Non-polar oil assisted DDA flotation of quartz II: Effect of different polarity oil components on the flotation of quartz

Accepted Manuscript Non-polar oil assisted DDA flotation of quartz II: Effect of different polarity oil components on the flotation of quartz

An Liu, Min-qiang Fan, Zhi-hong Li, Jin-chuan Fan PII: DOI: Reference:

S0301-7516(17)30186-2 doi: 10.1016/j.minpro.2017.09.003 MINPRO 3089

To appear in:

International Journal of Mineral Processing

Received date: Revised date: Accepted date:

3 February 2017 3 September 2017 7 September 2017

Please cite this article as: An Liu, Min-qiang Fan, Zhi-hong Li, Jin-chuan Fan , Nonpolar oil assisted DDA flotation of quartz II: Effect of different polarity oil components on the flotation of quartz, International Journal of Mineral Processing (2017), doi: 10.1016/ j.minpro.2017.09.003

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ACCEPTED MANUSCRIPT Non-polar oil assisted DDA flotation of quartz II: Effect of different polarity oil components on the flotation of quartz

An Liu a, *, Min-qiang Fan a, b, *, Zhi-hong Li a, Jin-chuan Fan b,

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a. College of Mining Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China

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b. College of Chemistry and Chemical Engineering, Taiyuan University of

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Technology, Taiyuan 030024, Shanxi, China

*Corresponding author. Tel.: +86 351 6014776.

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E-mail address: [email protected]

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Abstract

It is well accepted that non-polar oils can be used as collector extender in oil agglomeration flotation of many different types of mineral. In this paper, in order to

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study the influences of oil structures on the non-polar oil assisted dodecylamine

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(DDA) flotation of quartz, the research focuses on the interaction of DDA (collector) and non-polar oil (collector extender) in aqueous phase. In this work, saturated

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hydrocarbon dodecane and cyclohexane, unsaturated hydrocarbon dodecene, aromatic hydrocarbon dimethylbenzene and 1-methylnaphthalene were chosen as researched

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subjects, and the influences of oil structures on interfacial activity of oil–DDA–water ternary models were investigated by experimental and theoretical methods. The flotation results demonstrated that the collecting capability of the oil and DDA combination follows the order of 1-methylnaphthalene > dimethylbenzene > dodecene > cyclohexane > dodecane. Moreover, molecular dynamic (MD) simulation is performed to investigate the interfacial property of these five non-polar oil-DDA-water systems, and five parameters, radial distribution functions, interaction energy, density distribution, interfacial thickness and self-diffusion coefficient are proposed to reveal the influence mechanism of molecular structure on interfacial

ACCEPTED MANUSCRIPT activity of the ternary oil-DDA-water systems. The same trends are obtained from the parameters

described

above,

aromatic

hydrocarbon

dimethylbenzene

and

1-methylnaphthalene performance better than saturate hydrocarbon and unsaturated hydrocarbon. The researched results indicate the interaction between non-polar oil and DDA hydrophobic alkyl tail would enhance the mobility of oil component, which

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induce significant change of interfacial thickness and diffusivity. Additionally, the interaction between DDA hydrophilic head group and water molecules via strong

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hydrogen bonds, van der Waals interactions and weak electrostatic attraction would

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lead to water molecules penetrate through DDA monolayer and adsorb on oil surface. The researches enable us to obtain a deeper microscopic-level understanding of the

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interaction between oil-DDA-water ternary components, and maybe have some

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references for designing of oil-assisted flotation agent.

Keywords: oil-DDA-water ternary models; MD simulation; interfacial activity; oil

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1. Introduction

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structure

It is well accepted that oil agglomeration flotation, in which hydrophobic fine

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mineral particles aggregate together derived from the hydrophobic interaction of bridging liquid (non-polar oils), has recently attracted increasing attention. It is well

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accepted that fine mineral particles flotation can be dramatically improved by adding non-polar oil. The history of the addition of non-polar oil to the mineral fines applied in mineral processing can be traced back to 70 years ago, going as far back as 1950 when oil agglomeration flotation was developed to process manganese (Gates,1957; Runolinna, et al., 1960), ilmenite (Li, et al., 1960) and hematite (Mehrotra, et al., 1983) ores in fine size ranges. Recently, this technique has been widely used for the coal preparation from sulphur and ashes (Laskowski and Yu, 2000; Gray, et al., 2001; Aktas, 2002;

Alonso, et al., 2002; Cebeci and Sönmez, 2002; Cebeci and Sönmez,

2006; Sahinoglu and Uslu, 2002; Moses and Petersen, 2000), purification of fine

ACCEPTED MANUSCRIPT grained gold (Sen, 2005; Valderrama and Rubio, 2008; Sönmez and Cebeci, 2003a), agglomeration of oxide such as barite calcite (Sönmez and Cebeci, 2004a; Sönmez and Cebeci, 2003b; Sönmez and Cebeci, 2004b; Sönmez and Cebeci, 2004c) and celestite (Hwang, et al., 2008), and enhancing the utilization of waste-derived (Azevedo and Miller, 2000) and deinking of paper (Capes and Germain,1982).

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Up to now, the fundamental principles of oil agglomeration and effects of various operating parameters have been detailed researched by many investigators from

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different perspectives. Theoretical researches as well as experimental studies

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demonstrate that the various phases such as oil physical property (density and viscosity), oil concentration, and operating parameters (stirring time, intensity and

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flotation medium pH) greatly influence the oil agglomeration flotation process (Laskowski, 1992; Chary and Dastidar, 2010; Ozkan and Duzyol, 2014; Capes, et al.,

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1977; Ozkana et al., 2005). Previous research shows that only medium-density oils are appropriate for the oil agglomeration flotation process, for instance, kerosene and

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diesel. This is because lower density oils are too fluid to form hydrophobic

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agglomeration together, meanwhile, high density oils are too viscous to be sufficiently dispersed and cause agglomeration, respectively (Laskowski, 1992; Chary and Dastidar, 2010). According to Capes and Germain et al., oil concentration is also a

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significant factor in the oil agglomeration flotation process. On the one hand, this factor can affect the size of aggregates; on the other hand, it also can influence the

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agglomeration structure. They claimed that the aggregates only exist in the form of compact pellets at high oil concentration level, unconsolidated flocs at lower level of oil addition on the contrary, while micro-agglomerates at medium oil concentration level (Laskowski, 1992, Capes, et al., 1977). It is well known that oil agglomerate flotation process can be divided into a combination of an oil agglomeration operation and a froth flotation process. Basic researches have revealed that the improvement of non-polar oil to flotation process is originated from two aspects. On the one hand, the formation oil films on hydrophobic

ACCEPTED MANUSCRIPT particles can greatly increase the hydrophobization degree of mineral fines; on the other hand, the promotion by bridging the particles can cause compact aggregation in the aqueous (Chary and Dastidar, 2010; Schubert, 1979). Obviously, the premise behind this enhancement is mineral particle and oil drop spontaneous attachment. Whether the spontaneous attachment would occur depends on the total interaction

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potential energy of mineral particle and oil drop system (Dai and Lu, 1991). It is well known that the interaction between oil drops and mineral particles was mainly due to

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hydrophobic interaction for traditional oil agglomeration flotation (Dai and Lu, 1991;

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Glembotskii, 1981).

In our previous work, an improved version of oil agglomeration flotation processes

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has been proposed, which is based on introduction of the premixed combination of collector (DDA) and oil (kerosene) as mixture collector (Liu, et al., 2014; Liu, et al.,

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2016). Under this condition, spontaneous attachment would happen due to both the hydrophobic interaction and contribution of electric double layer attractive interaction.

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This is attributed to the adsorption of DDA on the oil/water interface, changing the

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zeta potential value of oil drop from negative to positive, which causes the electric double layer attractive interaction. In other words, the core issue is the adsorption behavior of DDA molecule on oil surface in aqueous phase. However, up to present,

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few papers have dealt with the fundamental aspects of the interaction between collector and non-polar oil to enhance oil agglomeration flotation.

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Additionally, it is well known that, in the oil agglomeration flotation process, most of the liquid hydrocarbons, such as kerosene, diesel, soybean and rapeseed oil, and other petroleum derivatives, are usually added industrially as bridging reagents (Laskowski and Yu, 2000; Gray, et al., 2001; Aktas, 2002;

Alonso, et al., 2002;

Cebeci and Sönmez, 2002; Cebeci and Sönmez, 2004b; Ozkan and Duzyol, 2014; Dai and Lu, 1991). Nevertheless, up to date, previous studies often considered the complex oil mixture as only one pseudo material without taking into account the varied molecular sorption behaviour of different oil fraction in this oil mixture. It is

ACCEPTED MANUSCRIPT well accepted that non-polar oil is a complex mixture of thousands of different compounds. For practical purposes, hydrocarbon group type analysis of saturated hydrocarbons, unsaturated and aromatics hydrocarbon fractions is widely used for the flotation of natural hydrophobic mineral and oil agglomerate flotation of ultrafine particles (Somasundaran and Wang, 2006). Although a large number of meaningful

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researches have been conducted from the experimental and theoretical aspects, it was still hard to observe the microscopic agglomeration process and hard to reveal which

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oil fraction was better for the oil agglomeration flotation.

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In order to study the influences of oil structures on the non-polar oil assisted dodecylamine (DDA) flotation of quartz, we are focuses on the interaction of DDA

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and non-polar oil in aqueous phase. Similar to a typical surfactant molecule, an typical ionic collector molecule usually consists of amphiphilic section the ‘‘head’’

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part and ‘‘tail’’ part. The hydrophilic head group possesses a polar functional group and the tail group owns a hydrophobic hydrocarbon chain. Due to the possession of

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the amphiphilicity character, they can easily adsorb at the oil/water interface to form a

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collector molecule monolayer (Somasundaran and Wang, 2006; Taggart, 1945). As a result, the amphiphilic collector can reduce the water/oil interfacial tension and stabilize the interface and would enable them to form various morphologies both at

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the interface and in the bulk phase. Based on this, we can treat DDA as a surfactant molecule in oil agglomeration flotation process, and the interfacial properties such as

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interfacial tension, interfacial thickness, and density distribution of multi-component systems can usually be modified by the DDA addition. Up to date, previous researches indicate that MD simulations are an ideal method to investigate the influence of surfactant to the interfacial properties, that is because on the one hand the interfacial region is too narrow for common experimental techniques to explore, on the other hand the chemical system is too complicated for liquid state theories to handle (Gray and Gubbins, 1984). In the past decades, abundant theoretical researches have been conducted to investigate the interfacial properties of surfactants adsorption

ACCEPTED MANUSCRIPT at water/oil systems based on the DPD simulation [Li, et al., 2011; Luu, et al., 2013], Monte Carlo (MC) simulations (Rekvig, et al., 2003), self-consistent field theory (SCFT) simulation (Ginzburg, et al., 2011) and molecular dynamics (MD) simulation (Ahn, et al., 2011; Hu, et al., 2013). In this paper, in order to illustrate the fundamental problem of the oil agglomeration

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flotation process, we treat DDA as surfactant and proposed a ternary model containing oil component, DDA molecules and water phase. Molecular dynamics simulation was

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adopted to study the interaction between oil and DDA molecules, and to reveal the

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influence of oil structures on interfacial activity of oil-DDA-water ternary systems. Here, dimethylbenzene (A), 1-methylnaphthalene (B), cyclohexane (C) dodecane (D)

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and dodecene (E) are selected as different structural components in non-polar oil as shown in Fig. 1. Dimethylbenzene and 1-methylnaphthalene are listed as aromatics

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molecule, cyclohexane and dodecene are listed as unsaturated hydrocarbon, dodecane is listed as saturated hydrocarbon. Through quantifying the molecular-scale structure,

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dynamic and energetic behavior of the oil-collector-water ternary system, it can be

flotation agent.

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believed that this will be helpful in guiding the screening and design of oil-assisted

2. Materials and methods

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2.1. Samples and reagents

In this work a pure quartz (SiO2) sample from Lingshou (Hebei Province, PR China)

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was used as mineral sample. The volume median diameter (d50) of the mineral sample was 27.6µm, and the specific surface area of the samples was 0.38m2/g, which was obtained by using a Quantachrome Autosorb-1 gas adsorption analyzer based on the BET method. In this study, DDA of analytical purity used was obtained from Tianjin Guangfu Chemical Research Institute, analytically grade dimethylbenzene, cyclohexane, 1-methylnaphthalene, dodecane and dodecene used as non-polar oil was supplied by Aladdin Reagent. The oil characteristics at 20 °C including density, bulk viscosity and

ACCEPTED MANUSCRIPT surface tension of the different oils were summarized in Table 1 (Liu et al., 2013). The premixed DDA and non-polar oil was prepared by dissolving different concentrations of DDA in the oil phase and stirred with a magnetic stirrer at 40 °C until a homogeneous sample was obtained. The DDA and oil emulsion collector was prepared by mixing different weight ratios of oil and DDA mixture (RDDA/oil=0.2, 0.4,

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0.6 and 0.8, and e.g., RDDA/oil=0.2 means the weight ratio of DDA and oil in the mixture collector was 20% and 80% respectively) with water in the concentration of 1

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wt%, followed by agitation for 30 min at the speed of 1500 rev/min.

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2.2. Methods 2.2.1. Flotation tests

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Flotation tests have been carried out in a XFG-1.5Ⅳ flotation machine with 1.5L cell. The stirring speed was set at 1500 rev/min, pulp density was controlled at 20%,

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the temperature of pulp was maintained at 20 °C and the air flow was cell self-aerated. Flotation tests adopted one-stage rough flotation, and the collector conditioning time

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was set at 2 min. After air was introduced into the pulp the flotation process lasts for 5

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min. The froth depth was approximate 2cm. In the flotation tests, different weight ratios of oil and DDA mixture (RDDA/oil =0.2, 0.4, 0.6 and 0.8) were used in flotation, while keeping the total collector dosage constant at 60 g/t. The water used in flotation tests

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was tap water and its pH was 7.6. 2.2.2 Oil droplets size analysis

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The size distribution of oil droplets were determined by using a S3500 particle size analyzer of Microtrac. The oil emulsion was prepared by mixing different weight ratios (RDDA/oil =0.2, 0.4, 0.6 and 0.8) of oils and DDA minxture with water in the concentration of 1 wt%, followed by agitation for 30 min at the speed of 1500 rev/min. The water used in the droplet size measurements was tap water and its pH was 7.6. 2.2.3. Molecular dynamics simulation Based on previous study, the interaction of the DDA at the different oil/water interface is investigated by molecular dynamics (MD) simulation on the basis of

ACCEPTED MANUSCRIPT COMPASS force field. The total potential energy function can be shown as Eq. (1): Etotal = EvdW + EQ + Ebond + Eangle + Etorsion

(1)

where Etotal represents the total energy, EvdW is the van der Waals, and EQ, Ebond, Eangle, and Etorsion are electrostatic, bond-stretching, angle-bending, and torsion-energy components, respectively (Xu, et al., 2013).

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2.2.3.1. Model. The built model was composed of collector layer, aqueous and oil box utilized the

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Amorphous Cell module, and the specific structure parameter of different systems was

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shown in Table S1. In order to describe the surface charge effect, based on collector ionization effect, the collector was described as protonation species DDA+ as

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previously discussed (Liu, et al., 2014; Liu, et al., 2015). Therefore, DDA are described as C12NH3+ in the simulation, for the purpose to ensure the system was

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electrically neutral; the system was neutralized by the addition of corresponding number of chloride ions.

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The simulation model systems consist of DDA, oil and water phases (Fig.2),

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possessing two DDA-dodecane interfaces, which have been widely used for the MD simulation of liquid-liquid interfaces.(Li, et al., 2011; Rekvig, et al., 2003, Ginzburg, et al., 2011; Ahn, et al., 2011). In order to construct this configuration, firstly, a

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monolayer consisting of 60 DDA molecules were prepared on an assumption of the head group (-CH2NH3+) under the ordered arrangement closed packing in an

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orthorhombic simulation box with the periodic boundary condition applied to all three spatial directions. Secondly, for relaxing this surfactant monolayer, an energy minimization module with the fixed cell dimensions (Lx, Ly, and Lz) were carried out. And then, the separate oil and water phases were prepared by means of the NVT module simulations on the basis of the experimental densities (at 300 K and 1 atm, 1.003 g/cm3 for 1-methylnaphthalene, 0.858 g/cm3 for dimethylbenzene, 0.765 g/cm3 for dodecene, 0.778 g/cm3 for cyclohexane, 0.725 g/cm3 for dodecane, and 0.997 g/cm3 for water) (Liu et al., 2013). Especially, the DDA molecules are placed with

ACCEPTED MANUSCRIPT favorable orientations all headgroups toward to the water phase, and the cell parameters of the simulation box were set to have the same Lx, and Ly dimensions. Finally, these three phases were integrated into one simulation box, and the oil and water phases set to having two interfaces perpendicular to the z-direction. In order to compare the following parameters: radial distribution functions, density distribution,

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interfacial thickness and self-diffusion coefficient, the cell parameters of the simulation box were set to have the similar Lz dimension as seen in the Table S1.

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Especially, the DDA molecules are placed with favorable orientations all head groups

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toward to the water phase. 2.2.3.2. MD simulation.

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The MD simulation based on COMPASS force field was performed with the Velocity Verlet algorithm (Verlet, 1967) at 298 K with a time step of 1.0 fs. The

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temperature and pressure was controlled by the Andersen thermostat (Buurenvan and Berendsen, 1994) and Berendsen method, respectively (Berendsen et al., 1984). The

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van der Waals interactions were calculated based on the Atom based method with

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cutoff distance of 12.5Å (Gunsteren and Berendsen, 1986). The electrostatic interactions were calculated based on the Ewald method (Gunsteren and Berendsen, 1986). As for the construction of the original configuration, in order to randomize the

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conformations of surfactant hydrocarbon chains, the NPT module followed by NVT module of MD simulations are conducted to obtain the equilibrate configuration.

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Firstly, an anneal progress of 10ps was carried out via NPT module simulation at 350 K, after that the systems’ temperature was controlled to gradually decrease to 298 K in another 10 ps. After the above simulations procedure, all the DDA head-groups and H2O molecules are frozen. And then, in order to overcome local minima derived from imposing thermal energy, and adjust the model to a more realistic density, the DDA headgroups and H2O molecules are all set to unfrozen, a 200 ps NPT module simulation is carried out. After the 200 ps NPT simulation, the model systems become more close to the real situation. Lastly, in order to obtain a good statistics simulation

ACCEPTED MANUSCRIPT data, a 1ns NVT module simulation was conducted for the statistical properties analysis. 3. Results and discussion 3.1 Experiment results 3.1.1 Flotation result

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Metal recovery is a universal index for expressing the effectiveness of single mineral separation. In order to clarify the synergistic effect of the combination

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collector, a comparative test of pure DDA and DDA-oil mixture was carried out. The

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preliminary studies showed that over 70% of quartz floated with 60 g/t DDA. To fully understand the interaction of DDA and oil component, the intermediate reagent

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dosage was selected. It is seen in Fig 3 that the collecting capabilities of the respective collectors follow the trend: 1-methylnaphthalene > dimethylbenzene > dodecene >

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cyclohexane > dodecane, and the recovery was significant higher than corresponding pure DDA (e.g. when the RDDA/oil =0.4, it means the pure DDA dosage is 24 g/t). The

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synergistic effect of DDA and oil was found compared with DDA alone which

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accorded well with Burkin and Soane’s research (Burkin and Soane, 1960). They demonstrated that quartz could float with the simultaneous addition of non-polar oil and DDA, when extremely small amounts of DDA was added, which was insufficient

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to cause the flotation of quartz by itself (Burkin and Soane, 1960). Similar result was found in the flotation of arsenic from the contaminated soils (Choi et al., 2016),

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recovery of fine sized gold from ores (Valderrama and Rubio, 2008) and agglomeration of molybdenite fines (Song et al., 2012; Fu et al., 2012); and the flotation experiments indicated the recovery of desired mineral increased with addition of nonpolar oils (e.g. kerosene, diesel, transformer and rapeseed oil). It should be pointed out that when the RDDA/oil exceeds 40/60, the recovery for quartz exceeds pure DDA (when the DDA dosage is 60 g/t, the quartz recovery is 71.62%). As shown in Fig.3, when the RDDA/1-methylnaphthalene is 0.8, the recovery value approximates to 95%. It is known that non-polar oil can’t collect natural hydrophilic

ACCEPTED MANUSCRIPT minerals; quartz can’t float with pure oil (Burkin and Soane, 1960). Although, synergistic effect of oil and DDA mixture collector was found, the most active ingredient was DDA in the mixture collector, oil component was just promoter. As a result, it can be imaged that the recovery was increasing with increases in the proportion of DDA in the oil-DDA mixture.

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3.1.2 Size distribution analysis of the oil emulsion According to Song’s research, the kerosene enhancement of the flotation closely

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correlated with droplet size of kerosene emulsion; and the smaller were the droplets,

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the better the flotation was (Song et al., 2012). In order to further understand the synergistic effect of oil-DDA emulsion on the flotation of quartz, the size distribution

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analysis of the oil droplets has been studied. Here, the size distribution data of D10, D50, D90 and Dav were exhibited in the Table 2, and the original data was showed in

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the Fig. S1-Fig. S5. As shown in the Table 2, the oil drop size of D10, D50, D90 and Dav of the five oil-DDA mixture emulsion decreases with the order of dodecane <

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cyclohexane < dodecene < dimethylbenzene < 1-methylnaphthalene, indicating that

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the droplet size of oil and DDA emulsion to be more smaller in this order. Besides, the effect of the composition of oil and DDA emulsion was also revealed, the results indicated that toward to the five oil emulsion, the size distribution data of D10, D50,

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D90 and Dav decreases with the order of (RDDA/oil =20%) < (RDDA/oil =40%) < (RDDA/oil =60%) < (RDDA/oil =80%), indicating that the oil droplet size decreasing with the DDA

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concentration increasing for the different oils. It is known that the decrease of oil droplets size corresponded to the increase of their specific surface area. Therefore, in the same dosage of oil, this resulted in a better availability of oil to the minerals’ surfaces, resulting in larger agglomerates formation and well flotation response (Burkin and Soane, 1960). This maybe another reason for the phenomenon that the recovery of quartz increased with increases in the proportion of DDA in the oil-DDA mixture. Similar result was found in the oil agglomeration of low-rank/oxidized coals; emulsification of kerosene in the presence of surfactants dramatically reduces the size

ACCEPTED MANUSCRIPT of the kerosene droplets, and reduces the oil consumption needed for efficient oil agglomeration (Laskowski and Yu, 2000). As a result, it can be deduced that the collecting capabilities of the respective oil-DDA mixture collector follow the trend: 1-methylnaphthalene > dimethylbenzene > dodecene > cyclohexane > dodecane. 3.2 Simulation results

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3.2.1 Interaction energy The relative affinity of DDA-mediated oil and water interface has significant

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impact on the interfacial properties. In order to compare the relative affinity of these

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five oil-DDA-water systems, the parameter ‘‘interaction energy’’ (Eint) which can be treated as the average intermolecular interaction measurement for per DDA molecule

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arising from the insertion of one DDA molecule into the oil-water interface is defined as following (Jang, et al., 2004):

Etotal -(n×EDDA, single +Eoil-water ) (1) n denoted the total interaction energy of the optimized simulation system,

where Etotal single

is the total energies of a free collector molecule, n is the number of the

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EDDA,

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Eint =

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DDA molecules, Eoil-water represented the energy a bare oil-water system. Single-point energy calculation can be represented as the following subsection: (1) for the oil– water–DDA supercell, Etotal would be obtained after the equilibrium state achieved;

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(2) after the equilibrium state simulation, removed DDA molecules, and a single-point

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energy calculation was carried out, from which Eoil-water was obtained; (3) EDDA was obtained from single-point energy calculation for single DDA molecule. The data used for the calculation of Eint and the results are listed in Table 3. It is well accepted that the more negative magnitude of the Eint indicates the more favorable interactions between DDA and oil component; the larger of Eint, the stronger of interaction between DDA/oil and DDA/water it is. From Table 3, the results indicated that the absolute value of interaction energy for these five oil components follow the order of 1-methylnaphthalene > dimethylbenzene > dodecene > cyclohexane > dodecane, which was consistent with the experiment results. The result

ACCEPTED MANUSCRIPT also indicated that aromatics oils were outperforming saturate hydrocarbon and unsaturated hydrocarbon, which was in accordance with Zhang’s (Zhang et al., 2015) study on the adsorption of oily collectors on model surface of Wiser bituminous coal and Zhong’s (Zhong et al., 2013) research on the adsorption mechanism of oil components on water-wet mineral surface.

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3.2.2 Structure of oil-water interface

3.2.2.1 Density distribution

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In order to analyze the interfacial property of the oil-DDA-water ternary system, a

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significant parameter the ‘‘interfacial density’’ of DDA molecules at the interface was investigated, and the density profiles results were shown in the Fig.4. It can be seen in

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the Fig. 4 that the density profiles of these five oil-DDA-water ternary systems distributed along the z-axis direction, which were obtained by dividing the system

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into 0.8 Å thick slabs parallel to the xy plane in the simulation box. For the symmetrical distribution of equilibrium configuration, Fig. 4 exhibits the extracted

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data in the range of 0-60 Å, and the oil surface was set as zero point. It is clearly

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shown that in the density profiles the oil and water phases have their own bulk densities, and the densities of each phase in the oil-DDA-water system (1.003± 0.005g/cm3 for 1-methylnaphthalene, 0.765 ± 0.005g/cm3 for dodecene, 0.858 ±

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0.005g/cm3 for dimethylbenzene, 0.725 ± 0.005g/cm3 for dodecane, 0.778 ± 0.005g/cm3 for cyclohexane and 0.997±0.005g/cm3 for water). These density profiles

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parameters agree well with those of the pure bulk phase, indicating that the simulation model of the oil-DDA-water system is considerably large enough to describe the interface between bulk phases and DDA molecules. As shown in Fig. 4, there was a well-defined interface exists between the oil and water phases in these five oil-DDA-water ternary systems. Firstly, addition of the DDA molecules increases the density of water and oil components at the interfacial region. Meanwhile, addition of DDA molecules also increases the interfacial width. It is clearly shown that the density at the cross point of the black line and the red line

ACCEPTED MANUSCRIPT has an increasing tendency with the addition of DDA in the interfacial zone, indicating that the water and oil phase is more and more remote from the interfacial center. Secondly, the density distributions of the DDA head group and tail group were also analyzed. The DDA head group is hydrophilic, and it could be seen in the density distribution of the DDA heads and H2O molecules are located at almost the same

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position as shown in Fig.4 a-e in the interfacial region, which indicated that water molecules can penetrate through DDA film and adsorb on oil surface. This

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phenomenon is because the strong interaction between positive charged DDA

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hydrophilic head and water molecules. Similar results were found in other surfactant-oil-water systems. According to Shi’s study, the addition of surfactants into

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the W/B/CPL system can drive CPL into the water-rich phase, increases the density of CPL at the interfacial region (Shi et al., 2015). As Hu reported that the addition of

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SDS increases the total density of dimethyl sulfoxide (DMSO) and hexane at the interfacial zone (Hu et al., 2015). The distance of density distributions for the DDA

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reflects the average extension of collector molecules along the z-direction. The results

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demonstrated that the distance of density distributions slightly increases with the order of 1-methylnaphthalene > dimethylbenzene > dodecene > cyclohexane > dodecane, indicating that the enhanced excluded-volume effect drives the DDA chain

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molecule to be more extended along the z-direction in this order. 3.2.2.2 Radial distribution functions (RDF)

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From the above detailed discussion, it is clear that the interaction between DDA hydrophilic head groups and H2O molecules is a significant parameter determining the activity of oil-DDA-water interface. In order to illustrate the interaction between the DDA heads and H2O molecules, another crucial parameter ‘‘radial distribution functions’’ (RDF) g(r) between following relevant atoms is calculated to investigated the interaction between DDA hydrophilic head groups and water molecules. The RDF functions between NDDA (nitrogen atoms in DDA molecules) and OH2O (oxygen atoms in H2O molecules) is calculated for equilibrium system, and the corresponding results

ACCEPTED MANUSCRIPT are shown in Fig. 5. The results indicated that the RDF functions of these five oil-DDA-water systems feature with the similar profile at the first sharp peak (2.1 Å) and second peak (4.1 Å). According to Xu et al., the formation of the first peak at 2.1 Å could be ascribed to the hydrogen bond interaction between the nitrogen atom of DDA molecule (NDDA) and hydrogen atom of H2O molecule (HH2O), and this strong

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hydrogen bond would naturally induce to form first hydration film around nitrogen atom of DDA molecule (Xu, et al., 2013). Besides, the existence of second peak is

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derived from the van der Waals interactions, electrostatic interaction and hydrogen

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bond interactions, which could be ascribed to the interaction of the hydrophilic head group with H2O molecules and first hydration film with H2O molecules (Xu, et al.,

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2013). Generally, the g(r) peak intensity index of H2O molecules with first hydration film is an effective reflection of the interaction between the polar group and water

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phase (Rosen and Kunjappu, 2004; Zhao, et al., 2010). It can be seen that for these five systems, the g(r) peak intensity of second peaks were similar to each other, while

D

the g(r) peak intensity of first peaks are quite different. The results demonstrated that

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the height for the first peaks of RDF profiles are 4.49 for 1-methylnaphthalene-water system, 4.26 for dimethylbenzene-water system, 3.91 for dodecene-water system, 3.74 for cyclohexane-water system, and 2.93 for dodecane-water system. So, the RDF

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results indicate that the interaction between DDA hydrophilic head groups and water molecules should follow the order of 1-methylnaphthalene > dimethylbenzene >

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dodecene > cyclohexane > dodecane, which is in agreement with the with the flotation results.

3.2.2.3 Interfacial thickness From the above analyses, it can be seen that the interaction between DDA hydrophilic head groups and water molecules would induce the modification of the oil-water interfacial width. It is well accepted that “interfacial thickness’’ is a crucial interfacial physical property that provides a quantitative measure for the size of the interface. In order to investigate how the addition of DDA molecules affects the width

ACCEPTED MANUSCRIPT of the oil-water interface, the interfacial thickness of different oil-water interface was calculated. Known from the previous study, the definition of “10-90” criterion is frequently used for assessing the interfacial thickness for the liquid-vapor interface. On the basis of the “10-90’’ criterion (Alejandre, et al., 1995; Rivera, et al., 2002), the interfacial

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thickness of oil (toil) and water (twater) are defined as following: interfacial thickness is the distance between two positions where the actual density of phase component

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varies from 10% to 90% of their bulk phase real density. For the parameter the oil and

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water interfacial thickness (ttotal), according to Jang (Jang, et al., 2004), which can be defined as the“90-90” interfacial thickness criterion, is the distance between two

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positions where the actual density of oil and water phase component are 90% of their bulk phase real density. And the specific definitions of toil, twater and ttotal were depicted

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in Fig. 6.

The interfacial thickness, ttotal, toil and twater of the five oils and pure

D

dodecane-water systems are calculated and exhibited in Table 4. The calculated results

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demonstrated that the interfacial thickness of the bare dodecane-water interface (in the absence of DDA) is 4.26 Å, which is consistent with the thickness (4.4±0.2 Å) from the synchrotron X-ray reflectivity experiment, indicating that our simulation results

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are believable (Mitrinovic et al., 2000). From the results in the Table 4, it is clear that the interfacial thicknesses of toil

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and twater presented an increasing trend with the addition of DDA, which indicated that the oil-water interfaces are broadened derived from the penetration of oil and water component into the hydrophobic tail groups and hydrophilic head groups. This phenomenon can be explained as follows. The addition of DDA molecules would induce to form a self-assembly DDA monolayer at oil-water interface, therefor due to the attraction of hydrophobic hydrocarbon tail of DDA molecule with oil component and hydrophilic head groups of amphipathic DDA with water phase would lead to spontaneous permeation between oil component and water phase. As a result, the

ACCEPTED MANUSCRIPT spontaneous permeation of component and water phase in the self-assembly DDA molecule membrane would result in the increase of interfacial thicknesses of toil and twater, and in addition the comprehensive interfacial thickness ttotal would also increase. As shown in the Table 4, compare with the thickness of these five systems, the thickness of twater and toil follows the order of 1-methylnaphthalene >

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dimethylbenzene > dodecene > cyclohexane > dodecane. Furthermore, the results indicate that the difference of interfacial thicknesses of twater is minor for the five

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systems; on the contrary, the difference of interfacial thicknesses of toil is large. As a

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result, the comprehensive of ttotal would keep the same order as toil. One of the most interesting phenomenons is that the interface broadening occurred mainly in the oil

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side, whereas the water interface was insensitive to the variation. According to Xu’s study, the interfacial thickness of oil-surfactant-water system could be ascribed to the

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molecular structure of the oil and surfactant molecule. Specifically, the interaction of surfactant hydrophobic alkyl chain and oil molecules decided the parameter toil; and

D

the interaction of hydrophilic head-group and water molecules decided the parameter

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twater (Xu et al., 2013). As a result, the most probable reason for this phenomenon is that, even though each system has the same components, DDA and water, while the different oil molecules would produce the different effective alkyl tail, which may

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affect the intermolecular interaction between DDA hydrophobic hydrocarbon tail and oil molecules result in a bigger difference of the parameter toil. In contrast, the same

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hydrophilic head group of DDA molecule would lead to a little difference of interaction intensity between water phase and hydrophilic head groups of amphipathic DDA, inducing to the similar interfacial thickness of twater. In general, the larger interfacial thickness of oil-water interface always accompanies with high interfacial activity of two phases. Therefore, the interfacial activity arise from this crucial parameter interfacial thickness follows the order of 1-methylnaphthalene > dimethylbenzene > dodecene > cyclohexane > dodecane, it well accords with the flotation results.

ACCEPTED MANUSCRIPT 3.2.2.4 Self-diffusion coefficient (D) Although the five oil-DDA-water system had similar interfacial activity of water component and DDA (Table 4), the interfacial activity of the different oil fraction was significantly different (Table 4). The higher interfacial activity always accompany with the higher mobility of oil molecules. The mobility of oil molecules can be

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assessed by the mean square displacement (MSD) functions described as following (Wu, et al., 2013):

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𝑀𝑆𝐷(𝑡) = 〈|𝑟𝑖 (𝑡) − 𝑟𝑖 (0)|2 〉

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where 𝑟𝑖 (𝑡) is the position of molecule i at time t and 𝑟𝑖 (0) is the initial position of molecule i. The larger slope of the MSD curve always reflects the higher diffusivity

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intensity of molecules. In can be seen in the Fig.7, as expected, the mobility of these five oil components was ranked in the order of dodecane < cyclohexane < dodecene <

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dimethylbenzene < 1-methylnaphthalene.

According to Einstein diffusion law, another crucial parameter “self-diffusion

D

coefficient” (D) of oil molecule which is determined from the slope of MSD curve

𝑁

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can be represented as following (Einstein, 1905): 1 𝑑 𝐷 = lim ∑〈|𝑟𝑖 (𝑡) − 𝑟𝑖 (0)|2 〉 6 𝑡→∞ 𝑑𝑡 𝑖=1

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where N is the number of diffusion particles. The trajectories of MD simulation were extracted and the self-diffusion coefficient D can be calculated through the slope of

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the MSD-time plot when Einstein diffusion was indeed happened. In general, the larger diffusion coefficient always reflects the stronger penetration intensity of the oil component. And this self-diffusion coefficient is on the basis of the assumption of a random walk for each simulated particle through the simulation matrix. Nevertheless, in our study, anomalous diffusion might occur with non-negligible impact on the pathway of oil molecules (Müller, et al., 1992). Therefore, in our study, the slope of log (MSD) vs. log (t) was not 1 as Einstein diffusion discussed before. As a result, it is impossible to utilize the simulated MD trajectories directly for the calculation of the

ACCEPTED MANUSCRIPT self-diffusion coefficient. Therefore, only where Einstein diffusion actually occurred the MD trajectories can be extracted for self-diffusion coefficient calculation. In order to illustrate this purpose, an example of dodecane diffusion is depicted in the Fig. 8. It can be seen that the slope of Fig. 8a increased from 0.79 to 1.04 (approximately equal to 1) after 120 ps indicating that the occurrence of anomalous diffusion followed by

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Einstein diffusion, which changed to 0.82 after 600 ps implying the noise portion. Correspondingly, it can be seen that in the Fig. 8b the trajectories between 120 and

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600 ps were sampled where the slope of MSD curve was 0.96, and the self-diffusion

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coefficient D value was calculated to be 1.59×10-9 m2s-1.

To further explore the transport properties of the oil-DDA-water interface

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influenced by DDA, the diffusion profile of the ternary system along the z-axis is monitored and self-diffusion coefficients of the oil component in the interfacial region

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are calculated on the basis of the supplementary data Fig.S6-Fig.S9 (in supplementary material). The calculated diffusion coefficients of cyclohexane, dodecane, dimethylbenzene, and 1-methylnaphthalene along z axis were 1.69×10-9m2s-1,

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D

2.06×10-9 m2s-1, 2.17×10-9 m2s-1 and 2.86×10-9 m2s-1, respectively. The results indicate that the diffusion coefficients of these five oil components followed the sequences order: 1-methylnaphthalene > dimethylbenzene > dodecene > cyclohexane >

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dodecane. As a consequence, the discussed penetration capability should also follow the specified order of 1-methylnaphthalene > dimethylbenzene > dodecene >

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cyclohexane > dodecane from the aspect of diffusion coefficient. Conclusions

In the present study, dodecane, cyclohexane, dodecene, dimethylbenzene and 1-methylnaphthalene were chosen as researched subjects, and the influences of oil structures on interfacial activity of oil–DDA–water ternary systems are investigated by the analyses of experimental tests and theoretical simulations. First, the combination of DDA and these five oils used as collector in quartz flotation. It is observed that the collecting capability of the five oils follows the order

ACCEPTED MANUSCRIPT of 1-methylnaphthalene > dimethylbenzene > dodecene > cyclohexane > dodecane. Second, MD simulations of the different oil–DDA–water ternary systems are performed to investigate the influence of oil structure on interaction energy, density profiles, radial distribution functions between DDA polar head group and water, interfacial thickness and self-diffusion coefficient, in which crucial factors are

described

above

are

all

following

the

order

of

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determining the interfacial activity. Simulation results demonstrate that the parameters 1-methylnaphthalene

>

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dimethylbenzene > dodecene > cyclohexane > dodecane, which accords well with

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flotation results. Specifically, the density profiles show that water molecules can penetrate through DDA monolayer and adsorb on oil surface, which is attribute to the

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strong interaction between positive charged DDA hydrophilic head and water molecules. The RDF between NDDA (nitrogen atoms in DDA molecules) and OH2O

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(oxygen atom in H2O molecules) indicates that the interaction of DDA and water is most favorable for 1-methylnaphthalene-DDA-water system via strong hydrogen

D

bonds, van der Waals and weak electrostatic attraction. The interfacial thickness

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results illustrate that the distinct different oil components lead to a large difference of interaction with the same hydrophobic alkyl chain of DDA molecule, inducing to significant change of toil. The large toil of 1-methylnaphthalene means high interfacial

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activity of oil-water phases. The self-diffusion coefficient profiles demonstrate that the mobility of the different oil fraction is significantly different, the diffusion ability

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follows the order of 1-methylnaphthalene > dimethylbenzene > dodecene > cyclohexane > dodecane. The results indicate that in molecular level aromatic fraction performance better than saturate and unsaturated hydrocarbon. This research will enable us to obtain a deeper microscopic-level understanding of the interaction between oil-DDA-water interfaces derived from DDA’s surface activity, which is beneficial to reveal the interaction mechanism between oil components and DDA alkyl tail, between water molecules and DDA head group, respectively. Our finding is helpful in guiding the oil-assisted flotation agent design.

ACCEPTED MANUSCRIPT

Acknowledgement The authors acknowledged the financial support provided by the National Natural Science Foundation of China (51704208).

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ACCEPTED MANUSCRIPT

Fig. 1. Sketch structure of research objects.

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(A)：Dimethylbenzene；(B)：1-Methylnaphthalene；(C)：Cyclohexane；(D)：Dodecane；

AC

CE

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D

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(E)：Dodecene

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Fig. 2. Initial configuration (a) and equilibrium configuration (b) of DDA adsorption on the dodecane-water interface.

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(Color representation is as follows: red, oxygen atoms; white, hydrogen atoms; blue, nitrogen atoms; light

AC

CE

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D

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green, chloride ions; and gray, carbon atoms. For clarity, only the skeletons of the water molecules are shown.)

ACCEPTED MANUSCRIPT

100

DDA DDA - Dodecane DDA - Cyclohexane DDA - Dodecene DDA - Dimethylbenzene DDA - 1-Methylnaphthalene

Recovery (%)

80

60

40

0

0.2

0.4

0.6

0.8

1.0

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Weight fraction of DDA, wDDA

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20

AC

CE

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D

MA

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Fig. 3. Effect of non-polar oil and DDA combination collector on flotation of quartz.

ACCEPTED MANUSCRIPT 1.2

(a)

H2O

Density (g cm )

1.0

Dodecene

DDA DDA Tail

DDA Head

-3

0.8

0.6

0.4

PT

0.2

0.0 0

10

20

30

40

50

60

z (Angstrom)

H 2O

Dodecane

DDA DDA Tail

DDA Haed

0.8

0.6

NU

-3

Density (g cm )

1.0

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(b)

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1.2

0.4

MA

0.2

0.0 20

30

40

50

z (Angstrom)

H2O DDA DDA Tail

0.8

DDA Head

0.4

0.2

0.0 0

10

20

30

z (Angstrom)

1.2

(d)

H2O

1.0

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0.6

AC

-3

Density (g cm )

1.0

Cyclohexane

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(c)

D

1.2

60

-3

10

Density (g cm )

0

Dimethylbenzene

DDA DDA Tail

DDA Haed

0.8

0.6

0.4

0.2

40

50

60

0.0 0

10

20

30

z (Angstrom)

40

50

60

ACCEPTED MANUSCRIPT 1.4

(e) H2O

1.2

1.0

DDA Head

-3

Density (g cm )

1-Methylnaphthalene

DDA DDA Tail

0.8

0.6

0.4

0.0 0

10

20

30

40

50

z (Angstrom)

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0.2

60

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Fig. 4. Density distribution profiles along z-axis direct of different system.

AC

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D

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a: Dodecene; b: Dodecane; c: Cyclohexane; d: Dimethylbenzene; e: 1-Methylnaphthalene

ACCEPTED MANUSCRIPT 5

Dodecene Cyclohexane Dodecane Dimethylbenzene 1-Merhylnaphthalene

g(r)

4

3

2

PT

1

0 0

2

4

6

8

10

12

14

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z / Angstrom 5

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g(r)

3

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Dodecene Cyclohexane Dodecane Dimethylbenzene 1-Merhylnaphthalene

4

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2

1

0 4

12

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z / Angstrom

14

D

2

Fig. 5. Radial distribution functions between NDDA (nitrogen atoms in DDA molecules) and OH2O

AC

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(oxygen atoms in H2O molecules) for each system.

ACCEPTED MANUSCRIPT 1.2

ttotal

H2 O

1.0

twater

-3

Density (g cm )

Dodecane 0.8

toil

0.6

0.4

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0.2

0.0 0

5

10

15

20

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z (Angstrom)

AC

CE

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D

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Fig. 6. Definition of interface thickness of water (twater), oil(toil) and total(ttotal).

ACCEPTED MANUSCRIPT 2000

Dodecane Cyclohexane Dodecene Dimethylbenzene 1-Merhylnaphthalene

2

MSD (Angstrom )

1500

1000

0 0

200

400

600

1000

RI

t (ps)

800

PT

500

AC

CE

PT E

D

MA

NU

SC

Fig. 7. MSD curves of five oil fractions.

ACCEPTED MANUSCRIPT 3.5

(a) 2

3.0

Slope:0.82, R =0.984

Log(MSD)

2.5

2.0 2

Slope:0.79, R =0.999

2

Slope:1.04, R =0.997 1.5

1.0

0.5

1.0

1.5

2.0

2.5

PT

0.5 3.0

Log(t)

RI

800

(b)

SC

600 2

MSD (Angstrom )

2

Slope:0.96, R =0.996

NU

400

MA

200

0 200

400

600

800

Time (ps)

1000

D

Fig. 8. Plots of dodecane (a) Log (MSD) vs. Log (time) and (b) MSD vs. time.

CE

PT E

(The circles are original data and the solid line is the linear fit.)

AC

0

ACCEPTED MANUSCRIPT Table 1. Oil physicochemical and interfacial properties at 20 °C Parameters Oil phase

Viscosity

Surface tension

g/cm3

mPa.s

dyne/cm

Dodecane

0.748

1.504

25.5

Dodecene

0.759

1.304

25.7

Cyclohexane

0.778

0.977

Dimethylbenzene

0.868

1.101

1-Methylnaphthalene

1.025

2.913

RI

SC NU MA D PT E CE AC

PT

Density

25.9 28.7 38.7

ACCEPTED MANUSCRIPT

Table 2. Size distribution data of different oil and DDA emulsion. DDA ratio

D (μm)

20% DDA

40% DDA

D10

D50

D90

Dav

D10

D50

D90

Dav

Dodecane

4.91

12.47

20.76

12.77

1.88

10.02

18.01

9.92

Dodecene

2.57

10.02

18.41

10.32

1.76

6.98

Cyclohexane

3.12

10.52

18.54

11.19

1.81

9.02

Dimethylbenzene

2.15

9.19

18.12

9.54

1.72

1.21

8.88

17.75

8.44

Oil phase

1-Methylnaphtha lene

C A

E C

PT

D E

1.57

6.35 4.05

SC

D10

U N

A M

D50

T P

I R

60% DDA

80% DDA

D90

Dav

D10

D50

D90

Dav

1.71

7.21

16.55

7.35

1.72

3.62

15.17

6.39

17.52

8.16

1.32

6.02

16.31

6.36

1.45

3.03

14.01

5.72

17.95

9.02

1.35

6.51

16.05

6.95

1.59

3.32

14.57

6.09

15.35

7.79

1.25

5.26

13.08

6.06

1.04

3.03

13.95

5.41

15.17

6.39

0.77

3.58

12.63

5.26

0.79

2.96

12.88

4.98

ACCEPTED MANUSCRIPT

Table 3. Interaction energy (Eint) of DDA at different alkanes/water system. Eint (kcal mol-1) Eoil-water

EDDA, single

Eint

Dodecane

-8825.327

-3658.775

-24.248

-61.861

Dodecene

-9584.539

-3617.136

-24.248

-70.292

Cyclohexane

-8927.782

-3532.015

-24.248

Dimethylbenzene

-9371.589

-3380.684

PT

-68.631

-24.248

-75.599

1-Methylnaphthalene

-9215.922

-3010.302

-24.248

-79.179

AC

CE

PT E

D

MA

NU

SC

Etotal

RI

Oil phase

ACCEPTED MANUSCRIPT Table 4. Interface thickness of DDA at different oil/water interface. Interface thickness (𝐴)̇ System toil

ttotal

Dodecane/water

3.93

3.21

4.26

Dodecane

29.23

18.86

31.21

Cyclohexane

29.65

21.53

32.42

Dodecene

29.77

22.06

Dimethylbenzene

29.96

24.10

1-Methylnaphthalene

30.10

26.03

AC

CE

PT E

D

MA

NU

SC

RI

PT

twater

32.85 33.53 34.61

RI

PT

ACCEPTED MANUSCRIPT

SC

100

NU

Recovery (%)

80

60

MA

40

20

0.2

0.4

AC

CE

Graphical abstract

PT E

D

0

DDA DDA - Dodecane DDA - Cyclohexane DDA - Dodecene DDA - Dimethylbenzene DDA - 1-Methylnaphthalene

0.6

0.8

Weight fraction of DDA, wDDA

1.0

ACCEPTED MANUSCRIPT

Highlights: ● Aromatic hydrocarbon dimethylbenzene and 1-methylnaphthalene performance better than saturate hydrocarbon and unsaturated hydrocarbon used as collector extender. ● Interaction between non-polar oil and DDA alkyl tail would enhance the mobility of

AC

CE

PT E

D

MA

NU

SC

RI

PT

oil component.