscovite After thesample muscovite was treated sample with was the treated DDA collector with the solution DDA collector at 1.0 × solution10−4 mol/L, at − 1.0the ×bubble10 4 mol/L,was successfully the bubble adhered was successfully on its surface, adhered as shown on its surface, in Figure as shown5(c1)–(c5), in Figure and the5(c1)–(c5), formed ◦ andcontact the angle formed (89.94°) contact was angle also (89.94close to) the was value also closein theto dynamic the value contact-angle in the dynamic test (Figure contact-angle 3(c1)–(c5)). test (FigureFor hydrophobic3(c1)–(c5)). talc, For hydrophobicthe bubble could talc, thebe stuck bubble on could its surface be stuck even on itsin surfacethe absence even of in the the collector absence ◦ ofsolution the collector (shown solution in Figure (shown 5(b1)–(b5)). in Figure However,5(b1)–(b5)). the formed However, contact the formedangle (57.06°) contact was angle slightly (57.06 too) wassmall slightly to seem too to smallhave toa tendency seem to haveto be a detach tendencyed. After to be treatment detached. with After the treatment DDA solution with the (Figure DDA solution5(d1)–(d5)), (Figure the5 (d1)–(d5)),talc surface the was talc hydrophobic surface was hydrophobic enough to make enough a tobubble make adhere a bubble steadily adhere steadilyand the andshaped the shapedangle was angle obviously was obviously bigger biggerthan that than of thatthe untreated. of the untreated. In comparing In comparing these thesetwo samples two samples with withdifferent different wettability, wettability, the collector the collector can effectively can effectively change change the hydrophobicity the hydrophobicity of a ofmineral a mineral surface. surface.

Minerals 2018, 8, 391 8 of 16 Minerals 2018, 8, x 8 of 16

Figure 5. 5. TheThe process process of bubble of bubble adhesion adhesion on different on different surfaces surfaces in water in atwater pH = 6 at ± pH0.5. ( =a) 6Natural± 0.5. muscovite;(a) Natural ( muscovite;b) natural talc; (b) natural (c) collector-treated talc; (c) collector-treated muscovite; muscovite;(d) collector-treated (d) collector-treated talc. talc.

3.2. AFM Imaging 3.2. AFM Imaging Tapping-mode AFM imaging was used to characterize the freshly cleaved muscovite and talc Tapping-mode AFM imaging was used to characterize the freshly cleaved muscovite and talc surfaces. The AFM images of bare muscovite and talc are illustrated in Figure6. Due to their unique surfaces. The AFM images of bare muscovite and talc are illustrated in Figure 6. Due to their unique crystal structures, the (001) crystal plane could be attained easily and both surfaces were very smooth, crystal structures, the (001) crystal plane could be attained easily and both surfaces were very smooth, shorter than 0.2 nm (see Figure6). Therefore, some factors that influence adsorption morphology shorter than 0.2 nm (see Figure 6). Therefore, some factors that influence adsorption morphology of of collectors on muscovite and talc surface can be excluded, making the results more representative collectors on muscovite and talc surface can be excluded, making the results more representative and and convincing. The adsorption morphology (1.5 µm × 1.5 µm) of muscovite and the height images of convincing. The adsorption morphology (1.5 μm × 1.5 μm) of muscovite and the height images of DDA on its surface were studied from Figure7, when its concentration ranged from 1 × 10−5 mol/L to DDA on its surface were studied from Figure 7, when its concentration ranged from 1 × 10−5 mol/L to 1 × 10−3 mol/L. As can be seen, different adsorption morphologies occurred at different concentrations. 1 × 10−3 mol/L. As can be seen, different adsorption morphologies occurred at different concentrations. At lower lower concentrations concentrations (Figure (Figure 7a7a to to Figure Figure 7b),7b), only only a a small small amount amount of of DDA DDA was was adsorbed adsorbed onon the muscovitethe muscovite surface surface and uniformly and uniformly spotted, spotted, indicating indicating that the that interaction the interaction was weaker was weaker between between lower concentrationslower concentrations of DDA of and DDA the and basal the plane basal of plane musc ofovite. muscovite. By increasing By increasing DDA concentrations DDA concentrations (Figure 7c(Figure to Figure7c to Figure7e), obvious7e), obvious changes changes in adsorption in adsorption morphology morphology appeared appeared that gradually that gradually increased increased both theboth density the density and andarea areaof adsorption, of adsorption, from from some some sing singlele spots, spots, sparsely sparsely flaky flaky distribution distribution to to compact compact sheets finally. finally. TakingTaking FigureFigure7 a7a as as the the example, example, a naturala natural adsorption adsorption state, state, at at the the lowest lowest concentration concentration of 1 × 10−5 mol/L, was observed and its adsorption height was about 1.5 nm. Considering the theoretical of 1 × 10−5 mol/L, was observed and its adsorption height was about 1.5 nm. Considering the theoreticallength of DDA length molecules of DDA (1.89molecules nm) [(1.8929,30 ],nm) the [29,30], height the profile height of aboutprofile 1.5 of nmabout could 1.5 benm explained could be explainedin that DDA in moleculesthat DDA were molecules adsorbed were obliquely adsorbed on muscoviteobliquely on surface. muscovite When surface. DDA concentrations When DDA × −4 concentrationsrose to 1 10 rosemol/L to 1 (Figure× 10−4 mol/L7c), one (Figure of the 7c), height one of profiles the height of about profiles 3.8 nm of about coincided 3.8 nm exactly coincided with exactlythe double-layer with the double-layer adsorption of adsorption DDA, while of mostDDA, of while the others most of were the stillothers monolayer. were still But monolayer. in Figure But7d, inthe Figure density 7d, and the area density of double-layer and area of increaseddouble-laye significantlyr increased and significantly only a few and molecules only a werefew molecules adsorbed werewith monolayer.adsorbed with At the monolayer. highest concentration At the highest (Figure concentration7e), the average (Figure heights 7e), the were average much heights higher were than muchthose ofhigher lower than concentrations. those of lower That concentrations. is to say, the adsorptionThat is to say, morphology the adsorption of DDA morphology changes gradually of DDA changesfrom monolayer gradually to bilayer,from monolayer and even toto multilayerbilayer, and as itseven concentration to multilayer keeps as its increasing. concentration keeps increasing.

Minerals 2018, 8, 391 9 of 16 MineralsMinerals 2018 2018, 8,, 8x, x 9 of9 of16 16

(a()a )

(b()b )

FigureFigureFigure 6. 6.Atomic Atomic 6. Atomic force force microscopy force microscopy microscopy (AFM) (AFM) (AFM) images images images of of (a )( abare of) bare (a )muscovite bare muscovite muscovite and and (b and()b talc) talc (b sheets.) talcsheets. sheets.

(a)( a) (b()b ) (c)( c)

1010 nm nm

Figure 7. Cont.

Minerals 2018, 8, 391 10 of 16 Minerals 2018, 8, x 10 of 16

Minerals 2018, 8, x 10 of 16

(d) (e)

FigureFigure 7. AFM 7. AFM images images (1.5 μ (1.5m ×µ 1.5m × μm)1.5 andµm) height and height profiles profiles of adsorp of adsorptiontion morphology morphology with withdifferent different − − DDADDA concentrations concentrations on (muscovited on) muscovite surface surface at pH at = pH6 ± =0.5. 6(e ±)( a0.5.) 1 × (10a)− 15 mol/L;× 10 5 (mol/L;b) 5 × 10 (−b5 )mol/L; 5 × 10 (c5) 1mol/L; × 10−4 mol/L;(c) 1 × (10d)− 54 ×mol/L; 10−4 mol/L; (d) 5 (×e)10 1 −× 410mol/L;−3 mol/L. (e ) 1 × 10−3 mol/L. Figure 7. AFM images (1.5 μm × 1.5 μm) and height profiles of adsorption morphology with different −5 −5 AdsorptionDDA concentrations morphology on muscovite (1.5 µm surface× 1.5 atµ pHm) = of6 ± talc0.5. ( anda) 1 × height10 mol/L; images (b) 5 × of10 DDA mol/L; collector (c) 1 × on its Adsorption10−4 mol/L;morphology (d) 5 × 10 −4(1.5 mol/L; μm (e )× 1 1.5× 10 −μ3 mol/L.m) of talc and height images of DDA collector on its surfacesurface are areshown shown in in Figure Figure 88.. LikeLike muscovite, muscovite, a similara similar situation situation was observedwas observed for talc; for the talc; adsorption the −5 adsorptionarea increased areaAdsorption increased gradually morphology gradually with increasing (1.5 with μm increasing × DDA1.5 μm) concentration. ofDDA talc concentration.and height At 1 images× 10 At of1mol/L, ×DDA 10−5 collectormol/L, a bit of a on DDAbit its of was DDAadsorbed wassurface adsorbed on are the shown talcon surfacethe in Figuretalc and surface 8. was Like uniformlyand muscovite, was uniformly spotted a similar like situationspotted muscovite. likewas observedmuscovite. Although for adsorptions Althoughtalc; the of adsorptionsmuscoviteadsorption of muscovite and area talc wereincreased and both talc gradually were monolayer, both with monola the increasing absorbedyer, the DDA amountabsorbed concentration. on amount talc wasAt on 1 much×talc 10 −was5 mol/L, more much thana bit more thatof on DDA was adsorbed on the talc−5 surface and was uniformly spotted like muscovite. Although thanmuscovite. that on muscovite. Increasing Increasing to 5 × 10 to 5 mol/L,× 10−5 mol/L, larger larger spots spots emerged emerge withd with the same the same adsorption adsorption heights, adsorptions of muscovite and talc were both monolayer, the− absorbed4 amount on talc was much more heights,which which tended tended to link to link together together to be to a be bulk. a bulk. Until Until 1 × 110 × 10−mol/L,4 mol/L, sparsely sparsely flaky flaky distribution distributionwas than that on muscovite. Increasing to 5 × 10−5 mol/L, larger spots emerged with the same adsorption −4 observed and the bulks were adsorbed with bilayer, while a little was monolayer. At 5 × −104 was observedheights, and which the tendedbulks wereto link adsorbed together to with be a bibulk.layer, Until while 1 × 10a −little4 mol/L, was sparsely monolayer. flaky distribution At 5 × 10 mol/L, closer bulks occurred showing clearly that the density and area of adsorption increased mol/L, closerwas observed bulks occurred and the bulks showing were clearlyadsorbed that with the bilayer, density while and a little area was of monolayer.adsorption At increased 5 × 10−4 evidently and the interaction was stronger between higher concentrations of DDA and the basal plane evidentlymol/L, and the closer interaction bulks occurred was stronger showing between clearly higher that the concentrations density and area of DDA of adsorption and the basal increased plane of talc.of talc. Whenevidently When DDA and DDA theconcentration interaction concentration was reached stronger reached the between themaximum, maximum, higher concentrationsthe theadsorption adsorption of DDA morphology and morphology the basal changed plane changed significantly.significantly.of talc. At Whenthis At time, thisDDA the time, concentration reagent the reagentadsorption reached adsorption onthe talc maximum, surface on talc was the surface veryadsorption uniform was morphology very and uniform tight, changedcovering and tight, −5 −3 almostcovering thesignificantly. entire almost surface At the this of entire time,the talc. the surface reagentRanging of adsorption from the talc. 1 × on 10 talc− Ranging5 mol/L surface to fromwas 1 × very10 1−3 ×mol/L,uniform10 muscovite andmol/L tight, tocovering and 1 ×talc 10 showedmol/L, aalmost similar muscovite the situation entire and surface with talc of showed the the DDA talc. a Ranging similarconcentrat from situationion 1 × rising; 10 with−5 mol/L however, the to DDA 1 × 10 the concentration−3 mol/L, adsorption muscovite rising;density and however, talcand areathe for adsorption showedtalc were a similar densitymuch situation andhigher area with than for the talcDDAthat were concentratof muscovite muchion higher rising; under than however, the that same ofthe muscovite adsorption concentration. underdensity theandThissame area for talc were much higher than that of muscovite under the same concentration. This phenomenonconcentration. can be This explained phenomenon by the can hydrophobicity be explained byof themineral hydrophobicity surfaces on of which mineral thesurfaces hydrophobic on which the hydrophobicphenomenon can attraction be explained of talc by isthe larger hydrophobicity than that of of mineral muscovite, surfaces making on which its the adsorption hydrophobic density attractionattraction of talc isof larger talc is thanlarger that than of that muscovite, of muscovite, making making its itsadsorption adsorption density density andand area ofof DDADDA and area of DDA higher. Similarly, this hydrophobic attraction can also generate its monolayer or higher. Similarly,higher. Similarly, this hydropho this hydrophobic attractionbic attraction can canalso also generate generate its its monolayer monolayer or multilayer multilayer adsorption on talc and muscovite surfaces. These results were consistent with the dynamic adsorptionadsorption on talc onand talc muscovite and muscovite surfaces. surfaces. These Th eseresults results were were consistent consistent withwith the dynamicdynamic wettabilitywettabilitywettability analysis analysis analysis above above abovethat thatthe that thehigher the higher higher the theconcentrationsthe concentrations of ofDDA of DDA DDA are, are, are,the the thelargerlarger larger the adsorbed theadsorbed adsorbed amountamount is,amount making is, making is, makingthe contact the the contact contact angles angles angles increasingly increasingly increasingly bigger. bigger.

(a) (b) (c)

Figure 8. Cont. (a) (b) (c)

Minerals 2018, 8, 391 11 of 16 Minerals 2018, 8, x 11 of 16

10 nm

(d) (e)

FigureFigure 8. AFM 8. AFM images images (1.5 (1.5µ mμm× × 1.51.5 µμm) and and height height profiles profiles of ofadsorp adsorptiontion morphology morphology with withdifferent different DDADDA concentrations concentrations on on talc talc surfacesurface at at pH pH = =6 ± 6 0.5.± (0.5.a) 1 × ( a10)− 15 mol/L;× 10 −(b5 ) mol/L;5 × 10−5 mol/L; (b) 5 ×(c) 101 ×− 105 −mol/L;4 −4 −3 (c) 1 mol/L;× 10− 4(dmol/L;) 5 × 10 ( dmol/L;) 5 × 10(e)− 14 ×mol/L; 10 mol/L. (e) 1 × 10−3 mol/L.

3.3. AFM3.3. AFM Force Force Analysis Analysis In aIn fine-grained a fine-grained flotation flotation system, system, thethe forces be betweentween particles particles can can be fitted be fitted according according to DLVO to DLVO theory where the total force, Ft, at a distance h between two parallel flat plates is decomposed into theory where the total force, Ft, at a distance h between two parallel flat plates is decomposed into two contributions from Van der Waals force, Fvdw, and electrostatic double-layer force, Fedl [13,31]: two contributions from Van der Waals force, Fvdw, and electrostatic double-layer force, Fedl [13,31]: (1) For two parallel basal planes of samples,Ft = theFvdw Van+ derFedl Waals force is written as: (1) For two parallel basal planes of samples, the Van der Waals force is written as: (2) 6 where A is the Hamaker constant, J; h is the distance betweenA the sample substrate and the sample F (h) = − (2) probe, m. For the current system [12,32]: vdw 6πh3 110 3.710 1.9310 where A is the Hamaker constant, J;, h is the distance between, the sample substrate and the(3) sample probe, m.Then, For thethe currentHamaker system constant [12 for,32 the]: interaction between samples in water can be calculated as [33,34]: −19 −20 −19 Amuscovite = 1 × 10 J, Awater = 3.7 × 10 J, Atalc = 1.93 × 10 J (3) (4) Then, the Hamaker constant for the interaction between samples in water can be calculated as [33,34]: Electrostatic double-layer force [35]: q 2 64 p (5) A131= Asample− Awater (4)

where NA is the Avogadro constant, 6.022 × 1023 mol−1; C is the molar concentration of solution, mol/L; kElectrostatic is the Boltzmann double-layer constant, 1.381 force × [1035−]:23 J/K; T is thermodynamic temperature, K; is Debye length, m; γ0 is calculated by: 2 Fedl = 64NACkTγ exp(−κh) (5) /20 1 (6) /2 1 23 −1 where NA is the Avogadro constant, 6.022 × 10 mol ; C is the molar concentration of solution, mol/L; 19 k is thewhere Boltzmann ψ0 is mineral constant, surface 1.381 potential,× 10 −V23; z Jis/ Kionic; T is valence; thermodynamic e is electron temperature, charge, 1.062 ×K 10; κ is C Debye. length, Figure 9 showed the interaction forces obtained with different concentrations of electrolytes m; γ0 is calculated by: (KCl) between sample probe and sample substrate. Each experimental curve (symbol) was fitted with exp(zeψ0/2kT) − 1 γ0 = (6) exp(zeψ0/2kT) + 1 19 where ψ0 is mineral surface potential, V; z is ionic valence; e is electron charge, 1.062 × 10 C. Minerals 2018, 8, 391 12 of 16

Figure9 showed the interaction forces obtained with different concentrations of electrolytes (KCl) between sample probe and sample substrate. Each experimental curve (symbol) was fitted with a DLVO model (corresponding solid lines). As shown in Figure9a, the monotonic repulsion was observedMinerals 2018,in 8, x ultrapure water for the whole range of studied forces due to negative12 charge of 16 on muscovite surface [13,36]. Differing from the crystal structure of muscovite, the chemical composition a DLVO model (corresponding solid lines). As shown in Figure 9a, the monotonic repulsion was of tetrahedral talc is relatively stable and the amount of its isomorphism is less, accounting for only few observed in ultrapure water for the whole range of studied forces due to negative charge on negative charges on talc surface and weaker electrostatic interaction in water, as shown in Figure9b. muscovite surface [13,36]. Differing from the crystal structure of muscovite, the chemical composition Afterof adding tetrahedral 1 mM talc·KCl is relatively solution, stable although and the the amount repulsion of its reduced isomorphism to a bit,is less, it was accounting still relatively for only large + becausefew negative K just neutralizedcharges on talc a surface few negative and weaker charges electrostatic on the interaction samples’ in surface. water, as A shown further in Figure increase of the electrolyte9b. After adding concentration 1 mM·KCl madesolution, the although repulsive the forces repulsion dramatically reduced to anda bit, progressively it was still relatively depressed. This islarge because because of moreK+ just and neutralized more K +a neutralizingfew negative charges most negative on the samples’ charges, surface. and the A increasing further increase electrolyte concentrationof the electrolyte compressed concentration the thickness made the of therepulsive electrical forces double dramatically layer on and the progressively mineral surface, depressed. resulting in greatlyThis reduced is because electrostatic of more repulsion.and more AtK+ 3neutralizing mM KCl concentration, most negative only charges, weak and repulsion the increasing was detected, and,electrolyte at 5 mM, noconcentration repulsion wascompressed observed the at thickness all. There of wasthe electrical even attractive double force,layer on though the mineral very weak, surface, resulting in greatly reduced electrostatic repulsion. At 3 mM KCl concentration, only weak within a short distance, which could be deemed to be the weak hydrophobic attraction between repulsion was detected, and, at 5 mM, no repulsion was observed at all. There was even attractive the probe and the substrate when they were very close to each other. force, though very weak, within a short distance, which could be deemed to be the weak hydrophobic attractionWhat’s more, between the ψthepotential probe and values the substrate of minerals, when whichthey were were very determined close to each by other. a Coulter Delsa 440sx Zeta potentialWhat’s analyzer more, the (Beckman ψ potential Coulter, values of Inc.), minerals, were which employed were asdetermined the approximation by a Coulter of Delsa the Stern potential440sx ofZeta samples, potential calibrating analyzer (Beckman the Stern Coulter, potential Inc.), were of the employed AFM tip as ofthe mineralapproximation probe of by the fitting the experimentalStern potential interactionof samples, calibrating force profiles the Stern [12]. potential Potential of the measurements AFM tip of mineral were probe conducted by fitting at least five timesthe experimental at the same interaction concentration force profiles and the[12]. average Potentialwas measurements calculated. were From conducted Figure at9a,b, least it five can be notedtimes that at the the experimental same concentration interaction and the force average profiles was calculated. fit really wellFrom into Figure the 9a,b, classical it can DLVObe noted theory at allthat concentrations the experimental of electrolytes interaction force for separation profiles fit really distances well shorterinto the thanclassical 5 nm DLVO of0 theory mM andat all 1 mM concentrations of electrolytes for separation distances shorter than 5 nm of 0 mM and 1 mM and and larger than 10 nm of 3 mM and 5 mM. The serious discrepancy within extremely short distances larger than 10 nm of 3 mM and 5 mM. The serious discrepancy within extremely short distances (less (lessthan 3 3 nm) nm) is is probably probably owing owing to to the the non-DLVO non-DLVO forces—the forces—the hydration hydration force force between between the mineral the mineral substratesubstrate and and probe, probe, which which is is not not included includedin in thethe classical DLVO DLVO model model [12]. [12 ].

2.0 2.0 (a) muscovite (b) talc K+ (mM) Ψ(mV) K+(mM) ψ (mV) 1.5 1.5 0 -42 0 -50 1 -36 ） ) 2 2 3 -22 1.0 1 -45 1.0 3 -20 5 -15 nN/m nN/m

5 -15 （ 2 （

2 0.5 0.5 F/H F/H

0.0 0.0

-0.5 -0.5 0 102030405060 0 102030405060 Separatation Distance（nm） Separatation Distance（nm） FigureFigure 9. Interaction 9. Interaction forces forces of of (a ()a muscovite) muscovite andand ( b)) talc talc measured measured in indifferent different concentrations concentrations of KCl of KCl solutionsolution at a at natural a natural pH. pH.

TheThe fitting fitting results results shown shown in in Figure Figure9 9are are good good enoughenough to to make make the the measured measured interaction interaction forces forces convinced deeply. As a result, the fitting process isn’t explored in the following study when the convinced deeply. As a result, the fitting process isn’t explored in the following study when electrolyte of the KCl solution is replaced by collectors. Interaction forces measured in different the electrolyte of the KCl solution is replaced by collectors. Interaction forces measured in different concentrations of DDA collector at a natural pH are shown in Figure 10. As shown, the repulsive concentrationsforce obtained of DDA at a lower collector concentration at a natural (1 pH × 10 are−5 mol/L) shown caused in Figure mainly 10. Asby shown,electrostatic the repulsiverepulsion force −5 obtainedwhere at the a lowerforce of concentration muscovite was (1 larger× 10 thanmol/L) talc due caused to its more mainly negative by electrostatic charges. With repulsion increasing where the forceDDA ofconcentration, muscovite wasthe electrostatic larger than repulsive talc due fo torce its reduced more negative and the charges.long-range With interaction increasing force DDA concentration,profiles changed the electrostatic from repulsive, repulsive at 5 × 10 force−5 mol/L, reduced to attractive and the with long-range a jump-in interactiondistance at around force profiles15 changednm of from muscovite repulsive, and 10 at nm 5 × of10 talc−5 atmol/L, 1 × 10−4 tomol/L. attractive After that, with for a the jump-in muscovite, distance a further at around increase 15 nm of muscovitein DDA concentration and 10 nm ofchanged talc at the 1 × suppressed10−4 mol/L. attractive After force that, and for it the even muscovite, became repulsive a further again. increase −4 in DDABut for concentration talc, the maximum changed attractive the suppressed force occurred attractive at 5 × 10 force mol/L and and it then even it becamewas suppressed. repulsive again. This situation for muscovite and talc can be explained that, at a lower concentration (1 × 10−5 But for talc, the maximum attractive force occurred at 5 × 10−4 mol/L and then it was suppressed. mol/L), only a small amount of DDA was adsorbed on the muscovite surface and the hydrophobic attraction was relatively small, still making the electrostatic repulsive force dominant. With

Minerals 2018, 8, 391 13 of 16

This situation for muscovite and talc can be explained that, at a lower concentration (1 × 10−5 mol/L), only a small amount of DDA was adsorbed on the muscovite surface and the hydrophobic attraction was relatively small, still making the electrostatic repulsive force dominant.Minerals With 2018 increasing, 8, x concentrations, the adsorbed amount of DDA rose up gradually,13 of 16 consistent with Figures7 and8 exactly and the surface of minerals also became more and more hydrophobic, causing theincreasing hydrophobic concentrations, attraction the adsorbed to appear. amount of The DDA maximum rose up gradually, of muscovite consistent with at 1 Figures× 10 −4 mol/L 7 and 8 exactly and the surface of minerals also became more and more hydrophobic, causing the correspondedMinerals to 2018 the, 8, xlarge-area monolayer adsorption and, after the maximum, the13 attractive of 16 force hydrophobic attraction to appear. The maximum of muscovite at 1 × 10−4 mol/L corresponded to the slowly declined due to the adsorption of the bilayer or multilayer, which made the hydrophobic increasinglarge-area monolayerconcentrations, adsorption the adsorbed and, after amount the maxi of DDAmum, rose the upattractive gradually, force consistent slowly declined with Figures due to −4 attractions7the and decrease. adsorption 8 exactly Nevertheless, andof the the bilayersurface or theof multilayer,minerals maximum also wh becameich of talcmade more at the 5 and ×hydrophobic 10more mol/Lhydrophobic, attractions was causing because decrease. theon the talc −4 surface therehydrophobicNevertheless, was a large attraction the maximum area to of appear. single-moleculeof talc The at 5 maximum × 10 mol/L adsorption of wasmuscovite because layer,at on 1 × the 10 showing −talc4 mol/L surface corresponded the there greatest was a to large the hydrophobic area of single-molecule adsorption layer, showing the greatest hydrophobic attraction that coincided attractionlarge-area that coincided monolayer with adsorption the AFM and, imaging after the maxi resultsmum, in the Figure attractive8. force slowly declined due to thewith adsorption the AFM imaging of the bilayerresults inor Figure multilayer, 8. which made the hydrophobic attractions decrease. −4 Nevertheless, the maximum of talc at 5 × 10 mol/L was because2.0 on the talc surface there was a large 2.0 area of single-molecule adsorption layer, showing the greatest hydrophobic attraction thatψ coincided (a) muscovite DDA (mol/L) ψ (mV) (b) talc DDA(mol/L) (mV) 1×10-5 -19.17 1.5 -5 with the1.5 AFM imaging results in Figure 8. 1×10 -12.64 -5 5×10 -3.43 5×10-5 3.27

-4 )

2 -4 ) 2×10 10.58 2.0 2 2×10 14.56 2.0 -4 1.0 1.0 5×10 18.95 -4 (b) talc DDA(mol/L) 5×10 ψ 23.23(mV) -3 ψ nN/m nN/m (a) muscovite DDA (mol/L) 1×10 23.08 (mV) -3

-5 （ 1×10 29.95 （ -5 2 2 1×10 -19.17 1.5 1×10 -12.64 1.50.5 0.5 -5 -5 F/H F/H 5×10 -3.43 5×10 3.27

-4 )

2 -4 ) 2×10 10.58 2 2×10 14.56 1.0 -4 1.0 0.0 5×10 18.95 0.0 5×10-4 23.23

-3 nN/m nN/m 1×10 23.08 -3

（ 1×10 29.95 （ 2 2 0.5 0.5 -0.5 -0.5 F/H F/H 0 102030405060 0 102030405060 Separation distance (nm) Separation distance (nm) 0.0 0.0 Figure 10.FigureInteraction 10. Interaction forces forces of (a of)muscovite (a) muscovite and and ( b)) talc talc measured measured in different in different concentrations concentrations of DDA of DDA -0.5 -0.5 collector0 at 102030405060pH = 6 ± 0.5. 0 102030405060 collector at pH = 6 ± 0.5.Separation distance (nm) Separation distance (nm) 3.4. Flotation Results 3.4. Flotation ResultsFigure 10. Interaction forces of (a) muscovite and (b) talc measured in different concentrations of DDA collectorThe floatability at pH = 6 of ± 0.5.muscovite and talc in different pH values was studied when the concentration The floatabilityof DDA was offixed muscovite at 1 × 10−4 andmol/L. talc As inshown different in Figure pH 11, values the recovery was studied of talc followed when thea similar concentration of DDA was3.4.trend Flotation fixed to that at Results 1 of× 10 muscovite.−4 mol/L. Talc As recovery shown remained in Figure relatively 11, the stable recovery in acid of talcconditions followed at a similar trend to thatapproximately ofThe muscovite. floatability 83%, of Talcand muscovite then recovery declined and remainedtalc slightly in differ toent relativelya minimum pH values stable at was pH studied = in 10 acid before when conditions rising the concentration again, at while approximately muscovite recovery decreased substantially with the increase of pH and rose at the same value with 83%, andof then DDA declined was fixed at slightly 1 × 10−4 mol/L. to a minimumAs shown in atFigure pH 11, = the 10 recovery before risingof talc followed again, a while similarmuscovite trendtalc. Overall, to that talc of recoverymuscovite. was Talc8% higher recovery than remainedthat of muscovite. relatively Apart stable from in pH, acid the conditionseffect of DDA at recoveryapproximately decreasedconcentration substantially on83%, muscovite and then anddeclined with talc the slightlyat pH increase =to 6a ±minimum 0.5 of is pH presented at and pH = rose 10in before Figure at the rising 12. same Asagain, for value whileDDA with talc. Overall, talcmuscoviteconcentration, recovery recovery the was variation decreased 8% higher trends substantially in than recovery thatwith between the of muscovite.increase muscovite of pH andand Apart talcrose were fromat the also same pH, similar value the where effectwith of DDA −4 concentrationtalc.recovery Overall, on muscovite increased talc recovery rapidly, and was talc up 8% atto pHhigher2 × =10 6than ±mol/L,0.5 thatis and of presented muscovite. then slowed inApart Figuredown from after 12 pH,. Asthat the for concentration.effect DDA of DDA concentration, Hence, results suggested that 2 × 10−4 mol/L is the optimum concentration for muscovite and talc the variationconcentration trends in on recovery muscovite between and talc muscovite at pH = 6 and± 0.5 talc is werepresented also in similar Figure where12. As recoveryfor DDA increased concentration,flotation. Taking the all variation the experiments trends in together, recovery the between higher muscovite concentrations and talcof DDA were generated also similar more where and × −4 rapidly, uprecoverymore to 2 adsorbed 10increased amountmol/L, rapidly, in and AFM up then imaging,to 2 slowed× 10 making−4 mol/L, down the and contact after then that anglesslowed concentration. increasingly down after bigger that Hence, concentration. ascribed results from suggested −4 that 2 × 10Hence,the hydrophobicmol/L results suggested is attraction the optimum that forces 2 × between10− concentration4 mol/L mineral is the surfaces.optimum for muscoviteAs concentration a result, a andhigher for muscovite talc flotation flotation. recoveryand talc Taking all the experimentsflotation.was finally together,Taking attained. all the the experiments higherconcentrations together, the higher of concentrations DDA generated of DDA generated more and more more and adsorbed amount inmore AFM adsorbed imaging, amount making in AFM the imaging, contact making angles the increasingly contact angles increasingly bigger ascribed bigger fromascribed the from hydrophobic the hydrophobic attraction forces between mineral surfaces. As a result, a higher flotation recovery 90 attractionwas forces finally between attained. mineral surfaces. As a result, a higher flotation recovery was finally attained.

90 60

Recovery/% 75 45 talc muscovite 60 30 Recovery/% 45 24681012 talc muscovite pH 30

24681012 pH Figure 11. Flotation recovery of muscovite and talc as a function of pH at DDA concentration of 1 × 10 −4 mol/L. Minerals 2018, 8, x 14 of 16

Figure 11. Flotation recovery of muscovite and talc as a function of pH at DDA concentration of 1 × Minerals 2018, 8, 391 14 of 16 10−4 mol/L.

100

40 talc Recovery/% muscovite

0 02468 -4 Concentration(10 mol/L) FigureFigure 12. 12. FlotationFlotation recovery recovery of of muscovite muscovite and talc asas aa functionfunction of of DDA DDA concentrations concentrations at at pH pH = 6= ±6 ±0.5. 0.5.

Overall, a dynamic contact angle was employed to illustrate how how wettability wettability changed changed over over time, time, while AFMAFM imagingimaging was was performed performed to to show show the the adsorbed adsorbed amount amount on on the the mineral mineral surfaces, surfaces, which which had hadan impact an impact on theon the wettability wettability of minerals,of minerals, and and then then explained explained the the phenomenon phenomenon of of dynamicdynamic contact angles atat the the last last 6 min.6 min. Besides Besides that, that, AFM AFM force measurementforce measurement was carried was out carried to explain out theto explain mechanism the mechanismof this phenomenon of this phenomenon from the perspective from the of interactionperspective force of interaction by using classical force by DLVO using theory. classical Therefore, DLVO theory.dynamic Therefore, wettability dynamic can directly wettability and visually can directly show and how visually the hydrophobicity show how the of hydrophobicity minerals changes of whenminerals common changes water when droplets common are wate substitutedr droplets withare substi a collectortuted with solution. a collector Regarding solution. the Regarding flotation theresponse, flotation the response, bigger the the final bigger contact the final angle contact and the angle more and the the adsorbed more the amount adsorbed in AFMamount imaging in AFM is, imagingthe higher is, the the flotation higher the recovery flotation is, showing recovery that is, allshowing the mechanism that all the interpretations mechanism interpretations are consistent with are consistentthe macroscopic with the flotation macroscopic results. flotation results.

4. Conclusions 4. Conclusions In the present study, the new dynamic wettability test and AFM imaging and force analysis were In the present study, the new dynamic wettability test and AFM imaging and force analysis were performed to characterize the hydrophobization mechanism of the DDA collector on a muscovite performed to characterize the hydrophobization mechanism of the DDA collector on a muscovite and and talc surface. The attachment of bubbles onto the muscovite and talc surface hydrophobizing at talc surface. The attachment of bubbles onto the muscovite and talc surface hydrophobizing at different concentrations of DDA was investigated. The AFM imaging and interaction forces were different concentrations of DDA was investigated. The AFM imaging and interaction forces were applied to explain the DDA adsorption mechanism on both minerals. To authenticate the effectiveness applied to explain the DDA adsorption mechanism on both minerals. To authenticate the of these techniques, ﬂotation tests were further carried out. The following major conclusions were effectiveness of these techniques, flotation tests were further carried out. The following major drawn from this detailed study: conclusions were drawn from this detailed study: (1) AfterAfter treatments treatments with with DDA, DDA, the the contact contact angles angles be becamecame much much larger larger compared compared to to initial angles, particularlyparticularly for for muscovite, muscovite, and and the the attachment attachment of of bubbles bubbles on on the talc surface was found to be muchmuch easier easier than than muscovite muscovite due due to to its its natural natural hydrophobicity. hydrophobicity. (2) FromFrom AFM AFM imaging, imaging, both both the the muscovite muscovite and talc showed showed a similar similar tendency: the higher higher the DDA DDA concentration,concentration, the the more more the the adsorbed adsorbed amount. amount. However, However, the adsorbed amount of DDA on talc surfacesurface was was obviously obviously more more than than that that on on muscovite. muscovite. (3) AsAs for for the the interaction interaction forces forces between between the the minerals minerals,, the the maximum maximum attractions attractions occurred occurred at different different concentrationsconcentrations respectivelyrespectively for for muscovite muscovite and an talcd talc and and agreed agreed well well with thewith AFM the imagingAFM imaging results. (4) results.Results obtained from ﬂotation tests were promising and quite in agreement with (4) Resultsthe phenomenon obtained offrom these flotation techniques. tests were promising and quite in agreement with the phenomenon of these techniques.

Author Contributions:Contributions:H.J. H.J. conceived conceived of andof designedand designed the experiments; the experiments; Y.G. and Y.G. W.J. performedand W.J. theperformed experiments the and analyzed the data; H.J. and Y.H. contributed reagents, materials and analysis tools; Y.G. and S.A.K. wrote experimentsthe paper. and analyzed the data; H.J. and Y.H. contributed reagents, materials and analysis tools; Y.G. and S.A.K. wrote the paper. Funding: Authors of this paper proudly acknowledge the ﬁnancial support from the National Natural Science Foundation of China (Grant Nos. 51674207, 50974134, and 51304162) and the Key Laboratory of Hunan Province for Clean and Efﬁcient Utilization of Strategic Calcium-Containing Mineral Resources (No. 2018TP1002). Minerals 2018, 8, 391 15 of 16

Conﬂicts of Interest: The authors declare no conﬂicts of interest.

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