Stronger Intermolecular Forces or Closer Molecular Spacing? Key

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Stronger Intermolecular Forces or Closer Molecular Spacing? Key Impact Factor Research of Gelator Self-Assembly Mechanism Si Chen,† Zhihang An,† Xiaoqian Tong, Yining Chen, Meng Ma, Yanqin Shi, and Xu Wang* College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou 310014, China S Supporting Information *

ABSTRACT: The benzene ring of low-molecular-weight gelators provides strong intermolecular forces but increases molecular spacing during self-assembly. To explore both of the above influences on the gel properties, we synthesize two gelators (Glu-CBZ and Glu-DPA) consisting of the same terminal long side chain but different aliphatic functional groups. The aliphatic functional groups are carbobenzoxy group and diphenyl phosphate group. The self-assembly driving forces, self-organization patterns, network morphologies, rheological properties, and the influences of solvents are researched through 1H NMR spectra, Fourier transform infrared spectra, field-emission scanning electron microscopy images, rheological characterizations curves, tube-inversion experiment, and calculation of van’t Hoff plots. The results show that the carbobenzoxy group of Glu-CBZ makes molecules pack more tightly such that it improves the gel properties during static equilibrium. Whereas the diphenyl phosphate group of Glu-DPA provides stronger intermolecular forces, performing outstandingly during dynamic equilibrium. It is advantageous to further investigate the competitive relationship in gel system between the increased number of functional groups and the consequent steric effect.

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INTRODUCTION Low-molecular-weight gelators (LMWGs) have caused a wide public concern for their special properties. As known to all, LMWGs can self-assemble into supramolecular gels in suitable solvents through noncovalent interactions, including ion−ion, dipole−dipole, hydrogen bonding, π−π stacking, van der Waals, host−guest, ion coordination, and so forth. The reversible supramolecular gels have been applied in many fields, including encapsulation and release of drugs,1,2 stimuli-responsive materials,3,4 optic and electronic materials,5,6 biomedical materials.7 What is more; they can also solve environmental problems, such as remediation of crude oil spillage and pollutant removal.8,9 On the basis of their application prospects, many research studies focused on broadening its applications and enhancing the properties of gels, such as mechanical strength10 and stiffness.11 However, the self-assembly processes of LMWGs are complicated because of varied influence factors, such as concentration,12 solvents,13−15 pH,16 chirality,17−19 and so forth. Among above influence factors, some studies have been carried out to investigate the influences of molecular structure on the self-assembly process.20−22 For example, Yasutomo Yamamoto replaced the terminal methyl groups of didodecanoyl ethylenediamine organogelator with carboxy groups and effectively enhanced the molecular self-assembly ability through intermolecular hydrogen bonding. Hence, the enhanced gelators could also form gel in water. In addition, the results of Fourier transform infrared (FTIR) spectra and scanning © XXXX American Chemical Society

electron microscopy (SEM) images indicated that intermolecular hydrogen bonding of the central amide groups was not interfered by terminal functional groups.23 A new class of peptoid-based low-molecular-mass organogelators with different terminal groups was prepared by Goutam Bis.24 Also, the results showed that only the gelator with tert-butyl group formed organogels in chlorinated solvents, suggesting that the molecular structure played an important role in the formation of gels. Although there are numerous qualitative research studies, few reports focus on the quantified analysis, for example, the effect of change in the number of functional groups on the selfassembly process because such a subtle change is hard to show observable influences. Whereas in this work, we have noticed that the tiny change in the structure of the gelator can cause a significant performance variation. Herein, two gelators (Glu-CBZ and Glu-DPA) with the same terminal long-side chain and different cephalic functional groups, carbobenzoxy group and diphenyl phosphate group, were synthesized (as shown in Figure 1). Although the increase in the number of benzene rings in the diphenyl phosphate group of Glu-DPA provides stronger intermolecular forces during self-assembly, it causes stronger steric effect which keeps the Glu-DPA molecules farther away from each other than GluReceived: November 10, 2017 Revised: November 24, 2017 Published: November 26, 2017 A

DOI: 10.1021/acs.langmuir.7b03873 Langmuir XXXX, XXX, XXX−XXX

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the mixture was heated in an oil bath until the gelator was dissolved completely. Next, the solution was statically cooled down at 25 °C for several minutes. The tube-inversion method19 was used to characterize the gelation abilities. Also, we studied the thermal stability of the gel according to the sol−gel transition temperature (Tgel). The Tgel was measured by increasing the heated temperature from 25 °C (1 °C at a time) until the gel was collapsed. Calculation of van’t Hoff Plot and Gibbs Constant of Gels. Equation 1 is the van’t Hoff equation, in which c denotes the concentration of the gelators, ΔH is the enthalpy change, R is the gas constant (8.314 J·mol−1·K−1), T denotes the Tgel, and ΔS is the entropy change. The van’t Hoff plot is created with the 1000/Tgel as the x coordinate and the ln(c) as the y coordinate, from which we can get the ΔH (from the slope) and the ΔS (from the intercept). Equation 2 is the Gibbs−Helmholtz equation and the ΔG is the Gibbs free energy. On the basis of the known ΔH and ΔS from the van’t Hoff plot, we further compare the Tgel of gels depending on ΔG.25−27

Figure 1. Structure of Glu-CBZ and Glu-DPA.

CBZ. Therefore, the self-assembly driving forces, selforganization patterns, network morphologies, rheological properties, and the influences of solvents were researched to investigate the self-assembly mechanism and gel properties, which could reveal that the key impact factors of gelator selfassembly mechanism is stronger intermolecular forces or closer molecular spacing.

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ln c = −ΔH /RT + ΔS /R

(1)

ΔG = ΔH − T ΔS

(2)

Rheology Measurements. Rheological characterizations of gels were performed on an Anton Paar MCR302 rheometer with a plate geometry (PP 25). The distance between the plates was set to 0.2 mm. Then, gels were scooped onto the rheometer plate. The angular frequency was between 0.01 and 10 rad·s−1 (with a strain γ = 0.1%) at 25 °C, and the oscillatory frequency sweep experiments were performed in the linear viscoelastic region to ensure that the calculated parameters correspond to an intact network structure. Dynamic strain sweep tests were carried out to increase the amplitude of oscillation from 0.1 up to 100% apparent strain shear (with a frequency ω = 6.28 rad·s−1) at 25 °C. The dynamic time sweep tests were carried out at a constant frequency of 6.28 rad·s−1 and an applied strain was changed from 2 to 0.1%. Morphology Test. The gels were carefully scooped onto the conducting resin on the platinum stubs and were allowed to dry

EXPERIMENTAL SECTION

Synthesis and Characterization of Gelators (Glu-CBZ and Glu-DPA). The details are present in section 1 in the Supporting Information. Gelation Abilities and Thermal Stabilities of the Gelators. The gelation abilities of these gelators in different solvents were tested. A certain amount of gelator was mixed with a solvent in a vial. Then,

Figure 2. Attenuated total reflection-FTIR spectra of the xerogel and powder of Glu-CBZ and Glu-DPA. B

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Langmuir overnight in air. The samples were further dried under vacuum for 8 h. After the xerogels were coated with a thin layer of gold, the morphologies were observed by using a Hitachi S-4700 field-emission SEM (FE-SEM, Hitachi, Japan) instrument. Self-Assembly Force and Self-Assembly Behavior Tests. FTIR spectroscopy was performed on a Nicolet NEXUS FTIR 6700 infrared spectrophotometer region by loading the samples into a KBr cuvette. Xerogels were prepared by lyophilization. The X-ray diffraction (XRD) patterns were obtained using an X’Pert Pro diffractometer with a Ni filter and Cu Kα (l = 1.54056 Å, voltage = 40 kV, and current = 40 mA). Each sample was scanned over the range from 3° to 40° with a sweep speed of 10°·min−1. In addition, the UV− vis spectra of 8-anilino-1-naphthalenesulfonic acid (ANS)-doped gelators were recorded on a LAMBDA 750S UV−vis spectrophotometer (PerkinElmer) by varying the solvent from a nongelating (tetrahydrofuran) to a gelating one (dimethyl sulfoxide). As the gelators of Glu-CBZ and Glu-DPA were devoid of any fluorescent moiety in its structure, we hydrophobically tagged one fluorescent probe, ANS ([gelators] = 3 mg/mL and [ANS] = 0.05 mg/mL).

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RESULTS AND DISCUSSION Self-Assembly Driving Forces Recognition of the Gels. FTIR and XRD were carried out to study the self-assembly driving forces. The results show that the hydrogen bond interaction and the π−π interaction are both the self-assembly driving forces of the gels of Glu-CBZ and Glu-DPA. The FTIR spectra of the xerogel and powder of Glu-CBZ and Glu-DPA are shown in Figure 2. Compared with the powder of Glu-CBZ, the N−H stretching vibration absorption peak of the xerogel is red-shifted from 3284.8 to 3281.4 cm−1, and the amide I band from 1687.1 to 1684.7 cm−1, suggesting that the N−H bond between the amide groups changes from a free state to an associated state. The FTIR spectra of Glu-DPA have similar results, and the PO stretching vibration absorption peak is red-shifted from 1251.77 to 1250.01 cm−1, indicating the H-bonding formation. However, the difference of the H-bondings formed by N−H···OC and N−H···OP is very small because of the same hydrogen bond donor and acceptor.28 Therefore, the results of the FTIR experiment show that the hydrogen bond interaction is one of the driving forces in the self-assembly process.29 Also, from XRD patterns, the diffraction peaks of 21.9° (Figure S1a) and 18.8° (Figure S1b) can be observed, which matches the interplanar space between the aromatic rings of the gelators owing to the π−π interaction.30 What is more; the results of UV−vis spectra also validate this conclusion (Figure S2).31 Consequently, the hydrogen bond interaction and the π−π stacking interaction are the self-assembly driving forces. As mentioned before, the two gelators are similar in structures but have different number of benzene rings. However, they both have amido-bonds and benzene rings that provide the H-bondings and the π−π interaction. On the basis of the negligible effect of phosphore atom for the selfassembly of molecular packing, we suspect that Glu-DPA has stronger intermolecular forces, whereas Glu-CBZ has a closer assembly mode owing to the smaller steric effect.30 Morphologies of the Gel Networks. The gel networks morphologies of the Glu-CBZ and Glu-DPA xerogels were investigated by SEM. The SEM images of the Glu-CBZ xerogels formed in styrene, MMA, and chlorobenzene show stereoscopic fibrous networks with fibers of 100−200 nm in diameter (Figure 3a,c,e). As shown in Figure 3b,d,f, the GluDPA forms fibrous networks in styrene and MMA but forms plicated intestine-like networks in chlorobenzene. Compared with Glu-CBZ, the gel networks for Glu-DPA show worse

Figure 3. SEM images of Glu-CBZ xerogels formed in (a) styrene, (c) MMA, and (e) chlorobenzene and the Glu-DPA xerogels formed in (b) styrene, (d) MMA, and (f) chlorobenzene.

stereoscopic properties, and the fiber diameters are larger (200−500 nm). The additional benzene ring in diphenyl phosphate group of Glu-DPA causes stronger steric hindrance effect, which further increases molecule distance and affects the orderly assembly of molecules. That is why the gel networks of Glu-DPA are less consummate than Glu-CBZ. It follows that the closer the molecule distance of gelators, the more complete is the selfassembly process. Self-Organization Models of Gels. From the previous discussions, we know that the gels of Glu-CBZ and Glu-DPA have similar self-assembly driving forces but obviously different morphologies. On the basis of the above-mentioned findings, we investigate their self-assemble ways through XRD patterns and further build the self-organization models of gels. As shown in Figure 4, the self-assembly aggregates of GluCBZ and Glu-DPA have different diffraction peaks. Typically, three major scattering peaks (2θ = 3.6°, 6.1° and 7.2°; Figure 4a) get the d spacing of 24.5, 14.4, and 12.3 Å, respectively. They appear in a ratio of 1:1/√3:1/2 in the Glu-CBZ scattering pattern, and this ratio corresponds to a hexagonal columnar structure.32 In the meanwhile, the scattering peaks of Glu-DPA (2θ = 3.5°, 7.5°, 10.5°; Figure 4a) get the d spacing of 25.2, 11.8, and 8.4 Å (the ratio = 1:√2/3:1/3) and correspond to a columnar square structure.33 What is more; the d spacing obtained from the XRD pattern is in good agreement with the length of a Glu-CBZ and Glu-DPA molecules (Scheme S1). Consequently, the single column is most likely formed by layerupon-layer stacking of gelators. Thus, these results imply the different self-assembly processes of gelators. As shown in Figure 4c, the moleculars of Glu-CBZ assemble to form single fibers through layer-uponlayer stacking. Then, single fibers interconnect to form fiber bundles in a hexagonal packing model. Finally the fiber bundles form 3D network structure. Also, the self-assembly process of C

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Figure 4. XRD patterns of (a) xerogel of Glu-CBZ and (b) xerogel of Glu-DPA. Schematic representation of the self-assembly process of (c) GluCBZ and (d) Glu-DPA.

Figure 5. Gels are all formed in styrene with a concentration of 2 wt %. (a) Plots of storage modulus G′ and loss modulus G″ vs angular frequency. (b) Plot of G′ and G″ vs oscillator stress at a constant frequency of 10 rad·s−1 at 25 °C. (c) Plot of G′ and G″ vs time in continuous step strain measurements for gels.

Glu-DPA is almost the same as the mode of Glu-CBZ; the difference is that single fibers assemble into fiber bundles in a cubic packing model in Figure 4d. Consistent with the above results, the difference of structures makes the two gelators self-assemble differently. Therefore, the

self-assembly process of Glu-CBZ is carried out in a more tightly hexagonal packing model, whereas Glu-DPA assembles in a cubic packing model because of the stronger steric hindrance effect. As a conclusion, the closer the distance, the more tightly packed. D

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Figure 6. Phase diagrams for Glu-CBZ and Glu-DPA from 0 to 4.0 wt % concentration in MMA (a), styrene (b), and chlorobenzene (c).

Rheological Properties. As we can see, closer molecular spacing plays a more important role than stronger intermolecular forces in previous discussions. However, the situation in rheological studies is exactly theopposite. The rheological properties of the two gels were measured. As shown in Figure 5a, the gels show a plateau region when the angular frequency was varied from 0.01 to 10 rad·s−1. The G′ value is higher than G″ over the entire frequency range, thus exhibiting a substantial elastic response. The ratio of G′ and G″ is 3.4−6.5 for Glu-CBZ and 7.8−11.2 for Glu-DPA. The G′/G″ values are both greater than 1, indicating that the hybrid gels retain their gel-like nature for a wide range of angular frequency amplitudes. The results of dynamic the strain sweep tests are shown in Figure 5b. When the gel structure is disrupted gradually, the value of G′/G″ decreases with the increase of strain. The G′ and G″ curves cross each other, and the G′/G″ ratio became less than 1 at a strain amplitude of 4.1% for Glu-CBZ and 6.8% for Glu-DPA. These results indicate that the gels can retain its gel-like character under a big strain amplitude. What is more, the gel of Glu-DPA gets a better ability of resistance to external forces than the gel of Glu-CBZ.34 The self-recovery properties of the gels are shown in Figure 5c. When the applied strain is 2% for 200 s, the G′ of Glu-CBZ decreases from 87.9 to 45.5 Pa, and the G′ of Glu-DPA decreases from 1230 to 980 Pa. It indicates that the gel network has been damaged under the impact of external force, resulting in a quasiliquid state and the disappearance of elasticity. When the applied strain decreases (γ = 0.1%) and frequency is invariant, G′ recovers to its initial value immediately and the gels return to a quasisolid state. Consequently, the gels of GluCBZ and Glu-DPA have good mechanical properties and selfrecovery properties, such that they have a potential application value in the self-recovery soft materials.

Obviously, the G′ of Glu-DPA is about 2000 Pa, whereas the G′ of Glu-CBZ is 300 Pa. What is more, the G′/G″ value of Glu-DPA is higher than Glu-CBZ. Also, the strain value of the sol−gel transformation of Glu-DPA is also higher than GluCBZ. These conclusions show that the gel of Glu-DPA is more robust than the gel of Glu-CBZ and get a better ability of resistance to external forces. Interestingly, the effect of structural difference on the rheological properties is different from the effect on morphologies and self-assembly processes. The stronger steric hindrance effect between molecules leads to the farer distance of gelators. Thus, the gel of Glu-DPA shows less consummate networks and less tightly packed than Glu-CBZ. Because the studies of morphologies and self-assembly processes are static processes but the studies of rheological properties are dynamic processes subjected to shear force, the gel of Glu-DPA shows better rheological properties. Besides, owing to the labile nature of the interactions (both hydrogen bond and π−π stacking) in the dynamic processes, the noncovalent interaction between molecules is a dynamic equilibrium of destruction and rebirth. The initial noncovalent interaction is broken by shear force, and more active sites are available to reconnect. Besides, the probability of molecules collisions increases, and it weakens the effect of steric hindrance effect. That is why the dynamic processes can change the importance of degree of the effect of closer molecular spacing and stronger intermolecular forces. Influence of Solvents on the Gelation Behaviors. The influence of solvents on gelation properties was also investigated in addition to the impact of gelator structure. In addition, gelatinization depends on the balanced relation between the interaction of gelators with gelators and the interaction of gelators with solvent molecules. The gelation behaviors of Glu-CBZ and Glu-DPA were tested in 15 kinds of E

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organic solvents. As shown in Table S1 in the Supporting Information, they form opaque gels in esters solvents and transparent gels in aromatic solvents. Remarkably, toluene and xylene are gelled by Glu-CBZ, but Glu-DPA dissolves in them. That is because the diphenyl phosphate group of Glu-DPA has stronger π−π stacking interaction with toluene and xylene. Similarly, the precipitation of Glu-DPA forms in hexane and cyclohexan owing to the stronger intermolecular forces, whereas Glu-CBZ succeeds to form gels owing to the closer molecular spacing and more tightly packed way. Influence of Solvents on Thermal Stabilities. Besides the gelation behaviors, the thermal stabilities of the gels are affected by solvents. Generally speaking, when the gel has lower critical gelation concentration, it has a better thermal stability. If the gel has a higher Tgel at the same concentration, it also has a better thermal stability. It is assessed by using the simple tubeinversion method, and the phase diagrams for gels are shown in Figure 6. The Tgel increases with the increase of the concentration of gelators in three solvents. However, the ranges of the increase of the Tgel in these three solvents are different (MMA > styrene > chlorobenzene). It is because MMA had a certain extent of hydrogen-bond-accepting ability, thus enhancing the hydrogen bonding strength between solvent molecules and gelators. Therefore, the thermal stability of the gel in MMA is better than the gels’ thermal stabilities in styrene and chlorobenzene. Obviously, the increase of consummate gel networks makes the thermal stability of Glu-CBZ superior to Glu-DPA in these three solvents, which matches the analysis result of the morphology test. Although Glu-DPA shows an outstanding performance in the process of dynamic equilibrium because of stronger intermolecular forces, the process of thermodynamic equilibrium depends on the gel networks mainly, on which closer molecular spacing has a better impact.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 86-0571-88320855. Fax: 86-0571-88320855. ORCID

Si Chen: 0000-0001-6371-616X Meng Ma: 0000-0001-6514-5495 Yanqin Shi: 0000-0001-6600-5750 Xu Wang: 0000-0003-4416-1538 Author Contributions †

S.C. and Z.A. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (grant no. 51773180) is gratefully acknowledged.

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CONCLUSIONS In this paper, two gelators with different number of benzene rings were synthesized. In addition, we studied the specific influence of increasing one benzene ring on the gelation systems. The results show that the gels of Glu-CBZ have more consummate networks and thermal stabilities, but the gels of Glu-DPA have better rheological properties. This means that the main effect of increasing one benzene ring is to provide stronger intermolecular force in the dynamic process, but it leads to steric hindrance effect in the static process. Therefore, this paper has been explored successfully that the key impact factors of the gelator self-assembly mechanism are stronger intermolecular forces or closer molecular spacing. This work is a creative quantified analysis of subtle structural changes of gelators and is helpful to probe into the influence of the structure of the gelators on the self-assembly mechanism.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b03873. Structure of different gelators; UV−vis spectra of (a) Glu-CBZ and (b) Glu-DPA in different solvents; gelation behaviors in different kinds of solvents; thermodynamic parameters of Glu-CBZ and Glu-DPA gels; van’t Hoff plots and Gibbs function curve for Glu-CBZ gel and GluDPA gel in chlorobenzene (PDF) F

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