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Facile preparation of monolithic k-carrageenan aerogels† Kathirvel Ganesan* and Lorenz Ratke To the best of our knowledge, it is the first study reporting the synthesis of monolithic k-carrageenan aerogels with meso- and macroporous structures, being unique in physical and chemical properties. We demonstrate a novel method to synthesize k-carrageenan aerogels in which potassium thiocyanate was used as the source of specific ions. Aerogels were characterized by envelope density analysis, scanning electron microscopy, nitrogen adsorption–desorption analysis, X-ray powder diffractometry and IR spectroscopy. By varying the concentration of k-carrageenan between 0.5 and 3 wt%, the envelope density can be linearly increased from 40 to 160 kg m3. The sulphate functional groups in the wet gel and the specific ions are the key factors controlling the volume shrinkage of aerogels which average about 66%. The aerogels exhibit a fibrillar structure similar to cellulose aerogels. The fibril thickness was observed to be 10–15 nm and the specific surface area was about 230 m2 g1. The existing meso- and
Received 12th November 2013 Accepted 24th January 2014
macroporous structures were confirmed by nitrogen adsorption–desorption isotherm analysis and scanning electron microscopy. The aerogels were completely pure, free of specific ions and confirmed
DOI: 10.1039/c3sm52862f
to be amorphous by powder X-ray diffraction. Hence, these porous materials can provide a matrix with a
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chelating function which can be used as a host in many applications.
Introduction Aerogels based on polysaccharides have attracted great interest in many research elds including drug delivery, separation techniques, tissue engineering and catalysis due to their very low toxicity, biodegradability and ease of production from natural resources.1–7 Among many aerogels based on polysaccharides, only some have cationic or anionic functional groups. They can be used as hosts for applications where high chemical affinity to guest atoms/molecules is required. For instance, the carboxylate anion in the alginate matrix coordinated to palladium metal can be used as a catalyst for the Suzuki reaction.4 Until now, only a limited number of aerogels with ionic functional groups have been explored. Herein, our research approach is to develop a procedure to prepare the aerogels of the anionic polysaccharide, k-carrageenan, and investigate their properties. To synthesize the aerogels of k-carrageenan, the careful and systematic understanding of its gelation is required. k-Carrageenan is a linear sulphated polysaccharide which belongs to the family of carrageenan (Fig. 1). It forms a hard gel in the presence of specic cations like potassium, rubidium or caesium. It is soluble in hot aqueous solution above 80 C. Aer cooling below mid-point temperature Tm, which is dened by
the concentration of specic cations, k-carrageenan forms a gel owing to its structural transition from coil to helix, double helix formation and aggregation of double helices.8–13 In this process the specic cations promote the structural transition and effectively stabilize the helices.13 In this specic cation-induced gelation, the co-ion of the specic cation, i.e., the specic anion, also plays a signicant role in dening the specic interaction of the polymer chain and the counter ion and can act as potent helix-stabilizers in the sequence of SO42 < F < Cl < Br # NO3 < I < SCN when K+ acts as the cation.13,14 NMR experiments and the Poisson–Boltzmann cell model suggest that the specic cation and specic anion bind to different classes of sites on the helix.13,15–17 However, the nature of binding sites of specic ions in the helix structure is still poorly understood. We could perceive the signicance of specic ions
Institut f¨ ur Materialphysik im Weltraum, Deutsches Zentrum f¨ ur Lu- und Raumfahrt, Linder H¨ ohe, 51170 K¨ oln, Germany. E-mail: [email protected] † Electronic supplementary 10.1039/c3sm52862f
information
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(ESI)
available.
See
DOI:
Fig. 1 The chemical structure illustrating the monomer unit of kcarrageenan.
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and understand that the specic cation and anion combinations have to be selectively chosen for the gelation process in order to achieve desired hydrogel properties. Because of the unique properties of k-carrageenan such as no toxicity in adequate concentration,18,19 ionotropic, thermotropic, chelating20–24 and encapsulating properties,25–27 the hydrogels have received great attention for applications especially as thickening and stabilizing agents in food industries, as scaffolds in tissue engineering,28,29 and in immobilizing enzymes, whole cells, proteins and biocatalysts.25 Although many studies have been carried out on the hydrogels of k-carrageenan, aerogels of k-carrageenan are less explored. Quignard et al.30 have reported a methodology in which aerogels of k-carrageenan in the form of micro-beads were prepared by the following procedure: the hot solution of k-carrageenan was dropped to the cold solution of potassium chloride, forming a hydrogel; solvent exchange with ethanol and nally the micro-beads were supercritically dried. The aerogel consists of a mesoporous structure with a narrow pore size distribution (20 nm), having a volume shrinkage of 95% from its wet gel volume.30–32 It is unclear how this ionic polysaccharide can lose the gel volume (95%) and the maximum porosity during the supercritical drying process. This huge shrinkage behaviour of the k-carrageenan aerogel makes this material ineffective for any kind of application. In this paper, we demonstrate a facile preparation method and characterization of monolithic k-carrageenan aerogels where the volume shrinkage is reduced to 66%. It is the rst study reporting monolithic k-carrageenan aerogels with meso- and macroporous properties. Potassium thiocyanate, KSCN, has been chosen as the source of specic ions because of its great interaction with the polymer units, increasing helix formation and stabilizing effectively the helix structures.13,14 Detailed analysis of the factors dening the properties of the new k-carrageenan aerogels will be carried out. Owing to their ionotropic, thermotropic and encapsulating properties, we believe that the aerogels of k-carrageenan would acquire an advantage over other wellknown polysaccharide aerogels like alginate and chitosan.
Experimental Materials and methods All chemicals were used as received. Potassium thiocyanate and k-carrageenan were purchased from Merck and Sigma-Aldrich respectively. The received k-carrageenan powder was analysed by EDX and found to be a potassium salt with a potassium to sulphur ratio (K/S) of 0.91. For the preparation of hydrogels, deionized water was used. Acetone with a grade of Normapur was used for solvent exchange. Supercritical drying was carried out in an autoclave using pure carbon dioxide (CO2), following the procedure reported by Hoepfner et al.33
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nitrogen adsorption–desorption isotherm analysis (Micromeritics – Tristar II 3020), Fourier transform infrared spectroscopy (FTIR; Bruker-Tensor 27 instrument) and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM: Merlin – Carl Zeiss Microscope; gold sputtered samples; SEM-EDX: LEO 1530 VP with EDX-Analyser; EDX detector: OXFORD; the samples were not sputtered). To examine the mole ratio of potassium and sulphur, K/S value, EDX analysis was employed. The EDX spectra and K/S values are given in the ESI (see Fig. SI-1 and Table SI-1†). The XRD measurements of monolithic and powder samples were performed on a Bruker D8 DISCOVER diffractometer using Cu-Ka ˚ UV-Vis absorption spectra were recorded radiation (l ¼ 1.54 A). using a Perkin Elmer Lambda 19 using 1 cm quartz cuvettes. Synthesis of k-carrageenan aerogels A homogeneous clear solution of k-carrageenan (175 mL) was prepared by warming an aqueous dispersion of k-carrageenan above 95 C under constant stirring. The pH of the solution was about 7.5 to 8. To this clear solution, 25 mL of an aqueous solution of KSCN (2.4 M) was added dropwise in a way that the temperature of the solution was not allowed to decrease below 90 C. During the addition of KSCN, it is essential to maintain the clear solution of k-carrageenan under stirring in order to have the homogeneous distribution of ions which can greatly inuence the monolithic gel formation. Aer complete addition of KSCN, the stirring of the homogeneous clear solution was continued for 5 minutes and then transferred to cylindrical polypropylene moulds. The moulds were sealed with a plastic Paralm, le undisturbed in a fume hood and cooled to room temperature. The concentration of KSCN was 0.3 M in the nal hydrogel. The concentration of k-carrageenan was varied between 0.5 and 3 wt%. Aer aging for 16 hours, solvent exchange was carried out with acetone until KSCN was completely removed. The removal of KSCN was conrmed by titrating the washed solvent (acetone) against an aqueous solution of iron(III) nitrate (0.1 M). Hence, thiocyanate ions can form a complex with iron(III) nitrate and the colour of the titrant, i.e., iron salt, changes to deep blood red, brown, orange, yellow, pale yellow or colourless depending upon the concentration of thiocyanate ions in acetone solution. The colourless or pale yellow solution of iron salt indicates the complete removal of thiocyanate ions from the wet gel body. This titration experiment with iron salt was combined with UV-Vis spectroscopy to track the complete removal of KSCN from the gel body, as the ferric thiocyanate complex can show an absorption band at 451 nm (see ESI – Fig. SI-2†).34 Aer conrming the complete removal of KSCN by UV-Vis spectroscopy, two more washing steps were carried out with acetone to ensure that there is no more trace of KSCN present in the sample. In the nal step, the acetogels were employed in the supercritical drying process to obtain the aerogels.
Characterization
Results and discussion
The products were characterized by envelope density measurement (Micromeritics – GeoPyc 1360), skeletal density (Micromeritics – Accupyc II 1340; Gas pycnometer – Helium), BET
The k-carrageenan aerogels obtained by supercritical drying are shown in Fig. 2. The samples are intact, cylindrical in shape, white and opaque.
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Fig. 2 Photographic image of k-carrageenan aerogels, prepared by adding 0.3 M of KSCN. The concentration of k-carrageenan (in wt%) is tagged below the corresponding sample.
The washing of hydrogels with acetone induces shrinkage of the wet gel body. Measurement of the volume aer washing gives the volume shrinkage of these wet acetogels. Aer supercritical drying further shrinkage is observed. The volume shrinkage of aceto- and aerogels with respect to the concentration of k-carrageenan is shown in Fig. 3. There was no major difference in the volume shrinkage of acetogels or aerogels while changing the concentration of k-carrageenan. The volume shrinkage was reduced only by about 2–3% if the concentration was increased by adding 1 wt% of k-carrageenan. To understand the shrinkage behaviour, the chemical composition of aerogels, i.e., the K/S mole ratio, was analysed and found to be in a range between 0.82 and 0.86. This K/S value is 6–10% less than native k-carrageenan (powder from Sigma-Aldrich, K/S ¼ 0.91). It suggests that the native potassium ions could be released from the hydrogels during the washing step. As this K/S mole ratio seems to be invariable and with very little difference from native k-carrageenan, it was excluded as the origin of the shrinkage behaviour. To elucidate the large shrinkage behaviour of k-carrageenan, the kind of interaction of solvents, acetone and supercritical
Fig. 3 Shrinkage behaviour of k-carrageenan gels. The K/S mole ratio value was obtained from EDX analysis.
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uid (CO2), with the sulphate functional group is taken into consideration. In the hydrogel form, the sulphate functional groups of k-carrageenan are hydrated enhancing the ionic strength and the repulsive force between the bres. During the solvent exchange process the diffusion of organic solvent can decrease gradually the solvated water molecules surrounding the sulphate ions and reduce the repulsive force between the bres. At the same time, KSCN is slowly washed out from the gel body. As a result, about 33% of volume shrinkage was observed from the hydrogel volume. Considering a 2–3 vol% difference as insignicant in the supercritical drying process, it was observed on average about 33% of volume shrinkage from the volume of acetogels. It may be because of the good chemical interaction of supercritical uid (CO2 gas) with the ionic groups of k-carrageenan. While degassing the CO2 gas, the bre strength could be destabilized. Here, increasing the concentration of k-carrageenan in unit volume does not prevent shrinkage behaviour. Due to this volume shrinkage behaviour of k-carrageenan, the supercritical drying process yielded only about 34 vol% of the aerogel, which is independent of the concentration of k-carrageenan. A linear relationship between envelope density and the concentration of k-carrageenan aerogels was observed (Fig. 4). The envelope density was obtained in the range between 40 and 160 kg m3 by varying the concentration of k-carrageenan. At zero concentration of k-carrageenan, a t to the data exhibits that there is a nite intercept of around 27 g L1. One can mathematically correct the measured density for the observed shrinkage, i.e., asking what would be the density, if the shrinkage had not occurred? Calculating this corrected density (rcorr), and naming the relative shrinkage (Sh), we obtain a simple relationship between the measured envelope density (re) and the shrinkage as rcorr ¼ re(1 Sh)
(1)
Taking the shrinkage data from Fig. 3 and the measured density data from Fig. 4, eqn (1) yields the corrected density
Fig. 4 Graph showing the linear dependence of envelope density on the concentrations of k-carrageenan.
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values which are shown in Fig. 4 as open circles. The decrease in the amount of k-carrageenan in the aerogel to zero gives an intercept value of zero. This shows that the intercept of the measured density values (lled circles in Fig. 4) is just an effect of shrinkage. It also demonstrates that any technique that could reduce shrinkage further, compared to our procedure here, would lead to even lighter aerogels in the range of 15 to 45 g L1. Scanning electron microscopy images of k-carrageenan aerogels are shown in Fig. 5. The fractured surface of the aerogel exhibited the interconnected nano-felt bre structure with a bre thickness about 10–15 nm. A broad pore size distribution
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ranging from meso- to macropores was also observed. Compared to the higher concentration of the k-carrageenan aerogel (see Fig. 5b and c), the lower concentration, 1 wt%, showed a loosely bound porous network (Fig. 5a), i.e., the intercrosslinking junctions of bres were less. Therefore the number of macropores and the pore size varied with the concentration of k-carrageenan. The porous structure and the specic surface area of aerogels were analysed from nitrogen adsorption–desorption isotherms (Fig. 6 and 7). For all the concentrations of k-carrageenan, the typical type IV isotherm (according to the IUPAC nomenclature) was observed. Fig. 6 shows one of the isotherms of an aerogel prepared with a concentration of 3 wt% of k-carrageenan. The isotherm showed a hysteresis loop caused by the capillary condensation of nitrogen in the mesopores (2–50 nm). The pore size distributions of 1 wt%, 2 wt% and 3 wt% of k-carrageenan aerogels are shown in the inset of Fig. 6. As BJH pore volume approximation is valid only for mesopores (2–50 nm), the inset shows a peak with a shoulder at about 35–40 nm, a rst maximum beyond 50 nm and a second one at 125 nm. The
Fig. 6 Nitrogen adsorption–desorption isotherm obtained from 3 wt % of the k-carrageenan aerogel. The inset shows the pore size distribution of 1 wt%, 2 wt%, and 3 wt% of k-carrageenan aerogels.
Scanning electron microscopy images of the fractured surface of k-carrageenan aerogels with concentrations of (a) 1 wt%, (b) 2 wt% and (c) 3 wt%.
Fig. 5
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Fig. 7 Graph illustrating the dependence of concentrations of
k-carrageenan on the specific surface area per mass (filled circles) and the specific surface area per volume (open circles).
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decrease beyond 55 nm reveals that the pore size distribution is rather broader agreeing with the results from scanning electron microscopy (see Fig. 5c). The BET specic surface area, i.e., the specic surface area per mass appeared to be almost independent of the concentration of k-carrageenan, holding a value of 230 m2 g1 (see Fig. 7 – lled circles). That means the surface area values obtained from nitrogen adsorption–desorption isotherms at low relative pressure (P/P0 0.3) are describing the surface area of the bre with a thickness about 10 to 15 nm which is constant for all concentrations of k-carrageenan (see Fig. 5). The number of bres and their inter-crosslinking junctions are excluded here. To get an indication of the function of the concentration of k-carrageenan, the specic surface area per volume (Sv) was computed by multiplying the BET specic surface area (Sm) with bulk envelope density (re) of the corresponding material (see eqn (2)). Sv ¼ Smre
(2)
In this case the specic surface area of the inter-crosslinking junctions was not taken into account and only the number of bres per volume was included. As a result, the value of specic surface area per volume is obtained which shows a linear relationship with the concentration of k-carrageenan (Fig. 7 – open circles), since the envelope density behaves in that way. This again conrms that the bre thickness is independent of the k-carrageenan concentration. If we compare this dependence of concentration on the value of specic surface area per volume and the percentage of volume shrinkage (compare Fig. 3 and 7), the specic surface area per volume would become independent of volume shrinkage as the number of bres and their intercrosslinking junctions have shown very little inuence on volume shrinkage (only 2–3%). Eqn (2) together with the wellknown relationship for the specic surface area, the bre or particle diameter and the solid fraction (4s) inside the aerogel,35 namely [Sv ¼ 44s/D], yields a relationship for the average bre diameter (D). D ¼ 4/(rsSm)
iron(III) nitrate. In comparison with the IR spectrum of commercial k-carrageenan, the vibrational bands were observed to be broader in the case of the aerogel, and the splitting of bands was not clear. This may be due to the difference in the crystallinity of the material. Factors affecting the monolithic form of k-carrageenan aerogels To understand the factors affecting the monolithic form of aerogels two experiments were carried out: (i) Inuence of the washing method. Two hydrogel samples with the same physico-chemical properties such as the shape, size, mass, volume, concentration of k-carrageenan (3 wt %) and KSCN (0.3 M) were prepared following the above procedure. One sample was washed with acetone until complete removal of KSCN and employed in supercritical drying. The shape of the aerogel was intact though it showed volume shrinkage as mentioned above (Fig. 10a). The other sample was
(3)
The skeletal density (rs) was measured to be 1.72 g cm3. Using the values for the specic surface area (see Fig. 7), we calculate an average bre diameter of 10 nm. This agrees well with the estimates from the SEM pictures. The powder X-ray diffraction data of the samples are shown in Fig. 8. The crystallinity of k-carrageenan mainly depends on the oriented packing of helices.8 The diffraction pattern of the k-carrageenan aerogel showed no pronounced peak. This indicates that the polymer lost its crystallinity while going through the synthetic pathway which may be caused by the random arrangement of helices. The sample purity was conrmed by FT-IR measurements. Fig. 9 shows the comparison spectra of KSCN, the aerogel of k-carrageenan and commercially available k-carrageenan. The absence of the KSCN vibrational band at 2049 cm1 (Fig. 9b) indicates that the aerogels prepared by this method are completely pure. This supports the titration experiment with
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Powder X-ray diffraction of commercially available k-carrageenan from Sigma-Aldrich and the k-carrageenan aerogel. The broad diffraction pattern indicates the amorphous nature of the aerogel.
Fig. 8
Comparison of FTIR spectra of KSCN (a), the k-carrageenan aerogel (b) and commercially available k-carrageenan from SigmaAldrich (c). The absence of the thiocyanate band at 2049 cm1 confirms the purity of the material. Fig. 9
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washed with water 6 times in 3 days, then with the acetone– water mixture and nally with acetone. Aer 3 days of exposure to water, the gel swelled but still intact with cylindrical shape. By washing with water, KSCN was almost removed from the gel. The swelled hydrogel failed to retain its shape and size when it was exposed to acetone. An enormous shrinkage was observed in the axial directions of the cylindrical shape (see Fig. 10b). Aer supercritical drying, the aerogels were analysed further to obtain physico-chemical properties. Both the samples have almost the same mass indicating no loss of k-carrageenan. The chemical composition obtained from
Fig. 10 Photographic image of k-carrageenan aerogels showing the influence of washing and drying processes. The hydrogels of these samples had the same physico-chemical properties. The washing process differs: (a) the sample was washed only with acetone and (b) the sample was washed with water (6 times in 3 days), the water– acetone mixture and acetone. The corresponding scanning electron microscopic images are compared at the bottom: image (c) obtained from sample (a); and image (d) obtained from sample (b).
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EDX analysis shows that there is a little difference in the K/S mole ratio. About 9% of potassium loss for the sample shown in Fig. 10a from its native k-carrageenan whereas 15% of potassium loss for the sample shown in Fig. 10b. Even though only 6% difference in the K/S mole ratio is observed between the samples shown in Fig. 10a and b, the deformation of the cylindrical sample (Fig. 10b) is incredible. Having noted this deformation of shape, considering the swelling properties and faster removal of KSCN while washing the gels with water, it was concluded that the stronger solvating effect of water molecules might have disintegrated or loosened the bre strength because of the weakening of helix structures, resulting in defect formation in the gel network. This could have been reected in the end product during the solvent exchange and supercritical drying. In comparison with the microstructure of a cylindrical sample (Fig. 10c), the deformed sample (Fig. 10d) has a closer packed bre structure with less macropores due to the enormous shrinkage and deformation of the aerogels in the axial directions of the cylindrical sample. (ii) Inuence of the concentration of specic ions. The KSCN concentration was varied from 0.3 M to 0.6 M or zero, setting the k-carrageenan concentration as constant (3 wt%). In the case of 0.6 M of KSCN, by the dropwise addition of KSCN to a hot, homogeneous solution of k-carrageenan, the hard gel pieces were immediately formed in solution at about 95–100 C which in turn leads to the inhomogeneous gel formation. In this case, the transition temperature for the gel pieces to have a coil structure can be more than 100 C. At zero concentration of KSCN hydrogels were prepared. The shrinkage behaviour of the k-carrageenan aerogel at zero concentration of KSCN is shown in Fig. 11. There is a lack of the specic ion and thereby the helix structures cannot be stabilized. During solvent exchange and the drying process the sample showed enormous shrinkage and deformed from its cylindrical shape like the sample shown in Fig. 10b. This result conrms that KSCN was not only involved in strengthening the hydrogel, but also played a signicant role in preparing the k-carrageenan aerogels with a specic shape and certain size when the hydrogels pass through the solvent exchange and drying process.
The shrinkage behaviour of the k-carrageenan aerogel at zero concentration of KSCN.
Fig. 11
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Conclusion We have successfully prepared monolithic k-carrageenan aerogels. The detailed analyses of these materials demonstrate that the sulphate functional groups in the wet gel and the specic ions are the key factors controlling the volume shrinkage of aerogels. Nitrogen adsorption–desorption isotherm analysis and scanning electron microscopy provide us an insight into the microstructure of the aerogels and it is found that the aerogels have an interconnected brillar structure with meso- and macroporous properties. The product is conrmed to be pure and free from any impurities or salt traces, and the original chemical composition of k-carrageenan is preserved at the end product. It can be stated that k-carageenan aerogels with mesoand macroporous properties could be promising candidates for applications as host materials.
Acknowledgements We thank Dr Matthias Kolbe for his help with scanning electron microscopy and EDX analysis, Dr Klemens Kelm for his assistance with powder X-ray diffraction data and Mrs Anke Nietsch for her timely help in UV-Vis spectroscopy.
References 1 M. A. Aegerter, N. Leventis and M. M. Koebel, Aerogels Handbook, Springer, New York, 2011. 2 F. Liebner, E. Haimer, A. Potthast and T. Rosenau, in Polysaccharide Building Blocks: A Sustainable Approach to the Development of Renewable Biomaterials, ed. Y. Habibi and L. A. Lucia, John Wiley & Sons, Inc., New Jersey, 2012. 3 T. Mehling, I. Smirnova, U. Guenther and R. H. H. Neubert, J. Non-Cryst. Solids, 2009, 355, 2472–2479. 4 A. Primo, M. Liebel and F. Quignard, Chem. Mater., 2009, 21, 621–627. 5 C. Garcia-Gonzalez, M. Alnaief and I. Smirnova, Carbohydr. Polym., 2011, 86, 1425–1438. ˇ. Knez, J. Biomater. Sci., Polym. 6 A. Veronovski, Z. Novak and Z Ed., 2012, 23, 873–886. 7 C. Gioia, A. Ricci, L. Bernardi, K. Bourahla, N. Tanchoux, M. Robitzer and F. Quignard, Eur. J. Org. Chem., 2013, 2013, 588–594. 8 N. S. Anderson, J. W. Campbell, M. M. Harding, D. A. Rees and J. W. B. Samuel, J. Mol. Biol., 1969, 45, 85–99. 9 R. P. Millane, R. Chandrasekaran, S. Arnott and I. C. M. Dea, Carbohydr. Res., 1988, 182, 1–17. 10 M. R. Mangione, D. Giacomazza, D. Bulone, V. Martorana and P. L. S. Biagio, Biophys. Chem., 2003, 104, 95–105. ¨ Pekcan and O. ¨ Tari, Int. J. Biol. Macromol., 2004, 34, 223– 11 O. 231.
3224 | Soft Matter, 2014, 10, 3218–3224
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12 M. Rinaudo, in Polysaccharides. structural diversity and functional versatility, ed. S. Dumitriu, Marcel Dekker, New York, 2005. 13 L. Piculell, in Food Polysaccharides and Their Applications, ed. A. M. Stephen, G. O. Phillips and P. A. Williams, CRC Press, Taylor & Francis Group, New York, 2006. 14 I. T. Norton, E. R. Morris and D. A. Rees, Carbohydr. Res., 1984, 134, 89–101. 15 P. S. Belton, V. J. Morris and S. F. Tanner, Int. J. Biol. Macromol., 1985, 7, 53. 16 S. Nilsson and L. Piculell, Macromolecules, 1991, 24, 3804– 3811. 17 W. Zhang, L. Piculell and S. Nilsson, Macromolecules, 1992, 25, 6165–6172. 18 S. M. Cohen and N. Ito, Crit. Rev. Toxicol., 2002, 32, 413–414. 19 A. J. Smit, J. Appl. Phycol., 2004, 16, 245–262. 20 F. Jones, H. C¨ olfen and M. Antonietti, Colloid Polym. Sci., 2000, 278, 491–501. 21 F. Jones, H. C¨ olfen and M. Antonietti, Biomacromolecules, 2000, 1, 556–563. 22 A. L. Daniel-da-Silva, T. Trinade, B. J. Goodfellow, B. F. O. Costa, R. N. Correia and A. M. Gil, Biomacromolecules, 2007, 8, 2350–2357. 23 M. Fujishima, H. Takatori, K. Yamai, Y. Nagao, H. Kitagawa and K. Uchida, J. Mater. Sci., 2008, 43, 3130–3134. 24 C. Kosanovic, G. Falini and D. Kralj, Cryst. Growth Des., 2011, 11, 269–277. 25 F. v. d. Velde, N. D. Lourenco, H. M. Pinheiro and M. Bakker, Adv. Synth. Catal., 2002, 344, 815–835. 26 A. J. Granero, J. M. Razal, G. G. Wallace and M. i. h. Panhuis, J. Mater. Chem., 2010, 20, 7953–7956. 27 A. Aldalbahi, J. Chu, P. Feng and M. i. h. Panhuis, Beilstein J. Nanotechnol., 2012, 33, 415–427. 28 V. E. Santo, A. M. Frias, M. Carida, R. Cancedda, M. E. Gomes, J. F. Mano and R. L. Reis, Biomacromolecules, 2009, 10, 1392–1401. 29 Y.-M. Lim, H.-J. Gwon, J.-H. Choi, J. Shin and Y.-C. Nho, Macromol. Res., 2010, 18, 29–34. 30 F. Quignard, R. Valentin and F. D. Renzo, New J. Chem., 2008, 32, 1300–1310. 31 M. Robitzer, A. Tourette, R. Horga, R. Valentin, M. Boissiere, J. M. Devoisselle, F. D. Renzo and F. Quinard, Carbohydr. Polym., 2011, 85, 44–53. 32 M. Robitzer, F. D. Renzo and F. Quignard, Microporous Mesoporous Mater., 2011, 140, 9–16. 33 S. Hoepfner, L. Ratke and B. Milow, Cellulose, 2008, 15, 121– 129. 34 G. Peintler, A. Nagy, A. K. Horvath, T. K¨ ortvelyesi and I. Nagypal, Phys. Chem. Chem. Phys., 2000, 2, 2575–2586. 35 E. E. Underwood, Quantitative Stereology, Addison-Wesley, New York, 1970.
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Facile preparation of monolithic k-carrageenan aerogels† Kathirvel Ganesan* and Lorenz Ratke To the best of our knowledge, it is the first study reporting the synthesis of monolithic k-carrageenan aerogels with meso- and macroporous structures, being unique in physical and chemical properties. We demonstrate a novel method to synthesize k-carrageenan aerogels in which potassium thiocyanate was used as the source of specific ions. Aerogels were characterized by envelope density analysis, scanning electron microscopy, nitrogen adsorption–desorption analysis, X-ray powder diffractometry and IR spectroscopy. By varying the concentration of k-carrageenan between 0.5 and 3 wt%, the envelope density can be linearly increased from 40 to 160 kg m3. The sulphate functional groups in the wet gel and the specific ions are the key factors controlling the volume shrinkage of aerogels which average about 66%. The aerogels exhibit a fibrillar structure similar to cellulose aerogels. The fibril thickness was observed to be 10–15 nm and the specific surface area was about 230 m2 g1. The existing meso- and
Received 12th November 2013 Accepted 24th January 2014
macroporous structures were confirmed by nitrogen adsorption–desorption isotherm analysis and scanning electron microscopy. The aerogels were completely pure, free of specific ions and confirmed
DOI: 10.1039/c3sm52862f
to be amorphous by powder X-ray diffraction. Hence, these porous materials can provide a matrix with a
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chelating function which can be used as a host in many applications.
Introduction Aerogels based on polysaccharides have attracted great interest in many research elds including drug delivery, separation techniques, tissue engineering and catalysis due to their very low toxicity, biodegradability and ease of production from natural resources.1–7 Among many aerogels based on polysaccharides, only some have cationic or anionic functional groups. They can be used as hosts for applications where high chemical affinity to guest atoms/molecules is required. For instance, the carboxylate anion in the alginate matrix coordinated to palladium metal can be used as a catalyst for the Suzuki reaction.4 Until now, only a limited number of aerogels with ionic functional groups have been explored. Herein, our research approach is to develop a procedure to prepare the aerogels of the anionic polysaccharide, k-carrageenan, and investigate their properties. To synthesize the aerogels of k-carrageenan, the careful and systematic understanding of its gelation is required. k-Carrageenan is a linear sulphated polysaccharide which belongs to the family of carrageenan (Fig. 1). It forms a hard gel in the presence of specic cations like potassium, rubidium or caesium. It is soluble in hot aqueous solution above 80 C. Aer cooling below mid-point temperature Tm, which is dened by
the concentration of specic cations, k-carrageenan forms a gel owing to its structural transition from coil to helix, double helix formation and aggregation of double helices.8–13 In this process the specic cations promote the structural transition and effectively stabilize the helices.13 In this specic cation-induced gelation, the co-ion of the specic cation, i.e., the specic anion, also plays a signicant role in dening the specic interaction of the polymer chain and the counter ion and can act as potent helix-stabilizers in the sequence of SO42 < F < Cl < Br # NO3 < I < SCN when K+ acts as the cation.13,14 NMR experiments and the Poisson–Boltzmann cell model suggest that the specic cation and specic anion bind to different classes of sites on the helix.13,15–17 However, the nature of binding sites of specic ions in the helix structure is still poorly understood. We could perceive the signicance of specic ions
Institut f¨ ur Materialphysik im Weltraum, Deutsches Zentrum f¨ ur Lu- und Raumfahrt, Linder H¨ ohe, 51170 K¨ oln, Germany. E-mail: [email protected] † Electronic supplementary 10.1039/c3sm52862f
information
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(ESI)
available.
See
DOI:
Fig. 1 The chemical structure illustrating the monomer unit of kcarrageenan.
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and understand that the specic cation and anion combinations have to be selectively chosen for the gelation process in order to achieve desired hydrogel properties. Because of the unique properties of k-carrageenan such as no toxicity in adequate concentration,18,19 ionotropic, thermotropic, chelating20–24 and encapsulating properties,25–27 the hydrogels have received great attention for applications especially as thickening and stabilizing agents in food industries, as scaffolds in tissue engineering,28,29 and in immobilizing enzymes, whole cells, proteins and biocatalysts.25 Although many studies have been carried out on the hydrogels of k-carrageenan, aerogels of k-carrageenan are less explored. Quignard et al.30 have reported a methodology in which aerogels of k-carrageenan in the form of micro-beads were prepared by the following procedure: the hot solution of k-carrageenan was dropped to the cold solution of potassium chloride, forming a hydrogel; solvent exchange with ethanol and nally the micro-beads were supercritically dried. The aerogel consists of a mesoporous structure with a narrow pore size distribution (20 nm), having a volume shrinkage of 95% from its wet gel volume.30–32 It is unclear how this ionic polysaccharide can lose the gel volume (95%) and the maximum porosity during the supercritical drying process. This huge shrinkage behaviour of the k-carrageenan aerogel makes this material ineffective for any kind of application. In this paper, we demonstrate a facile preparation method and characterization of monolithic k-carrageenan aerogels where the volume shrinkage is reduced to 66%. It is the rst study reporting monolithic k-carrageenan aerogels with meso- and macroporous properties. Potassium thiocyanate, KSCN, has been chosen as the source of specic ions because of its great interaction with the polymer units, increasing helix formation and stabilizing effectively the helix structures.13,14 Detailed analysis of the factors dening the properties of the new k-carrageenan aerogels will be carried out. Owing to their ionotropic, thermotropic and encapsulating properties, we believe that the aerogels of k-carrageenan would acquire an advantage over other wellknown polysaccharide aerogels like alginate and chitosan.
Experimental Materials and methods All chemicals were used as received. Potassium thiocyanate and k-carrageenan were purchased from Merck and Sigma-Aldrich respectively. The received k-carrageenan powder was analysed by EDX and found to be a potassium salt with a potassium to sulphur ratio (K/S) of 0.91. For the preparation of hydrogels, deionized water was used. Acetone with a grade of Normapur was used for solvent exchange. Supercritical drying was carried out in an autoclave using pure carbon dioxide (CO2), following the procedure reported by Hoepfner et al.33
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nitrogen adsorption–desorption isotherm analysis (Micromeritics – Tristar II 3020), Fourier transform infrared spectroscopy (FTIR; Bruker-Tensor 27 instrument) and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM: Merlin – Carl Zeiss Microscope; gold sputtered samples; SEM-EDX: LEO 1530 VP with EDX-Analyser; EDX detector: OXFORD; the samples were not sputtered). To examine the mole ratio of potassium and sulphur, K/S value, EDX analysis was employed. The EDX spectra and K/S values are given in the ESI (see Fig. SI-1 and Table SI-1†). The XRD measurements of monolithic and powder samples were performed on a Bruker D8 DISCOVER diffractometer using Cu-Ka ˚ UV-Vis absorption spectra were recorded radiation (l ¼ 1.54 A). using a Perkin Elmer Lambda 19 using 1 cm quartz cuvettes. Synthesis of k-carrageenan aerogels A homogeneous clear solution of k-carrageenan (175 mL) was prepared by warming an aqueous dispersion of k-carrageenan above 95 C under constant stirring. The pH of the solution was about 7.5 to 8. To this clear solution, 25 mL of an aqueous solution of KSCN (2.4 M) was added dropwise in a way that the temperature of the solution was not allowed to decrease below 90 C. During the addition of KSCN, it is essential to maintain the clear solution of k-carrageenan under stirring in order to have the homogeneous distribution of ions which can greatly inuence the monolithic gel formation. Aer complete addition of KSCN, the stirring of the homogeneous clear solution was continued for 5 minutes and then transferred to cylindrical polypropylene moulds. The moulds were sealed with a plastic Paralm, le undisturbed in a fume hood and cooled to room temperature. The concentration of KSCN was 0.3 M in the nal hydrogel. The concentration of k-carrageenan was varied between 0.5 and 3 wt%. Aer aging for 16 hours, solvent exchange was carried out with acetone until KSCN was completely removed. The removal of KSCN was conrmed by titrating the washed solvent (acetone) against an aqueous solution of iron(III) nitrate (0.1 M). Hence, thiocyanate ions can form a complex with iron(III) nitrate and the colour of the titrant, i.e., iron salt, changes to deep blood red, brown, orange, yellow, pale yellow or colourless depending upon the concentration of thiocyanate ions in acetone solution. The colourless or pale yellow solution of iron salt indicates the complete removal of thiocyanate ions from the wet gel body. This titration experiment with iron salt was combined with UV-Vis spectroscopy to track the complete removal of KSCN from the gel body, as the ferric thiocyanate complex can show an absorption band at 451 nm (see ESI – Fig. SI-2†).34 Aer conrming the complete removal of KSCN by UV-Vis spectroscopy, two more washing steps were carried out with acetone to ensure that there is no more trace of KSCN present in the sample. In the nal step, the acetogels were employed in the supercritical drying process to obtain the aerogels.
Characterization
Results and discussion
The products were characterized by envelope density measurement (Micromeritics – GeoPyc 1360), skeletal density (Micromeritics – Accupyc II 1340; Gas pycnometer – Helium), BET
The k-carrageenan aerogels obtained by supercritical drying are shown in Fig. 2. The samples are intact, cylindrical in shape, white and opaque.
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Fig. 2 Photographic image of k-carrageenan aerogels, prepared by adding 0.3 M of KSCN. The concentration of k-carrageenan (in wt%) is tagged below the corresponding sample.
The washing of hydrogels with acetone induces shrinkage of the wet gel body. Measurement of the volume aer washing gives the volume shrinkage of these wet acetogels. Aer supercritical drying further shrinkage is observed. The volume shrinkage of aceto- and aerogels with respect to the concentration of k-carrageenan is shown in Fig. 3. There was no major difference in the volume shrinkage of acetogels or aerogels while changing the concentration of k-carrageenan. The volume shrinkage was reduced only by about 2–3% if the concentration was increased by adding 1 wt% of k-carrageenan. To understand the shrinkage behaviour, the chemical composition of aerogels, i.e., the K/S mole ratio, was analysed and found to be in a range between 0.82 and 0.86. This K/S value is 6–10% less than native k-carrageenan (powder from Sigma-Aldrich, K/S ¼ 0.91). It suggests that the native potassium ions could be released from the hydrogels during the washing step. As this K/S mole ratio seems to be invariable and with very little difference from native k-carrageenan, it was excluded as the origin of the shrinkage behaviour. To elucidate the large shrinkage behaviour of k-carrageenan, the kind of interaction of solvents, acetone and supercritical
Fig. 3 Shrinkage behaviour of k-carrageenan gels. The K/S mole ratio value was obtained from EDX analysis.
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uid (CO2), with the sulphate functional group is taken into consideration. In the hydrogel form, the sulphate functional groups of k-carrageenan are hydrated enhancing the ionic strength and the repulsive force between the bres. During the solvent exchange process the diffusion of organic solvent can decrease gradually the solvated water molecules surrounding the sulphate ions and reduce the repulsive force between the bres. At the same time, KSCN is slowly washed out from the gel body. As a result, about 33% of volume shrinkage was observed from the hydrogel volume. Considering a 2–3 vol% difference as insignicant in the supercritical drying process, it was observed on average about 33% of volume shrinkage from the volume of acetogels. It may be because of the good chemical interaction of supercritical uid (CO2 gas) with the ionic groups of k-carrageenan. While degassing the CO2 gas, the bre strength could be destabilized. Here, increasing the concentration of k-carrageenan in unit volume does not prevent shrinkage behaviour. Due to this volume shrinkage behaviour of k-carrageenan, the supercritical drying process yielded only about 34 vol% of the aerogel, which is independent of the concentration of k-carrageenan. A linear relationship between envelope density and the concentration of k-carrageenan aerogels was observed (Fig. 4). The envelope density was obtained in the range between 40 and 160 kg m3 by varying the concentration of k-carrageenan. At zero concentration of k-carrageenan, a t to the data exhibits that there is a nite intercept of around 27 g L1. One can mathematically correct the measured density for the observed shrinkage, i.e., asking what would be the density, if the shrinkage had not occurred? Calculating this corrected density (rcorr), and naming the relative shrinkage (Sh), we obtain a simple relationship between the measured envelope density (re) and the shrinkage as rcorr ¼ re(1 Sh)
(1)
Taking the shrinkage data from Fig. 3 and the measured density data from Fig. 4, eqn (1) yields the corrected density
Fig. 4 Graph showing the linear dependence of envelope density on the concentrations of k-carrageenan.
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values which are shown in Fig. 4 as open circles. The decrease in the amount of k-carrageenan in the aerogel to zero gives an intercept value of zero. This shows that the intercept of the measured density values (lled circles in Fig. 4) is just an effect of shrinkage. It also demonstrates that any technique that could reduce shrinkage further, compared to our procedure here, would lead to even lighter aerogels in the range of 15 to 45 g L1. Scanning electron microscopy images of k-carrageenan aerogels are shown in Fig. 5. The fractured surface of the aerogel exhibited the interconnected nano-felt bre structure with a bre thickness about 10–15 nm. A broad pore size distribution
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ranging from meso- to macropores was also observed. Compared to the higher concentration of the k-carrageenan aerogel (see Fig. 5b and c), the lower concentration, 1 wt%, showed a loosely bound porous network (Fig. 5a), i.e., the intercrosslinking junctions of bres were less. Therefore the number of macropores and the pore size varied with the concentration of k-carrageenan. The porous structure and the specic surface area of aerogels were analysed from nitrogen adsorption–desorption isotherms (Fig. 6 and 7). For all the concentrations of k-carrageenan, the typical type IV isotherm (according to the IUPAC nomenclature) was observed. Fig. 6 shows one of the isotherms of an aerogel prepared with a concentration of 3 wt% of k-carrageenan. The isotherm showed a hysteresis loop caused by the capillary condensation of nitrogen in the mesopores (2–50 nm). The pore size distributions of 1 wt%, 2 wt% and 3 wt% of k-carrageenan aerogels are shown in the inset of Fig. 6. As BJH pore volume approximation is valid only for mesopores (2–50 nm), the inset shows a peak with a shoulder at about 35–40 nm, a rst maximum beyond 50 nm and a second one at 125 nm. The
Fig. 6 Nitrogen adsorption–desorption isotherm obtained from 3 wt % of the k-carrageenan aerogel. The inset shows the pore size distribution of 1 wt%, 2 wt%, and 3 wt% of k-carrageenan aerogels.
Scanning electron microscopy images of the fractured surface of k-carrageenan aerogels with concentrations of (a) 1 wt%, (b) 2 wt% and (c) 3 wt%.
Fig. 5
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Fig. 7 Graph illustrating the dependence of concentrations of
k-carrageenan on the specific surface area per mass (filled circles) and the specific surface area per volume (open circles).
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decrease beyond 55 nm reveals that the pore size distribution is rather broader agreeing with the results from scanning electron microscopy (see Fig. 5c). The BET specic surface area, i.e., the specic surface area per mass appeared to be almost independent of the concentration of k-carrageenan, holding a value of 230 m2 g1 (see Fig. 7 – lled circles). That means the surface area values obtained from nitrogen adsorption–desorption isotherms at low relative pressure (P/P0 0.3) are describing the surface area of the bre with a thickness about 10 to 15 nm which is constant for all concentrations of k-carrageenan (see Fig. 5). The number of bres and their inter-crosslinking junctions are excluded here. To get an indication of the function of the concentration of k-carrageenan, the specic surface area per volume (Sv) was computed by multiplying the BET specic surface area (Sm) with bulk envelope density (re) of the corresponding material (see eqn (2)). Sv ¼ Smre
(2)
In this case the specic surface area of the inter-crosslinking junctions was not taken into account and only the number of bres per volume was included. As a result, the value of specic surface area per volume is obtained which shows a linear relationship with the concentration of k-carrageenan (Fig. 7 – open circles), since the envelope density behaves in that way. This again conrms that the bre thickness is independent of the k-carrageenan concentration. If we compare this dependence of concentration on the value of specic surface area per volume and the percentage of volume shrinkage (compare Fig. 3 and 7), the specic surface area per volume would become independent of volume shrinkage as the number of bres and their intercrosslinking junctions have shown very little inuence on volume shrinkage (only 2–3%). Eqn (2) together with the wellknown relationship for the specic surface area, the bre or particle diameter and the solid fraction (4s) inside the aerogel,35 namely [Sv ¼ 44s/D], yields a relationship for the average bre diameter (D). D ¼ 4/(rsSm)
iron(III) nitrate. In comparison with the IR spectrum of commercial k-carrageenan, the vibrational bands were observed to be broader in the case of the aerogel, and the splitting of bands was not clear. This may be due to the difference in the crystallinity of the material. Factors affecting the monolithic form of k-carrageenan aerogels To understand the factors affecting the monolithic form of aerogels two experiments were carried out: (i) Inuence of the washing method. Two hydrogel samples with the same physico-chemical properties such as the shape, size, mass, volume, concentration of k-carrageenan (3 wt %) and KSCN (0.3 M) were prepared following the above procedure. One sample was washed with acetone until complete removal of KSCN and employed in supercritical drying. The shape of the aerogel was intact though it showed volume shrinkage as mentioned above (Fig. 10a). The other sample was
(3)
The skeletal density (rs) was measured to be 1.72 g cm3. Using the values for the specic surface area (see Fig. 7), we calculate an average bre diameter of 10 nm. This agrees well with the estimates from the SEM pictures. The powder X-ray diffraction data of the samples are shown in Fig. 8. The crystallinity of k-carrageenan mainly depends on the oriented packing of helices.8 The diffraction pattern of the k-carrageenan aerogel showed no pronounced peak. This indicates that the polymer lost its crystallinity while going through the synthetic pathway which may be caused by the random arrangement of helices. The sample purity was conrmed by FT-IR measurements. Fig. 9 shows the comparison spectra of KSCN, the aerogel of k-carrageenan and commercially available k-carrageenan. The absence of the KSCN vibrational band at 2049 cm1 (Fig. 9b) indicates that the aerogels prepared by this method are completely pure. This supports the titration experiment with
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Powder X-ray diffraction of commercially available k-carrageenan from Sigma-Aldrich and the k-carrageenan aerogel. The broad diffraction pattern indicates the amorphous nature of the aerogel.
Fig. 8
Comparison of FTIR spectra of KSCN (a), the k-carrageenan aerogel (b) and commercially available k-carrageenan from SigmaAldrich (c). The absence of the thiocyanate band at 2049 cm1 confirms the purity of the material. Fig. 9
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washed with water 6 times in 3 days, then with the acetone– water mixture and nally with acetone. Aer 3 days of exposure to water, the gel swelled but still intact with cylindrical shape. By washing with water, KSCN was almost removed from the gel. The swelled hydrogel failed to retain its shape and size when it was exposed to acetone. An enormous shrinkage was observed in the axial directions of the cylindrical shape (see Fig. 10b). Aer supercritical drying, the aerogels were analysed further to obtain physico-chemical properties. Both the samples have almost the same mass indicating no loss of k-carrageenan. The chemical composition obtained from
Fig. 10 Photographic image of k-carrageenan aerogels showing the influence of washing and drying processes. The hydrogels of these samples had the same physico-chemical properties. The washing process differs: (a) the sample was washed only with acetone and (b) the sample was washed with water (6 times in 3 days), the water– acetone mixture and acetone. The corresponding scanning electron microscopic images are compared at the bottom: image (c) obtained from sample (a); and image (d) obtained from sample (b).
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EDX analysis shows that there is a little difference in the K/S mole ratio. About 9% of potassium loss for the sample shown in Fig. 10a from its native k-carrageenan whereas 15% of potassium loss for the sample shown in Fig. 10b. Even though only 6% difference in the K/S mole ratio is observed between the samples shown in Fig. 10a and b, the deformation of the cylindrical sample (Fig. 10b) is incredible. Having noted this deformation of shape, considering the swelling properties and faster removal of KSCN while washing the gels with water, it was concluded that the stronger solvating effect of water molecules might have disintegrated or loosened the bre strength because of the weakening of helix structures, resulting in defect formation in the gel network. This could have been reected in the end product during the solvent exchange and supercritical drying. In comparison with the microstructure of a cylindrical sample (Fig. 10c), the deformed sample (Fig. 10d) has a closer packed bre structure with less macropores due to the enormous shrinkage and deformation of the aerogels in the axial directions of the cylindrical sample. (ii) Inuence of the concentration of specic ions. The KSCN concentration was varied from 0.3 M to 0.6 M or zero, setting the k-carrageenan concentration as constant (3 wt%). In the case of 0.6 M of KSCN, by the dropwise addition of KSCN to a hot, homogeneous solution of k-carrageenan, the hard gel pieces were immediately formed in solution at about 95–100 C which in turn leads to the inhomogeneous gel formation. In this case, the transition temperature for the gel pieces to have a coil structure can be more than 100 C. At zero concentration of KSCN hydrogels were prepared. The shrinkage behaviour of the k-carrageenan aerogel at zero concentration of KSCN is shown in Fig. 11. There is a lack of the specic ion and thereby the helix structures cannot be stabilized. During solvent exchange and the drying process the sample showed enormous shrinkage and deformed from its cylindrical shape like the sample shown in Fig. 10b. This result conrms that KSCN was not only involved in strengthening the hydrogel, but also played a signicant role in preparing the k-carrageenan aerogels with a specic shape and certain size when the hydrogels pass through the solvent exchange and drying process.
The shrinkage behaviour of the k-carrageenan aerogel at zero concentration of KSCN.
Fig. 11
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Conclusion We have successfully prepared monolithic k-carrageenan aerogels. The detailed analyses of these materials demonstrate that the sulphate functional groups in the wet gel and the specic ions are the key factors controlling the volume shrinkage of aerogels. Nitrogen adsorption–desorption isotherm analysis and scanning electron microscopy provide us an insight into the microstructure of the aerogels and it is found that the aerogels have an interconnected brillar structure with meso- and macroporous properties. The product is conrmed to be pure and free from any impurities or salt traces, and the original chemical composition of k-carrageenan is preserved at the end product. It can be stated that k-carageenan aerogels with mesoand macroporous properties could be promising candidates for applications as host materials.
Acknowledgements We thank Dr Matthias Kolbe for his help with scanning electron microscopy and EDX analysis, Dr Klemens Kelm for his assistance with powder X-ray diffraction data and Mrs Anke Nietsch for her timely help in UV-Vis spectroscopy.
References 1 M. A. Aegerter, N. Leventis and M. M. Koebel, Aerogels Handbook, Springer, New York, 2011. 2 F. Liebner, E. Haimer, A. Potthast and T. Rosenau, in Polysaccharide Building Blocks: A Sustainable Approach to the Development of Renewable Biomaterials, ed. Y. Habibi and L. A. Lucia, John Wiley & Sons, Inc., New Jersey, 2012. 3 T. Mehling, I. Smirnova, U. Guenther and R. H. H. Neubert, J. Non-Cryst. Solids, 2009, 355, 2472–2479. 4 A. Primo, M. Liebel and F. Quignard, Chem. Mater., 2009, 21, 621–627. 5 C. Garcia-Gonzalez, M. Alnaief and I. Smirnova, Carbohydr. Polym., 2011, 86, 1425–1438. ˇ. Knez, J. Biomater. Sci., Polym. 6 A. Veronovski, Z. Novak and Z Ed., 2012, 23, 873–886. 7 C. Gioia, A. Ricci, L. Bernardi, K. Bourahla, N. Tanchoux, M. Robitzer and F. Quignard, Eur. J. Org. Chem., 2013, 2013, 588–594. 8 N. S. Anderson, J. W. Campbell, M. M. Harding, D. A. Rees and J. W. B. Samuel, J. Mol. Biol., 1969, 45, 85–99. 9 R. P. Millane, R. Chandrasekaran, S. Arnott and I. C. M. Dea, Carbohydr. Res., 1988, 182, 1–17. 10 M. R. Mangione, D. Giacomazza, D. Bulone, V. Martorana and P. L. S. Biagio, Biophys. Chem., 2003, 104, 95–105. ¨ Pekcan and O. ¨ Tari, Int. J. Biol. Macromol., 2004, 34, 223– 11 O. 231.
3224 | Soft Matter, 2014, 10, 3218–3224
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12 M. Rinaudo, in Polysaccharides. structural diversity and functional versatility, ed. S. Dumitriu, Marcel Dekker, New York, 2005. 13 L. Piculell, in Food Polysaccharides and Their Applications, ed. A. M. Stephen, G. O. Phillips and P. A. Williams, CRC Press, Taylor & Francis Group, New York, 2006. 14 I. T. Norton, E. R. Morris and D. A. Rees, Carbohydr. Res., 1984, 134, 89–101. 15 P. S. Belton, V. J. Morris and S. F. Tanner, Int. J. Biol. Macromol., 1985, 7, 53. 16 S. Nilsson and L. Piculell, Macromolecules, 1991, 24, 3804– 3811. 17 W. Zhang, L. Piculell and S. Nilsson, Macromolecules, 1992, 25, 6165–6172. 18 S. M. Cohen and N. Ito, Crit. Rev. Toxicol., 2002, 32, 413–414. 19 A. J. Smit, J. Appl. Phycol., 2004, 16, 245–262. 20 F. Jones, H. C¨ olfen and M. Antonietti, Colloid Polym. Sci., 2000, 278, 491–501. 21 F. Jones, H. C¨ olfen and M. Antonietti, Biomacromolecules, 2000, 1, 556–563. 22 A. L. Daniel-da-Silva, T. Trinade, B. J. Goodfellow, B. F. O. Costa, R. N. Correia and A. M. Gil, Biomacromolecules, 2007, 8, 2350–2357. 23 M. Fujishima, H. Takatori, K. Yamai, Y. Nagao, H. Kitagawa and K. Uchida, J. Mater. Sci., 2008, 43, 3130–3134. 24 C. Kosanovic, G. Falini and D. Kralj, Cryst. Growth Des., 2011, 11, 269–277. 25 F. v. d. Velde, N. D. Lourenco, H. M. Pinheiro and M. Bakker, Adv. Synth. Catal., 2002, 344, 815–835. 26 A. J. Granero, J. M. Razal, G. G. Wallace and M. i. h. Panhuis, J. Mater. Chem., 2010, 20, 7953–7956. 27 A. Aldalbahi, J. Chu, P. Feng and M. i. h. Panhuis, Beilstein J. Nanotechnol., 2012, 33, 415–427. 28 V. E. Santo, A. M. Frias, M. Carida, R. Cancedda, M. E. Gomes, J. F. Mano and R. L. Reis, Biomacromolecules, 2009, 10, 1392–1401. 29 Y.-M. Lim, H.-J. Gwon, J.-H. Choi, J. Shin and Y.-C. Nho, Macromol. Res., 2010, 18, 29–34. 30 F. Quignard, R. Valentin and F. D. Renzo, New J. Chem., 2008, 32, 1300–1310. 31 M. Robitzer, A. Tourette, R. Horga, R. Valentin, M. Boissiere, J. M. Devoisselle, F. D. Renzo and F. Quinard, Carbohydr. Polym., 2011, 85, 44–53. 32 M. Robitzer, F. D. Renzo and F. Quignard, Microporous Mesoporous Mater., 2011, 140, 9–16. 33 S. Hoepfner, L. Ratke and B. Milow, Cellulose, 2008, 15, 121– 129. 34 G. Peintler, A. Nagy, A. K. Horvath, T. K¨ ortvelyesi and I. Nagypal, Phys. Chem. Chem. Phys., 2000, 2, 2575–2586. 35 E. E. Underwood, Quantitative Stereology, Addison-Wesley, New York, 1970.
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