Low Molecular Weight Organogels and Their Application in the

Nov 5, 2012 - (LMW) organogels, organic liquids get immobilized in a supramolecular structure created by a gelator, possibly via intermolecular H-bond...
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Low Molecular Weight Organogels and Their Application in the Synthesis of CdS Nanoparticles Pradipta Kumar,† Mahesh M. Kadam,† and Vilas G. Gaikar*,† †

Department of Chemical Engineering, Institute of Chemical Technology, Nathalal Parekh Marg, Matunga, Mumbai-19, India S Supporting Information *

ABSTRACT: Two new fatty acid based derivatives containing carboxylate and amide functions with different alkyl chain lengths were synthesized, and their properties as low molecular weight (LMW) gelators for organic solvents such as nitrobenzene, chlorobenzene, dichlorobenzene, and dioxane are evaluated. The xerogels under Scanning Electron Microscope (SEM) show self-assembled fibrillar and flowerlike structures. Rheological properties of the gelator forming an extended fibrillar network were studied to verify the solidlike behavior of the gel. The role of intermolecular hydrogen bonding in the formation of the supramolecular structures was confirmed through FTIR spectroscopy. The nitrobenzene based organogel superstructure was used as a template for synthesis of CdS nanoparticles. The nanoparticles were characterized through UV−vis spectroscopy and Transmission Electron Microscopy (TEM). nucleotides, and dendrimers,25,26 have been discovered. Among them, the fatty acid based gelators27−29 are the simplest organic molecules that are easy to synthesize at low cost. They are also capable of gelling a wide variety of organic solvents and intermolecular hydrogen bonding is the key force behind the process. Herein, we report the synthesis of N-(2hexadecanoylaminoethyl)succinamic acid (NHD-AES) as a potent gelator for nitrobenzene and the subsequent use of the nitrobenzene gel template for synthesis of CdS nanoparticles. NHD-AES was obtained by derivatization of methyl palmitate with ethylene diamine followed by condensation with succinic anhydride. The gelator contains both carboxylate and amide functions that help in forming multipoint attachment of strong intermolecular hydrogen bonds between neighboring molecules aligned axially resulting in the formation of very robust gels.30−33 Although, low molecular weight organogelators based on monochain derivatives of ethylenediamine27 and oxalyl amide29 have been synthesized and evaluated as nitrobenzene gelators, their respective minimum gelation concentration (mgc) values are quite high (Table 1). The present work also includes the synthesis and the gelling property of N,N′ bis-(hexadecanoyl)-1,2-diaminoethane (BHD-DAE) as another gelator. There exists a significant difference in the mgc values and the stability of the organogels derived from these two gelators.

1.0. INTRODUCTION Organogels are aggregates of gelators in organic liquids to produce viscoelastic semisolid materials that exhibit no flow property at the steady-state. In the low molecular weight (LMW) organogels, organic liquids get immobilized in a supramolecular structure created by a gelator, possibly via intermolecular H-bonding,1−3 van der Waal forces,4 and aromatic stracking.5,6 The nature and packing of the molecules in a gel decides its thermodynamical, mechanical, and optical properties. The stability, an important aspect of an organogel, depends upon the concentration of the gelator and storage temperature. The cooling rate while preparing the gel also alters the tertiary network structure of the gelators affecting the network’s ability to retain the solvent and the gel strength as well as the appearance of the gel.7 Due to the noncovalent nature of the interactions involved among the aggregates, the gelation is a thermoreversible process and a gelator is said to be efficient if it can immobilize the solvent at concentrations ≤2 wt %.4,8 The physical properties of a gel like the transition temperature (Tgel), viscosity, and elasticity change with the change in the nature of the network, responsible for gel formation. Gels with a long-range and cross-linked aggregation among gelator molecules result in a robust network compared to a short-range and unidirectional packing forming a transient gel.6,9 Due to their numerous applications in areas like photochemistry,10,11 organic templates for designing inorganic nanomaterials,12,13 electrochemistry,14,15 molecular recognition,16,17 cosmetics, lubrication, and healthcare,18−20 the organogelators contribute an important facet to the chemical industry. The progress in the field of nanomaterial synthesis using organogels is attracting greater attention from many researchers. The development of inexpensive and efficient organogelators is of greater interest. In the past few decades, a significant variety of LMW gelators, like steroids,21 carbohydrates,22,23 amino acids,24 organometallic compounds,6 urea,3 © 2012 American Chemical Society

2.0. EXPERIMENTAL SECTION 2.1. Materials. Palmitic acid, ethylene diamine (ED), palmitoyl chloride, succinic anhydride, pyridine, CdCl2, and Na2S·xH2O (55−58%) were purchased from s.d. Fine Chemicals and all are of AR grade. All organic solvents used Received: Revised: Accepted: Published: 15374

April 10, 2012 October 27, 2012 November 5, 2012 November 5, 2012 dx.doi.org/10.1021/ie300947x | Ind. Eng. Chem. Res. 2012, 51, 15374−15385

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Absorption spectrum was recorded on a Perkin-Elmer UV− vis 2700 double beam spectrophotometer with an optical path length of 1 cm. Fluorescence spectrum of as-prepared CdS was recorded on a JASCO-FP 6500 spectrofluorometer. The H NMR spectra were recorded on a Mercury plus 300 MHz NMR spectrometer (Varian, USA), where the chemical shifts are given in ppm. The H spectra were calibrated with an internal TMS signal (0.0 ppm). The phase transition temperatures (melting point) of the products were measured by Differential Scanning Calorimetry (SHIMADZU-DSC-60) with a heating/ cooling rate of 5 °C/min from 30 to 300 °C. The morphological analysis of the xerogels was performed by Scanning Electron Microscopy (SEM-JEOL-JSM). The xerogels were coated with platinum before the scanning process. The electric current was 15 mA, and the accelerating voltage was 20−30 kV. The Transmission Electron Microscope (TEM) images were collected using a Philips-CM200 microscope with an operating voltage of 200 kV. The sample 1 mg was added to 5 cm3 of spectroscopic grade methanol and allowed for sonication for 15 min. A drop of the sample was deposited and dried on a grid before visualization. An Anton Paar MCR-301 Rheometer was employed to analyze the rheological properties of the gels. Due to the highly viscous nature of these physical gels, dynamic tests were performed using parallel-plate (PP-25) geometry with a gap size of 1 mm. The amplitude sweep of the elastic and viscous moduli (G′ and G′′) was collected for strain values from 10−1 to 102%. In the frequency sweep test, G′ and G′′ were determined in the regime of linear viscoelasticity as a function of angular frequency. In the frequency sweep, the angular frequency was varied from 10−1 to 102 rad/s at constant 0.01% strain. The relaxation time spectrum was studied for G′ and G′′ at constant 1% strain at varied frequency from 10 rad/s to 600 rad/s. Hystersis area at shear rate 1 to 50 1/s was carried out for the samples to check the structural loss of the gel because of the applied stress. 2.3. Synthesis of Intermediates and Gelators. The syntheses of the desired products were carried out according to the procedure described below, following Scheme 1. Methyl palmitate was synthesized by passing dry HCl gas to the solution of palmitic acid in methanol.34

Table 1. Gelation Behavior of the Gelators in Various Solvents at Room Temperature (27−30 °C) against Their mgc Values and a Comparison with Recently Reported Literature for the Same Solvents gelators (mgc)a solvents chlorobenzene dichlorobenzene nitrobenzene 1,4-dioxane

NHDAES

BHDDAE

CG (1.0) CG (1.0) CG (1.0) OG (2.0)

OG (35)b OG (35)b OG (30)b OG (30)b

mgca oxalyl amide derivatives Suzuki et al.29 8−45 NT 10−40 18−45

a

Minimum gelation concentration (mg/cm3); OG (opaque gel); CG (clear gel). bOG (gels formed after cooling the respective solutions to 5−10 °C and with a low value of Tgel between 25−30 °C); NT (not tested), mgc values for nitrobenzene reported by Xuzhong Luo et al. (ref 27) is between 4 and 14 mg/cm3.

for carrying out reactions and for gelation tests were of AR grade from Spectrochem Pvt. Ltd. All reactions were conducted using oven-dried glassware under N2 atmosphere. The progress of the reactions was monitored by Thin Layer Chromatography (TLC) using Merck silica gel 60F254 precoated aluminum sheets and visualized by exposing the TLC plates to iodine vapors. 2.2. Methods. The characterization of all intermediates and the final products was carried out through spectroscopic techniques. FTIR spectra were measured with KBr pellets using a Bruker-VERTEX 80v vacuum FTIR spectrometer aligned with an Ultra−Scan interferometer (peak resolution of 1 cm−1). The mass spectra were recorded on a Finnigan LCQ Advantage Max mass spectrometer (LCQAD 30000, Thermo Electron Corp.) using methanol as a mobile phase, where dry nitrogen was used as a nebulizer gas as well as an auxiliary gas with a flow rate of 40 and 18 cm3/min, respectively. The capillary voltage, spray voltage, and the capillary temperature were set at 420 V, 5 kV, and 548 K with peaks noted by m/z values.

Scheme 1. Synthetic Scheme for Gelators NHD-AES and BHD-DAE

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(νas C−H of CH2), 2849.14 cm−1 (νs C−H of CH2), 1638.22 cm−1 (CO of amide), 1556.99 cm−1 (C−N bending), 1472.07 cm−1 (CH2 bending). MS (EI) m/z (%): 535.87 (100%,-ve mode) [M+], 1095.87 (100%) [2M+ + Na], 817.87 (50%) [M+ + 281], 281.47 (30%) [M+ − CH3(CH2)14 CONH2]. 1H NMR (300 MHz, C6D6, 25 °C, TMS) δ 5.31 (NH, S broad), δ 3.4 (4H, t, J = 5.8 Hz), δ 1.96 (4H, t, J = 6.9 Hz), δ 1.57 (4H, quintate, J = 6.9 Hz), δ1.24 (48H, m), δ 0.94 (6H, t, J = 6.9 Hz). 2.4. Gel Formation and Phase Transition Temperature (Tgel). A number of solvents were employed to evaluate the gelling ability of the newly synthesized gelators. A known quantity of the gelator was mixed with an organic solvent in a sealed test tube, and the mixture was heated in an oil bath between 80 to 90 °C till the solid dissolved in the solvent to get a clear solution. The resulting solution was then kept at room temperature (27−30 °C) for 15−20 min so as to allow the solution to go through a slow cooling process. Finally, the test tube was inverted to observe the gelation as indicated by no flow of the organic solvent out of the gel. The gel−sol phase transition temperature was measured using a standard falling ball method.35 2.5. Preparation of Xerogels. Xerogels were prepared by exposing the organogels to air for slow drying for 3 days at 27 ± 3 °C allowing minimum disturbance to the surface. 2.6. Synthesis of CdS. N-(2-Hexadecanoylaminoethyl)succinamic acid (NHD-AES) (600 mg, 1.5 mmol) was dissolved in nitrobenzene (60 cm3) at 100−110 °C and then allowed to cool slowly to 25−30 °C so as to form a gel. Then a solution of cadmium chloride (50 mg, 0.27 mmol) in water (0.5 cm3) was added, and the mixture was stirred at 25−30 °C for 30 min for uniform mixing. To the above mixture then a solution of sodium sulfide (50 mg, 0.33 mmol) in water (0.25 cm3) was added dropwise over a period of 2 h with constant stirring (700 rpm). The reaction mixture was stirred for further 4 h at the same temperature for complete precipitation of cadmium sulfide. The reaction mass was poured into DMF (100 cm3) to dissolve the gelator, separating out the solid CdS, which was centrifuged at 8000 rpm for 10−15 min using DMF and water as solvent separately to get rid of the gelator and soluble inorganic salts. The isolated CdS was then dried at 500−600 °C for 4 h (Yield, 30 mg) and characterized by TEM.

2.3.1. Synthesis of Hexadecanoic Acid-(2-Amino-ethyl)amide. Ethylene diamine (30 cm3, 0.44 mol) was added to methyl palmitate (10 g, 0.037 mol) at 25−30 °C, and then the solution was refluxed for 5 h. Water (100 cm3) was then added to the reaction mass after removing excess ethylene diamine by distillation under reduced pressure and was stirred for 30−45 min to get free-flowing solids. The solid product was filtered and washed with water till the pH of the filtrate became neutral. The crude solid was recrystallized once from toluene and twice from ethanol to get crystals of the desired product (Yield, 9.3 g). Rf 0.1 (methanol). DSC (mp 82−86 °C), IR: ν = 3303 cm−1 (νas N−H amide), 3209 cm−1 (νs N−H amide), 3085.24 cm−1 (overtone NH), 2953.73 cm−1 (C−H of CH3), 2918.14 cm−1 (νas C−H of CH2), 2849.69 cm−1 (νs C−H of CH2), 1637.25 cm−1 (CO of amide), 1555.73 cm−1 (C−N bending), 1468.64 cm−1 (CH2 bending). MS (EI) m/z (%): 299.33 (100%) [M+], 281.53 (30%) [M+ − NH2], 595.95 (30%) [2M+]. 1H-NMR (300 MHz, C6D6, 25 °C, TMS) δ 5.31 (NH, S broad), δ 3.08 (2H, qt, J = 5.86 Hz), δ 2.328 (2H, t, J = 5.86 Hz), δ 1.908 (2H, t, J = 6.9 Hz), δ 1.690 (2H, quintate, J = 6.9 Hz), δ1.324 (24H, m), δ 0.940 (3H, t, J = 6.9 Hz). 2.3.2. Synthesis of N-(2-Hexadecanoylamino-ethyl)-succinamic Acid (NHD-AES). Succinic anhydride (1.67 g, 0.016 mol) was added to hexadecanoic acid-(2-aminoethyl)-amide (5 g, 0.016 mol), suspended in toluene (50 cm3) at 25−30 °C, and then stirred at reflux for 7 h. Methanol (30 cm3) was added after removing toluene from the reaction mixture under reduced pressure to separate out crude solids. The product was filtered, recrystallized thrice from ethanol, and then dried under reduced pressure at 70 °C for 1 h (Yield, 5.4 g). DSC (mp 179−186 °C), IR: ν = 3303.18 cm−1 (νas N−H amide), 3208.81 cm−1 (νs N−H amide), 3081.94 cm−1 (overtone NH), 2953.74 cm−1 (C−H of CH3), 2920.20 cm−1 (νas C−H of CH2), 2850.45 cm−1 (νs C−H of CH2), 1695.47 cm−1 (CO of acid), 1638.49 cm−1 (CO of amide), 1556.34 cm−1 (C−N bending), 1472.02 cm−1 (CH2 bending). MS (EI) m/z (%): 420.67 (100%) [M+ + Na], 398.07 (40%) [M+], 298.60 (15%) [M+ − CO(CH2)2COOH], 1H NMR (300 MHz, DMSO−D6, 25 °C, TMS) δ12.07 (1H, S broad), δ 7.7 (2H, S, broad), δ 2.434 (4H, t, J = 6.23 Hz), δ 2.308 (4H, t, J = 6.59 Hz), δ 2.047 (2H, t, J = 7.3 Hz), 1.459 (2H, quintate, J = 7.3 Hz), δ 1.231 (24H, m), δ 0.871(3H, t, J = 6.6 Hz). 2.3.3. Synthesis of N,N′ Bis-(hexadecanoyl)-1,2-diaminoethane (BHD-DAE). Palmitoyl chloride (4.59 g, 0.016 mol) was added slowly to a solution of hexadecanoic acid-(2-aminoethyl)-amide (5.0 g, 0.016 mol) in pyridine (50 cm3) at 5−10 °C over a period of 30 min and then stirred at 60−70 °C for 5 h. Pyridine was distilled out under reduced pressure at 40−50 °C to separate out solids upon addition of water (100 cm3). The solid was filtered and then washed with 0.1 N HCl (30 cm3) to remove traces of pyridine and then recrystallized thrice from ethanol to get BHD-DAE (Yield, 7.2 g). In an alternate process, palmitoyl chloride (5.0 g, 0.018 mol) was added slowly to a solution of ethylene diamine (0.54 g, 0.009 mol) in pyridine (50 cm3) at 0−5 °C over a period of 30 min, and then the reaction mass was stirred at 60−70 °C for 5 h. The process for isolation of the crude product and its purification was the same as per the previous section (Yield, 3.8 g). DSC (mp 149−152 °C), IR: ν = 3299.17 cm−1 (νas N−H amide), 3208.64 cm−1 (νas N−H amide), 3085.15 cm−1 (overtone NH), 2953.73 cm−1 (C−H of CH3), 2916.93 cm−1

3.0. RESULTS AND DISCUSSION The minimum gelation concentration (mgc) and the transition temperature (Tgel) are the key properties of a gel. Table 1 summarizes the gelation capability of the gelators (NHD-AES and BHD-DAE) toward different organic liquids with their mgc values, where the sol-phase changes into a stable gel-phase at room temperature. Both the gelators form a gel with nonprotic organic solvents, such as chlorobenzene, dichlorobenzene, nitrobenzene, and dioxane. Whereas in protic solvents like ethanol and propanol, both gelators could be dissolved by heating but precipitate out on cooling instead of forming a gel. Protic solvents possibly prevent the potential supramolecular aggregation suggesting that the weak intermolecular hydrogen bonding is the driving force for the gelation. Aprotic solvents encourage the H-bonding and much better packing characteristics of the gel. The gelator molecules in the present study are designed to have multiple points for H-bonding that ensure lateral interactions among the functional groups, while van der Waal interactions among the neighboring molecules lead to closer packing of these structures. The presence of a long 15376

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Figure 1. Variation of Tgel with concentration of NHD-AES in different solvents.

Figure 2. Plot of log C of NHD-AES against the reciprocal of absolute temperature.

effective gelator for these solvents with mgc values ≤0.2 wt %. The respective values for gelator BHD-DAE are quite high i.e. between 30−35 mg/cm3. There is a considerable difference in their mgc values, and the gelator NHD-AES forms strong Hbonding compared to gelator BHD-DAE which is responsible for the gelation as is evidenced from their respective FTIR measurements discussed later in this report.

hydrocarbon chain in the molecule facilitates the solvation process. The lowest gelation concentration (mgc) test demonstrated that nitrobenzene, dioxane, chlorobenzene, and dichlorobenzene are the most easily gelled solvents with mgc values, as low as 1.0, 2.0, 1.0, and 1.0 mg/cm3, respectively, for the gelator NHD-AES. Thus gelator NHD-AES is considered to be an 15377

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Figure 3. SEM images of xerogels obtained using NHD-AES in (A) nitrobenzene, (B) chlorobenzene, (C) 1,4-dioxane, (D) dichlorobenzene, (E) nitrobenzene (higher magnification), and (F) powder form of the gelator showing the crystal nature.

other this suggests, however, that the role of the solvent is having less significance. A little higher enthalpy of disaggregation in the case of nitrobenzene is due to the higher solubility of the gelator NHD-AES compared to other solvents. The SEM images of the xerogels obtained from the gelator NHD-AES in different solvents show fibrillar networks responsible for the gelation process (Figure 3). The gelator spontaneously selfassemble through H-bonding into entangled fiberlike aggregates with an approximate diameter between 1−2 μm. The images also clearly illustrate the unidirectional growth of the structure resulting in the formation of fibers with an approximate length of more than 100 μm. Because of weak intermolecular interactions the gelator BHD-DAE, on the other hand, aggregates form well developed short-range flowerlike structures (Figure 4). A comparison of FTIR spectra of gelator NHD-AES in the powder form and its xerogel was made (Figure S2 of the Supporting Information). In the solid form, the IR bands, arising from non-hydrogen bonded amide groups, were observed at 3303 cm−1 (νas N−H str), 3208 cm−1 (νsy N−H str), 3081 cm−1 (overtone NH), 1638 cm−1 (CO amide str), and 1556 cm−1 (N−H bend), whereas the IR spectrum of the gelator in xerogel showed the respective bands at 3288 cm−1, 3203 cm−1, 3071 cm−1, 1631 cm−1, and 1554 cm−1, which are characteristics of hydrogen bonded amide groups.37 The peak

Optical properties of the gels also get altered on changing the nature of the solvent and concentration of the gelator. A clear distinction can be made with the change from a clear gel using nitrobenzene to an opaque gel using dioxane keeping the gelator NHD-AES constant (Figure S1 of the Supporting Information). The critical temperature (Tgel) of a gel system is the temperature at which the system experiences a gel−sol (or sol− gel) phase transition and is an important parameter to describe the gel stability. The thermal stability of the gels increased with the concentration of the gelator NHD-AES, in a nearly linear fashion as shown in Figure 1. A plateau or concentration independent region is reached at a much lower gelator concentration i.e. after 10 mg/cm3 in all the cases. The gels of nitrobenzene and dichlorobenzene were found to have higher Tgel values. The enthalpy of disaggregation was estimated from the plot of log C vs the reciprocal of Tgel (Figure 2) using the Schrader’s relation (eq 1)36 log[C ] = −

ΔH 1 * + constant 2.303R Tgel

(1)

where R is the universal gas constant, and ΔH is the melting enthalpy in kJ/mol. The estimated ΔH values are in the range of 41−44 kJ/mol. As the ΔH values are relatively close to each 15378

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Figure 4. SEM images of xerogels obtained using BHD-DAE in (A) nitrobenzene, (B) chlorobenzene, (C) dichlorobenzene, (D) dioxane, (E) powder form of the gelator, and (F) magnified image of the gelator.

at 1694 cm−1 of the xerogel, ascribed to the CO stretching mode of the CO2H groups is the characteristic of the formation of carboxylic acid dimers.38,39 An insignificant change was observed in the shift of the IR bands of gelator BHD-DAE indicating the presence of weak intermolecular interactions (Figure S2 of the Supporting Information). A comparison for the respective IR bands between both the gelators is shown in Table 2. The strong bands at 2916 (C−H asy) and 2849 (C−H sy) are due to trans (highly ordered) methylene groups in the chains of both the gelators.28,40

Rheological Study of Nitrobenzene Gel (NHD-AES). The amplitude sweep experiment was performed to find the resistance to deformation at a constant frequency of 5 rad/s. Figure 5 shows a comparison between two different gelator concentrations (10 and 20 mg/cm3). For a very low shear strain, the gel seems to behave linearly and G′′ is constant, but a subsequent increase in the shear strain, above a critical value, causes an increase in G′′ before it drops further, showing typical characteristics of a Maxwell fluid (Figure 5).41 The value G′ is much larger than that of G′′ before the strain reaches to a crossover point (flow point), indicating strong elastic character of the sample. Beyond the crossover point, the decrease in the modulus G′ value can be attributed to a partial breakup of the gel that begins to flow. The crossover point (G′=G′′) of 20 mg/cm3 gelator concentration is about 2 orders of magnitude higher than that of 10 mg/cm3. The crossover points (G′=G′′) were found at 0.033 and 0.067 MPa at corresponding shear stress (τ) of 0.24 × 10−3 and 0.92 × 10−3 MPa, respectively for the lower and higher gelator concentrations. Linear viscoelastic regions (LVE), which are independent of the magnitude of deformation, were found to be 0.0333% and 0.0356% strain (γ) for the gelator concentrations, 10 mg/cm3 and 20 mg/cm3, with corresponding shear stress (τ) of 0.056 × 10−3 and 0.015 × 10−2 MPa, respectively. Thus at higher gelator concentration

Table 2. Comparison of FTIR Frequencies in Support of HBonding NHD-AES IR bands N−H str. (as) N−H str. (s) N−H overtone CO amide N−H bending

BHD-DAE

powder form (cm−1)

Xerogel (cm−1)

powder form (cm−1)

Xerogel (cm−1)

3303

3288

3299

3295

3208 3081

3203 3071

3208 3085

3207 3072

1638 1556

1631 1554

1638 1556

1637 1556

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Figure 5. Determination of the linear regime: measurement of the evolution of G′ and G″ as a function of the applied strain at constant frequency 5 rad/s.

Figure 6. Measurement of the evolution of G′ and G″ as a function of the applied frequency at constant strain of 0.01.

magnitude higher than the G′′ over the entire frequency range. This type of dynamic response is a characteristic of gel-like materials.42,43 The elastic modulus (G′) of a gel system correlates with the rigidity (stiffness) of the network where G′ is independent of the frequency. Thus, we expect that more rigid structures were formed with increasing gelator concen-

the cross-linking density increases and the network of gels strengthens. Figure 6 shows that the storage modulus (G′) was always larger than the loss modulus (G′′) i.e., the elastic component is dominant over the viscous component of the gel. The frequency independent elastic modulus G′ is about 1 order of 15380

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Figure 7. Measurement of relaxation time at two different gelator concentrations as a function of H(λ) vs time.

Figure 8. Determination of hysteresis area (structural loss) of the gel as a function of the applied shear rate.

tration. The G′ increased linearly with a frequency from 10−1 to 102 Hz showing polymer-like viscoelasticity.42−44 The constant G′ at low frequencies mean that it is sufficient to form the entanglement or cross-links at even low gelator concentration of 10 mg/cm3.41 The monotonic increase in G′ at higher frequencies means the partial breakage of the interconnected network, inferred from the existence of the plateau region at lower frequencies, which represents a true cross-linked polymer gel network, reflecting the gel stability to external deforming forces. The stability of gel was also further proved by hysteresis and relaxation measurements.

Relaxation time spectrum is related to dynamic moduli G′ and G′′. The conversion from the frequency to the time domain takes advantage of the fact that G′′ probes the spectrum locally, while G′ is a more integral measure of the relaxation.45,46 A standard program was used to express the dynamic mechanical data.47 A typical relaxation time spectrum H(λ) of gel as a function of gelator concentration at 30 °C is shown in Figure 7. H(λ) is a parameter relating to the strength of the gel network formed due to gelator concentration. The higher relaxation time at 20 mg/cm3 was due to greater gelator network growth and cross-linking. The longest relaxation times 15381

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Figure 9. HR−TEM images of synthesized CdS (A and B), histogram showing the particle size distribution (C), and SAED pattern of as-prepared CdS (D).

(λ) was 0.37 s for 10 mg/cm3. At gelator concentration of 20 mg/cm3, it was 1 s indicating more elongated and entangled fibers which was also supported by the SEM images of the xerogels. Figure 8 shows the hysteresis area as structural loss of the gel after getting subjected to the stress. At increased gelator concentration of 20 mg/cm3, the hysteresis area decreased from −1,902 Pa/s to −2,347 Pa/s. As the shear rate was increased from 1 to 50 1/s, the viscosity of the sample decreased. On reversal of the shear rate from 50 to 1 1/s, the viscosity regained with minimum structural loss. The structural loss observed at the higher gelator concentration is also less as compared to that with at lower gelator concentration which suggests that the gel gets deformed by the initial shear stress but recovers its elastic property more easily after removal of the external stress at higher gelator concentrations. The rheological study of the nitrobenzene gel demonstrated about its viscoelastic nature and good strength against deforming stress. Its higher relaxation times makes it a suitable template for synthesizing nanoparticles based on particlecapping technique, where the extended network of the gel acts as a continuous phase spanning over the reacting liquid as the dispersed phase. The aim was to synthesize the CdS molecule inside the gel network so that the particles can grow in a controlled way. In this process, the first step involves the complete dispersion of the precursor (CdCl2 solution) into the continuous gel phase. In the second step, release of the aqueous solution of Na2S to the system initiates the growth of the CdS particles within the gel network. This giant gel matrix acts as a host to manage the growth of CdS particles to nanometer level. The high kinetic energy of the surface atoms of the nanoparticles makes them extremely reactive to undergo

particle agglomeration by self-coordination. The use of capping agents like gels reduces the surface energy of the particles through their coordination with the extended gel network and thus inhibiting the particle growth. The gels with the extended network, therefore, help in separating the process of nucleation from particle growth.48 The gelator concentration also plays an important role in the reduction of particle size. As the concentration was reduced to half, an increase in particle size (15−30 nm, Figure S3 of the Supporting Information) was observed suggesting the involvement of the gel network in the growth process. An attempt to decrease the particle size below 5 nm with the gelator concentration 20 mg/cm3 and above was unsuccessful. Further to confirm the participation of the gel suprastructure in regulating the particle size, a controlled experiment was carried out where CdS was synthesized first followed by formation of gel and then the whole system was kept for 6−7 h at 25−30 °C. The product obtained was isolated as per the experimental section 2.6 and analyzed through Scanning Electron Microscopy (SEM). SEM image (Figure S4 of the Supporting Information) shows the sizes of the particles obtained are between 50 μm−500 μm which is comparable to the bulk CdS. Effect of Agitation/Addition of Precursors to NHDAES-Nitrobenzene Gel. In the process for synthesis of CdS, the prepared NHD-AES-nitrobenzene gel (Section 2.4) was stirred at 700 rpm. The effect of stirring on gel deformation was studied through the rheological parameters (storage and loss modulus). The magnitude of G′ and G′′ was found to decrease by a factor of 1.5-fold (Figure S5 of the Supporting Information) but as G′ > G′′, the viscoelastic behavior in the stirred-gel is retained. Further, stepwise addition of precursors (solutions of CdCl2 and Na2S in water) has an insignificant 15382

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effect on the gel property as is evident from the figure (Figure S5 of the Supporting Information) which suggests that the aggregation pattern responsible for gel formation is unperturbed. Table S1 (Supporting Information) shows the drop in the viscosity component over the entire process. Characterization and Optical Properties of AsPrepared CdS. Figure 9 (A and B) shows the HR−TEM images of CdS particles synthesized in the nitrobenzene gel of NHD-AES, which clearly illustrates the presence of individual particles as well as aggregates of the nanoparticles. Aggregation was observed due to interaction among the particles even after allowing sufficient sonication time in methanol. These images also show that the particles are spherical and uniform in shape and size. The size distribution of CdS nanocrystals calculated from the TEM measurement is shown in Figure 9(C). The particle size ranges from 10−20 nm with an average or mean diameter of 15 nm. TEM image reflects mostly the particle aggregation due to high surface free energy which forbids the examination of a single particle. The Selected Area Electron Diffraction pattern of some nanoparticles shows the crystalline nature of the as-prepared CdS (Figure 9D). The sharp diffraction rings can be assigned to (111), (220), and (311) planes confirming the cubic phase (FCC) of CdS by comparison with the data from JCPDS 10-454. An attempt was made to correlate the size distribution obtained from TEM measurement with spectroscopic data. The UV−vis spectrum of well dispersed CdS solution in methanol showed an absorption maximum at 510 nm (Figure S6A of the Supporting Information). CdS nanoparticles display size quantization properties, where the absorption is related to the size of the particles. Thus from the UV−vis absorption spectrum one can calculate the mean particle size with the help of band gap energy. The band gap of a material is defined as the energy difference between the valence and conduction bands. The energy corresponding to this i.e. Egnano was calculated using eq 249 (σh υ)2 = K (h υ − Egnano)

peak at shorter wavelength. Literature data revealed the appearance of an emission band in the range of 450−750 nm as the crystal size of CdS varies.51,52 A broad emission spectrum from 450 to 650 nm centered at 530 nm (Figure S7 of the Supporting Information) was obtained for the as-prepared CdS nanoparticles with an excitation wavelength of 360 nm. The broadening of the fluorescence peak is due to a range of individual emissions which arises as a result of particle size distribution as is evidenced from the histogram.

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CONCLUSION We defined here some new gelators for nitrobenzene, chlorobenzene, dichlorobenzene, and dioxane. Among, the gelator N-(2-hexadecanoylaminoethyl)-succinamic acid (NHDAES) was found to be the most potent with quite low mgc values with significant elasticity, stability, and structural strength, which was successfully employed as a template for further synthesis of a spherical cadmium sulfide nanoparticle within a range of 10−20 nm.

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S Supporting Information *

Figure S1 showing the distinction between a clear and opaque gel formed by NHD-AES in nitrobenzene and 1,4-dioxane, respectively. A comparison of FTIR spectra between the powder form and its xerogel of NHD-AES and BHD-DAE in support of hydrogen bonding is shown in Figure S2. TEM image (Figure S3) showing the effect of gelator concentration on regulation of particle size of CdS. Figure S4 (SEM image) shows the formation of bulk CdS obtained after allowing the precursors to react before the gelation process. Effect of agitation/addition of precursors to NHD-AES-nitrobenzene gel is shown in Figure S5. Figure S6(A) showing the electronic spectrum of dispersed cadmium sulfide in methanol and S6(B) plot of (σhυ)2 vs energy (eV) to calculate Egnano. Fluorescence emission spectrum of as-prepared CdS nanocrystals is shown in Figure S7. This material is available free of charge via the Internet at http://pubs.acs.org.

(2)

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where σ is the molar absorption coefficient obtained from Beer−Lambert law, hυ is the photon energy, and K is a proportionality factor. The band gap energy (2.51 ev) was calculated by extrapolating the linear section of the curves to the x-axis in which (σhυ)2 = 0 (Figure S6B of the Supporting Information).50 Further, the Egnano is related to particle size by an electron−hole−box model with effective mass approximation, following eq 349 Eg nano = Eg bulk +

h2 ⎡ 1 1 ⎤ 3.6e 2 + ⎢ ⎥− mh ⎦ 4πεd 2d 2 ⎣ me

ASSOCIATED CONTENT

AUTHOR INFORMATION

Corresponding Author

*Phone: +91-022-33612013. Fax: +91-022-33611020. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.K. and M.M.K. are grateful to University Grant Commission (India) for financial assistance and SAIF-IIT-B for analytical support. The authors thank UGC Networking Centre in Chemical Engineering, ICT, Mumbai, for use of instruments for analysis.

(3)

where e is the charge of the element (1.60 × 10−19 C), ε is the relative permittivity (5.7ε0), ε0 is the permittivity of vacuum (8.85 × 10−12 C2/N/m2), mh is the effective mass of valence band hole (0.8 mo), effective mass of conduction band electron is me (0.19 mo), mo is the electron rest mass, h is Planck’s constant (6.63 × 10−34 J·s), Egnano (2.51 eV) is the band gap energy of the nanoparticles, and Eg bulk (2.5 eV) is the band gap energy of bulk. The diameter (d) of the CdS nanoparticle was calculated to be 9.4 nm which is in agreement with the data obtained from TEM measurements with minimum error. Hence, the decrease in particle size increases the effective band gap between the valence and conduction band resulting in a higher energy emitted photon which shows the fluorescence

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