Interactions of Divalent and Trivalent Metal Counterions with Anionic

Apr 20, 2016 - Centre for Bioengineering and Biotechnology, China University of ... 66 Changjiang West Road, Qingdao 266580, People's Republic of Chin...
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Interactions of Divalent and Trivalent Metal Counterions with Anionic Sulfonate Gemini Surfactant and Induced Aggregate Transitions in Aqueous Solution Zhang Liu,† Meiwen Cao,‡ Yao Chen,† Yaxun Fan,† Dong Wang,‡ Hai Xu,‡ and Yilin Wang*,† †

Key Laboratory of Colloid and Interface Science, Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ Centre for Bioengineering and Biotechnology, China University of Petroleum (East China), 66 Changjiang West Road, Qingdao 266580, People’s Republic of China ABSTRACT: Interactions of multivalent metal counterions with anionic sulfonate gemini surfactant 1,3-bis(N-dodecyl-Npropanesulfonate sodium)-propane (C12C3C12(SO3)2) and the induced aggregate transitions in aqueous solution have been studied. Divalent metal ions Ca2+, Mg2+, Cu2+, Zn2+, Mn2+, Co2+, and Ni2+ and trivalent metal ions Al3+, Fe3+, and Cr3+ were chosen. The results indicate that the critical micelle concentration (CMC) of C12C3C12(SO3)2 is greatly reduced by the ions, and the aggregate morphologies of C12C3C12(SO3)2 are adjusted by changing the nature and molar ratio of the metal ions. These metal ions can be classified into four groups because the ions in each group have very similar interaction mechanisms with C12C3C12(SO3)2: (I) Cu2+ and Zn2+; (II) Ca2+, Mn2+ and Mg2+; (III) Ni2+ and Co2+; and (IV) Cr3+, Al3+ and Fe3+. Cu2+, Mg2+, Ni2+, and Al3+ then were selected as representatives for each group to further study their interaction with C12C3C12(SO3)2. C12C3C12(SO3)2 interacts with the multivalent metal ions by electrostatic interaction and coordination interaction. C12C3C12(SO3)2 forms prolate micelles and plate-like micelles with Cu2+, vesicles and wormlike micelles with Al3+ or Ni2+, and viscous three-dimensional network structure with Mg2+. Moreover, precipitation does not take place in aqueous solution even at a high ion/surfactant ratio. The related mechanisms have been discussed. The present work provides guidance on how to apply the anionic surfactant into the solutions containing the multivalent metal ions, and those aggregates may have potential usage in separating heavy metal ions from aqueous solutions.

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interface induced by the formation of SLEnS/Al3+ complex. The initial formed multilayer includes a limited number of bilayers, usually smaller than 3. The number of bilayers increases to more than 20 when the AlCl3 concentration is high. Moreover, the binding of metal ions with the headgroups of anionic surfactants and the amphiphilic characteristic of surfactants can enrich metal ions at the aggregate/solution interface and the air/solution interface, which offers the opportunity of separating metal ions from aqueous solutions by foam flotation,19−21 adsorptive micellar flocculation,22−24 or micellar-enhanced ultrafiltration,25,26 and greatly promotes their catalyst activity.27−31 Thus multivalent metal ions can either improve or limit the performances of anionic surfactants. So far the effects of multivalent metal ions on traditional single-chain surfactants have been extensively studied. The CMC of sodium dodecyl sulfate (SDS) is reduced about 7-fold when the counterion is replaced by Mg2+, Mn2+, Co2+, Ni2+, and

INTRODUCTION Multivalent metal counterions greatly influence the surface and solution properties of anionic surfactants, which are closely related to the performance of surfactants in washing products, wastewater treatment, and mineral industry.1,2 Primary interaction between multivalent counterions and anionic surfactants is electrostatic attraction. The accompanying intensive neutralization and dehydration of anionic surfactants can lead to the precipitation or flocculation of surfactants,2−4 which restrains the performances of surfactants. However, the screening effect of multivalent metal ions to electrostatic repulsion between surfactant headgroups can reduce the area of headgroups,5 leading to reduction of critical micelle concentration (CMC),6 growth and morphology transition of aggregates,4,7−10 and enhanced adsorption of surfactants at interfaces.11−13 Penfold, Thomas, and co-workers14−18 have studied the interaction between Al3+ and sodium polyethylene glycol monoalkyl ether sulfate (SLEnS) with different alkyl chain lengths and different numbers of poly(ethylene oxide) group in aqueous solution using small-angle neutron scattering (SANS). They found that multilayer is formed at the air−water © 2016 American Chemical Society

Received: March 21, 2016 Revised: April 16, 2016 Published: April 20, 2016 4102

DOI: 10.1021/acs.jpcb.6b02897 J. Phys. Chem. B 2016, 120, 4102−4113

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The Journal of Physical Chemistry B Cu2+, regardless of the nature of the divalent counterions.32 The properties of anionic micelles of sodium dodecyl polyoxyethylene-2-sulfate in the presence of multivalent ions were investigated by means of light scattering methods.33 It was revealed that the strong binding of multivalent and particularly trivalent counterions with the surfactant triggers a sphere-tocylinder shape transition of the micelles and facilitates their further growth, even at very low ionic strength.34 Kralchevsky’s group4 studied the micelle size and shape of sodium dodecyl dioxyethylene sulfate as well as the Al3+/surfactant interactions, and found that Al3+ ions induce intensive growth of spherical micelles to rod-like micelles. Xu and co-workers17 found that a relatively low concentration of Al3+ ion promotes significant micellar growth of SLES, and at higher Al3+ concentration, the surfactant aggregate transits to lamellar structure and ultimately precipitate. As compared to anionic single-chain surfactants, anionic gemini surfactants show advantages in promoting hardness tolerance and constructing more diverse aggregate morphologies. However, the interactions of multivalent metal ions with gemini surfactants are rarely reported. Gemini surfactants,35−38 which have remarkably low CMC and strong self-assembling abilities, are the promising alternatives to conventional singlechain surfactants. Our previous work studied the interaction of a carboxylic gemini surfactant with Cu2+ ion and revealed that a small amount of Cu2+ ions transfer spherical micelles into vesicles at pH 12.0, while promote small vesicles to fuse into larger ones at 7.0.39 Another previous work40 found that anionic sulfonate gemini surfactant 1,3-bis(N-dodecyl-Npropylsulfonate sodium)-propane (C12C3C12(SO3)2) shows much better performance in the tolerance of calcium ions than sodium dodecyl sulfate (SDS), and the hardness tolerance is related to the aggregation ability and the morphologies of the surfactant aggregates. Because each 12-3-12(SO3)2 molecule contains two anionic charges, the area of the high charged headgroups of 12-3-12(SO3)2 is greatly reduced by the binding with Ca2+, which in turn greatly promotes the aggregation of the surfactant. The concentrated solution of the 12-312(SO3)2/Ca2+ mixture forms long entangled wormlike micelles with gel-like viscoelastic properties. Given that the two sulfonate groups can chelate with one or more multivalent metal ions, while the double alkyl chains may insert into one or two different aggregates, the interactions of various multivalent metal ions with the gemini surfactant may endow the surfactant with unexpected, unique aggregates and performances, and thus deserve to be systematically studied. Therefore, the present work has systematically studied the interaction of anionic sulfonate gemini surfactant C12C3C12(SO3)2 with divalent metal ions Ca2+, Mg2+, Cu2+, Zn2+, Mn2+, Co2+, and Ni2+, and trivalent metal ions Al3+, Fe3+, and Cr3+, and the corresponding aggregation behavior of C12C3C12(SO3)2 in aqueous solution. Al3+, Ca2+, Mg2+, and Zn2+ are widely distributed metal ions in hard water or wastewater, and Cr3+, Fe3+, Cu2+, Co2+, and Ni2+ are the paramagnetic transition metal ions of catalytic interest. The results indicate that the metal ions greatly reduce the CMC of C12C3C12(SO3)2. Prolate micelles, plate-like micelles, vesicles, wormlike, and three-dimensional network structure are constructed by changing the nature of the counterions and the molar ratios of counterions to C12C3C12(SO3)2. It is also found that C12C3C12(SO3)2 shows excellent hardness tolerance with Ni2+, Mg2+, Cu2+, and Al3+. The gathering of metal ions at the aggregate/solution interface and air/solution interface

offers the possibility of separating metal ions from aqueous solutions by foam flotation, adsorptive micellar flocculation, or micellar-enhanced ultrafiltration.

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EXPERIMENTS Materials. Anionic gemini surfactant 1,3-bis(N-dodecyl-Npropanesulfonate sodium)-propane (C12C3C12(SO3)2) was synthesized and purified as reported previously.41 Milli-Q water (18.2 MΩ cm) was used in all experiments. Hydrated metal chlorides, AlCl3·6H2O, FeCl3·6H2O, CrCl3·6H2O, ZnCl2· 6H2O, CuCl2·2H2O, MgCl2·6H2O, CoCl2·6H2O, NiCl2·6H2O, CaCl2·6H2O, and MnCl2·4H2O, were purchased from TCI with purities higher than 99.9%. Surface Tension Measurement. The surface tensions of the C12C3C12(SO3)2 solutions in the presence of these metal ions were measured by a 19.90 × 0.20 mm rectangle plate in a DCAT11 surface tensiometer from Dataphysics Instruments GmbH at 25.00 ± 0.05 °C with a standard error of surface tension within ±1 mN/m. Electrical Conductivity Measurements. Electrical conductivity of C12C3C12(SO3)2 with the metal ions in aqueous solutions was monitored by a JENWAY model 4320 conductivity meter at 25.0 ± 0.1 °C. Size Measurements. The size distribution of the metal ions/C12C3C12(SO3)2 aggregates in aqueous solutions was studied by the measurements of dynamic light scattering (DLS) at a scattering angle of 173° with a Nano ZS (Malvern Instruments) equipped with a 4 mW He−Ne laser (λ = 632.8 nm) and a thermostated chamber. The thermostated chamber was controlled at 25 °C. Potentiometric pH Titration. C12C3C12(SO3)2 was first dissolved in pure water at a concentration of 2.00 mM, and then the solution pH was adjusted to 8.0 with a small volume of 1.0 M NaOH standard solution. 2.00 mM of this solution was loaded in a double-walled glass container controlled at 25.0 ± 0.1 °C, and 10.00 mM metal ion solutions were gradually titrated into it in small portions. The pH values in the titration process were monitored with a pHS-2C acidity meter. Scanning Electron Microscopy (SEM). The morphologies of the Al3+/C12C3C12(SO3)2 and Cu2+/C12C3C12(SO3)2 and Mg2+/C12C3C12(SO3)2 mixtures were imaged with a fieldemission scanning electron microscope (Hitachi S-4800). All of the mixed solutions had the molar ratio of metal ions to C12C3C12(SO3)2 at Rm = 1.00 and the C12C3C12(SO3)2 concentration at Cs = 2.00 mM. The solutions were prepared at 25 °C, and then a small drop of each solution was frozen on a clean silica wafer with liquid nitrogen so that the aggregate structures in aqueous solution can be retained. Immediately afterward, the frozen samples were lyophilized under vacuum at about −50 °C. Finally, a 1−2 nm Pt coating completed the sample preparation. Cryogenic Transmission Electron Microscopy (CryoTEM). The solutions of the metal ions/C12C3C12(SO3)2 aggregates were embedded in a thin layer of vitreous ice on freshly carbon-coated holey TEM grids by blotting the grids with filter paper and then plunging them into liquid nitrogen. Frozen hydrated specimens were imaged by an FEI Tecnai 20 electron microscope (LaB6) operated at 200 kV in low-dose mode (about 2000 e/nm2) and at a nominal magnification of 50 000. For each specimen area, the defocus was set to 1−2 μm. Images were recorded on Kodak SO 163 films and then digitized by a Nikon 9000 with a scanning step of 2000 dpi corresponding to 2.54 Å/pixel. 4103

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Figure 1. Raw data and the observed enthalpy changes (ΔH) against the molar ratio of metal ions/C12C3C12(SO3)2 (Rm) for titrating 10.00 mM (I) Cu2+ and Zn2+, (II) Ca2+, Mn2+, and Mg2+, (III) Ni2+ and Co2+, and (IV) Cr3+, Al3+, and Fe3+ into 2.00 mM C12C3C12(SO3)2 solution. ΔH values are expressed in kJ/mol of metal ions. 1

H NMR. 1H NMR spectra were recorded by a Bruker Avance 400-NMR spectrometer operating at 400 MHz at room temperature (25 ± 2 °C). Deuterium oxide (99.9%) was used to prepare the stock solutions of C12C3C12(SO3)2 with the metal ions. The center of the HDO signal (4.790 ppm) was used as the reference. In the measurements, 32 scans were used, and the digital resolution was 0.04 Hz/data point. Rheology Measurement. The rheological properties of the Mg2+/C12C3C12(SO3)2 solution containing wormlike micelles were investigated at 25.00 ± 0.05 °C with a ThermoHaake RS300 rheometer. A solvent trap was used to

avoid water evaporation. Frequency spectra were conducted in the linear viscoelastic regime determined from dynamic strain sweep measurements. For the solutions with low viscosity, a double-gap cylindrical sensor with an outside gap of 0.30 mm and an inside gap of 0.25 mm was used. Isothermal Titration Microcalorimetry (ITC). Calorimetric measurements were carried out at 25.00 ± 0.01 °C on a TAM III microcalorimetric system with a stainless-steel sample cell of 1 mL. The cell was initially loaded with 0.6 mL of 2.00 mM C12C3C12(SO3)2 solution or water, and then 10.00 mM metal ion solution was injected consecutively into the stirred 4104

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The Journal of Physical Chemistry B sample cell in portions of 10 μL via a 500 μL Hamilton syringe controlled by a 612 Thermometric Lund pump until the desired concentration range had been covered. The observed enthalpy values (ΔH) were obtained from the areas of the calorimetric peaks after titrations. The ΔHobs values for 10.00 mM metal ion solution being titrated into water were very small and subtracted from the ΔH values for the same metal ion solution being titrated into the C12C3C12(SO3)2 solution. Thus, the final ΔH curves reflect the interactions of the metal ions with C12C3C12(SO3)2 and their effects on the C12C3C12(SO3)2 aggregates. Each ITC curve was repeated at least twice with a deviation of less than ±4%.

micellization and surface tension of C12C3C12(SO3)2 by fixing the surfactant concentration at 0.0020 mM. This surfactant concentration is far below the CMC of the surfactant without added metal ions, which means that the surfactant molecules exist as monomers. The surface tension of 0.0020 mM C12C3C12(SO3)2 solution is 62.43 mN/m. As shown in the surface tension curves (Figure 2a), with the additions of metal

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RESULTS AND DISCUSSION Interactions of Metal Ions with C12C3C12(SO3)2 in Solution. To have a general view on the interaction situations of different metal ions with C12C3C12(SO3)2, ITC was used to measure the heat absorbed or released in the processes of titrating 10.00 mM metal ions into 2.00 mM C12C3C12(SO3)2 solution. The corresponding ITC raw data and the curves of ΔH versus the molar ratio of metal ions/C12C3C12(SO3)2 (Rm) are shown in Figure 1. According to the features of the ITC curves and the phase states after the titrations, the metal ions are classified into four groups. In each group, the ITC curves display very similar varying patterns, and the differences only exist in the positions of the variations. Group I includes Zn2+ and Cu2+. With the addition of Zn2+ and Cu2+, the ΔH curves all show an exothermic peak at lower ion concentration, then an endothermic peak, and finally the endothermic ΔH decreases to near zero. Both Zn 2+ /C 12 C 3 C 12 (SO 3 ) 2 and Cu 2+ / C12C3C12(SO3)2 form precipitates after the ITC titrations. Group II includes Mg2+, Ca2+, and Mn2+. All of the ΔH curves for these ions show a sigmoid shape, exhibiting larger endothermic ΔHobs values at lower ion concentration, and then abruptly decreasing and reaching almost zero in final. All of the Mg2+/C12C3C12(SO3)2, Ca2+/C12C3C12(SO3)2, and Mn2+/C12C3C12(SO3)2 solutions are slightly viscous after titrations. Group III includes Ni2+ and Co2+. Upon the additions of the ions, ΔHobs increases from very low endothermic values to larger endothermic values, and then decreases back to very low values, shaping a peak. All of the Ni2+/C12C3C12(SO3)2 and Co2+/C12C3C12(SO3)2 mixtures form clear solutions after titrations. Group IV contains three trivalent ions Al3+, Fe3+, and Cr3+. They show the most complicated and fluctuated ΔH curves, and form precipitates after titrations. The fluctuations are caused by the precipitation. Because the CMC of C12C3C12(SO3)2 is 0.041 mM, and 2.00 mM used above is larger than the CMC, all of the ITC curves reflect the interactions of the metal ions with the C12C3C12(SO3)2 micelles and/or the resultant aggregate transition processes. Obviously, the metal ions in the four groups undergo different interaction processes with C12C3C12(SO3)2 and induce different aggregate transitions. To understand the physical meaning of these interactions and the detailed information on the aggregate transitions reflected in the ITC curves, Cu2+, Mg2+, Ni2+, and Al3+ are selected as representatives for each group, and the solution properties of their mixtures with C12C3C12(SO3)2 are further studied in the following text. Effects of Metal Ions on CMC and Surface Tension of C12C3C12(SO3)2. First, the surface tension experiments were carried out to study the effects of the metal ions on the

Figure 2. (a) Variation of the surface tension (γ) of the metal ions/ C 12 C 3 C 12 (SO 3 ) 2 solutions versus R m at the C 12 C 3 C 12 (SO 3 ) 2 concentration (Cs) of 0.0020 mM, and (b) γ versus the logarithm value of Cs at Rm = 1.00.

ions, the surface tension sharply decreases to the minimum values in the order of Al3+ (29.72 mN/m) < Ni2+ (31.12 mN/ m) ≈ Mg2+ (31.66 mN/m) < Cu2+ (33.43 mN/m), and then the surface tension values level off and become invariant. The inflection points correspond to the critical metal ion/ C12C3C12(SO3)2 molar ratios (Rm) required to induce the micellization of the surfactant. Beyond the molar ratios, the surfactant molecules form micelles, and further adding the metal ions cannot reduce the surface tension anymore. The critical Rm values for Al3+, Mg2+, and Ni2+ are approximately 0.17, while that for Cu2+ is 0.33. This suggests that a very small amount of metal ions have induced 0.0020 mM C12C3C12(SO3)2 to form micelles, and the Al3+, Mg2+, and Ni2+ show more efficient ability in lowering the surface tension of C12C3C12(SO3)2 than Cu2+. On the basis of the above results, the ability of the metal ions in reducing the CMC of C12C3C12(SO3)2 is studied by measuring the variation of surface tension with the C 12 C 3 C 12 (SO 3 ) 2 concentration (C s ). The metal ion/ C12C3C12(SO3)2 molar ratio is fixed at 1.00 to ensure the surface tension at each surfactant concentration has been reduced to the lowest value by metal ions. The corresponding γ-log Cs curves are shown together with the surface tension curve of C12C3C12(SO3)2 in the absence of the metal ions in Figure 2b. All of the surface tension curves with the four metal ions are very close with each other, indicating that all four metal ions show great and very similar ability of reducing the CMC and surface tension of C12C3C12(SO3)2 regardless of the nature of the metal ions. Moreover, the CMC value of 4105

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The Journal of Physical Chemistry B C12C3C12(SO3)2 (0.041 mM) is lowered by 30 times (0.0013− 0.0015 mM) by a small amount of Cu2+, Al3+, Mg2+, and Ni2+ (Rm = 1.00). As previously reported,32 it was found that the CMC of sodium dodecyl sulfate (SDS) is reduced to 1.20 mM from 8.25 mM by Mg2+, Mn2+, Co2+, Ni2+, and Cu2+, about 7fold. Obviously, as compared to the single chain surfactant, the multivalent metal ions are more efficient in reducing the CMC of gemini surfactants. In addition, the surface tension of water is reduced from ∼72 to ∼32 mN with no more than 0.0011 wt % metal ion/C12C3C12(SO3)2 mixture. Such a strong effect should result from the strong binding of the multivalent ions with anionic gemini surfactant molecules, which can enhance the adsorption of C12C3C12(SO3)2 at the air/water interface. It is expected that the multivalent metal ions will also affect the aggregate structures of the surfactant in solution. The following sections will reveal the aggregate transitions of the C12C3C12(SO3)2 micelles induced by Cu2+, Al3+, Mg2+, and Ni2+, respectively. Cu2+-Induced Aggregate Transitions. The aggregate transitions of the C12C3C12(SO3)2 micelles induced by Cu2+ have been studied by several techniques, and the results are shown in Figure 3. Figure 3a presents the changes of the observed enthalpy (ΔH) against the Cu2+/C12C3C12(SO3)2 molar ratio (RCu) by titrating 10.00 mM Cu2+ solution into 2.00 mM C12C3C12(SO3)2 solution at pH 8.0. The dilution enthalpy curve of 10.00 mM Cu2+ in water of pH 8.0 is also presented, which almost keeps constant close to zero. The corresponding changes of the aggregate size, electrical conductivity (κ), molar conductivity (Λ), and pH are presented in Figure 3b−d. Figure 3e shows the variations of the chemical shifts of the C12C3C12(SO3)2 protons in 1H NMR spectra against RCu. The variations of the chemical shifts (Δδ) are calculated from Δδ = δobs − δmic,42 where δobs is the observed chemical shift of the Cu2+/C12C3C12(SO3)2 mixture and δmic is the chemical shift of the protons in 2.00 mM C12C3C12(SO3)2 micelle solution without multivalent metal ions. Combining all of these results in Figure 3 indicates that the aggregate transitions of C12C3C12(SO3)2 micelles induced by Cu2+ are divided into four regions by RCu = 0.17, 0.41, and 0.67, respectively. The size results from DLS and the cryo-TEM and SEM images reveal that the four regions correspond to small micelles, prolate micelles (Figure 4a), plate-like micelles (Figure 4b), and plate-like precipitates (Figure 4c), respectively. The results will be further discussed in the following text. When RCu is below 0.17, the addition of Cu2+ does not cause any changes in ΔH, the aggregate size (∼4 nm), and the chemical shifts of the C12C3C12(SO3)2 protons, while leads to a sharp decrease of pH and molar conductivity. The unchanged ΔH, aggregate size, and the chemical shifts indicate that the C12C3C12(SO3)2 molecules still exist as small micelles and do not undergo aggregate transition in this region. The electrostatic binding leads to the sharp decrease in molar conductivity (Figure 3c). The decrease of pH is caused by the hydrolysis of Cu2+, and Cu(OH)+ is the main hydroxyl complex of copper ions in this pH region.43 When RCu is between 0.17 and 0.41, the continuous addition of Cu2+ makes ΔH start to change from zero to exothermic, and the variations of the chemical shifts of the C12C3C12(SO3)2 protons start to increase slightly. Meanwhile, the C12C3C12(SO3)2 aggregates start to display two size distributions, and the larger one begins to gradually grow up to ∼50 nm. The cryo-TEM image (RCu = 0.30, Figure 4a) reveals that the Cu2+/C12C3C12(SO3)2 micelles have transferred to prolate

Figure 3. Variation of (a) observed enthalpy change (ΔH), (b) distribution of hydrodynamic diameter, (c) electrical conductivity (κ) and molar conductivity (Λ), (d) pH of 2.00 mM C12C3C12(SO3)2 solution, and (e) chemical shift (Δδ) of protons Hc, Hd, He, Hf, and Hg, with the increase of the Cu2+/C12C3C12(SO3)2 molar ratio (RCu). The ΔH curves are obtained by titrating 10.00 mM Al3+ solution into 2.00 mM C12C3C12(SO3)2 solution or into water.

micelles. Because the pH slightly decreases from 6.80 to 6.20 in this region, Cu2+ is the main ion state.43 The exothermic ΔH suggests that Cu 2+ ions electrostatically bind with C12C3C12(SO3)2 headgroups. The slightly increased Δδ values show that the electrostatic interaction is not strong enough to generate significant changes in the chemical environment of the protons. However, the screening effect of Cu2+ ions to the electrostatic repulsion between the headgroups of C12C3C12(SO3)2 lessens the headgroup area, promoting the micellar growth from small spherical to prolate micelles. When RCu is between 0.41 and 0.67, with further addition of Cu2+ ions, ΔH begins to sharply change from exothermic to endothermic, and the aggregate size abruptly increases from ∼50 to ∼200 nm. The cryo-TEM image (RCu = 0.60, Figure 4106

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Figure 4. Images of cryo-TEM (the first and second columns, scale bar = 100 nm) and SEM (the third column, scale bar = 2 μm) of the ions/ C12C3C12(SO3)2 mixtures at different metal ion/C12C3C12(SO3)2 molar ratios: (a) RCu = 0.30, (b) RCu = 0.60, (c) RCu = 1.00, (d) RAl = 0.20, (e) RAl = 0.50, (f) RAl = 1.00, (g) RNi = 0.20, (h) RNi = 0.50, and (i) RNi = 1.00 at 2.00 mM C12C3C12(SO3)2.

the cryo-TEM and SEM images indicate that the four regions are small micelles, vesicles (Figure 4d), wormlike micelles (Figure 4e), and plate-like precipitates (Figure 4f), respectively. These results will be discussed in detail as follows. As compared to Cu2+, Al3+ carries more charges and owns stronger hydrolysis ability, so multiple equilibria including acid−base, hydrolysis, electrostatic, and coordination cooperatively control the state of Al3+. With the addition of the Al3+ solution into the C12C3C12(SO3)2 solution, the pH significantly decreases. Because in parallel titrating a Na+ solution at pH 3.6 into the C12C3C12(SO3)2 solution does not change the pH obviously, the significant pH decrease is mainly caused by the hydrolysis of Al3+. The two pH transition points (Figure 5d) just coincide with the first two boundaries of the aggregate transitions. The aggregates are small micelles above pH 6.2, the small micelles start to transfer into vesicles beyond pH 6.2, and the vesicles transfer into wormlike micelles below pH 4.5. As reported by Martin,44 in the whole pH region studied, the aluminum ions exist as a mixture of Al(OH)4−, Al(OH)3, Al(OH)2+, Al(OH)2+, and Al3+, and the amount of cationic components increases with the decrease of pH. The main components are Al(OH)4− and Al(OH)3 above pH 6.2, but Al(OH)2+ and Al3+ below pH 6.2. Al3+ becomes the main component below pH 4.5. Therefore, the aggregate transitions of C12C3C12(SO3)2 are strongly dependent on the states of aluminum ions. When RAl is below 0.17, the pH is above 6.2. With the addition of the aluminum ions, the larger endothermic ΔH (Figure 5a) indicates that a small amount of cationic components Al(OH)2+ and Al(OH)2+ electrostatically bind with C12C3C12(SO3)2, accompanied by the dehydration of all of the charges. The electrostatic binding leads to the sharp

4b) proves the existence of large plate-like aggregates. In this region, because the solution pH stays almost constant at 6.20, the copper ions exist as Cu2+, suggesting that electrostatic binding takes place between Cu2+ ions and the C12C3C12(SO3)2 headgroups. However, as is clearly shown in Figure 3e, the chemical shifts of the C12C3C12(SO3)2 protons start to significantly increase, and the largest change of chemical shift occurs in the Hf protons at the α positions of the N atoms, but the smallest change occurs in Hg at the α position of the sulfonate group. This means that the stronger interaction of Cu2+ ions with C12C3C12(SO3)2 occurs around the N atoms rather than near the sulfonate groups. Thus, it is concluded that the coordination interaction of Cu2+ ions with the N atoms of C12C3C12(SO3)2 becomes the domain interaction, which is also reflected in the enthalpy changes from exothermic to endothermic. Coordination interaction and electrostatic interaction jointly lead to the transition from the prolate micelles to the larger plate-like aggregates in this region. Finally, precipitation occurs when RCu exceeded 0.67 as marked by the shade (Figure 3). Final addition of Cu2+ only produces the dilution enthalpy of the Cu2+ ion solution. The SEM image (RCu = 1.00, Figure 4c) shows that the precipitates are plate-like. Al3+-Induced Aggregate Transition. The results about the Al3+-induced aggregate transitions of C12C3C12(SO3)2 are shown in Figure 5. According to the variations of ΔH, aggregate size, electrical conductivity (κ), molar conductivity (Λ), and chemical shifts (Δδ) of the protons of 2.00 mM C12C3C12(SO3)2 solution with the increase of the Al3+/ C12C3C12(SO3)2 molar ratio (RAl), the aggregate transitions can be divided into four regions: RAl < 0.17, 0.17 < RAl < 0.37, 0.37 < RAl < 0.65, and RAl > 0.65. The size values from DLS and 4107

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When RAl increases from 0.37 to 0.65, the pH only slightly decreases and Al3+ becomes the main state. In this region, the size distribution (Figure 5b) increases significantly, and wormlike micelles are shown in the cryo-TEM image (Figure 4e). So the vesicles have transferred into wormlike micelles in this region. Meanwhile, the ΔH still keeps larger endothermic values (Figure 5a), and the Δδ values (Figure 5f) increase slightly in this region, suggesting that added Al3+ ions continue to bind with C12C3C12(SO3)2 through electrostatic and coordination interactions. When RAl reaches 0.65, which is almost the electrostatic neutralization point between Al3+ ions and C12C3C12(SO3)2, plate-like precipitates formed (Figure 4f). Figure 5a shows very large endothermic enthalpy in this region, indicating that the electrostatic interaction between Al3+ ions and C12C3C12(SO3)2 becomes the dominate force and induces strong dehydration of the Al3+/C12C3C12(SO3)2 complexes. Ni2+-Induced Aggregate Transitions. By using a way similar to that above to analyze the results shown in Figure 6, it is concluded that the Ni2+/C12C3C12(SO3)2 mixture displays three regions, which are separated at the Ni2+/C12C3C12(SO3)2

Figure 5. Variation of (a) observed enthalpy changes (ΔH), (b) distribution of hydrodynamic diameter, (c) electrical conductivity (κ) and molar conductivity (Λ), (d) pH, and (e) chemical shifts (Δδ) of protons of 2.00 mM C12C3C12(SO3)2 solution with the increase of the Al3+/C12C3C12(SO3)2 molar ratio (RAl). The ΔH curves are obtained by titrating 10.00 mM Al3+ solution into 2.00 mM C12C3C12(SO3)2 solution or into water.

decrease in molar conductivity (Figure 5c). The unchanged size distribution and Δδ (Figure 5b and e) indicate that the interaction of the small amount of Al(OH)2+ and Al(OH)2+ with C12C3C12(SO3)2 is not enough to change the micelles. When RAl is between 0.17 and 0.37, the solution pH decreases from 6.20 to 4.50, so the aluminum ions mainly exist as Al(OH)2+ and Al3+. With the addition of the aluminum ions, the endothermic ΔH increases to a maximum at RAl = 0.37, and Δδ sharply increases to a very large value. Similar to the Cu2+/ C12C3C12(SO3)2 system, the most significant change in Δδ takes place on the Hf protons connecting to the N atoms of C12C3C12(SO3)2. These results indicate that in this region, the enhanced Al(OH)2+ and Al3+ contents strengthen the electrostatic bonds of the aluminum ions with C12C3C12(SO3)2, and the coordination bonds occur between the aluminum ions and the N atoms of C12C3C12(SO3)2. The binding process is endothermic because of the resulting dehydroxylation of Al(OH)2+ and the dehydration of C12C3C12(SO3)2 and Al3+. Meanwhile, a larger size distribution of ∼50 nm appears (Figure 5b), and the cryo-TEM image (Figure 4d) proves that the aggregates are vesicles.

Figure 6. Variation of (a) observed enthalpy change (ΔH), (b) distribution of hydrodynamic diameter, (c) electrical conductivity (κ) and molar conductivity (Λ), (d) pH, and (e) chemical shifts (Δδ) of protons of 2.00 mM C12C3C12(SO3)2 solution with the increase of the Ni2+/C12C3C12(SO3)2 molar ratio (RNi). The ΔH curves are obtained by titrating 10.00 mM Ni2+ solution into 2.00 mM C12C3C12(SO3)2 solution or into water. 4108

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Figure 7. Variation of (a) observed enthalpy changes (ΔH), (b) electrical conductivity (κ) and molar conductivity (Λ), and (c) pH of 2.00 mM C12C3C12(SO3)2 solution with the increase of the Mg2+/C12C3C12(SO3)2 molar ratio (RMg). (d) 1H NMR spectra and (e) distribution of hydrodynamic diameters of 2.00 mM C12C3C12(SO3)2 at different RMg. (f) Steady shear viscosity and (g) zero-shear viscosity η0 for Mg2+/ C12C3C12(SO3)2 solutions at 20.0 mM C12C3C12(SO3)2 and different RMg. The ΔH curves are obtained by titrating 10.00 mM Mg2+ solution into 2.00 mM C12C3C12(SO3)2 solution or into water. The SEM image (scale bar = 10 μm) inseted is from the Mg2+/C12C3C12(SO3)2 mixture at 20.0 mM C12C3C12(SO3)2 and RMg = 4.50.

When RNi is beyond 0.72, ΔH becomes zero, and all of the Δδ values, aggregate size, and molar conductivity remain almost constant. This means that the binding of Ni 2+ with C12C3C12(SO3)2 has reached a saturation. Besides, precipitation does not take place even when RNi reaches 1.00, that is, the charge equal point. Mg2+-Induced Aggregate Transitions. The aggregate transitions of the C12C3C12(SO3)2 micelles induced by Mg2+ ions are shown in Figure 7. Upon the addition of Mg2+ ions, the ITC curve (Figure 7a) is approximately sigmoidal in shape and changes gradually from endothermic to zero, reaching the saturation point of interaction at RMg = 0.88. The solution pH (Figure 7c) decreases only slightly from 8.0 to 7.0, indicating the weak hydrolysis ability of Mg2+. Meanwhile, the molar conductivity significantly decreases first, then keeps a relatively stable value, and finally increases slightly after the saturation point of interaction. The ΔH value starts from a larger exothermic value, while if no ions are added, the ΔH value should be zero. This means there is a very sharp increase in ΔH from zero to a large positive value at very low ion concentration. This sharp change is also reflected in the conductivity and DLS results. These results indicate that initial added Mg 2 + ions already strongly bind with the C12C3C12(SO3)2 micelles and induce the aggregate transition. The mixture maintains the same binding mode until the saturation point of interaction. This has been proved by the size increase and polymorphism distribution of the aggregates even at very low RMg (Figure 7e). The SEM image (Figure 4i) discloses that the small micelles have transferred into a threedimensional network structure, which may consist of entangled wormlike micelles. As compared to the other three systems, the Mg2+/ C12C3C12(SO3)2 mixtures show some viscosity, but still quite low to be precisely studied. So the rheological property of

molar ratios (RNi) of 0.33 and 0.72. Moreover, precipitation does not take place even when RNi reaches 1.00, that is, the charge equal point. Different from aluminum ions, with the additions of Ni2+ ions, the solution pH only decreases slightly from 8.0 to 7.2 (Figure 6d), showing that the hydrolysis of Ni2+ is relatively weak. When RNi is smaller than 0.33, the continuous and significant increases in endothermic ΔH and in Δδ demonstrate that the initial added Ni2+ ions already start to strongly interact with the C12C3C12(SO3)2 micelles, leading a large decrease in molar conductivity. Meanwhile, another kind of aggregates of 20−30 nm is generated beside the small micelles smaller than 10 nm (Figure 6b). The cryo-TEM image (Figure 4g) proves that they are vesicles. Moreover, the most significant change in Δδ also occurs in the Hf protons connecting to the N atoms of C12C3C12(SO3)2, suggesting the coordination between Ni2+ and the N atoms. The transition from micelles to vesicles is attributed to the reduction of the headgroup area of C12C3C12(SO3)2 due to the electrostatic and coordination interactions between Ni2+ and C12C3C12(SO3)2. As more Ni2+ ions insert into the headgroup area, the headgroup area may increase; thus the hydrodynamic diameter of the vesicles slightly decreases from ∼30 to ∼20 nm. When RNi exceeds 0.33 but is below 0.72, the increase of RNi leads to a decrease in the endothermic ΔH and an increase in the molar conductivity, which means that the electrical binding between Ni2+ and C12C3C12(SO3)2 is no longer the dominant effect in this region. Meanwhile, the Δδ value of the Hf protons markedly increases in this region, indicating that the coordination effect between them is enhanced. The aggregate size from DLS (Figure 6b) increases intensively, and the cryoTEM image (Figure 4h) indicated that the vesicles have transferred into wormlike micelles. 4109

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The Journal of Physical Chemistry B Mg2+/C12C3C12(SO3)2 was further studied at higher concentration. The steady shear viscosity for the Mg 2+ / C12C3C12(SO3)2 solution and the zero-shear viscosity η0 at 20.0 mM C12C3C12(SO3)2 and different RMg values are presented in Figure 7f and g. The solution viscosity increases with increasing RMg, and hydrogels form as RMg exceeds 4.00, which displays three-dimensional porous networks as shown in the SEM image. These results indicate that Mg2+ ions may play a bridging role connecting different wormlike micelles of C12C3C12(SO3)2. Different from other ions, as Mg2+ is added, the 1H NMR spectra of C12C3C12(SO3)2 do not show obvious chemical shift changes, but the 1H NMR peaks are broadened and the intensity of the peaks declines (Figure 7d). This further confirms the formation of large and closely packed aggregates as observed in other large aggregates.45,46 Comparisons of the Effects from Different Metal Ions. On the basis of the above results and discussion, the aggregate transitions of the C12C3C12(SO3)2 micelles upon the addition of different metal ions are summarized in Figure 8. With the

two significant driving forces in the aggregation of alkyldimethylamine oxide (CnDMAO) with divalent metal ion in aqueous solutions.52 Apparently, divalent and trivalent metal ions lead to much more abundant aggregate transitions in the gemini surfactant solution. As shown in Figure 8, the aggregate transitions of C12C3C12(SO3)2 are completely reflected in the enthalpy changes of the isothermal titration curves. For the Al3+/C12C3C12(SO3)2 and Ni2+/C12C3C12(SO3)2 systems, spherical micelles transit to vesicles when Rm is smaller than 0.33, that is, 1:3. Because of the electrostatic binding of the ions with the surfactant, the electrostatic repulsion between the surfactant headgroups is reduced, causing the decrease of the surfactant headgroup area and in turn inducing the transition of spherical micelles to vesicles. The electrostatic binding of Al3+ or Ni2+ with the surfactant headgroups is accompanied by strong dehydration of the surfactant and ions. Because the dehydration is endothermic, the transition of spherical micelles to vesicles leads to the gradual increase of the endothermic enthalpy from 0 to a maximum. The maximum of the endothermic ΔH value implies that the transition has ended. When Rm is between 0.33 and 0.67, vesicles transit to wormlike micelles, because more ions cause the increase of the headgroup area of the surfactant. Meanwhile, the endothermic enthalpy decreases from the maximum value to about zero. In this stage, the decrease of net charges of the mixture may weaken the dehydration, while the vesicle dissociation may be accompanied by hydration and ionization of the surfactant aggregates. The opposite changes result in the decrease of endothermic enthalpy, and finally the enthalpy becomes zero. As to Cu2+, it induces the spherical micelles to transfer into prolate micelles and plate-like aggregates. The two turning points are also 1:3 and 2:3 for Cu2+/C12C3C12(SO3)2. The different aggregate morphologies of the Cu2+/C12C3C12(SO3)2 systems might be caused by the special geometry of the Cu2+/ C12C3C12(SO3)2 complex. It is well-known that the complexes including Cu2+ are usually proteiform and show distorted geometries. The special configuration of the Cu2+/surfactant complex may induce the distortion of the spherical micelles. When the spherical micelles transfer into prolate micelles, the hydrocarbon chains are packing more tightly, which results in the growth of the micelles and the formation of prolate micelles. As the hydrophobic interaction is exothermic, the ΔH increases from 0 to a larger exothermic value. With the further addition of Cu2+, the prolate micelles transform into plate-like aggregates, the hydrophobic interaction may not change obviously, but the binding of Cu2+ may be accompanied by strong dehydration, which results in the increase in the endothermic enthalpy. Comparing the above three systems, the ΔH value of Mg2+/ C12C3C12(SO3)2 starts from a larger endothermic value when quite a small amount of Mg2+ is added in the C12C3C12(SO3)2 micelle solution. With the gradual addition of Mg2+, the ΔH value decreases gradually to zero, indicating the end of interaction. This is because the initial added metal ions already strongly bind with the C12C3C12(SO3)2 micelles and induce the aggregate transition from micelles to large aggregates. If the amount of added metal ions is small enough, the variation of the ΔH value should also experience an increase from 0 to a relatively larger value with the increase of Rm. However, as the quantity of Mg2+ needed to cause aggregate transition is very small, this variation process can hardly be detected. As proved

Figure 8. Variations of the metal ions/C12C3C12(SO3)2 aggregates against the molar ratio Rm. The curves are the ITC curves of titrating metal ions into the C12C3C12(SO3)2 solution.

additions of the metal ions to the small micelles of C12C3C12(SO3)2, Cu2+ induces the micelles to transfer into prolate micelle and plate-like aggregates, Al3+ and Ni2+ induce the micelles to transfer into vesicles and then wormlike micelles, while Mg2+ generates three-dimensional networks. Moreover, the C12C3C12(SO3)2 aggregates exhibit great hardness tolerance to Mg2+ and Ni2+ and do not produce precipitate even at charge equal point, while Al3+ and Cu2+ lead to precipitation when the metal ion/C12C3C12(SO3)2 ratios exceed 0.67, that is, 2:3. The addition of multivalent metal ions usually induces micelle growth, the formation of rod-like micelles,4 cylinder shape aggregates,47 lamellar phases,17 or vesicles.48 It is believed that the mechanism of the metal-induced phase transition is related to the coordination of the metal ions, the properties of the ligands, as well as the molecular geometry of the metal/ surfactant complexes.49,50 Hao and co-workers51 observed vesicle phase formed by surfactant complexes of double-chain anionic surfactant and divalent metal ions. They also found that metal−ligand coordination and hydrophobic interaction are 4110

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The Journal of Physical Chemistry B by the 1H NMR spectrum, Mg2+ does not form coordination bonds with the nitrogen atom on the surfactant. The transition of spherical micelles to three-dimensional networks is mainly caused by the electrostatic interaction of Mg2+ with the surfactant and the resulting association of the micelles. This process is accompanied by the reduction of net charge and dehydration leading to the decrease of the endothermic enthalpy. In summary, the aggregate transitions are controlled by the delicate balances between hydrolysis of the metal ions, and electrostatic and coordination interactions between the metal ions and the headgroups of C12C3C12(SO3)2. The hydrolysis of the metal ions affects the ionic states in solutions and in turn alters the electrostatic interaction and coordination interaction. The great hydrolysis ability of Al3+ and Cu2+ endows them with different ionic components as the hydrolysis goes on, while the relatively weak hydrolysis ability of Mg2+ and Ni2+ makes them mainly exist in the divalent state of Mg2+ and Ni2+. These ionic states determine that the electrostatic binding of the ions with C12C3C12(SO3)2 undergoes multiple stages, leading to the multiple aggregate transitions. Meanwhile, the 1H NMR spectra indicate that the Cu2+, Al3+, and Ni2+ ions show obvious coordination with the nitrogen atoms of C12C3C12(SO3)2, but Mg2+ ions do not show the coordination. However, as reported by Squattrito and co-workers,53 alkaline earth metal ion Ca2+ can coordinate to the sulfonate oxygen atoms, but transition metal ion Co2+ cannot. Thus, alkaline earth metal ion Mg2+ may coordinate with the sulfonate group of C12C3C12(SO3)2, although it cannot be distinguished from strong electrostatic interaction between them. So both electrostatic interaction and coordination interaction dominate the aggregate transitions of C12C3C12(SO3)2 in the presence of the Cu2+, Al3+, Ni2+, and Mg2+ ions. These interactions reduce the area of the C12C3C12(SO3)2 headgroups and thus induce the aggregate transitions from small micelles with larger curvature to prolate micelles, plate-like micelles, vesicles, and wormlike micelles with smaller curvature. The possible coordination between Mg2+ and the sulfonate groups may be another reason why the Mg2+/C12C3C12(SO3)2 mixture forms three-dimensional networks. Besides, because trivalent Al3+ becomes the main component only at higher ion/C12C3C12(SO3)2 molar ratio and aluminum ions consist of Al(OH)4−, Al(OH)3, Al(OH)2+, and Al(OH)2+ at lower ion/C12C3C12(SO3)2 molar ratio, the aggregate transitions induced by the aluminum ions are similar to those induced by divalent Ni2+. Their transition boundaries are consistent with each other.

electrostatic interactions but also coordination interaction. The hydrolysis of the metal ions affects the ionic states in solutions and in turn alters the electrostatic interaction and coordination interaction. Al3+ and Cu2+ have different ionic components due to the great hydrolysis, while Mg2+ and Ni2+ mainly exist as the divalent state because of their weak hydrolysis. These ionic states of metal ions determine that their electrostatic binding with C12C3C12(SO3)2 undergoes multiple stages, resulting in multiple aggregate transitions. The small micelles of C12C3C12(SO3)2 transfer to prolate micelles and plate-like micelles with Cu2+, vesicles and wormlike micelles with Al3+ or Ni2+, and viscous three-dimensional network structure with Mg2+. It is also found that C12C3C12(SO3)2 exhibits excellent hardness tolerance to Mg2+ and Ni2+, and forms precipitates with Cu2+ or Al3+ only when the molar ratio exceeds 0.67. This work helps to understand the interactions of multivalent metal ions with anionic gemini surfactants and the resultant aggregate transitions, and provides guidance on how to apply metal ions to control aggregate structures of surfactants.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-10-82615802. E-mail: [email protected] Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the National Natural Science Foundation of China (21025313, 21321063). REFERENCES

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CONCLUSION The present work has studied the interaction of multivalent counterions with anionic sulfonate gemini surfactant C12C3C12(SO3)2 and the induced aggregation behavior in aqueous solution. The results indicate that the CMC of C12C3C 12(SO3)2 is greatly reduced, and the aggregate morphologies of C12C3C12(SO3)2 are controlled by changing the nature and molar ratio of the metal ions. According to the ITC curves of the ion solutions being titrated into the surfactant solution, the metal ions show similar interaction processes in each of the following four groups: Cu2+ and Zn2+ (I); Ca2+, Mn2+, and Mg2+ (II); Ni2+ and Co2+ (III); and Cr3+, Al3+, and Fe3+ (IV). Cu2+, Mg2+, Ni2+, and Al3+ then were selected as representatives for each group to further study their interaction with C12C3C12(SO3)2 in detail. C12C3C12(SO3)2 interacts with the multivalent metal ions through not only 4111

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DOI: 10.1021/acs.jpcb.6b02897 J. Phys. Chem. B 2016, 120, 4102−4113

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DOI: 10.1021/acs.jpcb.6b02897 J. Phys. Chem. B 2016, 120, 4102−4113