pubs.acs.org/Langmuir © 2011 American Chemical Society
Self-Assembly Fibrillar Network Gels of Simple Surfactants in Organic Solvents Dong Wang†,‡ and Jingcheng Hao*,†,§ †
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China, ‡Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China, and §Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, PR China Received November 2, 2010. Revised Manuscript Received December 18, 2010 The self-assembled fibrillar network (SAFIN) organogels of a simple surfactant molecule, sodium laurate (C11H23COONa, SL), in organic solvents were investigated. The sol-gel transformation temperature depended on the SL concentration, the solvent, and the concentration of Naþ was evaluated. An important finding is that Naþ ions play an important role in forming organogels, which was regarded as the induction factor of gelation, but other cations, for instance, Liþ, Kþ, Ca2þ, and Mg2þ, do not have this capability. The observations by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) proved that the organogels were network structures with fibers and ribbons by trapping a certain amount of organic solvent. High-resolution TEM (HR-TEM) images indicated that each of the fibers or ribbons was composed of cylindrical micelles. The X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectra demonstrated that SL molecules in gels behave similarly to those in SL crystals. The mechanism of organogel formation was elaborated to provide a better understanding of fibrous surfactant gels in organic solvents.
Introduction Low-molecular-weight organic gels (LMOGs) are made of small organic molecules that trap organic or aqueous solvents.1,2 In recent years, the study of gels, especially those composed of amphiphilic molecules,3,4 has attracted attention because of wide-ranging applications in the in situ preparation and stabilization of nanomaterials5-9 and organic photochromatic materials,10 energy transfer, lightharvesting materials,11-13 drug-delivery systems,14-16 the preparation of dye-sensitized solar cells,17 and so forth.18,19 The general way to prepare solidlike materials (i.e., gels) is by cooling a solution that contains a small number of LMOGs below its gelation temperature (Tg). The formation of gels arises from noncovalent interactions including hydrogen bonds, π-π stacking, coordination *Corresponding author. E-mail:
[email protected]. Phone/Fax: þ86-53188366074 (o). (1) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133–3159. (2) Estroff, L. A.; Hamilton, A. D. Chem. Rev. 2004, 104, 1201–1217. (3) Fuhrhop, J. H. Chem. Rev. 1993, 93, 1565–1582. (4) Trickett, K.; Eastoe, J. Adv. Colloid Interface Sci. 2008, 144, 66–74. (5) Kobayashi, S.; Hamasaki, N.; Suzuki, M.; Kimura, M.; Shirai, H.; Hanabusa, K. J. Am. Chem. Soc. 2002, 124, 6550–6551. (6) Ray, S.; Das, A. K.; Banerjee, A. Chem. Commun. 2006, 2816–2818. (7) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435–445. (8) van Bommel, K. J. C.; Friggeri, A.; Shinkai, S. Angew. Chem., Int. Ed. 2003, 42, 980–999. (9) Llusar, M.; Sanchez, C. Chem. Mater. 2008, 20, 782–820. (10) Shumburo, A.; Biewer, M. C. Chem. Mater. 2002, 14, 3745–3750. (11) Ajayaghosh, A.; Praveen, V. K.; Vijayakumar, C. Chem. Soc. Rev. 2008, 37, 109–122. (12) Ajayaghosh, A.; George, S. J.; Praveen, V. K. Angew. Chem., Int. Ed. 2003, 42, 332–335. (13) Nakashima, T.; kimizuka, N. Adv. Mater. 2002, 14, 1113–1116. (14) Ray, S.; Das, A. K.; Banerjee, A. Chem. Mater. 2007, 19, 1633–1639. (15) Adhikari, B.; Palui, G.; Banerjee, A. Soft Matter 2009, 5, 3452–3460. (16) Bastiat, G.; Leroux, J. C. J. Mater. Chem. 2009, 19, 3867–3877. (17) Kubo, W.; Kitamura, T.; Hanabusa, K.; Wada, Y.; Yanagida, S. Chem. Commun. 2002, 374–375. (18) Wilder, E. A.; Wilson, K. S.; Quinn, J. B.; Skrtic, D.; Antonucci, J. M. Chem. Mater. 2005, 17, 2946–2952. (19) Palui, G.; Nanda, J.; Ray, S.; Banerjee, A. Chem.;Eur. J. 2009, 15, 6902– 6909.
Langmuir 2011, 27(5), 1713–1717
bonds, and van der Waals20-25 forces among the gelator molecules. These forces can induce the formation of supramolecular aggregates with various morphologies such as fibers, strands, and tapes. Those supramolecular aggregates are entangled with each other through junction zones to form a 3D network, within which the solvent molecules are immobilized in gel networks. However, the mechanism of the self-assembly of these small organic molecules into supramolecular structures in gels is not clear. Sodium carboxylate was reported to form gels in water, organic solvents, and ionic liquids, but the mechanism remains unknown.26-28 Marton et al. observed gel fibers with striations of sodium myristate (SM) in water.29 The striations were considered to be micellar fibers, which were attributed to hydrogen bonds due to hydration. Our group has observed hydrogels of sodium laurate in water at low concentrations.30 The formation of hydrogels was considered to be a result of the synergism of the hydrophobic force in the center and crystallization on the surface of the cylindrical nanofibers. Herein, to explore the mechanism of gel formation further, we prepared SL gels in different organic solvents. Our results show that SL fibers in organic solvents are center-divergent networks that are completely different from those in water.30 The fibers consisted of a bundle of cylindrical micelles with crystallization surfaces, and solvophobic centers were observed by TEM. Naþ ions play an important role in inducing the formation of gels (20) Basit, H.; Pal, A.; Sen, S.; Bhattacharya, S. Chem.;Eur. J. 2008, 14, 6534– 6545. (21) Mohmeyer, N.; Schmidt, H. W. Chem.;Eur. J. 2005, 11, 863–872. (22) Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263–2266. (23) Xing, B.; Choi, M.; Xu, B. Chem. Commun. 2002, 362–363. (24) Piepenbrock, M. M.; Lloyd, G. O.; Clarke, N.; Steed, J. W. Chem. Rev. 2010, 110, 1960–2004. (25) George, M.; Weiss, R. G. Acc. Chem. Res. 2006, 39, 489–497. (26) Liang, J.; Ma, Y.; Zheng, Y.; Davis, H. T. Langmuir 2001, 17, 6447–6454. (27) Heppenstall-Butler, M.; Butler, M. F. Langmuir 2003, 19, 10061–10072. (28) Jiang, W.; Hao, J. Langmuir 2008, 24, 3150–3156. (29) Marton, L.; McBain, J. W.; Vold, R. D. J. Am. Chem. Soc. 1941, 63, 1990– 1993. (30) Yuan, Z.; Lu, W.; Liu, W.; Hao, J. Soft Matter 2008, 4, 1639–1644.
Published on Web 01/07/2011
DOI: 10.1021/la104333x
1713
Article
Wang and Hao
and strengthening the gels, whereas other cations cannot. Some foundational data are presented, the effect of organic solvents on forming gels is also explained, and the microstructures and mechanism of gels in organic solvents are discussed. It could be expected that our work contributes to the cognition of crystallization gels.
Table 1. Gelation Range of 50 mmol 3 L-1 SL in Various Solvents at 25 ( 0.1 °C solvent
state
solvent
state
solvent
state
benzene toluene hexane cyclohexane
insoluble insoluble insoluble insoluble
methanol ethanol 1-propanol 1-butanol
soluble gel gel gel
1-nonanol 1-decanol diethylamide carbon disulfide
gel gel insoluble insoluble
tetrahydrofuran ethylene glycol acetone ether
insoluble soluble insoluble insoluble
1-pentanol 1-hexanol 1-heptanol 1-octanol
gel gel gel gel
Experimental Section Chemicals and Materials. Sodium laurate (SL) was purchased from Tokyo Kasei Kogyo Co., Ltd., and lauric acid (LA) was purchased from Acros. Sodium caprylate (SC), sodium myristate (SM), sodium palmitate (SP), and sodium stearate (SS) were purchased from Sinopharm Chemical Reagent Co., Ltd. All of the organic solvents;methanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, 1-heptanol, 1-octanol, 1-nonanol, 1-decanol, acetone, toluene, hexane, cyclohexane, benzene, carbon tetrachlotide, and so forth;were purchased from Sinopharm Chemical Reagent Co., Ltd. All of the inorganic reagents;lithium chloride, potassium chloride, calcium chloride, magnesium chloride, sodium hydroxide, sodium bromide, and so forth;were purchased from TianJin Kermel Reagent Co., Ltd. (China). All organic solvents were vacuum distilled. Other materials were used as received without any purification. The deionized water was distilled three times. Lithium laurate, potassium laurate, calcium laurate, and magnesium laurate were synthesized by mixing a 200 mmol 3 L-1 sodium laurate aqueous solution with 200 mmol 3 L-1 lithium chloride, 200 mmol 3 L-1 potassium chloride, 100 mmol 3 L-1 calcium chloride, and 100 mmol 3 L-1 magnesium chloride at 80 °C, respectively. The solutions were left standing for about 2 h and then filtered at room temperature, and the precipitates were rinsed with deionized water and ethanol many times and dried in a vacuum desiccator. Sample Preparation. After adding the required amounts of SL to various organic solvents and making sure that SL completely dissolves in the solvents, the samples were heated with a flat heater until the solutions were homogeneous. The gels formed upon cooling the samples to 25 °C. Gelation-Temperature Measurements. We used the method of tube inversion for gelation measurements by immersing the sealed tubes of SL in a water bath and heating the samples at a rate of 1 °C/10 min to near the solvents’ boiling points. The flow of every sample at every temperature was observed. Once the sample starts to flow, the temperature indicates that the transformation from gel to sol occurs. By repeating this measurement three times, we took the average gel-sol transformation temperature from the three values. TEM and High-Resolution TEM Observations. A drop of the sample (∼4 μL) at Tg was placed on a TEM grid (copper grid, 3.02 mm, 200 mesh). Most of the liquid was removed with blotting paper, leaving a thin film stretched over the holes. The gel formed upon cooling to 25 °C, and then the copper grids were put in a vacuum extractor to remove the organic solvent at T=25.0 ( 0.1 °C for 24 h. The copper grids were observed through a JEOL 100cx II TEM (Japan) at an acceleration voltage of 100 kV. The same samples were also observed through a JEOL JEM-2010 highresolution TEM (Japan) at an acceleration voltage of 200 kV. SEM Observations. Gel samples of SL in alcohols were dried for 10 h under high vacuum after being frozen with liquid nitrogen. Then the samples were observed using a JEOL JMS-6700 SEM (Japan). Small-Angle and Wide-Angle XRD Measurements. The X-ray diffraction (XRD) patterns were recorded using a Rigaku D/Max 2200-PC diffractometer with Cu KR radiation (λ = 0.15418 nm) and a graphite monochromator at ambient temperature. The same sample was used to test it from 1 to 10° and then from 10 to 80°. FT-IR Measurements. Fourier transform infrared spectra were measured on a Bruker TENSOR27 infrared spectrophotometer with a KBr pellet technique within the 4000-400 cm-1 1714 DOI: 10.1021/la104333x
Figure 1. Process of gelation of 50 mmol 3 L-1 SL in ethanol at T = 25.0 ( 0.1 °C. Gelation time: 0, 15, 30, 45, and 60 min from left to right. region. The gel samples of SL in ethanol were dried for 10 h under high vacuum after being frozen with liquid nitrogen.
Results and Discussion The gelation of sodium laurate (C11H23COONa, SL) in various organic solvents was carried out. It was found that 50 mmol 3 L-1 SL was soluble in methanol and ethylene glycol, partially soluble in other shorter straight-chain alcohols (i.e., from ethanol to 1-decanol), and insoluble in chloroform, benzene, toluene, hexane, cyclohexane, tetrahydrofuran, acetone, carbon disulfide, and ether at room temperature. After the samples were heated to reach temperatures up to the boiling points of the solvents, SL was soluble in shorter straight-chain alcohols but still insoluble in chloroform, benzene, toluene, hexane, cyclohexane, tetrahydrofuran, acetone, carbon disulfide, and ether. It is well known that the hydrogen bonds between alcohols and carboxylates can promote the dissolution of SL. However, the hydrocarbon chains of alcohols can prevent it. The balance of the solvophilic head and solvophobic tails plays a key role in the solubility of SL, which affects the gelation ability. The samples were kept for 1 h at T = 25.0 ( 0.1 °C, and white and opaque gels can be observed in shorter straight-chain alcohols, as listed in Table 1. The sample of 50 mmol 3 L-1 SL in ethanol can spontaneously start to form a gel at the bottom of the vial, and about an hour later, the gel has extended to the whole vial. We suspect that there is a nucleation procedure at the beginning of gelation. The center observed from the TEM and SEM photographs can provide evidence. Nucleates may be deposited by gravity during gelation. Therefore, gelation first occurred at the bottom of the vial. With respect to the formation of nucleates in bulk solution, we think that the bulk solution also reached high supersaturation as well as an interface at room temperature. The surface density effect is not obvious. The process of gel formation with time for 50 mmol 3 L-1 SL in ethanol is shown in Figure 1. The gels in different solvents are thermoreversible and thermodynamically stable. In our opinion, in molecular gels, supramolecular aggregation and SAFIN formation usually occur when a solution of gelator molecules is cooled below its characteristic gelation temperature (Tg). In this supersaturation concentration regime, microscopic phase separation Langmuir 2011, 27(5), 1713–1717
Wang and Hao
Article
Figure 2. TEM images of fibers for 50 mmol 3 L-1 SL in different alcohols: (a) ethanol, (b) 1-propanol, (c) 1-butanol, (d) 1-pentanol, (e) 1-hexanol, (f) 1-heptanol, (g) 1-octanol, (h) 1-nonanol, and (i) 1-decanol.
Figure 3. Gelation temperature (Tg, 9) and the minimum gelation concentration (MGC, 2) of SL gels in different alcohols.
occurs and fibers self-assemble because of the directional intermolecular interaction and solvophobic interaction. Solvents play an important role in the formation of SL gels. In Figure 2, one can see that the morphologies of SL fibers in different solvents are center-divergent. The fibers grow outward from the center and are entangled with each other. To this end, the fibrous networks come into being. However, with the reduction of the polarity of alcohols, the gelation temperature (Tg) increases and the minimum gelation concentration decreases, as shown in Figure 3. The gelation temperature was determined by placing the sealed screw-cap glass vials containing gels in a temperaturecontrolled water bath and visually observing the flow upon tilt for every degree. The measurements show that the alcohol chain lengths are longer and the gels are easier to form because the solvophobic force of SL decreases with the increasing length of alcohol molecules. As a result, SL fibers would dissolve easier at room temperature in longer-chained alcohols. More fibers would Langmuir 2011, 27(5), 1713–1717
Figure 4. SEM images of gels for 50 mmol 3 L-1 SL in 1-propanol
(a), 1-pentanol (b), 1-heptanol (c), and 1-nonanol (d).
entangle with each other, which can make the gels stronger. From Figure 4, we found that the fibers in alcohols with longer chains are shorter and denser. Therefore, higher temperature is necessary to dissolve the fibers. For the same reason, longer alcohols attain saturation more easily, so they would have a lower minimum gelation concentration (MGC). The gelation temperature can be also raised with the increase in SL concentration, as shown in Figure 5. It is easy to understand that higher SL concentration leads to more SL fibers. Detailed observations of the influence of inorganic salts on gelation show that only sodium salts play an important role in gelation. We added a series concentration of NaBr to a 50 mmol 3 L-1 DOI: 10.1021/la104333x
1715
Article
SL ethanol solution. The gelation temperature was raised with the increase in NaBr concentration (Figure 6). We also added NaOH, and the same results can be observed. We can conclude that the Naþ ions work in gelation but not the anions. In another experiment, Naþ in SL solution was substituted by Liþ, Kþ, Ca2þ, and Mg2þ. We cannot observe the formation of gels. Lithium laurate, potassium laurate, and calcium laurate (all 50 mmol 3 L-1) form precipitates in cool ethanol. Magnesium laurate (50 mmol 3 L-1) dissolved in cool ethanol to form a homogeneous solution, probably a micelle solution. More magnesium laurate (about 150 mmol 3 L-1) forms precipitates. It could be speculated that there is equilibrium in SL solutions (i.e., SL a Naþ þ L-). According to the solubility product principle, if the ion product Qi, where Qi = [Naþ] [L-], is larger than the solubility product constant, Ksp, leading to precipitates, then adding Naþ to a 50 mmol 3 L-1 SL ethanol solution could increase Qi and cause SL to precipitate more easily. Different from common precipitates, in the amphiphilic system, the precipitates
Figure 5. Gelation-temperature variation with SL concentration in ethanol.
Figure 6. Gelation temperature changes with NaBr concentration, 50 mmol 3 L-1 SL in ethanol.
Wang and Hao
are fibrous. Therefore, more fibers form to strengthen the gels by adding Naþ to SL solutions. Naþ ions have the ability to reduce the minimum SL gelation concentration. Not only can SL form gels, but sodium caprylate (C7H13COONa, SC) to sodium stearate (C17H35COONa, SS) can also form gels in alcohols. With the increase in the length of carbon chain of the carboxylate surfactant, the gelation temperature increases. The gelation temperatures of 50 mmol 3 L-1 sodium myristate (C13H27COONa, SM), sodium palmitate (C15H31COONa, SP), and SS are 55, 72, and 73 °C, respectively. The typical SEM images of SM, SP, and SS are shown in Figure 7. One can see that fibers of these gels are denser and shorter with increasing length of the carbon chain of the carboxylate surfactant. Formation Mechanism of SL Gels. In solution, SL is ionized. The headgroup of carboxylate ions’ mutual exclusion leads to the area of the headgroup extending. The main form of SL is spheroidal micelles above its cmc. According to the saturated solubility phenomenon, when the solution attains saturation, some Naþ and L- ions will precipitate in the form of an SL molecular crystal. However, in alkyl amphiphilic systems, the precipitates are not in the form of SL molecules but are in a kind of nanofiber structure. We concluded the following: (i) On the surface of the cylindrical nanofiber, Naþ and L- ions were arranged periodically. One Naþ ion can combine two L- ions, and one Lion can combine two Naþ ions, which likes a crystal lattice. (ii) The interior of the cylindrical nanofiber is filled with the hydrocarbon chains of sodium soaps, which likes cylindrical micelles. The HR-TEM images are shown in Figure 8. It can be concluded that the width of five cylindrical micelles is about 15 nm. Therefore, the diameter of a single cylindrical micelle is about 3 nm, which is about twice of the length of the SL molecule. (iii) Many cylindrical micelles bind together into a bundle (i.e., fibers or ribbons), and the fibers and ribbons interweave into the gel network structures. What are the driving forces for forming cylindrical micelles? We suspect, in the critical state, that Naþ ions try to approach L- ions, which are components of spheroidal micelles. This process causes the forces of ionic mutual exclusion to decrease. As a result, the area of the headgroup decreases. According to the theory of the packing parameter, P = v/la, v is the volume of the hydrophobic groups, l is the length of the hydrophobic groups, and a is the area of the hydrophilic headgroups; a reduction of the area of the headgroups, a, leads to an increase in the packing parameter, P. Therefore, spheroidal micelles produce a trend in transforming to cylindrical micelles (i.e., spheroidal micelles grow into long cylindrical micelles in a short time). Finally, Naþ and Lions combine on the surface of the cylindrical micelles to form a crystallization surface. The fibers consisting of nanofibers of SL micelles should separate out from the solution. Now it is an open question as to why only Naþ can induce this transformation. We suspect that the Naþ ionic radius precisely matches this crystalline lattice on the surface but other cations destroy the cylindrical micelles during the course of crystallization.
Figure 7. SEM images of (a) SM, (b) SP, and (c) SS fibers in ethanol. 1716 DOI: 10.1021/la104333x
Langmuir 2011, 27(5), 1713–1717
Wang and Hao
Article
Figure 8. High-resolution TEM images of SL fibers in ethanol. In the images, one can see the parallel nanofibers on the surfaces of several thin ribbons and can estimate the diameter of the nanofibers.
Figure 9. Small-angle and wide-angle XRD spectra for 50 mmol 3 L-1 SL in ethanol.
Figure 11. FT-IR spectra of SL gels and SL crystals.
Figure 10. Schematic drawing of the cylindrical micelle arrangement of SL gels.
Small-angle XRD spectra show two strong peaks at 2.7 and 3.7°, as shown in Figure 9. We can determine that the d spacing is 3.2 and 2.3 nm according to Bragg’s law. The relative positions of these reflections are 1:(2)1/2, indexing the sequence as the (100) and (110) reflections, respectively, indicating a columnar quadrate lattice with a lattice parameter of R = 3.2 nm. The value of 3.2 nm is just the diameter of a cylindrical micelle. Therefore, a schematic drawing is shown in Figure 10 for the arrangement of cylindrical micelles. Wide-angle XRD spectra also show the existence of peaks, proving that atoms are arranged periodically on the surfaces of cylindrical micelles. By comparing the FT-IR spectra of SL crystal and SL gel, as shown in Figure 11, we found that both of them have a peak at 1558 cm-1 that is attributed to the asymmetric stretching vibration Langmuir 2011, 27(5), 1713–1717
of -COO- in the crystallization of SL, indicating that the bonds between sodium and oxygen of the carboxylate ions exist. This result proves that the behavior of SL molecules in gels is similar to that in crystals, which is consistent with the XRD measurements and our hypothesis of the combination of Naþ ions and L- ions.
Conclusions SL gels in organic solvents were prepared. Gel formation can happen only in alcohols. Naþ ions that can induce the transition from spherical micelles to crystalline cylindrical micelles play an important role in the formation of SL gels. Other ions would destroy the formation of nanofibers. The structures of fibers were obtained, the nanofibers of which are like cylindrical micelles but with crystallization on the surface. The nanofibers have a quadrate arrangement in the cross section. Acknowledgment. This work was financially supported by the NSFC (grant no. 20625307 & 21033005), the National Basic Research Program of China (973 Program, 2009CB930103), and NFS of Shandong Province (Z2008B01). DOI: 10.1021/la104333x
1717
