Smectic Arrangement of Bentonite Platelets Incorporated in Gels of

Conformational and structural transitions in composite gels of poly(acrylamide) with incorporated suspensions of bentonite clay platelets induced by t...
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Langmuir 2003, 19, 10739-10746

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Smectic Arrangement of Bentonite Platelets Incorporated in Gels of Poly(acrylamide) Induced by Interaction with Cationic Surfactants S. G. Starodoubtsev,*,† A. A. Ryabova,† A. R. Khokhlov,† G. Allegra,‡ A. Famulari,‡ and S. V. Meille‡ Physics Department, Moscow State University, Leninsky Gory, Moscow 119992, Russia, and Materials and Chemical Engineering, “G. Natta”, Department of Chemistry, Polytechnic of Milan, Via L. Mancinelli 7, Milan, I-20131, Italy Received September 1, 2003. In Final Form: September 18, 2003 Conformational and structural transitions in composite gels of poly(acrylamide) with incorporated suspensions of bentonite clay platelets induced by the interaction with cationic surfactants were investigated. X-ray diffraction measurements demonstrate that due to the interaction with cationic surfactants initially disordered clay platelets can arrange into highly ordered smectic structures incorporating about 5-6 layers. The corresponding conformational transition is accompanied by shrinking of the gels. Small-angle X-ray scattering data show that the self-ordering of the platelets increases with both the concentration of the platelets and the length of the hydrophobic tail of the surfactant. While moderately cross-linked gels do not inhibit the formation of smectic organo-clay aggregates, larger concentrations of highly cross-linked gels clearly limit the process. It was demonstrated that the structure and properties of the gels depend on their previous history. In gels dried on glass surfaces, the smectic ordering becomes anisotropic and the platelets orient parallel to the substrate. Models of the smectic polymer/clay/surfactant complex in the gel phase are formulated and discussed.

Introduction The properties of hydrogels can be significantly modified through incorporation of colloids in a polymer network. One of the new classes of these composite systems are the gels loaded with dispersed clays. These composite materials combine the elasticity and permeability of gels with the high ability of clays to absorb different substances.1-4 The clay particles embedded in the cross-linked swollen polymer network strengthen the gel and prevent its collapse in bad solvents.3 On the other hand, immobilization in the cross-linked network prevents coagulation of dispersed clay particles. Among the other clays montmorillonite (MONT) and bentonite (BENT), which also has a high content of MONT, are the most important and widely used absorbents for organic compounds.5-8 Their key property is the ability to show extensive swelling in water. As a consequence, MONT has a large active surface area (700-800 m2/g). This peculiarity makes MONT and BENT the best candidates for use in gel-clay absorbers.7 The layer structure of clay minerals was deduced by Pauling.9 Each layer of MONT consists of two silica sheets * To whom correspondence should be addressed. † Moscow State University. ‡ Polytechnic of Milan. (1) Gao, D. Preparation and Property Improvements of a Superabsorbent Polymer Composite; Alberta Research Council: Edmonton, Canada, 1992; Chapter 1. (2) Gao, D.; Heimann, R. B. Polym. Gels Networks 1993, 1, 225. (3) Churochkina, N. A.; Starodoubtsev, S. G.; Khokhlov, A. R. Polym. Gels Networks 1998, 6, 205. (4) Starodoubtsev, S. G.; Churochkina, N. A.; Khokhlov, A. R. Macromol. Symp. 1999, 146, 193. (5) Grim, R. E. Clay Mineralogy; McGraw-Hill Series in Geology; McGraw-Hill: New York, 1953. (6) Hoffmann, U. Angew. Chem., Int. Ed. 1968, 7, 681. (7) Theng, B. K. G. Formation and Properties of Clay-Polymer Complexes; Elsevier: Amsterdam, 1979; Chapter 1. (8) Kukovsky, E. G. Structure and Properties of Clay Minerals; Naukova Dumka: Kiev, 1966.

with one aluminum sheet between them.6,7 A silica sheet has two planes of oxygen/hydroxyl ions, one of which consists of the bases and the other of the tips of linked Si(O, OH)4 tetrahedrons. Aluminum sheets contain planes of octahedral Al3+ ions, coordinated to the -O- and -OH groups. Due to isomorphic substitution of Si4+ by trivalent metal cations, the SiO- groups (whose charge is not compensated) form salt bonds with their counterions or undergo hydrolysis with the formation of SiOH groups. The counterions are capable of ion exchange with the cations in the solution. Figure 1a schematically shows a single platelet of the sodium salt of MONT surrounded with counterions. The lateral size of the sheets is large, ca. 100 nm. Due to this, the suspensions of MONT and BENT usually have high viscosity. In the dry state, the crystals of smectic clays are composed of multilayers, see Figure 1b. The swelling in water results in disordering of the platelets. Due to attraction forces, especially between the edges and the surface of the platelets, clays form highly viscous liquids (gel) with a card-house structure schematically illustrated in Figure 1c.6-8 At high dilution, the clay platelets form stable colloid dispersions with a disordered orientation of the platelets. Particles of BENT suspension embedded in poly(acrylamide) gel (PAAm) adsorb the cationic surfactant cetylpyridinium chloride (Py16).4,10-12 Adsorption is accompanied by shrinking of the gel composites. At high enough surfactant concentrations, the platelets arrange parallel to each other and smectic order is observed in the (9) Pauling, L. Proc. Natl. Acad. Sci. U.S.A. 1930, 16, 578. (10) Starodoubtsev, S. G.; Churochkina, N. A.; Khokhlov, A. R. Langmuir 2000, 16, 1529. (11) Evsikova, O. V.; Starodoubtsev, S. G.; Khokhlov, A. R. Vysokomol. Soedin., A 2002, 44, 1. (12) Starodoubtsev, S. G.; Ryabova, V. A. A.; Dembo, A. T.; Dembo, K. A.; Aliev, L. I.; Wasserman, A. M.; Khokhlov, A. R. Macromolecules 2002, 35, 6362.

10.1021/la0356248 CCC: $25.00 © 2003 American Chemical Society Published on Web 11/08/2003

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disordered platelets to form highly ordered smectic domains in cross-linked gels is not fully understood. In the previous studies,10-12 the investigated variables in the polymer/clay/surfactant systems were the concentration of the surfactant and, only to some extent, the chemical structure of the network. In the present paper, we attempt to understand better the effect of the network on the smectic arrangement of BENT platelets induced by the cationic surfactant. Our results make it possible to compare features and extent of the possible smectic arrangement of the clay platelets in aqueous suspensions of BENT and in the suspensions embedded in gels with two clearly different network structures. A second goal of the work was to investigate the effect of the chemical nature and of the hydrophobicity of the cationic surfactant on the disorder-order transition. Py16 and alkyl trimethylammonium bromides were used as the cationic surfactants. Finally we will discuss the anisotropic contraction of the composite gels resulting in preferred orientation of the system with respect to isotropic gel. Experimental Section

Figure 1. Schematic representation of the structure of the BENT suspension in water and in the gel phase: a single platelet (a); the dried clay powder (b); the card-house structure (c); the dispersion of BENT embedded in PAAm gel after ultrasonication (d); the structure of lamellae with intercalated cationic surfactant (e); the formation of surfactant aggregates on the surface of BENT platelets (f); the top (g) and the side (h) views of PAAm chains segregated from the surfactant in interlamellar space. Thick straight black lines, platelets and individual platelets layers; short black lines, negative charges of BENT platelets; winging black lines, PAAm chains; gray circles with tails, surfactant ions; open circles, sodium cations. Sodium cations are not shown in panels b-g. Each straight line in panels b-h represents the single platelet that is composed of three layers.

gels by X-ray diffraction. With even higher concentrations of the surfactant, overcharge of the platelet surface occurs and reentrant swelling of the gels is observed. It was also shown that variation of the cross-link density in the polymer network between 1/100 and 1/500 mol/mol does not affect the structure of the smectic domains. Since the dimensions of the clay platelets in the composite gel were large in comparison with the mesh size of the networks, they were unable to diffuse out of the gel. Without surfactant, the structure of the gel with embedded clay platelets showed no order in the positioning of the platelets. Such a structure (the card-house structure) is typical for semidiluted dispersions of layered clays. It is shown schematically in Figure 1d. The ability of the large

Materials. Acrylamide (AAm), N,N′-methylene-(bis)-acrylamide (BAA), and ammonium persulfate were purchased from Aldrich. Hexadecyltrimethylammonium bromide (R16), tetradecyltrimethylammonium bromide (R14), dodecyltrimethylammonium bromide (R12), and cetylpyridinium chloride (Py16) were obtained from Lancaster Synthesis Inc. BENT was purchased from Fluka Chemika-BioChemika Corp. The concentration of sodium in BENT was estimated at 4.35 wt %. The total chemical composition of BENT was as follows: Na, 4.35; Ca, 1.7; Fe, 1.6; Al, 11.4; Si, 29.8 wt %; O and H, the rest. (The chemical composition of BENT was determined in the Laboratory of Microanalyses of the Institute of Elementoorganic Compounds of the Russian Academy of Sciences.) The suspensions of the clays were prepared by mixing of the clay powder on a magnetic stirrer in closed flasks for several days. Then the suspensions were sonicated for 1 h using a SonoRex Super RK255 system (Volkswagen, Germany). Composite gels were synthesized by three-dimensional radical polymerization of monomers dissolved in the suspensions of the clays before polymerization.11,12 The concentrations of monomer, AAm, during polymerization were 6.0 and 15.0 wt %, the concentrations of BENT were 1.0, 2.4, and 4.0 wt %, and the molecular ratio of the cross-linker, BAA, to AAm was 1/100 and 1/500. Below we will identify the gels using these values and implying that they relate to the concentrations during the synthesis of the gels. The polymer/clay/surfactant complexes were prepared by dipping gels (generally the mass of the composite was about 0.2 g) in 0.01 M aqueous solutions of the surfactants (on the basis of 2 mL of surfactant solution per 10-6 mol Na+ initially contained in the gel) and allowing the gels to rest for 2 weeks at room temperature. All the solutions used were 0.01 M sodium chloride. Methods. The content of sodium ions in BENT was determined with reference to an external standard by flame photometry using the 589 nm wavelength at maximum of transmission. The measurements were performed using a Carl Zeiss Jena AAS1 atomic absorption spectrometer. Reference samples were prepared by dilution of standard Merck solutions. The swelling of the gels was characterized by the ratio F ) meq/md, where meq is the weight of the gel at equilibrium and md is the weight of the dried PAAm gel or composite. Gels were dried at 90 °C to constant weight. Drying of the gels for smallangle X-ray scattering (SAXS) measurements was carried out inside glass capillaries in a vacuum; the drying process was typically completed in 2 h. SAXS measurements were performed with a Bruker NANOStar diffractometer with Go¨bel mirrors under a vacuum at 0.154 06 nm. Samples were enclosed in glass capillaries for SAXS measurements. Acquisition times for diffraction patterns were typically about 2-3 h. The scattering vector is defined as q ) 4π sin θ/λ, where 2θ is the scattering angle. The mean longrange order dimension L in the gel/surfactant complexes was

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Table 1. Values of the Swelling Ratio for the Gels Swollen in Water after Preparation, F, and after Drying, Fdrsw BENT, wt %

PAAm, wt %

BAA/AAm, mol/mol

F

Fdrsw

2.4 2.4 4.0

6.0 15.0 4.0

1/500 1/100 1/500

84.0 10.5 78.5

40.5 10.4 27.0

Table 2. Swelling Ratio F and Its Experimental Error (in Brackets) for the Composite Gelsa Swollen in 0.01 M Solutions of Sodium Chloride and the Cationic Surfactants BENT, wt % 1.0 2.4 4.0 a

NaCl

R12

R14

R16

Py16

39.8((1.8) 38.8((2.3) 40.5((2.3) 40.4((2.2) 40.2((2.2) 34.1((1.3) 27.7((1.5) 29.0((1.5) 28.2((1.6) 28.0((1.5) 35.4((1.3) 23.7((2.0) 24.3((2.0) 25.1((2.1) 23.2((1.5)

Composition of the network: PAAm, 6.0%; BAA/AAm, 1/500.

estimated from half-maximum widths of the Bragg peaks in the diffraction patterns using the Scherrer formula:13

L)

λ βs cos θ

(1)

where βs is the full width at a half-maximum intensity of the peak (in radians) observed at the mean 2θ scattering angle. This procedure can still afford acceptable coherent domain size indications for scattering angles of about 2°, typical of the systems under study. Experimental full widths at half-maximum (fwhm’s) were empirically corrected for instrumental broadening (ca. 0.10°) determined from the line breadth in diffraction patterns of crystals of silver behenate.

Results Swelling Behavior of the Gels. The swelling ratio of the composite gels strongly depends on their previous history.11,12 The complete drying of gel composites followed by reswelling results in the decrease of the values of the swelling ratio F, due to the formation of the additional interactions between clay platelets in the dry state (Figure 1d),6,7 which partly remain after the gel swelling and decrease the gel swelling after drying. Values of the parameter F before and after the drying of the gels are listed in Table 1. They show that gels with lower acrylamide and lower cross-link concentrations are more sensitive to drying. “Dense” networks with higher content of the polymer and of cross-links prevent the formation of additional bonds between the edges and the flat surfaces of the clay platelets in the dry state, and their swelling is not changed as a consequence of drying. The effect of the repeated gel swelling is pronounced for the gel with a cross-link concentration of 1/500, while for the gel with a ratio of BAA to AAm of 1/100 it is not significant. It was previously shown that addition of low concentrations of the cationic surfactant Py16, that is, at concentrations much smaller than its critical micelle concentration (cmc) in water (cmc ∼ 9 × 10-4 M), results in a marked decrease of the gel swelling. Increase of the surfactant concentration (in the 0.001-0.1 M region) leads to a progressive gel swelling. However, the swelling ratio of the gels in the presence of the surfactant remains low in comparison with that in the solutions of sodium chloride. Shrinking of the gels in the presence of the cationic surfactants is observed also for the surfactants used in this study. Table 2 shows the values of F of the composite gels swollen in 0.01 M sodium chloride and in 0.01 M (13) Vainshtein, B. K. Diffraction of X-rays by Chain Molecules; Elsevier: Amsterdam, 1966.

Figure 2. SAXS profiles obtained from the composite gel with 4.0 wt % BENT 2 and 50 h after mixing with 0.1 M Py16 solution. The content of PAAm was 6.0 wt %, and the ratio of BAA to AAm was 1/500 mol/mol.

solutions of the surfactants. For the gel with 1.0 wt % BENT, the values of F do not depend on the surfactant hydrophobicity and coincide with that of the gel in the solution of sodium chloride. The increase of BENT concentration leads to a decrease of the swelling ratio of the gels modified with surfactants in comparison with that of the gels in 0.01 M NaCl. However, for the systems we investigated, the length of the hydrophobic tail and the chemical structure of the “head” of the surfactant ion do not affect their swelling degree. The anionic particles of BENT can absorb organic substances due to the ion exchange and also due to additional adsorption and intercalation of the organic substance between the platelets of the clays.5-7,14-17 In the previous study, it was shown that the isotherms of absorption of Py16 by the PAAm-BENT composites have a tendency to intermediate saturation; they display a plateau covering a wide region of surfactant concentration (ca. 3 × 10-3 to 3 × 10-2 M).10-12 The amount of the surfactant absorbed by the gel in the plateau region can be as much as 2 times higher than the number of sodium ions contained in the clay. At higher surfactant concentrations, a larger fraction of Py16 in the gel is not bound to the clay particles. In this study, most of the experiments were performed with the samples equalized in 0.01 M solutions of the surfactants. This is the region where the surface of the platelets is already covered by the cationic surfactants but the concentration of the “free” surfactant molecules and micelles is still low. Smectic Ordering: The Effect of BENT Concentration in the Gel-Clay Composites. Figure 2 shows two SAXS profiles obtained from the PAAm-BENT composite immersed in a 0.1 M solution of Py16. The relatively high concentration of the surfactant in this experiment was chosen because of the small diameter of the capillary. The data were measured at various different times after preparation of the samples: only the patterns measured 2 and 50 h after the addition of surfactant are shown in Figure 2. The experimental setup is shown in (14) Dekany, I.; Haraszti, T. Colloids Surf., A 1997, 123-124, 391. (15) Lagaly, G.; Weiss, A. Kolloid Z. Z. Polym. Sci. 1971, 243, 48. (16) Lagaly, G.; Weiss A. Kolloid Z. Z. Polym. Sci. 1971, 248, 979. (17) Lagaly, G.; Stange, H.; Weiss, A. Kolloid Z. Z. Polym. Sci. 1972, 250, 675.

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the right corner of Figure 2. A sample measuring ca. 1 × 1 × 1 mm was inserted in a 1.0 mm capillary between two layers of the surfactant solution. The scattering curves show two maxima located at 2θ ) 2.1 and 4.2°, respectively. The two reflections are the first and the second order of the same 4.0 nm periodicity and evidence the formation of the lamellar structure of the BENT-Py16 complex in the gel. The resulting structure is schematically shown in Figure 1e; it is formed by alternating bilayers of the surfactant and clay platelets.11,12 The hydrophobic tails of Py16 interact to reduce the contact area with water in the system. The van der Waals size of the fully extended Py16 molecule obtained from modeling is ca. 2.7 nm. The periodicity of the structures formed by Py16-BENT aggregates is about 4.2 nm, and the thickness of the single silica-aluminum-silica clay trilayer is about 0.8 nm if we subtract the plausible contribution of the free inorganic cations that will be exchanged with the cationic surfactant. Therefore, the thickness of the organic bilayer will be ca. 3.2 nm. This value is only about 60% of the spacing expected for a bilayer of fully extended Py16, which should correspond to about 5.4 nm (2 × 2.7 nm). The calculated value of the mean long-range order dimension L perpendicular to the platelet surface in the gel/Py16 complex can be determined from the fwhm of the diffraction maximum and is found to be ca. 25 nm. The ordered domains are therefore built of 5-6 alternating surfactant bilayers and clay platelets. Since the two scattering curves in Figure 2, recorded ca. 2 and 50 h after the addition of the surfactant, are congruent, the diffusion of Py16 in the gel is complete within 2 h. Based on this time value and on the characteristic size of the sample, R, of 1 mm, one can make a rough estimation of a lower limit for the diffusion coefficient D of Py16 into the gel phase: τ ) R2/D, and D > 1 × 10-6 cm2/s. It can be expected that at high enough dilution the platelets of BENT will not have any contact with each other in the gel phase and formation of smectic structures in the presence of cationic surfactants will not occur. Figure 3a shows the effect of BENT concentration on the X-ray diffraction profiles obtained from the gels having the same PAAm network composition. The gels were treated with Py16, and as already discussed, the gel with 4.0 wt % BENT shows an intense first-order peak at 2θ ) 2.1° and a less pronounced second-order maximum at 2θ ) 4.3°. The gel with 2.4 wt % BENT shows less intense first- and second-order maximums, but the periodicity within the lamellar structure in the composite remains the same. For the gel with 1.0 wt % BENT, there is no SAXS evidence of lamellar ordering. Formation of Smectic Structure in BENT Suspensions. Incorporation of the clay platelets in the crosslinked network should inhibit to some degree the formation of smectic structures in the gels. To estimate this effect, we studied the structure of the clay suspensions treated by the surfactants, but without addition of PAAm. We prepared 4.0 and 1.0 wt % concentration BENT suspensions in the same way as for the gel-clay composites. Then a layer of 0.01 M Py16 solution was accurately deposited on the surface of the suspensions. SAXS measurements were performed several days after the preparation of the suspensions, and the obtained scattering curves are presented in Figure 3b. The position and the breadth of the peaks in the diffraction profiles obtained from the 4.0 wt % suspension of BENT are practically identical to those obtained from the gel. This rather unexpected result shows that in the network with 6.0 wt % PAAm polymer chains do not provide a significant

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Figure 3. (a) SAXS profiles obtained from the composite gels with 4.0 wt % (1), 2.4 wt % (2), and 1.0 wt % BENT (3). (b) SAXS profiles obtained from the free suspensions of BENT with 4.0 wt % (1) and 1.0 wt % BENT (2). The content of PAAm was 6.0 wt %, and the ratio of BAA to AAm was 1/500 mol/mol. The gels were treated with 0.01 M Py16.

inhibition effect on the smectic organization of the clay platelets. In analogy to what was found in the gels, the scattering curve obtained from the clay suspension with 1.0 wt % BENT treated with Py16 does not show any maximum even several days after preparation. The most plausible explanation is that in diluted suspension the isolated clay platelets are overcharged by the excess of absorbed surfactant and the formation of the smectic aggregates is inhibited due to kinetics. The effect of the overcharge of individual MONT platelets in diluted suspension treated with cationic surfactants was demonstrated in recent work by Seguaris.18 Effect of Network Structure. The absence of marked effects of the network on the formation of the highly ordered smectic domains in the gel-clay composite is not a common case. Smectic orientation of the platelets should result in the appearance of mechanical tension in the network. Figure 4 shows SAXS profiles obtained from two gels with different content of PAAm and the cross-linker, BAA. The gel synthesized with 15 wt % content of the (18) Seguaris, J.-M. Langmuir 1997, 13, 653.

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Figure 4. SAXS profiles obtained from the composite gels with 2.4 wt % BENT. The content of PAAm was 6.0% (1) and 15.0% (2); the ratio of BAA to AAm was 1/500 mol/mol (1) and 1/100 mol/mol (2). The gels were treated with 0.01 M Py16.

monomer has a high concentration of elastically active chains due to higher initial concentration of the crosslinks and due to the large influence of entanglements between the interpenetrating polymer chains. As a result, the relative intensity of the maximum due to smectic aggregates in the scattering curve for this gel is clearly low in comparison with more diluted and less cross-linked gels. There is also a small shift of the maximum to smaller angles, that is, the distance between the platelets becomes slightly higher in the concentrated gel, suggesting incorporation of polymer chains in the galleries of the clay. However, in the region of lower PAAm concentration the effect of the network on the formation of smectic aggregates becomes small. For instance, for 6.0% PAAm gel with 2.4 wt % BENT the increase of the cross-link density from 1/500 to 1/100 mol/mol does not appear to affect the formation of smectic aggregates.12 Effect of the Chemical Structure of the Surfactant. The driving force for the aggregation of the clay platelets in the gels is in essence hydrophobic interactions between the surfactant bilayers arranged in the galleries between the flat sheets of BENT. The free energy of hydrophobic interactions depends strongly on the size of the hydrocarbon groups, that is, on the number of methylene units in the hydrophobic tail of the surfactant. Figure 5 shows the scattering curves obtained from the gel with 2.4 wt % BENT immersed in the solutions of R12-R16 and Py16. The scattering curves obtained from the gels treated with R16 or Py16 show intense first- and second-order maxima. The gel treated with R14 shows a rather weak first-order maximum, while the second order is not observed. R12 with its rather short tail hardly induces smectic ordering of the clay particles, and only a very broad and weak maximum in the scattering curve is observed in a region near 2θ ∼ 3.0°. This feature is absent in the scattering curves obtained from the same gel without R12. Thus, some organization of the clay platelets and their weak orientation occur even in this case. Table 3 shows the positions of the first- and the secondorder maxima, 2θ1 and 2θ2, the lamellar distance d, and the average long-range order dimension L representing the average size of coherent domains in the direction perpendicular to the surface of the clay platelets in the sample, for the swollen composite gels after treatment with 0.01 M solutions of the surfactants. The gels with 1.0

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Figure 5. SAXS profiles obtained from the composite gel with 2.4 wt % BENT and treated with 0.01 M solutions of different surfactants: Py16 (1), R16 (2), R14 (3), and R12 (4). The content of PAAm was 6.0%, and the ratio of BAA to AAm was 1/500 mol/mol. Table 3. Effect of BENT Concentration and of the Surfactant Structurea on the Positions of the First and the Second Maximums, 2θ1 and 2θ2 (deg), Interlamellar Distance d, and the Mean Long-Range Order Dimension L (i.e., the Lamellar Stacks Dimension) in the Swollen Gel-Clay Compositesb BENT, wt %

surfactant

1.0 2.4 2.4 2.4 2.4 4.0 4.0 4.0 4.0

R12-R16 Py16 R16 R14 R12 Py16 R16 R14 R12

2θ1

d,c nm

2θ2

L, nm

2.1((0.1) 2.2((0.1) 2.5((0.1)

4.2((0.05) 4.0((0.05) 3.5((0.05)

4.3((0.2) 4.5((0.2)

27((2) 21((2) 14((2)

2.1((0.1) 2.2((0.1) 2.4((0.1) 2.7((0.1)

4.2((0.05) 4.0((0.05) 3.5((0.05) 3.3((0.05)

4.3((0.2) 4.5((0.2)

24((2) 22((2)

a Composition of the network: PAAm, 6.0%; BAA, 1/500. b Estimated errors for 2θ1, 2θ2, d, and L are 0.1°, 0.2°, 0.1 nm, and 2 nm, respectively. c The d-spacing is given by the formula d ) 2π/q.

wt % BENT do not show any maxima on the scattering curves. The increase of the concentration of BENT leads to an increase of the region of the existence of the smectic state. Definite first maxima are observed for all of the surfactants used in this study. The calculated values of interlamellar distances d decrease with the decrease of the length of the surfactant but practically do not depend on the content of BENT in the gel matrix. This last result shows that the packing of the alternating bilayers of the surfactant and clay platelets in smectic crystals is rigidly determined by the size and geometry of the surfactant cations. For the gels containing the surfactants with shorter tails, the second maxima are not observed; thus the smectic areas in these gels are less ordered than in the gels treated with Py16 and R16. An analogous effect of the length of the hydrophobic tail of the surfactant is observed for the complexes of the free clays with cationic surfactants.14-17 In analogy to what already was discussed for Py16, the values of d reported in Table 3 for R16, R14, and R12 suggest the thickness of bilayers of the surfactant is only about 65% of the value expected for idealized bilayers

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Figure 7. 2-D SAXS profile obtained from the dried composite gel with 4.0 wt % BENT. The content of PAAm was 6.0%; the ratio of BAA to AAm was 1/500 mol/mol. The gel was treated with 0.01 M Py16. The capillary containing the sample of dried gel runs horizontally.

Figure 6. SAXS profiles obtained from the composite gels with 4.0 wt % (a) and 1.0 wt % (b) BENT in the swollen (curves 1 in panels a and b) and dried (curves 2 in panels a and b) state.

built of fully stretched, noninterdigitated, surfactant chains oriented perpendicular to the surface of the platelet. To account for the observed lamellar spacings, two simplified limit models of the organic bilayer can be taken into consideration: (i) the alkyl chains are not interdigitated but tilted with respect to the lamellar normal by an angle which must be about 50°; (ii) the two chains are orthogonal to the platelet surface and highly interdigitated with a central overlapped region of 1.7-1.9 nm (depending upon the system) and two noninterdigitated regions of about 0.3-0.6 nm each. The latter are occupied by the bulkier polar heads adjacent to the inorganic surface. Both limit models appear reasonable and have been used for the description of the structural features of different claysurfactant systems.14-17 Wide-angle X-ray scattering (WAXS) investigations are in progress to substantiate this question. The values given in Table 3 for the mean long-range order dimension L in the gel/surfactant complexes with Py16 and R16 show no marked difference with a BENT content increasing from 2.4 to 4.0 wt %. In both cases, the size of the ordered smectic regions in the composite gels can be estimated, and ordered domains are found to be built of 5-6 surfactant bilayers alternated with clay platelets. Decreasing the length of the hydrophobic tail of

the surfactant leads to a reduction of the size of the ordered smectic elements in the gel. In the case of R14, the determined value of L suggests that on average only about 4 bilayers form an aggregate. Effect of Gel Drying. The drying of the PAAm-BENT composites determines an increase of the clay and of the surfactant concentrations in the gel. Due to this, the fraction of the clay platelets intercalated in smectic structures should increase. On the other hand, the gel drying results in the increase of the concentration of polymer chains, which are bound to inhibit the selfordering of the platelets. Furthermore, since PAAm is in the glassy state at the temperature used in the study, the self-assembling of the smectic structures in the network should become impossible in completely dried composites. Figure 6a,b shows the scattering curves obtained from the swollen and dried gels with 6.0 wt % PAAm and treated with a 0.01 M solution of Py16. The gels contained 4.0 wt % (a) and 1.0 wt % (b) BENT during the synthesis. The drying of the gel with 4.0 wt % BENT has a negligibly small effect on the average relative intensity and the positioning of the SAXS peaks. The maximum of the first peak remains at ca. 2.1°, and also the fwhm and therefore the value of L are unchanged after drying. This behavior can be explained by assuming that 4.0 wt % is a high enough concentration to allow an essentially complete smectic arrangement of the platelets in the gel, so that upon drying no further change occurs. However, the drying of the gels in a capillary has a marked effect on the 2-D scattering pattern obtained from the sample. Figure 7 shows the 2-D SAXS image obtained for the dried gel that, in contrast to swollen gels (not shown), is strongly anisotropic. This anisotropy is probably due to nonuniform contraction of the gel during drying: clay platelets and smectic aggregates tend to orient parallel to each other and to the surface on which the gel is dried. The contraction of the gel must be a cooperative process influenced by anisotropy of the substrate. The scattering curves obtained from the swollen 1.0 wt % BENT gel do not show maxima as opposed to systems with higher BENT concentration. After drying, however, also the 1.0 wt % BENT composite gels present SAXS patterns with two maxima at 2θ1 ) 2.1° and 2θ2 ) 4.24.3° corresponding to a lamellar spacing d of 4.2 nm, identical to the value obtained for the composite with 4.0 wt % BENT. Moreover, the 2-D scattering pattern of the

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dried samples shows the same anisotropy as those of the 4.0 wt % BENT composites. Thus, for the gels with low content of the clay the increase of the concentration of BENT platelets and their orientation during drying result in rapid self-assembly of the smectic aggregates. Discussion PAAm gels with the embedded particles of BENT show a polyelectrolyte behavior.10-12 A decrease of the ionic strength of the solution results in a significant increase of their swelling ratio due to the osmotic pressure of the counterions. The free energy of the composite gel may be written as follows:10

∆G ) ∆Gel + ∆Gint + ∆Gtran + ∆Gc

(2)

In eq 2, ∆Gel describes the change in the elastic free energy; ∆Gint is the change in the free energy of the interactions between polymer segments, solvent molecules, and the surface of the clay particles; ∆Gtran is related to the change in the translational entropy of counterions of the clay and low molecular weight salt; and ∆Gc describes the change in the free energy of Coulomb interactions between the ions. For the case when the ionic surfactant is added to the solution, the expression for the free energy should include additional terms, the most essential being the term describing the free energy of formation of hydrophobic aggregates of the surfactant ∆Gm.19 The increase of the ionic strength leads to a decrease of the osmotic pressure in the composite gel. It was shown that a ∼0.01 M sodium chloride concentration is enough to eliminate the effect of the osmotic pressure of the counterions of the clay on the swelling ratio of the gel composite. The comparison of the swelling ratios for the composite gel in the solutions of sodium chloride and cationic surfactants (Table 2) shows that the addition of surfactants R16, R14, and R12 results in an additional marked shrinking of the gels. An analogous result was previously obtained only for Py16.10-12 It was shown by the spin-probe method that in the region of low surfactant (Py16) concentration, that is, below its cmc in water, the aggregates of the surfactant are already formed and they can solubilize hydrophobic molecules.12 Existence of such aggregates was also proven by SAXS. The scattering profiles obtained from the gel-clay composites treated with surfactant in this concentration range show positive curvature in the region of q, corresponding to d ∼ 3.5 nm. Such features are absent in the scattering curves obtained from the gels without the surfactant.12 Similar SAXS profiles corresponding to smaller d (d ∼ 2.8 nm) were obtained in this work for the composite gel with R12 and 2.4 wt % BENT (Figure 5, curve 4). The structures of the aggregates that are formed in a diluted solution of Py16 and in solutions of surfactants with short tails such as R12 are probably similar. How can one imagine the structure of these aggregates? The charge density on the surface of BENT crystals is about 1.0 × 10-7 mequiv/cm2, while the area occupied by a single ion of the cationic surfactant is about 0.25-0.30 nm2.7 The calculations show that the surfactant cations close-packed on the surface of BENT platelets give a charge density ca. 5-fold higher than the density of the negatively charged groups of the clay. When the surfactant concentration is low or the hydrophobic interactions are not strong enough (short hydrophobic tail), such aggregates will (19) Khokhlov, A. R.; Starodoubtsev, S. G.; Vasilevskaya, V. V. Adv. Polym. Sci. 1993, 109, 123.

occupy only partially the surface of clay platelets. The reason for the instability of continuous layers of surfactant on the surface of the platelets comes exactly from the large difference in the charge density of the layer of close-packed surfactant ions and the surface of the clay. Besides, due to their high hydrophobicity, the surfactant hydrocarbon tails should pack within the aggregates and form a doublelayer structure. We assume that the low-order aggregates of the surfactants on the surface of BENT particles in the gel phase can be represented as double-layer fragments schematically shown in Figure 1f. Aggregates with similar structure, “hemimicelles” or “superficial micelles”, are known to form on the external surface of the lamellar domains of layered clays.20-22 From this schematic representation, it follows that the appearance of the doublelayer fragments of the surfactant should result in a strong Coulomb attraction between the platelets because negatively charged areas of platelets will be attracted to the overcharged areas of platelets with double layers on them. This conclusion is supported by the fact that the gels begin to shrink in the region of surfactant concentration or alkyl chain length where no lamellar structure is observed by SAXS. Surfactants with concentrations close to 0.01 M together with high enough BENT concentrations form highly ordered smectic structures with alternating surfactant bilayers and clay platelets (Figure 1e). The reasons for the formation of such structures in the gel-clay composites in the presence of cationic surfactants have been recently discussed.12 The driving forces are the release of the clay counterions and the aggregation of the hydrophobic tails of the surfactant together with the electrostatic interaction between the charges of the surfactant and of the clay surface. At some critical length of hydrophobic tails and surfactant concentration, the total change in the free energy of the formation of smectic domains can become strongly negative. The fraction of the clay platelets that can be incorporated in smectic structures decreases with the decrease of the concentration of the clay platelets and of the length of the hydrophobic tail of the surfactant. The formation of smectic aggregates in the gel induces the appearance of the additional contractive forces that shrink the gel. At the same time, the formation of such structures is accompanied by an increase in the elastic free energy of the network ∆Gel. The unexpected result of this study is that at relatively low concentration of elastically active chains no retardation of the smectic arrangement of disordered clay platelets is observed. A large increase of the concentration of elastically active chains can on the other hand inhibit the formation of the ordered smectic structures in the gel (Figure 4). The volume fraction of the polymer in the composites is higher than or close to that of the clay with surfactant bilayers. Thus, highly polar PAAm chains should occupy a significant fraction of the galleries between the collapsed platelets in the smectic aggregates and coexist there with the hydrocarbon tails of the surfactants. A random distribution of the PAAm chains within the galleries should be unfavorable from the thermodynamic point of view because of the unfavorable interactions between the polar polymer chains and the hydrophobic residues of the surfactants. The PAAm chains localized between clay platelets loaded with surfactant should therefore segregate (20) Novich, B. E.; Ring, T. A. Langmuir 1985, 1, 701. (21) Harwell, J. H.; Hoskins, J. S.; Schechter, R. S.; Wade, W. H. Langmuir 1985, I, 251. (22) Rosen, M. J. Surfactants and Interfacial Phenomena; Wiley: New York, 1988; pp 122-132, 190.

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from the hydrocarbon surfactant residues. Such segregation together with partial local orientation of the polar chains and hydrogen bond formation between them should significantly compensate the positive free energy inputs in the smectic ordering of the platelets induced by cationic surfactants. A reasonable model of the arrangement of surfactant and PAAm domains in the galleries between the ordered clay platelets is schematically shown in Figure 1g,h. We have discussed elsewhere11,12 the structural changes that occur in the PAAm gel-clay composites treated by the cationic surfactant (Py16) in the region of high surfactant concentration. It is however important to point out that the increase of the surfactant concentration leads to the overcharging of the surface of the smectic aggregates and free platelets. The overcharging leads to Coulomb repulsion between the clay particles and their partial separation and, hence, to the partial reentrant swelling of the composite gel. In particular, the effect of the overcharging can explain the absence of the smectic

Starodoubtsev et al.

arrangement in suspensions and composite gels of BENT with 1.0 wt % clay concentration. Conclusion The interaction of the composite PAAm-BENT gels with cationic surfactants leads to the formation of smectic aggregates composed of alternating layers of BENT platelets and bilayers of the surfactants. The observed transition occurs despite the relatively large size of clay platelets as compared to the mesh size of the PAAm network. It was demonstrated that the existence and extent of the transition depend on the concentration of BENT in the gel, the length of the hydrophobic tail of the surfactant, and the structure of the PAAm network. Acknowledgment. S. G. Starodoubtsev gratefully acknowledges the CARIPLO Foundation and Landau Network-Centro Volta for financial support and the opportunity to work in Italy. LA0356248