Interactions of Divalent and Trivalent Metal Counterions with Anionic

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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 J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b02897 • Publication Date (Web): 20 Apr 2016 Downloaded from http://pubs.acs.org on April 21, 2016

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The Journal of Physical Chemistry

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, P. R. China ‡

Centre for Bioengineering and Biotechnology, China University of Petroleum (East China), 66

Changjiang West Road, Qingdao 266580, P. R. China

ACS Paragon Plus Environment

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ABSTRACT: Interactions of multivalent metal counterions with anionic sulfonate gemini surfactant 1, 3-bis(N-dodecyl-N-propanesulfonate 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+; Ca2+, (II) Mn2+ and Mg2+; (III) Ni2+ and Co2+; Cr3+, (IV) Al3+ and Fe3+. Then Cu2+, Mg2+, Ni2+ and Al3+ 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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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, 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.1113

Penfold, Thomas and coworkers14-18 have studied the interaction between Al3+ and sodium

polyethylene glycol monoalkyl ether sulfate (SLEnS) with different alkyl chain length and different number of polyethylene oxide group in aqueous solution using small-angle neutron scattering (SANS). They found that multilayer is formed at air-water 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 aggregate/solution interface and 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 So multivalent metal ions can either improve or limit the performances of anionic surfactants.


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So far 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 Cu2+, regardless of the nature of the divalent counterions.32 The properties of anionic micelles of sodium dodecyl polyoxyethylene-2sulfate 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-to-cylinder shape transition of the micelles and facilitates their further growth, even at very low ionic strength.34 Kralchevsky group4 studied the micelle size and shape of sodium dodecyl dioxyethylene sulfate as well as the Al3+/ surfactant interactions, and found that Al3+ ions induces intensive growth of spherical micelles to rod-like micelles. Xu and coworkers17 found that 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. Comparing with 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 single-chain 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-N-propylsulfonate sodium)-propane (C12C3C12(SO3)2) shows much better performance in the tolerance of calcium ions than sodium dodecyl sulfate


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(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 12-3-12(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 deserves 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 behaviour 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 catalystic 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 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-N-propanesulfonate 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 were measured 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 were 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 thermostatted chamber. The thermostatted chamber was controlled at 25 °C. Potentiometric pH Titration. C12C3C12(SO3)2 was firstly dissolved in pure water at a concentration of 2.00 mM, 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


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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 field-emission scanning electron microscope (Hitachi S-4800). All 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 (Cryo-TEM). The solutions of the metal ions/C12C3C12(SO3)2 aggregates were embedded in a thin layer of vitreous ice on freshly carboncoated 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 a nominal magnification of 50 000. For each specimen area, the defocus was set to 1 to 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. 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.


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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 2.00 mM C12C3C12(SO3)2 solution or water, and then 10.00 mM metal ion solution was injected consecutively into the stirred 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 (∆H) were obtained from the areas of the calorimetric peaks after titrations. The ∆Hobs values for 10.00 mM metal ion solution being titrated into water were very small and subtracted from the ∆H values for the same metal ion solution being titrated into the C12C3C12(SO3)2 solution. Thus the final ∆H 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 less than ± 4%.

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


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C12C3C12(SO3)2 solution. The corresponding ITC raw data and the curves of ∆H 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 ∆H curves all show an exothermic peak at lower ion concentration, then an endothermic peak, finally the endothermic ∆H decreases to near zero. Both Zn2+/C12C3C12(SO3)2 and Cu2+/C12C3C12(SO3)2 form precipitates after the ITC titrations. Group II includes Mg2+, Ca2+ and Mn2+. All the ∆H curves for these ions show a sigmoid shape, exhibiting larger endothermic ∆Hobs values at lower ion concentration, and then abruptly decreasing and reaching almost zero in final. All 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, ∆Hobs increases from very low endothermic values to larger endothermic values, and then decreases back to very low values, shaping a peak. All 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 ∆H curves, and form precipitates after titrations. The fluctuations are caused by the precipitation.


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Figure 1. The raw data and the observed enthalpy changes (∆H) 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. ∆H values are expressed in kJ/mol of metal ions. Because the CMC of C12C3C12(SO3)2 is 0.041 mM and 2.00 mM used above is larger than the CMC, all the ITC curves reflect the interactions of the metal ions with the C12C3C12(SO3)2


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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. In order to understand the physical meaning of these interactions and the detailed information of 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. Firstly, the surface tension experiments were carried out to study the effects of the metal ions on the 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 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 inflexion 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+ shows more efficient ability in lowering the surface tension of C12C3C12(SO3)2 than Cu2+.


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Figure 2. (a) Variation of the surface tension (γ) of the metal ions/C12C3C12(SO3)2 solutions versus Rm at the C12C3C12(SO3)2 concentration (Cs) of 0.0020 mM, and (b) γ versus the logarithm value of Cs at Rm = 1.00. On the basis of the above results, the ability of the metal ions in reducing the CMC of C12C3C12(SO3)2 are studied by measuring the variation of surface tension with the C12C3C12(SO3)2 concentration (Cs). 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-logCs curves are shown together with the surface tension curve of C12C3C12(SO3)2 in the absence of the metal ions in Figure 2b. All the surface tension curves with the four metal ions are very close with each other, indicating that all of the four metal ions show great and very similar ability of reducing the CMC and surface tension of C12C3C12(SO3)2 in regardless of the nature of the metal ions. Moreover, the CMC value of C12C3C12(SO3)2 (0.041 mM) is lowered by thirty times (0.0013 ~ 0.0015 mM) by a small amount of Cu2+, Al3+, Mg2+ and Ni2+ (Rm = 1.00). As previous 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+


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and Cu2+, about 7-fold. Obviously, comparing with 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 mN 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 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 (∆H) 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+ into 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, 3c and 3d. 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 (∆δ) are calculated from ∆δ = 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 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


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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

∆H (kJ/mol)

results will be further discussed in the following text. 0.0 6

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g (e) f e d c

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Figure 3. Variation of (a) observed enthalpy change (∆H), (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 (∆δ) of protons Hc, Hd, He, Hf and Hg, with the increase of the Cu2+/C12C3C12(SO3)2 molar ratio (RCu). The ∆H curves are obtained by titrating 10.00 mM Al3+ solution into 2.00 mM C12C3C12(SO3)2 solution or into water.


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Figure 4. The 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 ratio: (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. When RCu is below 0.17, the addition of Cu2+ does not cause any changes in ∆H, 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 ∆H, aggregate size and the chemical shifts indicate that the C12C3C12(SO3)2 molecules still exist as small micelles and do not undergo


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aggregate transition in this region. The electrostatic bind leads to the sharp decrease in molar conductivity (Figure 3c). The decrease of pH are 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 ∆H 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 micelles. Because the pH slightly decreases from 6.80 to 6.20 in this region, Cu2+ is the main ion state.43 The exothermic ∆H suggests that Cu2+ ions electrostatically bind with C12C3C12(SO3)2 headgroups. The slightly increased ∆δ 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 adding Cu2+ ions, ∆H begins to sharply change from exothermic to endothermic, and the aggregate size abruptly increases from ~50 nm to ~200 nm. The Cryo-TEM image (RCu = 0.60, Figure 4b) proves the existence of large platelike aggregates. In this region, since 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 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 in Hg at the α


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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 ∆H, aggregate size, electrical conductivity (κ), molar conductivity (Λ), and chemical shifts (∆δ) 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 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.


17

∆H (kJ/mol)

45

300

(a)

30 15

Al3+/C12C3C12(SO3)2 Al3+/H2O

0

(b)

200 100

0.8

κ

(c) 500

Λ

400 300

0.6

200 0.4 8

2

κ (mS/cm)

0

(d)

7

pH

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Λ (S cm /mol)

3+

Al /C12C3C12(SO3)2

6

+

Na /C12C3C12(SO3)2

5 4

∆δ (ppm)

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

size (d.nm)


g (e) f e d c

0.3 0.2 0.1 0.0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

RAl

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


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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 aluminium 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 aluminium ions. When RAl is below 0.17, pH is above 6.2. With the addition of the aluminium ions, the larger endothermic ∆H (Figure 5a) indicates that a small amount of cationic components Al(OH)2+ and Al(OH)2+ electrostatically bind with C12C3C12(SO3)2, accompanying with the dehydration of all the charges. The electrostatic bind leads to the sharp decrease in molar conductivity (Figure 5c). The unchanged size distribution and ∆δ (Figure 5b and Figure 5e) 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 aluminium ions mainly exist as Al(OH)2+ and Al3+. With the addition of the aluminium ions, the endothermic ∆H increases to a maximum at RAl = 0.37, and ∆δ sharply increases to a very large value. Similar to the Cu2+/C12C3C12(SO3)2 system, the most significant change in ∆δ 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 binds of the aluminium ions with C12C3C12(SO3)2, and the coordination bonds occur between the aluminium


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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. When RAl increases from 0.37 to 0.65, 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 ∆H still keeps larger endothermic values (Figure 5a) and the ∆δ 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 the similar way above to analyse 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 molar ratios (RNi) of 0.33 and 0.72. Moreover, precipitation does not take place even when RNi reaches 1.00, i.e., the charge equal point. Different from aluminium 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.


20

∆H (kJ/mol)

15

size (d.nm)

80

(a) Ni2+/C12C3C12(SO3)2

10

Ni2+/H2O

5 0 (b)

60 40 20

S S

S

S

S S

0.7

(c)

240

0.6

κ

220

0.5

Λ

200

0.4

180 (d)

8.0

pH

2

κ (mS/cm)

S

Λ (S cm /mol)

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


7.5 7.0 0.4

∆δ (ppm)

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(e)

g f e d c

0.3 0.2 0.1 0.0 0.0

0.3

0.6

0.9

1.2

1.5

RNi

Figure 6. Variation of (a) observed enthalpy change (∆H), (b) distribution of hydrodynamic diameter, (c) electrical conductivity (κ) and molar conductivity (Λ), (d) pH, and (e) chemical shifts (∆δ) of protons of 2.00 mM C12C3C12(SO3)2 solution with the increase of the Ni2+/C12C3C12(SO3)2 molar ratio (RNi). The ∆H curves are obtained by titrating 10.00 mM Ni2+ solution into 2.00 mM C12C3C12(SO3)2 solution or into water. When RNi is smaller than 0.33, the continuous and significant increases in endothermic ∆H and in ∆δ 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 are 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


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significant change in ∆δ 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 and more Ni2+ ions insert in the headgroup area, the headgroup area may increase, thus the hydrodynamic diameter of the vesicles slightly decreases from ~30 nm to ~ 20 nm. When RNi exceeds 0.33 but below 0.72, the increase of RNi leads to a decrease in the endothermic ∆H and an increase in the molar conductivity, which means that the electrical binding between Ni2+ and C12C3C12(SO3)2 is no longer the dominate effect in this region. Meanwhile, the ∆δ 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 Cryo-TEM image (Figure 4h) indicated that the vesicles have transferred into wormlike micelles. When RNi is beyond 0.72, ∆H becomes zero, and all the ∆δ values, aggregate size and molar conductivity keep almost constant. This means that the binding of Ni2+ with C12C3C12(SO3)2 has reached a saturation. Besides, precipitation does not take place even when RNi reaches 1.00, i.e., 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 firstly, then keeps a relatively stable


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value and finally increases slightly after the saturation point of interaction. The ∆H value starts from a larger exothermic value, while if no ions are added, the ∆H value should be zero. This means there is a very sharp increase in ∆H 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 Mg2+ ions already strongly binds 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 three-dimensional network structure, which may consist of entangled wormlike micelles.

Figure 7. Variation of (a) observed enthalpy changes (∆H), (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)

1

H 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


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and different RMg. The ∆H 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) inserted is from the Mg2+/C12C3C12(SO3)2 mixture at 20.0 mM C12C3C12(SO3)2 and RMg = 4.50. Compared with 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 Mg2+/C12C3C12(SO3)2 was further studied at higher concentration. The steady shear viscosity for the Mg2+/C12C3C12(SO3)2 solution and the zero-shear viscosity η0 at 20.0 mM C12C3C12(SO3)2 and different RMg are presented in Figure 7f and 7g. 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. Based on the above results and discussions, the aggregate transitions of the C12C3C12(SO3)2 micelles upon the addition of different metal ions are summarized in Figure 8. With the additions of the metal ions to the small micelles of C12C3C12(SO3)2, Cu2+ induces the micelles to transfer into prolate micelle and platelike 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


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at charge equal point, while Al3+ and Cu2+ lead to precipitation when the metal ion/ C12C3C12(SO3)2 ratios exceed 0.67, i.e., 2:3. Metal ion/C12C3C12(SO3)2 molar ratio 2:3 1:3 Prolate micelles

Cu2+

Al

Plate-like aggregates

Precipitates

3+

Spherical micelles

Wormlike micelles

Vesicles

Ni2+ Three dimentional network

Mg2+ 0.0

0.2

0.4

0.6

0.8

1.0

Rm

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. The addition of multivalent metal ions usually induce micelle growth, the formation of rodlike micelles,4 cylinder shape aggregates,47 lamellar phases17 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 coworkers51 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 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.


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For the Al3+/C12C3C12(SO3)2 and Ni2+/C12C3C12(SO3)2 systems, when Rm is smaller than 0.33, i.e., 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 with strong dehydration of the surfactant and ions. Since 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 ∆H 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 with hydration and ionization of the surfactant aggregates. The opposite changes result in the decrease of endothermic enthalpy, 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 ∆H increases from 0 to a larger exothermic value. With the further


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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 with strong dehydration, which results in the increase in the endothermic enthalpy. Comparing with the above three systems, the ∆H 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. And with the gradual addition of Mg2+, the ∆H value decreases gradually to zero, indicating the end of interaction. This is because the initial added metal ions already strongly binds 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 ∆H 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 by the 1H NMR, 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 with the reducing 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, electrostatic and coordination interaction 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


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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 coworkers,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 aluminium 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 aluminium ions are similar to those induced by divalent Ni2+. Their transition boundaries are consistent with each other.

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


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results indicate that the CMC of C12C3C12(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); Cr3+, Al3+ and Fe3+ (IV). Then Cu2+, Mg2+, Ni2+ and Al3+ 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 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. AUTHOR INFORMATION Corresponding Author *(Y.L.W.) E-mail: [email protected]


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Address: Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China Telephone number: +86-10-82615802 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We are grateful for financial support from National Natural Science Foundation of China (21025313, 21321063). REFERENCES (1) Pereira, R. F. P.; Tapia, M. J.; Valente, A. J. M.; Burrows, H. D. Effect of Metal Ion Hydration on the Interaction between Sodium Carboxylates and Aluminum (III) or Chromium (III) Ions in Aqueous Solution. Langmuir 2011, 28, 168‒177. (2) Somasundaran, P.; Ananthapadmanabhan, K. P.; Celik, M. S. Precipitation-Redissolution Phenomena in Sulfonate-Aluminum Chloride Solutions. Langmuir 1988, 4, 1061‒1063. (3) Xing, F.; Niu, J.; Liu, X.; Wang, X. Effect of a Spacer Group on Surface Activity, Salinity and Hardness Tolerance, Mimic Oil Washing Efficiency of Monododecyl Diaryl Disulfonate. J. Surfactants Deterg. 2014, 17, 95‒100. (4) Alargova, R. G.; Danov, K. D.; Kralchevsky, P. A.; Broze, G.; Mehreteab, A. Growth of Giant Rodlike Micelles of Ionic Surfactant in the Presence of Al3+ Counterions. Langmuir 1998, 14, 4036‒4049. (5) Carlsson, I.; Edlund, H.; Persson, G.; Lindström, B. Competition between Monovalent and Divalent Counterions in Surfactant Systems. J. Colloid Interface Sci. 1996, 180, 598‒604.


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Interactions of Divalent and Trivalent Metal Counterions with Anionic

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