- Email: [email protected]
SA + additive ion) gels
Colloids and Surfaces B: Biointerfaces 38 (2004) 197–200
Nanostructural characterization of the dehydrated (NIPA/SA + additive ion) gels K. Haraa,∗ , M. Sugiyamab , M. Annakac , Y. Soejimad a
Institute of Environmental Systems, Faculty of Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan b Department of Physics, Fuculty of Sciences, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan c Department of Chemistry, Fuculty of Sciences, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan d Research and Development Center for Higher Education, Kyushu University, 4-2-1 Ropponmatsu,Chuo-ku, Fukuoka 810-8560, Japan Received 20 February 2004; accepted 9 April 2004
Abstract In the present study, the authors have investigated ion-absorption effects on a nanoscopic structure of a dehydrated Nisopropylacrylamide/sodium acrylate (NIPA/SA) gel. First of all, the authors compared small-angle X-ray scattering (SAXS) profile of a dehydrated NIPA/SA gel with that dehydrated after absorbing Cu2+ ion. Then, in order to examine copper-ion distribution structure in the NIPA/SA gel dehydrated after absorption of copper-ion, an incident-X-ray energy-dependence of a small-angle X-ray scattering profile was observed, in which the anomalous dispersion effect was clearly perceived especially around a distinct SAXS-peak. Because the SAXS-peak is thought to come from a dehydration-induced microphase separation between hydrophilic and hydrophobic network-polymers in the NIPA/SA gel, such a feature indicates that the copper-ions gather in the dehydration-induced hydrophilic domains. In addition to this interesting copper-ion nanostructure in the dehydrated NIPA/SA gel, a difference in the SAXS-peak position between the dehydrated NIPA/SA gels with and without absorbing the copper-ion has shown a possibility of a controlling method of the nanostructure in relatively gentle conditions without special instruments. Along this line, in order to get further information on the ion-absorption effects on the nanostructure, the authors have compared the SAXS profiles of the several NIPA/SA gels which were dehydrated after absorbing respectively different kinds of ions. In the observation, the SAXS-peak positions have shown characteristic features which are different with the kind of the absorbed ions and found to be classified into several kinds according to the periodic-table group of the absorbed ion. © 2004 Elsevier B.V. All rights reserved. Keywords: Metal-ion-capture; Nanoscopic; N-Isopropylacrylamide; NIPA/SA; Gel
1. Introduction In both of the industrial technology and fundamental science, nanoscale structures have attracted much attention. In these days, several methods are taken to materialize the nanostructure with the structural-scales needed. Among them, the self-organization technique is usually utilized for realizing the structure in a range from few-nm’s to ∼50 nm, which can be the most realistic method for the mass∗
Corresponding author. Tel.: +81 92 642 3815; fax: +81 92 633 6958. E-mail address: [email protected] (K. Hara).
0927-7765/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2004.04.012
production without requiring precision instruments. Among the self-organizing techniques, there are still many concrete creating methods; the authors have been much interested in one of these methods which make use of constituent compatibilities. It is well-known that, in a polymer-solvent system, constituents’ compatibilities affect and change its internal structure such as a domain occurrence by a phase separation, of which the feature is governed by several thermodynamic quantities [1]. In addition to these parameters, the electrostatic force will also have an influence on the system balance. One of the well-known examples is the microphase separation which is usually realized in weakly charged polymer
198
K. Hara et al. / Colloids and Surfaces B: Biointerfaces 38 (2004) 197–200
solution systems. In relation to this phenomenon, the authors have been interested in the nanostructure of the heteropolymer hydrogels of which the network is composed of partly hydrophilic (charged) and partly hydrophobic polymers. States and properties of the gels are considerably influenced by interaction changes between solvent and polymernetwork with environmental conditions. For example, the property change in N-isopropylacrylamide (NIPA) gel with increasing temperature is well-known and interesting: there is a small and continuous change in volume in the pure NIPA gel at 36 ◦ C with the hydrophilic-to-hydrophobic change of its isopropyl-group, while, in the case that the networkpolymer has additional ionized groups inside such as the N-isopropylacrylamide/acrylic acid (NIPA/AAc) copolymer gel, the volume change becomes more drastic with the effect of the Donan potential [2]. With existence of an expanding force originated from an electrostatic interaction against the hydrophobic shrinking one, the NIPA/AAc gel exhibits a discrete volume change at a higher temperature than the pure NIPA gel [3,4]. Such a drastic volume change is referred as a volume phase transition [2,5]. In addition to this macroscopic feature, there also occurs a nanostructural change by introducing the ionized group in the gel: an emergence of a prominent peak in a small-angle neutron scattering (SANS) profile indicating realization of the microphase transition induced by interactions between the hydrophilic carboxyl and hydrophobic isopropyl-groups [6,7]. Such a competition of the internal forces will occur in a condition other than the volume phase transition. As such a phenomenon, the authors have been investigating property change of the hydrogels by dehydration, which is still interesting because the dehydration is one of the most general phenomena by which the gels show a drastic volume change. Besides, by dehydration, there occur property changes indicating that some gels becomes glass-like substances [8], which have been confirmed by viscoelastic measurements [9,10], Raman scattering [11] and inelastic neutron scattering experiments [12]. Moreover, by a small-angle X-ray scattering (SAXS) study, a distinct nanostructure has been also found in a dehydrated N-isopropylacrylamide/sodium acrylate (NIPA/SA) gel, which is a congeneric hydrogel of the NIPA/AAc gel, indicating occurrence of the microphase separation by the dehydration [13]. In a series of the nanostructural observations of the dehydrated NIPA/SA gel, the authors carried out detailed SAXS profile change examinations with the sodium-acrylic concentration and those on SANS profile change with heavy-water-content, in which some characteristic properties has been revealed on the nanoscale domain structure caused by the dehydration-induced microphaseseparation: the distinct SAXS and SANS peaks can be only observed in a restricted parameter region [14]. This feature indicates that the characteristic structure in the dehydrated NIPA/SA gel can be realized on a delicate balance of related interactions [15,16], conversely, the nanostructural control can be sufficiently conducted with manipulating macroscopic conditions without much energy and complicated equipments
because essentially the dehydration process requires no special processes and instruments such as those usually utilized in the lithography techniques. Next, let us go ahead on other properties of the heteropolymer gel which contains both of the ionized and hydrophobic side chains. It is well-known that some ionized polymers can capture multivalent ions in cooperation with several ionized side chains, namely by the chelation mechanism. Such a phenomenon can be harnessed for detecting and/or capturing toxic multivalent ions in waste solutions. Actually, Jackson et al. [17] devised an ionized-gel sensor sensitive to multivalent metal ions. The features suggest that the ion-capture property of the ionized-gel should be also very useful in the environmental purification technology. However, to the authors’ knowledge, there have been almost no investigations on the ion-capturing feature of the ionized-gel from a nanostructural point of view; the authors have been convinced that such investigations should be very important because such systems are stabilized with a balance of the interactions in a nanoscopic scale as mentioned above. Under these circumstances, the authors have been investigating the nanostructure of the dehydrated NIPA/SA gel. In the present study, the authors have investigated ionabsorption effects on a nanoscopic structure of a dehydrated N-isopropylacrylamide/sodium acrylate (NIPA/SA) gel. First of all, the authors compared SAXS profile of a dehydrated NIPA/SA gel with that dehydrated after absorbing Cu2+ ion. Then, in order to examine copper-ion distribution structure in the copper-ion-captured-and-dehydrated NIPA/SA gel, an incident-X-ray energy-dependence of small-angle X-ray scattering (SAXS) profile was observed. Next, in order to get further information on the ion-absorption effects on the nanostructure, the authors have compared the SAXS profiles of the several NIPA/SA gels which were dehydrated after absorbing respectively different kinds of ions.
2. Experimentals 2.1. Comparison of SAXS profiles of the dehydrated NIPA/SA gel with and without additive Cu2+ The NIPA/SA gels were synthesized by a free radical polymerization of the mixture {NIPA, 500 mM (5.66 g), sodium acrylate, 200 mM (1.88 g), N,N -methylenebisacrylamide, 8.6 mM (0.132 g), N,N,N ,N -tetramethylethylenediamine (240 L) in pure water (100 g)}, which was fully saturated with nitrogen, initiated by adding a small amount of ammonium persulfate (40 mg). The gelation was carried out 24 h at 0 ◦ C. After then, the jelled samples were washed in pure water for a week. The purified gel was cut into three portions; two of them were put into 0.2 and 1.0 mM’s aqueous solutions of CuCl2 for six days. Then all of the gels were gently dried in the atmosphere for six days. Then the SAXS profiles of these three kinds of dehydrated NIPA/SA gels were observed.
K. Hara et al. / Colloids and Surfaces B: Biointerfaces 38 (2004) 197–200
Observation of the SAXS profiles were carried out at room temperature with a SAXS apparatus (SAXES) installed at BL10C of Photon Factory in Institute of Materials Structure Science (IMSS), High Energy Accelerator Research Organization (KEK), Tsukuba, Japan. An X-ray beam from a syn˚ in wave-length) was used chrotron orbital radiation (1.3918 A as a light source of SAXES and the intensity distribution of the scattered X-ray was sensed with a scattering vector ranged ˚ −1 . The observed X-ray infrom 6.0 × 10−3 to 2.0 × 10−1 A tensity was corrected for the cell scattering and absorption, and then normalized with the thickness of the sample and irradiated time. By these correction and normalization, the value of final data straightforwardly corresponds to the scattering efficiency. 2.2. Observation of a copper ion distribution structure utilizing X-ray anomalous dispersion effect The preparation of the specimen was conducted in the same manner mentioned above. Observation of the SAXS profiles were also carried out at room temperature with the above-mentioned SAXS apparatus. Incident-X-ray energy dependence of the SAXS profile was observed with tuning an incident X-ray energy in a range from 8.91 to 9.04 keV so as to across the cabsorption edge (8.98 keV) with a resolution of E/E = 10−4 utilizing a channel-cut silicon-monochromator. The data correction procedure was also performed in the same manner mentioned above. 2.3. SAXS-peak variations with the several kinds of additive ions
199
Fig. 1. The SAXS profiles of a dehydrated NIPA/SA (500 mM/200 mM) gel and of those dehydrated after absorbing Cu2+ ion. Numerals in a square at the upper-right corner are Cu2+ concentrations of aqueous solutions in which NIPA/SA gel lumps were immersed before dehydration.
peak shift ceases at the concentration above 0.2 mM, which can be interpreted that because the carboxyl-group number in the gel is finite, superfluous Cu2+ ions can not interact with the stuffed ionized side chain polymers and, consequently, do not affect the nanoscopic structure. Other characteristic change is a broadening of the SAXS-peak indicating an increase in a heterogeneity of the domain structure. This feature can reflect a heterogeneous distribution of the region where the chelation occurs, however because there exists still a clear SAXS-peak, the heterogeneity may be small. Fig. 2 shows a distribution of the incident-X-ray energy dependence of the SAXS-intensity difference from that far
The preparation procedure of the wet NIPA/SA gel was almost same with that mentioned in Section 2.1 except for the concentration of NIPA (400 mM) and SA (300 mM). After preparing the purified wet gel, several lumps were cut out, after that, each gel block was immersed in a 0.2 mM aqueous solution of LiCl, NaCl, KCl, RbCl, CsCl, MgCl2 , CaCl2 , SrCl2 , BaCl2 , AlCl3 , FeCl3 , CoCl3 , NiCl2 and CuSO4 for 12 h. Then, the gel blocks were dried gently for six days in the atmosphere. Observation of the SAXS profiles of these dehydrated specimens were carried out in the same manner mentioned in Section 2.1.
3. Results and discussions Fig. 1 shows SAXS profiles of both the dehydrated NIPA/SA gels with and without additive copper ion. With absorbing copper ion, the SAXS-peak shifts toward the lower-q region demonstrating a correlation length of the hydrophilic domains becomes small. This feature can come from a condition that each copper ion (Cu2+ ) in the hydrophilic domains attracts two ionized side chains (negatively charged carboxyl groups) dangling from separated main chains by making a chelate-connection. It has been also found that the SAXS-
Fig. 2. The incident-X-ray energy dependence of the SAXS-intensity difference from that far (at 9.04 keV) from the Cu K␣ absorption edge. A thick line extruding from the base shows the energy of Cu K␣ absorption edge.
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• Aluminum ion CL of Al3+ locates near {Co3+ , Fe3+ }, indicating that the ionic charge number has important role to determine the correlation length.
Fig. 3. Ion-radius (reported by Shanon [20]) dependence of the correlation length derived from SAXS profiles of NIPA/SA (400 mM/300 mM) gels dehydrated after absorbing ions indicated in the figure.
(at 9.04 keV) from the Cu K␣ absorption edge. As can be easily seen, at the energy of the Cu K␣ absorption edge, the intensity difference takes a minimum around the SAXS-peak position. Because the anomalous dispersion effect only occurs by the copper element [18,19], this characteristic feature directly demonstrates a close relation between the Cu2+ distribution and hydrophilic domain structure. This result indicates a localized distribution of the copper ions in the hydrophilic domain. In the SAXS experiments mentioned in Section 2.3, a clear and intense SAXS-peak was also observed in all of the specimens with additive ions, regardless of their ionic charge numbers, which demonstrates that the fundamental nanoscale structure in the dehydrated NIPA/SA gel is not destroyed by the ion addition. Besides, it was also found that the position of the SAXS-peak shows systematic change with the kinds of the additive ions. Fig. 3 shows ion-radius (reported by Shannon [20] in the six-coordination structure) dependences of a correlation length (CL) derived from the SAXS-peak positions. In the figure, five sets of systematic variation of CL are perceived of which the behaviors seem mainly differ with the periodic table groups of the additive ions. Therefore, let us classify their behaviors from that point of view: • Alkali metal-ion group A large variation in CL with a maximum between the sodium and potassium ions. • Alkaline-earth metal-ion group A large variation in CL. Similar ion-radius dependence with the alkali metal-ion group. • Transition metal-ion groups {Cu2+ , Ni2+ }, {Co3+ , Fe3+ } CL’s with the same ionic charge number does not differ so much, while those with different ionic charge numbers ({Cu2+ , Ni2+ } and {Co3+ , Fe3+ }) locate with a distance in the figure.
Because the periodic table groups are defined by the valence of elements, the most effective condition of the SAXSpeak shift seems the ionic charge number, then, the next important condition can be the ion-radius. For the more detailed discussion, further investigations with more quantitative information may be needed such as an absolute amount of the absorbed ion and so on, which are now in preparation. From these SAXS observations, drastic systematic changes in the SAXS-peak have been revealed, which should be not only interesting from a fundamental science viewpoint but also valuable as a nanostructural controlling technique as mentioned above.
Acknowledgments The authors would like to express their thanks to Dr. H. Inoko of Osaka University for his kind help on the SAXS experiments. The SAXS experiments were performed under the approval of the Photon Factory Advisory Committee (proposal No. 2002G094).
References [1] P.G. de Gennes, Scaling Concepts in Polymer Physics, Cornell University Press, Ithaca, NY, 1979. [2] T. Tanaka, Phys. Rev. Lett. 45 (1980) 1636. [3] S. Hirotsu, Y. Hirokawa, T. Tanaka, J. Chem. Phys. 87 (1987) 1392. [4] S. Hirotsu, Phase Transitions 47 (1994) 183. [5] T. Tanaka, Phys. Rev. Lett. 40 (1978) 820. [6] M. Shibayama, T. Tanaka, J. Chem. Phys. 97 (1992) 6842. [7] M. Shibayama, T. Tanaka, J. Chem. Phys. 102 (1995) 9392. [8] E. Takushi, L. Asato, T. Nakada, Nature 345 (1990) 298. [9] T. Masuike, S. Taki, K. Hara, S. Kai, Jpn. J. Appl. Phys. 34 (1995) 4997. [10] N. Koshoubu, H. Kanaya, K. Hara, S. Taki, E. Takushi, K. Matsushige, Jpn. J. Appl. Phys. 32 (1993) 4038. [11] K. Hara, T. Masuike, A. Nakamura, H. Okabe, N. Hiramatsu, Jpn. J. Appl. Phys. 34 (1995) 5700. [12] K. Hara, A. Nakamura, N. Hiramatsu, A. Ma-tsumoto, Physica B 263–264 (1999) 350. [13] M. Sugiyama, S. Kuwajima, Y. Soejima, A. Na-kamura, N. Hiramatsu, T. Kikukawa, A. Suzuki, K. Hara, Jpn. J. Appl. Phys. 38 (1999) L1360. [14] M. Sugiyama, M. Annaka, K. Hara, M.E. Vigild, G.D. Wignall, J. Phys. Chem. 107 (2003) 6300. [15] V. Borue, I. Erukhimovich, Macromolecules 21 (1988) 3240. [16] J.F. Joanny, L. Leibler, J. Phys. (France) 51 (1991) 545. [17] D.K. Jackson, S.B. Leeb, A. Mitwalli, D. Fusco, C. Wang, T. Tanaka, J. Intell. Mater. Syst. Struct. 8 (1997) 184. [18] Y. Soejima, K.F. Fischer, Z. Kristallogr. 184 (1988) 71. [19] Y. Soejima, Z. Kristallogr. 184 (1988) 87. [20] R.D. Shannon, Acta Crystallogr., Sect. A 32 (1976) 751.
Nanostructural characterization of the dehydrated (NIPA/SA + additive ion) gels K. Haraa,∗ , M. Sugiyamab , M. Annakac , Y. Soejimad a
Institute of Environmental Systems, Faculty of Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan b Department of Physics, Fuculty of Sciences, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan c Department of Chemistry, Fuculty of Sciences, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan d Research and Development Center for Higher Education, Kyushu University, 4-2-1 Ropponmatsu,Chuo-ku, Fukuoka 810-8560, Japan Received 20 February 2004; accepted 9 April 2004
Abstract In the present study, the authors have investigated ion-absorption effects on a nanoscopic structure of a dehydrated Nisopropylacrylamide/sodium acrylate (NIPA/SA) gel. First of all, the authors compared small-angle X-ray scattering (SAXS) profile of a dehydrated NIPA/SA gel with that dehydrated after absorbing Cu2+ ion. Then, in order to examine copper-ion distribution structure in the NIPA/SA gel dehydrated after absorption of copper-ion, an incident-X-ray energy-dependence of a small-angle X-ray scattering profile was observed, in which the anomalous dispersion effect was clearly perceived especially around a distinct SAXS-peak. Because the SAXS-peak is thought to come from a dehydration-induced microphase separation between hydrophilic and hydrophobic network-polymers in the NIPA/SA gel, such a feature indicates that the copper-ions gather in the dehydration-induced hydrophilic domains. In addition to this interesting copper-ion nanostructure in the dehydrated NIPA/SA gel, a difference in the SAXS-peak position between the dehydrated NIPA/SA gels with and without absorbing the copper-ion has shown a possibility of a controlling method of the nanostructure in relatively gentle conditions without special instruments. Along this line, in order to get further information on the ion-absorption effects on the nanostructure, the authors have compared the SAXS profiles of the several NIPA/SA gels which were dehydrated after absorbing respectively different kinds of ions. In the observation, the SAXS-peak positions have shown characteristic features which are different with the kind of the absorbed ions and found to be classified into several kinds according to the periodic-table group of the absorbed ion. © 2004 Elsevier B.V. All rights reserved. Keywords: Metal-ion-capture; Nanoscopic; N-Isopropylacrylamide; NIPA/SA; Gel
1. Introduction In both of the industrial technology and fundamental science, nanoscale structures have attracted much attention. In these days, several methods are taken to materialize the nanostructure with the structural-scales needed. Among them, the self-organization technique is usually utilized for realizing the structure in a range from few-nm’s to ∼50 nm, which can be the most realistic method for the mass∗
Corresponding author. Tel.: +81 92 642 3815; fax: +81 92 633 6958. E-mail address: [email protected] (K. Hara).
0927-7765/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2004.04.012
production without requiring precision instruments. Among the self-organizing techniques, there are still many concrete creating methods; the authors have been much interested in one of these methods which make use of constituent compatibilities. It is well-known that, in a polymer-solvent system, constituents’ compatibilities affect and change its internal structure such as a domain occurrence by a phase separation, of which the feature is governed by several thermodynamic quantities [1]. In addition to these parameters, the electrostatic force will also have an influence on the system balance. One of the well-known examples is the microphase separation which is usually realized in weakly charged polymer
198
K. Hara et al. / Colloids and Surfaces B: Biointerfaces 38 (2004) 197–200
solution systems. In relation to this phenomenon, the authors have been interested in the nanostructure of the heteropolymer hydrogels of which the network is composed of partly hydrophilic (charged) and partly hydrophobic polymers. States and properties of the gels are considerably influenced by interaction changes between solvent and polymernetwork with environmental conditions. For example, the property change in N-isopropylacrylamide (NIPA) gel with increasing temperature is well-known and interesting: there is a small and continuous change in volume in the pure NIPA gel at 36 ◦ C with the hydrophilic-to-hydrophobic change of its isopropyl-group, while, in the case that the networkpolymer has additional ionized groups inside such as the N-isopropylacrylamide/acrylic acid (NIPA/AAc) copolymer gel, the volume change becomes more drastic with the effect of the Donan potential [2]. With existence of an expanding force originated from an electrostatic interaction against the hydrophobic shrinking one, the NIPA/AAc gel exhibits a discrete volume change at a higher temperature than the pure NIPA gel [3,4]. Such a drastic volume change is referred as a volume phase transition [2,5]. In addition to this macroscopic feature, there also occurs a nanostructural change by introducing the ionized group in the gel: an emergence of a prominent peak in a small-angle neutron scattering (SANS) profile indicating realization of the microphase transition induced by interactions between the hydrophilic carboxyl and hydrophobic isopropyl-groups [6,7]. Such a competition of the internal forces will occur in a condition other than the volume phase transition. As such a phenomenon, the authors have been investigating property change of the hydrogels by dehydration, which is still interesting because the dehydration is one of the most general phenomena by which the gels show a drastic volume change. Besides, by dehydration, there occur property changes indicating that some gels becomes glass-like substances [8], which have been confirmed by viscoelastic measurements [9,10], Raman scattering [11] and inelastic neutron scattering experiments [12]. Moreover, by a small-angle X-ray scattering (SAXS) study, a distinct nanostructure has been also found in a dehydrated N-isopropylacrylamide/sodium acrylate (NIPA/SA) gel, which is a congeneric hydrogel of the NIPA/AAc gel, indicating occurrence of the microphase separation by the dehydration [13]. In a series of the nanostructural observations of the dehydrated NIPA/SA gel, the authors carried out detailed SAXS profile change examinations with the sodium-acrylic concentration and those on SANS profile change with heavy-water-content, in which some characteristic properties has been revealed on the nanoscale domain structure caused by the dehydration-induced microphaseseparation: the distinct SAXS and SANS peaks can be only observed in a restricted parameter region [14]. This feature indicates that the characteristic structure in the dehydrated NIPA/SA gel can be realized on a delicate balance of related interactions [15,16], conversely, the nanostructural control can be sufficiently conducted with manipulating macroscopic conditions without much energy and complicated equipments
because essentially the dehydration process requires no special processes and instruments such as those usually utilized in the lithography techniques. Next, let us go ahead on other properties of the heteropolymer gel which contains both of the ionized and hydrophobic side chains. It is well-known that some ionized polymers can capture multivalent ions in cooperation with several ionized side chains, namely by the chelation mechanism. Such a phenomenon can be harnessed for detecting and/or capturing toxic multivalent ions in waste solutions. Actually, Jackson et al. [17] devised an ionized-gel sensor sensitive to multivalent metal ions. The features suggest that the ion-capture property of the ionized-gel should be also very useful in the environmental purification technology. However, to the authors’ knowledge, there have been almost no investigations on the ion-capturing feature of the ionized-gel from a nanostructural point of view; the authors have been convinced that such investigations should be very important because such systems are stabilized with a balance of the interactions in a nanoscopic scale as mentioned above. Under these circumstances, the authors have been investigating the nanostructure of the dehydrated NIPA/SA gel. In the present study, the authors have investigated ionabsorption effects on a nanoscopic structure of a dehydrated N-isopropylacrylamide/sodium acrylate (NIPA/SA) gel. First of all, the authors compared SAXS profile of a dehydrated NIPA/SA gel with that dehydrated after absorbing Cu2+ ion. Then, in order to examine copper-ion distribution structure in the copper-ion-captured-and-dehydrated NIPA/SA gel, an incident-X-ray energy-dependence of small-angle X-ray scattering (SAXS) profile was observed. Next, in order to get further information on the ion-absorption effects on the nanostructure, the authors have compared the SAXS profiles of the several NIPA/SA gels which were dehydrated after absorbing respectively different kinds of ions.
2. Experimentals 2.1. Comparison of SAXS profiles of the dehydrated NIPA/SA gel with and without additive Cu2+ The NIPA/SA gels were synthesized by a free radical polymerization of the mixture {NIPA, 500 mM (5.66 g), sodium acrylate, 200 mM (1.88 g), N,N -methylenebisacrylamide, 8.6 mM (0.132 g), N,N,N ,N -tetramethylethylenediamine (240 L) in pure water (100 g)}, which was fully saturated with nitrogen, initiated by adding a small amount of ammonium persulfate (40 mg). The gelation was carried out 24 h at 0 ◦ C. After then, the jelled samples were washed in pure water for a week. The purified gel was cut into three portions; two of them were put into 0.2 and 1.0 mM’s aqueous solutions of CuCl2 for six days. Then all of the gels were gently dried in the atmosphere for six days. Then the SAXS profiles of these three kinds of dehydrated NIPA/SA gels were observed.
K. Hara et al. / Colloids and Surfaces B: Biointerfaces 38 (2004) 197–200
Observation of the SAXS profiles were carried out at room temperature with a SAXS apparatus (SAXES) installed at BL10C of Photon Factory in Institute of Materials Structure Science (IMSS), High Energy Accelerator Research Organization (KEK), Tsukuba, Japan. An X-ray beam from a syn˚ in wave-length) was used chrotron orbital radiation (1.3918 A as a light source of SAXES and the intensity distribution of the scattered X-ray was sensed with a scattering vector ranged ˚ −1 . The observed X-ray infrom 6.0 × 10−3 to 2.0 × 10−1 A tensity was corrected for the cell scattering and absorption, and then normalized with the thickness of the sample and irradiated time. By these correction and normalization, the value of final data straightforwardly corresponds to the scattering efficiency. 2.2. Observation of a copper ion distribution structure utilizing X-ray anomalous dispersion effect The preparation of the specimen was conducted in the same manner mentioned above. Observation of the SAXS profiles were also carried out at room temperature with the above-mentioned SAXS apparatus. Incident-X-ray energy dependence of the SAXS profile was observed with tuning an incident X-ray energy in a range from 8.91 to 9.04 keV so as to across the cabsorption edge (8.98 keV) with a resolution of E/E = 10−4 utilizing a channel-cut silicon-monochromator. The data correction procedure was also performed in the same manner mentioned above. 2.3. SAXS-peak variations with the several kinds of additive ions
199
Fig. 1. The SAXS profiles of a dehydrated NIPA/SA (500 mM/200 mM) gel and of those dehydrated after absorbing Cu2+ ion. Numerals in a square at the upper-right corner are Cu2+ concentrations of aqueous solutions in which NIPA/SA gel lumps were immersed before dehydration.
peak shift ceases at the concentration above 0.2 mM, which can be interpreted that because the carboxyl-group number in the gel is finite, superfluous Cu2+ ions can not interact with the stuffed ionized side chain polymers and, consequently, do not affect the nanoscopic structure. Other characteristic change is a broadening of the SAXS-peak indicating an increase in a heterogeneity of the domain structure. This feature can reflect a heterogeneous distribution of the region where the chelation occurs, however because there exists still a clear SAXS-peak, the heterogeneity may be small. Fig. 2 shows a distribution of the incident-X-ray energy dependence of the SAXS-intensity difference from that far
The preparation procedure of the wet NIPA/SA gel was almost same with that mentioned in Section 2.1 except for the concentration of NIPA (400 mM) and SA (300 mM). After preparing the purified wet gel, several lumps were cut out, after that, each gel block was immersed in a 0.2 mM aqueous solution of LiCl, NaCl, KCl, RbCl, CsCl, MgCl2 , CaCl2 , SrCl2 , BaCl2 , AlCl3 , FeCl3 , CoCl3 , NiCl2 and CuSO4 for 12 h. Then, the gel blocks were dried gently for six days in the atmosphere. Observation of the SAXS profiles of these dehydrated specimens were carried out in the same manner mentioned in Section 2.1.
3. Results and discussions Fig. 1 shows SAXS profiles of both the dehydrated NIPA/SA gels with and without additive copper ion. With absorbing copper ion, the SAXS-peak shifts toward the lower-q region demonstrating a correlation length of the hydrophilic domains becomes small. This feature can come from a condition that each copper ion (Cu2+ ) in the hydrophilic domains attracts two ionized side chains (negatively charged carboxyl groups) dangling from separated main chains by making a chelate-connection. It has been also found that the SAXS-
Fig. 2. The incident-X-ray energy dependence of the SAXS-intensity difference from that far (at 9.04 keV) from the Cu K␣ absorption edge. A thick line extruding from the base shows the energy of Cu K␣ absorption edge.
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K. Hara et al. / Colloids and Surfaces B: Biointerfaces 38 (2004) 197–200
• Aluminum ion CL of Al3+ locates near {Co3+ , Fe3+ }, indicating that the ionic charge number has important role to determine the correlation length.
Fig. 3. Ion-radius (reported by Shanon [20]) dependence of the correlation length derived from SAXS profiles of NIPA/SA (400 mM/300 mM) gels dehydrated after absorbing ions indicated in the figure.
(at 9.04 keV) from the Cu K␣ absorption edge. As can be easily seen, at the energy of the Cu K␣ absorption edge, the intensity difference takes a minimum around the SAXS-peak position. Because the anomalous dispersion effect only occurs by the copper element [18,19], this characteristic feature directly demonstrates a close relation between the Cu2+ distribution and hydrophilic domain structure. This result indicates a localized distribution of the copper ions in the hydrophilic domain. In the SAXS experiments mentioned in Section 2.3, a clear and intense SAXS-peak was also observed in all of the specimens with additive ions, regardless of their ionic charge numbers, which demonstrates that the fundamental nanoscale structure in the dehydrated NIPA/SA gel is not destroyed by the ion addition. Besides, it was also found that the position of the SAXS-peak shows systematic change with the kinds of the additive ions. Fig. 3 shows ion-radius (reported by Shannon [20] in the six-coordination structure) dependences of a correlation length (CL) derived from the SAXS-peak positions. In the figure, five sets of systematic variation of CL are perceived of which the behaviors seem mainly differ with the periodic table groups of the additive ions. Therefore, let us classify their behaviors from that point of view: • Alkali metal-ion group A large variation in CL with a maximum between the sodium and potassium ions. • Alkaline-earth metal-ion group A large variation in CL. Similar ion-radius dependence with the alkali metal-ion group. • Transition metal-ion groups {Cu2+ , Ni2+ }, {Co3+ , Fe3+ } CL’s with the same ionic charge number does not differ so much, while those with different ionic charge numbers ({Cu2+ , Ni2+ } and {Co3+ , Fe3+ }) locate with a distance in the figure.
Because the periodic table groups are defined by the valence of elements, the most effective condition of the SAXSpeak shift seems the ionic charge number, then, the next important condition can be the ion-radius. For the more detailed discussion, further investigations with more quantitative information may be needed such as an absolute amount of the absorbed ion and so on, which are now in preparation. From these SAXS observations, drastic systematic changes in the SAXS-peak have been revealed, which should be not only interesting from a fundamental science viewpoint but also valuable as a nanostructural controlling technique as mentioned above.
Acknowledgments The authors would like to express their thanks to Dr. H. Inoko of Osaka University for his kind help on the SAXS experiments. The SAXS experiments were performed under the approval of the Photon Factory Advisory Committee (proposal No. 2002G094).
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