Thixotropic Organogels Based on a Simple N-Hydroxyalkyl Amide

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Articles Thixotropic Organogels Based on a Simple N-Hydroxyalkyl Amide: Rheological and Aging Properties M. Lescanne,† P. Grondin,† A. d’Ale´o,‡ F. Fages,‡ J.-L. Pozzo,*,‡ O. Mondain Monval,† P. Reinheimer,§ and A. Colin† Centre de Recherche Paul Pascal CNRS, UPR 8641, 33 Avenue A. Schweitzer, 33600 Pessac, France, Supramolecular Chemistry Biomimetism and Nanosciences, UMR 5802, Universite´ Bordeaux 1, 351 Cours de la Libe´ ration, 33405 Talence, France, and Thermo Electron Corporation, Dieselstrasse 4, D-76227 Karlsruhe, Germany Received July 7, 2003. In Final Form: February 6, 2004 The thixotropic properties of thermoreversible organogels composed of N-3-hydroxypropyl dodecanamide and various apolar fluids have been investigated by X-ray scattering, light microscopy, and rheo-optics experiments. This revealed that gel formation occurs via a precipitation process. Depending upon the cooling rate, large interconnected aggregates are formed and induce an elastic behavior. When submitted to a shear flow, these aggregates disentangled and became aligned in the direction of the velocity. Nevertheless, shear does not alter the structure of the individual aggregate and connections between the aggregates are quickly rebuilt due to gravity and thermal fluctuations when the applied flow is stopped. The alignment under flow and the reformation of the connections after the cessation of the shear induces the thixotropic behavior.

In recent years, a wide variety of organic compounds have been reported to form gels with various organic fluids.1 Organogels are thermoreversible, viscoeleastic materials consisting of low molecular weight gelators (LMOGs) self-assembled into complex three-dimensional structures. At high temperature, LMOGs are quite soluble in organic solvents and their solutions are liquid. At low temperature, multiple nonconvalent interactions such as hydrogen bonding, donor-acceptor, and hydrophobic interactions between the organogel building blocks give birth to a three-dimensional network1 of entangled and connected fibers. This process is reversible; that is, the gel can melt again at high temperature and be reformed under cooling. In recent years, these organogels have been the subjects of increasing interest mainly due to the fundamental questions raised by the gel structures and assembly mechanisms.2 Although these gels could also offer potential uses for the food industry, cosmetics, and chromatography, their high sensibility to external mechanical stress greatly reduces the range of their ap†

Centre de Recherche Paul Pascal CNRS. Universite´ Bordeaux 1. § Thermo Electron Corp. ‡

(1) (a) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133-3159. (b) Abdallah, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 1237-1245. (c) Van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263-2266. (d) Terech P.; Weiss, R. G. In Surface Characterization Methods; Milling, A. J., Ed.; Marcel Dekker: New York, 1999. (e) Shinkai, S.; Murata, K. J. Mater. Chem. 1998, 8, 485. (f) van Esch, J.; Schoonbeek, F.; De Loos, M.; Veen, E. M.; Kellogg, R. M.; Feringa, B. L. Supramolecular science: Where it is and where is going; Nato ASI Series C: Mathematical and Physical Sciences, Vol. 527; Kluwer Academic: Dordrecht, 1999; p 233. (g) Brenninger Physiol. Chem. 1892, 16, 552. (2) (a) de Vries, E. J.; Kellogg, R. M. J. Chem. Soc., Chem. Commun. 1993, 238. (b) Sobna, J.-E.; Fages, F. Chem. Commun. 1997, 372. (c) Hanabusa, K.; Maesaka, Y.; Kimura, M.; Shirai, H. Tetrahadron Lett. 1999, 40, 2385. (d) Abdallah, D. J.; Weiss, R. G. Langmuir 2000, 16, 352. (e) Terech, P.; Countin, A.; Giroud, A. M. J. Phys. Chem. B 1997, 101, 6810. (f) Ahmed, S.; Sallenave, X.; Fages, F.; Gundert, G. M.; Muller, W. M.; Muller, U.; Vogtle, F.; Pozzo, J. L. Langmuir 2002, 18, 70967101.

Figure 1. N-3-Hydroxypropyl dodecanamide (NHD) molecule.

plications. Indeed, when they are submitted to flow, the solvent is irreversibly expelled from the 3-D network which is at least partially disrupted. When the mechanical solicitation is stopped, the sample behaves as a solid suspension and has lost its elastic properties. The only way to obtain a gel again is to heat it and cool it once more. In this article, we report the synthesis of a very simple LMOG: the N-3-hydroxypropyl dodecanamide (called hereafter NHD for convenience) (see Figure 1). This molecule, derived from our previous study on N-acyl-1,ωamino acid based gelators,2e forms a gel in various fluids such as toluene and some mixtures of silicone oil in toluene and of pentanol in dodecane. When a tube filled with these gels is shaken, the gel is broken and the sample flows very rapidly. However, when the shearing is no longer applied, the sample recovers its elastic properties and does not flow any more. These gels are defined as “thixotropic systems”. As defined in 1975 by the British Standards institution, the viscosity of these organogels decreases under stress, but when the stress is removed the viscosity and the elasticity of the sample gradually recover their initial values. As they support shear stress, these gels are particularly attractive for industrial applications.3c Such a behavior is indeed quite unusual for organogels derived from LMOGs. To our knowledge, only very few examples of such behavior have been reported in the literature.3a-c Following an experimental approach based on a combined use of techniques (rheological, rheo-optical, and X-ray scattering measurements and light microscopy observations), we investigate the mechanism of gel formation of these systems and their aging and rheological properties. We discuss the mech-

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anism of gel formation and aging within the general theory of crystallization.4-6 Based on the analysis of the gel microstructure, we give an explanation for their thixotropic behavior and present an analysis of their rheological properties. Experimental Section Materials. The synthesis of NHD was previously described.7 To a solution of aminopropanol (Sigma) (1 mol equiv) in 40 mL of 0.2 N NaOH (Aldrich) at 0 °C is added dropwise lauryl chloride (1 mol equiv). The reaction is stirred for 1 night at room temperature. The precipitate is filtered and dissolved in dichloromethane (Aldrich). The organic layer is washed several times with water and dried over Na2SO4 (Sigma). After evaporation, the white solid is triturated with pentane, filtered, and dried under a vacuum. Toluene (Aldrich) is used as received. IR (KBr): 3301 cm-1 (NH), 2919 cm-1 (CH), 1638 cm-1 (amide I), 1555 cm-1 (amide II). 1H NMR (CDCl3): 0.87 ppm (t, 3H, CH3), 1.25-1.30 ppm (m, 16H, CH2), 1.72 ppm (m, 4H, CH2CH2CO and CH2CH2NH), 2.22 ppm (t, 2H, CH2CO), 3.42 ppm (m, 2H, CH2NH), 3.63 ppm (t, 2H, CH2OH), 5.85 ppm (1H, NH). Light Microscopy Experiments. NHD molecules are dissolved at high temperature in the solvent (toluene). Warmed solutions of the samples are introduced into thin capillary tubes (100 µm thick, 1 mm large, and 20 mm long), that are then sealed at both ends. Using a temperature-controlled table Linkam (including a Peltier element), high-rate temperature quench can be performed while measuring the gelling temperature and looking at the microscopic formation of the gel using polarizing light microscopy (Nikon). The cooling rate can be adjusted between 10 and 0.1 °C/min. A fast cooling rate (ca. 20 °C/min) can be achieved by quickly displacing the capillary tube from a 50 °C oven to the microscope table that is maintained at 15 °C. Taking into account the volume of the tube and the diffusity heat coefficient, the cooling rate was estimated, even if not linear, to be greater than 20 °C/min. X-ray Scattering. X-ray experiments are performed using a rotating copper anode generator. A graphite monochromator is used to select the copper K wavelength ()1.5418 Å). The incident beam is collimated by a set of rectangular slits that define an irradiated area of the sample of 0.8 × 1 mm2. We use a 2D-image plate detector; the detection area, of circular shape with diameter of 18 cm, is an assembly of square pixels of 150 × 150 cm2 providing a 16-bit dynamic range. The direct beam is stopped in front of the detector by a beam stop. The sample holders used are glass cylindric capillaries. They are sealed with a torch once filled with the samples. Rheological Measurements. Rheological experiments are performed on a stress-controlled rheometer (TA instrument AR1000) equipped with a cone (60 mm diameter, 4° cone angle, and 0.118 mm gap truncation) and plate geometry. The plate temperature is controlled and can be adjusted accurately between 2 and 99 °C using fast temperature ramps due to the use of a Peltier element. A solvent-trapping device is placed above the cone to avoid evaporation. Rheological experiments are particularly sensible to sliding of the gel layer during the measurements. To avoid this, a striated cone and a rough plate are used. The following procedure is used to load the sample: a warmed NHD solution in toluene is introduced in the shearing gap of the (3) (a) Brinksma, J.; Feringa, B. L.; Kellog, R. M.; Vreeker, R.; Van Esch, J. Langmuir 2000, 16, 9249-9255. (b) Ihara, H.; Sakurai, T.; Yamada, T.; Hashimoto, T.; Takafuji, M.; Sagawa, T.; Hachisako, H. Langmuir 2002, 18 (19), 7120-7123. (c) Livoreil, A; Baghdadli, N. (FR) L’OREAL Patent EP1264589. (d) Hotten, B. W.; Birdsall, D. H. J. Colloid Sci. 1952, 7, 284-294. (e) Murdan, S.; Gregoriadis, G.; Florence, A. T. Eur. J. Pharm. Sci. 1999, 8, 177-186. (4) Burton, W. K.; Cabrera, N.; Frank, F. C. Philos. Trans. R. Soc. London 1951, A243, 299. (5) Bennema, P. J. Cryst. Growth 1984, 69, 182. Sunagawa, I. J. Cryst. Growth 1999, 1156. (6) Grandum, S.; Yabe, A.; Nakagomi, K.; Tanaka, M.; Takemura, F.; Kobayashi, Y.; Frivik, P. E. J. Cryst. Growth 1999, 205, 382. (7) Kato, H.; Ogawa, Y.; Choshi, Y. (New Japan Chemical Co., Ltd., Japan). Jpn. Kokai Tokkyo Koho 1988, 6 pp. CODEN: JKXXAF JP 63215789 A2 19880908 Showa. Patent written in Japanese. Application: JP 87-49790 19870303. CAN 110:138559 AN 1989:138559 CAPLUS (Copyright 2003 ACS).

Figure 2. Phase diagram of NHD in toluene. The bold line refers to the gel temperature. The square indicates the gel temperature of an NHD solution of 37.5 mmol/L. The bullet indicates the concentration of NHD soluble in toluene at 15 °C. rheometer at 50 °C and then rapidly cooled to 5 °C at a given rate. The first step of an experiment consists of determining the so-called linear regime of the gel. This is done by measuring the storage modulus G′, associated with the energy storage, and the loss modulus G′′, associated with the loss of energy, as a function of the stress amplitude. The linear regime is such that both dynamic moduli are independent of the stress amplitude and reflect the properties of the unperturbed network. In that regime, G′ and G′′ are measured as functions of the angular frequency which is varied from 0.1 to 10 Hz. The second type of experiments are flow experiments. A constant shear stress is applied to the sample for 2 min, and the shear rate is recorded as a function of time. After 2 min, the shear stress is then slightly increased and the same procedure is applied. The same experiments are also performed by imposing the shear rate. In this latter case, a constant shear rate is applied and the shear stress is recorded as a function of time. After 10 min, the shear rate is then slightly increased and the same procedure is applied. To control the evolution of the structure of the gel in the rheometer cell, the sample is carefully taken from the rheometer cell and observed under an optical microscope at the end of each experiment. As the structure of the gel is a crucial parameter in the understanding of the rheological properties, we also performed some optical observations of the sample under shear. Rheo-optics Measurements. To perform this analysis, we use a new scientific instrument called “RheoScope 1” made by Thermo Hacke. A universal air-bearing rheometer is combined with an optical microscope and a video camera. With this combination, the rheo-mechanical properties and the observation of microstructures can be performed at the same time. Whereas most of the existing optical methods such as flow-birefringence or flow-dichroism give information on the structural changes at the molecular level, the instrument described here provides some visual information on structural changes within the micrometer range. During a measurement in rotation or oscillation, the rheological behavior is recorded simultaneously with the corresponding picture of the microstructure.

Results Macroscopic Observations. Various solutions of NHD in toluene, with concentrations ranging from 10 to 60 mmol/L, are prepared and heated at 50 °C in order to totally dissolve the solid powdered sample. The samples are introduced in a temperature-controlled bath and then cooled rapidly at a given temperature. Observations are then performed 1 h after the quench, that is, when the system temperature and its state are stable and homogeneous throughout the whole experimental cell. Each point in the phase diagram corresponds to a different tube. Figure 2 presents the obtained phase diagram. The sample

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Figure 3. Microscopy picture showing the formation of the gel (cooling rate, 20 °C/ min, [NHD] ) 3.75 × 10-2 mol/L). Observations are performed at T ) 15 °C. Full scale, 400 microns × 280 microns. Time t after the quench: (a) t ) 1 s; (b) t ) 2 s; (c) t ) 3 s; (d) t ) 0.4 s; (e) t ) 5 s.

is a liquid at high temperature and a gel at low temperature. The transition temperature between the liquid phase and the suspension of solid crystals increases as a function of concentration. Let us now focus on the rheological behavior of these samples. A 37.5 mmol/L solution of NHD in toluene is prepared and heated at 50 °C in order to totally dissolve the solid powdered sample. At T ) 50 °C, the sample is liquid. The tube is introduced in a temperature-controlled bath and cooled rapidly at 15 °C. The sample is then maintained at 15 °C, and observations are performed at this temperature. Observation, 20 s after the quench, points out that the sample is a homogeneous gel. When this tube is shaken, the gel breaks and flows. At rest, the gel reforms. This sample has thixotropic properties. One week after the quench, observations reveal the presence of a few rodlike crystals. Two weeks after, these rodlike crystals are much more numerous and fall to the bottom of the tube. The crystals and the solvent phase separate; the sample is not a gel anymore, it has aged. The gel may be obtained again by heating and cooling the sample again. To get a better understanding of this evolution, we perform the same experiments using thin capillary tubes observed with a light microscope. Microscopic Observations. High Cooling Rate Velocities. A 37.5 mmol/L solution of NHD in toluene is heated at 50 °C and cooled to 15 °C at a cooling rate roughly equal to 20 °C/min. Pictures are taken during the whole cooling process and at the final temperature of 15 °C. These pictures (Figures 3 and 4) allow the identification of two distinct processes: first the gel is formed, and then the gel ages. One observes the gel formation with a light microscope (Figure 3). It occurs via a nucleation-growth process. A small dense aggregate nucleates. Then, fibers grow from it. During their growth, one single fiber may split to give

birth to two other fibers. The gel is then formed of very big aggregates composed of interconnected branches or fibers that have grown from the same center. The rate of fiber growth is 200 µm/s. The micrographs are separated by time interval of 1 s (see Figure 3). These interconnected aggregates are responsible for the elastic behavior of the gel. The junctions between fibers that have grown from the same center are strong. However, light microscopy allows secondary junctions to be observed. Indeed, above a given threshold of NHD concentration, the aggregates interdigitate through the entanglement of the branched fibers. This process leads to the formation of cavity domains that confine the solvent molecule. The relative strength of these two types of junctions will be discussed in more detail at the end of this article. At this stage, the structure of the NHD gel looks like the structure of another organogel obtained with 2,3-din-decyloxyanthracene (DDOA).8 However, the NHD aggregates are much more branched than the one previously observed with DDOA. This first step in the gel formation is very fast and lasts less than 5 min at the studied concentrations. Strikingly, this primary structure is not stable and is going to age. In Figure 4, we now observe the aging of the gel. Clearly, some parts of the primary gel fibers melt and reform at other places with a different morphology. The mean diameter of the fibers increases from 4 to 10 microns. The new structure that is obtained is less branched and smoother. It looks like interconnected rods with a few strong connections remaining at the initial nuclei. It is important to note that aging does not stop after 2000 s but carries on at a lower rate. After 1 week, we will be left with a suspension of very large unconnected rods remaining in the system. This qualitative study leads to the following conclusions. First, the gelation of toluene with NHD arises from a multistep process involving (i) the formation of tiny nuclei, (8) Lescanne, M.; Colin, A.; Mondain-Monval, O.; Fages, F.; Pozzo, J.-L. Langmuir 2003, 19, 2013-2020.


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Figure 4. Microscopy picture showing the aging of the gel (cooling rate, 20 °C/min, [NHD] ) 37.5 mmol/L). Observations are performed at T ) 15 °C. Full scale, 400 microns × 280 microns. Note that aging continues after 2820 s. Time t after the quench: (a) t ) 10 s; (b) t ) 540 s; (c) t ) 1680 s; (d) t ) 2820 s.

(ii) the growth of branched fibers from these nucleation points, (iii) the formation of a three-dimensional network via the entanglement of star-shaped crystallites, and (iv) the aging of the gel. The initial gel state is metastable, its final state (after 2 weeks) being a suspension of independent and unconnected rods. Second, the initial gel material can be viewed as displaying two kinds of junction modes at the microscopic level. The central nucleus plays the role of a strong junction, while the entanglements of the arms originating from two neighboring crystallites are junctions with lower strength. This behavior is similar to that previously obtained in other systems.8 Influence of the Cooling Rate. We performed the same experiments by varying the cooling rate first used to form the initial gel material. The initial structures are presented in Figure 5 for various cooling rates ranging from 1 to 0.01 °C/min. The aging phenomenon becomes very slow when the cooling velocity is decreased. This occurs because the initial structure obtained is less branched when the cooling velocity is decreased and the aging phenomenon is less pronounced or observable. At very low cooling velocity, it is impossible to distinguish the two previously described processes. The structure looks like rods weakly entangled. A few strong connections may however be observed. The sample has lost the main part of its elastic properties. In the following, we will focus on gels obtained under high cooling velocities. Our experiments are made either around 10 min or 2 h after the cooling. The studied gel will be a suspension of interconnected rods with a few

strong connections. Typically, its structure will vary between an assembly of entangled dendritic aggregates (see Figure 4a) and an assembly of interconnected rods with a few strong connections (see Figure 4d). X-ray Experiments. X-ray scattering experiments are performed in order to characterize the molecular ordering. The spectra are presented between q ) 0.048 and q ) 0.3 Å. The lower q domains are not presented because the intensity is screened by the beam stop. For the neat phase composed of NHD crystalline powder (Figure 6), two peaks are observed. We believe that the second weak peak (q ) 0.19 Å) corresponds to impurities that are slightly soluble in toluene. For the gel samples, the experiment deals with a gel obtained under high cooling velocity. The exposure to X-rays begins 10 min after the quench and lasts for 5 h. The experiment is performed at 15 °C. Aging has thus begun, and we study a gel made with interconnected rods with a few strong connections. A bump shape and a peak are observed on the X-ray spectrum. The bump shape is related to the form factor of the fibers. One peak is observed with q values slightly smaller than the one corresponding to the lowest q peak evidenced in the powder. In agreement with previous results,8-10 these experiments confirm that molecules are arranged in the gel through a crystalline ordering reminiscent of that of the pure molecules. The mean distance between (9) (a) Terech, P.; Bouas-Laurent, H.; Desvergne, J. P. J. Colloid Interface Sci. 1995, 174, 258. (b) Terech, P.; Allegraud, J. J.; Garner, C. M. Langmuir 1998, 14, 3991-3998. (10) Abdallah, D. J.; Weiss, R. G. Chem. Mater. 2000, 12, 406.

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Figure 7. Evolution of the shear moduli G′ (filled circles) and G′′ (open circles) as a function of time after the gel formation. The applied shear stress is 1 Pa, and the frequency of the measurement is 1 Hz.

Figure 5. Microscopy picture showing the structure of the gel 1 h after the quench for various cooling rates. [NHD] ) 37.5 mmol/L. Full scale, 250 microns × 175 microns. The cooling rate is equal to (a) 0.1 °C/min and (b) 1 °C/min.

Figure 6. X-ray scattering spectra. From down to up, the two different curves correspond respectively to the NHD powder and to a gel phase of NHD (c ) 75 mmol/L) obtained with a high cooling rate (20 °C/min). The two time exposures are different. The time exposure is equal to 1 h for the NHD power and 5 h for the gel phase.

the molecules in the gel is larger than in the powder. One possible explanation may be that the crystalline network is slightly swollen by the solvent.14 (11) Ho¨pken, J.; Pugh, C.; Richtering, W.; Mo¨ller, M. Makromol. Chem. 1988, 189, 911.

Moreover, we do not notice on the gel sample the presence of a second or a larger peak than the one obtained on the neat phase. This suggests that the ordering of the molecule is not affected by the aging process or at least that this effect is not measurable. Rheology. A solution of NHD in toluene with a concentration of 75 mmol/L is prepared and heated at 50 °C in order to totally dissolve the solid powdered sample. The hot solution is introduced between the flat and the conical plates of the rheometer cell and then cooled to 5 °C with a cooling rate of 20 °C/min. Effect of Aging during the First Hour. To analyze the effect of the aging on the rheological properties during the first hour of the gel life, we measured the elastic properties of a gel as a function of time just after the cooling. Five minutes after its formation, the gel was submitted to a periodic stress (0.5 Pa) at a constant frequency f ) 1 Hz. Figure 7 presents the evolution of the shear moduli as a function of time. Clearly, the shear moduli are constant. The aging phenomenon does not modify the elastic properties at least in a period of 1 h. The structures presented in Figure 3 and Figure 4 have the same shear moduli. However, when one waits for more than 1 week, aging has dramatic consequences, that is, the sample lost most of its elastic properties. In the following, the rheological experiments will be performed during the first 2 h after the formation of the gel and we will assume that aging has no effect on the rheological properties. In the case of longer experiments, we will check that the structure did not vary too much. The gel will thus have a structure varying between dendritic stars (Figure 3) and interconnected rods with a few connections (Figure 4). As pointed out by the above experiment, we assume that this evolution has no impact on the rheological properties. Rheological Study. A gel, formed under high cooling rate conditions (i.e., 20 °C/min) and studied during the first 2 h after the quench, is submitted to a varying periodic stress at a constant frequency f ) 1 Hz (Figure 8) in order to determine the linear regime. At low stress values, the G′ parameter is more than 1 order of magnitude greater than G′′, which shows the dominant elastic character of the material. Both moduli remain roughly constant below a critical stress value of around 1 Pa, known as the yield stress value that represents the upper limit of the linear regime. Above this stress value, a decrease of G′ is (12) Beginn, U.; Keinath, S.; Mo¨ller, M. Macromol. Chem. Phys. 1998, 199, 2379-2384. (13) Furman, I.; Weiss, R. G. Langmuir 1993, 9, 2084-2088. (14) Laan, S.; Feringa, B. L.; Kellog, R. M.; Esch, J. v. Langmuir 2002, 18, 7136-7140.


Figure 8. Determination of the linear regime. Measurement of the evolution of G′ (open circles) and G′′ (filled circles) as a function of the applied shear stress. The frequency of the measurement is f ) 1 Hz. The sample is a gel of NHD (c ) 75 mmol/L). The cooling rate is 20 °C/min, and the experiment is performed at 5 °C.

Figure 9. Evolution of G′ (open symbols) and G′′ (filled symbols) as a function of the frequency.The applied shear stress is equal to 0.5 Pa. The experiment is performed in the linear regime. The sample is a gel of NHD (c ) 7.5 × 10-2 mol/L). The cooling rate is 20 °C/min, and the experiment is performed at 5 °C.

observed, which can be attributed to a partial breakup of the gel that begins to flow. A new gel, formed under high cooling rate conditions (i.e., 20 °C/min) and studied during the first 2 h after the quench, is prepared and is subjected to a nondestructive frequency sweep experiment with a constant stress of 0.5 Pa. Four different experiments performed on the same gel are presented in Figure 9. A slight dependency of the elastic modulus G′ as a function of the frequency is noticed (from 8000 Pa at 0.1 Hz to 1.5 × 104 Pa at 10 Hz), which is consistent with a viscoelastic behavior. At this stage, it is important to note that the reproducibility of the measurements is excellent on the same sample but not as perfect when we compare the properties of two gels loaded and formed in the rheometer. The numerical values of G′ and G′′ may vary from 40% from one experiment to another. This behavior is common to many organogels. We believe that these discrepancies are related to a dispersion in the aggregate number density from one

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Figure 10. Evolution of G′ (open symbols) and G′′ (filled symbols) as a function of the time after an initial shear stress of 20 Pa for 1 min stopped at t equals zero. The applied stress is 1 Pa, and the frequency of the measurement is 1 Hz. The sample is a gel of NHD (c ) 75 mmol/L). The cooling rate is 20 °C/min, and the experiment is performed at 5 °C.

gel to another. Indeed, this parameter depends on cooling velocities, impurities, and filling of the geometry which are difficult to control very precisely. Note that the same behavior was found using a Couette cell of 2 mm gap rather than a cone plate. These discrepancies thus do not seem to be related to confinement effects. However, all the experiments show that G′ is greater than G′′ at all frequencies in the whole frequency range investigated: clearly the sample is a gel. Moreover, in all experiments we noticed an increase of the elastic modulus as a function of frequency. To study the thixotropic properties of the gel and its recovery, a new gel, formed under high cooling rate conditions (i.e., 20 °C/min) and studied during the first 2 h after the quench, is sheared at a constant stress of 20 Pa for 1 min. At the beginning of the experiment, the mechanical stress is released and we monitor the gel recovery by measuring the time evolution of the elastic modulus. During this experiment, we apply an oscillating shear stress of 1 Pa at a frequency of 1 Hz. We chose to apply a low shear stress in order to avoid destruction or a perturbation of the gel by the mechanical stress. Figure 10 presents the evolution of the shear moduli as a function of time. We note that the initial shear stress has destroyed the elastic properties of the gel. At t equals zero, G′ is less than 3 Pa, and during the first 200 s, the sample is more viscous than elastic. However, the sample gradually recovers its elastic properties. After 300 s, it behaves as gel, and after 1000 s G′ remains constant and nearly equal to its value in the initial gel (104 Pa instead of 1.2 × 104 Pa before shearing). We checked that this experiment may be performed many times (at least three times) on the same gel and that each time the gel recovered its elastic properties without being heated and cooled. This gel clearly exhibits thixotropic behavior: it breaks when submitted to a shear stress and heals when the shear stress is released. This thixotropic behavior occurs at the very beginning of the gel life (structure presented in Figure 3) as well as 2 h after (structure presented in Figure 4). We now study the gel behavior under constant applied shear stress and shear rate. A constant shear stress is applied to the sample for 2 min, and the shear rate is recorded as a function of time. After 2 min, the shear stress is then slightly increased and the same procedure is applied. Figure 11 presents the evolution of the measured shear rate and of the applied

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Figure 11. Evolution of the measured shear rate as a function of the time for different applied shear stresses. For t less than 94 s, the applied shear stress is 16 Pa. For t greater than 94 s, the applied shear stress is 18 Pa.

shear stress as a function of time. Clearly, two regimes are observed. When the applied shear stress is less than 18 Pa, the shear rate remains very low and is not precisely measurable. When the applied shear stress is equal to 18 Pa, the shear rate increases and reaches a very high value. As these values exceed the maximal velocity value of the rheometer, the experiments stop. To give a description of the rheological curve, we now impose the shear rate and measure the shear stress. In this case, a constant shear rate is applied to the sample for 10 min and the shear stress is recorded as a function of time. This procedure is applied for different shear rate values ranging from 0.1 to 1500 s-1. Figure 12a-c presents typical evolution of the measured signal as a function of time. The experiments performed at high shear rate lead to stationary values (Figure 12c). By contrast, experiments performed at low shear rate (shear rate of less than 200 s-1) reveal either a noisy or an oscillatory behavior. Figure 13 presents the evolution of the measured shear stress as a function of the applied shear rate. The measured shear stress values are obtained by averaging the recorded signal on the last 300 s of the peakhold. In Figure 13, one may divide the signal into three regimes. First, in region 1 the measured shear stress decreases as a function of the shear rate in the case of an applied shear rate of less than 2 s-1. Second, in region 2 the shear stress does not significantly vary as a function of the applied shear rate when it is greater than 2 s-1 and less than 100 s-1. Third, in region 3 the measured shear stress increases as a function of the applied shear rate for a shear rate higher than 100 s-1. To summarize all the rheological results, a continuous line at the shear stress of 18 Pa has been added on the diagram of Figure 13. For an applied shear stress lower than 18 Pa, the sample does not flow. For an applied shear stress bigger than this value, the sample flows but the shear rate is too high to be measured by the rheometer in this geometry. Rheo-optics Experiments. To get some insights into the gel flowing behavior, we performed some rheo-optics measurements. We were able to study simultaneously the rheological behavior of the gel under shear rate while looking at the evolution of its structure using a light microscope. These experiments reveal several important points. First, during all the experiments, the structure of the rods does not change. Shear does not break them. In Figure 14 is shown a picture of the needles at the beginning (a) and at the end (b) of the experiment. The observed texture clearly does not change during the experiment.

Figure 12. Evolution of the measured shear stress as a function of the time for different applied shear rates. The applied shear rate is equal to (a) 0.5 s-1, (b) 100 s-1, and (c) 1500 s-1.

Second, at low shear rate, the aggregates located near the cone do not seem to flow whereas the cone clearly moves. This indicates the presence of sliding. Third, at higher shear rate, by focusing at a different level in the sample, we note that the flow is nonhomogeneous (see Figure 15). Some aggregates flow while others do not. Fourth, for the highest shear rate, nearly all the gel flows, and the flowing aggregates are aligned along the flow direction (see Figure 16). Discussion Gel Morphology: Mechanism of Formation and Aging of the Gel. As found previously on other systems,8 the formation of the gel occurs via a nucleation growth mechanism. The observed influence of the cooling rate on the gel morphology can be interpreted in terms of the general theory of crystallization through a mechanism of nucleation-growth. Our X-ray scattering results show


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Figure 13. Evolution of the shear stress as a function of the shear rate. The filled circles correspond to stationary states, and the open circles correspond to nonstationary or oscillating behavior.

that the fiber network possesses a high degree of crystallinity, with a crystal order strongly related to that of the crystalline powder. The parameter responsible for the crystal growth is the driving force σ. In the case of a solute dissolved in a solvent, σ is also called the supersaturation and is defined as

σ)

(

)

C0 - Ceq(T) ∆µ ) ln kT Ceq(T)

where C0 is the solute concentration in the solvent, Ceq(T) is the solubility of the solute at the temperature T, and ∆µ is the difference in chemical potential of the solute in the liquid and in the solid phase. If C0 is greater than Ceq, crystals appear and grow in the solvent. They coexist with a solution of concentration Ceq when equilibrium is reached. The frequency of nuclei apparition is nearly zero for small supersaturation and increases dramatically as a function of σ above a threshold value. The mechanism of crystal growth has been widely studied. As shown in the famous paper by Burton, Cabrera, and Franck,4 the morphologies of crystals depend on the supersaturation. For low supersaturation, the crystal grows following a screw mechanism or a two-dimensional nucleated growth mechanism. In these cases, the surface of the crystal remains flat. Above a critical supersaturation, normal growth occurs and the surface of the crystal becomes rough. For very high supersaturation, dendritic growth may be observed. For example, in diamond or snow formation,4-6 the morphologies of the precipitate change,

going from a ball-like to a star-like shape as the driving force increases. The same processes occur during gel formation. The supersaturation of an NHD solution increases when the temperature decreases (see Figure 2). We recall that the gel transition temperature increases when the NHD concentration increases. For a very low cooling rate (0.01°/min), the sample remains for a long time close to the gel temperature and thus the main part of the crystallization process occurs at the gel temperature. In this case, the driving force is low, the number of nuclei is small, and large rodlike aggregates are formed. The sample is a suspension of NHD crystals. It is not a gel. By contrast, for a high cooling rate the main part of the crystallization process occurs at low temperature. Hence, due to the dimensions of the sample and to the heat diffusivity coefficient, the equilibrium temperature is reached rapidly. Then the nucleation and the growth mechanism occur at low temperature. In this case, the driving force is important. A lot of nuclei appear. Strands grow from the rodlike structure, and dendritic aggregates grow. A gel is formed. In the case of the pictures presented in Figures 3 and 4, the supersaturation is equal to ln((37.5 - 12)/12) ) ln(2). Figure 2 shows that Ceq(15 °C) ) 12 mmol/L. This value is rather high and induces dendritic growth. After gel formation, we observed an aging of the gel. It seems that the first formed structures are unstable and that they melt to reform some new structures that are flat. This mechanism looks like an Ostwald ripening mechanism.15,16 The structures formed by the gelification process have different curvature radius. Due to the Laplace pressure effect, the chemical potential of the molecules lying in the region of high curvature is higher than that of the molecules lying in the region of low curvature. As these molecules are soluble in the solvent, this chemical potential difference induces a migration of the molecule from the high-curvature region of the interface to the lowcurvature region. This implies the disappearance of dendritic structure and the formation of flat needles. Rheological Properties. This system is a gel. Its linear viscoelastic properties are usual for organogel systems. The elastic modulus is higher than the loss modulus. The elastic modulus varies as a function of the frequency. This dependence can be explained by taking into account the nature of the junctions present in the gel network. Usually, a frequency dependence of the dynamic moduli, G′ and G′′, is attributed to the temporary character of the bonds that form the network. If the frequency of the mechanical stress is greater than the frequency associated with the lifetime of the bonds, the bonds are not untied at this time

Figure 14. Microscopy picture of the aggregates before (a) and after (b) the whole experiment presented in Figure 13. The aggregates are not broken by the flow. Full scale, 400 microns × 280 microns.

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Figure 15. Rheo-optic picture showing the inhomogeneous flow. The applied shear rate is equal to 10 s-1. Full scale, 400 × 280 microns. The direction of the flow corresponds to the diagonal of the rectangle.

Figure 16. Rheo-optic picture showing the aligned aggregates. The applied shear rate is equal to 500 s-1. Full scale, 400 × 280 microns. The direction of the flow corresponds to the diagonal of the rectangle.

scale. The material then behaves as a gel. By contrast, for time scales such that the frequency of the stress is lower than the bonds’ characteristic frequency, the bonds do not exhibit any mechanical response and the sample behaves as a liquid (G′′ is greater than G′ in this case). The observed dependence of the G′ modulus with frequency indicates that at least some bonds are not permanent. In a previous study dealing with DDOA organogel,8 we have shown that this time scale was associated with the strong junctions of the gel. The main originality of this system is its thixotropic properties. To characterize this behavior, we need to understand why the system melts when it is submitted to a flow and why, when the flow is stopped, the gel reforms. Let us recall that this behavior is unusual for LMOGs. The first question is, why does the system melt under shear flow? In the range of intermediate shear rate (regions 1 and 2), rheo-optics experiments show that the flow is nonhomogeneous in the cone geometry. Such inhomogeneous flow has already been encountered in wormlike micelles, paste, and gels.17,18 Some parts of the sample flow, whereas some others are jammed. The volume of the sample that (15) Ostwald, W. Z Phys. Chem. 1900, 34, 495.

Lescanne et al.

flows increases as a function of the applied shear rate. Aggregates are aligned at high shear rate. Shear modifies the texture of the gel and aligns aggregates along the velocity field. By contrast with the DDOA sample,8 shear does not break the microscopic structure of the gel and does not change the morphology of the aggregates. At this stage, we can say that shear induces a phase transition between a jammed phase and an aligned phase. In the aligned phase, the aggregates are not interconnected which induces a sudden decrease of the viscosity. The melting of the gel is thus related to a phase transition between a jammed phase and an aligned phase. The second question is, why does the gel reform when the flow is stopped? When shear is stopped, thermal fluctuations and gravity rebuild the connections between the aggregates. This process is possible because shear does not change the structure of the aggregates and does not break them. Note that this indicates that the entanglements between aggregates are weaker junctions than the strong chemical junctions between rods. Indeed, chemical junctions unlike the entanglements are not broken by the flow. Let us now come back to the analysis of the first part of the rheological curve. This part presents two kinds of responses as a function of the applied shear rate. First, the averaged shear stress seems to decrease as a function of the applied shear rate; second, it seems to become constant and not to depend on the applied shear rate. The decrease of the shear stress as a function of the shear rate does not indicate a region of negative velocity. It simply shows that the measurement of the viscosity is not valid because the flow is nonhomogeneous. Such a behavior has already been encountered in a previous study dealing with a thixotropic gel made with aggregated nanoparticles.19 Pignon et al. assume that one of the reasons the stress falls at low shear rate is connected with the fact that flow occurs in those situations in a layer whose thickness tends toward the characteristic dimensions of the suspended objects or interparticle forces. A high stress is required to induce flow when the size of the flowing layer is smaller than the characteristic size of the suspended objects. When the applied shear rate is increased, the thickness of the flowing layer increases and a smaller stress is required to induce flow because the friction between the two jammed structures is lower. The same explanation may be also valid for our system. Let us note that the structure of these gels and of our gel involved the same length scale since the Laponite nanoparticles are aggregated20 and that the authors claimed that the rheological properties are governed by the large length scales (of the order of a few microns).21 These rheo-optics measurements reveal a phase transition between a jammed phase and a fluid one. However, some questions remain open. What is the value of the shear rate in the flowing part of the sample? Is this value the same in the entire intermediate shear rate zone? These questions require local measurements of the velocity. Such (16) (a) Servi, I. S.; Turnbull, D. Acta Metall. 1966, 4, 161. (b) Berriman, R. W. J. Photogr. Sci. 1964, 12, 121. (c) Stabel, A.; Heinz, R.; de Shryver, F.; Rabe, J. P. J. Phys. Chem. 1995, 99, 505-507. (d) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 1389-1395. (17) Larson, R. The Structure and Rheology of Complex Fluids; Oxford University Press: New York, 1999. (18) da Cruz, F.; Chevoir, F.; Bonn, D.; Coussot, P. Phys. Rev. E 2002, 66, 051305. (19) Pignon, F.; Magnin, A.; Piau, J. M. J. Rheol. 1996, 40 (4), 573. (20) Pignon, F.; Magnin, A.; Piau, J. M.; Cabane, B.; Linder, P.; Diat, O. Phys. Rev. E 1997, 56, 3281. (21) Pignon, F.; Magnin, A.; Piau, J. Phys. Rev. Lett. 1997, 79 (23), 4689-4692.


measurements are under way in our laboratory using light scattering measurements in heterodyne geometry.22 Conclusion This study leads to a better understanding of the gel formation mechanism and of its rheological properties. Microscopy experiments demonstrate that gel formation occurs through a nucleation-growth mechanism. Nuclei first appear as strong, crystalline junctions, and fiberlike objects emanate from these nuclei to give rise to dendritic structures. Two kinds of junctions, that we here refer to as “connected” and “entangled”, between the fibers are shown to occur with different characteristic energies and times. The mechanism of gel formation appears to be due to a precipitation process. An aging phenomenon occurs after the formation of the gel. Ostwald ripening induces the formation of flat needles. After 2 weeks, the sample is no longer a gel but looks like a rod suspension. (22) Salmon, J. B.; Manneville, S.; Colin, A.; Pouligny, B. Eur. Phys. J.: Appl. Phys. 2003, 22, 143.

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During the first week, the gel is in a metastable state. During this period, it exhibits thixotropic properties. The structural determination of the gel under shear led us to a better understanding of its viscoelastic behavior. By contrast with DDOA gels, NHD aggregates support mechanical stress and do not break under flow. The existence of thixotropic properties is due to a phase transition under shear between a jammed phase of anisotropic aggregates and an aligned phase of these aggregates. At rest, some connections are rebuilt due to gravity and thermal fluctuations. Our current efforts are directed toward the stabilization of the gel in its metastable state. The use of a mixture of organogels or of a molecule less soluble in the solvent may reduce the Ostwald ripening.23 LA035219G (23) Gandolfo, F. G.; Rosano, H. L. J. Colloid Interface Sci. 1997, 194, 31.

Thixotropic Organogels Based on a Simple N-Hydroxyalkyl Amide

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