Experimental and Molecular Dynamics Simulation Study of Specific

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An Experimental and Molecular Dynamics Simulation of Specific Ion Effect on the Graphene Oxide Surface and Investigation of Their Influence on Reactive Extraction of Model Dye Molecule at Water/Organic Interface Priyakshree Borthakur, Purna Kanta Boruah, Najrul Hussain, Bhagyasmeeta Sharma, Manash R. Das, Sara Mati#, David Reha, and Babak Minofar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02787 • Publication Date (Web): 09 Jun 2016 Downloaded from http://pubs.acs.org on June 11, 2016

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The Journal of Physical Chemistry

An Experimental and Molecular Dynamics Simulation of Specific Ion Effect on the Graphene Oxide Surface and Investigation of their Influence on Reactive Extraction of Model Dye Molecule at Water/Organic Interface Priyakshree Borthakura,b, Purna K. Boruaha,b, Najrul Hussaina,b, Bhagyasmeeta Sharmaa, Manash R. Dasa,b* Sara Matić,c, David Řehad and Babak Minofard,e*

a

Advanced Materials Group, Materials Science and Technology Division, CSIR-North East Institute of Science and Technology, Jorhat 785006, Assam, India. b c d

Academy of Scientific and Innovative Research (AcSIR), India

Faculty of Science, University of Zagreb, Roosevelt Sq. 6, Zagreb, Croatia

Institute of Physics and Biophysics, Faculty of Science, University of South Bohemia, Branišovská 1760, 37005 ČeskéBudějovice, Czech Republic

e

Center for Nanobiology and Structural Biology, Institute of Microbiology, Academy of Sciences of the Czech Republic, Nové Hrady, Czech Republic

Corresponding Author (s): Dr. Manash R. Das Advanced Materials Group, Materials Science and Technology Division, CSIR-North East Institute of Science and Technology, Jorhat 785006, Assam, India. Tel: +91- 9957178399, Fax: +91−376−2370011, E Mail: [email protected], [email protected] Dr. Babak Minofar Center for Nanobiology and Structural Biology, Institute of Microbiology, Academy of Sciences of the Czech Republic and Faculty of Science, University of South Bohemia, České Budějovice, Czech Republic Phone: +420 389 033 801 Mobile: +420 777 874 938 Email: [email protected], [email protected] 1 ACS Paragon Plus Environment


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ABSTRACT The influence of different inorganic anions (Cl−, Br−, SCN−, NO3− , SO42− and CH3COO− ) and cations (Ca2+, Mg2+, Na+ and NH4+) on the surface potential of graphene oxide (GO) suspension has been investigated both experimentally and computationally. The hydrophilic GO surface has negative surface potential (zeta potential) which can be varied by changing the pH of the suspension as well as adding external inorganic ions. The surface of GO is hydrophilic in basic medium and becomes hydrophobic in acidic medium due to the protonation and deprotonation of the surface functional groups. The presence of inorganic ions affect the electrophoretic mobility of the dispersed phase within the GO suspension and influences it zeta potential. This is due to the formation of a double layer of charge at the interface of GO and ionic salt solution. Molecular dynamics (MD) simulations were used to understand the interactions of ions within the slipping plane of GO which influences its zeta potential in salt solutions. The results suggested that the influence of the various inorganic ions on the electrokinetic potential of GO is ion specific and depends on polarizability of the ions.

Having high specific surface area, amphiphilic and biocompatible, GO was

successfully utilized in the reactive extraction technique of methyl blue (MB) dye molecule at the water/toluene interface. The present study demonstrates that presence of highly polarizable ions increases the zeta potential as well as hydrophobicity of GO which facilitates the extraction of MB from aqueous to the organic phase.

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INTRODUCTION In recent years, the colloidal systems are widely used in various industrial, biological and commercial applications and the stability of the colloidal systems is a key factor which influences their application in different fields. Zeta potential, an important surface property, plays the key role in the study of stability of a colloidal system.The structure of the electrical double layer at the particle-dispersion medium interface influences the zeta potential and other surface properties of a colloidal dispersion. In colloidal science, the presence of salt solutions plays an important role on the variation of zeta potential at particle-electrolyte solution interface due to the variation of the distribution of ions near the interface and pH of the colloidal suspension which are mainly a function of ionic charge and size. In case of a colloidal dispersion of oxide materials, on addition of salt solutions, the acid-base surface functional groups of oxide surface react with the ions of the salt and result in generation of electric charge at the interface.1,2 The magnitude of the electrical charge on the surface and the structure of the electrical double layer at the interface depend upon the concentration of the electrolyte solution as well as on the nature of the ions. Ionic species plays an important role in different chemical processes such as electrochemical reactions at electrode surfaces, atmospheric chemistry on aerosol surfaces, extraction processes between organic and aqueous phases, ion transfer through biological membranes etc.3 In 1888, Franz Hofmeister discovered that the salting out efficiency of different ions on the precipitation of hen egg white proteins are different, and based on this observation he arranged the ions within a series known as Hofmeister Series (HS).4,5 From the earlier studies it was realized that the Hofmeister salts have significant ion-specific effect on the surface tension of water/air and oil/water interfaces, stability of emulsions, solubility of gases and in catalysis of chemical reactions etc.6 In colloidal systems the HS ions show a distinguishable influence on the surface charge and polarity. Hydrophobic anionic colloids show direct HS effect and the 3 ACS Paragon Plus Environment


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cationic colloids show reverse HS effect.7,8 In case of hydrophilic colloids, the anionic colloids show reverse HS but the cationic colloids show direct HS effect.7,9 The effect of Hofmeister ions on the colloidal systems can be discussed based on the variation of nonelectrostatic potentials between the ions and the surfaces and the ion hydration forces. Furthermore the polarizability of the ion also plays an important role in this study. Das et al. studied the effect of inorganic anions like NO3− (aq), I− (aq), Br− (aq), Cl− (aq), SO42− (aq) and S2O32− (aq) and cations Ca2+ (aq) and Mg2+ (aq) on the zeta potential and isoelectric point of α-alumina as well as their effect on adsorption density of the p-hydroxy benzoate on αalumina in aqueous medium.10 They observed that, at very low electrolyte concentration, the polarizability of ions plays an important role for the specific ion adsorption onto the αalumina surfaces in addition to ion-ion, ion-solvent interactions in the presence of different salts. Graphene oxide (GO) sheets have ionizable (−COOH and −OH) edge groups on the surface and produce a colloidal dispersion in aqueous medium due to its hydrophilic nature. The specific ion effect on the stability of GO colloidal system and the zeta potential are associated with the electrical charge formed at the GO/electrolyte solution interface. To best of our knowledge, there is no literature available on the discussion about the influence of HS ions on surfaces of GO.

In this paper we mainly focus on the measurement of the zeta potential

value (ζ) of GO colloidal suspension in presence of different anionic and cationic ions like Cl−(aq), Br−(aq), NO3−(aq), SCN−(aq), SO42−(aq), CH3COO−(aq), Na+ (aq), Ca2+ (aq), Mg2+ (aq) and NH4+ (aq) of two different concentration (0.01M and 0.1M) in the pH range 2−10 of the medium. From the results obtained, it is found that with the variation of pH of the colloidal systems, the polarity of GO surface is varied and hence the effect of ions on the GO surface at different pH is also different which depends on the surface polarity as well as on the polarizability of the ion. 4 ACS Paragon Plus Environment

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Precisely, our objective is to investigate the specific ion effect on the GO surface through both experimentally and molecular dynamic (MD) simulation which will play a major role in the reactive extraction of the dye molecules for environmental remediation. Reactive extraction technique refers to a liquid-liquid extraction technique in which a suitable carrier is used to extract toxic materials from the aqueous to the organic phase. The material and the carrier react with each other in the aqueous phase and then the resultant complex is extracted to the organic phase. Reactive extraction technique is highly effective for biodiesel production, enantioselective separation, extraction of organic acids from the aqueous phase etc.11-15 Reactive extraction technique also has attracted a considerable attention for the removal of dye molecules from the industrial waste before being discharged into normal water stream. Among the various industrially utilized dye molecules, MB is a toxic material for its carcinogenic and mutagenic effects16 and hence removal of MB from the industrial effluents is an important field of study. Our research group has recently utilized the reactive extraction technique for the removal of both the cationic and anionic dye molecules using GO as an efficient carrier.17 It was found that the extraction of dye molecules mainly depends on the surface charge (zeta potential) of the surfactant molecules. Thus the variation of the zeta potential of GO surface as a function pH in presence of different anionic and cationic ions shows significant effect on the reactive extraction analysis of dye molecules at the water – organic interface. The presence of both hydrophilic polar groups at the edges and the hydrophobic aromatic structure makes GO amphiphilic in nature. Because of the amphiphilicity, GO can be used as a surfactant in the formation of pickering emulsions consisting of an aqueous phase and an organic phase.18 In this paper we have explored that the zeta potential value of GO depends on the pH of the medium as well as on the presence of electrolyte ions. Thus, the influence of the presence of a series of inorganic ions and pH of the medium on the adsorption of methyl blue (MB) at the GO–electrolyte interface seems to



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be an uninvestigated study which is yet to be explored. Our present work includes the variation of the surface properties in presence of different electrolytes and the effect of surface charge on the reactive extraction analysis of carcinogenic methyl blue an anionic dye in toluene-water interface. The structure of MB is shown in Figure 1.

Figure 1. Structure of Methyl Blue (MB) dye molecule

EXPERIMENTAL SECTION Materials: Graphite powder (99%, E Merck, India), NaOH (99%; Qualigens), Toluene (>99.5%, S.D. Fine-Chem, Mumbai, India), Methyl blue (Loba Chemie, Reagent grade), NaCl (>99%, E Merck, India), NaBr (>99%,LR, Indian Drugs & pharmaceuticals Ltd), NaNO3(>99%, E Merck, India), NaSCN (>98, Sigma Aldrich), CH3COONa anhydrous (>99%, Sarabhai M. Chemicals), Na2SO4 (>99%E Merck, India), MgCl2 (>99%, E Merck, India), CaCl2 (>99%, Sd fine-Chem limited), NH4Cl (>99.5%, Qualigens, India), Deionized water . All salts are used after recrystallization. Synthesis and characterization of graphene oxide (GO)


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GO was synthesized using Hummers and Offemann method and details characterization (XRD, UV-Vis spectra, TGA, HRTEM etc.) were illustrated in our previous publications.19,20

Measurement of Zeta Potential of GO suspension in presence of different electrolyte solutions The potential difference between the two phases which are in contact with each other can be determined by four different techniques such as electrophoresis, electro-osmosis, streaming potential and sedimentation potential. In the present study, the surface potential was determined by utilizing electrophoresis method. In electrophoresis method, the particles move under the influence of an applied electric field. The zeta potential of GO

suspensions was

calculated by measuring the electrophoretic mobility using Zetasizer analyzer (Model: Nano ZS, Malvern, UK) instrument and Smoluchowski equation.21 The relation between zeta potential and electrophoretic mobility is

ζ= 4πη/Dt× µ………. (1)

Where, ζ is the zeta potential, η is viscosity of the medium, µ is electrophoretic mobility at particular temperature, Dt is the dielectric constant of the medium. Thus the effect of different monovalent and divalent salts on the surface charge of particles can be studied by investigating the electrophoretic mobility as a function of pH in presence of different salts. The effect of pH, electrolyte concentration and presence of different ionic species on the zeta potential of GO suspension was evaluated.

The specific ion effect on the Zeta potential of GO (1 mg/mL) was evaluated by measuring the zeta potential at two different concentration of salt solutions (0.01M and 0.1M)



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separately with the help of Zetasizer as a function of pH of the medium of the range of 2 to 10. GO suspension was prepared by ultrasonication of 1 mg/mL GO suspension with a fixed salt concentration at the desired pH value for zeta potential measurement. The pH of the suspension was adjusted with 0.1M HCl and 0.1M NaOH by using pH meter. The 1M stock solution of the various electrolytes (NaCl, Na2SO4, NaSCN, NaNO3, NaBr, CH3COONa, NH4Cl, MgCl2 and CaCl2) were prepared by dissolving in deionised water. The total volume of the GO suspension was adjusted to 5 mL with DI water in presence of electrolyte solution at desired pH for the zeta potential experiment.

Reactive Extraction analysis The reactive extraction of MB at water-organic interface was studied by using GO as carrier and toluene as model organic solvent in the pH range 2−10 and at 25 oC. The effect of time on the reactive extraction analysis was performed by stirring 5 mL of aqueous solution containing 1 mM MB dye solution and 1 mg mL−1 GO with 5 mL of toluene for 10, 30, 40, 60, 80, 100, 120, 140,160, 180, 200 and 220 s. The mixtures were then allowed to settle for another 1 h and the bubbles were imaged using microscope. The average size of the bubbles was measured using Image J software. The amount of extracted dye molecules from the aqueous phase to the organic phase was evaluated by determining the UV-Vis spectra of the aqueous dye solution at λmax = 595 nm. The adsorption capacity of the dye molecule at a particular time was determined by using the following equation

qt = (Co-Ct)V/m

(2)

Where, Co is the initial dye concentration and Ct is the concentration of the dye molecule at time t, respectively. V is the total volume of the aqueous phase and m is the mass of the GO.


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The percentage of MB extraction at a particular time was calculated from the following equation

Extraction (%) = (Co−Ct)/Co× 100

(3)

The effect of different ions on the reactive extraction analysis of the dye molecule was investigated adopting same experimental procedure on the mixture containing 1 mg/mL GO, 1 mM MB dye, 0.01M and 0.1M salt solution in 5 mL aqueous phase and 5 mL of toluene as organic phase.

Computational Section In order to support experimental findings and understand at molecular level the interactions of ions within the slipping plane, influencing the zeta potential of GO in salts solutions; classical MD simulations have been employed. MD simulation is powerful technique to identify the interaction of solute-solvent as well as solvent–solvent molecules in different solutions. For construction of GO C186H24O24 which has COOH and OH groups as functional groups has been used. The ratio of COOH:OH assumed to be 1:1 and no epoxy groups used in the structure as the structure of GO used in experimental work was not identified in terms of different functional groups. MD simulations of a single GO molecule in both acidic and basic forms (protonated and deprotonated) solvated in pure water and aqueous solutions of NaCl, Na2SO4, NaSCN, NaNO3, NaBr, CH3COONa, NH4Cl, MgCl2 and CaCl2 with the concentration of 0.1M have been performed. For preparation of the solution boxes, Packmol package was used and the GO molecules have been placed in the centre of the cubic box.22,23 Periodic boundary conditions in all three dimensions has been used for all simulations and for



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simulated molecules and ions the General Amber Force Field (GAFF) parameter set was employed .24

Partial charges for all molecules and ions were obtained by ab initio quantum mechanical calculations based on geometry optimization by using the Gaussian 03 package25 by employing the Density Functional Theory (DFT) method with B3LYP/cc-pVDZ level of theory. Optimized structures were used to perform single point calculations for further partial charges calculations with the Restrained Electrostatic Potential (RESP)26 fitting procedure by applying the Antechamber program package.27 Prepared systems were energy minimized by steepest descent minimization procedure in order to remove unfavourable contacts in the solutions then equilibrated by 100ps restrained NVT (canonical ensemble) followed by 100ps restrained NPT (isothermal–isobaric ensemble) molecular dynamics simulations.

Linear

constraint solver (LINCS) algorithm28 was employed for all bonds involving hydrogen atoms. The short-range non-bonded interactions were truncated to zero with the cut-off distance of 1.2 nm and the long-range part of the electrostatic interactions was calculated by the Particle Mesh Ewald method.29All the atoms in the systems were given initial velocities according to Maxwell–Boltzmann distribution at 300 K. V-rescale coupling algorithm was used 30 with the coupling constant of 0.1 ps in order to maintain constant temperature and pressure for all simulated systems. All production runs were done in NPT ensemble for 20 ns at 300 K and a time step of 2 fs was used for all simulations. Coordinates, velocities and energies were saved for analysis every 5 ps. All simulations were performed employing Gromacs 4.5.5 program package. 30-33 In order to quantify the distribution of each ion species around the GO molecule in the solutions the radial distribution function (RDF) of GROMACS program package has been employed. Visual Molecular Dynamics (VMD) program was used for visualizations of the trajectory and preparation of snapshots.34


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RESULTS AND DISCUSSION Influence of monovalent and divalent ions on the zeta potential of GO sheets In this study we have used GO as a base material which zeta potential value is found to be negative in the pH range 2−10. Without the effect of any monovalent and divalent ions the zeta potential has the lowest value ~−40 mV in the pH range 4−10 and the highest value ~−23 mV at pH 2 for GO surface. The variation of the zeta potential of GO in the presence of 0.01M and 0.1M of NaCl (aq), Na2SO4 (aq), NaNO3 (aq), NaBr (aq), CH3COONa (aq), NaSCN (aq), CaCl2 (aq), MgCl2 (aq), NH4Cl (aq) as a function of pH is shown in the Figure 2 and 3, respectively. It is observed from the Figure 2 that the zeta potential value is found to be negative in the pH range 2 −10 in presence of the 0.01M Cl− (aq), Br−(aq), SCN− (aq), NO3− (aq), SO42− (aq) and CH3COO− (aq). With increasing the pH of the medium the zeta potential of GO tends towards more negative value due to the more ionization of the surface edging groups.35 At the low pH, the zeta potential of GO has low negative values due to the adsorption of H+ ions from the acidic media on the negatively charged surface.36 When a salt of Na+ ion is introduced into the GO suspension, the Na+ ions are adsorbed onto the GO surface by electrostatic interactions and formed a double layer. But the polarizability of the anions of the salt effect the extent of adsorption of the cation on the negatively charged surface of GO. In the Figure 2(a), it is shown that at lower pH range, where surface becomes hydrophobic as compared to the higher pH, small number of positively charged cations (Na+) are adsorbed on the GO surface and the anions and resting Na+ ions are distributed around the fixed layer of adsorbed cations on the GO surface. The hydrophobic GO surface attract the smaller anions more strongly by electrostatic interactions than the larger anions. Hence the zeta potential of the GO surface has less negative value in presence of lager anions (in comparison to without salt) than the smaller anions.37 The polarizability of the anions plays the key role in the variation of zeta potential of GO in presence of inorganic salts. Smaller



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anions are adsorbed on the hydrophobic positively charged GO surface more strongly than the larger ions. Hence, it is observed from Figure 2(a) that in acidic and also in basic pH range the zeta potential of GO follows the order SO42− (aq) < GO < Cl− (aq) < SCN− (aq) < Br− (aq) ~NO3− (aq) < CH3COO− (aq) at the background electrolyte concentration of 0.01 M. In high pH range the GO surface is hydrophilic in nature, when the salt solution is introduced into the GO suspension, Na+ ions are adsorbed on the GO surface and thus becomes positive. On hydrophilic positively charged surface larger anions are more strongly adsorbed than the smaller anions.

37

Hence the surface potential of GO in presence of Na2SO4 salt solution is

more negative than the GO surface without salt. The other anions do not show considerable effect on the surface potential.

However, the zeta potential value of the GO shifted towards positive in presence of the 0.1M concentration of the background electrolyte solutions. It is observed from the Figure 3(a) that in acidic pH range (e.g. pH 4) the zeta potential of GO follows the order GO < SO42− (aq) ~ SCN− (aq) < Br− (aq) < NO3− (aq) < Cl− (aq) < CH3COO− (aq) because of the adsorption of anions on the positively charged GO surface after formation of a double layer by Na+ ions at the GO-electrolyte interface which is mainly depend on the size and polarizability of the anions. The highly polarizable anions are adsorbed more strongly on the GO surface after formation of double layer by Na+ ions at the interface due to higher surface active properties and hence the zeta potential of GO follows the as mentioned order as a function of the polarizability of anions.38 The zeta potential value of the GO in presence of the 0.1M background electrolyte is found to be in the range ~ − 22 mV to ~ −16 mV which is more positive than original GO suspension. This phenomenon is observed due to the adsorption of more Na+ ions on the GO surface due to electrostatic interaction between the negative GO


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surface and positive Na+ ions. Almost similar zeta potential variation of GO is observed in the neutral as well as the basic pH range.

(a)

(b)

Figure 2. Variation of zeta potential of GO as a function of pH in presence of 0.01M solutions of (a) sodium salt of different anions and (b) chloride salt of different cations



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The variation of the zeta potential of GO in presence of NH4Cl (aq), MgCl2 (aq) and CaCl2 (aq) at 0.01M and 0.1M concentration as a function of the pH is depicted in Figure 2(b) and 3(b), respectively. From the Figure 2(b), it is seen that the zeta potential value of GO suspension in presence of different chloride salts namely NH4Cl (aq), NaCl (aq), MgCl2 (aq) and CaCl2 (aq) (same anion but different cations) are less negative than the absence of salt solution and follows the order Ca2+ (aq) > Mg2+ (aq) >Na+ (aq) >NH4+ (aq) due to the adsorption of cations on the negatively charged surface. Divalent cations are more strongly bound to the negative surface and hence give rise to a relatively small negative zeta potential value whereas the monovalent cations Na+ (aq) and NH4+ (aq) are weakly bound to the surface. Moreover, the size of the hydrated ions in the Stern layer also influences the thickness of the layer as well as the zeta potential value. The Ca2+ ions have small hydrated ionic radii and as a result Ca2+ ions can easily enter into the Stern layer and the zeta potential of the GO dispersion as compared to the dispersions containing other salts was less negative. The same effect is seen in the case of monovalent Na+ (aq) and NH4+ (aq) ions where weakly hydrated Na+ (aq) can easily enter into the Stern layer than the highly hydrated NH4+ ions. The NH4+ (aq) ion can form hydrogen bonding with the H2O molecules and hence get highly hydrated. The concentration of the salt solutions has a great influence in the variation of the zeta potential of GO. The zeta potential value of the GO suspension becomes less negative with an increase in concentration of the salt solutions from 0.01M to 0.1M. It is due to the more counter-ions can accumulate on the GO surface and thus the electrical double layer is get compressed. The zeta potential value of GO is found to be ~ −10 mV in presence of the 0.01 M divalent cations of Ca2+ (aq), Mg2+ (aq), NH4+ (aq). Interestingly, we have found the isoelectric point of the GO at pH ~7.5 and ~8.3 in presence of the 0.1M Concentration of the divalent cations Mg2+ (aq) and Ca2+ (aq), respectively.


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(a)

(b)

Figure 3. Variation of zeta potential of GO as a function pH in presence of 0.1M solutions of (a) sodium salt of different anions and (b) chloride salts of different cations



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Computational section MD simulations were performed with both acidic and basic forms of GO solvated in water and aqueous salt solutions of NaCl (aq), Na2SO4 (aq), NaNO3 (aq), NaBr (aq), CH3COONa (aq), NaSCN (aq), CaCl2 (aq), MgCl2 (aq) and NH4Cl (aq) in order to better understand the pH dependence of the interaction of ions with the surface of GO. Radial distribution function (RDF) was used to demonstrate the distribution of ions around a single molecule of GO in order to quantify the interactions of ions with the surface of GO (Figure 4). Snapshots of MD simulations (Figure 4) show the interactions of individual ions with either the hydrophobic surface or polar COOH and OH functional groups on the surface of GO. The interaction of Na+ cations with GO were observed to occur in a pH dependent manner due to binding and electrostatic interaction to the anionic form of carboxylic groups on the surface of GO, which was also observed experimentally by zeta potential measurements. The binding of cations to the carboxylate groups causes a double layer of charged surface which results in less negative total charge at the surface of GO. Additionally, some anions showed specific interactions to the hydrophobic surface of GO. In all MD simulations it was assumed that all carbons of the GO molecule have zero partial charge thus surface of GO has hydrophobic character. Ions which prefer hydrophobic surfaces tend to be attracted to the surface of GO. Anions such as NO3− (aq), SCN− (aq) and CH3COO− (aq) showed surface propensity to the air/water interface which have been observed both experimentally and theoretically in many studies. It seems that the behaviour of these anions at the surface of GO is the same as at air/water interface. Also, it is shown, both experimentally and theoretically, that by increasing the hydrophobicity of ions, their surface propensity at the air/water interface is enhanced, as it is observed for benzene dicarboxylic dianions when CH3 groups to benzene ring.39-48


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From the comparison of RDFs of anions around acidic and basic forms of GO it can be observed that more bulky, polarizable and hydrophobic anions such as NO3− (aq), SCN− (aq) and CH3COO− (aq) show stronger affinity for binding to the acidic form of GO which is hydrophobic and has a zero charged surface. However, they show weaker interactions with the basic and negatively charged form of GO. The reason for such phenomena is the interactions and ion pair formation of these ions with cations attracted to the CH3COO− (aq) groups of GO. Thus less anions are found close to the hydrophobic surface of GO. On the other hand, doubly negative charged SO42− (aq) anions are not bound to the surface of GO but they are binding with high affinity to the anionic form of GO when the double layer with sodium cations is formed, which causes more negative zeta potential for solution of GO with Na2SO4 (aq). Singly charged Cl− and Br− anions show very low affinity for binding to either the hydrophobic surface of GO or to the Na+ cations in the double layer at the surface. Even though the distribution of Cl− is showing a slight increase around the basic form of GO in RDF calculations.



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(b)

(a)

Figure 4. Radial distribution function of anions around the surface of GO, in (a) acidic and (b) ionic forms in the solutions of NaNO3 (aq), NaSCN (aq), CH3COONa (aq), NaCl (aq), Na2SO4 (aq) and NaBr (aq). Complexes formed at the surface of GO in ionic form upon binding of sodium cations to carboxylic groups of GO or binding to the hydrophobic surface of GO represented for (c) CH3COO−, (d) SCN−, (e) SO42− and (f) NO3− anions. (Color coding: C-cyan, O-red, N-dark blue, H-white, S-yellow, Na-light blue.)

The interaction of ions with the surface of GO is responsible for the changes of experimentally measured zeta potential. Therefore to explain the pH dependence of zeta potential in different solutions of sodium salts (Figure 2), it is essential to investigate the 18 ACS Paragon Plus Environment

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nature of the ions binding to the surface. It was observed experimentally that in low pH values the GO surface has the most negative charge in aqueous solutions without any salt. At these low pH values the carboxylic groups are protonated, therefore no sodium ions are binding to the surface of GO. However, the hydrophobic, bulky and polarizable anions such as NO3− (aq), SCN− (aq) and CH3COO− (aq) can bind to the hydrophobic surface of GO thus generating of partially positive charge on the surface which could lead to less negative zeta potential as it is observed experimentally. Moreover, these polyatomic anions are increasing the bulkiness of the complex which is retarding the diffusion of the complex through the medium. Therefore, presence of these anions results in decreased mobility of complexes in the solutions thus making the zeta potential values even less negative. Moreover, GO has the least negative value in aqueous solution of CH3COONa which is due to the binding of CH3COO− anions in a very specific manner at the surface of GO. The acetate anions are closely bound to the surface of GO in which the methyl group pointing to the hydrophobic surface and carboxylic group is oriented towards the solution. The propensity for the solution/vapor interface of acetate ions has previously been described by Minofar et al.44-46 Same manner of orientation of acetate anions at the interface of solutions to vapor or surface of GO is confirming the affinity of their methyl moiety for hydrophobic phase. This binding mode can especially influence the volume and mobility of the complex during the zeta potential measurement, resulting in highest values of zeta potential observed in experiment (Figure 2).

Therefore, it is clear that the specific binding modes of anions are responsible for the observed variation of zeta potential of GO in sodium salts solutions even at low pH values in which the sodium ions are not contributing to the less negative charge of GO surface. The generation of positively charged double layer can be followed by binding of anions to the 19 ACS Paragon Plus Environment


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surface of GO at higher pH values, at which the carboxylic groups become deprotonated. This is especially favourable in case of doubly charged SO42− (aq) anion which was observed as a formation of a complex represented in Figure 4. The binding of doubly charged SO42− (aq) anions increases the amount of negative charge at the surface therefore contribute to even lower zeta potential of GO in the solutions of Na2SO4 (aq). Other anions which are singly charged and not so closely bound to the double layer formed by sodium cations do not show such strong effect on zeta potential as sulfate ions. Cl− (aq) anions are weakly bound to Na+ cations while Br− anions are not found in the first solvation shell around GO. This is the reason for more positive surface potential of GO in NaCl (aq) and NaBr (aq) salt solutions. However, it must be taken into account that bromide anions have slightly stronger binding affinity to the hydrophobic surface of GO. Thus the observed zeta potential is in agreement with computational findings.

The values of zeta potential obtained in higher concentrations of salts in solutions which are shifted to more positive values correspond to the presence of more Na+ (aq) anions attracted to the carboxylate groups. However, one of the factors affecting the measurement of zeta potential is the viscosity of the medium.49 Therefore, the presence of large anions close to the surface of GO can influence the obtained values even more in higher concentrations of salts. The interactions of cations with GO depend strongly on the deprotonation of carboxylic groups at higher pH values. However, the intensity of these interactions and the number of cations attracted to the surface of GO depend on their properties such as charge of ions, size and solvation properties. At low pH values, cations are not very likely to interact with GO as it can be observed in Figure 5. However, at higher pH values these interactions are increased significantly, especially in the case of Na+ (aq) and Ca2+ (aq) cations. Due to their similar size they were found to bind to the carboxylic groups more closely and forming more stable 20 ACS Paragon Plus Environment

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complexes than Mg2+ (aq) which is a smaller cation. Therefore, the double layer of charge at the surface of GO is more positive in the solutions of NaCl (aq) and CaCl2 (aq). Positive charge at the interface of double layer with the solution can also lead to attraction of Cl− (aq) ions into the outer solvation shell of GO. Polyatomic cation NH4+ (aq) is also closely bound to the carboxylic groups of GO forming a bulky complex on the surface. The electrostatic interactions of NH4+ (aq) cation with GO are additionally supported by the formation of hydrogen bonds between the hydrogen atoms of NH4+ (aq) and oxygen of COO− (aq) group which can be observed from the first peak at very low r values in RDF for this cation with both acidic and basic form of GO. Due to the hydrophobicity, NH4+ (aq) cations show surface propensity to the air/water interface, proposed by recent publication of Tian et al.41 NH4+ (aq) also showed affinity for binding to the hydrophobic surface of GO during MD simulations, which can contribute to even more positive surface charge.



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(b)

(a)

Figure 5. Radial distribution function of ionic species around the surface of GO, in (a) acidic and (b) ionic state, in the aqueous solutions of NaCl (aq), MgCl2 (aq), CaCl2 (aq) and NH4Cl (aq). Complexes formed at the surface of GO in ionic form by binding of cations: (c) Na+, (d) Mg2+, (e) Ca2+ and (f) NH4+. (Color coding: Cl-green, Mg-tan, Ca-purple, Na-light blue)

The zeta potentials measured in chloride salts solutions containing different cationic species can be also explained by observing the nature of their binding to GO during MD simulations. As it was previously mentioned, Cl− (aq) anions bind weakly to the first solvation shell of GO therefore they are not contributing significantly to the surface potential. This is due to high 22 ACS Paragon Plus Environment

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solubility of Cl− (aq) and its preference for staying in the aqueous solution. Therefore, the effect of cations to zeta potential of GO in chloride salts solutions can be attributed solely to their different binding affinities and charge contributions to the double layer formed at GO surface. As it can be seen in Figure 3 that the doubly charged cations such as Ca2+ (aq) and Mg2+ (aq) are significantly increasing the zeta potential of GO, which is leading even to positive values at very high pH values. These values are slightly higher for Ca2+ (aq) due to higher preference for closer binding to deprotonated carboxylic groups, which was observed in RDF analysis of MD simulations. Singly positively charged Na+ (aq) cations were shown to increase the zeta potential by only half of the values of difference between pure GO and solutions containing Ca2+ (aq) and Mg2+ (aq). Similar zeta potential values have been observed in high concentration solutions with NH4+ (aq) cations. However, the interactions of these cations with GO are not as strong as the ones of Na+ (aq), which was observed in RDF analysis of MD simulations as well as the zeta potential measurements in lower concentration of NH4Cl (aq) solutions.

Reactive Extraction Analysis In this study we have investigated the extraction of toxic carcinogenic and mutagenic MB dye molecules from aqueous medium using GO as suitable carrier and toluene as organic solvent. The MB adsorbed on GO surface can be easily extracted from the aqueous phase to the organic phase and thus the dye molecule can be easily separated and recovered for further applications. Due to the amphiphilic nature GO has ability to form stable bubbles at the water-organic interface and bind to the dye molecules through electrostatic and van der Waals interactions.50-52

Apul et al. and Li et al. suggested that the π electrons of the C=C

bonds and aromatic rings of the adsorbed organic molecules can bind through π˗π bonding with the delocalized π electrons of the benzene ring of graphene like structures.53,54 MB dye 23 ACS Paragon Plus Environment


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molecules contain π-electrons in its aromatic region which facilitates the binding of dye molecule through π˗π interaction with the π conjugation region of GO.51,52 Sharma et al. reported that after adsorption of MB on GO, the intensity of the FTIR bands due to C=C and ˗SO3− groups decreased in comparison to the free dye molecule. This result suggested that the π˗π interactions took place between the dye molecule and the GO sheet.20 Thus π˗π coupling along with the electrostatic interaction are responsible for the adsorption of dye molecules on the GO sheets.

Toluene is used as model organic solvent due to strong π-π interactions

between the unoxidized GO basal plane and aromatic toluene molecules which promote the transfer of GO from aqueous phase to organic phase and highest extraction was achieved.50, 55 Herein, we have investigated the effect of different reaction conditions such as pH of the medium, presence of electrolytes on the bubble formation at the water-toluene interface as well as on the extraction process.

Effect of pH The effect of pH on the reactive extraction process was investigated by stirring a colloidal system consists of 5 mL aqueous phase containing GO (1 mg/mL) with 1 mM MB dye and 5 mL toluene (organic phase) for 10, 30, 40, 50, 60, 90, 120, 150, 200 and220 s at different pH (pH 2, 5, 7, 9) of the medium. Due to the amphiphilic behaviour of GO, it can be used as a surfactant between the aqueous and the organic phase and results in the formation of bubbles at the interface which are stable up to one week. At low pH range, with increase in the hydrophobicity of GO, it can easily transferred to the organic phase which facilitates the bubble formation and hence more bubbles are formed at the water-toluene interface. It was also found that with increase in the pH of the medium, the bubble size decreases due to the decrease in tendency of aggregation of hydrophilic GO.56 In our experiment, it is depicted in Figure 6 that the average bubble size was found to be around 443.5 µm, 263.3 µm, 206.54


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µm and 143.3 µm at pH 2, 5, 7 and 9, respectively. We observed that (Figure 2) with increase the pH of the medium, the surface of GO becomes more negative (the zeta potential of GO-water suspension was ~ −39 mV in basic medium) due to the more ionisation of the – COOH groups present on the GO surface by the OH− ions present in the medium. Hence the adsorption of the anionic dye molecules retarded by the repulsion between the negative surface and the dye molecule. Moreover, in high pH range, GO sheet becomes hydrophilic and it tends to stay in the water phase and the extraction of dye molecule from the water phase to the organic phase decreases. Whereas at low pH range the surface charge of GO surface tends towards positive value (zeta potential of GO-water suspension was ~ −23 mV in acidic medium) due the presence of unionized −COOH groups and thus the electrostatic attraction between the GO surface and the dye molecules increases and hence more adsorption takes place and extraction of dye molecule is also increased. The effect of pH on the reactive extraction process is depicted in the Figure 7. At low pH the GO surface becomes hydrophobic and tends to stay in organic phase and thus the transfer of GO from water to organic phase is become feasible hence extraction of dye molecules to the organic phase is facilitated by forming larger size bubbles at the interface. The amount of dye molecule transferred from the aqueous phase to the organic phase was found to be about 85.19% at pH 2 whereas at pH 9 the extraction amount is decreased to 76.45%.

Figure 6. Microscopy images of the droplets formed at water-toluene interface at (a) pH 2, (b) pH 5, (c) pH 7 and (d) pH 9 respectively. 25 ACS Paragon Plus Environment


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Figure 7. Extraction of MB (%) on GO surface at different pH

It is observed from the Figure 7 that the effect of stirring time also has significant influence on extraction process. At pH 2 the GO surface becomes more positive and π-π interaction and electrostatic interactions between the three negatively charged SO3− parts with the oxygen containing surface functional groups of GO are the driven forces for the adsorption. For extraction process, formation of bubbles at the water-organic interface plays a significant role. Less number of bubbles is formed at the water-toluene interface at low stirring time. (The photographs of the bubble formation at the water-toluene interface in different stirring time are shown in the supporting information, Figure S1). Hence, the extraction efficiency is very low at low stirring time. With increase the stirring time, more number of dye molecules come into contact with the GO sheets and more bubbles are formed at the interface hence extraction efficiency increases. Due to hydrophobicity, GO tends to remain at the organic phase and thus show more extraction at pH 2 in comparison to higher pH. At high pH, 26 ACS Paragon Plus Environment

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although dye molecules can bind with GO through both electrostatic interactions and π-π interaction, but due to hydrophilicity of GO it tends to remain at the aqueous phase. Therefore the extraction efficiency is maximum at the lowest pH (pH 2).

Effect of Cations The effect of cations on the extraction of dye molecules was evaluated by analysing the extraction of MB dye molecules from the aqueous phase to organic phase in presence of different salts NaCl (aq), MgCl2 (aq), CaCl2 (aq) and NH4Cl (aq) (same anion but different cations) at pH 2, 5, 7 and 9. Figure 8 shows the effect of different cations on the extraction process of the dye molecules. It is seen from the Figure 8 that the extraction efficiency of MB dye molecule in presence of 0.01M salt solutions is followed the order GO (aq) < NH4Cl (aq) < NaCl (aq) < MgCl2 (aq) < CaCl2 (aq). It is due to the surface potential (zeta Potential) of GO in presence of different cations increases from NH4+ (aq) to Ca2+ (aq) ions due to their polarizability effect. Because of the presence of positive surface charge, GO can bind more strongly with the negatively charged MB dye molecule and the transfer of the dye adsorbed GO from water to organic phase is increased by the forming more bubbles at the interface due to more hydrophobic nature of GO (positive zeta potential). It was observed from Figure 8 that percentage of extraction of MB was found 99.01% and 94.16% in presence of Ca2+ (aq) and NH4+ (aq) ions, respectively at pH 2 within 200 s but at pH 9, percentage of extraction decreases to 92.68% in presence of Ca2+ (aq) and to 86.43% in presence of NH4+ (aq) ions within 200 s because of the increase in hydrophilic nature of the GO sheets in basic medium. The binding of negative MB dye molecule with the GO sheet in presence of different cations also depends on the polarizability of the ions. Highly polarizable cations bind more strongly with the negative dye molecules through electrostatic interactions and hence facilitate the extraction to the organic phase than the less polarizable cations. Hence extraction of MB in



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presence of divalent Ca2+ (aq) and Mg2+ (aq) ions is larger than the monovalent Na+ (aq) and NH4+ (aq) ions. The above experimental results show that, the pH of the medium also plays an important role in the extraction process of negatively charged MB dye molecule on the GO surface in presence of inorganic cations.

Figure 8.

Extraction of MB (%) on the GO surface in presence of 0.01M chloride salt

solutions of different cations at pH 2, pH 5, pH 7 and pH 9 in 200 s.

We have also investigated the extraction of the MB in presence of 0.1M salt solutions and it was observed that the extraction process is enhanced with increasing the concentration of the salt solutions. The effect of 0.1M Na+ (aq) and NH4+ (aq) ions on the extraction process is similar to the effect of 0.01M salt solutions (Figure 9). But the presence of 0.1M Ca2+ (aq) and Mg2+ (aq) ions the extraction of MB dye molecule from aqueous phase to organic phase


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increases with the increase of pH of the medium due to the more hydrophobicity of GO in basic medium (Figure 9). Both these cations show a significant effect before and after their isoelectric point. For Mg2+ (aq) ion, before its isoelectric point (pH ~7.5) the GO sheet remains slightly hydrophilic (negative zeta potential below ~ −9 mV with decrease in pH) whereas after the isoelectric point the GO surface becomes completely hydrophobic due to its positive surface charge (positive zeta potential above ~ 1 mV with increase in pH) (Figure 3b). Due to its high hydrophobic nature GO tends to remain at the organic phase and hence transfer of GO from the water to toluene phase facilitated due to strong π-π interactions between the unoxidized basal plane of GO and aromatic ring of toluene. Therefore, the extraction of MB from the water to toluene phase in presence of Mg2+(aq) ions takes place up to 99.73% at pH 9. Similarly, presence of Ca2+ (aq) ions also plays a significant role on the extraction process of MB before and after its isoelectric point (pH ~ 8.3) (Figure 10). Due to increase in hydrophobicity of GO with high surface charge it tends to remain in the organic phase and GO completely transferred to the organic phase through the formation of bubbles. In presence of Ca2+(aq) ion maximum extraction (99.33%) takes place at pH 9 (Figure 10). The variation in the extraction process in presence of Ca2+ (aq) and Mg2+ (aq) ions is due to the different polarizability of the cations.



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Figure 9. Extraction of MB (%) on GO surface in presence of 0.1M chloride salt solutions of different cations at pH 2, pH 5, pH 7 and pH 9 in 200 s.

Figure 10. Effect of 0.1M chloride salt solution of Mg2+ (aq) and Ca2+ (aq) ions on extraction of MB before and after their isoelectric point at pH 7.5 and 8.3 respectively in 200 s.


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Effect of anions The extraction of the dye molecule from the aqueous phase to the organic phase is also effected by the different salt solutions present in the mixture. The variation of the extraction with the presence of 0.01M different salt solutions namely NaCl (aq), NaBr (aq), NaNO3 (aq), Na2SO4 (aq), CH3COONa (aq), NaSCN (aq) (same cation and different anions) is shown in the Figure 11. It is seen from the Figure 11 that the percentage of extraction increases in the order Br− (aq) < Cl − (aq) < SCN− (aq) < NO3− (aq) < SO42− (aq) < CH3COO− (aq) at pH 2 which depends on the surface potential of GO. Within 200 s, the maximum extraction of MB dye molecule (about 95.32%) was observed in presence of CH3COO− (aq) anions because of the low negative surface charge (~ −16mV) and hence increase in hydrophobicity of GO, it easily transfer to the organic phase whereas the lowest extraction of dye (88.32%) was observed in presence of NaBr (aq) due to the hydrophilicity nature of GO with high negative surface charge (~ −20mV) which is larger than the GO alone (extraction 85.13%) due to the presence of Na+ of the salt solution and less negative surface charge of GO. Less negative the surface charge of GO (high zeta potential value), the electrostatic repulsion of the SO3− (aq) part of MB dye molecule with the oxygen containing functional groups of GO is become less effective hence results in the more extraction of the dye molecules. Moreover in presence of the salt solutions, the GO surface becomes more positive and more hydrophobic. As a result, the transfer of GO-dye complex from the aqueous phase to the organic phase is increased. With the increase in pH of the medium the variation of surface charge in presence of different salt solutions mainly depends upon the ion-ion interactions, polarizability of ions and hence the extraction of MB also effected by the different ions significantly at different pH. Figure 11 shows the variation of the extraction of MB dye molecules in presence of different anions at different pH. At high pH value (pH 9) the extraction follows the order Na2SO4 (aq) < GO (aq) ~ NaSCN (aq) ~NaCl (aq) ~NaNO3 (aq) < NaBr (aq) < CH3COONa 31 ACS Paragon Plus Environment


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(aq) which is related to the surface potential of GO in presence of different salts. The extraction of dye molecule in presence of CH3COONa (aq) salt was about 86.27% (due to low negative surface charge of GO ~ −29 mV) within 200 s. Where as in presence of Na2SO4 (aq) the extraction decreases to 73.55% due to the more hydrophilicity of GO with surface charge ~ −42 mV and the presence of more polarizable divalent SO42− (aq) ions, strong repulsions between the negative ions and the negative dye molecules. Thus it is seen from the above observations that the pH of the medium as well as the presence of different anions effect on the reactive extraction process due to the variation of the surface property of GO as a function of pH in presence of different salt solutions. Similar observations are occurred in presence of 0.1M salt solutions which is shown in Figure 12. It was observed that the extraction process is more facilitated in presence of 0.1M salt solutions due the presence of more cations at the interface and hence more less negative value of the surface charge of GO and hence strong interaction between the GO and the dye molecules takes place which results in the more extraction of the dye to the organic phase


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Figure11. Extraction of MB (%) on the GO surface in presence of 0.01M sodium salt solutions of different anions at pH 2, pH 5, pH 7 and pH 9 in 200 s.

Figure12. Extraction of MB (%) on GO surface in presence of 0.1M sodium salt solutions of different anions at pH 2, pH 5, pH 7 and pH 9 in 200 s. 33 ACS Paragon Plus Environment


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Reusability study The stability of the adsorbent has a great importance in the field of environmental remediation. To investigate the stability of the adsorbent, reusability of the adsorbent materials is carried out to study the adsorption behavior. The variation of the surface property of adsorbent in presence of different inorganic ions has considerable effect on the adsorption efficiency of an adsorbent. Keeping in view the importance of this investigation, the reusability of GO sheets towards the extraction of methyl blue was performed with the GO of concentration 1 mg/mL and 1mM MB dye solution in presence of 0.01M solutions of CaCl2 (aq), NaCl (aq), Na2SO4 (aq) and CH3COONa (aq) inorganic salts at pH 2 and temperature 25 o

C. The reusability study was performed upto four cycles with stirring time 200 s. After each

cycle, GO was separated by centrifugation and repeatedly washed with hot water and acetone and then dried in an air oven at 60 oC for 4h.The extraction efficiency was found around 99.01%, 86.54%, 78.43% and 72.76% for 1st, 2nd, 3rd and 4th cycle in presence of CaCl2 (aq) salt solution whereas the efficiency of GO was about 95.72%, 84.54%, 75.54% and 67.43% for 1st, 2nd , 3rd and 4th cycle in presence of NaCl (aq) salt solution. Similarly, the extraction efficiency of GO was found to be about 95.45%, 73.75%, 70.43% and 61.54% ; 95.32%, 88.12%, 72.12% and 63.45 % in presence of Na2SO4 (aq) and CH3COONa (aq) salt solution respectively. It is found that the extraction efficiency of GO towards MB extraction in each repeated cycle is decreased. The adsorption of dye molecule on the GO surface may occupy the active sites of the adsorbent in each cycle and thus retard the adsorption of dye molecules on the adsorbent surface. The reusability study of GO towards MB dye molecule upto 4th cycle is shown in the Figure 13.


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Figure 13: Reusability study of GO towards reactive extraction of MB in presence of 0.01M solutions of different inorganic salts NaCl (aq), CaCl2 (aq), Na2SO4 (aq) and CH3COOH (aq).

CONCLUSIONS This study investigated the effect of different inorganic ions on zeta potential at the GOelectrolyte interface and the extraction of MB dye from the aqueous phase to the organic phase using amphiphilic GO as a carrier. The zeta potential of GO surface is significantly affected by the different inorganic ions which mainly depend upon the surface charge and size of the specific ions of the salt and the pH of the suspension. MD simulations were used to study the interactions between cations and anions with the surface of GO and correlate to the measured zeta potential to the presence of ions in molecular level. The influence of 35 ACS Paragon Plus Environment


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different ions on the zeta potential of GO suspension predicted from the determination of RDF of the background inorganic ions in the colloidal suspension is very similar to the experimental measurement. MD simulation also proposes that the concentration of the background electrolyte also play an important role on zeta potential of a colloidal suspension. This study suggests that MD simulation is an advantageous tool to predict the specific ion effect on the surface potential of colloidal suspensions. Presence of a salt increases the extraction efficiency of MB from aqueous phase to organic phase due to the increase in intermolecular interactions between the adsorbent and adsorbate as well as the hydrophobicity of GO which is depend on the surface potential of GO. From the comparison of zeta potential measurements and extraction efficiency of methyl blue (MB) in presence of different salt solutions it can be concluded that extraction increases when zeta potential becomes more positive. This is due to the accumulation of positive charge around carboxylic side groups of GO which is favourable for binding of negatively charged SO3− groups of MB. Electrostatic interactions of these groups together with hydrophobic interactions with the surface of GO lead to formation of a neutral and highly hydrophobic complex between MB and GO

which is important for effective extraction from aqueous media into the organic

phase. Furthermore, the positive charge of the amino group in the central part of MB can bind with the polarizable anions present on the hydrophobic GO surface. Moreover, hydrogen atoms of amino group can bind with the –COOH groups of GO through hydrogen bonding in basic medium

which makes the extraction of dye more efficient. From the above

experimental findings it can be concluded that the surface property of an adsorbent is driven by the presence of different specific ions which is an essential factor in reactive extraction process. This work provides a new route for the study of reactive extraction technique using different adsorbent in presence of different salts which is influenced by the surface property of the adsorbent.


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ASSOCIATED CONTENT Supporting Information The images of bubbles formation at water – toluene interface in presence of GO in different times are shown in supporting information. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected], [email protected]. Tel.: +91- 9957178399 *E-mail: [email protected]. Tel.: +420 389 033 801 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors are grateful to the Director, CSIR−NEIST, Jorhat, for his interest to carry out the work. PB acknowledges DST, New Delhi, India for DST-INSPIRE Fellowship grant. NH acknowledges UGC, New Delhi, India for JRF grant. BM acknowledges support from the Grant Agency of the Czech Republic Czech Science Foundation (Project 13-08651S). Also, access to instruments and other facilities was supported by the Czech research infrastructure for systems biology C4SYS (project no LM2015055)



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REFERENCES

(1)

Wiese, G.R.; James, R.O.; Yates, D.E.; Healy, T.W. Electrochemistry of the Colloid Water Interface. Int. Rev. Sci., Phys. Chem. Ser. 2, Bockris, J.O’M., Ed, Butterworths London 1976, 6, 53-102.

(2)

James, R.O.; Parks, G.A.

Characterization of Aqueous Colloid by Their Electrical

Double-Layer and Intrinsic Surface Chemical Properties. Surface and Colloid Science, Matijevic, E., Ed, Wiley-Interscience, New York 1982, 12, 119-216. (3)

Imamura, T.; Mizukoshi, Y.; Ishiyama, T.; Morita, A. Surface Structures of NaF and Na2SO4 Aqueous Solutions: Specific Effects of Hard Ions on Surface Vibrational Spectra. J. Phys. Chem. C 2012, 116, 11082−11090.

(4)

Hofmeister, F. About the Science of the Effect of Salts. Arch. Exp. Pathol. Pharmakol. 1888, 24, 247-260.

(5)

Kunz, W.; Henle, J.; Ninham, B.W. About the Science of the Effect of Salts. Franz Hofmeister’s Historical Papers. Curr.Opin.Colloid Interface Sci. 2004, 9, 19-37.

(6)

Traube, J. The Attraction Pressure. J. Phys. Chem. 1910, 14, 452-470.

(7)

Lopez-Leon, T.; Santander-Ortega, M. J.; Ortega-Vinuesa, J.L.; Bastos- Gonzalez, D. Hofmeister Effects in Colloidal Systems: Influence of the Surface Nature.

J.Phys.Chem.C 2008, 112, 16060-16069. (8)

Lopez-Leon, T.; Jodar-Reyes, A.B.; Bastos-Gonzalez, D.; Ortega-Vinuesa, J.L. Hofmeister Effects in the Stability and Electrophoretic Mobility of Polystyrene Latex Particles. J. Phys. Chem. B 2003, 107, 5696-5708.

(9)

Lopez-Leon, T.; Jodar-Reyes, A. B.; Ortega-Vinuesa, J. L.; Bastos- Gonzalez, D. Hofmeister Effects on the Colloidal Stability of an IgG-coated Polystyrene Latex.

Colloid Interface Sci. 2005, 284,139-148.


J.

Page 39 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(10) Das, M. R.; Borah, J. M.; Kunz, W.; Ninham, B. W.; Mahiuddin, S. Ion Specificity of the Zeta Potential of α-Alumina, and of the Adsorption of p-Hydroxybenzoate at the αAlumina−Water Interface. J. Colloid Interface Sci. 2010, 344, 482−491. (11) Bizek, V.; Horacek, J.; Rericha, R.;

Kousova, M.

Amine Extraction of

Hydrocarboxylic Acids. 1. Extraction of Citric acid with 1-octanol/n-heptane Solutions of Trialkylamine. Ind. Eng. Chem. Res. 1992, 31, 1554-1562. (12) Juang, R.S.; Huang, R.H. Equilibrium Studies on Reactive Extraction of Lactic acid with an Amine Extractant. Chem. Eng. J. 1997, 65, 47–53. (13)

Tang, K.; Wu, G.; Zhang, P.; Zhou, C.; Liu, J. Experimental and Model Study on

Enantioselective Reactive Extraction of p-hydroxyphenylglycine Enantiomers with Metal Phosphine Complexes. Sep. Purif. Technol. 2013, 115, 83-91. (14)

Jiang, Y.; Li, D.; Li, Y.; Gao , J.; Zhou, L.; He, Y. In situ Self-catalyzed Reactive

Extraction of Germinated Oilseed with Short-chained Dialkyl Carbonates for Biodiesel Production. Bioresour.Technol. 2013, 150, 50-54. (15)

Gu, H.; Jiang, Y.; Zhou, L.; Gao, J. Reactive Extraction and in situ Self-catalyzed

Methanolysis of Germinated Oilseed for Biodiesel Production. Energy Environ. Sci. 2011, 4, 1337-1344. (16) Fan, L.; Luo, C.; Sun, M.; Li, X.; Lu, F.; Qiu, H. Preparation of Novel Magnetic Chitosan/Graphene Oxide Composite as Effective Adsorbents toward Methylene blue.

Bioresour. Technol. 2012, 114, 703-706. (17) Sharma, P.; Borah, D.J.; Das, M.R. Graphene Oxide Nanosheets at the Water- Organic Solvent Interface: Utilization in One-Pot Adsorption and Reactive Extraction of Dye Molecules. Chem Phys Chem 2014, 15, 4019-4025. (18) Pickering. S.U. Emulsions. J. Chem. Soc., Trans. 1907, 91, 2001-2021.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(19) Das, M.R.; Sarma, R.K.; Saikia, R.; Kale, V.S.; Shelke, M.V.; Sengupta, P. Synthesis of Silver Nanoparticles in an Aqueous Suspension of Graphene Oxide Sheets and its Antimicrobial Activity. Colloids Surfaces B: Biointerfaces 2011, 83, 16–22. (20)

Sharma, P.; Hussain, N.; Borah, D.J.; Das, M.R. Kinetics and Adsorption Behavior of the Methyl Blue at the Graphene Oxide/Reduced Graphene Oxide Nanosheet−Water Interface: A Comparative Study. J. Chem. Eng. Data 2013, 58, 3477−3488.

(21) Callaghan, I.C.; Ottewill, R.H. Interparticle Forces in Montmorillonite Gels. Faraday

Discussions of the Chemical Society 1974, 57, 110-118. (22) Martínez, J.M.; Martínez, L. Packing Optimization for Automated Generation of Complex System's Initial Configurations for Molecular Dynamics and Docking. J.

Comput. Chem. 2003, 24, 819-825. (23) Martínez, L.; Andrade, R.; Birgin, E.G.; Martínez, J.M. Packmol: A Package for Building Initial Configurations for Molecular Dynamics Simulations. J. Comput. Chem. 2009, 30, 2157-2164. (24) Wang, J.; Wolf, R. M.; Caldwell, J. W.; Kollman, P. A.; Case, D. A. Development and Testing of a General AMBER Force Field. J. Comput.Chem.2004, 25, 1157-1174. (25) Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Montgomery Jr. J.A.; Vreven, T.; Kudin, K.N.; Burant, J.C.; et al. Gaussian 03 (Revision D.01), Gaussian, Inc., Wallingford CT, 2004. (26) Bayly, C.I.; Cieplak, P.; Cornell, W.; Kollman, P.A. A Well Behaved Electrostatic Potential Based Method using Charge Restraints for Deriving Atomic Charges: the RESP Model. J. Phys. Chem. 1993, 97, 10269–10280.


Page 40 of 45

Page 41 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(27) Wang, J.; Wang, W.; Kollman P. A.; Case, D. A. Automatic Atom Type and Bond Type Perception in Molecular Mechanical Calculations. J. Mol. Graph. Model. 2006, 25, 247-260. (28) Hess, B.; Bekker, H.; Berendsen, H.J.; Fraaije, J.G. LINCS: A Linear Constraint Solver for Molecular Simulations. J. Comput. Chem. 1997, 18, 1463-1472. (29) Darden, T.; York, D.; Pedersen, L. Particle mesh Ewald: An N⋅ log (N) Method for Ewald Sums in Large Systems. J. Chem. Phys. 1993, 98, 10089-10092. (30) Bussi, G.; Donadio, D.; Parrinello, M. Canonical Sampling through Velocity Rescaling.

J. Chem. Phys. 2007, 126, 014101. (31) Van der Spoel, D.; Lindahl, E.; Hess, B.; Groenhof, G.; Mark, A.E.; Berendsen, H. J. C. GROMACS: Fast, Flexible and Free. J. Comput. Chem. 2005, 26, 1701-1719. (32) Lindahl, E.; Hess, B.; van der Spoel, D. GROMACS 3.0: A Package for Molecular Simulation and Trajectory Analysis. J. Mol. Mod. 2001, 7, 306-317. (33) Berendsen, H.J.C.; van der Spoel, D.; Van Drunen, R. GROMACS: A Message-Passing Parallel Molecular Dynamics Implementation. Comp. Phys. Comm. 1995, 91, 43-56. (34) Humphrey, W.; Dalke, A.; Schulten, K. VMD: Visual Molecular Dynamics. J. Mol.

Graph. 1996, 14, 33–38. (35) Miller, Y.; Thomas, J.L.; Kemp, D.D.; Finlayson-Pitts, B.J.; Tobias, D.J.; Gerber, R.B. Structure of Large Nitrate–Water Clusters at Ambient Temperatures: Simulations with Effective Fragment Potentials and Force Fields with Implications for Atmospheric Chemistry. J. Phys. Chem. A 2009, 113, 12805– 12814.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(36) Grim R.E. Clay Mineralogy 2nd edition, McGraw-Hill Book Company, New York, 1968, 34-35. (37) Vane, L.M.; Zang, G.M. Effect of Aqueous Phase Properties on Clay Particle Zeta Potential and Electroosmotic Permeability: Implications for Electro-kinetic Soil Remediation Processes. J. Hazard. Mater. 1997, 55, 1-22. (38) Schwierz, N.; Horinek, D.; Netz, R.R. Anionic and Cationic Hofmeister Effects on Hydrophobic and Hydrophilic Surfaces. Langmuir 2013, 29, 2602−2614. (39) Para, G.; Jarek, E.; Warszynski, P. The Hofmeister Series Effect in Adsorption of Cationic Surfactants-Theoretical Description and Experimental Results. Adv. Colloid

Interface Sci. 2006,122, 39-55. (40) Wingen, L. M.; Moskun, A. C.; Johnson, S. N.; Thomas, J. L.; Roeselova, M.; Tobias, D. J.; Kleinman, M. T.; Finlayson-Pitts, B. J. Enhanced Surface Photochemistry in Chloride-Nitrate Ion Mixtures. Phys. Chem. Chem. Phys. 2008, 10, 5668–5677. (41) Tian, C.; Byrnes, S.J.; Han, H.L.; Shen, Y.R. Surface Propensities of Atmospherically Relevant Ions in Salt Solutions Revealed by Phase-Sensitive Sum Frequency Vibrational Spectroscopy. J. Phys. Chem. Lett. 2011, 2, 1946–1949 (42) Salvador, P.; Curtis, J. E.; Tobias, D. J.; Jungwirth, P. Polarizability of the Nitrate Anion and Its Solvation at the Air/Water Interface. Phys. Chem. Chem. Phys. 2003, 5, 37523757. (43) Petersen, P. B.; Mucha, M.; Jungwirth, P.; Saykally, R. J. Enhanced Concentration of Polarizable Anions at the Liquid Water Surface: SHG Spectroscopy and MD Simulations of Sodium Thiocyanide. J. Phys. Chem. B 2005, 109, 10915-10921.


Page 42 of 45

Page 43 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(44) Minofar, B.; Jungwirth, P.; Das, M. R.; Kunz, W.; Mahiuddin, S.: Jungwirth, P. Propensity of Formate, Acetate, Benzoate, and Phenolate for the Aqueous Solution/Vapour Interface: Surface Tension Measurements and Molecular Dynamics Simulations. J. Phys. Chem. C 2007, 111, 8242-8247. (45) Minofar, B.; Vacha, R.; Wahab, A.; Mahiuddin, S.; Kunz, W.; Jungwirth, P. Propensity for the Air/Water Interface and Ion Pairing in Magnesium Acetate vs. Magnesium Nitrate

Solutions:

Molecular

Dynamics

Simulations

and

Surface

Tension

Measurements. J. Phys. Chem. B 2006, 110, 15939-15944. (46) Wahab, A.; Mahiuddin, S.; Hefter, G.; Kunz, W.; Minofar, B.; Jungwirth, P. Ultrasonic Velocities, Densities, Viscosities, Electrical Conductivities, Raman Spectra, and Molecular Dynamics Simulations of Aqueous Solutions of Mg(OAc)2 and Mg(NO3)2: Hofmeister Effects and Ion Pair Formation. J. Phys. Chem. B 2005, 109, 24108-24120. (47) Minofar, B.; Vrbka, L.; Mucha, M.; Jungwirth, P.; Yang, X.; Wang, X.B.; Fu, Y.J.; Wang, L.S. Interior and Interfacial Aqueous Solvation of Benzene Dicarboxylate Dianions and Their Methylated Analogues: A Combined Molecular Dynamics and Photoelectron Spectroscopy Study. J. Phys. Chem. A 2005, 109, 5042-5049. (48) Harper, K.; Minofar, B.; Sierra-Hernandez, M.R.; Casillas-Ituarte, N.N.; Roeselova, M.; Allen, H.C. Surface Residence and Uptake of Methyl Chloride and Methyl Alcohol at the Air/Water Interface Studied by Vibrational Sum Frequency Spectroscopy and Molecular Dynamics. J. Phys. Chem. A 2009, 113, 2015-2024. (49) Giupponi, G.; Pagonabarraga, I. Determination of the Zeta Potential for Highly Charged Colloidal Suspensions, Philos. Trans. A Math. Phys. Eng. Sci. 2011, 369, 2546-2554.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(50) Kim, J.; Cote, L.J.; Kim, F.; Yuan, W.; shull, K.R.; Huang, J. Graphene Oxide Sheets at Interfaces. J. Am. Chem. Soc. 2010, 132, 8180-8186. (51) Ramesha, G.K.; Kumara, A.V.; Muralidhara, H.B.; Sampath, S. Graphene and Grapheme Oxide as Effective Adsorbents toward Anionic and Cationic Dyes. J. Colloid

Interface Sci. 2011, 361,270-277. (52) Bradder , P.; Ling, S.K.; Wang, S.; Liu, S. Dye Adsorption on Layered Graphite Oxide.

J. Chem. Eng. Data 2011, 56, 138-141. (53) Apul, O. G.; Wang, Q.; Zhou, Y.; Karanfil, T. Adsorption of Aromatic Organic Contaminants by Graphene Nanosheets: Comparison with Carbon Nanotubes and Activated Carbon. Water Res. 2013, 47, 1648−1654. (54) Li, Y.; Du, Q.; Liu, T.; Sun, J.; Jiao, Y.; Xia, Y.; Xia, L.; Wang, Z.; Zhang, W.; Wang, K.; Zhu, H.; Wu, D. Equilibrium, Kinetic and Thermodynamic Studies on the Adsorption of Phenol onto Graphene. Mater. Res. Bull. 2012, 47, 1898−1904. (55) He, Y.; Wu, F.; Sun, X.; Li, R.; Guo, Y.; Li, C.; Zhang, L.; Xing, F.; Wang, W.; Gao, J. Factors that Affect Pickering Emulsions Stabilized by Graphene Oxide. ACS Appl.

Mater. Interf. 2013, 5, 4843-4855. (56) Shih, C.J.; Lin, S.; Sharma, R.; Strano, M.S.; Blankschtein, D. Understanding the pHDependent Behavior of Graphene Oxide Aqueous Solutions: A Comparative Experimental and Molecular Dynamics Simulation Study. Langmuir 2012, 28, 235241.


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Graphical Abstract


Experimental and Molecular Dynamics Simulation Study of Specific

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