Langmuir 2006, 22, 349-352
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Liesegang Pattern Formation in K-Carrageenan Gel Takayuki Narita* and Masayuki Tokita Department of Physics, Faculty of Science, Kyushu UniVersity, 4-2-1, Ropponmatsu, Fukuoka 810-8560, Japan
Langmuir 2006.22:349-352. Downloaded from pubs.acs.org by UNIV OF EDINBURGH on 01/23/19. For personal use only.
ReceiVed August 17, 2005. In Final Form: October 23, 2005 We report a new class of the spatial pattern formation process in which the gel plays essential roles. The system studied here is the solution of κ-carrageenan in which potassium chloride is diffused. The solution transforms into the gel state with the diffusion of potassium chloride. Then the stripe pattern, which is perpendicular to the direction of the diffusion of potassium chloride, appears within the gel. The pattern thus formed in the gel is studied as a function of the concentration of the solution of potassium chloride. We find that the dense region of the stripe pattern consists of the liquid crystalline gel, whereas the dilute region is the amorphous gel. The transition from the amorphous gel to the liquid crystalline gel, hence, occurs in the gel state of κ-carrageenan. The gel behaves as a pattern-forming substance as well as the supporting medium of the pattern in this system. The period and the thickness of the layers of liquid crystalline gel are analyzed. Both the period and the thickness of the layers are found to depend strongly on the concentration of the solution of potassium chloride.
Introduction The formation of patterns in time and space is a universal phenomenon, which can be observed in the wide variety of the chemical and biological systems.1,2 A class of the spatial patterns that appear in the gel, and hence studied in detail, is the Liesegang pattern in which the precipitate of inorganic salt forms the concentric circle pattern.3-9 The pattern is formed as a result of the coupling of the diffusion and the precipitation of metal ions in the gel. The gel is, hence, only used as a supporting medium of the spatial patterns that appear in the system. Here, we report a new class of Liesegang pattern in which the gel plays the roles of the supporting medium of the pattern as well as the pattern forming substance. The system studied here consists of κ-carrageenan and potassium chloride. The repeating unit of κ-carrageenan is shown in Figure 1. It has been reported that the solution of κ-carrageenan transforms into the gel under the presence of the gel-promoting ions.10,11 One of the typical ions that promote the gelation is the potassium ion. The lowest concentration of potassium ion that is required for the gelation is of the order of 10 mM. The gels thus prepared are isotropic in space. On the other hand, the liquid crystalline gels are obtained at much higher concentration regions of the gel-promoting ions.12,13 The liquid crystalline gel thus obtained shows strong polarized light under the crossed-nicols. In this system, hence, the spatial distribution of the gel-promoting ions will create the spatial pattern in the gel. The detailed study * Corresponding author. E-mail:
[email protected]. (1) Cross, M. C.; Hohenberg, P. C. ReV. Mod. Phys. 1993, 65, 851. (2) Shinbrot, T.; Muzzio, F. J. Nature 2001, 410, 251. (3) Stern, K. H. Chem. ReV. 1954, 54, 79. (4) Henisch, H. K. Periodic Precipitation; Pergamon Press: Oxford, 1991. (5) Jablczynski, K. Bull. Soc. Chim. Fr. 1923, 33, 1592. (6) Antal, T.; Droz, M. Magnin, J.; Racz, Z.; Zrinyi, M. J. Chem. Phys. 1998, 109, 9479. (7) Mu¨ller, S. C.; Kai, S.; Ross, J. J. Phys. Chem. 1982, 86, 4078. (8) Racz, Z. Physica A 1999, 274, 50. (9) Matalon, R.; Packter, A. J. Colloid Sci. 1955, 10, 46. (10) Morris, V. J. In Functional Properties of Food Macromolecules, 2nd ed.; Hill, S. E., Ledward, D. A., Mitchell, J. R., Eds.; Aspen Publishers: Gaithersburg, MD, 1998; pp 143-226. (11) Piculell, L. In Food Polysaccharides and Their Applications; Stephen, A. M., Ed.; Marcel, Dekker: New York, 1995; pp 205-244. (12) Borgstro¨m, J.; Quist, P. O.; Piculell, L. Macromolecules 1996, 29, 5926. (13) Borgstro¨m, J.; Egermayer, M.; Sparrman, T., Quist, P. O.; Piculell, L. Langmuir 1998, 14, 4935.
Figure 1. Chemical structures of repeating unit of κ-carrageenan. κ-Carrageenan consists of (f3)-β-D-Galp-4-sulfate-(1.4)-3,6-anhydro-R-D-Galp-(1f.
of the pattern, hence, will promote a better understanding of the spatial structure formation under the gelation. Furthermore, the gels in which the spatial patterns are frozen in have potential application in drug delivery systems, optical switches, and so forth.14,15 In the following sections, we describe the way in which the pattern is created in the gel, and then the results are discussed in terms of the Liesegang phenomenon. Experimental Section Sodium salt of κ-carrageenan and potassium chloride (reagent grade) were purchased from Wako Pure Chemical Co. Ltd. and used without further purification. κ-Carrageenan was dissolved into distilled and deionized water at a temperature of 50 °C. The concentration of κ-carrageenan was fixed at 1 wt %. The solution was then transferred into a glass capillary of 1.9 mm in diameter and 40 mm in length. The one end of the capillary was sealed with the cellulose membrane to diffuse the potassium chloride into the solution. The pore size of the cellulose membrane used here was 2.5 nm (Japan Medical, Science, Japan). In the case of such a membrane, the potassium chloride could pass through the membrane easily while κ-carrageenan could not. As shown in Figure 2, the one end of the capillary, which was sealed with the membrane, was kept in contact with the potassium chloride solution for 1 day at a temperature of 50 °C. The volume of the potassium chloride solution was more than 100 times larger than that of κ-carrageenan solution. The concentrations of potassium chloride solution, to which the sample (14) Makino, K.; Idenuma, R.; Ohshima, H. Colloids Surf., B 1996, 8, 93. (15) Dobashi, T.; Nobe, M.; Yoshihara, H.; Yamamoto, T.; Konno, A. Langmuir 2004, 20, 6530.
10.1021/la0522350 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/03/2005
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Figure 2. Schematic illustration of the preparation of the κ-carrageenan gel. Figure 4. Polarized microscope images of κ-carrageenan gel. The concentration of the solution of potassium chloride solutions are 0.2, 0.5, 2.0, 3.0, and 4.0 M from top to bottom.
Figure 3. (a) Microscope image of the κ-carrageenan gel. The gel is observed under the natural light. The left edge is the diffusing end of the potassium ion. (b) The polarized microscope image of the gel shown in (a). (c) The expanded view of (b). (d) The brightness profile along the axial direction of the image (c). cells were in contact, were changed from 0.1 to 4.0 M. In this system, hence, only the potassium chloride diffused into the κ-carrageenan solution. The gels thus obtained were taken out of the capillary and observed under polarized microscope (Nikon SMZ-U with the crossed-nicols). The polarized microscope images of the gels were gained by using a digital camera (Casio QV-2300UX). The intensity profile of the images was measured by using image analyzing software (IPLab spectrum, Scanalitics Inc.). The phase diagram of κ-carrageenan in potassium chloride solution was also determined. The uniform solution of κ-carrageenan and potassium chloride was prepared at 90 °C, and then the temperature was lowered to 50 °C. The phase diagram was determined at 50 °C after 3 days from the preparation of the solutions. The concentration of κ-carrageenan was changed from 0.1 to 1.6w/w% at various concentrations of potassium chloride from 0.1 to 0.6 M. The phase boundary between the sol and the gel states was determined by the fluidity of the solution. The phase boundary between the amorphous gel and the liquid crystalline gel was also determined by the appearance of the polarized light.
Results In Figure 3, the images of the rod shape gel obtained here are shown. As shown in Figure 3a, the gel is almost transparent under the natural light. It, however, shows strong polarized light when observed under the crossed-nicols as shown in Figure 3b. The intensity of the polarized light changes when the gel is rotated on the stage of a polarized microscope. It strongly suggests
that the brighter regions consist of the liquid crystal and that the liquid crystal domains are oriented to a certain direction. The direction of orientation, however, could not be determined at present. It is clear from this image that the periodical stripe pattern is created within the gel. The stripe pattern is formed perpendicular to the axial direction of the rod shaped gel. It strongly suggests that the pattern is formed by the concentration gradient of potassium ion that is created along the axial direction of the rod shaped gel. The expanded view of the Figure 3b is shown in Figure 3c with the intensity profile of the image, which is shown in Figure 3d. The distance between neighboring brighter layers increases with the distance from the diffusing end of the potassium chloride. This is one of the characteristics of the Liesegang pattern. In Figure 4, the patterns in the gel that was obtained under various concentrations of the potassium chloride are shown. Both the period and the thickness of the layers depend on the concentration of the solution of potassium chloride. It is clear from these images that the period of the stripe pattern becomes shorter with the concentration of the solution of potassium chloride. On the other hand, the thickness of the liquid crystalline layers becomes thinner when the concentration of the solution of potassium chloride is increased. Hence, the stripe pattern that formed in the higher concentration region of the potassium ion becomes finer. The period of the liquid crystalline layers and the width of them are the measure that characterizes the stripe pattern in the gel. First, the distance between two adjacent liquid crystalline layers, ∆xn, is measured and plotted as a function of the distance from the diffusing end in Figure 5.The results are well explained as follows:
∆xn ) p xn+ const.
(1)
Here, xn represents the distance between the nth liquid crystalline layer and the diffusing end. The coefficient p is called as the spacing coefficient. Second, the thicknesses of the liquid crystalline layers, wn, are measured from the half width of the each peaks of the intensity profile that is shown in Figure 3d and plotted in Figure 6 as a function of the distance from the diffusing end. To avoid the lens effect due to the rod shaped gel, the intensity profile is measured at the central portion of the gel. It is clear from Figure 6 that the width of the liquid crystalline layers depends on the distance
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Figure 5. Relationship between the spacing ∆xn and the distance form the diffusing end xn. The concentration of potassium chloride in the outer solution is 0.2 M, O; 0.5 M, 3; 2.0 M, 0; 3 M, ); and 4 M, 4, respectively. The lines are the least-squares fit to the results.
Figure 8. Phase diagram of κ-carrageenan and the potassium chloride system. The open symbols represent the isotropic gel phase and the closed symbols correspond to liquid crystalline phase. The arrow in this figure indicates the path along which the system changes with time.
the width coefficient is expressed as follows:
R ) 0.53p
(3)
Discussion
Figure 6. Relationship between the width wn and the distance form the diffusion end xn. The symbols are the same with that of Figure 5. The lines are the least-squares fit to the results.
Figure 7. Double logarithmic plot of the spacing coefficient p, O, and the width coefficient R, ∆, as a function of the concentration of potassium chloride. The slopes of the lines are -1.
from the diffusing end. The results are well expressed by the following equation:
wn ) R xn + const.
(2)
The coefficient R is called as the width coefficient. It is clear from Figure 6 that the thickness of the liquid crystalline layers linearly increases with the distance from the diffusing end. These results shown in Figures 5 and 6 indicate that the spacing coefficient and the width coefficient depend on the concentration of the solution of potassium chloride. In Figure 7, the spacing coefficients and the width coefficients are plotted as a function of the concentration of the solution of potassium chloride in a double logarithmic manner. It is found from these figures that both the spacing coefficient and the width coefficient are inversely proportional to the concentration of the solution of potassium chloride. The relationship between the spacing coefficient and
The phase diagram of κ-carrageenan in potassium chloride solution is given in Figure 8. It is clear from this figure that the phase boundary between the sol and the gel states lays at a lower concentration region of the potassium chloride of about 10 mM. The results obtained here are not too far from the previous results because the exact phase boundary strongly depends on the molecular weight of κ-carrageenan.13,16 On the other hand, the phase boundary between the amorphous gel and the liquid crystalline gel appears at a higher concentration region of the potassium chloride of about 0.15-0.2 M. In this study, the concentration of κ-carrageenan is fixed at 1.0%. The phase of the present system, hence, changes along the arrow given in Figure 8 with time. The initial concentration of the potassium chloride in the κ-carrageenan solution is less than 10 mM because it is solution. The concentration of the potassium ion, however, increases with time by the diffusion from the reservoir. The solution of κ-carrageenan, hence, first transforms from the sol to the gel state when the concentration of potassium chloride in the solution becomes higher than 10 mM. The gel formed in this state is the amorphous gel since the concentration of potassium chloride is not much higher. Then the gel transforms into the liquid crystalline gel when the concentration of potassium ion is increased above 0.15 M. The liquid crystal state of the gel may occur by the alignment of the double helical zones of κ-carrageenan.10,11 The κ-carrageenan, hence, behaves as a supporting medium of the pattern as well as a pattern forming substance. The creation of the liquid crystal, which is the dense phase of the polymer network, also indicates the creation of the dilute phase of the polymer network according to the conservation of mass. Both phases are spontaneously fixed in space because the formation of liquid crystal and the growth process occur in the gel state. Once the liquid crystal layer is formed in space, it triggers the formation of layers along the direction of diffusion. These may be the origin of the stripe pattern in the present system. The aspects of the pattern formation are also confirmed by a preliminary computer simulation and will be reported elsewhere. Similar studies of gelation have been made so far using the alginate system.17 However, the formation of the spatial pattern within the gel has yet to be reported. In the case of alginate, the gel collapses when the gelation takes place. It suggests that the (16) Watase, M.; Nishinari, K. J. Texture Stud. 1981, 12, 427. (17) Smidsrød, O.; Skja˚k-bræk, G. Trends Biotechnol. 1990, 8 (3), 71.
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density of the polymer network is much higher than the κ-carrageenan gel. In such a system, the elastic modulus of the gel is large, and hence, the mobility of polymer chain is not enough to create the liquid crystalline region after gelation. This may be the main reason for the difference in the κ-carrageenan system and the alginate system. The appearance of the spatial pattern observed here is quite similar with that of the Liesegang pattern.3-9 The relationship between the spacing, the width, and the distance from the diffusing end is the characteristic of the Liesegang pattern.5-8 In our case, however, the system is a quasi-one-dimensional gel, and hence, the results are the stripe pattern that grows along the axial direction of the rod shaped gel. It has been studied so far that the spacing coefficient, p, in the Liesegang pattern is well described by the following equation:6,9
CB0
p ) F(B0) + G(B0) CA0
wn )
wn )
CLC - CAG
∆xn
(6)
CB0 - CAG CLC - CAG
pxn
(7)
By comparing eq 7 with eq 2, the width coefficient is simply expressed by the spacing coefficient as follows:
(4)
(5)
CB0 - CAG
Substitution of eq 1 into above equation yields following equation:
R)
Here, CA0 is the initial concentration of the salt A that diffuses into the gel and CB0 is the initial concentration of the salt B that presence in the gel (hereafter, A and B are called as the diffusant and the reactant for the sake of simplicity). The coefficients F and G are the constants that depend on the concentration of the reactant. The substances A and B correspond to potassium chloride and κ-carrageenan in the present system. The coefficients F and G and also CB0 are reasonably assumed to be constant because the initial concentration of reactant is fixed at 1.0% in this study. The spacing coefficient is, hence, only a function of the concentration of diffusant. It is clear from eq 3 that the spacing coefficient should depend on the concentration of diffusant inversely. The results shown in Figure 7 can be thus explainable in terms of the Liesegang phenomenon. The spatial change of the width of the liquid crystal layers is also an important parameter that characterizes the aspect of the Liesegang pattern. The spatial change of the width of the Liesegang pattern has yet to be clarified in detail though it is discussed theoretically.8 Consider now the successive layers of xn and xn+1. The distance between these layers is expressed by ∆xn. The concentration of κ-carrageenan before pattern formation is CB0 and is uniform in space. After the pattern is formed, the region ∆xn is split into two regions. One region consists of the liquid crystalline gel whose width is wn with the average concentration of CLC. The other is the amorphous gel, and its width is (∆xn - wn) with average concentration CAG. Both the amorphous and the liquid crystalline regions of the patterned gel solely consist of κ-carrageenan in the present case. The decrease of the concentration of the polymer network in the amorphous gel is, hence, compensated by the increase of the concentration of liquid crystalline gel in the layer. The conservation of mass yields following relationship:
CB0 ∆xn ) (∆xn - wn)CAG + wnCLC
The width of the pattern can be written as follows:
CB0 - CAG CLC - CAG
p
(8)
The front factor of the spacing coefficient in eq 8 is only a function of the concentration of the reactant. The spacing coefficient and the width coefficient, hence, depend on the concentration of the diffusant in the same manner. Furthermore, eq 3 indicates that the front factor of eq 8 is about one-half. The results are realized when the thickness of the liquid crystalline region of the gel is almost the same with that of the amorphous region of the gel. The results are confirmed by the images of the patterned gel that are shown in Figure 4 and the intensity profile of the gel, Figure 3d.
Conclusion The gel in which the liquid crystalline layers are frozen is obtained by a simple method. Such a gel is formed by diffusing potassium chloride into the solution of κ-carrageenan. The spacing and the width of liquid crystalline layers, which characterize the stripe pattern, are analyzed. It is found that the pattern thus created in the gel is expected of the Liesegang phenomenon. The results obtained here indicate that the solution of κ-carrageenan first transforms into the gel state from the sol state. The gel thus formed is uniform in space. Then the liquid crystalline layers are formed with time. The pattern is spontaneously frozen into the polymer network of the gel because the formation of the liquid crystalline gel occurs in the gel state. The gel, hence, behaves as a supporting medium of the spatial pattern as well as the pattern forming substance. Acknowledgment. The authors thank Professor Isamu Ohnishi of Hiroshima University for the fruitful discussions and the preliminary computer simulation of the system. One of the authors (T.N.) thanks financial support from Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists. LA0522350
