Article Cite This: Langmuir XXXX, XXX, XXX−XXX
pubs.acs.org/Langmuir
Competitive Adsorption of Ions at the Oil−Water Interface: A Possible Mechanism Underlying the Separation Selectivity for Liquid−Liquid Solvent Extraction Pan Sun,†,§ Kun Huang,*,†,‡ and Huizhou Liu†
Langmuir Downloaded from pubs.acs.org by UNIV OF TEXAS AT EL PASO on 10/24/18. For personal use only.
†
CAS Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, P. R. China § University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *
ABSTRACT: Adsorption, especially competitive adsorption of ions at the interfaces, governs a wealth of physicochemical processes. Understanding the mechanism behind these interfacial behaviors is crucial for developing novel strategies to intensify reactions or transfer processes. Herein, as an example, we found that in the case of liquid−liquid transport of V(V) and Cr(VI) ions, the competitive adsorption of V(V) and Cr(VI) ions against coexisting SO42− ions at the oil−water interface exhibits a significant impact on the selective separation behaviors of V(V) and Cr(VI) ions. The transport of Cr(VI) ions would be hindered by adding Na2SO4 into the aqueous solutions because of the competitive adsorption of SO42− ions at the interface being stronger than that of Cr(VI) ions, whereas the transport of V(V) ions would not be affected because of the stronger affinity of V(V) ions to the interfaces compared to that of SO42− ions. The present work provides new inspirations for developing efficient strategies to improve the separation efficiency of target ions with similar physic-chemical properties by regulating their adsorption behaviors at the interface. It is beneficial to get a deeper understanding into the microscopic nature of competitive adsorption behaviors of ions at interfaces from the interface-molecular level.
■
mass transfer.11,12 During the transport of ions from the aqueous phase into the organic phase, interfacial self-assembly, adsorption, and distribution behaviors of organic extractant molecules extensively occurred, because of their amphiphilic structural characters.13−15 However, interface propensity of the target ions to be extracted and the effects from other coexisting interfering ions were scarcely reported. Generally, the ability of different ions adsorbed at the interface depends on their hydrophilicity and/or hydrophobicity.16 Therefore, only the ions prefer to be absorbed at the interface could interact with the organic extractant molecules, and then be extracted into the organic phase. As reported in previous works, the adsorption behaviors of ions at interfaces could be estimated by their distribution behaviors in the interfacial region.17 However, direct
INTRODUCTION
Adsorption and distribution of inorganic ions at gas−liquid, liquid−liquid, and solid−liquid interfaces have attracted considerable attentions because of its scientific and technological importance toward various physical and chemical processes.1−3 Examples include heterogeneous chemical reactions at the interfaces,4 ions separation by liquid−liquid extraction,5 membranes separation,6 and interaction at the biomembranes.7,8 Although great efforts have been made to elucidate the competitive adsorption behaviors of different ions at the interfaces and their impact on the changes in interfacial molecule−ion interaction, understanding into the microscopic mass transfer mechanism of ions across the interface is still relatively poor, particularly at the molecular level. The interface between two immiscible liquid phases is a place where various heterogeneous chemical reactions and transport occur.9,10 The chemical reactivity of reaction species, organic extractant molecules, and target ions at the interface play a crucial role in determining the efficiency of reactions and © XXXX American Chemical Society
Received: August 8, 2018 Revised: September 24, 2018 Published: October 14, 2018 A
DOI: 10.1021/acs.langmuir.8b02691 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir monitoring of distribution of ions in the interfacial region is extremely difficult. Numerous studies were performed by concerning the changes in structures of interfaces induced by the adsorption of ions at the interfaces, such as the measurement of interface tension,18−20 surface potentials,21−23 and water structures at interfaces.24−26 Therefore, an in situ study of distribution of ions at the interface was urgent for the understanding of the interfacial adsorption behavior of ions. Attenuated total reflectance−Fourier transform infrared (ATR−FTIR) is an interface selective spectroscopy and has wide application in the study of interfacial behavior of ions. For example, the adsorption of sulfate ions on goethite in the presence of copper ions was studied by ATR−FTIR.27 Besides, the adsorption of perchlorate, sulfate, and thiosulfate ions onto chromium(III) oxide hydroxide thin films was also elucidated by in situ ATR−FTIR spectroscopy.28 The experimental results demonstrated that ions could be divided into two categories: kosmotropic ions and chaotropic ions, according to their affinity to the interfaces.29,30 Generally, kosmotropic ions are strongly hydrated and are repelled from the interface. By contrast, chaotropic ions are easy to lose their hydration shell and prefer to be adsorbed at the interface. When these ions coexist in aqueous solutions, the distinct interface propensity of different ions toward the oil−water interface always induces competitive adsorption behaviors of ions at the interface, which is of pivotal importance to the selectivity of interfacial reactions and the efficiency of mass transfer processes. In our previous work, we found that addition of Na2SO4 could selectively inhibit the transport of Cr(VI) ions from the aqueous solution into the organic phase. By contrast, the transport of V(V) ions would not be affected by the addition of Na2SO4.31 Taken into account the distinct interface propensity of those ions, we supposed that the transport selectivity of V(V) and Cr(VI) ions across the interface resulted from the competitive adsorption behaviors of those coexisting ions at the interface. Notably, the competitive adsorption behaviors of ions at the interface were actually a dynamic process. However, the study of kinetic effect on the competitive adsorption of ions at the interface was extremely difficult. In the present work, the adsorption behaviors of V(V) and Cr(VI) ions at the air/aqueous solution interface were investigated by ATR− FTIR spectroscopy. It was expected to provide a basis for the understanding of adsorption behaviors of ions at the liquid− liquid interface. ATR experimental results indicated that V(V) and Cr(VI) ions exhibited different adsorption selectivity with the increase of Na2SO4 concentration in the aqueous solutions, which was responsible to explain the different transport behaviors of V(V) and Cr(VI) ions. Molecular dynamics (MD) simulation was employed to obtain molecule-level understanding into the competitive adsorption behaviors of various coexisting ions at the interfaces.
■
Scheme 1. Structures of Extractant N1923 Used in This Study
on a Bruker Vector 22 FTIR spectrometer with a substrate of ZnSe crystal in the region from 500 to 2000 cm−1 using a deuterotriglycine sulfate detector at room temperature (25 °C). In each experiment, ATR−FTIR spectra for the surface of ultrapure water were used as the background. The spectra were recorded from 128 coadded scans at 2 cm−1 resolution and analyzed using OPUS spectroscopic software. MD Simulation Details. MD simulations were performed with the program NAMD, version 2.10.32 The CHARMM force field33 was used for the ions, and the TIP3P model was employed to describe water molecules.34 The force-field parameters and geometry parameters of anions were taken from the works reported in the previous literature.35,36 Graphical force-field toolkit available in VMD package37 was utilized to compute the missing parameters. All quantum-chemical calculations were carried out using the Gaussian 09 suite of programs, and the optimal structure of ions is given in Figure S1 in the Supporting Information.38 Snapshot of initial simulation box is given in Figure S2 in the Supporting Information. The components of different simulated systems are shown in Table S1 in the Supporting Information. Other simulation details are also provided in the Supporting Information.
■
RESULTS AND DISCUSSION The selective transport of V(V) and Cr(VI) ions across the oil−water interface during the increase of Na2SO4 concentrations. Figure 1 gives the effect of Na2SO4 concentration on the transport efficiency of V(V) and Cr(VI) ions from the aqueous
Figure 1. The effect of Na2SO4 concentration on the transport efficiency of V(V), Cr(VI) ions, and separation factor βV/Cr. The experimental condition: volume ratio of oil to aqueous phase was 1:20. The concentration of V(V) and Cr(VI) in the aqueous solutions was 5 mmol/L,. The volume fraction of organic extractant, primary amine N1923, in the heptane organic phase was 20%, v/v. Other experimental conditions were same as that stated in the ref 31.
EXPERIMENTAL SECTION
Materials. Na2CrO4 and Na2SO4 were purchased from Aladdin Industrial Corporation. NaVO3 was purchased from Sinopharm Chemical Reagent Co., Ltd., China. Primary amine N1923 (93%) was kindly supplied by Shanghai Rare-earth Chemical Co., Ltd. The structure of primary amine N1923 is shown in Scheme 1. All aqueous solutions were prepared using ultrapure water (18.2 MΩ) from a millipore simplicity purification system. The pH of the aqueous solutions was adjusted by addition of small quantities of H2SO4 or NaOH solutions. Attenuated Total Reflectance Infrared Spectroscopy. ATR− FTIR spectra for the surface of the aqueous solutions were recorded
solution into the organic phase. As can be seen, 98.52% of V(V) ions and 90.04% of Cr(VI) ions transport into the organic phase when Na2SO4 is not added into the aqueous solutions. However, with the increase of Na2SO4 concentrations, the transport efficiency of Cr(VI) ions decreases significantly. When Na2SO4 concentration is above 0.20 mol/ L, the transport efficiency of Cr(VI) ions decreases sharply to B
DOI: 10.1021/acs.langmuir.8b02691 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Figure 2. (a) ATR−FTIR spectra of V(V) ions in the alkaline aqueous solutions containing different concentrations of V(V) (the aqueous pH was 12. The curves 1−7 represent the V(V) concentration of 1, 2, 4, 10, 15, 20, and 30 mmol/L, respectively.); (b) ATR−FTIR spectra of Cr(VI) ions in the alkaline solutions containing different concentrations of Cr(VI). (The aqueous pH was 12. The curves 1−7 represent the Cr(VI) concentration of 1, 2, 4, 10, 15, 20, and 30 mmol/L, respectively.).
Figure 3. (a) ATR−FTIR spectra of V(V) aqueous solutions with addition of different concentrations of Na2SO4. (The aqueous pH was 12. The concentration of V(V) was 0.025 mol/L. The curves 1−10 represent the Na2SO4 concentration of 0, 0.01, 0.04, 0.1, 0.15, 0.3, 0.5, 0.7, 0.9, and 1.13 mol/L, respectively.); (b) Enlarged view of the ATR−FTIR spectra in (a) from 750 to 950 cm−1.
SO42− ions, could change the existing form of V(V) and Cr(VI) ions based on the following reactions: HVO42− + HSO4− = H2VO4− + SO42− or CrO42− + HSO4− = HCrO4− + SO42−.39 However, under such high pH conditions in the present work, HSO4− ions did not exist in the aqueous solutions. Thus, the existing form of V(V) and Cr(VI) ions in the aqueous solutions would not be affected by SO42− ions. According to the thermodynamic analysis, in our experimental conditions, V(V) and Cr(VI) ions existed in the forms of HVO42− and CrO42−, respectively, in the aqueous solution. As depicted in Figure 2, the peaks located at about 860 and 875 cm−1 were attributed to HVO42− and CrO42− ions, respectively.40,41 Obviously, the intensity of ATR absorption peaks of V(V) and Cr(VI) ions increases gradually with the increase of their corresponding concentrations in the aqueous solutions. According to the Gibbs adsorption equation, the concentration of ions at the interface would increase with increasing the bulk concentration of them. Hence, the increase in intensity of ATR peaks of V(V) and Cr(VI) ions might be resulted from the increase in their surface concentrations. The variation in the intensity of ATR peaks reflects the change in the surface concentration of V(V) and Cr(VI) ions, that is, adsorption behaviors of V(V) ions and Cr(VI) ions at the interface. The competitive adsorption behaviors of V(V) and Cr(VI) ions. Figure 3a gives the ATR−FTIR spectra of V(V) solutions containing different concentration of Na2SO4. The ATR peak located at about 1100 cm−1 was attributed to the sulfate ions.40 Its intensity increases with the increase of Na2SO4 concentration. The ATR peak of V(V) ions depicted in Figure 3b has
14.32%, whereas that of V(V) ions has no obvious change. As a result, the separation factor of V(V) to Cr(VI), that is, βV/Cr, increases significantly with the increase of Na2SO4 concentrations. The transport behaviors of V(V) and Cr(VI), during the increase of Na2SO4 concentrations, might be closely related to their competitive adsorption behaviors at the oil−water interface. It is well known that the adsorption behaviors of ions at the interfaces are highly dependent on their hydrophobicity. Generally, the stronger is the hydrophobicity of the ion, the easier is the adsorption of the ion at the interfaces. Herein, we supposed that sulfate ions could compete with V(V) ions and Cr(VI) ions for the adsorption site at the interface, so that the efficient separation between V(V) and Cr(VI) ions could be achieved. However, experimental evidence needs to be provided to verify our hypothesis. The ATR−FTIR spectra of V(V) and Cr(VI) ions at the interface. In the present work, the ATR−FTIR spectroscopy was employed to detect the adsorption behaviors of V(V) and Cr(VI) ions at the interface during the increase of Na2SO4 concentrations. Figure 2 depicts the ATR−FTIR spectra of the alkaline aqueous solutions containing different concentrations of V(V) and Cr(VI) ions. The existing form of V(V) and Cr(VI) ions in the aqueous solutions have been widely investigated. It was demonstrated that under the present experimental condition (aqueous pH was 12), V(V) existed in the form of HVO42−, whereas Cr(VI) ions existed in the form of CrO42−. Other salt anions, such as C
DOI: 10.1021/acs.langmuir.8b02691 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Figure 4. (a) ATR−FTIR spectra of Cr(VI) solutions with addition of different concentrations of Na2SO4. (The aqueous pH is 12. The concentration of Cr(VI) is 0.025 mol/L. The curves 1 to 10 represent the Na2SO4 concentration of 0, 0.01, 0.04, 0.1, 0.15, 0.3, 0.5, 0.7, 0.9, and 1.13 mol/L, respectively); (b) Enlarged view of the ATR−FTIR spectra in (a) from 750 to 950 cm−1.
significantly decreases to about 50% of their initial values, when the concentration of Na2SO4 increased to 1.13 mol/L. While the ATR intensity of V(V) ions remained nearly unchanged, even the concentration of Na2SO4 increased to 1.13 mol/L. As we have concluded previously, the intensity of ATR peaks of V(V) and Cr(VI) ions could be employed to demonstrate their adsorption behaviors at the interface. It is clear that the addition of Na2SO4 has no obvious influence on the intensity of ATR peaks of V(V) ions, which indicates that the adsorption behavior of V(V) ions at the interface would not be affected by the addition of Na2SO4. On the contrary, the intensity of ATR peaks of Cr(VI) ions decreases abruptly with the increase of Na2SO4 concentrations. This could be understood by the change in the adsorption behavior of Cr(VI) ions during the increase of Na2SO4 concentration. The Cr(VI) ions adsorbed at the interface might be replaced by sulfate ions and that resulting in the decrease in the intensity of ATR peaks of Cr(VI) ions. Compared to the change trends in the transport efficiency of V(V) and Cr(VI) depicted in Figure 1, we supposed that the different transport behaviors of V(V) and Cr(VI) during the increase of Na2SO4 concentration might be originated from the competitive adsorption behaviors of V(V) and Cr(VI) ions during the increase of Na2SO4 concentration in the aqueous solutions. MD simulation of the adsorption behaviors of V(V), Cr(VI), and sulfate ions. To gain more molecular-level insights into the effects of competitive adsorption behaviors of V(V) and Cr(VI) ions on the transport selectivity of V(V) and Cr(VI) during the increase of Na2SO4 concentration, MD simulation was employed in the present work. Figure 6 gives the density curves of V(V) (a) and Cr(VI) (b) ions across the air/liquid interface with increasing concentration of Na2SO4. The peaks in density profiles near the interface represent the enrichment of V(V) and Cr(VI) ions at the interface. As can be seen in Figure 6a, the peak in the density profile of V(V) ions remains nearly unchanged as increasing the concentration of Na2SO4. It indicates that the adsorption of V(V) ions at the interface will not be affected by Na2SO4. However, a different change was found in the density profile of Cr(VI) ions. As depicted in Figure 6b, the peak decreases first and then be away from the interface, which indicates that the adsorption of Cr(VI) ions will be inhibited due to the competition from the sulfate ions for the adsorption sites at the interface. In addition, snapshots of the air/aqueous interface below the density profile of V(V) and Cr(VI) ions also verify our
no obvious change, despite the increase of concentration of Na2SO4. Figure 4a gives the ATR−FTIR spectra of Cr(VI) solutions containing different concentrations of Na2SO4. As can be seen that the change trend of the ATR peak of sulfate ions is similar to that of V(V) solutions containing different concentrations of Na2SO4. However, obvious decrease in the ATR peak of Cr(VI) ions was observed during the increase of concentration of Na2SO4, which is different from that trend of the ATR peak of V(V) ions as depicted in Figure 3b. To verify that the variation of the ATR peak of Cr(VI) ions originated from the decrease in the surface concentration of ions, and not from the variation of Cr(VI) ions species caused by the sulfate ions, the transmission IR spectra of Na2CrO4 solutions containing different concentrations of Na2SO4 were measured, and the result is given in Figure S3 in the Supporting Information. As can be seen, transmission IR spectra of Cr(VI) ions would not be affected by the addition of Na2SO4. In addition, the transmission UV−vis spectra of Na2CrO4 aqueous solutions containing different concentrations of Na2SO4 were also recorded. The result is depicted in Figure S4 in the Supporting Information. The results further demonstrated that the variation in the ATR spectra of Na2CrO4 aqueous solutions containing different concentrations of Na2SO4 did not result from the changes in the species of Cr(VI) ions in the solutions. For further analyzing the different effects of Na2SO4 concentration on the ATR peaks of V(V) and Cr(VI) ions, here, the variation in the absorbance intensity of the ATR peaks of V(V) and Cr(VI) ions during the increase of Na2SO4 concentration in the aqueous solutions, is depicted in Figure 5. As can be seen in Figure 5, the ATR intensity of Cr(VI) ions
Figure 5. Effect of Na2SO4 concentration on the intensity of ATR peaks of V(V) and Cr(VI) ions. D
DOI: 10.1021/acs.langmuir.8b02691 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Figure 6. (a) Density curves of V(V) ions across air/liquid interface for different concentration of Na2SO4; (b) Density curves of Cr(VI) ions across air/liquid interface for different concentration of Na2SO4; (The curves 1 to 9 represent the Na2SO4 concentration of 0.01, 0.04, 0.1, 0.15, 0.3, 0.5, 0.7, 0.9, and 1.130 mol/L, respectively.) (c) Snapshot of the air/liquid interface for the V(V)−SO42− system, in which the Na2SO4 concentration was 1.130 mol/L; (d) Snapshot of the air/liquid interface for the Cr(VI)−SO42− system, in which the Na2SO4 concentration was 1.130 mol/L. (Water molecules are represented by the red line; V(V) ions are represented by the blue spheres; Cr(VI) ions are represented by the red spheres; and SO42− is represented by the yellow spheres).
HVO42− ions was depicted in Figure 7c. According to the location of first peaks, the average coordination number (CN) of water molecules around SO42− ions, CrO42− ions, and HVO42− ions were 8.90, 8.56, and 7.62, respectively. As can be seen, the average coordination number (CN) of water molecules around SO42− ions was approximately equal to that of CrO42− ions, whereas the average coordination number (CN) of water molecules around HVO42− ions was relatively smaller. According to the previous literatures, the affinity of ions to the water molecules could be used to estimate the hydrophilicity or hydrophobicity of those ions. The stronger is the affinity of ions to the water molecules, the stronger is the hydrophilicity of ions. Obviously, the strong hydrophilicity of ions suggested their poor hydrophobicity. Therefore, we could conclude that the hydrophobicity of SO42− ions was similar to that of CrO42− ions, whereas both of them were lower than that of HVO42− ions. It was demonstrated that the interface propensity of ions was determined by their hydrophilicity. The strong hydrophilicity of ions was in favor of the adsorption of ions at the interface. As a result, CrO42− ions adsorbed at the interface would be replaced by SO42− ions when the concentration of SO42− ions increases to much higher than that of CrO42− ions, whereas the adsorption of HVO42− ions remains nearly unchanged due to their much stronger hydrophobicity compared to that of SO42− ions.
conclusions about the difference in adsorption behaviors of V(V) and Cr(VI) ions. It is obvious that Cr(VI) ions adsorbed at the interface could be replaced by sulfate ions as increasing the concentration of sulfate ions. While V(V) ions exhibit strong affinity to the interface, its adsorption behaviors would not be affected by the addition of sulfate ions. As reported in various literatures, the affinity of ions to the interface is closely related to their hydrophobicity. In addition, the hydrophobicity of ions could be estimated from their hydration ability.42 Figure 7a gives the radial distribution function (RDF) of the O atom in H2O around Na+ ions in different aqueous solutions. Similar RDF curves have been observed for Na+ ions in the Na2SO4, Na2CrO4, and Na2HVO4 solutions. Figure 7b gives the RDF of H atoms in H2O around S atoms in SO42− ions, Cr atoms in CrO42− ions, and V atoms in HVO42− ions. The hydration of Na+ ions was stronger than that of SO42− ions, CrO42− ions, and HVO42− ions, which is consistent with the reports in other literatures that cations prefer to be hydrated than anions.43 Notably, two peaks appear in the RDF of water molecules around SO42− ions, CrO42− ions, and HVO42− ions. From SO42− ions to HVO42− ions, the intensity of the peaks in the RDF gradually decreases. In addition, the location of the first peaks gradually increases. It suggests that the hydration of the ions gradually decrease from SO42− ions to HVO42− ions. Furthermore, the average coordination number (CN) of water molecules around SO42− ions, CrO42− ions, and E
DOI: 10.1021/acs.langmuir.8b02691 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Figure 7. (a) RDF of O atoms in H2O around Na+ ions in different aqueous solutions (the black line represents Na2SO4 aqueous solutions, the red line represents Na2CrO4 aqueous solutions, and the blue line represents Na2HVO4 aqueous solutions); (b) RDF of H atoms in H2O around S atoms in SO42− ions, Cr atoms in CrO42− ions and V atoms in HVO42− ions; (c) Average coordination number (CN) of water molecules around SO42− ions, CrO42− ions, and HVO42− ions.
■
■
CONCLUSIONS
Corresponding Author
We found that the selectivity for the separation of V(V) from Cr(VI) during the increase of Na2SO4 concentration was originated from the competitive adsorption behaviors of V(V) and Cr(VI) ions against coexisting sulfate ions at the oil−water interface. The adsorption behaviors of ions obtained by ATR− FTIR spectroscopy indicated that sulfate ions could selectively replace Cr(VI) ions to be adsorbed at the interfaces, whereas the adsorption of V(V) ions would not be affected. MD simulations further verified the competitive adsorption behaviors of V(V) and Cr(VI) ions against coexisting sulfate ions at the interface. In addition, the RDF analysis indicated that distinct interface propensity of V(V), Cr(VI), and sulfate ions depend upon their different hydrophobicities. The present study highlights a new insight into the transport and separation behaviors of different ions from the view of their competitive adsorption behaviors at the oil−water interface. It is possible for us to develop new strategies to intensify the separation of ions with similar physic-chemical properties by regulating their adsorption behaviors at the interface.
■
AUTHOR INFORMATION
*E-mail:
[email protected]. ORCID
Pan Sun: 0000-0002-6128-8656 Kun Huang: 0000-0002-6933-6480 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (nos. 51574213 and 51074150), and the Key Project of Chinese National Programs for Fundamental Research and Development (973 Program nos. 2013CB632602, 2012CBA01203). We thank the Supercomputing Center of Chinese Academy of Sciences for allowing us to use the ScGrid for theoretical calculations.
■
REFERENCES
(1) Tamashiro, M. N.; Constantino, M. A. Ions at the water−vapor interface. J. Phys. Chem. B 2010, 114, 3583−3591. (2) Jungwirth, P.; Tobias, D. J. Specific ion effects at the air/water interface. Chem. Rev. 2006, 106, 1259−1281. (3) Hua, W.; Verreault, D.; Allen, H. C. Surface electric fields of aqueous solutions of NH4NO3, Mg(NO3)2, NaNO3, and LiNO3: implications for atmospheric aerosol chemistry. J. Phys. Chem. C 2014, 118, 24941−24949. (4) Ohtani, N.; Ohta, T.; Hosoda, Y.; Yamashita, T. Phase behavior and phase-transfer catalysis of tetrabutylammonium salts. interfacemediated catalysis. Langmuir 2004, 20, 409−415. (5) Scoppola, E.; Watkins, E.; Li Destri, G.; Porcar, L.; Campbell, R. A.; Konovalov, O.; Fragneto, G.; Diat, O. Structure of a liquid/liquid interface during solvent extraction combining X-ray and neutron
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b02691. Simulation details and transmission IR and UV−vis spectra of Na2CrO4 aqueous solutions containing different concentrations of Na2SO4 (PDF) F
DOI: 10.1021/acs.langmuir.8b02691 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
the presence of copper ions. Environ. Sci. Technol. 2008, 42, 9191− 9196. (28) Degenhardt, J.; McQuillan, A. J. In situ ATR-FTIR spectroscopic study of adsorption of perchlorate, sulfate, and thiosulfate ions onto chromium(III) oxide hydroxide thin films. Langmuir 1999, 15, 4595−4602. (29) Azam, M. S.; Weeraman, C. N.; Gibbs-Davis, J. M. Specific cation effects on the bimodal acid−base behavior of the silica/water interface. J. Phys. Chem. Lett. 2012, 3, 1269−1274. (30) Hopkins, A. J.; Schrödle, S.; Richmond, G. L. Specific ion effects of salt solutions at the CaF2/water interface. Langmuir 2010, 26, 10784−10790. (31) Sun, P.; Huang, K.; Liu, H. Z. Separation of V and Cr in alkaline aqueous solution using acidified primary amine N1923. Hydrometallurgy 2016, 165, 370−380. (32) Kalé, L.; Skeel, R.; Bhandarkar, M.; Brunner, R.; Gursoy, A.; Krawetz, N.; Phillips, J.; Shinozaki, A.; Varadarajan, K.; Schulten, K. NAMD2: Greater scalability for parallel molecular dynamics. J. Comput. Phys. 1999, 151, 283−312. (33) Mackerell, A. D. Empirical force fields for biological macromolecules: Overview and issues. J. Comput. Chem. 2004, 25, 1584−1604. (34) Jorgensen, W. L.; Chandrasekhar, J.; Madura, J. D.; Impey, R. W.; Klein, M. L. Comparison of simple potential functions for simulating liquid water. J. Chem. Phys. 1983, 79, 926−935. (35) Wernersson, E.; Jungwirth, P. Effect of water polarizability on the properties of solutions of polyvalent ions: simulations of aqueous sodium sulfate with different force fields. J. Chem. Theory Comput. 2010, 6, 3233−3240. (36) Ding, Y.-Q.; Chen, C.-l.; Gu, Q.-R.; Fu, L.-H. Molecular simulation study of the effects of inorganic anions on the properties of chrome-crosslinked collagen. Modell. Simul. Mater. Sci. Eng. 2014, 22, 065008. (37) Mayne, C. G.; Saam, J.; Schulten, K.; Tajkhorshid, E.; Gumbart, J. C. Rapid parameterization of small molecules using the force field toolkit. J. Comput. Chem. 2013, 34, 2757−2770. (38) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al., Gaussian 09, Revision D.01; Gaussian, Inc.: Wallingford CT, 2009. (39) Gong, X.; Ning, P.; Cao, H.; Zhang, C. Measurement and modeling of the solubility of NH4VO3 in the Na2HPO4−H2O and (NH4)(2)HPO4−H2O Systems. J. Chem. Eng. Data 2016, 61, 628− 635. (40) Frederickson, L. D.; Hausen, D. M. Infrared spectra-structure correlation study of vanadium-oxygen compounds. Anal. Chem. 1963, 35, 818−827. (41) Hoffmann, M. M.; Darab, J. G.; Fulton, J. L. An infrared and Xray absorption study of the equilibria and structures of chromate, bichromate, and dichromate in ambient aqueous solutions. J. Phys. Chem. B 2001, 105, 1772−1782. (42) Potapova, E.; Grahn, M.; Holmgren, A.; Hedlund, J. The effect of calcium ions and sodium silicate on the adsorption of a model anionic flotation collector on magnetite studied by ATR-FTIR spectroscopy. J. Colloid Interface Sci. 2010, 345, 96−102. (43) Ganguly, P.; Hajari, T.; van der Vegt, N. F. A. Molecular simulation study on hofmeister cations and the aqueous solubility of benzene. J. Phys. Chem. B 2014, 118, 5331−5339.
reflectivity measurements. Phys. Chem. Chem. Phys. 2015, 17, 15093− 15097. (6) Ren, C. E.; Hatzell, K. B.; Alhabeb, M.; Ling, Z.; Mahmoud, K. A.; Gogotsi, Y. Charge- and size-selective ion sieving through Ti3C2Tx MXene membranes. J. Phys. Chem. Lett. 2015, 6, 4026− 4031. (7) Lotan, O.; Fink, L.; Shemesh, A.; Tamburu, C.; Raviv, U. Critical conditions for adsorption of calcium ions onto dipolar lipid membranes. J. Phys. Chem. A 2016, 120, 3390−3396. (8) Rydberg, J.; Cox, M.; Musikas, C.; Choppin, G. R. Solvent Extraction Principles and Practice; Marcel Dekker: New York, 2004. (9) Scoppola, E.; Watkins, E. B.; Campbell, R. A.; Konovalov, O.; Girard, L.; Dufrêche, J.-F.; Ferru, G.; Fragneto, G.; Diat, O. Solvent extraction: structure of the liquid-liquid interface containing a diamide ligand. Angew. Chem., Int. Ed. 2016, 55, 9326−9330. (10) Sahu, P.; Ali, S. M.; Shenoy, K. T. Structural and dynamical properties of Li+-dibenzo-18-crown-6(DB18C6) complex in pure solvents and at the aqueous-organic interface. J. Mol. Model. 2014, 20, 2413. (11) Diss, R.; Wipff, G. Lanthanide cationextraction by malonamideligands: from liquid−liquid interfaces to microemulsions: A molecular dynamics study. Phys. Chem. Chem. Phys. 2005, 7, 264− 272. (12) Sieffert, N.; Chaumont, A.; Wipff, G. Importance of the liquid− liquid interface in assisted ion extraction: New molecular dynamics studies of cesium picrate extraction by a Calix[4]arene. J. Phys. Chem. C 2009, 113, 10610−10622. (13) Knight, A. W.; Qiao, B.; Chiarizia, R.; Ferru, G.; Forbes, T.; Ellis, R. J.; Soderholm, L. Subtle effects of aliphatic alcohol structure on water extraction and solute aggregation in biphasic water/ndodecane. Langmuir 2017, 33, 3776−3786. (14) Beaman, D. K.; Robertson, E. J.; Richmond, G. L. Unique assembly of charged polymers at the oil−water interface. Langmuir 2011, 27, 2104−2106. (15) Benay, G.; Wipff, G. Liquid-liquid extraction of uranyl by an amide ligand: interfacial features studied by MD and PMF simulations. J. Phys. Chem. B 2013, 117, 7399−7415. (16) Schnell, B.; Schurhammer, R.; Wipff, G. Distribution of hydrophobic ions and their counterions at an aqueous liquid−liquid interface: A molecular dynamics investigation. J. Phys. Chem. B 2004, 108, 2285−2294. (17) Jungwirth, P.; Tobias, D. J. Ions at the air/water interface. J. Phys. Chem. B 2002, 106, 6361−6373. (18) Slavchov, R. I.; Novev, J. K. Surface tension of concentrated electrolyte solutions. J. Colloid Interface Sci. 2012, 387, 234−243. (19) Onuki, A. Surface tension of electrolytes: Hydrophilic and hydrophobic ions near an interface. J. Chem. Phys. 2008, 128, 224704. (20) Marcus, Y. Individual ionic surface tension increments in aqueous solutions. Langmuir 2013, 29, 2881−2888. (21) Marcus, Y. Specific ion effects on the surface tension and surface potential of aqueous electrolytes. Curr. Opin. Colloid Interface Sci. 2016, 23, 94−99. (22) Slavchov, R. I.; Novev, J. K.; Peshkova, T. V.; Grozev, N. A. Surface tension and surface Δχ-potential of concentrated Z+:Z− electrolyte solutions. J. Colloid Interface Sci. 2013, 403, 113−126. (23) Markin, V. S.; Volkov, A. G. Quantitative theory of surface tension and surface potential of aqueous solutions of electrolytes. J. Phys. Chem. B 2002, 106, 11810−11817. (24) Gopalakrishnan, S.; Liu, D.; Allen, H. C.; Kuo, M.; Shultz, M. J. Vibrational spectroscopic studies of aqueous interfaces: Salts, acids, bases, and nanodrops. Chem. Rev. 2006, 106, 1155−1175. (25) Hua, W.; Verreault, D.; Allen, H. C. Relative order of sulfuric acid, bisulfate, hydronium, and cations at the air-water interface. J. Am. Chem. Soc. 2015, 137, 13920−13926. (26) Marcus, Y. Effect of ions on the structure of water: structure making and breaking. Chem. Rev. 2009, 109, 1346−1370. (27) Beattie, D. A.; Chapelet, J. K.; Gräfe, M.; Skinner, W. M.; Smith, E. In situ ATR FTIR studies of SO4 adsorption on goethite in G
DOI: 10.1021/acs.langmuir.8b02691 Langmuir XXXX, XXX, XXX−XXX
