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Two Gelation Mechanisms of Deoxycholate with Inorganic Additives: Hydrogen Bonding and Electrostatic Interactions Jin Zhang, Haiqiao Wang, Xiaoyang Li, Shasha Song, Aixin Song, and Jingcheng Hao J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b04140 • Publication Date (Web): 22 Jun 2016 Downloaded from http://pubs.acs.org on June 24, 2016

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The Journal of Physical Chemistry

Two Gelation Mechanisms of Deoxycholate with Inorganic Additives: Hydrogen Bonding and Electrostatic Interactions

Jin Zhang, Haiqiao Wang, Xiaoyang Li, Shasha Song, Aixin Song,* Jingcheng Hao

Key Laboratory of Colloids and Interface Chemistry & Key Laboratory of Special Aggregated Materials (Shandong University), Ministry of Education, Jinan 250100, China

* To whom correspondence should be addressed. E-mail: [email protected]; Tel: +86-531-88363532; Fax: +86-531-88364750(o)

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ABSTRACT: This work describes the gelation behaviors of a biological amphiphile, deoxycholate (DC), in aqueous solution by adding inorganic salts and modulating pH. Electrostatic interaction and hydrogen bonding can separately act as the controlling interaction for the hydrogel formation. The hydrogels formed at higher pH (above 8.5) through introducing monovalent inorganic cations (Na+) are mainly driven by electrostatic interaction between deoxycholate species and Na+ ions. When pH is decreased, with the formation of DCA molecules, hydrogen bonding between DCand DCA come into being another leading role to construct the hydrogels, which can induce the gels within an appropriate pH region (4.7-7.3) without inorganic cations. Gels constructed through the self-assembly of deoxycholate present diverse properties according to the difference in the main driving force. Moreover, the combination of the two important interactions can significantly enhance the gelation ability.

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1. INTRODUCTION Supramolecular hydrogels as a fascinating type of soft materials have attracted extensive attention1-2 for applications in fields of biology,3-4 materials,5 environment6 and biomedicine science.7-8 They are a kind of solid-like materials constructed by three-dimensional network, immobilizing a large number of water molecules.2 The complex networks are formed by self-assembly of amphiphilic molecules via multiple non-covalent interactions, such as hydrogen bonding, electrostatic interaction, stacking, metal coordination and hydrophobic effect. The hydrogels formed from biological molecules with low molecular weight gelators are of significant importance because of their smart responsiveness to external stimuli9-13 and excellent biocompatibility and biodegradability, such as amino acids,14-16 peptides,17-19 bile acids19-22 and their derivatives.23,24 Among the small biological molecules, bile acids and their salts are key biological surfactants in vertebrates. Different from a small hydrophilic head and a flexible hydrophobic tail in common surfactants, bile salts are rigid and almost flat molecules with weakly separated hydrophobic and hydrophilic faces, which endows them unique physiochemical properties and distinct aggregate properties.25 The gelation behaviors of bile acids or bile salts, including sodium deoxycholate (NaDC),26-28 sodium lithocholate29-31 and sodium cholate,32-34 have been adequately studied. The gels consisting of well-defined nanotubes, functional helical structures, or nanofibers were obtained via molecular self-assembly by adding organic molecules or inorganic additives.20,21,32,35,38 3



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The addition of inorganic salts is a simple and easy method to construct gels formed by bile salts, for which the gelation behaviors are rather different according to the structures of bile salts. The cholate can form super gels with some lanthanide ions in aqueous solution.33,34,36 For lithocholate, gels can be formed when a series of positively charged ions, e.g., alkali metal ions, NH4+, divalent and multi-valent metal ions, are introduced.29,31 The investigation on gels formed by NaDC mainly focused on the gelation at pH values close to neutrality under the introduction of alkali metal ions and lanthanide ions.26,38 Though abundant gel systems and rich microstructures were obtained, it is a pity that the gelation mechanism has been still fuzzy at different conditions. Generally, the combination of bile salts with metal ions through electrostatic interaction was considered to be the driving force. In our previous reports, two gelation processes, the electrostatic interaction and metal coordination driving mechanisms were discussed for gels formed by lithocholate with different metal ions.29,31 For construction of gels formed by NaDC, the common process is to mix NaDC with metal ions in the buffer solutions around the pH value closing to pKa of deoxycholic acid, 6.6. However, a clear mechanism of the roles of pH and metal ions in the formation of hydrogels is still lack, which is adverse to design hydrogels formed by DC- at different conditions. The purpose of the present work is to reveal the different formation mechanisms of hydrogels of DC- at different conditions. Three systems were selected for the comparison of the gelation behavior and the properties of hydrogels formed affected by three important factors: hydrogen bonding, electrostatic interaction and the 4


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cooperative effect of the two interactions. The results show hydrogels can be formed in all the designed conditions. However, the appearance, mechanical strength, microstructures, thermal stability and critical gelation concentrations (the minimum concentration to form gels) are rather different, indicating the difference in the formation mechanism.

2. METHODS AND MATERIALS 2.1 Materials. Sodium deoxycholate (NaDC) and tetramethylammonium hydroxide (TMAOH) were purchased from J&K Chemical Company (China, purity > 98%). Deoxycholic acid (DCA) was purchased from Acros Organics Company (USA, purity > 98.5%). Sodium chloride (NaCl) and acetic acid were purchased from Sinopharm Chemical Reagent Co. (China). All the chemicals were used without further purification. Ultrapure water with a resistivity of 18.25 MΩ·cm was acquired using a UPH-IV ultrapure water purifier (China). 2.2 Gelation Behavior Study. Three different systems were studied in the present work. For NaDC/NaCl system, NaDC and NaCl were weighed in a test tube and were dissolved in 5 mL ultrapure water. For the DCA/TMAOH system, DCA was dissolved TMAOH solution in equimolar ratio, and then acetic acid was added to adjust pH. The final volume of the total solution was 5 mL. For the 75 mM NaDC/NaCl system, NaDC and NaCl were dissolved in 5 mL ultrapure water, then acetic acid was added to adjust pH under stirring. The samples were stirred mildly to get homogenous samples and were equilibrated at 25.0 ± 0.5 ºC for at least 4 weeks until they did not change any 5



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more. 2.3 Methods and Characterizations. Transmission Electron Microscope (TEM) Observation.

Samples were prepared

by dropping about 5 μL of sample solution on the carbon-coated copper grids (400 meshes) and the redundant solution was wiped off with filter paper. The samples freeze-dried in a vacuum extractor were observed on a JEOL JEM-1400 TEM operating at 120 kV with a Gatanmultiscan CCD for recording images. Rheological Measurements. The rheological measurements were operated on a HAAKE RS6000 rheometer with a cone-plate system (C35/1Ti L07116). In oscillatory measurements, an amplitude sweep at a fixed frequency of 1 Hz was carried out prior to the following frequency sweep in order to ensure the selected stress was in a linear viscoelastic region. Fourier Transform Infrared Spectroscopy (FT-IR) Measurements. The Fourier transform infrared (FT-IR) spectra were measured using a VERTEX-70/70v 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). X-ray Diffraction (XRD) Measurements. The XRD patterns of the freeze-dried samples were recorded between 1 and 10º in the 2θ mode (1°·min−1) using a DMAX-2500PC diffractometer with Cu Kα radiation (λ = 0.15418 nm) and a graphite monochromator.

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3. RESULTS AND DISCUSSION 3.1 Gelation Behavior. To investigate the different effect on gelation, three systems were designed: In the first system, only NaCl was introduced to NaDC solution to study the effect of electrostatic interaction on gelation process; In the second system, tetramethylammonium hydroxide (TMAOH) was added to dissolve deoxycholic acid (DCA), and the pH of solution was adjusted with acetic acid to study the effect of pH, for which Na+ was avoided (We have proved that the electrostatic interaction induced by TMA+ cannot gelate DC in aqueous solution); In the third system, NaCl was added to NaDC solution at different pH, for which the effect of both pH and Na+ was considered. Figure 1 shows the gel phase region of the three selected systems, NaDC/NaCl, DCA/TMAOH (modulating pH), and 75 mM NaDC/NaCl (modulating pH). One can find that the hydrogels can be formed in all these three systems at different conditions. For NaDC/NaCl system (Figure 1a), the concentrations of NaCl and NaDC have positive effect on hydrogel formation. When the concentration of NaDC was below 90 mM, the transition from sols to gels occurs with the increasing NaCl concentration. As the NaDC concentration increases, the minimum concentration of NaCl to form gel decreases. When the concentration of NaDC was above 90 mM, with the addition of NaCl, no gels can be formed, only a transition from sol to sol-precipitate two-phase occurs. For DCA-TMAOH system (Figure 1b), pH shows a significant influence on the formation of hydrogels. The mixtures of DCA and TMAOH at equimolar ratio form transparent solution with pH being about 8.5, for which almost all DCA 7



molecules transform to DC- ions. When acetic acid is added, gels are found at the pH region of about 6.7-7.3, which is consistent with previous report.38 When pH value is below 6.7, the mixtures of gels and white precipitates are produced. While at pH above 7.3, the gels are destroyed to transform to sols gradually. It should be noted that the formation of gels occurs at the DCA concentration higher than 160 mM.

For 75

mM NaDC/NaCl system at different pH (Figure 1c), the minimum NaCl concentration to form gels decreases with the decrease of pH within the studied region, indicating the significant effect of pH on the gels formation. As a general view, NaDC can form hydrogels at rather low concentrations (20 – 90 mM) under the addition of NaCl at pH of 8.5. At the absence of Na, DC- can also form hydrogels at higher concentrations, above 160 mM, and lower pH, 6.7-7.3, for which DC-DCA complexes are existed. When the two factors (Na+ and pH) are both considered, hydrogels can be produced at lower concentrations of both DC (e.g., 75 mM) and Na+ (e.g., 30 mM at pH = 7.0). 400

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Figure 1. Gelation behavior of three studied systems at 25.0 ± 0.5 ºC. (a) NaDC/NaCl system, (b) DCA/TMAOH system at different pH, and (c) 75 mM NaDC/NaCl system at different pH.

3.2 Microstructures of Hydrogels. The microstructures of gels of three systems were detected by TEM. As shown in Figure 2, all the gels are composed of three dimensional networks formed by stacking one dimensional nanofibers. However, the nanofibers exhibit different morphologies for different systems, which correlate with the appearance of the gels. For the transparent hydrogels formed in NaDC/NaCl system, the necklace-like nanofibers composed of nanospheres can be observed with very small diameter, about 23 nm (Figure 2a). Nanofibers with necklace-like morphology are often found in gels formed by NaDC, in which the deprotonated 9



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carboxylate groups (−COO−) of NaDC are considered to concentrate the cations (Na+) to create the local supersaturation followed by the nucleation and growth of crystalline nanospheres.26-28,39 In DCA/TMAOH (pH = 7) system, the opaque white hydrogels are produced. Figure 2b shows that the gels consist of continuous nanofibers with larger diameter (about 33 nm) which are linked to form concentrated three dimensional networks. For hydrogels formed in NaDC/NaCl system at pH around 7, the gels are slightly turbid (Figure 2c and d). Interestingly, the TEM images show that the gels are composed of slim nanofibers, along which the very small nucleated nanospheres are dotted. Further study shows that the nanofibers in gels formed at higher pH, 7.35, are fragmented, while those at lower pH, 7.0, are continuous. Thus, we can say that the gels formed by NaDC with NaCl at pH = 7 exhibit the significant effect of the two factors, Na+ and pH.

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Figure 2. TEM images of hydrogels formed by 20 mM NaDC/180 mM NaCl(a), (200 mM DCA/200 mM TMAOH at pH = 7 (b), 75 mM NaDC/50 mM NaCl at pH = 7 (c) and 7.35 (d). The photo of each gel sample is inserted in its TEM image.

3.3 Rheological Properties. The rheological measurements were carried out to evaluate the mechanical strength of gels. The network structures of gels will be destroyed at a critical shear stress (τ*), showing a fast decrease of the elastic modulus (G'). The critical stress is called “yield stress”, which reflects the mechanical strength of the network. Figure 3 shows the stress sweep results of the three systems. As shown in Figure 3a, for gels formed by NaDC with NaCl, the mechanical strength can be enhanced by increasing concentrations of both NaDC and NaCl, which increases both τ* and G' values. For DCA system (pH = 7) modulated by TMAOH and acetic acid, the τ* and G' increase as the DCA concentration increases (Figure 3b). Moreover, the mechanical strength also changes with pH. When pH decreases from 7.35 to 7, τ* and G' increase from 8 Pa and 2.5 Pa to 10 Pa and 7 Pa, respectively (Figure 3c). Figure 3d shows that for gels formed by NaDC with NaCl at pH = 7, τ* and G' also increase with the increasing NaDC concentration. Thus, we can conclude that the 11



modulation of the mechanical strength can be realized by changing the concentration of gelator (NaDC or DCA), the concentration of additive (NaCl), and pH. The gelator concentration is considered to influence the density of nanofibers for the construction of network structures, while the NaCl concentration and pH mainly affect the tightness of the arrangement of the nanofibers, for which both the two factors show significant contribution to the gel strength.

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Figure 3. Stress sweep results of hydrogels: (a) NaDC/NaCl system; (b) DCA/TMAOH system with different concentrations of DCA at pH = 7; (c) 75 mM NaDC/200 mM NaCl at different pH; (d) different concentrations of NaDC with 50 mM NaCl at pH = 7.

The comparison of mechanical property between different systems is shown in Figure 4. From Figure 4a one can find that gels formed by NaDC and NaCl exhibit 12


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stronger strength at pH = 7 than those without modulating pH, i.e., at the pH of NaDC solution, about 8.5, even if the gels formed at pH = 7 have much lower NaCl concentrations. While in Figure 4b, when pH = 7, the gels formed in 200 mM DCA/TMAOH, 200 mM NaDC and 200 mM NaDC/50 mM NaCl have the similar τ* value and G' value, indicating the more significant influence of pH than the additional salts on the strength of gels. However, without the introduction of Na+, for DCA/TMAOH system at pH = 7, gels can only be formed at DCA concentration above 160 mM, while NaDC can form gels at above 20 mM by adding NaCl and shows a increasing strength with NaCl concentration. As a summary, we can say that the pH value and the concentration of gelators, DC or DCDCA, show great influence to the mechanical strength of gels, while the addition of salts, e.g., NaCl, mainly affects the critical gelation concentration and the mechanical strength at conditions without pH modulation. 2

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3.4 Formation Mechanism of Hydrogels. To detect the formation mechanism of gels in different conditions, FT-IR measurements and small angle XRD were carried out. As shown in Figure 5a, DCA has a symmetric stretching peak of the carbonyl on carboxyl group at 1704 cm−1. For NaDC and gels of NaDC/NaCl system (Figure 5b), no peak can be found around 1704 cm-1, while the peaks at 1563 cm-1 and 1408 cm-1 appears, which is ascribed to the asymmetric and symmetric stretching vibration of carboxylate group, suggesting the deprotonation of carboxyl groups and the formation of deoxycholate anions. For hydrogels of DCA/TMAOH at pH = 7 (Figure 5c), the two peaks at 1693 cm-1 and 1647 cm-1 are assigned to the C=O stretching vibrations of –COOH andCOO- groups, respectively, indicating the co-existence of DCA and DC-. Moreover, the stretching vibration mode of COOH show a bathochromic shift from 1704 cm-1 to 1693cm-1, reflecting the formation of hydrogen bonding between the COOH group on DCA and the COOgroup on DCions. For hydrogels of NaDC/NaCl at different pH (Figure 5d), with the decreasing pH, a peak at 1693 cm-1 appears, being ascribed to the stretching vibration of COOH with a bathochromic shift due to the hydrogen bonding. The peaks at 1563 cm-1 and 1408 cm-1 also demonstrate the existence of DC. Thus, we can conclude that, with the decrease of pH, DCA molecules are produced and form hydrogen bonding with DC ions. The electrostatic interaction and hydrogen bonding with molecular structures are illustrated in Figure 6.

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Figure 6. Illustration of the electrostatic interaction between DC and Na(a) and hydrogen bonding between DCand DCA (b).

Small-angle X-ray diffraction (XRD) patterns were measured to reveal more detail

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information of the microstructures. A single peak can be found from the hydrogels formed by 75 mM NaDC with different concentrations of NaCl (Figure 7a). According to Bragg’s law, the reflection peaks are calculated correlate to the d value of 1.90 nm, 1.78 nm and 1.77 nm with the increasing NaCl concentration, being a little larger than a deoxycholate backbone length, 1.5 nm,37 which is related to the cycled units of the ordered arrangements of deoxycholate anions with Na+. The results show that the addition of Na+ can induce the closer packing of the arrangement ascribed to the decrease of electrostatic repulsion between the deoxycholate anions. For NaDC/NaCl system at different pH, as shown in Figure 7b, gels of 160 mM NaDC/50 mM NaCl at pH of 6.70, 7.03, and 7.35 were calculated having the d value of 1.90 nm, 1.73 nm, and 1.74 nm, respectively, showing the effect of pH on the arrangement of DC and DCA species, for which a certain pH region of around 7 was exhibited to induce the more close arrangement. As for the hydrogels formed by 160 mM DCA at pH = 7 modulated by TMAOH and acetic acid (Figure 7c), the XRD pattern are quite difference. A weak peak at 5.34°and a strong peak at 8.76°can be found, correlating to the d values of 1.66 nm and 1.01 nm, respectively, a little larger than the length of a deoxycholate backbone and twice as the width of a deoxycholate backbone.37 The increasing concentration of gelator induces the slight closer stacking, showing the d values of 1.65 nm and 1.00 nm, respectively. Figure 7d shows the difference in the three systems. The hydrogel formed by of DCA at pH of 7 without NaCl exhibits a relatively ordered stacking (curve 1), as discussed above. When 50 mM NaCl was introduced (curve 2), the peak reflected by the arrangement of the 16


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width of two deoxycholate backbones (d value of 1.01 nm) disappears, and another peak, correlating to the d value of 1.66 nm shifts to that of d = 1.73 nm, indicating the looser stacking. For hydrogels formed by 75 NaDC/200 mM NaCl (curves 3 and 4), d value is larger (1.90 nm) at the natural pH, 8.5, than that at pH = 7 (1.85 nm). Thus, as a general view, we can conclude that the deoxycholate species in the three systems possess the similar arrangement mode, for which the gels driven by hydrogen bonding hold a closer stacking, exhibiting the ordered array both in parallel and perpendicular directions, respectively shown as d1 and d2 in Figure 8b.

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Figure 7. XRD results of hydrogels formed in (a) 75 mM NaDC/NaCl system, (b) NaDC/NaCl system at different pH, (c) 200 mM DCA/200 mM TMAOH at pH = 7, and (d) the three different systems.

Based on above results and the relative reports of supramolecular structures formed 17



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by bile acids and their salts, the mechanism of hydrogels was proposed. Deoxycholate is a well-known facial amphiphile with a polar face composed of hydroxyl groups and a nonpolar face composed of two methyl groups. The deoxycholate species adopt a lamellar type of arrangement in the hydrogels, for which the deoxycholate pairs are stacked together at their hydrophilic edges, forming the continuous hydrophilic cavities between each two connected DC species (or a DC species and a DCA molecule) with a nonpolar face-to-nonpolar face arrangement driven by hydrophobic effect. Figure 8a shows the formation mechanism of hydrogels formed by NaDC and NaCl. The driving force of gelation is mainly considered to be the electrostatic interaction induced by Na+ ions, which reduce the electrostatic repulsion between the hydrophilic groups by compressing the thickness of electric double layer.29 Besides the electrostatic interaction, the carboxylate of a DC species engages itself in hydrogen bonding with the hydroxyl group of another DC species.30,40 As for the gels formed by DCA at pH around 7 modulated by TMAOH and acetic acid, part of DCA molecules are deprotonated to form DCions. The DC ions cannot form hydrogels with TMA+ ions mainly driven by electrostatic interaction probably due to the large size of TMA+ ions. Thus, the hydrogen bonding formed between the carboxyl group on DCA molecule and carboxylate group on deprotonated DCspecies act as the main driving force for the gelation process, as shown in Figure 8b. For NaDC/NaCl system at pH around 7, both the hydrogen bonding between DCand DCA and the electrostatic attraction between DC and Na+ play important roles in the formation of gels, which 18


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is shown in Figure 8c. In a word, the hydrogels can be formed under the synergistically driving of hydrogen bonding, electrostatic interaction, hydrophobic effect and van der Waals. The hydrogen bonding and electrostatic interaction are the crucial driving forces in gelation process, for which the main driving force can be modulated by changing the external conditions.

Figure 8. Schematic representation of the self-assembly mechanism in different systems: (a) NaDC/NaCl; (b) DCA/TMAOH at pH around 7; (c) NaDC/NaCl at pH around 7.

3.5 Response to Temperature. Hydrogels formed by bile acids or their salts show different response to temperature. Two are two types of responses: one type is heating enhancement with a gel-gel' transition, being reported in mixtures of cholate or lithocholate with metal ions;29,31,36 the other type is the gel-sol transition under heating, which is generally occurred in gels formed mainly through hydrogen bonding due to 19



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the destruction of hydrogen bonds by heating.20-22 Herein, the thermostability of the hydrogels in the present systems were studied. As shown in Figure 9, all hydrogels were found to transform to sols by heating. This process can be reversed by cooling the sols to room temperature, for which gels are regained. One can also find that the samples become a little opaque than those before heating, which may be ascribed to the formation and coalescence of the nanofibers during the cooling process.

a

b

c

Figure 9. Photos of the reversible gel-sol transition of hydrogels formed in different systems: (a) NaDC/NaCl; (b) DCA/TMAOH at pH = 7; (c) NaDC/NaCl at pH = 7.

The gel-sol transition temperature, i.e., the melting temperature, Tm, was also detected. It is a pity that we did not get the DSC signals probably due to the minor thermal effects. However, through visual observation of the reversed gel samples, the 20


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Tm values of the gels were all found and shown in Table 1. For gel of 75 mM NaDC/200 mM NaCl at the natural pH, 8.5, Tm was found to be 41 °C. When pH was decreased to be 7.29 and 7.0, Tm increased to 46 °C and 48 °C, respectively, which was considered that more energy was needed to damage the networks of gels due to the formation of hydrogen bonding. For gels formed by 160 mM DCA (pH was adjusted to be 7.0 by TMAOH and acetic acid), Tm is 44 °C. While for gels of 160 mM NaDC/50mM NaCl at pH = 7.0, Tm is slightly higher, 45 °C, being ascribed to the electrostatic interaction besides the hydrogen bonding. The results showed that under the double main driving forces, hydrogen bonding and electrostatic interaction, more energy was needed to break the network structures of gels. Table 1. The Gel-Sol Transition Temperature of Hydrogels Formed in Different Systems Sample

Concentration (mM)

T (oC)

NaDC/NaCl

75/200

41

NaDC/NaCl (pH = 7.29)

75/200

46


75/200

48

DCA/TMAOH (pH = 7.0)

160

44


160/50

45

4. CONCLUSIONS Hydrogels formed by deoxycholate with inorganic cations possess different formation mechanisms through modulating external conditions. The supramolecular gels can be obtained in the presence of additional salts having monovalent inorganic cations or at 21



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appropriate pH value, for which both the electrostatic interaction and hydrogen bonding act as the driving force in the formation of hydrogels. At higher pH value, above 8.5, the deoxycholate species form hydrogels with monovalent inorganic cations mainly through electrostatic interaction. While when pH decreases, the formation of hydrogen bonding between DCA and DC shows obvious effect on the gelation process, which can induce the production of hydrogels without inorganic cations at pH around 7 (6.7-7.3). Due to the different formation mechanism, the gels also exhibit significant difference in gelation ability, microstructure, mechanical strength and appearance. The enhancement of gelation ability can be achieved by combining the two kinds of driving forces. Hopefully the present results are useful in the design of hydrogel systems of deoxycholate with inorganic salts through modulating the synergy of multiple driving forces according to the specific requirements.

ACKNOWLEDGEMENTS This work was funded by the NSF for Distinguished Young Scholars of Shandong Province (JQ201303) and National Natural Science Foundation of China (21573134 and 21420102006).

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Table of Contents (TOC)

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Hydrogen Bonding and Electrostatic Interactions - American Chemical

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