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SAXS Study of ι-Carrageenan-Surfactant Complexes Eleonora Shtykova and Alexander Dembo Institute of Crystallography, Russian Academy of Sciences, 59 Leninsky Prospekt, Moscow 117333, Russia
Elena Makhaeva and Alexei Khokhlov Physics Department, Moscow State University, Moscow 117234, Russia
Guennady Evmenenko*,† and Harry Reynaers Laboratory of Macromolecular Structural Chemistry, Catholic University of Leuven, Celestijnenlaan 200F, B-3001 Heverlee, Belgium Received October 14, 1999. In Final Form: March 6, 2000 Small-angle X-ray scattering (SAXS) was used to study the structure of ι-carrageenan gels formed in the presence of cationic surfactant cetylpyridinium chloride (CPC) at various concentrations of the polysaccharide and as a function of the surfactant concentration. A physical gel of low ι-carrageenan concentration displays no characteristic peak at small angles. After addition of CPC to the ι-carrageenan solution central scattering and equidistant characteristic peaks appear due to ordering in the gel structure. This ordering is the result of complex hydrophobic and electrostatic interactions between the polymer chains and the charged surfactants. The ι-carrageenan gelation in the presence of CPC leads to selfassembly of surfactant molecules inside the physical gel similarly to micellization in a solution. In this case the shrinking or collapse of polyelectrolyte gels takes place. The additional central scattering emerges due to the process of surfactant micellization. The relative positions of the main and secondary peaks (1:2) correspond to the formation of a lamellar structure of the complexes. The mean long-range order dimensions, the radii of interaction, and the degree of disorder in the systems are calculated from the Bragg peaks on the SAXS patterns.
1. Introduction Carrageenans are water soluble, sulfated polysaccharides extracted from different species of marine red algae. On the basis of their chemical structure, one can discern two main groups: κ- and ι-carrageenans. They differ in the extent to which they carry sulfate groups and in the positioning of these groups along the polymer chain (see Figure 1).1 This study is restricted to the iota component. Due to the presence of sulfate groups, carrageenans are strong polyelectrolytes. A sharp volume transition, wellknown as the polymer network collapse, is observed for polyelectrolytes interacting with oppositely charged surfactants.2 The obtained complexes have recently been shown to exhibit highly regular self-assembled nanostructures.3-7 Interaction between an anionic polysaccharide and an oppositely charged surfactant was studied in refs 8-10. It was shown that their interaction in aqueous * Corresponding author. † Present address: Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Rd., Evanston, IL 602083112. (1) Rees, D. A. Polysaccharides Shapes; Chapman and Hall: London, 1977. (2) Khokhlov, A. R.; Kramarenko, E. Yu.; Makhaeva, E. E.; Starodubtsev, S. G. Macromolecules 1992, 25, 4779. (3) Bronstein, L. M.; Platonova, O. A.; Yakunin, A. N.; Yanovskaya, I. M.; Valetsky, P. M.; Dembo, A. T.; Makhaeva, E. E.; Mironov, A. V.; Khokhlov, A. R. Langmuir 1998, 14, 252. (4) Khandurina, Yu. V.; Dembo, A. T.; Rogacheva, V. B.; Zezin, A. B.; Kabanov, V. A. Vysokomol. Soedin. 1994, 36A, 235. (5) Okuzaki, H.; Osada, Y. Macromolecules 1995, 28, 380. (6) Yeh, F.; Sokolov, E. L.; Khokhlov, A. R.; Chu, B. J. Am. Chem. Soc. 1996, 118, 6615. (7) Sokolov, E. L.; Yeh, F.; Khokhlov, A. R.; Chu, B. Langmuir 1996, 12, 6229.
Figure 1. Chemical structure of (a) κ-carrageenan and (b) ι-carrageenan copolymeric repeating units.
solutions leads to phase separation if an insufficient amount of salt is added. The extent of this interaction is controlled mainly by the ionic strength of the solution and by the concentration of surfactant.8 Polyelectrolytesurfactant complexes are of potential interest for practical applications; e.g., they can be used as a nanostructured environment (matrix) for a controlled nanoparticle formation, allowing one to monitor the process of nanoparticle growth.11 From this viewpoint the polysaccharide matrix (8) Herslof-Bjorling, O.; Sundelof, L.-O.; Porsch, B.; Valtcheva, L.; Hjerton, S. Langmuir 1996, 12, 4628. (9) Caram-Lelham, N.; Sundelof, L.-O. Int. J. Pharm. 1995, 115, 103. (10) Caram-Lelham, N.; Sundelof, L.-O. Biopolymers 1996, 39, 387.
10.1021/la991357n CCC: $19.00 © 2000 American Chemical Society Published on Web 04/27/2000
SAXS Study of ι-Carrageenan-Surfactant Complexes
is of special interest as a nontoxic natural material for applications in the food industry. Structural transformations of carrageenan molecules in solution have been investigated in detail as a function of temperature, ionic strength and the type of counterions.12-24 One of the basic properties of carrageenans is their ability to induce gelation of solutions. Comprehensive light scattering and SAXS studies revealed that, in contrast to common knowledge in the literature, the gelation of the carrageenans is a two-step process involving a conformational transition of the type coil-to-single helix, followed by a side-by-side association of the single helices to form a three-dimensional water-swollen network resulting in a weakly elastic physical gel.17-19,25-27 This gelation is ion-induced and thermoreversible. Since the hydrogel matrices are designed for the use in aqueous media, nondestructive analysis methods are required to study the structure of the gels in their native environment. Electron and atomic force microscopies need a special pretreatment of the specimen which might distort the structure of the sample. In contrast, small-angle X-ray scattering (SAXS) allows one to study native hydrogels. This experimental approach is widely used to determine size distribution functions for disperse systems of different physical nature.28-30 In the present paper this method is used to study the interaction of physical gels of ι-carrageenan with a cationic surfactant cetylpyridinium chloride (CPC) at various concentrations of polysaccharide and as a function of surfactant concentration. The effect CPC has on the swelling behavior and the structure of polymersurfactant complexes formed by networks of ι-carrageenan and oppositely charged surfactants is investigated. 2. Experimental Section 2.1. Materials. Purification of Commercial ι-Carrageenan. Since the behavior of this polysaccharide depends crucially on the type of counterions, at first the commercial sample of (11) Svergun, D. I.; Shtykova, E. V.; Dembo, A. T.; Bronstein, L. M.; Platonova, O. A.; Yakunin, A. N.; Valetsky, P. M.; Khokhlov, A. R. J. Chem. Phys. 1998, 109, 11109. (12) Paoletti, S.; Smidsrod, O.; Grasdalen, H. Biopolymers 1984, 23, 1771. (13) Slootmaekers, D.; De Jonghe, C.; Reynaers, H.; Varkevisser, F. A.; Bloys van Treslong, C. J. Int. J. Biol. Macromol. 1988, 10, 160. (14) Slootmaekers, D.; Mandel, M.; Reynaers, H. Int. J. Biol. Macromol. 1991, 13, 17. (15) Vanneste, K.; Mandel, M.; Paoletti, S.; Reynaers, H. Macromolecules 1994, 27, 7496. (16) Viebke, C.; Piculell, L.; Nilsson, S. Macromolecules 1994, 27, 4160. (17) Turquois, T.; Rochas, C.; Taravel, F.-R.; Doublier, J. L.; Axelos, M.-A.-V. Biopolymers 1995, 36, 559. (18) Denef, B.; Mischenko, N.; Koch, M. H. J.; Reynaers, H. Int. J. Biol. Macromol. 1996, 18, 151. (19) Mischenko, N.; Denef, B.; Koch, M. H. J.; Reynaers, H. Int. J. Biol. Macromol. 1996, 19, 185. (20) Denef, B.; Gamini, A.; Delben, F.; Paoletti, S.; Reynaers, H.; Vanneste, K. Biopolymers 1998, 45, 105. (21) Viebke, C.; Borgstrom, J.; Carlsson, I.; Piculell, L.; Williams, P. Macromolecules 1998, 31, 1833. (22) Hjerde, T.; Smidsrod, O.; Stokke, B. T.; Christensen, B. Macromolecules 1998, 31, 1842. (23) Bongaerts, K.; Reynaers, H.; Zanetti, F.; Paoletti, S. Macromolecules 1999, 32, 675. (24) Bongaerts, K.; Reynaers, H.; Zanetti, F.; Paoletti, S. Macromolecules 1999, 32, 683. (25) Piculell, L. Curr.t Opin. Colloid Interface Sci. 1998, 3, 643. (26) Guenet, J.-M. Trends Polym. Sci. 1996, 4, 6. (27) Wittgren, B.; Borgstrom, J.; Carlsson, I.; Piculell, L.; Williams, P. Macromolecules 1998, 31, 1833. (28) Small-Angle X-ray Scattering; Glatter, O., Kratky, O., Eds.; Academic Press: London, 1982. (29) Feigin, L. A.; Svergun, D. I. Structure Analysis by Small-Angle X-ray and Neutron Scattering; Plenum Press: New York, 1987. (30) Modern Aspects of Small-Angle Scattering; NATO ASI Series C, Vol. 451; Brumberger, H., Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1995.
Langmuir, Vol. 16, No. 12, 2000 5285 ι-carrageenan was purified from potassium and calcium ions and obtained in the pure Na+-form. To purify ι-carrageenan from K+ and Ca2+ and to obtain it in pure Na+-form, a 0.5 wt % aqueous solution of the commercial ι-carrageenan was filtered through an ion-exchange column with a cationic ion-exchange resin in the Na+-form (“IOS KB-4”). Determination of the Ion Content in ι-Carrageenan Solutions. The Ca2+ content in ι-carrageenan solutions was determined by the reaction with the sodium salt of ethylenediaminetetraacetic acid (EDTA).31 Eriochrome black was used as a metal indicator. The analysis was performed as follows. A 3 mL aliquot of the initial 0.5 wt % aqueous solution of ι-carrageenan was diluted with distilled water to obtain the total volume of the solution equal to 100 mL. Then 5 mL of ammonium buffer solution and 7-10 drops of the indicator were added to this solution. The prepared solution was titrated with EDTA solution up to the change of color from red to blue. The results of this analysis showed that the concentration of calcium ions in the initial solution of ι-carrageenan (before purification) is 0.02% (which is consistent with the data reported by the manufacturer), while the solution after the ion-exchange column contains less than 0.001% of Ca2+. Preparation of ι-Carrageenan Gels Containing Surfactant. The gels were prepared from ι-carrageenan solutions of concentrations 0.5, 1.0, and 1.5 wt % in 0.1 M KCl. For this purpose 0.1 mL of 3 M KCl was mixed with 3 mL of the polymer solution and stirred 30 min on a magnetic stirrer at 40 °C. Then the calculated amount of the surfactant, cetylpyridinium chloride, was added to the hot solution (40 °C) of the polymer and was stirred during one more hour. Gels were obtained by cooling of hot solutions to 20 °C. The addition of CPC during stirring leads either to the formation of a cohesive turbid gel or to precipitation. The gel formed keeps its volume and could be characterized by the certain elasticity modulus. The precipitate is powderlike. Both the gels and the precipitates could be easily separated from the solution. Within 2 days three experiments were done: determination of the total mass of the gels as a function of the ratio [CPC]/[charg.i-Car.], the SAXS measurements, and the absorption of CPC. The efficiency of the latter process by the gels was characterized by the molar ratio, ϑ, being the number of absorbed surfactant molecules (Ψ) over the number of polysaccharide charged groups (mol/mol). The Ψ values were measured spectrophotometrically and were calculated using the formula Ψ ) (M - DVs-1), where M is the initial amount of surfactant molecules in the whole solution (mol), is the extinction at the absorption maximum (a wavelength λ ) 260 nm), Vs is the solution volume, and D is the optical density of the solution at equilibrium. The measurements were performed with a Hewlett-Packard 8452A spectrophotometer. 2.2. Methods. X-ray scattering measurements were done on the diffractometer “AMUR-K” (made at the Institute of Crystallography, Russian Academy of Sciences) involving a linear position-sensitive detector and a single-crystal monochromator at a wavelength of λ ) 1.542 Å. The Kratky-type geometry was used with a sample-to-detector distance of 673 mm and an entrance slit width of 0.2 mm to cover the range of momentum transfer 0.012 < q < 0.55 Å-1 (here, q ) 4π sin θ/λ, where 2θ is the scattering angle). The windows of the sample holder are poly(ethylene terephthalate) foils of a thickness of 0.01 mm; the thickness of the sample was about 1 mm. The latter varied along the length of the sample holder (10 mm), excluding the possibility of absolute calibration. The data are normalized to the intensity of the incident beam and corrected for the detector response following standard procedures.29
3. Results and Discussion The ι-carrageenan gelation in the presence of the cationic surfactant, cetylpyridinium chloride, was studied at various concentrations of polysaccharide. For all systems under study the features of gel formation and properties depend on the CPC concentration. The addition of a small amount of CPC leads to the formation of a turbid gel. An increase of the CPC concentration results in the formation (31) Alexeev, B. N. Quantitative Analysis; Chemistry: Moscow, 1978.
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Figure 2. Dependence of the mass change of ι-carrageenan gels, m/m0, on the initial CPC concentration for various concentrations of ι-carrageenan: 0.5 (1), 1 (2), and 1.5% (3) ι-carrageenan gels.
of an inhomogeneous gel, with dense white “complex” regions coexisting with more swollen regions. The gel is shrinking in comparison with the gel formed by the surfactant-free system. In this case the system consists of a macroscopic gel and an external solution that can be easily separated from the gel phase. The gel formed keeps its volume, and it can be characterized by the certain elasticity modulus. Figure 2 shows the dependencies of the relative mass of ι-carrageenan gels formed in the presence of CPC, m/m0, as a function of the CPC concentration. The increase of the CPC fraction during ι-carrageenan gelation results in a decrease of the gel mass. In some cases addition of CPC to the physical gel leads to formation of a precipitate (Figure 2). The amount of CPC molecules [CPC] per one charged group of the gel [charg.i-Car.] at which the allin-one gel system is destroyed and the precipitate is observed depends on the concentration of the polysaccharide. In the case of 0.5 wt % gel a powderlike sediment is formed at a [CPC]/[charg.i-Car.] ratio equal to 0.5 (1 CPC molecule/2 charged groups of the polysaccharide) and 0.8 (Figure 2, curve 1). For a 1 wt % gel such a process takes place at [CPC]/[charg.i-Car.] ) 1 and at the ratio ) 1.5 (Figure 2, curve 2). In contrast, for a 1.5 wt % gel the addition of up to [CPC]/[charg.i-Car.] ) 1.5 does not result in the precipitation. The precipitation also does not take place at lower concentrations of CPC under the same concentrations of the gel. In this case the shrinking of the gel is more pronounced for the 0.5 wt % gel (Figure 2). Above some CPC concentration the polymer collapse takes place. Here it should be noted that the behavior of ι-carrageenan in the presence of CPC (the formation of shrinked gel) differs from properties of the mixtures of nongelling polyelectrolyte with oppositely charged surfactant. In the latter case the increase of the concentration of the oppositely charged surfactant does not lead to the continuous shrinking of the polyelectrolyte system. Instead, the formation of a dense precipitate is observed.32,33 The behavior of the gels can be explained both by the formation (32) Chandar, P.; Somosundaran, P.; Turro, N. J. Macromolecules 1988, 21, 950. (33) Hayakawa, K.; Murata, H.; Satake, I. Colloid Polym. Sci. 1990, 268, 1044.
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Figure 3. Dependence of CPC absorption on the initial CPC concentration by ι-carrageenan based gels: 0.5 (1), 1 (2), and 1.5% (3) ι-carrageenan gels.
of complexes between the anionic groups of the polymer and the surfactant cations and by the aggregation of the bound surfactant molecules in the micelles. Figure 3 demonstrates the absorption of surfactant ions by the polymer. It can be seen that for all the systems under study the amount of the absorbed CPC per one charged group of the polysaccharide [CPC]abs/[charg.i-Car.] is proportional to the amount of added surfactant expressed as a ratio [CPC]/[charg.i-Car.] up to [CPC]/[charg.iCar.] ) 1. On further increase of the CPC content, one observes the saturation of the CPC absorbed by the polymer. The value of [CPC]abs/[charg.i-Car.] reaches the ultimate level close to 1 (0.9 in the case of the 1.5 wt % gel and 1.0 in the case of the 1 wt % gel). Here it should be noted that the amount of absorbed CPC is proportional to the amount of the charged groups of polymer. However, the amount of absorbed CPC leading to the destruction of the gel system and converting it to a powderlike precipitate depends on the polymer concentration in the system. The fact that ι-carrageenan gel formed from the initial solution containing 1.5% of polymer does not shrink despite significant absorption of the surfactant (Figures 2 and 3) may be connected with different structures of the physical gel for this case. This question needs further investigation. The structure of the polysaccharide-surfactant complexes is studied by SAXS. Figures 4-6 show the SAXS dependencies for the studied systems. For the present concentration range, the physical gels of pure K+-carrageenate (Figures 4-6, curves 1) display practically no central scattering and no characteristic peak emerges (which could point to some ordering in the gel structure). For the latter case, the scattering curves can be fitted well by the Ornstein-Zernike equation, which is valid for the scattering of an idealized gel.34 The structural heterogeneity of such a gel is described by a correlation function in the form
γ(r) ) 〈c〉2
r ξ exp r ξ
( )
(1)
where 〈c〉2 is the mean-square amplitude of the concentration fluctuations in the gel and ξ is the correlation length of polymer-polymer interactions between the (34) De Gennes, P. G. Scaling Concepts in Polymer Physics; Cornell University Press: Ithaca, New York, 1979.
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Figure 4. SAXS patterns for different 0.5% ι-carrageenan gels charged with an increasing amount of CPC: physical gel (1); [CPC]/[Car.] ) 0.2 (2); [CPC]/[Car.] ) 0.9 (3); [CPC]/[Car.] ) 1.8 (4).
Figure 5. SAXS patterns for different 1% ι-carragenan gels charged with increasing amounts of CPC: physical gel (1); [CPC]/[Car.] ) 1 (2); [CPC]/[Car.] ) 2 (3); [CPC]/[Car.] ) 3 (4); [CPC]/[Car.] ) 4 (5).
fluctuating chains of the gel network. The Fourier transform of the former equation gives the well-known Ornstein-Zernike scattering function35
I(q) ) IL(0)/(1 + q2ξ2)
(2)
where IL(0) ) 〈c〉2ξ2(2/π)1/2. The fit of the experimental data according to this equation makes clear that the structure of the ι-carrageenan gel could be characterized by the size of heterogeneity ξ ) 40.5 Å, which can be attributed to domains of constant local concentrations (35) Ornstein, L. S.; Zernike, F. Proceedings Academy Sciences: Royal Netherlands Academy of Sciences and Letters: Amsterdam, 1914; Vol. 17, p 793.
Figure 6. SAXS patterns for different 1.5% ι-carragenan gels charged with increasing amounts of CPC: physical gel (1); [CPC]/[Car.] ) 0.62 (2); [CPC]/[Car.] ) 1.85 (3).
around the physical junctions consisting of helical chain segments along the polysaccharide chains. The data obtained by SAXS for the gel-surfactant systems show that for the studied contents of surfactant in the gel phase highly ordered structures are formed. After addition of CPC to the systems under investigation (0.5, 1, and 1.5 wt % ι-carrageenan gels) central scattering and a characteristic peak at about q ) 0.14 Å-1 emerge due to surfactant ordering within the gel. The scattering curves obtained from the 0.5 and 1 wt % ι-carrageenan gel-surfactant samples have one main sharp peak and a weaker-resolved secondary peak at q ) 0.27-0.29 Å-1 (Figures 4 and 5). The relative positions of the main and secondary peaks (1:2) point to the formation of regions with a lamellar structure of the CPC. If one compares the scattering curves obtained for the different CPC contents, one observes a marked increase of the relative intensity as well as a sharpening of the main and of the secondary peaks at least for systems obtained at higher CPC concentrations (Figures 4 and 5). The main and secondary peaks do not change positions. These results demonstrate a high degree of order in the complexes obtained in the presence of CPC. Further increasing of the carrageenan content in the system contributes to a substantial increase of the central scattering and leads to a decrease of the main Bragg peak, whereas the second maximum practically disappears (Figure 6). Such observation manifests the formation of nanoscale heterogeneities due to a partial disruption of the gel-surfactant complex. Hence these heterogeneities can be interpreted as fragments of the polymer chains, being in a state of conformational order and involved in interchain junctions. The formation of the nanoscale fragment is more pronounced for more concentrated systems. These conclusions are in agreement with further analysis of the SAXS data. In the insets of Figures 4-6 the scattering from surfactant molecules in the region of the interference maximum after subtracting the background is shown. The mean long-range order dimension, L, in the gel-surfactant systems can be estimated from the Bragg peaks on the
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Table 1. Structural Characteristics of the Gel-Surfactant Complexes Obtained from SAXS Curves gel (%)
[CPC]/[Car.]
qmax (Å-1)
a j (Å)
L (Å)
rm (Å)
∆/a j
0.5
0.2 0.9 1.8a 1.0 2.0a 3.0a 4.0a 0.62 1.85
0.14 0.14 0.13 0.13 0.14 0.14 0.13 0.14 0.14
44.9 44.9 48.3 48.3 44.9 44.9 48.3 44.9 44.9
131 123 131 134 170 185 165 113 130
207 194 207 211 268 292 261 179 205
0.19 0.19 0.19 0.19 0.16 0.16 0.17 0.20 0.19
1.0
1.5 a
Precipitates.
SAXS patterns by the Scherrer formula36
L)
λ βscos θ
(3)
where βs is the full width at a half-maximum intensity of the Bragg peak observed at a mean scattering angle of 2θ. From the SAXS curves a radius of interaction, rm, and a degree of disorder in the system ∆/a j can be calculated36
rm ) ∆/a j)
2
(2.5π )
λ βs
[ ]
j 1 βsa π λ
(4)
1/2
(5)
Here, a j ) 2π/qm is the characteristic size of the longest periodicity in the gel-surfactant complex and ∆ is the mean-square deviation of distances between the neighboring molecules. These results obtained from our experimental data are presented in Table 1. As one can see from Table 1 the dimension L increases with the increasing content of CPC. This appears to be most pronounced for the 1 wt % gel. On subsequent addition of surfactant (up to [CPC]/[Car.] ) 4) a slight contraction of the ordered elements of the complex and a decrease of their characteristic sizes are observed. At the same time the degree of disorder, ∆/a j , in the system increases. Hence the size of the mean long-range order L, the radii j are of interaction rm, and the degree of disorder ∆/a dependent on the concentration of both the polysaccharide and the surfactant in the systems. The statistically disordered physical gel seems to promote the formation of more or less perfect carrageenan-surfactant selfassemblies. This effect can be explained by the surfactant concentration dependent inhomogeneous spatial distribution in the gel; apparently the surfactant molecules prefer those regions in the gel structure which allow for optimal surfactant assembly fitting, similar to micellization in solution.37-39 (36) Vainshtein, B. K. Diffraction of X-rays by Chain Molecules; Elsevier Publishing Co.: Amsterdam-London-New York, 1966; pp 203-254. (37) Dembo, A. T.; Yakunin, A. N.; Zaitsev, V. S.; Mironov, A. V.; Starodoubtsev, S. G.; Khokhlov, A. R.; Chu, B. J. Polym. Sci., Part B: Polym. Phys. 1996, 34, 2893. (38) Chu, B.; Yeh, F.; Sokolov, E. L.; Starodoubtsev, S. G.; Khokhlov, A. R. Macromolecules 1995, 28, 8447.
On the other hand, interaction of the surfactants with the polyelectrolyte chains induces shrinking of the gel. Hence, surfactant molecules serve as additional crosslinks. Further addition of CPC increases the degree of disorder in the complex, because of a lack of carrageenan units involved in gel-surfactant interaction. Increasing CPC-ι-carrageenan ratios in the system leads to partial distortion of the surfactant-gel complex and to a significant increase of the central scattering resulting from additional aggregation of free surfactant molecules in micelles. All obtained complexes exhibit highly regular selfassembled nanostructures. One can see from Table 1 that the structural characteristics of the gel-surfactant complexes are weakly different from each other for the shrunken gels and for the precipitates. These precipitates can be interpreted as microgel particles involving the same ordered nanoorganization. Thus the increase of the CPC concentration at the gelation of ι-carrageenan leads to the change of the gel macrostate (shrinking and collapse gel, powderlike precipitate); however the parameters of the nanostructure for the all studied systems practically do not change. 4. Conclusions Interaction of a physical ι-carrageenan gel with an ionic surfactant leads to more or less or complete shrinking of the gel and to the formation of ordered periodic structures. The ordering formation is the result of hydrophobic and electrostatic interactions in the polymer network-surfactant system. The surfactant molecules in conjunction with the carrageenan molecules self-assemble in the gel similarly to micellization in a solution resulting in a lamellar type of organization involving gel contraction and ultimately, depending on the CPC-carrageenan ratio, to gel collapse. At low polymer concentrations one observes sedimentation of microgel particles. An increase of the polymer concentration for a given surfactant concentration improves the stability of the physical gel; however the degree of shrinking of such gels after the addition of extra surfactant is much less pronounced. For the former more stable and tighter gels, one observes a decrease of the intensity of the SAXS patterns. Similar structures have been observed for slightly crosslinked oppositely charged polyelectrolyte gel-surfactant systems.32-34 In these reports it is demonstrated that regular supramolecular structures are formed depending on the polyelectrolyte charge density and the chemical nature of the swollen network as well as on the chemical nature and the length of the hydrophobic residues of amphiphilic molecules and their concentration. Acknowledgment. The authors are grateful to Dr. N. B. Feropontov for help in purifying the commercial carrageenan. The work was supported by the Russian Foundation for Basic Research (Grant No. 97-03-32770a) and by the INTAS Grant No. 96-1115. G.E. is grateful to the Research Council of K.U. Leuven for a research fellowship. LA991357N (39) Mironov, A. V.; Starodoubtsev, S. G.; Khokhlov, A. R.; Dembo, A. T.; Yakunin, A. N. Macromolecules 1998, 31, 7698.
