Using a Solvent-Induced Self-Assembly Approach to Fabricate and Tune the Organogels and Hydrogels

doi:10.21203/rs.3.rs-3333580/v1

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Using a Solvent-Induced Self-Assembly Approach to Fabricate and Tune the Organogels and Hydrogels

https://doi.org/10.21203/rs.3.rs-3333580/v1

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In the present work, stable organogels and hydrogels could be formed by dimeric-dehydrocholic acid derivative (DDAD) in different solvents. Compared with the organogels, the hydrogels formed by DDAD were found to be thermal reversible and had higher gel-to-solution transition temperature. The supramolecular structures in the organogels and hydrogels were further studied by using transmission electron microscopy (TEM) and atomic force microscopy (AFM). TEM and AFM images of the supramolecular gels showed that the solvent effects played a crucial role in morphological structures. Specifically, the organogel had a three-dimensional porous network structure. While, the hydrogel had a supramolecular structure made up of long fibers. Fourier transformation infrared spectroscopy (FT-IR) showed that multiple hydrogen bonds among the gelator molecules were the main driving forces in gel formation. On this base, the solvent effects on the gelation abilities and thermal stability were discussed. Thus, the present study provides a solvent-induced self-assembly approach and contributes substantially to the development of the supramolecular gels as soft materials.

soft materials

organogels

hydrogels

low-molecular-weight gelators

self-assembly

Supramolecular gels, driven by molecular self-assembly and composed of low-molecular-weight gelators (LMWGs), have attracted widespread attention for the construction of functional soft materials [1, 2]. Generally, the gelator molecules self-assemble into one-dimensional (1D) nanofibres through weak non-covalent interactions, such as hydrogen bonding, π–π stacking, dipole–dipole, vander Waals interactions, hydrophobic interactions, metal coordination. These nanofibres further wrap around each other to give a continuous three-dimensional (3D) network structure. Solvent molecules are immobilized in the 3D network, resulting in the formation of the supramolecular gels [3]. Because of their reversible nature and unique behavior, supramolecular gels are well known for their potential applications in materials science, photoactive device, biosensor, template synthesis, pollution control, etc [4].

For the development of supramolecular gels, much effort is devoted to preparing the gels, which exhibit responsive properties in the presence of various external stimuli (e.g., pH, light, temperature, redox, ultrasound, solvent effect, and/or their combination) [5]. Various stimuli have been used to investigate the responsiveness of the gelators, and among the available stimuli, the most commonly adopted stimulus is pH tuning or the use of an optical trigger [6, 7]. For example, the morphology of phenylalanine-substituted cholic acid has been reported to show narrow surfactant nanotubes at pH = 1.1, but small micelles at pH = 10.0 [8]. The structure and transparency of a two-component organogel could be reversibly changed by alternative UV light irradiation [9]. Another simple and effective stimulus is regulating temperature due to that the noncovalent interactions (e.g., hydrogen bonding, π–π stacking, van der Waals interactions) can be largely affected by temperature [10]. We have recently conducted a preliminary experiment and found that a polyoxometalate–cholesterol conjugate in mixed N, N-dimethylformamide/toluene solvents could assemble into elongated nanoribbons and ultralong microrods at temperatures of 20.0 and 40.0°C, respectively [11]. The external stimuli (e.g., redox, ultrasound, and/or their combination) used in preparation of the gels, are new, smart, and highly powerful methods [12]. For instance, a ferrocene–peptide conjugate has been reported to show ultrasound-induced morphological transformation from a fibrous network to spherical micelles [13]. In addition, other external stimuli in preparation of the gels with unusual properties include solvent effects [14], metal coordinations [15], host–guest interactions [16], addition of chemicals [17], and so on. Until now, various gelators that possess responsive properties have been reported, but the rational design of gelators together with an in-depth investigation of the external stimuli is still limited. Therefore, it is still meaningful and challenging to priori design new gelators and probe their gelation behaviors in the presence of certain external stimuli.

Supramolecular gels based on cholic acid derivatives have been known for a long time, due to the molecular structure of cholic acid containing a rigid steroidal backbone and three polar hydroxyl groups [18]. This structure facilitates the self-assembly of gelator molecules into ordered supramolecular structures, leading to the formation of organogels or hydrogels. For example, structurally various organo- and hydrogelators (e.g., molecular structure with alkyl amide [19], urea [20], aromatic ring [21], cationic salt [22]) have been reported in the literatures. However, such gelators derived from cholic acid derivatives are far from satisfactory. Namely, the effective gelators are still limited so far.

In the present work, dehydrocholic acid was selected for the synthesis of dimeric-dehydrocholic acid derivative (DDAD), which was used as a potential gelator (Scheme 1), since the supramolecular gels fabricated with dehydrocholic acid or its derivatives have been rarely reported. In the molecular structure of DDAD, it contains two dehydrocholic acid moieties and alkyl chain linkers as well as one carboxyl unit. With such a design, the gelation behaviors, thermal stability and supramolecular structures of the supramolecular gels were found to be greatly influenced by the solvent effects.

Materials

DDAD has been prepared as reported previously [23]. All starting materials were purchased from commercial sources and used directly. The organic solvents are analytical grade. Water and N, N-dimethylformamide (DMF) were directly distilled prior to use.

Sample preparation

Typically, a known weight of DDAD was mixed in 1.0 mL solvent. The mixture was heated until the DDAD was dissolved. The resulting solution was then cooled spontaneously down to room temperature. The gel–sol transition temperature (T_gel) values of the organogels and hydrogels at different concentrations were tested by the inverse flow method [24].

Techniques

Transmission electron microscopy (TEM) images were collected by a 200kv field emission transmission electron microscope (FEI-Tecnai G2-F20). As for TEM measurement, the sample was prepared by slightly touching the gel onto a carbon-coated copper grid (400 mesh), and dried in a vacuum drying chamber at 25°C for 48h. Atomic force microscopy (AFM) images were recorded using a multimodel atomic force microscope (Digital Instrumental Nanoscope IV), and performed in a tapping mode at room temperature on a glass slide. For the characterization of AFM, the sample was prepared by slightly touching the gel onto a newly peeled mica surface, and dried in a vacuum drying chamber at 25°C for 48 h. Fourier transformation infrared spectroscopy (FT-IR) was performed on a Bio-Rad FTS-135 spectrometer using KBr pellet technology.

gelation behaviors studies

Table 1

Gelation ability of DDAD (c = 20 mg mL^–1) in various solvents
Entry	Solvent	DDAD
1	n-Hexane	I
2	Cyclohexane	I
3	Toluene	I
4	Ethyl acetate	I
5	CH₂Cl₂	G
6	CHCl₃	G
7	Ethanol	P
8	Methanol	S
9	Acetonitrile	S
10	DMF	S
11	H₂O	I
12	NH₃·H₂O	G
I insoluble, G gel, P precipitate, S solution

The chemical structure of DDAD is shown in Scheme 1, and DDAD is prepared through a simple three-step synthesis. In our experiments, DDAD (20 mg) was mixed with various solvents (1 mL) in a screw-cap vial, and the mixture was heated until a clear solution was obtained. The resulting solution was cooled in air to room temperature and aged for two days. Based on the observation of gelation behaviors, we found an interesting phenomenon that is the sample was almost insoluble in low polarity solvents or water (H₂O), but it could form stable gels in dichloromethane (CH₂Cl₂), trichloromethane (CHCl₃) and ammonia water (NH₃·H₂O) under the experimental conditions.

After the aforementioned research, the gelation behaviors of DDAD were further investigated in CHCl₃ and NH₃·H₂O solutions using a DDAD concentration of 20 mg mL^–1 as a standard concentration. As shown in Fig. 1, the organogel is transparent, whereas the hydrogel is opaque. The heating-cooling cycle showed that the hydrogel had reversible thermal behavior, but the organogel has no thermal reversibility. Furthermore, the hydrogel was still solidified and non-fluid after adding excessive amounts of hydrochloric acid (HCl) for several days. At the same time, the hydrogel was found to be more stable than the organogel. The hydrogel could keep stable for several months at room temperature, but the organogel showed a phase separation within a month. The above studies imply that the solvent effects have an important influence on the gelation behavior.

Thermal stability studies

The aforementioned findings encouraged us to further examine the gel properties. The relationships of the gel–sol transition temperature (T_gel) versus different concentrations of DDAD (c_DDAD) were measured using the inverse flow method. As shown in Fig. 2, the formation of a stable organogel or hydrogel was observed only at c_DDAD ≥ 10 mg mL^–1 or c_DDAD ≥ 14 mg mL^–1, indicating that the minimum concentration of the organogel and hydrogel was 10 and 14 mg mL^–1, respectively. The thermal stabilities of organogels and hydrogels were dependent on the concentrations of DDAD. With increasing the concentrations of DDAD, T_gel values of both organogels and hydrogels were increased. Obviously, the hydrogel was more thermally stable than the organogel. For example, at c_DDAD = 20 mg mL^–1, the T_gel value of the organogel was only 41°C, and that of the hydrogel was 62°C. These results suggest that the thermal stabilities of the supramolecular gels can be modulated by the solvent effects.

Morphology studies

For a more detailed understanding of the solvent effects on the formation of the organogels and hydrogels at the molecular level, the supramolecular structures of the organogels and hydrogels formed by DDAD were surveyed at c_DDAD = 20 mg mL^–1. The supramolecular structures in dried xerogel samples of the organogel and hydrogel were first examined by TEM analysis. Seen from Fig. 3a, TEM characterization shows that the supramolecular structures in the dried xerogel sample of the organogel make up of a densely porous network structure. In contrast, the morphologies in the dried xerogel sample of the hydrogel show a densely fibrillar network consisting of entangled longer fibers (Fig. 3b).

With an aim to measure dimensions of the morphologies, AFM analysis was further utilized to explore the supramolecular structures of the organogel. For the dried xerogel sample (c_DDAD = 20 mg mL^–1), the supramolecular structures shown in Fig. 4a are not very clear. The amplified image in Fig. 4b shows that the organogel has a densely porous network structure. The average widths and heights of porous structures are about 350 nm and 7 nm, as shown by the height profile across the line in Fig. 4c.

The further studies were conducted on the dried xerogel sample of the hydrogel (c_DDAD = 20 mg mL^–1). As shown in Fig. 5a, the supramolecular structures of the hydrogel are ambiguous, because the high-density fibers impede the clarity of the single fiber. Its amplified image shows an entangled 3D network structure composed of long fibres (Fig. 5b). The height profile analyses across the fibers demonstrate that the fibers are ~ 90 nm in width and ~ 8 nm in height (Fig. 5c, d). From the above results, it can be concluded that the solvent effects have high influence on the supramolecular structures of the supramolecular gels.

Driving force studies

With an aim to deeply investigate the main driving forces in the gelation process, the FT-IR spectra of DDAD powders and dried xerogel samples of the organogel and hydrogel were studied at c_DDAD = 20 mg mL^–1. As shown in Fig. 6, the stretching vibrations of OH and C = O groups in DDAD powders appear at 3319, 1667 and 1648 cm^− 1, respectively. In contrast, the corresponding peaks shift respectively to 3315, 1659 and 1641 cm^− 1 in dried xerogel sample of the organogel, and these peaks of the hydrogel are 3311, 1654 and 1636 cm^− 1. These results imply the formation of hydrogen bonding upon gelation. According the results shown above, we believe that the main driving force of gelation is the hydrogen-bond interactions. XRD characterizations and analyses were also carried out for the xerogel samples, and no valid data is obtained.

Discussion of solvent effects

In the previous subsection of the gelation studies, we observed an interesting phenomenon that the formation of a stable supramolecular gel and its thermal stability were greatly influenced by the solvent effects. It is worth mentioning that DDAD is basically insoluble in apolar solvent, and a solution in polar solvent. It can form a stable organogel in low solvents, such as CH₂Cl₂ and CHCl₃. It is well known that stable hydrogels or organogels are formed only when there are subtle balances between gelator–gelator and gelator–solvent interactions. DDAD powders have strong gelator–gelator interactions among their own molecules. DDAD solubility is enhanced with the increase of solvent polarity, and such improved solubility may weaken the gelator–gelator interactions while enhance the gelator–solvent interactions (typically intermolecular van der Waals forces), resulting in experimental phenomena of insoluble, organogel and solution in various solvents. However, the balance between gelator–gelator and gelator–solvent interactions must be met so that a stable organogel can be formed.

Compared with the organic solvents, DDAD as an organic compound is insoluble in water. The addition of ammonia water further enhances the DDAD solubility in water, such enhanced solubility weakens the gelator–gelator interactions (typically intermolecular hydrogen bonding), leading to the formation of hydrogels at subtle balances between gelator–gelator and gelator–solvent interactions. This explanation can be well demonstrated by the thermal stability study. With the increase of test temperature, the solubility of dehydrocholic acid group gradually increases in CHCl₃ solvent, which greatly increases gelator–solvent interactions, resulting in low T_gel values. While, the solubility of dehydrocholic acid group is not increased in ammonia water, since it is poor solubility. Therefore, high T_gel values are observed, due to the gelator–solvent interactions are not affected.

Gel mechanism study

On the basis of the molecular structure of DDAD, morphological studies, FT-IR characterizations and discussion as mentioned above, a probable mechanism for the formation of the supramolecular gel has been proposed and is represented in Scheme 2. The gelator molecules firstly form a primary structure via intermolecular hydrogen bonds among amide bonds, when DDAD powders are dissolved in solution. With the increase of self-assembly time, these primary structures further grow into a secondary structure (typically fiber). Lots of secondary structures finally form a tertiary structure, which entraps solvent molecules, resulting in the formation of a supramolecular gel.

In conclusion, a stable organogel or hydrogel system consisting of dimeric-dehydrocholic acid derivative was designed and prepared, so that we could well understand the influence of solvent effects on the gel formation, thermal stability, and morphological structures. Compared with the organogels, the hydrogels were observed to have better thermal stability and higher gel-to-solution transition temperature. The solvent effects on the gelation behaviors and morphological structures were investigated by using the various methods, such as TEM, AFM and FT-IR characterizations. Furthermore, the influence of solvent effects was also demonstrated and discussed. Therefore, the present study provides a solvent-induced self-assembly approach and substantially contributes to develop the supramolecular gels as soft materials.

Ethical Approval No human or animal subjects were used in this research.

Competing interests The authors declare no conflict of interest.

Authors' contributions Ruicong Wang: funding acquisition, conceptualization, methodology, and writing original draft. Xiaoting Hao: data curation, investigation, and experiment. Haikuan Yang: funding acquisition, supervision, writing - review & editing. All authors contributed to the general discussion. All authors have read and agreed to the published version of the manuscript.

Funding We greatly appreciate the financial support of the Scientific and Technological Innovation Plan of Higher Education Institutions of Shanxi Province (No. 2021L609),the Fundamental Research Program of Shanxi Province (No. 202203021211102) and National Natural Science Foundation of China (No. 21802127).

Availability of data and materials All data generated or analyzed during this study are included in this article.

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Schemes 1 and 2 are available in the Supplementary Files section.

No competing interests reported.

SupportingInformation.doc
Scheme1.png
Scheme 1 Molecular structure of the dimeric-dehydrocholic acid derivative.
Scheme2.png
Scheme 2 Schematics represent the formation process of a hydrogel.

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Editorial decision: Major revision
02 Oct, 2023
Reviews received at journal
19 Sep, 2023
Reviewers agreed at journal
19 Sep, 2023
Reviewers agreed at journal
19 Sep, 2023
Reviewers invited by journal
19 Sep, 2023
Editor assigned by journal
11 Sep, 2023
Submission checks completed at journal
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First submitted to journal
07 Sep, 2023

You are reading this latest preprint version

Using a Solvent-Induced Self-Assembly Approach to Fabricate and Tune the Organogels and Hydrogels

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Version 1

Abstract

Figures

Introduction

Experimental Section

Materials

Sample preparation

Techniques

Results and discussion

gelation behaviors studies

Thermal stability studies

Morphology studies

Driving force studies

Discussion of solvent effects

Gel mechanism study

Conclusions

Declarations

References

Schemes

Additional Declarations

Supplementary Files

Status:

Version 1