Langmuir 1994,10,3406-3418
3406
Organogels and Aerogels of Racemic and Chiral 12-HydroxyoctadecanoicAcid P. Terech,*>tJV. Rodriguez,tsg J. D. Barnes,” and G. B. McKenna” Institut Laue Langevin, 156X,38042,Grenoble Cedex, France, and National Institute of Standards and Technology, Polymers Division, Structure and Mechanics Group, Gaithersburg, Maryland 20899 Received October 26,1993. I n Final Form: May 3, 1994@ Gels were prepared by cooling dilute solutions of 12-hydroxyoctadecanoicacid (12-HOA),a fatty acid surfactant, in a variety of organic solvents. Both the racemic (DL)and chiral (D)forms of 12-HOAwere used. Small angle neutron scattering, small angle X-ray scattering, and wide-angle X-ray scattering investigations demonstrated basic similarities between the chiral and the racemic gels. The gels are three-dimensional networks of fibers of rectangular cross-sectional shape with varying aspect ratio and thickness. The fiber network is strengthened by junction points where the molecules pack in a monoclinic crystallineform. “Headto head contactsbetween the carboxylic acid groups and the formation of multiple hydrogen sequences are ubiquitous in these gels.
I. Introduction Various surfactants in a semidilute range of concentrations (typically of the order of 1%) are known to gel some organic solvents’ and give thermoreversible plastic materials. These gels are viscoelastic solids formed through a sharp sol-gel threshold during the kinetics of aggregation of the amphiphilic molecules. Among the few examples available, the 12-hydroxyoctadecanoic acid (HOA) is of special interest because of the exceptionally large variety of solvents which can be gelled. In addition, the HOA molecule has a unique asymmetric center ( C d , making accessible the optically active enantiomer D-HOA and the racemic mixture DL-HOA. HOA is part of a chemical class of fatty acids and derivatives which still receive a great deal of attention because their study provides fundamental information on the physics of surfactant aggregation in aqueous solutions2and organic solvents and also because of their related industrial applications (i.e. as detergents and lubricants). In this context, the chiral HOA organogels have been well a n a l y ~ e d . These ~ surfactant-made physical gels are heterogeneous materials formed with a 3D network of aggregated HOA molecules drowned in a fluid phase. The aggregates are very long (up to several micrometers) and rigid fibers. Within the fibers, the hydroxyl groups on the chiral carbons CIZare connected to the neighboring molecules by unidirectional hydrogen bonding sequences. The fibrillar aggregates merge into microcrystalline nodes of variable shape depending on the solvent type and c~ncentration.~ The present study of the racemic DL-HOAI organic solvent systems add to the understanding of this class of gels by answering some basic questions addressing (1)the ability of racemic system to form organogels
* Author to whom correspondence should be addressed. Institut Laue Langevin.
* Present address (member of CNRS): CEA-DBpartement de +
Recherche, Fondamentale sur la Matiere Condensee, SESAM-PCM, 17,rue des Martyrs, 38054,Grenoble Cedex 9,France. 8 Present address: Laboratorie de Spectroscopie Molkculaire et Cristalline, Universite de Bordeaux I, 351,cours de la Liberation, 33405 Talence Cedex, France. National Institute of Standards and Technology. Abstract published in Advance A C S Abstracts, September 1, 1994. (1)Terech, P. A m . Inst. Phys. Conf. Proc. 1991,226,518. (2)Charvolin, J. J . Chim. Phys. (Paris) 1983,80,15. (3)Tachibana, T.; Mori, T.; Hori, K. Bull. Chem. SOC.Jpn. 1980,53, 1714;1981 54,73. (4)Terech, P. J . Phys. 11 1992,2,2181. @
(considering that it has been observed that aqueous racemates could not produce gels5), (2) the structural features of the related racemic aggregates, and (3) the structural correlations between the gel, aerogel, and crystalline solid states which distinguish the crystallization and gelation processes with either chiral or racemic compounds. Linear hydrogen bonding has already been observed to be responsible for the growth of helical fibers either in water (aspartic acid derivative6) or in organic solvents (such as D-homoandrostanyl steroid7). Enantiomer discrimination (for stereamidess in water) and enantiomorphic relations (for HOA in organic solvents3)have also been proposed. A “chiral bilayer effect” was introduced on the basis of studies on D-gluconamide aqueous gels.5 This effect would explain the stability of chiral gels compared to the racemic homologues, which precipitate as crystals instead of giving gels made up of fibers of bilayers. Electron microscopy and differential scanning calorimetry (DSC) were used in these earlier studies5 to probe the structure and longevity of the aggregates. The “chiral bilayer effect” is based on a tendency for rearrangement from “head to tail” conformations of the enantiopolar planes of the crystal toward “tail to tail” conformations in the aqueous chiral gels. The related racemates appear to be denser in the solid state and form enantioapolar bilayers. The transition from chiral gels to crystals takes a long time because it involves major molecular rearrangements from enantiopolar to apolar bilayers. This necessity for molecular rearrangement exerts a stabilizing effect on the chiral metastable gels. As for organic solvents, the conclusions drawn from the above studies5s9are not so overwhelming, and no fine structural investigation is yet available. The situation is expected to be fundamentally different as only dipolar interactions between nonionic surfactants prevail in the medium in contrast to aqueous systems where various strong ionic interactions, hydrophobic effects, or specific hydrogen bondings between the solute and water can develop. ( 5 ) Fuhrhop, J. H.; Schnieder,P.; Rosenberg,J.;Boekema, E. J.A m . Chem. SOC.1987,109,3387. (6)Imae, T.; Takahashi,Y.; Muramatsu, H. J . A m . Chem. SOC.1992,
114,3414. (7)Terech, P. J. Colloid Interface Sci. 1985,107,244. (8)Amett, E.M.; Thompson, 0. J.A m . Chem. SOC.1981,103,968. (9)Fuhrhop,J.H.; Schnieder,P.;Rosenberg,J.;Boekema,E.;Helfrich, W . J . A m . Chem. SOC.1968,110,2861.
0743-746319412410-3406$04.5010 0 1994 American Chemical Society
Organogel and Aerogel Formation
Langmuir, Vol. 10, No. 10, 1994 3407 1s
1\
1.o
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0.5
0
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loo0
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25
3000
time (s)
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Figure 1. Rheological time sweep experiments for DL-HONnitrobenzene gels: (A) the warmed nitrobenzene solution (C = 0.345 wt %) is introduced between the cone and plate of the rheometer and cooled to cross the sol-gel threshold; (B) corresponding G ( T ) at various concentrations (1, C = 0.345 wt %; 2, C = 0.7 wt %; 3, C = 4.6%). Frequency = 1 r a d s , strain = 1.02%.
The present paper discusses the validity of the "chiral bilayer effect" concept for fiberlike aggregation in organic media on the basis of accurate structural investigations on HOA organogels. The results reported concern mainly the racemic derivative DL-HOA, but complementary studies of the chiral D-HOAcompound are also presented. Molecular arrangements within the aggregates are studied by wide-angle X-ray scattering (WAXS)experiments while the long-range correlations, describing the shape of the aggregates, are studied by small angle scattering (SANS and SAXS) experiments. A calorimetric study of the aerogels is performed to differentiate the solid materials. Correlations are drawn between the HOA optical activity, the solvent type used in the sol-gel process, and results from the DSC, SANS, and SAXS techniques. The experimental results indicate that the short-range organization inside the fibers, the shape of their cross section, and the thermal behavior of the aerogels are interconnected.
11. Experimental Section The optically active D-HOA compound was obtained as previously d e ~ c r i b e d .The ~ racemic DL-HOAand the solvents (benzene, cyclohexane, cis-decaline, tetralin, octane, nitrobenzene, hexafluorobenzene) were obtained from Aldrichlo (99% purity). Gel samples were prepared by dissolving the surfactant in the solvent at high temperature followed by cooling the solution a t room temperature. The 0.34-13 wt % concentration range was investigated. The aerogels were obtained by very slow evaporation of the solvent from gels; the sample was allowed to progressively shrink into a spherical solid mass of very reduced volume. Attempts of crystallization in nongelling solvents (methanol)gave only poor quality crystals and so powder samples were used in the WAXS studies. The diffraction pattern of the nonsubstituted octanoic acid (OA) homologue (Aldrich) was recorded and used as a reference. The SANS investigation of the racemic gels was made using the 20-MW reactor a t NIST (Gaithersburg, MD) using the 8-m and 30-m" spectrometers. A common wavelength of 10 A was used with a mean triangular wavelength distribution of 0.25 and 0.31 for the 8- and 30-mspectrometers, respectively. The monochromators were mechanical velocity selectors with variable speeds and pitches. The bidimensional detectors were 64 x 64 cm2 3He position-sensitive proportional counters giving 128 x 128 arrays of raw data. Absolute intensities were obtained by calibration with the standard scatterings of a silica gel and a polystyrene sample for the 8-m and 30-m spectrometers, respectively. For the 8-m spectrometer, the detector was pivoted by 7.5" about the sample to extend the angular range, and a multibeam converging collimation system was used. The SANS
data reduction and usual corrections for the detector response, subtractions of the various transmission corrected backgrounds, masking steps of the detector, and absolute calibration were performed with the NIST standard programs. The momentum transfer Q (A-l) was defined as Q = (4dd)sin 0 as usual for pure elastic scattering, where 0 is half the scattering angle. The complementary SAXS study of some samples was made a t NIST usin a 10-m camera (sample-detector distance = 5 m) a t 1 = 1.54 (copper target of a 12 kW rotating anode generator) with a two-dimensional position-sensitive proportional counter (25 x 25 cm) delivering 128 x 128raw dataarrays. Data were corrected for the empty beam signal and a polyethylene standard was used to normalize scattered intensities. Additional neutron scattering experiments on chiral systems have been performed on the 58MW high flux reactor of ILL (Grenoble, France). The D11 spectrometer has been used at four different distances and the calibration was made with a light water standard. A planar square (BF3) multidetector with 64 x 64 elements (1cm linear resolution in both dimensions) was used to get 64 x 64 arrays of raw data. The instrumentation and software used at ILL have been extensively described else~here.'~J~ Wide-angle X-ray scattering (WAXS) experiments used an Elliot GX13lorotating anode generator equipped with a double focusing system of two Frenk's mirrors for pointlike collimation. Experiments were done at the European Molecular Biology Laboratory (EMBL) Grenoble outstation. Powders were irradiated in identical conditions within spun 1.0 mm diameter glass capillaries. Relative intensities were measured with a Photoscan P l O O O densitometer (Optronics Inc.10) from photographic films. The racemic systems were characterized rheologically using the RheometricslORMS 800 rotatory rheometer at NIST. Acone and plate geometry (25 mm diameter, 0.0982 radian cone angle, 0.047 mm gap) was used for the experiments and the transducer was a force rebalance transducer with a 100 gcm torque capacity a t full scale. The dynamic mechanical response was examined in the frequency range from 0.001 to 100 rad/s. Calorimetric analysis of the aerogels was performed at CEA-Grenoble using a Perkin-Elmer DSC-2C'O differential scanning calorimeter. Experiments were performed a t 0.31 K min-I and instrument calibration with standard compounds gave reproducibility in the
1
(10) Certain commercial materials and equipment are identified in this paper to specify adequately the experimental procedure. In no instance does such identification imply recommendation or endorsement by the National Institute of Standardsand Technologyor CEA-Grenoble, nor does it imply necessarily that the product is the best available for the purpose. (11)Glinka, J.; Rowe, J. M.; LaRock, L. G. J.App1. Crystallogr. 1986,
19,427.
(12) Ibel, K. J . Appl. Crystallogr. 1976, 9, 296.
(13)Guide to the neutron research at the ILL, ILL, Grenoble,France (1988).
Terech et al.
3408 Langmuir, Vol. 10, No. 10, 1994 I
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frequency (radh) Figure 2. Rheology of a DL-HOMnitrobenzene gel sample. Frequency sweep experiments, C = 0.345 wt %. The complex viscosity v*, the storage modulus G , and the loss modulus G are plotted. ?5ol
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'c..,. . +, 1 Figure 3. Cross-sectional intensity normalized by the concentrationC scatteredby fiberlike HOA aggregates in benzene ,1 , gels. Q is in A-l andZin cm-'. Curve 1: optically active D-HOA, 1o . ~ , C = 0.0235 g - ~ m -SANS, ~, ordinates have been divided by 10 1 0.001 0.01 0.1 Q for the sake of clarity. Curve 2: Racemic DL-HOA,C = 0.0147 Curve 3: racemic DL-HOA,C = 0.0268 g ~ m - ~ Figure 5. SANS cross-sectionalintensity scattered bylamellarg ~ m (SANS). -~ (SANS). Curve 4: racemic DL-HOA, C = 0.030 g - ~ m(SAXS). -~ like aggregates in HONnitrobenzene gels. Q is in A-1 and Z Full lines are theoretical adjustments given by eqs 1for square in cm-'. Curve 1: optically active D-HOA,C = 0.0115 g ~ m - ~ , cross sections. Fittin parameters are as follows: 2, a = 97 A, ordinates divided by 100 for clarity. Curve 2: optically active E = 0.08, 4,a = 100 , E = 0.1. ~, divided by 10. Curve 3: D-HOA,C = 0.0355 g ~ m -ordinates . 4: racemic DLracemic DL-HOA,C = 0.0081 g ~ m - ~Curve HOA, C = 0.158 g - ~ m - Full ~ . lines are theoretical adjustments temperature determination of the melting transition of apgiven by eqs 2 for ribbonlike aggregates. Fitting arameters proximately 0.05 K. are as follows: 2, T = 300 A, E = 0.35; 4,T = 300 E = 0.35. 111. Results G' and G (loss modulus) are roughly constant over 4 We observed no difference between the gel forming decades of the angular frequency and G is much greater ability of the racemic DL-HOAand chiral D-HOA. This than G . observation contrasts with previous claims that racemates Typical SANS and SAXS scattering curves of HOA gels do not form gels because they should produce platelets3r5 in benzene, cyclohexane, cis-decalin, tetralin, and niinstead of forming fibers. There are also no differences trobenzene are reported in Figures 3, 4, and 5. The between the chiral and the racemic gels4 with respect to graphical representations &I, classically used for fibrillar (1)the aspect ofthe samples (benzene gels are much more scatterers,l4v4shows the cross-sectionalscattered intensity transparent than the cyclohexane, nitrobenzene, or versus Q. A plateau at low angles is good evidence that hexafluorobenzene gels), (2) the variety of organic solvents scatterers can be described as rigid fibers (see expression which can be gelled, (3)the range of concentrations, and 1-1). Figure 3 compares the scatteringbehavior ofbenzene (4)the longevity of the samples-no special metastability gels as a function ofthe HOA concentration and the optical being encountered with the racemic systems of HOA. activity and demonstrates that the profile is not strongly Figures 1 and 2 show some preliminary mechanical altered by these parameters. In all cases, the &-I characterizations of the gel state observed for the racemic asymptotic low-angle plateau and the wide-angle Bragg HOA. Time sweep experiments, recorded at low frequency peak (Q x 0.14 A-l, more clearly seen with the X-ray (1 radk) for solutions of increasing concentration, show experiments) are observed. When the concentration is that the elastic modulus G (real part of the complex shear increased, the flatness of the plateau is improved. The modulus) settles a t a given time (or temperature) of the intermediate &-rangeof the sharp intensity decay displays kinetic curve. The threshold is clearly seen for the dilute sample and is strongly dependent upon the concentration (14)Glatter, 0.;Kratky, 0.InSmallAngleX-ruyScuttering;Academic Press: London, 1982. (Figure 2). For gels at equilibrium, Figure 2 shows that a**..
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Organogel and Aerogel Formation
Langmuir, Vol. 10, No. 10, 1994 3409 I
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Figure 6. Thermograms of racemic and optically active HOA aerogels for a selection of organic solvents: A, benzene, (1)(DL) crystallinesolid, (2) (DL)aerogel, (3) (DL) aerogel after being heated between TAGand T,, (4) (D)aerogel; B, octane, (1)(DL) aerogel, (2) (D)aerogel; c, CsFs (1)(DL) aerogel, (2) (D)aerogel; D, nitrobenzene, (1)(DL) aerogel, (2) (D)aerogel.
two oscillations (curves 1 and 4 of Figure 3). Figure 4 shows the scattering curves (SANSand SAXS)of racemic HOA gels in hydrocarbons like cyclohexane, cis-decalin, and tetralin for which a Q-' decay at low-angles is also observed. For the nitrobenzene gels of Figure 5 , the slope d(logO/d(log Q )of the scattering curve at low angles is -2 and a Q21 uersus Q representation, typical of very rectangular or lamellar shapes (see expression 2-1), is preferred. Comparison of the dependence of the intensity decay upon the HOA concentration for racemic and chiral nitrobenzene gels reveals that no major qualitative changes can be observed between the samples. Figure 6 depicts the thermal analysis results for the HOA aerogels. Here the behaviors of the chiral and racemic organoaerogels are compared with each other and with their behaviors in different solvents. As a reference we also show the crystalline powder results obtained from a solvent which cannot be gelled (methanol, Figure 6A). There are several points to be made in discussing these thermograms. First, in comparing behaviors, the largest differences are to be seen between thermograms obtained from different solvents, e.g., Figure 6A(benzene)vs Figure 6D (nitrobenzene). However, this is not the major point of the discussion and we turn to the subtle differences seen in the thermograms for the chiral and racemic systems in the same solvent. The interpretation of these subtle effects is consistent with the scattering results. The temperature positions of the thermal events are collected in Table 1. The final melting temperature TM of the solids, seen as a rather sharp endothermic peak, seems t o be weakly sensitive to the optical activity: TM in the crystalline powder changes from 353.8 K in the
chiral form to 353.3 K for the racemic. From Table 1 there is a suggestion that the racemic forms have a TM value slightly lower than for the chiral forms, but the difference is very weak. For the organogels, the melting peak is preceded by a broad endothermic peak whose shape and position depend strongly upon the solvent type and weakly on the HOA optical activity. For many chiral organogels (see Figure 6B and ref 3) the broad peak (position TAG) is immediately followed by a sharp exothermic peak, a behavior that is not as readily distinguished in the racemic organogels. With CeF6 a single endotherm is observed in both cases (D or DL) while with nitrobenzene no clear endotherm a t TAGis seen. When the racemic organogel is heated to a temperature between the two endothermic peaks and cooled, a subsequent temperature scan results in a thermogram that exhibits only the final melting endothermic peak a t TM. The irreversibility of the related phase transition in the solid state a t TAGis illustrated in Figure 6A (curve 3) for a benzene racemic aerogel. The magnitude of the temperature difference AT = TM - TAGvaries from 0 K (nitrobenzene) to 3.7 K (octane) for the chiral aerogels and from 0.9 K (nitrobenzene) to 3.7 K (benzene) for the racemic aerogels. The same series of organic solvents has been used for WAXS studies of both chiral and racemic organogels and aerogels. Typical powder diffraction patterns are shown in Figure 7 for a crystalline solid (DL-HOA)and a benzene gel and aerogel. The diffraction patterns of all the materials are comparable and show Bragg peaks typical of very crystalline systems. The intense and sharp (001) and (003)reflections are slightly shifted depending on the
Terech et aZ.
3410 Langmuir, Vol. 10,No. 10, 1994 Table 1. Calorimetric and Structural Data for Racemic and Chiral HOA Organogelsa
solvent
surfactant chirality
none
D DL D DL D DL D
cyclohexane benzene
hexafluorobenzene
350.61-351.2 350.75 350.251-350.9 350.21351.1 351.4 351.1 350.71-351.2 350.71i-352.75
DL
octane
D DL
nitrobenzene
D
TAG(K)
352.1
DL
TM(K) 353.8 353.3 354.0 353.1 353.8 353.9 354.1 353.9 354.4 353.5 353.1 353.0
AT(K) 0.00 0.00 3.40 2.95 3.55 3.65 2.70 2.80 3.30 2.80 0.00 0.90
L(A)
k
Mgeo
0.0 190
0.36
1.43
200
1.0
1.00
200
0.0
0.10
150
0.0
0.18
300
0.0
0.045
a For each type of solvent, the maximum of the broad endothermic peak, TAG, and the final melting temperature, TM,are indicated along with the longest length of the side (A = 2a) of square or rectangular cross sections (for fibers) or thickness T (for ribbons). k is the anisometric ratio (see eq 1)of the cross section of the fibers or ribbons. l/kge,, is a parameter describing the ability to develop large cross-sectional areas of the unidirectional aggregates (k:k'f" = &AB2or k $ p = l o p ) . For ribbons, the minimum size f, of the lateral extension in very rectangular cross sections is taken as L E 10T. This approximation is the limit beyond which no distinction can be made between a finite or infinite rectangular cross section, as seen by SANS.4
maximum for the solid (0.367 8)and follows the sequence solid/benzene/octane/cyclohexane/nitrobenzene~exafluorobenzene in decreasing order (see Table 2).
w
0
I
1
0.5
2
1.5
Q(A-') Figure 7. Experimental WAXS patterns of racemic DL-HOA (1)crystalline powder, upper trace is a blow up of the high Q region; (2) aerogel from benzene, upper trace is a blow up of the . high Q region; (3) gel in benzene at C = 0.025 g ~ m - ~Curve 4 is the corresponding diffise scatteringof the solvent (benzene). solvent type in which the gel or aerogel has been prepared (see also Table 2). At larger &-values, a fine analysis of the aerogel diffractograms shows subtle differences between the various patterns depending on the solvent type or optical activity (not shown). Clearly, the 1-1.8 8-1 range (or 19-26' in 28 units), more resolved in the crystalline or aerogel states, is largely smeared out in the gel state within a broad line. Only rare (hkZ)reflections can still be distinguished for the most crystalline gels (concentratedsamples). If variations of the mean reticular distance D = 2~d/((&(001) &(003))/4)are considered as a function of the HOA optical activity and solvent type (from Table 21, the particular behavior of benzene aerogels is noticed. With benzene the sense of variation from racemic to chiral aerogels is identical and ofcomparable amplitude. A correlation with the cross-sectional shape of the aggregates from SANS measurements will be drawn in the following. It can be pointed out that the intensity ratio increases from the racemic to the chiral derivative in all solvents except hexafluorobenzene. In the latter, the HOA/C&'6 gel will stand apart from the average behavior of the HOA organogels. This observation can probably be correlated with the fact that the distribution of the aromatic rings in the liquid structure of benzene and hexafluorobenzene is opposite (respectively orthogonal15 and parallel16). Comparing the D and DL derivatives, the amplitude of the decrease of the spacing D from the D to DL derivatives, is
+
IV. Analysis and Discussion JY-1. Racemic DL-HOA/OrganicSolventsSystems. (a) Rheological Studies. Dynamic mechanical measurements were performed to examine the gelation and elastic properties ofthe different systems. A more detailed report of the rheological behavior is the subject of a future study. Here are simply reported results that indicate approximately when gelation occurs and that arguably demonstrate that gelation has occurred. To this end, the storage modulus G is reported as a function of time after the solution is prepared, indicating the onset of gelation. Also, once gelation has occurred G and G , - t h e loss modulus, are reported as a function of frequency and, as described below, their behavior is consistent with that of a gel. A discussion of the definitions of G and G as well as of gel-like behavior is given in ref 17. When the warmed racemic solution is cooled, the gelation of the HOA aggregates is evidenced by an abrupt increase of G a t some characteristic time that depends on concentration, as shown in Figure 1. The solid or gel-like state is evidenced by the fact that both G and G" are virtually independent of the frequency over the range tested (figure 2). Furthermore, the magnitude of G is much larger than what would be found for entangled polymeric solutions (which are viscoelastic) in the same concentration range even though the concentration is very low (C= 0.345 wt %, G = 3700 Pa). These results indicate that the material can be considered as a gel and the physical junctions in the network appear as "permanent" cross-links with lifetimes that are much longer than those of the entanglements which result in rubbery behavior in viscoelastic fluids. The racemic HOMorganic solvent systems give authentic thermoreversible gels which behave as viscoelastic solids over the range of frequencies studied. The mechanical response, as evidenced here, is similar to that exhibited by chiral HOA18and suggests the existence of microcrystalline nodes similar to those of the related chiral organogels, as discussed subsequently. (15)Bartsch, E.; Bertagnolli, H.; Schulz, G.; Chieux, P. Ber. BunsenGes. Phys. Chem., 1986,89, 147. (16) Bartsch, E.; Bertagnolli, H.; Chieux, P. Ber. Bunsen-Ges. Phys. Chem., 1986,90, 34. (17) Ferry, J. D. In Viscoelastic Properties of Polymers, 3rd ed.; J. Wiley and Sons: New York, 1980. (18)Mansot, J. L.; Terech, P.; Martin, J. M. Colloids Surf. 1969,39, 321.
Organogel and Aerogel Formation
Langmuir, Vol. 10,No. 10,1994 3411
Table 2. WAXS Results for Racemic and Chiral HOA Aerogelsa racemic chiral AH" ZDL DDL ID DD
no.
solvent
0 1 2 3 4 5
solid cyclohexane benzene hexafluorobenzene octane nitrobenzene
3.21 4.23 3.63 4.55 4.86 3.16
30.05 30.73 31.67 34.43 40.77
45.93 46.38 46.268 45.696 45.997 46.268
4.29 4.61 4.19 4.20 5.60 4.03
45.563 46.577 45.93 46.064 45.963 46.508
AI
AD
1.336 1.09 1.154 0.923 1.152 1.275
$0.367 -0.197 +0.338 -0.368 +0.034 -0.240
a The sequence of solvents is that of their increasing vaporization enthalpy (kJ*mol-l). Z is the ratio of the intensities Zool/z003 and D is the corresponding long Bragg spacing (A). AZ is the ZdZDL ratio and AD is the DDL- DDdifference (A).
Table 3. Comparison of the Structural Parameters Obtained by SANS Experiments on Racemic and Optically Active4 HOA Benzene Gelsa racemic (DL) optically active (D) C( g ~ m - ~ ) 0.0147 0.0235 overall shape fiber fiber 214 A, E = 0.065 cross-sectional shape size A (square) 194 A,E = 0.08 molecular packing n L (mo1.A-1) 41.7 39 a E is the relative monodispersity of the cross-section proportional to the half width a t half height4 of a Gaussian distribution which also includes the instrumental resolution.
(b)Shapes of the HOA Aggregates. The long-range characteristics of the aggregates were investigated by SANS. The analytical procedure used to deduce the sizes and shapes of the scatterers in the chiral HOA gels4 is also valid for the racemic gels. Long unidirectional aggregates are observed in benzene, cyclohexane, and cisdecalin solvents as demonstrated by the typical Q-l low angle asymptotic behavior of the scattered intensity. For a dilute system, the scattered intensity is proportional to the amount of surfactant aggregated in the fibrillar structures. The angular dependence of the scattered intensity is described by expression 1-1which shows a Q-l low-angle asymptoticbehavior. In the Q-range of the sharp Gaussian intensity decrease, the so-called "Guinier" region for the cross-section of fibers, the corresponding dimensions can be deduced (expressions 1-1and 1-4) in the range QR, < 1and &Ip > 1,with 1, being the persistence length of the fiber. From all previous studies (EM3,SAS4), and HOA fibers appear very rigid over thousands of angstroms. For benzene gels, a good fit to the experimental data is obtained for a square cross-sectional geometry (full side A = 2a = 200 A), very comparable to that of the chiral benzene gels.4 Identical conclusions are drawn from cyclohexane and cis-decalin gels (see Figure 4). If the concentration is increased, the Q-' asymptotic behavior ofthe benzene and cyclohexane gels is preserved as for the chiral gels. For the cyclohexane aggregates, a better fit is also obtained if a slight anisometry of the cross section is introduced. The structural determinations, especially the molecular weight per unit length of fiber (ML) and the cross-sectional radius of gyration (Rc),are made only with the most dilute sample for each solvent so that the influence of the structure factor fo the interacting rigid fibers is minimized. The theoretical scattered intensities are calculated as f01lows~~J~ 2c-2
I(&) = -Ab
Q
MLLzn sin(Qa cos g,) sin(Qka cos g,) Qa cosg, Qka cosg,
[ ~~~~~
~
~
] dg, (1-1) 2
~
(19)Mittelbach, P.; Porod, G. Acta Phys. Austriaca, 1961, 14, 185.
where a and b are half the sides of the rectangular (or square) cross section, k is its anisometry k = bla = BIA (k = 1for a square, k 0 for lamellar shapes). C is the surfactant concentration in g . ~ m - ~ , is the specific neutron contrast (cm0g-l) of the fiber, and esis the neutron density of the solvent (cm~cm-~). b2 is the specific neutron density of the surfactant b2 = N A E z b i / M where the summation is extended over the neutron scattering length values (bi) of the 57 atoms of HOA. N A is Avogadro's number and M the HOA molecular weight (300.49). v2 is the specific volume of the surfactant in the fiber. The neutron contrasts of the racemic and chiral HOA derivatives are taken to be equal because the isotopic composition is identical (C18H3603) and the densities of D and DL compounds can be taken as equal (v2 x 0.98 cm3/g for HOA4) as observed for homologous fatty acids.20 The assumption that u2 within the fibers is very close to v2 in the crystalline state is supported by the extreme similarity of the related diffraction patterns. Experimentally, the HOA specific contrasts in deuterated solvents are -5.222 x 10-lo c d g in benzene, -6.564 x 10-lo cmlg in cyclohexane, and -5.420 x 10-lo c d g in nitrobenzene. Figure 3 emphasizes the similarity of the scattering properties of the chiral and racemic HOA benzene organogels. The mean values using the "Guinier plots" In QI versus Q2 (expression 1-3), measured for the dilute systems, are: number of aggregated HOA per A of linear fiber nL x 41.7 mo1.A-l and R, x 83.6 A giving a = 102 A for fibers with a square cross section (while the best adjustment to the whole scattering curve was obtained with a = 97 A, expression 1-1).The results are collected in Table 3. For nitrobenzene gels (Figure 5 ) , a Q-2 intensity decay at low angles is observed throughout the entire concentration range-an identical observation was made for the chiral HOA gels.4 This behavior is attributed to very flat aggregates which scatter like lamellae of thickness T.The
-
(20)For example: u2 SZ 1.0cmVgfor D- and DL-2-methylodadecanoic acids.21
3412 Langmuir, Vol. 10, No. 10, 1994
Terech et al.
Table 4. Comparison of the Structural Parameters Obtained by SANS Experiments between Racemic and Optically Active Forms of HONCyclohexane and HOA/Nitrobenzene Gelsa cyclohexane nitrobenzene D
DL
0.0125 fiber slightly rectangular k = 0.36 B = 100 nL = 38.5
0.01345
D
DL
~~
C(g~m-~) overall shape cross-sectional shape sizes (cross section) molecular packing
0.0355 ribbon lamellar (k
k = 0.8 B = 150 nL = 49.6
T=300A nA = 0.23
0.0081
-
0)
T=300A nA = 0.23
a
a The molecular packing is expressed as nL (mol..&-1) for fibers and TZA mol.A-2) for ribbons. The geometrical features of cyclohexane gels correspond to approximatly the same radius of gyration R , ca. 86.5 (expression 1-4) and are two equivalent adjustments.
intensity is defined as
where AQ is the volumic neutron contrast (cdcm3)of the scatterer, S is the cross section, and Rt the transverse radius of gyration of the lamellar aggregate. If n L is used to describe the 1D aggregation mechanism in fibers, nA (the number of aggregated HOA molecules per unit area) is preferred for the 2D aggregates. The thickness T i s obtained from the slope of In Q21versus Q2 plot (expression 2-3) in the appropriate Q-range (QT < 1 and QL > 1, L being a dimension characteristic of the lateral extension of the lamellar particle). The mean values (Table 3) are nA = 0.23 mol-A-2 and Rt x 85.3 A, while T is sensitive to the solvent type (from 150 A in octane to 300 A in nitrobenzene). The long-range correlations found in the aggregates of racemic HOA gels are very similar to those of chiral gels (ref 4 and Tables 3 and 4). The 3D solid-like network of HOA gels can be visualized as a random distribution of very long rigid fibers interconnected by junction zones. Considering the concentration and solvent type effects on the low-angle asymptotic behavior of the scattering (see Figures 3,4, and 51, it is reasonable to simulate the shapes of the junction zones as bundles of fibers (see also previous electron microscopy studies3) with an overall crosssectional shape being square or more or less rectangular (lamellae). Appendix 1 describes the small angle scattering of the equivalent composite material at various stages of the growth of suchjunction zones. This procedure could be used, complementary to the common method measuring the width of the diffraction peaks, to evalulate the crystallinity of the samples. Advantageously, the distinction between fibers and lamellae (increased spatial coherence) can be obtained by scattering methods. The “real” network pattern is probably complicated with the presence of diffuse scattering due to the “structure factor” of more or less randomly distributed junction zones and heterogeneities of various shapes. In addition, an idealized situation with an isotropic and periodic network is considered in Appendix 2. Nevertheless, the current model accounts for the concentration effect observed for benzene gels where an extra intensity in the intermediary Q-range leads to an improved flatness of the Q-l plateau. (c) Internal Packing of the Fibers and Their Junction Zones. The short-range length features of the HONorganic solvent systems, defining the internal
molecular arrangements of the fibers and their junction zones,were systematically investigated by WAXS and DSC (see Table 2) to attempt to correlate the chiral and racemic HOA organogels (see section lV-2).As already mentioned, the diffraction patterns of the aerogels and the crystalline powder are very comparable in the 0-2 A-l Q-range, except for a broadening of all the peaks observed in the gel state (see Figure 7). In the gel, the first two strong reflections (001) and (003) remain even at low concentration while at higher concentration (not shown), some high-Q reflections emerge from a broad peak which is mainly due to the solvent (benzene) a t Q 1.4 A-l. In fact, the same features have already been observed for the chiral 12-R-HOAhenzene gel.3 The Bragg peaks indicate the existence of crystalline organizations reminiscent of the solid state and arise from the fibers and their junctions zones organized in a 3D gel network. X-ray analyses for single crystals of DL-HOAcould not be carried out on account of the difficulty of obtaining good quality single crystals. However, reasonable structural assumptions can be made for the molecular packing of DL-HOA. The long spacing of HOA in the crystal, aerogel, or gel states varies slightly around D x 46.2 A (Table 2) and is a function of the solvent type. The periodicity is interpreted as the distance between double plane layers of oriented amphiphilic molecules with a “head to head” packing as observed in many fatty acids and, especially, in the non-substituted octadecaonic acid homologue (OA).21-24Like most fatty acids, OA appears in different crystal forms. The B form, obtained by crystallization in various organic solvents, is the most stable at room t e m p e r a t ~ r e .The ~ ~ structure is monoclinic with a P21la crystallographic space group and four molecules per unit cell. The long spacing is 43.86 A and corresponds to a “head to head” arrangement of the molecules in layers through dipolar interactions between the carboxylic groups. Such a situation can occur in DLHOA and the presence ofthe asymmetric and hydroxylated carbon Clz introduces slight structural modifications allowing the formation of gels in a wide range of organic solvents. The ability of racemic DL-HOAamphiphilic molecules to form gels arises from the propagation oflinear hydrogen bonding sequences within fibrillar aggregates. As OA does not form gels, the carboxylic groups are not the only polar groups involved in the aggregation mechanism of the HONorganic solvent systems for which the hydroxyl groups also participate. The assumption that HOA racemates cannot produce gels, as reported previ(21)Wyckoff,R.G.In Crystal Structures: The Structures ofAliphatic Compounds; Interscience Publishers: New York, 1969;Vol. 5,Chapter IVc, p 589. (22)Dahlen, B.;Lunden, B. M.; Pascher, I. Acta Crystallogr. 1976, B32,2059. (23)Dahlen, B.Acta Crystallogr. 1972,828,2555. (24)Abrahamsson,S.A.; Lunden, B. M. Acta Crystallogr. 1972,B28, 2562. (25)Goto, M.;ksada, E. Bull. Chem. Soc. Jpn. 1978,51, 2456.
Organogel and Aerogel Formation
Langmuir, Vol. 10,No.10,1994 3413
Y
9
IB
I A
Figure 8. Simulated molecular ordering in HOA aggregates (monoclinic symmetry with 2 = 4). "he atomic coordinates of the nonsubstituted OA homologuez5are used and the cell parameters are slightly modified to take into account the WAXS specifities of the HOA systems (see text): A, racemic DL-HOA,B, chiral D-HOA. The crystallographic cell and the traces of the planes containing the aliphatic chains are indicated. The dotted line is the H-bond sequence: (A) regular bonds in the sequence between two orthogonal traces; (B) two bond distances connecting parallel traces.
ously,26 is erroneous. Usually, racemate fatty acids crystallize in the triclinic centrosymmetric space group P1 as 2-~~-hydroxytetradecanoic acid with two molecules per unit Indeed, for DL-HOA,preliminary investigations of the cell constants from WAXS powder data are much in favor of a set of parameters which resembles that of OA, containing four molecules per unit cell. A P21/a2' crystallographic space group or any subgroup with 2 = 4 can be assumed for HOA (like OA). In a previous it has been claimed that such a structural arrangement does not allow hydrogen bonding sequences. The Eresent analysis indicates that this is true only along the b axis while a H-bond sequence along the Fi axis remains possible. Figure 8 shows the molecular ordering obtained when the OA atomic parameters from ref 25 are used and supplemented by a hydroxyl group on the Cl2 position and a slight increase of the 2 parameter of OA from 49.3 to 52.0A to account for the increased long spacing leading to dz 1.04 g/cm3. A zigzag c h a g of hydrogen bonds connects the Clz positions in the G,b) planes with a ca. 2.8 A oxygen-oxygen distance, which is a configuration which appears to generate_the strongest hydrogen bonds. The values of the 7i and b cell parameters, as well as the internal angle of rotation of the molecule along its long axis2, are defining the hydrogen bond lengths which can be drawn in this model. In the worst rotational configuration possible of the HOA molecules,the hydrogen bond length goes up to 4.1 8. In any case, the b direction is unfavorable for the development of a hydrogen bond sequence as, in the best rotational situation, the H-bond length would be ca. 5.1 Consequently, the 1D aggregation mechanism in DL-HOAorganogels most probably arises along the shortest crystallographic direction Fi and involyes two molecules per crystallographic cell in a P21/a (or P1) symmetry with 2 = 4.
A.
(26) Tachibana, T.; Yoshizumi, T.; Hori, K. Bull. Chem. SOC.Jpn. 1979, 52, 34. (27) The space groupP2llais centrosymmetricand has four symmetry operations,therefore one molecule generates the three others.28 If the starting molecule is the L enantiomer, the helicoidal 2-fold axis will generatea moleculewith the same chiralitywhereas the inversion center and the gliding mirror plane along zi will generate two D enantiomers. (28) Znternational Tables for Crystallography; Reidel, D., Eds.; Dordrecht, Holland, 1983; Vol. 4.
The geometrical quantities obtained by SANS can be linked with the specific features of the molecular arrangement in the gel state by
-n~_ -A, nA
mnL =
with
NA2
Mv,
and
M L = n,M
mn -NaT A - Mu,
(3-2)
and MA = n,M
where m is the linear molecular multiplicity, MA is the mass per unit area of the flat particle, and nL takes into account the molecular organization along with the fiber axis. The mechanism of linear aggregation is specific for each system and depends upon the chemical functionality of the surfactant. For the crystalline solid, the concept of n L has no meaning since u2 stands for a tridimensional crystal. Within the above described structural assumptions, the fiber axis is along the 7i direction and the hydrogen bond sequence involves two molecules per crystallographic cell (see Figure 8) and the subsequent linear molecular multiplicity m is therefore equal to 2 A-l. In gels, the expected theoretical values (expression 3-2) for such a crystalline organization in the HOA aggregates are n L = 40.9 m o l k ' (fibers withA = 200 n A = 0.31 mol.A-2 (lamellae with T = 300 A),and ndnA = 133. The agreement with the SANS experiments is satisfactory ( n r p 41.7 mol-A-l, nFpx 0.23 mol.A-,, nJ n A x 180). In summary, Figure 9 illustrates the molecular organization within the HOA fiber. The proposed structural model allows the interpretation of the anisotropic scattering of oriented D-HOA/CGF~ gets detailed in Figure 10 of ref 4. The aligned ribbonlike aggregates are in a plane perpendicular to the incident neutron beam and generate a Q-2 anisometric (elliptical) low-angle scattering while the (001) Bragg diffraction peak is observed as an anisotropic equatorial scattering. Additionally, a detailed contrast profile of the cross section of the aggregates, including the bimolecular
A),
Terech et al.
3414 Langmuir, Vol. 10,No. 10, 1994
eters of the solvent (or their combination). As an example, the thickness of the lamellar aggregates (see also ref 4) is doubled from octane (To = 150 A)to nitrobenzene (To = 300 A)gels while strictly isometric shapes of the cross l sections are observed with benzene (200 A side). These I I effects cannot be summarized by only the dielectric I I constant (or polarity) of the solvents (€benzene = 2.28; €nitrobenzene = 34.82)because lamellae can also be found in octane (€octane = 1.95) or hexafluorobenzene. To attempt structural correlations between racemic and chiral HOA organogels, it was necessary to perform complementary experiments with D-HOA. The difficulty in obtaining single crystals of D-HOAfor X-ray analysis remains as for DL-HOA. Nevertheless, it turns out that the racemic form exhibits a greater crystallinity (polycrystallinity)than the chiral one, as previously observed.26 WAXS patterns of crystalline powder, aerogels and gels of D-HOA(not shown) can reveal only subtle differences with equivalent DL-HOAsystems (see Table 2). (b) Correlations Using SANS, WAXS, and DSC Techniques According to the Optical Activity and SolventType. Calorimetricmeasurements demonstrate significant differences for the aerogels formed in different solvents. Additionally, the DSC thermograms exhibit -‘ -I---subtle effects that can be attributed to the chiral and racemic structures. For the systems formed from nitrobenzene, the D or DL aerogels are made up of lamellae where the coherence extension is intermediate between Figure 9. Structural model of the HOA fibrillar or ribbonlike fibers (l-D) and crystals (3-D) and not surprisingly the aggregates in organic solvents. The crystallographic axis and the fiber geometry are indicated. The dimensions are propormeasured AT (K) of the related thermograms (see Table tional to the crystallographic cell parameters as shown. 1)are very small. In addition, a comparison of D (A) measured on the diffraction patterns for the D and DL organization, is described in Appendix 3. Figure 13 shows compounds in nitrobenzene aerogels shows that racemic how the internal heterogeneity can modify the scattering organization is closer to that of the crystalline powder profile in the Porod region and improve the adjustments than that of the D-HOA (see Table 2). In appropriate of Figures 3, 4,and 5. solvents, the lateral extension of the square (benzene) or IV-2.Tentative Structural Correlationsbetween slightly rectangular cross section (cyclohexane) of the Racemic and Chiral12-HOAOrganogels. (a)Chiral fibers implies a molecular ordering that differs somewhat D-HOAOrganogels. The previous structural ~ t u d i e s ~ > ~more than that in the crystalline solid as evidenced in the have shown that the 3D gel network is formed by endothermic behavior preceding the melting endotherm microcrystallites interconnected by very long and rigid a t 2 ’ ~ For the chiral HOA, the thermograms were fibers. On the basis of diffraction studies of fatty acids,21 previously interpreted3as resulting from the restructuring stearic derivative^,^^>^^*^^ and HOA, a monoclinic ordering of the aerogel through a cold crystallization (exothermic using the identity and screw axis operators is most peak) to a more ordered crystalline state. The present probable (Le. crystallographic space group P21). In this study shows that the racemic aerogels are either somewhat framework, the D-HOAmolecules are arranged “head to more crystallized than the chiral systems or have mohead” in layers through dipolar interactions between the lecular arrangements closer to those of a reference carboxylic groups with the bimolecules defining the long crystalline state (crystal powder obtained from the nonspacing seen as the (001) reflection. Similarly with DLgelling methanol solution). HOA, a unidirectional hydrogen bonding process is It is known that optically active and racemic crystals developed between the hydroxylated C12 positions to of surfactants can crystallize in different crystallographic generate the fiber axis of the aggregates. The shape of space groups.31 Frequently, a primitive monoclinic space the cross section and/or the junction zones can vary group is observed for the D or L derivative and the cell according to the solvent type and/or concentration from parameters can be drastically different from those of the squares (benzene or octane at low concentrations)to very racemic compound. Nevertheless, SANS and WAXS anisometric rectangles (nitrobenzene or octane a t high experiments show that both chiral and racemic HOA give concentrations). As shown by SANS4, the length of one organogels, aerogels, and crystalline powders with almost side of the cross section is a multiple of the bimolecular the same structural features. Therefore, their molecular length. As for the racemic compound, this side is assumed packing should lead to the same kind of hydrogen bond to be almost parallel to the (E) crystallographic axis (see sequences. In section IV-l-c, a probable molecular packing Figure 9). The dipole moments of the OH and COOH is proposed for the racemic form and an equivalent groups are lying at the interface with the solven4. This approach for the chiral forms leads to the noncentrosymarrangement allows the growth of the other side ( b )of the metric space-group P21 with four molecules per unit cell square by similar but weaker dipolar interactions ac(2 = 4). Indeed, most of the chiral derivatives of fatty cording to the solvent type. The cross-sectional size and acids crystallizein this space group but with two molecules shape of the fibers and their junction zones in the gel per unit As for the racemic form, the powder state are obviously correlated with some physical paramdiffraction patterns of D-HOAcan be simulated with a set (29)Lunden, B. M.Acta Crystullogr. 1976,B32,3149. (31)Benedetti, E.;Pedone,C.; Sirigu,A.Actu Crystullogr. 1973,B29, (30)Malta, V.;Celotti, G.; Zannetti, R.; Martelli, A. F. J. Chem. SOC.
Fiber axis (1.a)
e -
-
B 1971,548.
730.
Organogel and Aerogel Formation
Langmuir, Vol. 10, No. 10, 1994 3415
I
1
I
I
I
0
0.5
1
1.5
2
Q ( K ‘1 Figure 10. Simulated diffraction patterns of DL-HOA(curve 1)and D-HOA(curve 2) using the atomic coordinates of OA25 and a sli htly modified c cell parameter (see text). a = 5.59 A, b = 7.385, c = 52 A, p = 117.24’. The resolution is that of the present experimental conditions. Crystallographic space groups: DL-HOA= Pda;D-HOA= PZI. of cell parameters very close to that of OA with 2 = 4.The most favorable molecular packing in terms of H-bonding possibility and density value, for both chiral and racemic derivatives, occurs when the CIZ-0 bonds are aligned along the b axis with a sequence “up-down” involving two molecules per unitsell. Projections of these structural models on a plane (E,b)perpendicular to the chain axis are reported in parts A and B of Figure 8. Along the zigzag chains of hydrogen bonds, the planes containing the aliphatic chains (see their traces on Figure 8) of the successive molecules are perpendicular for the DL-HOA and parallel for D-HOA. The indicative simulated WAXS powder diffraction patterns32of the two structures (Figure 10) are very similar and quite comparable to the corresponding experimental data. In addition, the simulated structure appears more homogeneous in the racemic than in the chiral form as the distances between layers are more comparable in the three directions and particularly along the hydrogen bonding sequence (Ti direction). For D-HOA,the two typical distances of interaction between the oxygen atoms of the Clz hydroxyl groups are do-0 = 2.5 and 3.1 8,whereas for the racemate only one intermediate distance at do-0 = 2.8 A is observed. These molecular considerations are consistent with the differences seen in the calorimetric measurements but are not strong enough to influence the shapes of the cross section of the fibrillar aggregates as studied by SANS. The structures slightly evolve according to the solvent type and temperature of preparation in a way which could be compared to that observed between the two crystalline forms B and C of the OA homologue24~29 where slight specific interreticular and angular shifts lead to small variations of the cell parameters and the related intensity variations described above (Figures 7 and 10 and Table 2). A correlation between the molecular rearrangements, the cross-section shapes, and the profiles of the thermograms can be made. Upon considering the sequence of solvents (based on enthalpy of vaporization) cyclohexane/ benzenehexafluorobenzene/octane/nitrobenzene,we can see how each technique reveals the tendency of the chiral and racemic molecules to assemble themselves. SANS (32) The pattern simulations were performed using the program Fullprof (Rietveld method33). (33) Rodriguez-Carvajal, J. Proceedings of the XV Congress of the International Union of Crystallography; Toulouse, France, 1990, 127.
experiments show that the ability to form anisometric lamellar structures is the same for the racemic and chiral organogels. This ability has been discussed above on a molecular level (Figure 8). Depending on the solvent type, the dipolar interactions in organic media impose a more or less compact and homogeneous fibrillar aggregate according to the strength of the developed H-bonds. The shape of the related cross sections, the calorimetric behaviors, and some differentiating diffraction features appear linked in a complex dependence on some physical constants or their combinations of the solvents. The sequence noted in Table 2 (solidhenzene/octane/cyclohexanehitrobenzenehexafluorobenzene) is also close to the tendency of organogels to develop anisometric structures in the fibers and junction zones of its organogels. In the solid 3-D isotropic organization, benzene develops isotropic structures as in the case for octane at low HOA concentrations,4cyclohexane produces slightly rectangular shapes (also the case of cis-decalin samples), and nitrobenzene and hexafluorobenzene give ribbonlike aggregates with lamellar nodes. Again, it is interesting to note that in the pure solvents benzene and hexafluorobenzene the molecular aromatic planes are either parallel or perpendicular to each other. Interestingly, this singularity (due to opposite quadrupole moments15J6) seems to be correlated with the development of square or rectangular cross sections of the fibers in HONorganogels observed respectively in the benzene or hexafluorobenzene solvents.
V. Conclusion The driving forces for surfactant aggregation in organic solvents are different from those in aqueous systems where the so-called “hydrophobic effects’’ dominate. The important molecular rearrangements from “head to tail” to “tail to tail” observed in some aqueous systems5 during the crystal to gel transition are not observed with the HONorganic solvents systems. Dipolar interactions between the carboxylicpolar heads anchor the dimensions of the rectangular cross sections of the fibers to a multiple of the HOA bimolecular length and are complemented by the growth of unidirectional hydrogen bonds between the hydroxyls on the Clz along the fiber axis. The initial organization in the aerogel state evolves to that of a crystalline state reached by an irreversible solid-solid transition (corresponding to the endothermic peak at TAG). It can be compared to the B to C transition in the OA compound. In organic solvents, the “chiral bilayer effect” observed in aqueous systems could only play a role if the balance of the hydrophobiclpolar interactions of the surfactant aggregateholvent couple could be such that a significant gain of free energy is possible. Furthermore, in water, the hydrophobic interactions are frequently aided by H-bonds with the solvent. In this respect, the situation is simpler in organic solvents. Still, a greater “crystallinity’’ of the racemic HOA gel samples is observed in a system where the ”head to head” conformations, already existing in the solid state, are preserved in the organogels. By contrast to aqueous systems, stable micellar fibers are produced with the racemate HOA derivative in organic solvents. Moderate solvent-dependent molecular reorganizations were investigated by calorimetry and WAXS. The solvent type influences the anisometry of the cross sections by favoring the lateral growth (_fibersor ribbons) or the aggregation of fibers along the b axis within the junction zones. Interestingly, observations of ribbonlike shapes by electron microscopy3* have been previously (34)Tachibana, T.; Kambara, H. Bull. Chem. SOC. Jpn. 1969, 42, 3422.
Terech et al.
3416 Langmuir, Vol. 10,No. 10,1994 made on metal shadowed aerogels. Because the gel network of surfactant aggregates is very fragile, major restructuring a t different scales occurs during the solvent evaporation of the gel to aerogel transformation. Consequently, structural determinations of the shape of aggregates in aerogel samples cannot be extrapolated to the situation in gel samples. The present SAS study developed here is a direct determination of the structures of the HOA aggregates in the gel systems in the presence of the solvent. The racemic network which could be theoretically seen as two interpenetrated and independent chiral networks appears in fact to result from a new mixed structure. A relationship between the molecular packings of racemic and optical enantiomers has already been reported for amino acid isoleucine31and also for valine and tyrosine. In both racemic and optically active forms, molecular layers are associated in pairs by hydrogen bonding and are crystallographically related by inversion centers or mirror planes in the racemic forms and by binary screw axes in the optically active compounds. The understanding of the physical chemistry of this class of materials adds to the knowledge of the structure of the thermoreversible surfactant-made networks and also presents a great interest for derived applications. For instance, in the field of polymer crystallization, the highly dispersed fibers of some aerogels have been used to promote the crystallization of some polyolefins via epitaxy.35 The present study has produced some structural correlations for a variety of HOA organogels. Fatty acids are not the only candidates from this class of materials and future studies will focus on other gelators displaying the typical “thermal signature” of the mesomorphic organization of the aerogels. For instance, an optically active D-homoandrostanyl steroid derivative clearly shows an identical thermal behavior (see ref 36) while having very different diffraction patterns for the gel, aerogel, and crystalline states.
Acknowledgment. We acknowledge NIST (USA)and ILL (France) neutron facilities for providing the neutron beams and all technical and financial supports. NATO and the Polymers Division a t NIST are also thanked for a financial support (for P.T.) and the exchange between ILL and NIST. The “Centre National de la Recherche Scientifique” (CNRS, France) has approved the project and is acknowledged. The EMBL outstation of Grenoble is thanked for the use of the WAXS camera. We thank Professor R. G. Weiss for bringing to our attention ref 5. Appendices: Scattering by Crystalline Gels The simplified formalism of expressions 1and 2 used to analyze the scattering of crytalline gels (type of HOA organogels) is valid for very dilute samples. Some modifications of the scattered intensity can occur when the concentration is increased. These effects are due to the interferences of the neutron waves scattered by the fibers but also by their junction zones. In the following, basic approaches are developed to deal with the effects of (i) the concentration upon the scattered intensity (concentrated systems) and (ii) the internal inhomogeneity of fibers upon their form factor (isolated fibers). 1. Low-AngleScattering: Simulationof the Scattering by Two Populationsof Scatterers. The ability of the cross-sectional shapes to laterally develop from (35)Thieny, A.;Straupe,C.; Lotz, B.;Wittman, J.C .Polym. Commun. 1990.31. 299. (36) Terech, P.Mol. Cryst. Liq.Cryst., 1989,166, 29.
?
i
w
8
0.0001
0.001
Q
0.01
0.1
0,0001
0.001
Q
0.01
0. I
Figure 11. Simulation ofthe low-anglescattering(Qis in A-’) of a gel consisting of two populations of scatterers made up of fibers and junction zones. Two types of gels are considered according to the symmetry of their junction zones: type I = rods + rods and type I1 = rods + lamellae. “he graphic representation QZ uersus Q is appropriate for unidirectional aggregates (fibers). A fibers with different square cross sections (A = 200 A and 800 A). B: rods = square cross section, A = 200 A + lamellae, T = 300 A. Intensities for different fraction x of the constitutive rodlike fibers (1,x = 1.0;2 x = 0.9;3, x = 0.7;4 , x = 0.5) of cross-section size A = 200 A. square to rectangular (from fibers to lamellae or ribbons) is observed both for the optically active and racemic HOA. If in benzene, only a Q-l asymptotic behavior is observed, this is no longer the case in nitrobenzene where an exclusive Q-2behavior is seen. Meanwhile, in some other solvents a mixed situation can be observed according to the concentration (see octane4). A gel network can be formed in the appropriate thermodynamic conditions (concentration, temperature, solvent type) of fibers interconnected by crystalline junction zones whose crosssectional symmetry can be isotropic (the case of benzene) or more or less anisometric (octane, nitrobenzene). The scattering can be described as that of a distribution of two populations of scatterers: the fibers and the nodes. The first system (eq 4-11, which models the rodlike geometry of the nodes, is made up of fibers having a square cross section (A = 200 A, like the fibers in dilute benzene samples) and rodlike nodes with much larger square cross sections (B being the full length of the side). The second system (eq 4-21, which models the lamellar-like geometry of the nodes, is made up offibers having a square cross section (A = 200 A) and ribbon-like nodes (T= 300 A, as in nitrobenzene samples). In both models, the fraction of fibrillar nodes is varied (Figure 11). In the low and intermediate Q-ranges (the so-called “Guinier-range”), the respective scattered intensities Ip, and IL are
where 1 - x is the fraction of surfactant involved in the nodes. Figure 11shows the corresponding scattering curves. If system 1is considered (expression 4-1), the increase of the fraction of the nodes (bundles) leads to (i)a conservation of the Q-l asymptotic behavior while the absolute level of scattering is increased, (ii)an apparent increase of the cross-sectional radius of gyration, and (iii) occasionally, the appearance of an intensity oscillation for very high fraction of rodlike nodes (1- x = 0.7). This oscillation should not be confused with a cross-sectional form factor oscillation (differences in intensity and position). Most of these observations have been confirmed
Organogel and Aerogel Formation experimentally with the benzene gels. If system 2 is considered (expression 4-21, an increase of the number density of lamellae affects significantly the Q-l asymptotic behavior especially at very low-angles which rapidly turns to a QW2decay-a behavior which has been observed for octane and fluorobenzene gels4when the concentration is increased. 2. Low-AngleDiffraction of Statistical Gels. The HOA 3D network consists of randomly distributed rigid fibers. Despite that the HOA gel network scatterers are infinitely long rods, the situation can be idealized as an isotropic distribution of finite rods within a cubic symmetry (Pm3mcrystallographic space group = 221). The goal of this oversimplifiedmodel is to approach the understanding of the concentration effects on the low-angle part of the scattering of these crystalline gels. As an example, it has been observed (see Figure 3 ) that concentrated HOA/ benzene gels exhibit improved low-angle asymptotic Q-l behavior. The Rietveld algorithm3’fits (or simulates) the observed powder diffraction pattern using as variables the instrumental characteristics (resolution curve of the diffractometer) and the structural parameters of the sample material (cell constants, atomic and displacement parameters, etc.). The program Fullprofbased on theYoung and Wiles program (Rietveld refinement39, offers the possibility of adding subroutines for structure factor calculation^.^^ A specialized subroutine for form factor calculations has been utilized.39 As far as the 3D crystalline organization is concerned, the structure factor can be decomposed into the product of the form factor of the molecular aggregates and the network interference function
Langmuir, Vol.10, No. 10, 1994 3417
1
Figure 12. Structure factor for a periodic cubic (Pm3,J distribution of rods (squarecross section,A = 200 A) to symbolize the gelnetwork. Curve 1,“dilutegel”,L= 307OAcorrespondin to 9 = 0.0127; curves 2, 3, “concentrated gels”, L = 1000 corresponding to 9 = 0.12. The junction zones are either isotropic (curve 2) and described by spheres (diameter = 400 A) or anisotropic (curve 3) and described by ellipsoids (half large axis = 400 A and eccentricity = 2.5). Abscissas have the same scale (Q is in A-1) as the experimental spectra (Figures 3, 4,and 5).
R
nodes (case of benzene gels) and f, is given by eq 5-3.
with x = QrJ(sin
wherex,, y,, andz, are the coordinates ofthe nth aggregate in the unit cell, (hkl)are the Miller indices ofthe considered reflection, and N is the number of aggregates per unit cell. The nth form factor f, is expressed according to the shape of the nth aggreggte with respect to the orientation of the scattering vector Q(hkZ)defined by the angles 0 and p (spherical coordinates) within the internal reference system of the aggregate. f, has been calculated for two geometries. ( 1 )for parallelepipeds (sides 2a and 2b and long axis 21) forming the periodic skeleton of the network
fn(hkZ)= 8(abZ) AQ 16sin(Qa sin 8 cos 9 )sin(Qb sin 8 sin 9 )sin(QZ cos 6 )
where A@ is the mean contrast of the aggregate with respect to the solvent. ( 2 )for ellipsoids (radius r and eccentricity e ) describing the nodes or junction zones of the network, only the spherical angle 0 is necessary since the circular plane of the cross section is perpendicular to the z axis of the internal aggregate reference. When e > 1,the ellipsoids are oblate and describe anisometric nodes (case of nitrobenzene gels) while for e = l,spheres describe isotropic (37) Rietveld, H.M.Acta Crystallogr. 1967,22,151. Rietveld, H.M . J . Applied Crystallogr. 1969,2, 65. (38)Wiles, D. B.; Young, R. A. J.Appl. Crystallogr. 1981, 14, 149. (39) Software by V. Rodriguez available upon request.
e)2+ (e cos el2
Numerically, the I, a,b, e values are chosen to account for the situation in benzene and nitrobenzene gels. The distribution of rods determining the mesh size ( E ) of the network is calculated as follows: (E) =L = A ( 3 / 4 ~(where )~~ L = 21 is the length of the equivalent rods, 2a is the full side of their square cross section ( a = b), and #J is the volumic fraction. Figure 12 compares dilute and concentrated gels the nodes of which are either spherical (benzenegels, curve 12-2)or ellipsoidal (nitrobenzene gels, curve 12-3). As shown in Figure 12, the resulting diffraction peaks of the periodic structure affect mainly the intermediary Q-range where the concentration effects are experimentally observed. 3. Simulation of the Scattering by Inhomogeneous Fibers. Because of the very character of amphiphility of the “monomeric” surfactant species, heterogeneous areas exist within the aggregate. For a “head to head” aggregation mechanism, defining the long spacing in the WAXS patterns, an heterogeneous form factor can be calculated in the Q > 2nIA range where the scattering profile is, sensitive to the internal cross section contrast details. The contrast of the cross section is calculated for a square the side ofwhich (also theE sinP crystallographic axis) is made up of four bimolecules (the case of benzene gels). A periodic modulation of the contrast, each twice the lipophilic length (Ls,) of the HOA molecule and twice the carboxylic length (Lpol), is calculated. The form factor Z(Q) of the fibers is the product of an axial term (proportional to Q-1) and a cross-sectional term Z, (see expressions 1and 2). For the above defined heterogeneous
3418 Langmuir, Vol. 10, No. 10, 1994
Terech et al.
,
square cross section, Z, is given by
iI / " II 4
I
1
I
I="' 'I =
sin(Qa cos p) Qa c o s p sin(Q(a-lipo) cos p) Q(a-lipo) cos p I
IV = -A'2
+
Q(a-lipo-2pol-2lipo) cos p
sin(Q(a-lipo-2pol-21ipo-2pol) cos p) Q(a-lipo-2pol-2lipo-2pol)cos p
with lip0 pol =LHOA and AQIand Ae2 being the contrasts with respect to the solvent of the lipophilic (length lipo) and polar (length pol) parts, respectively. Figure 13 shows that the contribution of the crosssectional inhomogeneity is localized in the intensity oscillations area arising from the cross-sectional form factor. Typically, the level of these oscillations (mainly the first three), defined with respect to the intensity level
0.2
/\
.....
_.
. l
L
\
.
1
sin(Q(a-lipo-2pol) cos p) I11 = -Ae2 &(a-lipo-2pol) cos p
sin(&(a-lipo-2pol-2lipo) cos p)
0.1
0
1
0
0.04
0.08
0.12
0.16
Q
Figure 13. Effects of the heterogeneity of the cross section on the SAXS Porod form factor intensity oscillations(Qis in A-1). HOAhenzene gel, C = 0.030 g ~ m - ~The . insert shows the proximity ofthe (001)Bragg reflection. Curve 1, homogeneous cross section, a = 110 A; curve 2,heterogeneous cross section, a = 110 A, contrast ratio (pol/lipo)= 1/3,L,,I = 5 A, Lbp0= 22.5
A.
of the asymptotic Q-l plateau, can be modified. An example of an improved fit (better visualized in Porod representation Q4Z versus Q enhancing the cross-sectional oscillations), is given in Figure 13where the homogeneous situation is also shown. Still, a slight discrepancy remains as a shift in the third oscillation which could be explained by the vicinity of the (001) Bragg reflection.
