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Interaction between low molecular weight carboxylic acids and muscovite: Molecular dynamic simulation and experiment study
Colloids and Surfaces A 559 (2018) 8–17
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Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Interaction between low molecular weight carboxylic acids and muscovite: Molecular dynamic simulation and experiment study ⁎
Xue Xiaopenga,b, Xu Zhonghaoa,b, Israel Pedruzzia,b, Li Pinga,b, , Yu Jianguoa,b, a b
T
⁎
State Key Laboratory of Chemical Engineering, School of Chemical Engineering, East China University of Science and Technology, Shanghai, 200237, China National Engineering Research Center for Integrated Utilization of Salt Lake, East China University of Science and Technology, Shanghai, 200237, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Muscovite Carboxylic acid Interface phenomena Molecular dynamic simulation Adsorption Contact angle
Interface phenomena between low molecular weight (LMW) carboxylic acids and muscovite was investigated through molecular dynamic simulation and experiment, where the typical monocarboxylic acids including formic acid (C1), acetic acid (C2), propionic acid (C3) and butyric acid (C4) were used as models. Density distribution, adsorption energy, root mean square dynamic (RMSD) of carboxylic acids on water-muscovite interface were calculated through molecular dynamic simulation, and the advanced characterization methods, such as ATR-FTIR spectra, AFM images and contact angle were performed to test and verify the relative simulation findings. The molecular simulation showed that carboxylic acids adsorbed on surface of muscovite through hydrogen bond between H atom of eCOOH functional group of carboxylic acid and O atom of muscovite, belong to outer sphere adsorption, and ATR-FTIR spectra and AFM images confirmed this finding. Adsorption energy for long carbon chain carboxylic acid (C4) was higher than that for short carbon chain carboxylic acid (C1, C2 and C3) due to the effect of carboxylic acid diffusion on water-muscovite interface. The hydrophilic functional group eCOOH of carboxylic acids preferably adsorbed on muscovite surface, while the hydrophobic functional groups eCH3 and eCH2 of carboxylic acids were far from the muscovite surface. So, the hydrophobicity on muscovite surface increased due to the adsorption of carboxylic acids, which resulted in the increase of contact angle of water on muscovite surface.
⁎ Corresponding authors at: State Key Laboratory of Chemical Engineering, School of Chemical Engineering, East China University of Science and Technology, Shanghai, 200237, China. E-mail addresses: [email protected] (P. Li), [email protected] (J. Yu).
https://doi.org/10.1016/j.colsurfa.2018.09.033 Received 8 July 2018; Received in revised form 11 September 2018; Accepted 13 September 2018 Available online 14 September 2018 0927-7757/ © 2018 Elsevier B.V. All rights reserved.
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1. Introduction
images and contact angle. These research would provide significant information for hydrometallurgy, flotation and geochemical science.
Low molecular weight (LMW) carboxylic acids are most common organic matters in natural world, especially in soil [1–3], where the concentration of aliphatic LMW carboxylic acids has reached the range of 0.1–1.0 mM [4]. There are various sources to accumulate LMW carboxylic acids in the natural world. Wastewaters from biological fermentation process or chemical industries contain a certain amount of LMW carboxylic acids [5,6], sometimes which are seeped into soil to result in the accumulation of LMW carboxylic acids. Degradation of organic matters from dead plants and animals would release LMW carboxylic acids to accumulate slowly in soil. In addition, strong activities of various microorganisms (bacteria and fungi) would also lead to enrichment of different LMW carboxylic acids in soil [3,4,7]. In the natural world, there exists also a large amount of minerals, such as feldspar, mica, apatite, pyrite etc, so the interaction between the enriched LMW carboxylic acids and minerals occurs frequently. Moreover, the interaction would promote the release of valuable elements from these minerals. For example, the carboxylic acids promoted dissolution of P bearing mineral [8–10] and K bearing mineral [11,12] to release the nutrient elements of K and P as fertilities for plant growth. On the other hand, LMW carboxylic acids could react and complex with many minerals to release toxic metals (Cd2+, Al3+, Pb2+, Cu2+) into soil, which resulted in heavy metals pollutions. Moreover, in the view point of engineering, interaction between LMW carboxylic acids and minerals also plays a key role in hydrometallurgy, for example, LMW carboxylic acid could be applied as depressant in mineral flotation [13,14] and as extraction solvent [15–17]. Therefore, the investigation on the interaction between LMW carboxylic acids and minerals is more significant. Generally, interaction mechanism between organic acids and minerals is divided into outer-sphere adsorption and inner-sphere adsorption. Outer-sphere adsorption is organic matter adsorbed on mineral surface by physical electrostatic force, Van der Waals force and hydrogen bond through O (F, N, S or C) and H atoms [18,19], and adsorption energy is lower due to weak interaction force. On the contrary, inner-sphere adsorption is organic matter adsorbed on mineral surface through organic ligands (mainly COOe) combined directly with metal elements (Ca, Fe, Al, Mg) [20–22], and adsorption energy is higher due to chemical reaction and formation of organic ligand-metal complex. With the development of molecular simulation and advanced characterization methods, the interaction between organic acids and minerals can be explored at the molecular or atomic level. For example, Based on the molecular dynamic simulation, it has found that the mixed organic agents (oleic acid and octadecanoic acid) with good performance of muscovite flotation [23–25]. Through adsorption energy calculations and adsorption behavior analysis, organic acid mechanism was be figured out exactly. At the same time, many advanced instruments (ATR-FTIR and AFM) have been frequently used to validate the molecular simulation results. AFM detection has been adopted to validate if organic acid is adsorbed visibly [26,27], while ATR-FTIR detection has been used to distinguish outer-sphere adsorption and innersphere adsorption [28–30]. Muscovite with KAl2(Si3Al)O10(OH)2 formula, is a layered dioctahedral aluminosilicate mineral, the most common mineral in soil. The layer phyllosilicate consists of two tetrahedral (T) sheets with an octahedral (O) sheet in between (TOT structure). The octahedral unit is linked to the other units via shared octahedral edges. Due to a large amount of muscovite distributed in soil, interaction between LMW carboxylic acids and muscovite can’t be avoidable. Moreover, the release of K element from muscovite by adsorption and dissolution of organic acids could be used as fertilizer for plant growth. Therefore, in this paper, the interaction of muscovite with four kinds of LMW monocarboxylic acids, such as formic acid (C1), acetic acid (C2), propionic acid (C3) and butyric acid (C4), were investigated through molecular dynamic simulation combined with the advanced characterization methods, such as ATR-FTIR spectra, AFM
2. Theoretical and experimental sections 2.1. Theoretical section 2.1.1. Molecular structures of carboxylic acids, muscovite and adsorption models Molecular dynamic simulation was performed in Material Studio 6.0 software (Accelrys, USA). Four kinds of monocarboxylic acids were chosen as models, formic acid (HCOOH, FA), acetic acid (CH3COOH, AA), propionic acid (CH3CH2COOH, PA) and butyric acid (CH3CH2CH2COOH, BA). Molecular structures of carboxylic acids were firstly optimized in Dmol3 module based on DFT methods to obtain accurate structure and charge assignments. Generalized gradient approximation (GGA) with the Perdew–Burke − Ernzerhof (PBE) [31] of the exchange − correlation functional and basis set DNP 3.5 were applied. Convergence criteria were SCF tolerance 1 × 10−6, energy change 1 × 10-5Ha, max force 0.002Ha/ Å and max displacement 0.005 Å [32]. Molecular structures of water and muscovite were built and optimized as same described methods. Final optimized models were shown in Fig. 1. Crystal lattice parameters of unit of muscovite were: a = 5.187 Å; b = 8.995 Å; c = 19.502 Å; α = 90.00°；β = 95.87°; γ = 90.00°. Adsorption model of carboxylic acids on muscovite surface were built, as shown in Fig. 2, where a box containing 50 carboxylic acid molecules and 300 water molecules was placed between two muscovite super cells, and facet (0 0 1) of muscovite was cleaved and expanded to a 5*3*1 super cell. 2.1.2. Molecular dynamic simulation methods Before carrying out molecular dynamic simulation, adsorption models built in Fig. 2 should be optimized geometrically to obtain stable initial models in Forcite module. Universal force field (UFF) was applied in MD simulation. UFF had been proved to be suitable for organic molecules–mineral interface system [25,33]. After geometrical optimization, adsorption models were carried out MD simulation at 298.15 K for 1.2 ns in NVT ensemble (time step: 1 fs). Only NVT ensemble was adopted because initial structures were very stable and muscovite surface were frozen (NPT can’t be running when atoms were frozen in Forcite module). Besides, thermostat was velocity scale by 10 K and energy deviation was 50,000.0Kcal/mol. Electrostatic was described by Ewald methods while van der waals was described by atom based method. Three important parameters including density of numbers, adsorption energy and Root mean square dynamic (RMSD) were calculated in MD simulation to evaluate interaction between carboxylic acids and muscovite. Adsorption energy Ea was calculated by the following equation [34]:
Ea =
E1 −E2−E3 + E4 N
Where E1 was the total energy of adsorption models after MD simulations; E2 stood for energy of adsorption models excluding carboxylic acid molecules; E3 designated energy of adsorption models excluding muscovite super cells; E4 demonstrated total energy of water molecules; N was number of carboxylic acid molecules, respectively. Density of numbers was defined as following equation [35]:
ρz =
M × Nz − δz , z + δz 2
2
δz × S
Where pz was density of numbers of carboxylic acid; M stood for atom mass; N(z-δz/2, z+δz/2) designated average numbers of occurring in interval δz; S was basal surface area. Density of numbers could be used to analyze density of carboxylic acid in a specific position. If density of carboxylic acid was particularly higher near muscovite surface than other places, carboxylic acid could be adsorbed. 9
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Fig. 1. Optimized molecular structures of carboxylic acids, water and muscovite.
Fig. 2. Adsorption models of carboxylic acids on muscovite surface (green sticks were water molecules). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
2.2. Experimental section
Root mean square displacement (RMSD) can be used to determine the adsorption dynamics of carboxylic acid on water-muscovite interface and it was defined as following equations [36]:
Pure muscovite sheets were purchased from Sinopharm Chemical Reagent Company (China). Rough muscovite sheets were cut into 8 mm × 8 mm and removed by adhesive tape to expose new (0 0 1) surface which was cleaned by alcohol and water. When muscovite sheet was dried, they were mixed with 100 ml 0.1 mol/L different carboxylic acids (AR, > 99.5%) into plastic bottles. Then plastic bottles were put into a shaker at 25 °C and 180 rpm for 2 h. After interactions, muscovite sheets were dried at 25 °C in a vacuum drying oven naturally. Dried
N
RMSD =
1 ∑ [ri (t )−ri (0)]2 N i
Where N was total number of carboxylic acid molecules; ri(t) and ri(0) stood for position of carboxylic acid at time t and initial position. 10
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Fig. 3. Density of numbers of carboxylic acid adsorption on muscovite surface at initial state before MD simulation and at equilibrium state after MD simulation.
automatically optimized [27]. Furthermore, contact angle of muscovite sheets were measured by JC2000DE (Shanghai zhongchen, China). One drop of water was dropped into muscovite sheet, after 30 s, contact angle was collected by CCD camera
muscovite sheets were analyzed by ATR-FTIR (Nicolet, 6700, USA). All spectra were obtained using KBr pellets at 25 °C with a resolution 256 scans and 4cm−1. Besides, muscovite sheets were observed by AFM (Vecco/DI, USA) with peak force mode at 25 °C. Scan rate were 11
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Fig. 4. Density of water molecules on muscovite surface in the presence and absence of carboxylic acids.
Fig. 6. RMSD curve of carboxylic acid on water-muscovite interface during MD simulation.
MD simulation, density distribution curves of carboxylic acids between two muscovite surfaces are calculated, and the simulated results are shown in Fig. 3. It is found that density of carboxylic acid becomes obviously higher when compared with the initial random distribution curve near muscovite surface. Density distribution curve of carboxylic acid looks like “M” shape, that is to say the concentration of carboxylic acid on muscovite surface higher than that in the middle part, which indicates that carboxylic acids can adsorb on muscovite surface. Besides, it can be seen that the high density distributions for four carboxylic acids on muscovite surface are approximate 2.5–3.2 Å−3. Because the surface properties of minerals would be different in the presence and absence of water [37,38], the presence of water will change adsorption behaviors of carboxylic acids on muscovite surface. Fig. 4 shows density of water molecules on muscovite surface in the presence and absence of carboxylic acids. Density of water molecules decreases obviously when carboxylic acids adsorb competitively on muscovite surface. The adsorption ability of carboxylic acid is stronger than that of water. In order to study whether four carboxylic acids adsorption on muscovite surface is spontaneous or not, the free energy △G is calculated. According to the literature [39], △G is the minimum value of W (r), where W(r) is the potential mean force (PMF), describing the free energy surface along the chosen coordinate, J/mol. W(r)=-RTlog(nz), where R is thermodynamic constant, 8.3145 J/mol K, T is temperature of adsorption system, K, nz is distribution density of adsorption matter along Z axis. Fig. 5 demonstrates the calculated W(r) along Z axis for four carboxylic acids interaction on muscovite, where the minimum value of W(r) can be found and then △G value is obtained. △G1 of formic acid adsorption on muscovite surface is −0.96 kJ/mol, △G2 of acetic acid adsorption on muscovite surface is −1.16 kJ /mol, △G3 of propionic acid adsorption on muscovite surface is −1.13 kJ /mol and △G4 of butyric acid adsorption on muscovite surface is −1.19 kJ /mol. Since △G1-4 < 0, it can be concluded that adsorption of four carboxylic acids on muscovite surface is spontaneous totally.
Fig. 5. Calculated W(r) values for four carboxylic acids adsorption on muscovite surface.
3. Results and discussions 3.1. Adsorption of carboxylic acids on muscovite surface Adsorption behavior of four carboxylic acids, including formic acid (C1), acetic acid (C2), propionic acid (C3) and butyric acid (C4), on muscovite (0 0 1) surface is investigated by molecular dynamic simulation. The adsorption model used for MD simulation contains 300 water molecules and 30 carboxylic acid molecules between two muscovite surfaces in a box or a unit for MD simulation. Before MD simulation, distribution of carboxylic acid in water phase is random as shown in Fig. 2. After 1.2 ns
Table 1 Adsorption energies of carboxylic acids on muscovite surface. Adsorption system
Ea/kJ mol−1
E1/kJ mol−1
E2/kJ mol−1
E3/kJ mol−1
E4/kJ mol−1
Formic-muscovite Acetic- muscovite Propionic-muscovite Butyric-muscovite
−9.49 −17.70 −16.95 −19.19
−610806.55 −616568.77 −631949.12 −702200.83
−607291.13 −605483.66 −627917.09 −696741.29
−3952.40 −11101.70 −4010.46 −5173.08
−911.53 −901.50 −825.72 −673.19
12
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Fig. 7. AFM images for adsorption phenomena of carboxylic acids on muscovite surface. A, original muscovite surface; B, formic acid ; C, acetic acid; D, propionic acid; E, butyric acid.
Fig. 8. AFM images with 3D for adsorption of carboxylic acids on muscovite surface；A, original muscovite surface; B, formic acid ; C, acetic acid; D, propionic acid; E, butyric acid.
Generally, adsorption energy is used to investigate interaction between carboxylic acid and muscovite. Table 1 lists adsorption energies of four carboxylic acids on muscovite surface calculated by MD
simulation. Adsorption energies of four carboxylic acids on muscovite surface are −9.49 kJ/mol for formic acid, −17.70 kJ/mol for acetic acid, −16.95 kJ/mol for propionic acid and −19.19 kJ/mol for butyric 13
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Fig. 9. Schematic diagram illustrating interaction between the functional groups of propionic acid and active sites on muscovite surface.
Fig. 10. Adsorption of carboxylic acid on muscovite surface at equilibrium state. A, formic acid ; B, acetic acid; C, propionic acid; D, butyric acid.
dynamics of carboxylic acid on muscovite. The rank of RMSD for four carboxylic acids is formic acid (C1) > propionic acid(C3) > acetic acid (C2) > butyric acid (C4). The rank of RMSD is in opposite with the rank of the absolute value of adsorption energy for four carboxylic acids. For LMW carboxylic acid adsorption, the dynamic diffusion of carboxylic acid in water phase will has negative effect on the adsorption behaviors on muscovite, for example, formic acid would prefer moving in water phase rather than being adsorbed on muscovite surface [36]. RMSD of butyric acid is much lower, as a result, butyric acid is inclined to be adsorbed on muscovite surface instead of in water phase. Lower RMSD
acid, respectively. These adsorption energies are negative, which means that four carboxylic acids can adsorb on muscovite surface. From Table 1, it is found that the absolute value of adsorption energy increases with the increase of carbon chain length of carboxylic acid, which means the adsorption of carboxylic acid with long carbon chain becomes easier than that with short carbon chain. Moreover, it is noticed that the absolute value of adsorption energy of propionic acid (C3) is lower than that of acetic acid (C2). And then, RMSD value for each carboxylic acid adsorption is calculated by MD simulation, as shown in Fig.6. RMSD value is frequently used to evaluate adsorption 14
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surface due to carboxylic acid adsorption become bigger with the increase of carbon numbers, especially when comparing formic acid adsorption with butyric acid adsorption. However, what should be noticed the coverage area of propionic acid on muscovite is lower than that of acetic acid. It also is in agreement with the above mentioned explanation for adsorption energy and RMSD values. Furthermore, 3D vertical AFM images of adsorption of four carboxylic acids are shown in Fig. 8. From Fig. 8, it is found that adsorption layer thickness of formic acid on muscovite surface is about 8.8 nm (> 2.818 Å), adsorption layer thickness of acetic acid is about 7.7 nm (> 3.423 Å), propionic acid is about 12.2 nm (> 4.941 Å) and adsorption layer thickness of butyric acid is about 2.9 nm (> 5.868 Å). The adsorption layer thicknesses for four carboxylic acids are all higher than maximum length of their molecules. Compared to original height of muscovite (0.4 nm), it can deduced the adsorption of carboxylic acids on muscovite surface is multilayer. 3.2. Interface phenomena between carboxylic acid and muscovite Fig. 11. ATR-FTIR spectra of adsorption of carboxylic acids on muscovite surface.
Both MD simulation result and AFM detected result demonstrate that carboxylic acids adsorb on muscovite surface through hydrogen bond. However, there are three candidate adsorption sites on muscovite surface, Al site, Si site and O site, and carboxylic acid has three candidate functional groups to provide H atom for hydrogen bond formation, namely eCOOH, eCH3 and eCH2 functional groups. Fig. 9 gives a schematic diagram illustrating interaction between the functional groups of carboxylic acid and active sites on muscovite surface, where propionic acid is taken as example. Where and how carboxylic acids are adsorbed on muscovite surface? Fig. 10 demonstrates the molecular structural diagrams of carboxylic acid adsorption on muscovite surface at equilibrium state obtained by MD simulation. According to the MD simulation data, the distance between H atom of eCOOH functional group and O atom of muscovite surface is calculated as 2.509 Å for formic acid, 2.860 Å for
value will result in higher adsorption energy that gives an understanding why adsorption energy of propionic acid lower than acetic acid. Through MD simulation, it is found that four carboxylic acids can adsorb on muscovite surface. Here, AFM detector is adopted to observe the adsorption phenomena of carboxylic acids on muscovite surface, and the detected AFM images are shown in Fig. 7. The original surface of muscovite without the adsorption of carboxylic acids is smooth, as shown in Fig. 7a. After 2 h adsorption experiment, the muscovite surface becomes not smooth, part of the surface is covered with the adsorbed carboxylic acids, as shown in Fig. 7b–e. According to the detected AFM images, it is found that the coverage area on muscovite
Fig. 12. Density of numbers of carboxylic acid with different functional group adsorption on muscovite surface. A, formic acid ; B, acetic acid; C, propionic acid; D, butyric acid. 15
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Fig. 13. Contact angles of water on muscovite surface with and without carboxylic acid adsorption. A, original muscovite; B, with formic acid adsorption; C, with acetic acid adsorption; D, with propionic acid adsorption E, with butyric acid adsorption.
acetic acid, 2.595 Å for propionic acid and 2.816 Å for butyric acid, respectively. At the same time, the bond angle of O⋯H⋯O is also calculated as 124°for formic acid, 123°for acetic acid, 141°for propionic acid and 150°for butyric acid, respectively. According to the literature [39], the hydrogen bond length is normally in the range of 2.5 Å to 3.0 Å and its bond angle is bigger than 90°. The calculated results from Fig. 10 satisfy the conditions of hydrogen bond formation, so it is deduced that carboxylic acids adsorb on muscovite surface through hydrogen bond. To verify the MD simulation findings, ATR-FTIR spectra detection is carried out to test if carboxylic acids are adsorbed through hydrogen bond, belong to outer-sphere adsorption. Fig. 11 shows the detected ATR-FTIR spectra after adsorption of carboxylic acids on muscovite surface. There exist three main frequencies which reflect crystal structure of muscovite, frequency at 748 cm−1 due to vibration of AleO bond, frequency at 1026cmn−1 due to vibration of SieO bond, and frequency at 3624 cm−1 due to vibration of eOH. Compared with the original muscovite without adsorption of carboxylic acids, there are no any new peaks appearing or disappearing during adsorption of carboxylic acids, which indicates that adsorption of carboxylic acid on muscovite surface belongs to outer-sphere adsorption rather than innersphere adsorption, and adsorption of carboxylic acid happens mainly through hydrogen bond. Based on moleculor strutural diagrams of carboxylic acids in Fig. 1, there are three types of funtional groups to povide H atom for hydrogen bond formation during adsorption of carboxylic acid on muscovite surface, namely eCOOH, eCH2 and eCH3, respectively. Which functional group will preferentially adsorb on muscovite surface? To answer this question, adsorption density for each carboxylic acid with different funtional group for hydrogen bond formation is calculated by MD simulation. Fig. 10 shows the simulated results, through compared with each other, it turns out that density of numbers of carboxylic acid with eCOOH funtional group adsoprtion is obvisouly higher than that with for eCH3 and eCH2 funational group adsorption. This leads to adsorption prefrence beahviour of carboxylic acid on muscovite surface [40]. Furthermore, what should be noticed the difference of the preferential adsorption among −COOH, −CH3 and −CH2 functional
groups become obvious with the carbon chain length increase. As shwon in Fig. 12, the difference of the preferential adsorption is not obvious for formic acid adsorption on muscovite surface, while the difference of the preferential adsorption of the functional group is obvious when butyric acid adsorbs on muscovite surface. According to the MD simulation results, it is found that carboxylic acids adsorb preferentially on muscovite surface through hydrogen bond between H atom of eCOOH functional group of carboxylic acid and O atom of muscovite surface, which will result in higher concentration of eCH3 and eCH2 functional groups on upper part of muscovite surface contacted with water phase. As known, eCOOH functional group is hydrophilic, while eCH3 and eCH2 functional groups are hydrophobic. So contact angles of water on muscovite surface should increase obviously due to the adsorption of carboxylic acids. Fig. 13 shows the measured results for contact angles of water on muscovite surface with and without adsorption of carboxylic acids. The contact angle of water on original muscovite surface is 10.26°, which means that muscovite has affinity with water. After carboxylic acid adsorption, the contact angles are measured as 26.76° with formic acid adsorption, 31.35° with acetic acid adsorption, 27.02°with propionic acid adsorption and 36.14° with butyric acid adsorption, respectively. This measured results are in agreement with MD simulation findings, high concentration of hydrophobic groups eCH3 and eCH2 gathering in upper part of muscovite surface will result in bigger contact angle of water on muscovite surface [41,42]. Contact angle tests validate MD simulation results and confirm the preferable adsorption of carboxylic acids. 4. Conclusions Muscovite is a layered dioctahedral aluminosilicate mineral, and LMW carboxylic acids, such as formic acid, acetic acid, propionic acid and butyric acid, can adsorb spontaneously on muscovite surface through hydrogen bond between H atom of eCOOH functional group of carboxylic acids and O atom of muscovite. Moreover, carboxylic acid with long carbon chain will be adsorbed easier than that with short carbon chain due to the effect of carboxylic acid molecular diffusion on water-muscovite interface. 16
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The hydrophilic functional group eCOOH of carboxylic acids preferably adsorb on muscovite surface, while the hydrophobic functional groups eCH3 and eCH2 of carboxylic acids are far from the muscovite surface. So, the hydrophobicity on muscovite surface increases due to the adsorption of carboxylic acids, which results in the increase of contact angle of water on muscovite surface. With the increase of the alkyl carbon chain length of carboxylic acids, the hydrophobicity on muscovite surface becomes stronger and contact angle becomes bigger due to adsorption of carboxylic acids, which will facilitate the flotation of minerals.
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Acknowledgement The authors wish to acknowledge National Nature Science Foundation of China (No. 21776089, No. U1610102, No. 21376085) and the International S&T Cooperation Program of China (Grant 2016YFE0132500) References [1] O.O. Onireti, C. Lin, J. Qin, Combined effects of low-molecular-weight organic acids on mobilization of arsenic and lead from multi-contaminated soils, Chemosphere 170 (2017) 161–168. [2] J. Li, R. Xu, D. Tiwari, G. Ji, Effect of low-molecular-weight organic acids on the distribution of mobilized al between soil solution and solid phase, Appl. Geochem. 21 (2006) 1750–1759. [3] D.L. Jones, P.G. Dennis, A.G. Owen, P.A. VanHees, Organic acid behavior in soils – misconceptions and knowledge gaps, Plant. Soil 248 (2003) 31–41. [4] B.W. Strobel, Influence of vegetation on low-molecular-weight carboxylic acids in soil solution—a review, Geoderma 99 (2001) 169–198. [5] H. Zhang, X. Lan, P. Bai, X. Guo, Adsorptive removal of acetic acid from water with metal-organic frameworks, Chem. Eng. Res. Des. 111 (2016) 127–137. [6] M.P. Zacharof, S.J. Mandale, P.M. Williams, R.W. Lovitt, Nanofiltration of treated digested agricultural wastewater for recovery of carboxylic acids, J. Clean. Prod. 112 (2016) 4749–4761. [7] B. Mohite, Isolation and characterization of indole acetic acid (iaa) producing bacteria from rhizospheric soil and its effect on plant growth, J. Soil. Sci. Plant. Nut. 13 (2013) 638–649. [8] J.I. Drever, L.L. Stillings, The role of organic acids in mineral weathering, Colloid. Surf. A 120 (1997) 167–181. [9] W. Wei, X. Zhang, J. Cui, Z.G. Wei, Interaction between low molecular weight organic acids and hydroxyapatite with different degrees of crystallinity, Colloid Surf. A 392 (2011) 67–75. [10] M.H. Feng, B.T. Ngwenya, L. Wang, W. Li, V. Olive, R.M. Ellam, Bacterial dissolution of fluorapatite as a possible source of elevated dissolved phosphate in the environment, Geochim. Cosmochim. Acta 75 (2011) 5785–5796. [11] D.E. Lazo, L.G. Dyer, R.D. Alorro, Silicate, phosphate and carbonate mineral dissolution behavior in the presence of organic acids: a review, Miner. Eng. 100 (2017) 115–123. [12] Z. Balogh-Brunstad, C.K. Keller, J.T. Dickinson, F. Stevens, C.Y. Li, B.T. Bormann, Biotite weathering and nutrient uptake by ectomycorrhizal fungus, suillus` tomentosus, in liquid-culture experiments, Geochim. Cosmochim. Acta 72 (2008) 2601–2618. [13] L.Y. Xia, B. Hart, K. Douglas, The role of citric acid in the flotation separation of rare earth from the silicates, Miner. Eng. 74 (2015) 123–129. [14] X. Liu, G.Y. Huang, C.X. Li, R.J. Cheng, Depressive effect of oxalic acid on titanaugite during ilmenite flotation, Miner. Eng. 79 (2015) 62–67. [15] A. Martínez-Luévanos, M.G. Rodríguez-Delgado, A. Uribe-Salas, F.R. CarrilloPedroza, J.G. Osuna-Alarcón, Leaching kinetics of iron from low grade kaolin by oxalic acid solutions, Appl. Clay. Sci. 51 (2011) 473–477. [16] W. Astuti, T. Hirajima, K. Sasaki, N. Okibe, Comparison of effectiveness of citric acid and other acids in leaching of low-grade Indonesian saprolitic ores, Miner. Eng. 85 (2016) 1–16. [17] R. Larba, I. Boukerche, N. Alane, N. Habbache, S. Djerad, L. Tifouti, Citric acid as an alternative lixiviant for zinc oxide dissolution, Hydrometallurgy 134 (2013) 117–123. [18] S.B. Johnson, T.H. Yoon, B.D. Kocar, Gordon E. Brown, Adsorption of organic matter at mineral/water interfaces. 2. Outer-sphere adsorption of maleate and
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Contents lists available at ScienceDirect
Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Interaction between low molecular weight carboxylic acids and muscovite: Molecular dynamic simulation and experiment study ⁎
Xue Xiaopenga,b, Xu Zhonghaoa,b, Israel Pedruzzia,b, Li Pinga,b, , Yu Jianguoa,b, a b
T
⁎
State Key Laboratory of Chemical Engineering, School of Chemical Engineering, East China University of Science and Technology, Shanghai, 200237, China National Engineering Research Center for Integrated Utilization of Salt Lake, East China University of Science and Technology, Shanghai, 200237, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Muscovite Carboxylic acid Interface phenomena Molecular dynamic simulation Adsorption Contact angle
Interface phenomena between low molecular weight (LMW) carboxylic acids and muscovite was investigated through molecular dynamic simulation and experiment, where the typical monocarboxylic acids including formic acid (C1), acetic acid (C2), propionic acid (C3) and butyric acid (C4) were used as models. Density distribution, adsorption energy, root mean square dynamic (RMSD) of carboxylic acids on water-muscovite interface were calculated through molecular dynamic simulation, and the advanced characterization methods, such as ATR-FTIR spectra, AFM images and contact angle were performed to test and verify the relative simulation findings. The molecular simulation showed that carboxylic acids adsorbed on surface of muscovite through hydrogen bond between H atom of eCOOH functional group of carboxylic acid and O atom of muscovite, belong to outer sphere adsorption, and ATR-FTIR spectra and AFM images confirmed this finding. Adsorption energy for long carbon chain carboxylic acid (C4) was higher than that for short carbon chain carboxylic acid (C1, C2 and C3) due to the effect of carboxylic acid diffusion on water-muscovite interface. The hydrophilic functional group eCOOH of carboxylic acids preferably adsorbed on muscovite surface, while the hydrophobic functional groups eCH3 and eCH2 of carboxylic acids were far from the muscovite surface. So, the hydrophobicity on muscovite surface increased due to the adsorption of carboxylic acids, which resulted in the increase of contact angle of water on muscovite surface.
⁎ Corresponding authors at: State Key Laboratory of Chemical Engineering, School of Chemical Engineering, East China University of Science and Technology, Shanghai, 200237, China. E-mail addresses: [email protected] (P. Li), [email protected] (J. Yu).
https://doi.org/10.1016/j.colsurfa.2018.09.033 Received 8 July 2018; Received in revised form 11 September 2018; Accepted 13 September 2018 Available online 14 September 2018 0927-7757/ © 2018 Elsevier B.V. All rights reserved.
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1. Introduction
images and contact angle. These research would provide significant information for hydrometallurgy, flotation and geochemical science.
Low molecular weight (LMW) carboxylic acids are most common organic matters in natural world, especially in soil [1–3], where the concentration of aliphatic LMW carboxylic acids has reached the range of 0.1–1.0 mM [4]. There are various sources to accumulate LMW carboxylic acids in the natural world. Wastewaters from biological fermentation process or chemical industries contain a certain amount of LMW carboxylic acids [5,6], sometimes which are seeped into soil to result in the accumulation of LMW carboxylic acids. Degradation of organic matters from dead plants and animals would release LMW carboxylic acids to accumulate slowly in soil. In addition, strong activities of various microorganisms (bacteria and fungi) would also lead to enrichment of different LMW carboxylic acids in soil [3,4,7]. In the natural world, there exists also a large amount of minerals, such as feldspar, mica, apatite, pyrite etc, so the interaction between the enriched LMW carboxylic acids and minerals occurs frequently. Moreover, the interaction would promote the release of valuable elements from these minerals. For example, the carboxylic acids promoted dissolution of P bearing mineral [8–10] and K bearing mineral [11,12] to release the nutrient elements of K and P as fertilities for plant growth. On the other hand, LMW carboxylic acids could react and complex with many minerals to release toxic metals (Cd2+, Al3+, Pb2+, Cu2+) into soil, which resulted in heavy metals pollutions. Moreover, in the view point of engineering, interaction between LMW carboxylic acids and minerals also plays a key role in hydrometallurgy, for example, LMW carboxylic acid could be applied as depressant in mineral flotation [13,14] and as extraction solvent [15–17]. Therefore, the investigation on the interaction between LMW carboxylic acids and minerals is more significant. Generally, interaction mechanism between organic acids and minerals is divided into outer-sphere adsorption and inner-sphere adsorption. Outer-sphere adsorption is organic matter adsorbed on mineral surface by physical electrostatic force, Van der Waals force and hydrogen bond through O (F, N, S or C) and H atoms [18,19], and adsorption energy is lower due to weak interaction force. On the contrary, inner-sphere adsorption is organic matter adsorbed on mineral surface through organic ligands (mainly COOe) combined directly with metal elements (Ca, Fe, Al, Mg) [20–22], and adsorption energy is higher due to chemical reaction and formation of organic ligand-metal complex. With the development of molecular simulation and advanced characterization methods, the interaction between organic acids and minerals can be explored at the molecular or atomic level. For example, Based on the molecular dynamic simulation, it has found that the mixed organic agents (oleic acid and octadecanoic acid) with good performance of muscovite flotation [23–25]. Through adsorption energy calculations and adsorption behavior analysis, organic acid mechanism was be figured out exactly. At the same time, many advanced instruments (ATR-FTIR and AFM) have been frequently used to validate the molecular simulation results. AFM detection has been adopted to validate if organic acid is adsorbed visibly [26,27], while ATR-FTIR detection has been used to distinguish outer-sphere adsorption and innersphere adsorption [28–30]. Muscovite with KAl2(Si3Al)O10(OH)2 formula, is a layered dioctahedral aluminosilicate mineral, the most common mineral in soil. The layer phyllosilicate consists of two tetrahedral (T) sheets with an octahedral (O) sheet in between (TOT structure). The octahedral unit is linked to the other units via shared octahedral edges. Due to a large amount of muscovite distributed in soil, interaction between LMW carboxylic acids and muscovite can’t be avoidable. Moreover, the release of K element from muscovite by adsorption and dissolution of organic acids could be used as fertilizer for plant growth. Therefore, in this paper, the interaction of muscovite with four kinds of LMW monocarboxylic acids, such as formic acid (C1), acetic acid (C2), propionic acid (C3) and butyric acid (C4), were investigated through molecular dynamic simulation combined with the advanced characterization methods, such as ATR-FTIR spectra, AFM
2. Theoretical and experimental sections 2.1. Theoretical section 2.1.1. Molecular structures of carboxylic acids, muscovite and adsorption models Molecular dynamic simulation was performed in Material Studio 6.0 software (Accelrys, USA). Four kinds of monocarboxylic acids were chosen as models, formic acid (HCOOH, FA), acetic acid (CH3COOH, AA), propionic acid (CH3CH2COOH, PA) and butyric acid (CH3CH2CH2COOH, BA). Molecular structures of carboxylic acids were firstly optimized in Dmol3 module based on DFT methods to obtain accurate structure and charge assignments. Generalized gradient approximation (GGA) with the Perdew–Burke − Ernzerhof (PBE) [31] of the exchange − correlation functional and basis set DNP 3.5 were applied. Convergence criteria were SCF tolerance 1 × 10−6, energy change 1 × 10-5Ha, max force 0.002Ha/ Å and max displacement 0.005 Å [32]. Molecular structures of water and muscovite were built and optimized as same described methods. Final optimized models were shown in Fig. 1. Crystal lattice parameters of unit of muscovite were: a = 5.187 Å; b = 8.995 Å; c = 19.502 Å; α = 90.00°；β = 95.87°; γ = 90.00°. Adsorption model of carboxylic acids on muscovite surface were built, as shown in Fig. 2, where a box containing 50 carboxylic acid molecules and 300 water molecules was placed between two muscovite super cells, and facet (0 0 1) of muscovite was cleaved and expanded to a 5*3*1 super cell. 2.1.2. Molecular dynamic simulation methods Before carrying out molecular dynamic simulation, adsorption models built in Fig. 2 should be optimized geometrically to obtain stable initial models in Forcite module. Universal force field (UFF) was applied in MD simulation. UFF had been proved to be suitable for organic molecules–mineral interface system [25,33]. After geometrical optimization, adsorption models were carried out MD simulation at 298.15 K for 1.2 ns in NVT ensemble (time step: 1 fs). Only NVT ensemble was adopted because initial structures were very stable and muscovite surface were frozen (NPT can’t be running when atoms were frozen in Forcite module). Besides, thermostat was velocity scale by 10 K and energy deviation was 50,000.0Kcal/mol. Electrostatic was described by Ewald methods while van der waals was described by atom based method. Three important parameters including density of numbers, adsorption energy and Root mean square dynamic (RMSD) were calculated in MD simulation to evaluate interaction between carboxylic acids and muscovite. Adsorption energy Ea was calculated by the following equation [34]:
Ea =
E1 −E2−E3 + E4 N
Where E1 was the total energy of adsorption models after MD simulations; E2 stood for energy of adsorption models excluding carboxylic acid molecules; E3 designated energy of adsorption models excluding muscovite super cells; E4 demonstrated total energy of water molecules; N was number of carboxylic acid molecules, respectively. Density of numbers was defined as following equation [35]:
ρz =
M × Nz − δz , z + δz 2
2
δz × S
Where pz was density of numbers of carboxylic acid; M stood for atom mass; N(z-δz/2, z+δz/2) designated average numbers of occurring in interval δz; S was basal surface area. Density of numbers could be used to analyze density of carboxylic acid in a specific position. If density of carboxylic acid was particularly higher near muscovite surface than other places, carboxylic acid could be adsorbed. 9
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Fig. 1. Optimized molecular structures of carboxylic acids, water and muscovite.
Fig. 2. Adsorption models of carboxylic acids on muscovite surface (green sticks were water molecules). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
2.2. Experimental section
Root mean square displacement (RMSD) can be used to determine the adsorption dynamics of carboxylic acid on water-muscovite interface and it was defined as following equations [36]:
Pure muscovite sheets were purchased from Sinopharm Chemical Reagent Company (China). Rough muscovite sheets were cut into 8 mm × 8 mm and removed by adhesive tape to expose new (0 0 1) surface which was cleaned by alcohol and water. When muscovite sheet was dried, they were mixed with 100 ml 0.1 mol/L different carboxylic acids (AR, > 99.5%) into plastic bottles. Then plastic bottles were put into a shaker at 25 °C and 180 rpm for 2 h. After interactions, muscovite sheets were dried at 25 °C in a vacuum drying oven naturally. Dried
N
RMSD =
1 ∑ [ri (t )−ri (0)]2 N i
Where N was total number of carboxylic acid molecules; ri(t) and ri(0) stood for position of carboxylic acid at time t and initial position. 10
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Fig. 3. Density of numbers of carboxylic acid adsorption on muscovite surface at initial state before MD simulation and at equilibrium state after MD simulation.
automatically optimized [27]. Furthermore, contact angle of muscovite sheets were measured by JC2000DE (Shanghai zhongchen, China). One drop of water was dropped into muscovite sheet, after 30 s, contact angle was collected by CCD camera
muscovite sheets were analyzed by ATR-FTIR (Nicolet, 6700, USA). All spectra were obtained using KBr pellets at 25 °C with a resolution 256 scans and 4cm−1. Besides, muscovite sheets were observed by AFM (Vecco/DI, USA) with peak force mode at 25 °C. Scan rate were 11
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Fig. 4. Density of water molecules on muscovite surface in the presence and absence of carboxylic acids.
Fig. 6. RMSD curve of carboxylic acid on water-muscovite interface during MD simulation.
MD simulation, density distribution curves of carboxylic acids between two muscovite surfaces are calculated, and the simulated results are shown in Fig. 3. It is found that density of carboxylic acid becomes obviously higher when compared with the initial random distribution curve near muscovite surface. Density distribution curve of carboxylic acid looks like “M” shape, that is to say the concentration of carboxylic acid on muscovite surface higher than that in the middle part, which indicates that carboxylic acids can adsorb on muscovite surface. Besides, it can be seen that the high density distributions for four carboxylic acids on muscovite surface are approximate 2.5–3.2 Å−3. Because the surface properties of minerals would be different in the presence and absence of water [37,38], the presence of water will change adsorption behaviors of carboxylic acids on muscovite surface. Fig. 4 shows density of water molecules on muscovite surface in the presence and absence of carboxylic acids. Density of water molecules decreases obviously when carboxylic acids adsorb competitively on muscovite surface. The adsorption ability of carboxylic acid is stronger than that of water. In order to study whether four carboxylic acids adsorption on muscovite surface is spontaneous or not, the free energy △G is calculated. According to the literature [39], △G is the minimum value of W (r), where W(r) is the potential mean force (PMF), describing the free energy surface along the chosen coordinate, J/mol. W(r)=-RTlog(nz), where R is thermodynamic constant, 8.3145 J/mol K, T is temperature of adsorption system, K, nz is distribution density of adsorption matter along Z axis. Fig. 5 demonstrates the calculated W(r) along Z axis for four carboxylic acids interaction on muscovite, where the minimum value of W(r) can be found and then △G value is obtained. △G1 of formic acid adsorption on muscovite surface is −0.96 kJ/mol, △G2 of acetic acid adsorption on muscovite surface is −1.16 kJ /mol, △G3 of propionic acid adsorption on muscovite surface is −1.13 kJ /mol and △G4 of butyric acid adsorption on muscovite surface is −1.19 kJ /mol. Since △G1-4 < 0, it can be concluded that adsorption of four carboxylic acids on muscovite surface is spontaneous totally.
Fig. 5. Calculated W(r) values for four carboxylic acids adsorption on muscovite surface.
3. Results and discussions 3.1. Adsorption of carboxylic acids on muscovite surface Adsorption behavior of four carboxylic acids, including formic acid (C1), acetic acid (C2), propionic acid (C3) and butyric acid (C4), on muscovite (0 0 1) surface is investigated by molecular dynamic simulation. The adsorption model used for MD simulation contains 300 water molecules and 30 carboxylic acid molecules between two muscovite surfaces in a box or a unit for MD simulation. Before MD simulation, distribution of carboxylic acid in water phase is random as shown in Fig. 2. After 1.2 ns
Table 1 Adsorption energies of carboxylic acids on muscovite surface. Adsorption system
Ea/kJ mol−1
E1/kJ mol−1
E2/kJ mol−1
E3/kJ mol−1
E4/kJ mol−1
Formic-muscovite Acetic- muscovite Propionic-muscovite Butyric-muscovite
−9.49 −17.70 −16.95 −19.19
−610806.55 −616568.77 −631949.12 −702200.83
−607291.13 −605483.66 −627917.09 −696741.29
−3952.40 −11101.70 −4010.46 −5173.08
−911.53 −901.50 −825.72 −673.19
12
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Fig. 7. AFM images for adsorption phenomena of carboxylic acids on muscovite surface. A, original muscovite surface; B, formic acid ; C, acetic acid; D, propionic acid; E, butyric acid.
Fig. 8. AFM images with 3D for adsorption of carboxylic acids on muscovite surface；A, original muscovite surface; B, formic acid ; C, acetic acid; D, propionic acid; E, butyric acid.
Generally, adsorption energy is used to investigate interaction between carboxylic acid and muscovite. Table 1 lists adsorption energies of four carboxylic acids on muscovite surface calculated by MD
simulation. Adsorption energies of four carboxylic acids on muscovite surface are −9.49 kJ/mol for formic acid, −17.70 kJ/mol for acetic acid, −16.95 kJ/mol for propionic acid and −19.19 kJ/mol for butyric 13
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Fig. 9. Schematic diagram illustrating interaction between the functional groups of propionic acid and active sites on muscovite surface.
Fig. 10. Adsorption of carboxylic acid on muscovite surface at equilibrium state. A, formic acid ; B, acetic acid; C, propionic acid; D, butyric acid.
dynamics of carboxylic acid on muscovite. The rank of RMSD for four carboxylic acids is formic acid (C1) > propionic acid(C3) > acetic acid (C2) > butyric acid (C4). The rank of RMSD is in opposite with the rank of the absolute value of adsorption energy for four carboxylic acids. For LMW carboxylic acid adsorption, the dynamic diffusion of carboxylic acid in water phase will has negative effect on the adsorption behaviors on muscovite, for example, formic acid would prefer moving in water phase rather than being adsorbed on muscovite surface [36]. RMSD of butyric acid is much lower, as a result, butyric acid is inclined to be adsorbed on muscovite surface instead of in water phase. Lower RMSD
acid, respectively. These adsorption energies are negative, which means that four carboxylic acids can adsorb on muscovite surface. From Table 1, it is found that the absolute value of adsorption energy increases with the increase of carbon chain length of carboxylic acid, which means the adsorption of carboxylic acid with long carbon chain becomes easier than that with short carbon chain. Moreover, it is noticed that the absolute value of adsorption energy of propionic acid (C3) is lower than that of acetic acid (C2). And then, RMSD value for each carboxylic acid adsorption is calculated by MD simulation, as shown in Fig.6. RMSD value is frequently used to evaluate adsorption 14
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surface due to carboxylic acid adsorption become bigger with the increase of carbon numbers, especially when comparing formic acid adsorption with butyric acid adsorption. However, what should be noticed the coverage area of propionic acid on muscovite is lower than that of acetic acid. It also is in agreement with the above mentioned explanation for adsorption energy and RMSD values. Furthermore, 3D vertical AFM images of adsorption of four carboxylic acids are shown in Fig. 8. From Fig. 8, it is found that adsorption layer thickness of formic acid on muscovite surface is about 8.8 nm (> 2.818 Å), adsorption layer thickness of acetic acid is about 7.7 nm (> 3.423 Å), propionic acid is about 12.2 nm (> 4.941 Å) and adsorption layer thickness of butyric acid is about 2.9 nm (> 5.868 Å). The adsorption layer thicknesses for four carboxylic acids are all higher than maximum length of their molecules. Compared to original height of muscovite (0.4 nm), it can deduced the adsorption of carboxylic acids on muscovite surface is multilayer. 3.2. Interface phenomena between carboxylic acid and muscovite Fig. 11. ATR-FTIR spectra of adsorption of carboxylic acids on muscovite surface.
Both MD simulation result and AFM detected result demonstrate that carboxylic acids adsorb on muscovite surface through hydrogen bond. However, there are three candidate adsorption sites on muscovite surface, Al site, Si site and O site, and carboxylic acid has three candidate functional groups to provide H atom for hydrogen bond formation, namely eCOOH, eCH3 and eCH2 functional groups. Fig. 9 gives a schematic diagram illustrating interaction between the functional groups of carboxylic acid and active sites on muscovite surface, where propionic acid is taken as example. Where and how carboxylic acids are adsorbed on muscovite surface? Fig. 10 demonstrates the molecular structural diagrams of carboxylic acid adsorption on muscovite surface at equilibrium state obtained by MD simulation. According to the MD simulation data, the distance between H atom of eCOOH functional group and O atom of muscovite surface is calculated as 2.509 Å for formic acid, 2.860 Å for
value will result in higher adsorption energy that gives an understanding why adsorption energy of propionic acid lower than acetic acid. Through MD simulation, it is found that four carboxylic acids can adsorb on muscovite surface. Here, AFM detector is adopted to observe the adsorption phenomena of carboxylic acids on muscovite surface, and the detected AFM images are shown in Fig. 7. The original surface of muscovite without the adsorption of carboxylic acids is smooth, as shown in Fig. 7a. After 2 h adsorption experiment, the muscovite surface becomes not smooth, part of the surface is covered with the adsorbed carboxylic acids, as shown in Fig. 7b–e. According to the detected AFM images, it is found that the coverage area on muscovite
Fig. 12. Density of numbers of carboxylic acid with different functional group adsorption on muscovite surface. A, formic acid ; B, acetic acid; C, propionic acid; D, butyric acid. 15
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Fig. 13. Contact angles of water on muscovite surface with and without carboxylic acid adsorption. A, original muscovite; B, with formic acid adsorption; C, with acetic acid adsorption; D, with propionic acid adsorption E, with butyric acid adsorption.
acetic acid, 2.595 Å for propionic acid and 2.816 Å for butyric acid, respectively. At the same time, the bond angle of O⋯H⋯O is also calculated as 124°for formic acid, 123°for acetic acid, 141°for propionic acid and 150°for butyric acid, respectively. According to the literature [39], the hydrogen bond length is normally in the range of 2.5 Å to 3.0 Å and its bond angle is bigger than 90°. The calculated results from Fig. 10 satisfy the conditions of hydrogen bond formation, so it is deduced that carboxylic acids adsorb on muscovite surface through hydrogen bond. To verify the MD simulation findings, ATR-FTIR spectra detection is carried out to test if carboxylic acids are adsorbed through hydrogen bond, belong to outer-sphere adsorption. Fig. 11 shows the detected ATR-FTIR spectra after adsorption of carboxylic acids on muscovite surface. There exist three main frequencies which reflect crystal structure of muscovite, frequency at 748 cm−1 due to vibration of AleO bond, frequency at 1026cmn−1 due to vibration of SieO bond, and frequency at 3624 cm−1 due to vibration of eOH. Compared with the original muscovite without adsorption of carboxylic acids, there are no any new peaks appearing or disappearing during adsorption of carboxylic acids, which indicates that adsorption of carboxylic acid on muscovite surface belongs to outer-sphere adsorption rather than innersphere adsorption, and adsorption of carboxylic acid happens mainly through hydrogen bond. Based on moleculor strutural diagrams of carboxylic acids in Fig. 1, there are three types of funtional groups to povide H atom for hydrogen bond formation during adsorption of carboxylic acid on muscovite surface, namely eCOOH, eCH2 and eCH3, respectively. Which functional group will preferentially adsorb on muscovite surface? To answer this question, adsorption density for each carboxylic acid with different funtional group for hydrogen bond formation is calculated by MD simulation. Fig. 10 shows the simulated results, through compared with each other, it turns out that density of numbers of carboxylic acid with eCOOH funtional group adsoprtion is obvisouly higher than that with for eCH3 and eCH2 funational group adsorption. This leads to adsorption prefrence beahviour of carboxylic acid on muscovite surface [40]. Furthermore, what should be noticed the difference of the preferential adsorption among −COOH, −CH3 and −CH2 functional
groups become obvious with the carbon chain length increase. As shwon in Fig. 12, the difference of the preferential adsorption is not obvious for formic acid adsorption on muscovite surface, while the difference of the preferential adsorption of the functional group is obvious when butyric acid adsorbs on muscovite surface. According to the MD simulation results, it is found that carboxylic acids adsorb preferentially on muscovite surface through hydrogen bond between H atom of eCOOH functional group of carboxylic acid and O atom of muscovite surface, which will result in higher concentration of eCH3 and eCH2 functional groups on upper part of muscovite surface contacted with water phase. As known, eCOOH functional group is hydrophilic, while eCH3 and eCH2 functional groups are hydrophobic. So contact angles of water on muscovite surface should increase obviously due to the adsorption of carboxylic acids. Fig. 13 shows the measured results for contact angles of water on muscovite surface with and without adsorption of carboxylic acids. The contact angle of water on original muscovite surface is 10.26°, which means that muscovite has affinity with water. After carboxylic acid adsorption, the contact angles are measured as 26.76° with formic acid adsorption, 31.35° with acetic acid adsorption, 27.02°with propionic acid adsorption and 36.14° with butyric acid adsorption, respectively. This measured results are in agreement with MD simulation findings, high concentration of hydrophobic groups eCH3 and eCH2 gathering in upper part of muscovite surface will result in bigger contact angle of water on muscovite surface [41,42]. Contact angle tests validate MD simulation results and confirm the preferable adsorption of carboxylic acids. 4. Conclusions Muscovite is a layered dioctahedral aluminosilicate mineral, and LMW carboxylic acids, such as formic acid, acetic acid, propionic acid and butyric acid, can adsorb spontaneously on muscovite surface through hydrogen bond between H atom of eCOOH functional group of carboxylic acids and O atom of muscovite. Moreover, carboxylic acid with long carbon chain will be adsorbed easier than that with short carbon chain due to the effect of carboxylic acid molecular diffusion on water-muscovite interface. 16
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The hydrophilic functional group eCOOH of carboxylic acids preferably adsorb on muscovite surface, while the hydrophobic functional groups eCH3 and eCH2 of carboxylic acids are far from the muscovite surface. So, the hydrophobicity on muscovite surface increases due to the adsorption of carboxylic acids, which results in the increase of contact angle of water on muscovite surface. With the increase of the alkyl carbon chain length of carboxylic acids, the hydrophobicity on muscovite surface becomes stronger and contact angle becomes bigger due to adsorption of carboxylic acids, which will facilitate the flotation of minerals.
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