Hydrogels Facilitated by Monovalent Cations and Their Use as

Apr 7, 2014 - (1-5) Surfactant gels show solid-like properties and are constructed ... DNA,(15) peptides,(16, 17) lipids,(18) carbohydrates,(19) and s...
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Hydrogels Facilitated by Monovalent Cations and Their Use as Efficient Dye Adsorbents Haiqiao Wang, Wenlong Xu, Shasha Song, Lei Feng, Aixin Song,* and Jingcheng Hao Key Laboratory of Colloid and Interface Chemistry & Key Laboratory of Special Aggregated Materials (Shandong University), Ministry of Education, Jinan 250100, China S Supporting Information *

ABSTRACT: Gelation behavior of lithocholate (LC−) mixed with different monovalent cations in water was detected. The hydrogels consisting of tubular networks were formed by introducing alkali metal ions and NH4+ to lithocholate aqueous solutions at room temperature. The formation of tubular structures was considered to be mainly driven by the electrostatic interaction with the assistance of a delicate balance of multiple noncovalent interactions. It is interesting that the increase in temperature can induce a significant enhancement in strength of the hydrogels, accompanied by the formation of bundles of tubules and larger size aggregates. The mechanism of the temperature-induced transition can be explained by the “salting-out” effect and the electric double layer model. The hydrogels showed very high adsorption efficiency and adsorption capability for the cationic dyes and were promising to act as toxic substance adsorbents.

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INTRODUCTION As a kind of soft matter, surfactant gels have attracted broad attention because of their wide applications.1−5 Surfactant gels show solid-like properties and are constructed by threedimensional networks formed by gelators to immobilize solvent molecules.6−9 In the process of gel formation, noncovalent interactions such as hydrogen bonding, electrostatic interaction, π−π stacking, and van der Waals play important roles.10−13 Gelators usually contain functional groups such as carboxyl, hydroxyl, and amino, which are easy to interact with other ones. Tubular structures are especially interesting since tubules have biological relevance. They have been observed in several systems of biomolecules such as proteins,14 DNA,15 peptides,16,17 lipids,18 carbohydrates,19 and so on, and have received extensive consideration for applications, including controlled release and drug delivery20 and being used as templates for materials synthesis.21,22 Tubules can be obtained from various synthetic routes; one of the most common ones is to use the process of self-assembly.23 For fabricating tubular structures, bile acids or their salts consisting of rigid steroidal backbone with polar hydroxyl groups and carboxyl group are particularly useful in building blocks. The exclusive facial amphiphilicity leads to unique aggregation behavior.24−27 The hydroxyl and carboxyl groups result in the intermolecular hydrogen bonding,28 and moreover, they can also interact with the functional groups of other molecules.11,29,30 Hydrogels have been found to be formed by cholate salts with a series of transition metal ions.12,31,32 The hydrogels were considered to be induced by the metal-coordinated interaction between the anions of bile salts and the transition metal ions.32−34 When monovalent cations were introduced, in the © 2014 American Chemical Society

case of the weakness or even absence of metal-coordinated interaction, the gelation properties exhibited a distinct difference due to the difference in the number and position of hydroxyls in the steroidal backbone of bile salts. Herein, we report a series of hydrogel systems formed by lithocholate (LC−) with different monovalent cations, M+ (M+ = Li+, Na+, K+, Rb+, Cs+, NH4+). Among the studied monovalent cations, unlike the transparent solutions of cholate salts and the weak gels formed by deoxycholate with only Na+, we obtained hydrogels with high gelation capability using lithocholate with all above cations. The hydrogels were formed only by introducing M+ to sodium lithocholate (SLC) solutions at room or even lower temperatures. The tubular microstructures were observed in all hydrogel systems. Compared with fibrous and helical structures, tubular structures were relatively rarely reported in hydrogel systems. The main driving force was considered to be the electrostatic interaction, which was affected by the ionic radii of the cations, leading to different gelation capability. It was notable that increasing temperature induced the enhancement of mechanical strength of the hydrogels instead of the general gel−sol transition, accompanying with the increase in the size of the tubular structures. The gels showed excellent adsorption capability for positively charged dyes, such as methylene blue and rhodamine 6G, with the maximum adsorption efficiency and adsorption capacity among reported results, and was hopeful to act as water-purifying agents in an environmentally friendly way. Received: January 6, 2014 Revised: April 7, 2014 Published: April 7, 2014 4693

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Zeta Potential Measurements. The zeta potential was measured on a Zeta PALS potential analyzer instrument (Brookhaven, USA) with parallel-plate platinum black electrodes spaced 5 mm apart and a 10 mm path length rectangular organic glass cell. All samples were measured at a sinusoidal voltage of 80 V and a frequency of 3 Hz. UV−Visible Absorption Spectra Measurements. The UV−vis absorption spectra measurements were performed on a HITACHI U-4100 spectrophotometer using 10 mm path length quartz cell. The scan rate for each measurement was 300 nm·min−1.

MATERIALS AND METHODS Materials. Lithocholic acid (LCA, 98%) was purchased from Acros Organics (USA) and used without further purification. NaOH, LiCl, NaCl, KCl, CsCl, NH4Cl, NaF, NaBr, NaNO3, Na2S, NaNO2, Na2CO3, Na2SO4, Na2SO3, Na3PO4, methylene blue, rhodamine 6G, amido black 10B, chrome azurol S, and methyl orange were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and were of p.a. quality. Ultrapure water with a resistivity of 18.25 MΩ·cm was obtained by a UPH-IV ultrapure water purifier (China). Gelation Behavior Study. A desired amount of LCA was weighed accurately into test tubes, and then aqueous solutions containing equimolecular NaOH were added. LCA was dissolved to form SLC solutions. Then salts with M+ as cations were added to SLC solutions under stirring. The sample solutions were equilibrated for at least 4 weeks until they did not change any more. Transmission Electron Microscope (TEM) and Cryogenic (Cryo)-TEM Observations. The TEM and cryo-TEM observations were carried out on a JEOL JEM-1400 TEM (Japan) operating at 120 kV. The images were recorded on a Gatan multiscan CCD (USA) and processed with a Digital Micrograph. For TEM observations, about 5 μL of sample solution was dropt on TEM grids (copper grid, 3.02 mm, 400 meshes, coated with Formvar film). The excess solution was wiped away with filter paper, and the copper grids were then freeze-dried in a vacuum extractor at −55 °C. For the observation of hydrogels formed at 60 °C, 5 μL of sample solution was dropt on TEM grids at 60 °C. After being wiped away with filter paper for excess solution, the copper grids were frozen quickly in liquid Nitrogen and freeze-dried in the vacuum extractor at −55 °C. The TEM observations were carried out at room temperature. For cryo-TEM observations, samples were prepared in a controlled-environment vitrification system (CEVS) at room temperature. A micropipet was used to load about 5 μL of solution onto a TEM copper grid, which was coated with carbon-grid support film. The solution was blotted with two pieces of filter paper, forming a thin film suspended on the mesh holes. After waiting for about 5 s, the samples were quickly plunged into liquid ethane (cooled by liquid nitrogen) at −165 °C. The vitrified samples were then stored in the liquid nitrogen until they were transferred to a cryogenic sample holder (Gatan 626) and observed on TEM at −174 °C. Rheological Measurements. Rheological measurements were performed on a HAAKE RS6000 rheometer with a cone− plate system (C35/1°Ti L07116). In oscillatory measurements, an amplitude sweep at a fixed frequency of 1 Hz was carried out prior to the following frequency sweep, making sure that the selected stress was in the linear viscoelastic region. Fourier Transform Infrared Spectroscopy (FT-IR) Measurements. The FT-IR spectra were measured on a VERTEX-70/70v FT-IR spectrometer (Bruker Optics, Germany). Spectra over 4000−400 cm−1 were obtained by taking 64 scans with a final resolution of 4 cm−1. Spectral manipulation was performed by the OPUS 6.5 software package (Bruker Optics, Germany). Small Angle X-ray Diffraction (XRD) Measurements. The XRD patterns were recorded on a DMAX-2500PC diffractometer with Cu Kα radiation (λ = 0.15418 nm) and a graphite monochromator. The samples were examined at room temperature within 1−10° in the 2θ mode (1°·min−1).

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RESULTS AND DISCUSSION Gelation Behavior. Tubular structures were reported in aqueous solutions of lithocholate with Na+ and NH4+ as counterions.24−27,33,34 In this work, hydrogels were obtained just by mixing chlorides of monovalent cations (LiCl, NaCl, KCl, RbCl, CsCl, and NH4Cl) with SLC aqueous solutions. The concentration of SLC solution was fixed at 20 mM. The minimum gelation concentrations of salts needed for gelating 20 mM SLC (cgel) were confirmed by an inverted test tube method. As shown in Figure 1, these hydrogels have different

Figure 1. Photographs of hydrogels formed by 20 mM SLC with 200 mM M+: (a) NH4+; (b) Li+; (c) Na+; (d) K+; (e) Rb+; (f) Cs+. T = 25 °C.

Table 1. Bare Radii (r) and Hydrated Radii (rh) of Different Cations35 and Their Minimum Gelation Concentrations (cgel) at a Fixed SLC Concentration of 20 mM at 25 °C ion

r (nm)

rh (nm)

cgel (mM)

NH4+ Li+ Na+ K+ Rb+ Cs+

0.148 0.094 0.117 0.149 0.163 0.186

0.331 0.382 0.358 0.331 0.329 0.329

75 60 95 113 125 130

appearances at room temperature. Table 1 presents the bare radii, r, and the hydrated radii, rh, of the cations35 and the cgel values. Li+ showed the strongest gelation ability, while Cs+ was the weakest, demonstrating that the cation with a larger rh (smaller r) has the greater gelation ability. The ability to hold the network of different cations was determined by the differential of cations for binding to the carboxylate of LC−. For alkali metal ions, the difference in the ability of cations to induce gelation should be relevant to their interaction with LC−, which was influenced by their radii, showing an increasing ability with the decrease in the bare radii or increase in the 4694

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Figure 2. Microscopic images of hydrogels. TEM images of hydrogels formed by 20 mM SLC/200 mM M+: (a) NH4+ (inset: magnified image of one broken tubule); (b) Li+. Cryo-TEM images of hydrogels formed by 20 mM SLC/120 mM M+: (c) Na+; (d) K+; (e) Cs+.

hydrated radii. Among the studied monovalent cations, NH4+ is an exception. Microstructures. The microstructures of SLC/M+ hydrogels were detected by TEM and cryo-TEM measurements. In microscopic images shown in Figure 2, we observed threedimensional networks formed by the entanglement of onedimensional nanotubes. For the hydrogels formed in SLC/ NH4+ and SLC/Li+ systems, the breakage and hollow cavity can be seen clearly as the wall of tubules collapse when they are freeze-dried (Figures 2a and S1), verifying the tubular character. Diameters of tubules are not uniform, ranging from 50 nm to 1 μm (Figure 2a,b). As for hydrogels formed by SLC with Na+, K+, and Cs+ (Figure 2c,d,e), plenty of stiff nanotubes with extremely uniform diameter of about 50 nm can be observed clearly in the cryo-TEM images, which are consistent with the microstructure of SLC solutions.24,26,27 One can find that the opaque hydrogels induced by NH4+ and Li+ are composed of nanotubes with larger diameters, while the transparent hydrogels are composed of the uniform nanotubes in smaller size. The difference in the tubular size may explain their different appearance. Mechanical Strength. The solid-like network structures of gels will be broken suddenly above the yield stress (τ*) under shear. The τ* value reflects the strength of the network structures.36 As shown in Figure 3, at the fixed concentration of 20 mM SLC/200 mM MCl, for hydrogels induced by NH4+ and Li+, τ* and the elastic modulus (G′) are about 500 Pa and 105 Pa, respectively, which are much larger than those induced by Na+, K+, Rb+, and Cs+, with τ* and G′ around 10 and 100 Pa, respectively. The mechanical strength of the hydrogels is in accordance with the ability of cations to induce gelation, as shown in Table 1. Figure 4 shows G′ at a fixed SLC concentration of 20 mM with different cation concentrations. For all gels, G′ increases with cation concentrations and become stable when the concentrations reach certain values. This result further proves that the hydrogels are induced by cations, and the increase in cation concentration makes the tubules arrange more tightly.

Figure 3. Elastic modulus (G′) as a function of oscillatory stress of hydrogels of 20 mM SLC/200 mM M+ at 25 °C.

Figure 4. Elastic modulus (G′) as a function of M+ concentration at 25 °C. The SLC concentration is 20 mM.

Effect of Different Anions. To detect the role of anions in gelation, sodium salts with different anions were introduced to 20 mM SLC aqueous solutions. Hydrogels with almost the same appearance as those formed by NaCl were obtained. The minimum concentrations of Na+ (cNa+) needed for gelating 20 mM SLC, around 95 mM (see Table 2), are very close to the 4695

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Table 2. Minimum Concentrations of Salt (csalt) and the Corresponding Concentrations of Na+ (cNa+) Needed for Gelating 20 mM SLC at 25 °C salt

csalt (mM)

cNa+ (mM)

NaCl NaF NaBr NaNO3 NaNO2 Na2S Na2SO4 Na2SO3 Na2CO3 Na3PO4

95 95 95 90 93 47 48 50 53 32

95 95 95 90 93 94 96 100 106 96

Figure 6. Small angle XRD patterns of hydrogels formed by 20 mM SLC/M+ at 25 °C. The M+ concentration is (a) 80 mM Li+, (b) 100 mM Na+, (c) 160 mM K+, (d) 150 mM Rb+, and (e) 160 mM Cs+.

hydrogel system induced by NaCl. Figure 5 shows the dynamic rheological results of hydrogels formed by NaNO3, Na2SO4,

Based on above results, the mechanism of the tubular structures in hydrogels was proposed and shown in Scheme 1. Scheme 1. Schematic Representation of the Self-Assembly Mechanism of LC− with Cations

Figure 5. Elastic modulus (G′) as a function of oscillatory stress of hydrogels of 20 mM SLC/200 mM Na+ with corresponding anions at 25 °C.

Na2CO3, and Na3PO4. The τ* and G′ values are found to be very close for different salts, and are also similar to the results of NaCl induced hydrogels. According to these results, we can conclude that the anions seem to play an insignificant role in the gelation. Mechanism Explanation. FT-IR measurements were used to investigate the interaction between LC− and cations. As shown in Figure S2a, LCA in the solid state has a carbonyl symmetric stretching peak at 1705 cm−1. In hydrogels formed by SLC and alkali metal ions (Figure S2b), the double peaks of the carboxylate vibration at 1565 cm−1 (antisymmetric vibration) and 1409 cm−1 (symmetric vibration) in the spectra indicates the formation of the salts, for which the −COOH groups of LCA convert to carboxylate anions.37 Detailed information on the microstructure was revealed by small-angle X-ray diffraction (XRD) patterns shown in Figure 6. A peak was found for all hydrogels. According to Bragg’s law, we can get the d values from the reflection peaks, which are considered to be related to some cycled units induced by the arrangements of LC− species. The d value is found within the range of 1.7 to 2.1 nm, slightly larger than a cholate backbone length (1.5 nm).38 When the concentration of cations varies, as shown in Figure S3, the position of the reflection peaks almost does not change, indicating that the amounts of cations exhibit very little effect on the arrangement of LC− species in nanotubes.

The lithocholate species and monovalent ions adopt a lamellar type of arrangement in the hydrogels. The carboxylate of a lithocholate species engages itself in hydrogen bonding with the hydroxyl group of another lithocholate species and forms the continuous hydrophilic cavities between the two connected lithocholate species.11,28 Such interior cores can act as the pocket in which water molecules are held through interfacial tension. Moreover, the carboxylate groups interact with the cations through electrostatic force and extend to give rise to aggregates of nanodimensional tubules. As for the hydrogels formed by SLC with NH4+, the situation exhibits rather difference. Different from alkali metal ions, NH4+ hydrolyzes and increases the acidity of the solution, which leads part of LC− species turn to lithocholic acid (LCA). As shown in Figure 7a, the peaks at 1708 and 1550 cm−1 are correlated to the carbonyl stretching of −COOH and −COO−, respectively, indicating the coexistence of LCA and LC−. Small angle XRD results also reflect the special nature of the SLC/NH4+ system. At 2θ = 4.14, a weak peak appears, from which the d spacing distance calculated is 2.13 nm, close to the value of gels induced 4696

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Figure 7. FT-IR spectrum (a) and small angle XRD curve (b) of a hydrogel of 20 mM SLC/80 mM NH4+ at 25 °C.

by Li+. At larger diffraction angles, four strong peaks appear, corresponding to the d values of 1.50, 1.35, 1.06, and 0.95 nm. These data show that the hydrogels formed in SLC/NH4+ system bear the same microstructure, as shown in Scheme 1. However, the arrangement in other directions is more ordered than that parallel to LCA molecules. As the existence of LCA, the lamellar aggregates are less negatively charged, leading to a lower membrane curvature. The reason might be that the tubules in NH4+ induced hydrogels are of larger size than those induced by Li+, Na+, K+, Rb+, and Cs+. Since different amounts of cations are required to form hydrogels, the ability of different cations to hold the networks is determined by differential cation binding ability to LC−. The sequence of cations to associate to LC−, Li+ > Na+ > K+ > Rb+ > Cs+, is in the order of decreasing hydrated radius, which has been found in many surfactant systems.39−41 Gel−Gel′ Transition Induced by Temperature. The sol−gel transition as a result of thermal stimuli was observed in most gel systems. The sol−gel behavior is usually correlated to the aggregation fashion that small surfactant molecules selfassemble into complex three-dimensional networks, mainly directed by hydrogen bonding or other weak interactions which were significantly affected by temperature. Besides the gel−sol transition, the temperature triggered gel−gel′ transition, for which the La3+−cholate hydrogel system worked as a typical example of “heating-promoted gelation” behavior, was believed to originate from the lower hydrophilicity and higher bending rigidity.31 Interestingly, for hydrogels obtained from the present systems, the gels are also not destroyed by heating; on the contrary, they exhibit gel−gel′ transition, an obvious enhancement in strength at higher temperature, accompanied by an appearance change from transparent to opaque. As an example, the appearance of the hydrogel of 20 mM SLC/200 mM NaCl at 25 and 40 °C is shown in Figure 8. For gels induced by different cations, the temperature triggered transition is also different, as shown in Figure 9. One can see clearly that the transition temperature from transparent to opaque occurs in the sequence of Rb+ > K+ > Na+ > Li+ > NH4+. The hydrogels induced by Li+ and NH4+ are opaque at room temperature (25 °C), while the transparent hydrogels induced by Na+ and K+ transformed to opaque ones at temperature no less than 40 and 60 °C, respectively. As an exception, the hydrogels induced by Cs+ maintain the transparent appearance until being destroyed above 75 °C. The transition process is irreversible and slow and can hardly be detected by DSC determination. The mechanical properties of the hydrogels with the increase in temperature were investigated by dynamic rheological

Figure 8. Photos of hydrogel of 20 mM SLC/200 mM NaCl at 25 °C (left) and 40 °C (right).

Figure 9. Transition of hydrogels formed by 20 mM SLC/200 mM M+: (a) NH4+; (b) Li+; (c) Na+; (d) K+; (e) Rb+; (f) Cs+ within 5−80 °C. Different states are labeled as transparent gel (striped); opaque gel (diagonal crosshatched); turbid liquid (open); separation of gel and solvent (vertical crosshatched).

determination. The gels of 20 mM SLC/200 mM M+ (M+ = Li+, Na+, K+) were selected to be detected. As shown in Figure 10, the hydrogels exhibit typical solid-like rheological behavior, for which G′ and G″ are nearly independent of the oscillatory frequency, and G′ exceeds G″ over the investigated frequency range. With an increase in temperature, G′ and G″ increase significantly, along with an appearance change from transparent gels to opaque ones, indicating a remarkable enhancement in the strength. The microstructures also changed with temperature. The TEM images in Figure 11 show the microstructures of gels induced by Na+ and K+ at 60 °C. Compared with the transparent gels at room temperature (Figure 2c,d), the tubules 4697

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Figure 10. Frequency sweep of hydrogels formed by 20 mM SLC/200 mM M+: (a) Li+; (b) Na+; (c) K+ at different temperature.

Figure 11. TEM images of hydrogels formed by (a) 20 mM SLC/200 mM Na+ and (b) 20 mM SLC/200 mM K+ at 60 °C.

formation of the bundles of tubules to a dynamic mechanism besides the thermodynamic “salting-out” process. Another perspective is the electric double layer theory. The surface of colloidal particles in solutions generally shows double layer structure, for which the surface charges are surrounded by the counterions. As for tubular aggregates, a similar model can be established. The surface of aggregates is negatively charged because of the deprotonated carboxylate species. The cations surround the tubules to form the Stern layer and diffuse layer via the Coulombic force. Similar to the colloidal particles, in the present systems, the cation concentration gradually decreases from the surface of aggregates to bulk solutions. The stability of aggregates and the association of cations can be reflected by the zeta potential,39 which indicates the repulsion force between the adjacent aggregates with same type charges. Table 3 shows the zeta potential of tubular aggregates formed by LC− in the presence of Li+, Na+, and K+ before gelation concentration. At 25 °C, the 20 mM SLC/M+ systems show the zeta potential sequence of K+ > Na+ > Li+ at the same cations concentration. The SLC/Li+ system show the smallest zeta potential,

are of much larger sizes instead of the uniform smaller ones. Some tubes are found to aggregate into bundles (pointed by arrows in Figure 11). From the above results we can conclude that temperature increasing causes the growth of the aggregates, leading to the change of visual and mechanical properties. For the temperature-induced gel−gel′ transition, the probable explanation can be discussed from two aspects: the “salting-out” effect on the hydrophobic moieties of amphiphiles, and the model of electric double layer based on tubular aggregates. As mentioned above, the LC− species self-assemble to form a three-dimensional network, which is directed by noncovalent interactions including hydrophobic interaction. When being subjected to higher temperature, the solubility of hydrophobic moieties of LC− decreases in the presence of salts, resulting in the tendency to form larger aggregates and bundles of tubules. The so-called “salting-out” effect was observed by Feng et al. in a thermo-switchable surfactant gel system.42 The difference is that in the present system, the gel−gel′ transition induced by temperature process is irreversible. We ascribe the 4698

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Table 3. Zeta Potential of 20 mM SLC/30 mM M+ Systems M+ Li (25 °C) Na+ (25 °C) K+ (25 °C) Na+ (40 °C) K+ (40 °C) +

zeta potential (mV) −21.16 −86.77 −99.69 −20.17 −34.26

± ± ± ± ±

3.69 6.68 4.73 2.92 3.47

illustrating the tightest association of Li+ to LC−. The increasing amount of cations causes the stronger chargeshielding effect on the Stern layer and the higher probability for tubular aggregates to collide and coalesce. As the cations in the Stern layer, which are electrostatically attracted by tubules formed by LC−, are partially dehydrated, the size of the unhydrated cations is important.39 Combining Figure 9 and Table 1, one can find that the anions with smaller radius induce the stronger gel−gel′ transition, which sustains our interpretation. Moreover, from Table 3 we also find that the zeta potential decreases with temperature. The lower zeta potential indicates that more cations enter the Stern layer, or even access into the aggregates, being adsorbed between LC− anions through electrostatic attraction. This phenomenon was partly because higher temperature promotes the cations dehydration, which enhances the electrostatic attraction between cations and LC− species and decreases the stability of aggregates. Thus, the transition of the gels from transparent to opaque occurs with temperature is consistent with the results of zeta potential determination. From the perspective of microstructures, the smaller tubules gather into bundles or fuse to form larger size aggregates. Meanwhile, the shielding effect on lamellar aggregates leads to the decrease in membrane curvature and the formation of tubules with larger size. As the small angel XRD results show (Figure S4), the d value only increases very little at higher temperature, which implies that the cycled units induced by the arrangements of LC− species remain the same style at different temperatures. We consider that the slight increase in d value is caused by the more intense thermal motion of both LC− species and monovalent cations. In this way, the visual observation of the gel−gel′ transition with temperature can be satisfactorily explained. Dye Adsorption. The pH responsive hydrogels and metallo-hydrogels have been used to adsorb different types of toxic dyes with the intention to purify dye contaminated wastewater.2,3,43 In this paper, the hydrogels formed by SLC/ MCl were tested for adsorbing the positively charged dyes, methylene blue and rhodamine 6G. For a typical process, the hydrogels were submerged to the dye solutions under stirring and then left undisturbed. The adsorption process was finished within 20 min, and the bluish-black or crimson solutions become crystal clear, as shown in Figure 12. The UV−visible spectroscopy was used to monitor the adsorption of dye during 1 h with various time intervals. The dye concentration decreased sharply soon after the addition of hydrogels, and then decreased gradually, reaching very low value after 20 min (Figure 13a,c). Combined with the calibration curves of methylene blue and rhodamine 6G solutions (Figure S5), we calculated the concentration of residual dyes in solutions with time. As shown in Figure 13b,d, the dyes can be adsorbed almost completely within 20 min. The adsorption capacities of gels induced by different cations are shown in Table 4. The adsorption capacities are similar to that of hydrogels induced by

Figure 12. The methylene blue (a) and rhodamine 6G (b) solutions before (left) and after (right) adsorption.

different cations, within the range of 0.87−1.10 g·g−1 LCA for methylene blue and 1.12−1.42 g·g−1 LCA for rhodamine 6G, which are excellent results among the reports of other dye adsorption agents.44 The SLC concentration was fixed at 20 mM, and the MCl concentrations were 80 mM NH4Cl, 80 mM LiCl, 160 mM NaCl, 160 mM KCl, 150 mM RbCl, and 160 mM CsCl. The adsorption capacities were calculated using the amount of dyes adsorbed by per unit mass LCA in hydrogels. The adsorption of amido black 10B, chrome azurol S, and methyl orange was also studied. We found that the adsorption efficiency and adsorption capacity were much lower than those of methylene blue and rhodamine 6G. In order to detect the driving force of the adsorption, FT-IR measurements were carried out (Figure S6). The results show that there is no obvious change of the vibration of functional groups in dye molecules before and after the adsorption, indicating that no hydrogen bonding forms between the hydrogels and dyes in the adsorption process. Considering that the hydrogels show excellent adsorption capability for the positively charged dyes and inferior adsorption capability for negatively charged dyes, the possible reason could be mostly attributed to the strong electrostatic attraction between these positively charged dyes and the negatively charged tubular aggregates. Moreover, the appropriate hydrophobic/hydrophilic balance which typically exists between the dye adsorption agents and dye molecules43,45 might also contribute to the fast and efficient adsorption. Because no other additives were added in the gelation process, the repollution of water by the introduction of heavy metals, acids, and alkalis during the adsorption process was prohibited.

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CONCLUSIONS In summary, we reported the hydrogels formed by lithocholate with different monovalent cations in aqueous solutions. The hydrogels were composed of tubular structures with different sizes. The gelation ability and the size of tubules are relevant to the radii of cations, which result in the various electric double layer structures. The driving forces of the gelation are mainly considered to be the electrostatic interaction induced by the cations of salts, cooperating with the hydrogen bonding and the hydrophobic interaction, which lead to the formation of the continuous tubular structures and eventually give rise to the formation of hydrogels. Interestingly, the hydrogels obtained expressed an obvious enhancement in mechanical strength with the increase in temperature. Due to the negatively charged tubules, the hydrogels showed excellent adsorption capacity for cationic dye molecules. These hydrogels can be used as a class of environmentally friendly water-purifying agents. We hope 4699

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Figure 13. The UV−vis spectra of methylene blue (a) and rhodamine 6G (c) solutions and dye concentrations of methylene blue (b) and rhodamine 6G (d) with time after the addition of hydrogels of 20 mM SLC/160 mM NaCl. The solutions for (a,c) at 0 and 2 min were diluted 200 and 3 times, respectively, and the solution for (b,d) at 0 min was diluted 50 times before determination.

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ACKNOWLEDGMENTS This work is financially supported by the NSF for Distinguished Young Scholars of Shandong Province (JQ201303), the NSFC (21173132), and the Independent Innovation Foundation of Shandong University (IIFSDU, 2012TS001).

Table 4. Adsorption Capacities of Hydrogels Induced by Different Salts salts

methylene blue (g/g)

rhodamine 6G (g/g)

NH4Cl LiCl NaCl KCl RbCl CsCl

0.87 0.93 1.03 1.09 1.08 1.10

1.28 1.12 1.42 1.36 1.27 1.35

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our results will provide useful information in understanding the effect of external conditions on aggregates fabrication and applications in biological and materials science areas.

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ASSOCIATED CONTENT

S Supporting Information *

AFM images of tubules of 20 mM SLC/200 mM LiCl system; FT-IR spectra of LCA and the xerogels formed by 20 mM SLC/M+ at 25 °C; small angle XRD patterns of hydrogels formed by 20 mM SLC with different concentrations of MCl at 25 °C and different temperature; calibration curves of methylene blue and rhodamine 6G in aqueous solutions; FTIR spectra of methylene blue and rhodamine 6G before and after the adsorption. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel: +86-531-88363532; Fax: +86-531-88364750. Notes

The authors declare no competing financial interest. 4700

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