Design of the gelatorsIt has been proved in our previous work that malonoester structures possessing amide groups can act as low-molecular-weight organogelators and efficiently gel organic solvents23. Cholesteryl derivatives, known for their ability to gel organic solvents including oils, are promising for environmental applications such as pollutant remediation due to their biodegradable and renewable nature24. Introducing cholesteryl units into the malonoester structure is expected to lead to robust supramolecular gel formation, leveraging multiple non-covalent interactions. This approach aligns with green chemistry principles and sustainable development. The synthesis of the cholesteryl-containing malonoester structure is described as feasible through a simple and efficient multi-component reaction, expected to result in enhanced gelation properties due to the combined effects of cholesteryl units and existing non-covalent interactions.Gelation behavior of the compoundsThe gelation behaviors of compound dicholesteryl gelator 5a were investigated in non-polar organic solvents found in petroleum as oil pollutants, detailed in (Table 1). The compound was found to form stable gels with a lower minimum gelation concentration (MGC) value, making it the preferred gelator. At just 1% (w/v) MGC, this compound effectively gels n-dodecane, petroleum, diesel, kerosene, and silicone oil solvents, as depicted in. Additionally, a stable gel was achieved in n-dodecane at 5 °C with only 0.5% (w/v) MGC. However, the compound resulted in an unstable gel in n-octane and a partial gel in n-heptane. These findings position compound dicholesteryl gelator 5a as an excellent candidate for gelation in a range of petroleum-based solvents, with potential implications for various practical applications. Indeed, incorporating cholesterol into the gelator structure aligns with the principles of green chemistry due to its renewable and biodegradable nature. The ability to form stable gels without the need for additional solvents further supports its environmentally friendly profile. These characteristics make the gelator a promising candidate for applications such as absorbing crude oil in seawater, where minimizing environmental impact is crucial. Furthermore, the addition of an oily solvent in conjunction with water and the powder of a gelator resulted in the observation of phase-selective gelation (Fig. 2). Further observation revealed that the majority of the resultant gels exhibited thermostable characteristics, maintaining their gel state for several weeks. Additionally, as illustrated in Fig. 3a, the gels formed in oil products were found to be thermo-reversible, meaning that they could be liquefied by heating the gel above its Tgel and then solidified below Tgel (the gel-to-solution transition temperature, Tgel, is 58 °C). This transition can be repeated at least ten times (Fig. 3b). This behavior is attributed to the reversible interactions, such as hydrogen bonding and π-π stacking, which are responsible for the gelation process5. Dicholesteryl gelator 5d forms weaker gels compared to dicholesteryl gelator 5a when used with the same solvents. The similar intramolecular interactions between the amide and ester groups, van der Waals interactions between cholesteryl moieties, and π-π stacking interaction of the phenyl groups in these four structures indicate that the difference in gelation behavior is due to the van der Waals interactions of the different R groups in the isocyanide derivatives. It is concluded that the cyclohexyl group in dicholesteryl gelator 5a and the n-butyl group in dicholesteryl gelator 5d lead to networked assemblies that enclose alkyl solvents through capillary forces and interface tensions25. However, the t-butyl and the 2,4,4-trimethyl pentane groups in dicholesteryl gelator 5b and 5c do not lead to networked assemblies due to their branching. The difference in gelation ability between the cyclohexyl and n-butyl groups is related to conformational freedom. The larger and bulkier nature of the cyclohexyl group compared to the n-butyl group restricts conformational freedom, resulting in better preorganization of the gelator structures, which improves the gelation process26. The data in Table 1 indicates that as the solvent becomes less polar and heavier, gel formation increases due to the growing van der Waals interactions. The derivatives were tested in a range of polar solvents, including aprotic (DMSO, DMF, THF) and protic solvents (acetonitrile, ethyl acetate, ethanol, methanol), but none of them demonstrated the ability to form an appropriate gel (Table S 1). This suggests that these gelators are selectively able to form gels in nonpolar aliphatic solvents, such as those found in petroleum.Fig. 2The phase-selective gelation behavior of dicholesteryl gelator 5a in petroleum in the presence of seawater. The gelator is in its (a) solution state, (b,c) gel state. (A video file (Video-Fig. 2) in appendix supports this Figure).Fig. 3The thermo-reversibility behavior of the gel 5a in petroleum (a) solidification by cooling below Tgel and liquefication by heating the gel above its Tgel, (b) The gel-to-solution transition cycle.The ability of dicholesteryl gelator 5a as oil spill treatmentOne of the reported applications of low-molecular-weight organogelators is in the treatment of oil spills. In this context, the organogelator needs to have the capability to solidify oil products. The gelation ability of dicholesteryl gelator 5a was investigated in non-polar organic solvents commonly found in petroleum (as shown in Table 1). The results indicated that dicholesteryl gelator 5a can effectively gel oil products at low concentrations of the gelator (1 and 2% (w/v)). To simulate the collection of oil from the water surface, the organogelator should be insoluble in water, as it is unable to enter the water and selectively immobilize oil in biphasic oil–water mixtures6,8. Some petroleum as oil phase was poured onto the surface of seawater (Fig. 4a), after which the powdered gelator was applied on top (Fig. 4b). The container was then agitated gently to simulate the sea waves, after which the dissolution of the gelator in the oil was observed at room temperature. As the gelator dissolved, the viscosity of the oil on the water’s surface increased until complete gelation occurred (Fig. 4c). Accordingly, by solidifying the leaked oil and separating it from the water surface (Fig. 4d,e, the gelator can be separated and reused. This demonstrated that the organogel can be formed at the seawater surface at room temperature without heating–cooling and co-solvent and stably remain on the water surface without absorbing water due to its hydrophobic nature. The hydrophobicity was confirmed using the water contact angle method in Sect. “Poor wetting”. By distilling the solvent from the removed gel layer, the gelator was recovered and found to be reusable.Fig. 4Selective gelation of petroleum in the presence of seawater (a) biphasic solution of 20 ml water and 2 ml petroleum, (b) adding gelator 5a on the water surface, (c) waiting for gelation after dissolving at room-temperature, (d) collecting the gelled oil by spatula in one side, (e) removing the gel from the water surface (cleaning the surface of the seawater from petroleum). (A video file (Video-Fig. 4) in appendix supports this Figure).Prior to utilization of the synthesized derivative as an absorber for oil spillage remediation, it was necessary to ascertain its biocompatibility, which is defined as the lack of toxicity to organisms. For this purpose, a fish species was selected as the test organism and 12 mg of gelator was added to water to test for toxicity. The lack of mortality among the fish over the 40-day test period indicates that the gelator does not exhibit toxic effects on aquatic organisms (Figure S17).Thermal stability of the gelThe thermal properties of gels, particularly organogel, are important for understanding their behavior and potential applications. One of the key thermal properties of gels is gelation temperature (Tgel). This is the temperature at which the gel-sol transition occurs. It indicates the thermal stability of the gel, which determines the range of temperatures over which the gel remains stable. One of the simplest and quickest techniques for determining Tgel is the ‘tube inversion’ method. This method involves visually observing the point at which a sample begins to flow, and is based on the principle that as the strength of the network increases, the temperature of the gel transition to solution increases, and eventually the gel becomes capable of flowing. In this study, different concentrations of dicholesteryl gelator 5a were prepared, ranging from 4 to 50 mg/ml. The Tgel was obtained by heating each sample at a rate of 2 °C per minute and using the inverted test tube technique. As depicted in Figure S18a, it was observed that as the concentration of the gel increased, the Tgel also increased. This trend continued until the concentration reached 30 mg/ml, after which the Tgel value remained constant. This behavior is consistent with previous findings in the literature for organogel, indicating a saturation point in the relationship between gel concentration and Tgel5,23,27.Another method for investigating the transfer of gel to solution is using differential scanning calorimetry (DSC) technique. DSC thermal curves provide valuable information about phase transfer, stability, and energy storage/release properties, indicating the behavior of gels under thermal changes. In general, the peaks in the DSC curve, which are created due to temperature changes, indicate changes in the system’s structure. In DSC curves, two types of peaks appear: endothermic peaks that occur due to an increase in temperature and exothermic peaks that are created during the temperature decrease stage28. Multiple endothermic peaks indicate the conversion of gel to solution or reflect the polymorphism resulting from temperature changes in the network. This phenomenon occurs when a substance exhibits different crystalline structures at different temperatures. On the other hand, multiple exothermic peaks in DSC curves indicate gel formation or crystallization. In some cases, the peaks in DSC curves become very broad. The broadening of the endothermic peaks can be interpreted by considering an organogel as a binary system. The broadening of the peak reflects the distribution of energy required for the transition from the gel phase to the solution phase, which may be due to the presence of gradient compositions in the binary system. This phenomenon results in a range of temperatures for gel melting instead of a single temperature. The multiple endothermic peaks shown in Figure S19 clearly demonstrate these aspects. The first endothermic peak observed indicates the beginning of molecular mobility in the gel network, representing the transition from a glassy state to a rubbery state29. At this stage, with the weakening of the weakest hydrogen bonds, the stable gel network begins to lose its rigidity and becomes more fluid. If the onset temperature (resulting from the corresponding peak) of the first endothermic peak is assigned to Tg, this temperature corresponds to what is observed in the “tube inversion” method. According to DSC curves, increasing the concentration of the gelator from 2 to 8% (w/v) leads to an increase in the value of Tg. This result indicates an increase in the strength of the gel network with the increase in gelator concentration up to 8% (w/v). Furthermore, based on the DSC curves shown in Figure S19–S21, with the increase in gelator concentration, the intensity of the second endothermic peak decreases, and this peak merges with the first endothermic peak. This can be attributed to a decrease in the probability of polymorphism30,31. Because as the gelator concentration increases, the space between fibrils becomes more restricted, reducing the gel’s ability to reform multiple crystalline structures (so-called thermotropic polymorphism). Therefore, these two endothermic peaks appear as a very broad and extensive peak. Another key property of gels, in addition to Tg, is the temperature of the highest endothermic peak, known as Tm, where the solid phase completely dissolves. Literature studies indicate that the reported values for Tg and Tm using DSC method differ from those obtained using tube inversion method and rheological measurements. This difference is mainly attributed to different heating rates, signal-to-noise ratios, and experimental conditions (such as sample weight or volume)32. As shown in Table 2, the DSC technique shows a higher value for Tg compared to other techniques (around 10 degrees Celsius).Table 2 Summary of the thermotropic properties of 2% (w/v), 4% (w/v), and 8% (w/v) organogels in the n-dodecane solvent.In addition to the mentioned properties, investigating the thermal reversibility of organogel, which is achieved by reducing the temperature of the sample in the solution phase, can also help in describing the behavior of gels. In this process, the first endothermic peak is related to the freezing process32. This temperature is called Tc, another important feature of the gel. As shown in Figure S19, the Tc temperature obtained from the endothermic peak in the cooling process is lower than both endothermic peak temperatures in the heating process. This phenomenon is called hysteresis heat and is observed in many reversible heat systems33. In gels that are thermally reversible, there are crystalline and amorphous regions. Therefore, it can be assumed that the formation of a new ordered structure with stable heat transfer zones is the reason for the existence of hysteresis heat.Reports indicate that the temperature at which the rheology shows the transition of a gel system to a solution (Tgs) is between two temperatures, Tg and Tm31. This temperature is the same temperature at which the rheology analysis shows G’ and G” to be equal34. Figure S18b confirms this phenomenon quite well.The interactions between the organogelator and the solvent during self-assemblyOne of the powerful tools for studying the intermolecular interactions between the organogelator and the solvent in the self-assembly process is FT-IR spectroscopy35. Therefore, to achieve an internal view of the self-assembly process and investigate the intermolecular interactions in our organogelator molecule, the FT-IR spectroscopies of the original powder of dicholesteryl gelator 5a were compared with the gel state and the solution state of this compound in the n-dodecane solvent. The FT-IR spectroscopy of pure n-dodecane solvent (Fig. 5a) was also compared with other FT-IR spectrums. The FT-IR spectrum of the original powder (Fig. 5e) shows the stretching vibration of NH and C=O functions of the amide group in 3387 and 1675 cm−1, respectively. These amounts shifted to 3299 and 1639 cm−1 in the spectrum of the gel state with 10% (w/v) and 25% (w/v) concentrations of the gel (Fig. 5c,d). Furthermore, other shifts to 3299 and 1637 cm−1 in the spectrum of the solution state are observed (Fig. 5b). These observed shifts represent the participation of the NH and C=O groups of the amide structure in hydrogen bonding36. Other spectral shifts are observed for C=O groups of the ester and the ketone in the structure (Table 3), which approve the participation of these groups in the assembly process. As seen in Table 3, another piece of evidence for forming the gel state is a vibration shift of the CH2 group of n-dodecane solvent from 2958 in the solvent to 2962 in the gel state. The reason for this spectral shift is related to the vibrational freedom, that is lower energy is needed for the vibration of solvent molecules in a liquid state than in the gel state37.Fig. 5FT-IR spectra of (a) solvent, (b) solution, (c) gel 10% (w/v), (d) gel 25% (w/v), (e) powder of dicholesteryl gelator 5a.Table 3 FT-IR data of dicholesteryl gelator 5a in gel, and solution states in n-dodecane solvent and powder.The self-assembled nano-fiber structuresThe microstructures of the surface of the gel network of dicholesteryl gelator 5a was studied by optical microscopic images taken on an OLYMPUS U-AN360P (Figure S22) in a petroleum and n-dodecane solvent. As shown in Figure S22a,b, the xerogels show a uniform texture.The morphology of the gel was also characterized by atomic force microscopy (AFM). As shown in Figure S22, the presence of the entangled network inclusions of fibrous or tubular structure shown by the AFM images of the xerogel is in agreement with the SEM images.To obtain a more detailed understanding of the microstructure of the gel 5a, xerogel was studied by SEM observation. Figure 6 illustrates SEM images of the xerogel. As depicted in Fig. 6, a three-dimensional network was created by aggregating vigorous self-assembled nano-fiber structures with fiber diameters of 32–116 nm which trapped the solvent molecules in their three-dimensional spaces. Nano-structured gels have a high surface area which leads to closer contact with the liquid phase and quick internal diffusion kinetic. Therefore, nano-structured gels are superior to micro-structured gels6. Furthermore, because of the importance of nanofiber structures in biomedical applications such as wound dressings, drug delivery systems, enzyme immobilization, and carriers for cell cultivation, a large number of studies have been dedicated to these structures38. It is interesting to note that one of the methods for producing nanofiber structures is the molecular self-assembly technique39. Referring to Table 4, having a nanofiber structure is a significant property of our gelator rather than the micro-structures reported in the literature. Further investigation of the SEM images shows that the morphology of the xerogel is characterized by thin ribbons which makes an entangled network structure. This morphology of the xerogel is consistent with the structures of organogels18. Since these gels are obtained by low-molecular-weight organic molecules with non-covalent interactions, they can be considered supramolecular gels.Fig. 6SEM images of the micro and nanostructures of xerogel of 2% (w/v) of gelator 5a in n-dodecane solvent. (a) 500 nm, (b) 2 µm.Table 4 Gelling performance of various cholesteryl-based gelators reported in the literature.XRD analysisThe structure of the xerogel was studied by PXRD analysis. According to the FT-IR and SEM results and the chemical structure of the compound, it can be said that there are interactions that are responsible for assembling the plane of the compound. These interactions are caused by the aromatic groups stacked over each other by π-π stacking interaction and the amide and ester groups of each molecule which interact with another molecule by hydrogen bonding interaction. Referring to Figure S23, the d spacing of the most intense peak in the PXRD pattern of xerogel is 32.91489 A°, which appear in 2.6820 [°2Theta]. The amorphous peak, which continues as a broad hump, indicates a decrease in the crystallinity of the interlayer distance of the xerogel29. As shown in the SEM images, the 3D entangled network which is the result of the interconnection of thin fibers in the interlayer causes to amorph peak.The solid-like viscoelastic behavior of the gelHaving no regular stereochemical configuration rather than perfect regular spheres leads to specific rheological properties as reported in the literature21,40. Therefore, rheological properties were carried out on the gel in the n-dodecane solvent. The storage modulus G’, related to the energy storage, and the loss modulus G”, associated with the loss of energy, were measured as functions of stress amplitude at room temperature. Firstly, stress sweep measurements at a constant angular frequency of ω = 10 rad/s and varied shear stress τ were performed to determine the linear viscoelastic range of the organogel. As the figure shows, both G’ and G” values for gel are nearly constant until a critical stress value called dynamic yield stress. After this stress, an abrupt decrease in both G’ and G” occurred that represents the destruction of the gel network structure. Figure 7 also shows that the storage modulus G’ is greater than the loss modulus G” below the critical stress value. This demonstrates that the samples under study mainly exhibit elastic behavior41.Fig. 7Variation of storage modulus and loss modulus with applied (a) shear stresses (Pa), and (b) angular frequency (rad/s).To detect the tolerance performance of the gel against external forces, a frequency sweep test was conducted18. A frequency sweep was conducted in a 0.1–500 rad s−1 angle frequency range at a definite strain of 1%. The results are shown in (Fig. 7). The storage modulus G’ was greater than loss modulus G” in the entire frequency range. This observation is another confirmation that the networks of the gel show solid-like viscoelastic properties. Moreover, there is almost weak dependence on frequency, specifically, within a range of frequencies from ~ 0.1 to ~ 100 rad/s. This behavior suggests that samples have powerful physical junctions in the frequency range. Therefore, this leads to showing good endurance against external forces.The role of the concentration of the gelator in the rheological properties was studied. For this purpose, the behavior of the G’ value for different concentrations of 1 and 2% (w/v) of dicholesteryl gelator 5a versus the angular frequency was investigated (Figure S24). The used solvent was n-dodecane. It can be understood that the G’ value for the gel with a 2% (w/v) concentration is higher than that of a 1% (w/v). This observation states that the gel with a 2% (w/v) concentration of the gelator is elastically stronger. This influence of the concentration of the gelator on the stability of the gel network is reported in the literature18,42. Furthermore, the storage modulus of the gel of 1% (w/v) gelator concentration shows more dependency on the frequency change, rather than that of 2% (w/v), which is nearly constant in the frequency range shown.Poor wettingSurface wettability is a critical property in the context of oil/water separation processes43. In this regard, the surfaces of oil absorbents utilized for oil/water separation need to be lipophilic, meaning they should attract oil while repelling water, making them hydrophobic. Soft materials like gels exhibit varying surface wettabilities due to their diverse nanostructures35. To investigate this, a gel composed of dicholesteryl gelator 5a in n-dodecane was applied onto glass and dried under laboratory conditions. Subsequently, the pendant drop contact angle method was employed to assess the wettability of the dried gel surface. The contact angle of a water droplet on a flat surface was found to be 118°, as illustrated in (Fig. 8). In accordance with the literature, a contact angle between 90 and 150° indicates a hydrophobic surface. Therefore, our xerogel surface can be classified as hydrophobic.Fig. 8Water contact angle test of xerogel of dicholesteryl gelator 5a in n-dodecane solvent.
