Combined Experimental and Computational Study of the Gelation of

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Combined Experimental and Computational Study of the Gelation of Cyclohexane-Based Bis(acyl-semicarbazides) and the Multi-StimuliResponsive Properties of Their Gels Sravan Baddi,† Sita Sirisha Madugula,‡ D. Srinivasa Sarma,§ Yarasi Soujanya,*,‡ and Aruna Palanisamy*,† †

Polymers and Functional Materials Division, ‡Centre for Molecular Modelling, CSIR-Indian Institute of Chemical Technology, Hyderabad-500007, Tarnaka, India § Geochemistry Division, CSIR-National Geophysical Research Institute, Hyderabad-500007, Telangana, India S Supporting Information *

ABSTRACT: The current study reports the one-step synthesis and gelation properties of cyclohexane-based bis(acyl-semicarbazide) gelators with an additional −NH group incorporated into urea moieties and carrying hydrophobic chains of varying length (C8− C18). The gels exhibited thermoreversibility and could be tuned in the presence of anions at different concentrations in addition their the ultrasound-responsive nature, thus making them multi-stimuli-responsive. The combined experimental and computational study on these gels reveals that the balance between two noncovalent interactions, viz., hydrogen bonding between the amide groups in acyl-semicarbazide moieties and van der Waals forces between long hydrocarbon tails, is found to be the determining factor in the process of organogelation. A systematic increase in alkyl chain length leads to equilibrium between these two types of noncovalent forces that is manifested in the spectral and thermal properties of the gels. The H-bonding interactions dominated up to a certain chain length, and further increases in the alkyl chain length led to increased van der Waals interactions as observed by IR, XRD, and thermal studies. Computational calculations were carried out on dimer structures of C8−C18 to understand the variation in noncovalent forces responsible for aggregate formation in the gel state as a function of the alkyl chain length. The results indicate that both intermolecular and intramolecular hydrogen bonding stabilize the aggregate structures. Supramolecular aggregation in the gel state led to the viscoelastic nature of the gels, and the addition of anions led to the disruption of self-assembly, which was studied by rheology. due to their self-assembling properties. Hanabusa et al.9,10 were the first to synthesize a series of bis-urea cyclohexane organogelators including trans-(1S,2S)-bis(ureidododecyl)cyclohexane (SS-BUC). Thereafter, Feringa’s group11−13 extensively studied the thermotropic and rheological properties of trans-(1R,2R)-bis(ureidododecyl)cyclohexane (RR-BUC), which is the enantiomer of SS-BUC and has the same physical properties as SS-BUC except for the helicity of the gel fiber. Loos et al. have worked extensively on cyclohexane core-based gelators, viz., the gelating capacity of polymerizable bis(amido)cyclohexane and bis(ureido)cyclohexane derivatives in organic solvents.11 Studies on hydrogelation by a simple modification of the peripheral substituents of cyclohexane bis-urea organogelators with hydrophilic hydroxy or amino functionalities14 and chiral recognition through cooperative interactions in 1,2bis(ureido) cyclohexane derivatives10 have also been carried out. The gelation of room-temperature nematic liquid crystals by the self-aggregation of low-molecular-weight molecule trans(1R,2R)bis(dodecanol amino)cyclohexane through hydrogen

1. INTRODUCTION Smart gels, also known as stimuli-responsive gels, respond to external stimuli by changing their physico−chemical properties. Smart materials based on stimuli-responsive low-molecularweight organic gelators (LMWOGs) have potential applications in photoswitches, sensors, molecular logic gates, and other soft functional materials. A wide variety of organogels that can respond to light, temperature, sound, mechanical stimuli (sonication-aided gelation), anions, metal ions, redox, proton/ pH, and small molecules have been investigated.1−6 It is well known that, for sensing and the recognition of different anions, −NH units such as amides, (thio)ureas, indole, and pyrazole could be utilized as binding sites for anions.3 The anion tuning of gels has been studied in depth by Steed et al. in urea-based systems, and they have shown that the competition between anion−gelator binding and gelator self-assembly can be precisely used to tune the gel properties.7 Recently, Mahapatra et al. reported a multi-stimuli-responsive L-cysteine-derived amphiphile containing a poly(ethylene glycol) tail wherein the gel structure was disrupted by the addition of a tetrabutyl ammonium fluoride salt.8 A number of cyclohexane derivatives have been investigated so far with respect to their organogelation and hydrogelation © XXXX American Chemical Society

Received: October 30, 2015 Revised: December 31, 2015

A

DOI: 10.1021/acs.langmuir.5b03987 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir Scheme 1. Schematic Representation of the Synthesis of the Bis(acylsemicarbazide) Gelators

Figure 1. Digital photograph of the multiple switching process of C12 under the alternate effects of ultrasound, temperature, and anions.

bonding is yet another report on a cycloaliphatic gelator.15 The gelation behavior of a series of bisamide- and bisurea -based low-molecular-weight gelators, built into a transoid-cyclohexyldiamine core with a systematic variation in the length of the two linear alkyl moieties, has been examined in a range of polar and apolar solvents.16−18 In both of these cases, gelation occurred collectively as a result of van der Waals and hydrogen bonding interactions and the gels resulted from anisotropic selfassembly.19 We have designed cyclohexane-based bis(acyl-semicarbazide) gelators, the gels of which respond to multiple stimuli-like anions, temperature, and ultrasound. These compounds were found to be excellent gelators, giving transparent gels in organic solvents such as xylene, chlorobenzene, toluene, and benzene. The influence of the spacer length on gelation properties such as the minimum gelator concentration, time of gelation, morphology of xerogels, and gel melting temperatures was investigated. Experimental findings are further substantiated with DFT (density functional theory)-based computational studies. The anion tuning properties of the gels were investigated with 1H NMR and rheological studies.

gelator−solvent interaction was determined by IR spectroscopy using a PerkinElmer spectrum 100 Fourier transform infrared spectrometer (FT-IR). The gel and the solution of the gelator in a specific solvent were taken in KBr discs and scanned in the 4000−400 cm−1 range. Nuclear magnetic resonance spectra (1H NMR) were recorded in DMSO-d6 (dimethylsulfoxide-d6) on a Bruker 500 MHz NMR spectrometer, and tetramethylsilane (TMS) was used as the internal reference. The gel melting temperatures were determined by differential scanning calorimetry (DSC) with a Q 100 series instrument (TA Instruments) from room temperature to 130 °C at a heating rate of 5 °C/min under a nitrogen atmosphere (flow rate 50 mL/min), with a weighed quantity of the gel (10−15 mg) taken in a hermitic pan and sealed. The xerogel for X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies was obtained by freezedrying the corresponding organogels (in benzene) at −100 °C. XRD spectra for the xerogels were obtained by using a Siemens/D-5000 Xray diffractometer with Cu Kα radiation of a wavelength of 1.54 Å and a continuous scanning speed of 0.045/min. Diffraction data were recorded at room temperature in the range of 2° ≤ 2θ ≤ 65°. The microstructure of the xerogels was studied by using a Hitachi S-3400N field-emission scanning electron microscope energy-dispersive spectrometer (FESEM-EDS). The acceleration voltage was 15 kV, and the emission was 10 mA. For sonication, a bath-type sonicator (Sisco Lab maa ultrasonicator) of frequency 35 kHz and ultrasound peak output 320 W was used. Rheological measurements were performed with a strain-controlled rheometer (102 series modular compact rheometer, Anton Paar, Graz, Austria) equipped with a parallel plate. The gap distance was fixed at 1 mm. The measurements were conducted via a dynamic frequency sweep method at a constant strain (0.05%) with varying angular frequency (ω) from 0.1 to 100 rad s−1; at the same time, a strain amplitude sweep was performed at a constant frequency ( f) of 1.0 Hz with strain ranging from 0.01 to 100 at 25 °C. 2.3. Synthesis of Fatty Acid Hydrazides. The methyl esters were prepared from the corresponding fatty acids by refluxing a mixture of fatty acid (1 mM), methanol (10 mM), and 1% concentrated sulfuric acid at 75 °C for 4 h. The fatty acid hydrazides were prepared from the corresponding methyl esters by reaction with hydrazine hydrate according to the procedure already reported.20,21 2.4. Synthesis of Gelator. IPDI and fatty acid hydrazides (C8− C18) are subjected to a simple addition reaction to yield bis(acylsemicarbazide) as shown in Scheme 1. In an emblematic experiment, to a stirred solution of fatty acid hydrazide (2.1 mM) in 30 mL of DMF, DBTDL (0.01% by weight of reactants), and IPDI (1 mM) is

2. EXPERIMENTAL SECTION 2.1. Materials. Isophorone diisocyanate (IDPI 98%), dibutyltin dilaurate (DBTDL 95%), tetrabutyl ammonium fluoride (TBAF 98%), tetrabutyl ammonium chloride (TBACl 97%), tetrabutyl ammonium bromide (TBABr 98%), tetrabutyl ammonium iodide (TBAI 98%), tetrabutyl ammonium acetate (TBAOAc 97%), tetrabutyl ammonium nitrate (TBANO3 97%), and tetrabutyl ammonium tetrafluoroborate (TBABF4 99%) purchased from Sigma-Aldrich were used as received. Octanoic acid 98%, decanoic acid 98%, dodecanoic acid 98%, tetradecanoic acid 97%, hexadecanoic acid 98%, octadecanoic acid 95%, hydrazine hydrate 98%, and the organic solvents used for gelation experiments purchased from Sd Fine Chemicals Limited (Mumbai, India) were used as received. 2.2. Techniques. Electrospray ionization mass spectra (ESI-MS) were recorded with a Waters e2695 separators module (Waters, Milford, MA, USA) mass spectrometer. Qualtro micromass was used with mass Lynx software. The conditions maintained were as follows: capillary voltage 3.28 kV, source temperature 120 °C, and desolvation temperature 350 °C with ESI positive ion mode. The nature of the B


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Langmuir

Table 1. MGC (Minimum Gelator Concentration, mg/mL), Gelation Behavior at Room Temperature,a and Thermal Properties of Gels in Different Organic Solventsb alkyl chain length

toluene

benzene

chloro benzene

o-xylene

m-xylene

p-xylene

Tgel (°C)c

C8 C10 C12 C14 C16 C18

PG 30(60) 30(20) 25(20) 25(20) 40(30)

30(30) 30(15) 30(10) 30(10) 30(10) 30(10)

PG 30(30) 30(15) 30(15) 30(15) PG

S S S 30(20) S S

S S S 25(20) S S

S S 30(25) 30(20) 30(20) PG

101.5 87.6 73.8 57.2

a

PG, partial gel; S, solution. Each number in parentheses denotes the time of gelation (min) in the particular solvent. bAll gels are transparent (Figure S2). cThe melting point of the gels was determined by DSC in chlorobenzene.

Figure 2. (a) DSC curves of C10−C16 gelators in chlorobenzene at a concentration of 30 mg/mL. (b) DSC curves of the C12 gelator at concentrations of 40 and 60 mg/mL in chlorobenzene. studying the stimuli response. A solution of C12 gelator (30 mg/mL in chlorobenzene) was subjected to ultrasound treatment and left undisturbed for some time to check the gelation. To check the gel− sol transition on addition of anions, fixed amounts of tetrabutylammonium salts were added to gels of a specific concentration (30 mg/ mL) and the phase transition was monitored visually by checking the flow at regular time intervals.

added 10 mL of DMF dropwise under a nitrogen atmosphere. The reaction mixture is stirred at room temperature for 1 h and then at 60−65 °C until the NCO peak disappears completely (as monitored by IR and TLC (thin layer chromatography)). The mixture is then cooled and poured into water, and the compound precipitates. The product is separated by filtration, washed with water, and dried in a vacuum oven at 50 °C. Purification is done by boiling the product in ethyl acetate and then filtering to remove the unreacted hydrazide, and the purity is checked by TLC. The melting point of the compounds was determined by DSC, and the structure was confirmed by 1H NMR and ESI-MS (Figure S1 and Table S1 Supporting Information). 2.5. Gelation Test and Determination of the Gel-to-Sol Transition Temperature. For a typical gelation test, a weighed quantity of the gelator was added to a specific volume (1 mL) of the organic solvent in a test tube. The tube was sealed and heated until a clear solution was obtained, which was left undisturbed to cool on its own to room temperature. The gelation abilities of the samples were determined by using the heating−cooling−inverting method22 as illustrated in Figure 1. The samples were considered to be a gel when there was no flow (observed by immobilization of the solvent) on inverting the tube. The minimum gelator concentration (MGC), which is the minimum amount of the gelator necessary to gel 1 mL of the solvent, was determined by trial and error. The gelation time (GT) is the minimum time taken for the gel to form, from the time it is left undisturbed to cool on its own. The gel state was denoted as G, partial gelation as PG, solution state as S, and undissolved/insoluble as I. The gel-to-sol transition temperature (Tgel) was determined by the dropping ball method in which steel balls were placed on the gel in a glass vial (i.d. 12 mm) maintained in a thermostatic oil bath and the temperature was slowly raised at a rate of 2 °C min−1. Tgel was recorded when the ball reached the bottom of the test tube. The sol− gel phase-transition temperature (Tgel) indicates the thermal stability of the gel as shown in Figure 1. All of the gels (C10, C12, and C14) had thermoresponsive, mechanoresponsive, and chemoresponsive properties; however, the C12 gelator was chosen as a representative for

3. RESULTS AND DISCUSSION 3.1. Gel Properties. The gelation abilities of six gelators (C8−C18) were tested in different organic solvents, among which xylene, toluene, benzene, and chlorobenzene were found to be favorable to gelation at an MGC of 25−30 mg/mL. GT varied between 10 and 30 min depending on the nature of the solvent and the chain length of the spacer as shown in Table 1. Even though GT is smaller for benzene-based gels, it is not possible to determine Tgel because Tgel of benzene-based gels is close to the boiling point of benzene. All of the gels obtained were thermoreversible, stable to several heating−cooling cycles, and stable to mechanical agitation without any significant effect on the gelation ability and gel properties. These gels are stable for a period of 5 to 6 months at ambient temperature. The studies with o-xylene, m-xylene, and p-xylene show that the positions of methyl groups in the solvent have a significant effect on MGC. The gelation of C12−C16 was favored by pxylene, but o-xylene and m-xylene favored the gelation of the C14 gelator only. This could be due to the match in the symmetries of the concerned gelators and p-xylene. In the present study, Tgel was initially determined by the dropping ball method (Table S1). The melting point of the gels (defined as the temperature above which macroscopic flow appears) was determined by the dropping ball method in a C


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Figure 3. (a) Collapse of the C12 gel with 1 equiv of F− at different time intervals. (b) State changes of the chlorobenzene gel of C12 (30 mg/mL) in response to different anions.

thermally controlled bath. The effect of chain length on Tgel was studied at a fixed concentration of the gelator (30 mg/mL) for C10−C16, which was found to be in the 75−95 °C range. To study the effect of concentration on gelation, the C12 gels were prepared in chlorobenzene at different concentrations (40 and 60 mg/mL). When the concentration was increased from 40 to 60 mg/mL, Tgel increased from 92.2 to 101.7 °C and GT decreased from 13 to 6 min. However, the thermal behavior of the gels was studied by using DSC to understand the nature of thermal transitions taking place in the gel. Chlorobenzene, being a high-boiling-point solvent, was chosen to prepare the gels for studying the thermal transitions. The gels showed a sharp endothermic transition in the first heating cycle, which corresponds to the melting point of the gel. The gel-to-sol phase-transition temperature found in these experiments for C10−C16 gels is in the 101 to 57 °C range at a concentration of 30 mg/mL. When the chain length increased from C10 to C16, the endothermic transition shifted from higher to lower temperature (Table 1 and Figure 2a). C8 and C18 gelators showed partial gelation in chlorobenzene and hence were not subjected to DSC. The C12 gelator in chlorobenzene was chosen to study the effect of concentration on the gel properties. As the concentration increased from 30 to 60 mg/ mL, the transition shifted to higher temperatures as shown in Figure 2b. The increase in Tgel with increasing gelator concentration reflects the higher stability of the gel networks formed as a result of increased noncovalent interactions, viz., hydrogen bonding and hydrophobic interactions at higher concentrations. The absence of reversibility is also observed in the subsequent heating scans, which may be due to crystal imperfection.18 However, the reversibility of the sol−gel transition was observed during gelation studies by the test tube inversion method. Ultrasound treatment of the suspension of C12 gelator (30 mg/mL) in chlorobenzene for a period of 5 min and then leaving it undisturbed for 40 min at room temperature resulted in a stable translucent gel (Figure 1). Increasing the concentration of gelator from 30 to 60 mg/mL led to a decreasing resting time from 40 to 20 min. When benzene was used as the solvent, the resting time was further reduced to 10 min at 30 mg/mL concentration and 4 min at 60 mg/mL. The gels that formed reverted to the solution state upon heating and again to the gel state upon cooling the warm solutions. When tetrabutylammonium salts were added to the thermally formed gels, the gel−sol transition was observed after 20 min of the addition of anions, and this transition was not reversible upon addition of protic solvents such as methanol. The anion binding property of C12 in the gel state was investigated by adding fixed amounts of different salts, viz., TBAF, TBACl, TBABr, TBAI, TBAOAc, TBANO3, and TBABF4, to the chlorobenzene gel. Interestingly, the addition of 1 equiv of TBAF, TBACL, and TBAOAc resulted in a rapid

transition from a gel to a homogeneous solution. The addition of 1 equiv of TBAF to the gel converted it to a partial gel after 10 min and further to solution after 20 min as shown in Figure 3(a). However, the gel state of C12 could be destroyed only by adding 3 equiv of other halide anions (Br−, I− and NO3−). When BF4− ions were used, the gel phase could not be disturbed even after the addition of 3 equiv. The results indicate that C12 possesses a high selectivity toward F−, Cl−, and AcO− that could be visually observed by the gel−sol transition (Figure 3(b)). The binding capability and the minimum concentration of anions required for the gel−sol transition were investigated with rheological studies. 3.2. FT-IR Spectroscopy. The involvement of noncovalent interactions such as hydrogen bonding in the self-assembly of C8−C18 gels was investigated by FT-IR spectroscopy. FT-IR spectra of these gels were recorded in benzene. The spectral data corresponding to N−H, CO, and CH2 groups are summarized in Table 2. FT-IR spectra of the gelators in Table 2. FT-IR Absorption Bands (ν, cm−1) for the N−H, CO, and CH2 Groups for the Bis(acyl-semicarbazide)s with Different Alkyl Chain Lengths CH2 stretching sample code

H-bonded N−H stretch

H-bonded CO (amide I)

N−H bend(amide II)

ν-asym

ν-sym

C8 C10 C12 C14 C16 C18

3280 3289 3294 3295 3292 3286

1660 1658 1655 1655 1656 1656

1557 1557 1556 1555 1557 1559

2933 2926 2926 2924 2922 2922

2857 2855 2854 2853 2852 2852

solution (benzene) were recorded to compare the position of the bands with those in the gel state. Two other strong absorption bands corresponding to the symmetric and asymmetric stretching vibrations of CH2 groups were observed at ∼2855 and 2927 cm−1, respectively. In the solution state, the spectra showed bands at approximately 3341, 1650, and 1577 cm−1, which are the characteristic non-hydrogen-bonded N−H stretching band and the CO (amide I) and N−H bending (amide II) frequencies, respectively. However, in the gel state, the N−H signals appeared as a broad band at around 3219 cm−1 rather than as a distinct peak. In addition, the CO (amide I) and N−H (amide II) bending bands shifted to approximately 1643 and 1528 cm−1, respectively. This lowering of the stretching bands for CO (amide I) and the bending band for N−H (amide II) indicates the presence of intermolecular hydrogen bonding between the carbonyl group and the amide N−H group in the gel state23,24 as shown in Figure 4. Thus, intermolecular hydrogen bonding through the D


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Langmuir urea-like moieties carrying an additional NH group is crucial to the self-assembly leading to organogelation.

Figure 4. FT-IR spectra of the C16 gelator in gel and solution states in benzene at a concentration of 30 mg/mL.

Characteristic FT-IR bands of xerogels (C8−C18) are shown in Table 2 and Figure S3. It is evident from Table 2 that the N−H stretching frequencies are shifted to higher wavenumbers (3280 to 3295 cm−1) with an increase in chain length from C8 to C14, indicating a decrease in the strength of hydrogen bonding. However, there is no significant change in the CO stretching and N−H bending frequencies as we move from C8 to C14, indicating the same type of hydrogen bonding as mentioned above. The CH2 infrared stretching frequencies reflect the extent of packing of the alkyl chains. The spectra of xerogels showed a decrease in the absorbance frequency of symmetric and asymmetric CH2 vibrations, indicating increased van der Waals interactions with increasing alkyl chain length. This type of harmony within the two noncovalent interactions leads to the formation of globular and folded sheetlike morphology in the gel.25 These observations reveal that Hbonding interactions within semicarbazide groups keep the cyclohexane core together in shorter alkyl chains (C8−C14). However, with the increase in chain length, van der Waals interactions become predominant (C16 and C18). For this reason, the N−H stretching frequencies were shifted to lower wavenumbers in the cases of C16 and C18 compared to C12. 3.3. Microstructure. An examination of the SEM images reveals that the xerogel morphologies are quite different from each other, viz., flowerlike morphology (C8), disclike aggregates (C12 and C14), and folded sheets (C10, C16 and C18)25 as shown in Figure 5. The discs have a diameter of about 2.3−3 μm. The difference between the morphologies of the xerogels is due to differences in the chain length and the different extents of secondary forces operating in the molecules. These results show that a small change in the structure of the gelators leads to a significant change in the extent of H bonding and van der Waals interactions and hence a drastic difference in the mode of self-assembly. 3.4. X-ray Diffraction Study. The WAXS diffractograms of all of the xerogels were similar and exhibited a series of welldefined, sharp low-angle reflections around 2θ values of 2−5°, suggesting a lamellar organization. The reflections have periodicities of 1/1, 1/2, 1/3, and so forth. The d spacings and relative intensities of C8−C18 xerogels are shown in Figure

Figure 5. SEM images of xerogels of C8−C18 gels from benzene.

6 and listed in Table 3 and Table S2. All of the bis(acylsemicarbazides) exhibited no significant peaks at 2θ > 30° (d < 2.97 Å), and the most intense peak was observed below 2θ = 5°. The d spacing of the most intense peak increased with the length of the alkyl side chain, from 18.22 for C8 to 33.53 Å for C18, which corresponds to the molecular length of the gelators. In all of the diffractograms, there is a broad diffraction at 2θ ≈ 20−21° (d = 4.4−4.2 Å) that corresponds to the plane of hydrogen bonding.26,27 The distance between the planes was found to increase with increasing chain length of the spacer up to C14 and then to decrease. This variation is reflected as a change in the extent of H bonding and van der Waals interactions as discussed earlier in IR studies. The peak observed at 2θ ≈ 23° (d = 3.8 Å) is assigned to the distance between the alkyl chains within the molecule and the neighboring molecules. All of these results were substantiated with molecular modeling studies that showed a similar trend of intermolecular distances (Table S4 in the Supporting Information). Figure 6 and Table 3 illustrate the crystallinity calculated from the X-ray diffractograms as a function of the number of carbons at the alkyl side chain derived from fatty acid. The lamellar size is obtained from the largest peak, which is the reflection from the (002) plane. The size of the lamellae is given by the length of the two fatty acid chains, and in the present case (002), the d position was multiplied by 2 to obtain the size of the lamellae. Sizes of the lamellae can be used to elucidate the longitudinal packing of the gelator molecular on the basis of its fatty acid chain length. Many such lamellae stacked together in the (002) direction make up the thickness, or height, of a triglyceride nanocrystallite. In a triglyceride E


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Figure 6. X-ray diffractograms of C8−C18 xerogels.

chain length and from then on an increasing trend. The number of lamellae was also found to decrease up to C14 and from then on slightly increased. This trend further substantiates the predominance of H bonding in the case of gelators with shorter chain lengths (C8−C14) and the selective predominance of van der Waals interactions for the others (C14−C18). 3.5. Rheological Studies. The rheological performance is a significant factor in the latent applications of the gels. It is strongly influenced by the supramolecular aggregation of gelator molecules. For the characterization of the general rheological behavior and to provide information about the stiffness of aggregation, we have examined the organogels of C10−C16 gels in chlorobenzene at room temperature. 3.5a. Frequency Sweep Studies on Gels. The viscoelastic property of the gel samples was characterized by rheological measurements. The rheological behavior of C10−C16 gels in chlorobenzene with 30 mg/mL MGC was studied. In a typical frequency sweep experiment, the storage modulus G′ and loss modulus G″ were plotted as a function of angular frequency (ω) at a constant strain of 0.05%. In our study, G′ was weakly depended on ω and all gels have a storage modulus G′ greater than the loss modulus G″; this rheological behavior evidenced the viscoelastic nature of the soft elastic gel. Frequency sweep is an important method for detecting the tolerance performance of a material to external forces. Accordingly, the test was conducted for the materials mentioned above. The results are shown in Figure 7, which clearly shows that the G′ values of the four gels display a weak dependence on frequency, with values being almost the same at 100 and 0.15 rad s−1. This is typical viscoelastic behavior, suggesting that there has been no phase transition during the test and the system is a true gel, and all four gels have good tolerance to external forces. A comparison of the G′ value of the gels indicates that the storage modulus of the C10 gel is greater than that of C12, C14, and C16 gels, showing that the C10 compound is a more effective gelator and forms stronger gels. The differences in the four gels revealed by frequency sweep measurements demonstrate, as expected, that the chain lengths of the gelators have a significant effect on the

Table 3. Crystallinity and Number of Lamellae Calculated from the X-ray Diffractograms as a Function of the Number of Carbons at the Alkyl Side Chain chain length

d spacing (Å)

C8 C10 C12 C14 C16 C18

18.2268 20.7418 25.6156 28.3890 31.8008 33.5324

fwhm

Θ

0.598 0.903 0.891 0.745 0.632

2.420 2.128 1.724 1.554 1.388 1.316

crystal domain

lamellar size

no. of lamellae

133.00 88.00 89.16 106.63 125.69

23.17 15.34 15.50 18.60 21.90

6.30 3.69 3.03 1.70 1.74

chain, carbon atoms are joined in a zigzag form, maintaining a 120° angle, and three carbons are separated by 2.54 Å in the β polymorphic form.28,29 Because the fatty acid unit present in the C8 gelator contains eight carbons, the total carbon chain length is 2.54 Å × 3.5 = 8.89. This result indicates that a 2L packing will give a value close to 18 Å whereas a 3L packing will not. The thickness of the crystal can be also be computed from the data obtained by the X-ray pattern, for which the Scherrer equation must be used. Accordingly, the (002) peak was chosen for this calculation. The copper wavelength, λ = 1.54 Å, was used together with a value of K = 0.9. When there is no strain or imperfection in the sample, the Scherrer equation can be used to find the thickness of the crystallite, thickness =

Kλ B cos θ

(1)

where B is the half-width at half-maximum of the peak under study on the 2θ scale in radians (1.5B), λ is the wavelength (in nm), and K is a constant approximately equal to unity and related both to the crystallite shape and to the way that B and the thickness are defined. As observed in Table 3, the lamellar size decreased from C8 to C12 and then slowly increased from C14 onward. This behavior is similar to the results from IR and DSC studies where there was a decreasing trend up to a certain F


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Figure 7. (a) Evolution of G′ and G″ as a function of the angular frequency of C10−C16 gels. (b) Evolution of G′ and G″ as a function of the angular frequency of the C12 gel in different solvents. (c) Evolution of G′ and G″ as a function of the angular frequency of the C12 gel at different concentrations (30, 60, and 90 mg/mL).

finally the G″ value is greater than G′ after a critical shear stress, revealing the collapse of the 3D gel networks and the dominant fluid character of the organogel. This critical stress value is referred to as the yield stress (τy). To evaluate the effect of alkyl chain length upon the mechanical properties of the gels formed in the same solvent, rheological properties of C10, C12, C14, and C16 gels in chlorobenzene (30 mg/mL) were examined. The results are shown in Figure 8a. Comparison of the G′ values of the gels indicates that the storage modulus of a gel decreases with increasing chain length of the gelator. Among them, it is apparent that the value of C10 is larger than that of C12, and C16. Furthermore, the yield stresses of the gels are also dependent on chain length of gelators. Similarly, longer alkyl chain length corresponds to lower value of the yield stress. However, yield stress of the gel system of C14 is more than that of C10 and C12 gel. These results demonstrate clearly that the alkyl chain length of the gelators has a significant effect on the mechanical properties of the resulting gels.29,30,34,35 The effect of gelator concentration on the rheological properties of the supramolecular gels is shown in Figure 8b. The G′ value at a gelator concentration of 30 mg/mL is 20 695 Pa, that at 60 mg/mL is 41 823 Pa, and that at 90 mg/mL is 76 897 Pa. The corresponding yield stress increased from 1579 Pa (30 mg/ mL) to 4225 Pa (90 mg/mL), indicating that the concentration of the gelator has a great effect on the mechanical strength and the elasticity of the gels. These results suggest that the stability of the gel network depends on the concentration of the gelator and rheological properties such as the viscoelastic behavior of the gel, which is also enhanced by increasing gelator concentration.35 To study the effect of different solvents on

mechanical properties of the resulting gels.30−34 To study the effect of concentration, the C12 gels were prepared in chlorobenzene at various concentrations (30, 60, and 90 mg/ mL). Figure 7b shows that when the concentration was increased from 30 to 90 mg/mL, the G′ of the gel increased from 67 631 to 1.83 × 105 Pa, indicating that the 90 mg/mL gel exhibits more stiffness and a good tolerance to external force. The storage modules G′ and complex viscosity η* were found to decrease progressively with increasing chain length and increase with increasing concentration. To study the effect of solvent on the gel strength, the C12 gelator was chosen and gels (30 mg/mL) were prepared in different solvents such as benzene, toluene, chlorobenzene, and xylene. Benzene-based gels had larger G′ and G″ values, indicating that they are stronger than other gels. The order of strength is as follows: benzene > chlorobenzene > toluene > p-xylene. The results are shown in Figure 7c. Complex viscosity (η*) versus angular frequency (ω) graphs of gels at all variations are shown in Figure S4. 3.5b. Amplitude Sweep Studies on Gels. To investigate the effects of the alkyl chain length and concentrations of gelator and different solvents on the mechanical properties of the gels, G′ and G″ of the gel systems were measured as functions of shear stress (τ) at a constant frequency (f) of 1.0 Hz at 25 °C. The results are summarized in Figure 8 and Table S3. It can be seen that the value of G′ is much larger than the value of G″ with the small shear force at the beginning, suggesting the stability and the dominant elastic nature of the gel. Both G′ and G″ remain almost unchanged with the increase in the shear stress, and in a definite stress value they cross each other and G


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Figure 8. (a) Evolution of G′ as a function of shear stress for C10−C16 gels. (b) Evolution of G′ and G″ as a function of shear stress for the C12 gel in different solvents. (c) Evolution of G′ and G″ as a function of shear stress for the C12 gel in different concentrations (30, 60, and 90 mg/mL).

the mechanical properties of the gels, we chose the C12 gelator and prepared gels in benzene, chlorobenzene, toluene, and pxylene at 30 mg/mL. Figure 8c reveals that the values of G′ and τy of the gel decrease gradually, as we see from top to bottom. Benzene-based gel had the largest G′ and τy values, which means that the stiffness is greater than for the other gels. All of these observations demonstrate that the nature of the solvent has a significant influence on the mechanical properties of the corresponding gel.36 G′ and G′ versus strain graphs of gels at all variations are shown in Figure S5. The Tgel values determined are consistent with the phase-transition temperature values determined by DSC. 3.5c. Effect of Stimuli on Gel Strength. The strength of gels obtained by thermal and ultrasound treatment were compared through rheological studies. The results are summarized in Figure 9. The G′ of both organogels was greater than their corresponding G″ in the applied frequency range, suggesting solidlike behavior of the gels. However, the G′ and τy (48 199 and 2075 Pa) values of the C12 organogel obtained by ultrasound treatment were much greater than those of the organogel obtained by temperature treatment (G′ = 20 695 and 1040 Pa). This implies that the mechanical strength of the organogel resulting from ultrasound treatment is greater than that obtained by thermal treatment. 3.5d. Anion Tuning Properties of Gels. It was seen that anion binding by amide and urea can have a significant and controlled effect on the gel rheology; the strongest bound anions exhibited the most marked inhibition of gel strength. The effects of anions added as their tetrabutyl ammonium salts on gels of C12 in chlorobenzene were studied in greater detail.

Figure 9. Variation of G′ and G″ with shear stress (τ) for the C12 ultrasound- and temperature-treated organogel in chlorobenzene (30 mg/mL).

The salts were dissolved in 50 μL of acetonitrile, and these solutions were added to the gels to study the tuning properties. The presence of AN did not affect the stability of the gels; however, it aided the dissolution of the salt in the gel. The effect of added anion was monitored by the change in the storage modulus G′, which in all cases was found to change proportionally to the yield stress. From the obtained data, it could be inferred that the disintegration of gels was strongly H


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with varying spacer length and their influence on gelation properties were also confirmed by this study. Dimer structures of C8−C18 have been built in AMPAC,37 and a systematic conformational search was carried out using the random sampling method. The final geometries were selected from the most stable conformation, and their geometries were optimized at the M06-2X/6-31G (d)38 level of theory as implemented in the G0939 package. Frequency calculations were carried out on the optimized structures to ensure that the stationary points are minima and to compare the results with the experimental FTIR studies. The calculated molecular length shows an increasing trend ranging from 17.97 Å for C8 to 31.98 Å for C18, which is in agreement with the experimental WAXS diffractograms of all of the xerogels. (A summary of the molecular length of all of the dimers is given in Table S4 of the Supporting Information). The optimized geometry of the C8 dimer showing major hydrogen bond distances is shown in Figure S8, and the optimized structures of all other dimers with their hydrogen bond distances are given in the Supporting Information. As clear from Figure S8, there exist multiple intermolecular and intramolecular interactions (NH···OC and CO···HC) between two monomers. The fact that both of these forces work together in stabilizing the dimer structure is demonstrated by comparing the energetics of truncated dimer structures. The dimer structures were truncated in such a way that the gels have either the cyclohexane ring with only the functional groups responsible for H bonding or the cyclohexane ring with only the alkyl chains responsible for van der Waals forces. The results are compiled in Table S5. It is evident from this table that the major part of the stabilization energy of the dimer is due to the strength of H bonding, which is more or less constant at 25 kcal/mol. Interestingly, as the alkyl chain length increases from C8 to C18, the contribution of van der Waals forces toward overall stabilization energy is consistently increased. All of these observations further elucidate that the gelation process is enhanced up to short distances by the predominance of hydrogen bonding and with increasing chain length the van der Waals interactions will also cooperate in stabilizing the gelation network.40 It is well known that intermolecular interactions are the forces that are mainly responsible for aggregate formation in the gel state.40 The corresponding intermolecular hydrogen bond distances found in all of the dimers are summarized in Table 5. (For C8, see Figure S8, and for the other gels, see Table S6.) It is clear from the table that these distances increase

correlated with the anion binding strength. With the addition of 0.2 equiv of F− to the gel, τy decreased by 378 Pa, and on further increasing the anion concentration of F−, τy decreased gradually. The maximum concentration of anions incorporated (1 equiv) gave a τy of 30 Pa, and a further increase in concentration led to the solution state (Figure S6a, Table 4). Table 4. Rheological Properties of the C12 Gel (30 mg/mL) with a Varying Concentration of TBAF and Varying TBA Salts at the Same Concentration sample code C12 + 0 equiv of TBAF C12 + 0.2 equiv of TBAF C12 + 0.4 equiv of TBAF C12 + 0.6 equiv of TBAF C12 + 0.8 equiv of TBAF C12 + 1 equiv of TBAF C12 + 1.2 equiv of TBAF C12 + 1.5 equiv of TBAF

yield stress (Pa) 1040 378 212 201 56 30

sample code C12 + 0.5 equiv of TBAF C12 + 0.5 equiv of TBAOAC C12 + 0.5 equiv of TBACl C12 + 0.5 equiv of TBABr C12 + 0.5 equiv of TBANO3 C12 + 0.5 equiv of TBAI

yield stress (Pa) 215 219 294 399 748 847

sol sol

The strength of anions on N−H binding was studied by fixing 0.5 equiv of different types of anions (Figure S6b and Table 4). Interestingly, F− and AcO− had almost the same τy values (215 and 219 Pa), whereas Cl− and Br− had τy ≈ 294 and 399 Pa, respectively. Anions NO3− and I− had progressively weaker effects, as shown by the fact that these anions displayed higher τy values (748 and 847 Pa). It can be concluded that the anion binding for the C12 gel decreases in the sequence F− > AcO− > Cl− > Br− > NO3− > I− > BF4−, correlating to decreasing τy. The strong binding of F− on the C12 gel is caused by the high basicity of the anion, which can be due to the disruption of hydrogen-bonding interactions as a result of the deprotonation of the hydrogen-bonding moiety. These results clearly indicate the selective chemical response of the C12 gel toward the fluoride anion, resulting in a gel-to-sol transition. 3.6. 1H NMR Studies. To further understand the interaction between the C12 gelator of N−H and F− of TBAF, the 1H NMR spectra of the C12 gelator were recorded by adding different amounts of F− as shown in Figure S7. With 0.5 equiv of F− added, the proton signal assigned to the urea and amide group disappeared but the N−H (beside the cyclohexane ring) was found to be less intense with a signal shifted downfield from 5.9 to 6.8 ppm. Subsequently increasing the concentration of F− to 1 equiv led to the complete disappearance of the N−H signal. This implied that the amide and urea groups either interacted with F− through hydrogen bonding or were deprotonated by excess amounts of F−. Thus, it is reasonable to assume that the gel−sol transition of C12 originated from the disruption of the hydrogen-bonding interaction between the urea and amide groups in the presence of F−. 3.7. Computational Studies. Computational calculations have been carried out to understand the nature and role of noncovalent interactions responsible for the formation of aggregates in gels. The extent of these noncovalent interactions

Table 5. Summary of Intermolecular and Intramolecular Bond Distances (Å) and CO and N−H Bond Stretching Frequencies (cm−1) in the Six Gel Dimersa Sl no. b

1 2b 3b 4c 5d 6d

atoms involved

C8

C10

C12

C14

C16

C18

O130−H26 O137−H34 N25−H108 O45−H74 C129−O130 N127−H128

2.09 2.55 2.44 2.54 1762 3606

2.12 2.47 2.46 2.40 1761 3623

2.12 2.41 2.46 2.39 1761 3629

2.14 2.62 3.01 2.38 1761 3635

2.10 2.57 2.57 2.44 1761 3607

2.10 2.56 2.52 2.38 1759 3613

a

The numbering of the atoms is with respect to the C8 dimer in Figure S8, whereas for the rest of the dimers, the numbering of the respective atoms is the same as in Table S6. bIntermolecular hydrogen bond distance. cIntramolecular hydrogen bond distance. dMajor Hbond stretching frequencies.

I


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The rheological studies confirmed the viscoelastic nature of the gels and the influence of gelator concentration and spacer length on the stiffness of the gels. Self-assembly was through the structures that tend to associate side by side so as to maximize both the possible van der Waals interactions between the aliphatic chains and the hydrogen bonds. The length of the aliphatic chains has a great influence not only on the gelation ability but also on the thermal and rheological behavior.

until C14, after which they decrease and remain almost unchanged. Consequently, the intramolecular hydrogen bond distance is found to decrease until C14 and then to increase for C16 and C18. This trend is in agreement with the experimental FT-IR and XRD studies. These observations further substantiate that the gelation process is enhanced up to short distances and then gelates improperly as the chain length increases. A summary of the computationally calculated molecular lengths of all of the dimers is given in Table S4 of the Supporting Information Frequency vibrational analysis is done on dimer structures to understand the pattern of hydrogen bonding in N−H···OC bonds. It is seen in the FT-IR studies that as the chain length increases, the wavenumber of the N−H bond stretch increases until C14 and then decreases for C16 and C18. A similar trend is observed in the computationally calculated N−H bond stretch shown in Table 5, which increases until C14, decreases steeply at C16, and increases further for C18. However, for the CO bond stretch, in both experimental and theoretical studies there is a decreasing trend in wavenumber from C8 to C18 as seen in atoms C129−O130 in C8. A schematic representation of the possible mode of aggregation is as shown in Figure 10.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03987. 1 H NMR gelators, digital photograph of C8−C18 gels, IR overlay of C8−C18 xerogels, properties of gelators and gels table, rheology graphs of gels, storage modulus and loss modulus as a function of strain, anion tuning graphs overlay by stress sweep experiments, XRD data, computationally calculated molecular length and plane of hydrogen bonding table, optimized geometries of C10−C18 gelators, and summary of stabilization energies of C8−C18 dimer structures (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail:[email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS S.B. is indebted to CSIR, India, for senior research fellowship. REFERENCES

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Figure 10. Schematic representation of the C12 gelator showing the (a) molecular length, (b) model of the molecular aggregate, (c) model of the probable mode of packing within the gel, (d) cross section, and (e) xerogel SEM image.

4. CONCLUSIONS A novel class of bis(acyl-semicarbazide) gelators with varying chain length of side groups (C8−C18) were found to exhibit gelation in some aromatic solvents. These organogels exhibited multi-stimuli-responsive behavior to external stimuli such as temperature, ultrasound, and anions. The gel systems exhibited a gel−sol transition because of the disruption of intermolecular hydrogen bonds and the selective recognition of F− and AcO− stimuli at 1 equiv concentration. Other halide anions (Br−, I−, and NO3−) led to gel disruption at higher concentration. The mechanical strength of the ultrasound-derived organogel was observed to be higher than that of the thermally derived gel. The spectral, thermal, and diffraction studies confirmed the mechanism of gelation, which strongly depended on the balance between the two secondary forces of H bonding and van der Waals interactions. Minute variations in the side-chain length led to drastic changes in the morphology, and the computational studies substantiated the experimental results. J


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Combined Experimental and Computational Study of the Gelation of

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