- Email: [email protected]
Hybridization of sugar alcohols into brucite interlayers via a melt intercalation process
Journal of Colloid and Interface Science 368 (2012) 578–583
Contents lists available at SciVerse ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Hybridization of sugar alcohols into brucite interlayers via a melt intercalation process Kazuya Morimoto a, Kenji Tamura b,⇑, Tamao Hatta c, Seiko Nemoto c, Takuya Echigo b, Jinhua Ye b, Hirohisa Yamada b a b c
Graduate School of Science and Engineering, Ehime University, 2-5, Bunkyo-cho, Matsuyama 790-8577, Japan National Institute for Materials Science (NIMS), Environmental Remediation Materials Unit, 1-1 Namiki, Tsukuba 305-0044, Japan Japan International Research Center for Agricultural Sciences (JIRCAS), 1-1, Ohwashi, Tsukuba 305-8686, Japan
a r t i c l e
i n f o
Article history: Received 10 August 2011 Accepted 19 November 2011 Available online 1 December 2011 Keywords: Brucite Xylitol D-sorbitol Intercalation
a b s t r a c t We report the preparation of organic-brucite (BR) hybrids using harmless sugar alcohols (xylitol, XYL, and sorbitol, SOR). Since XYL and SOR are solid materials at room temperature, the hybridization was investigated by comparing two separate methods, hydrothermal treatment and melt mixing. BR-sugar alcohol hybrids were successfully prepared by a melt intercalation method at 175 °C. X-ray diffraction and Fourier transform infrared spectroscopy analyses indicated that organic molecules were intercalated into the brucite layers, overcoming the barrier of hydroxyl bonds between the BR layers. Moreover, X-ray photoelectron spectroscopy and thermal analyses showed that the intercalated materials at 175 °C resulted in the formation of covalent Mg–O–C bond linkages on the interlayer surface of BR. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Brucite, BR [magnesium hydroxide, Mg(OH)2] is a hydrous layer mineral, which has been used as a non-toxic industrial material in applications such as pharmaceutical substances, cosmetic products [1], catalysts [2], flame retardants [3], and inorganic absorbents [4]. Despite its many practical applications, there have been remarkably few attempts to construct hybrids of brucite using host–guest complexation. When brucite is organically modified as a nano-hybrid material, its applicability becomes broader. For example, in terms of ‘‘harmless’’ materials, the possibility of using an intercalation compound based on brucite (an aluminum-free biomedical material) as a molecular container for drug delivery systems (DDS) is very attractive. However, it is difficult to expand each layer by host–guest intercalation reactions because the octahedral layers of brucite carry no electrical charge, instead being held together tightly through hydrogen bonding. Nevertheless, a few reports based on the glycothermal reaction of layered hydroxides can be found. One approach for preparing organic-brucite hybrids is through the formation of covalent linkages on the hydroxyl groups of a layer surface. The incorporation of glycols such as ethylene glycol and glycerol at higher temperatures has been reported [5]. In another report, the formation of an ethylene glycol–gibbsite [Al(OH)3] intercalation compound was studied [6]. The reaction of gibbsite (one of the polymorphs of aluminum ⇑ Corresponding author. Fax: +81 29 860 4667. E-mail address: [email protected] (K. Tamura). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.11.048
hydroxide) in ethylene glycol at high temperatures yielded a glycol derivative of boehmite. In each of these cases, only small molecular polyols such as ethylene glycol and glycerol have been used to form the intercalation compounds. XYL and SOR, which belong to the sugar family, are not hazardous materials according to the Regulation of Hazardous Substances and are found in numerous food products. If these sugar alcohols could be fixed into brucite interlayers, the resultant hybrids would have applications as harmless materials in controlling the stability of an intercalant and in sustained release applications (properties required for DDS). The purposes of this study are to find a preparative methodology for sugar alcohol-brucite hybrids and to characterize them in detail. In this new effort, XYL and SOR were used as the nontoxic intercalants. This paper reports the intercalation and interlamellar grafting of these polyols. The intercalation was performed directly in the melt of the sugar alcohol at temperatures above its melting point.
2. Experimental 2.1. Materials preparation BR reagent (Rare Metallic Co., Ltd.) was used as a starting material. XYL (melting point: 92–96 °C) and SOR (melting point: 97– 100 °C) were used as sources of sugar alcohols and were purchased from Wako Pure Chemicals (Japan). Each sugar alcohol powder (2 g) was melted at 175 °C in a glass vial, and then mixed well with
K. Morimoto et al. / Journal of Colloid and Interface Science 368 (2012) 578–583
579
Fig. 1. Powder XRD patterns of brucite with sugar alcohols prepared by (A) the melt intercalation method and (B) hydrothermal treatment. Profiles of: (a) starting brucite, (b) xylitol supplemented brucite, and (c) D-sorbitol-supplemented brucite.
brucite powder (0.1 g). The mixtures were held at 175 °C in a thermostatic oven for 24 h under atmospheric conditions. The products obtained were washed with methanol and centrifuged. No removal of sugar alcohols was observed through this washing treatment. Thereafter, the solid products were dried using a vacuum freezedrier. The samples are denoted XYL-BR and SOR-BR. For comparison, samples treated with an aqueous solvent were prepared. For that purpose, 4 g of a sugar alcohol was dissolved in 4 mL distilled water. The solution was mixed with 0.1 g of BR. The suspension was hydrothermally aged using a TeflonÒ-lined stainless steel crucible at 175 °C for 24 h in a thermostatic oven. After treatment, the solid product obtained was isolated by centrifugation and washed with methanol.
2.2. Characterization The samples were subjected to X-ray diffraction (XRD; Ultima IV, Rigaku) measurements using Cu Ka radiation (k = 0.15406 nm) under conditions of 40 kV, 30 mA, and 2°/min scanning. The morphology of the sugar alcohol-brucite samples was observed using an SEM (JSM-6700FT, Jeol) at 7 kV. Fourier transform infrared spectroscopy (FTIR) spectra were obtained with an ATR–FTIR spectrometer (IR Affinity, Shimadzu) on a diamond crystal from 400 to 4000 cm1. Thermogravimetric-differential thermal analysis (TG-DTA) was performed using a Thermoplus TG8120 thermogravimetric analyzer
Fig. 3. The ATR–FTIR spectra of brucite with sugar alcohols prepared by (A) the melt intercalation method and (B) hydrothermal treatment: (a) starting brucite, (b) brucite with xylitol, and (c) brucite with D-sorbitol.
from Rigaku under a nitrogen flow at a uniform heating rate of 10 °C/min up to 600 °C. X-ray photoelectron spectroscopy (XPS) was conducted to further investigate the electronic state of the starting material and the intercalated samples prepared by the melt intercalation method. The XPS spectra were collected using a VG-Scienta ESCA-300 (Sweden) with a monochromatized Al Ka X-ray source (hv = 1486.6 eV) at a power of 1.0 kW and a base pressure of 7.3 108 Pa in the analytical chamber. The analysis chamber was equipped with a sputter gun, in which Ar gas was used to sputter clean the sample [Mg(OH)2] surface. Electrostatic charging due to the poor electrical conductivity of sugar alcohol samples was minimized by using a flood gun. Using a takeoff angle of 90°, survey scans were performed to identify the C, O, and Mg elements. Narrow region XPS spectra of the C1s and O1s were acquired with an analyzer pass energy of 75 eV, a step energy of 0.02 eV, a time per step of 0.5 s, a slit of 0.3 mm, with three sweeps, and a flood gun of 4 eV. Electron binding energies were cal-
Fig. 2. SEM images of (a) BR, (b) XYL-BR, and (c) SOR-BR powders.
580
K. Morimoto et al. / Journal of Colloid and Interface Science 368 (2012) 578–583
Fig. 4. (A) TG curves, (B) DTA profiles, and (C) DTG profiles under a nitrogen flow at a uniform heating rate of 10 °C/min for (a) starting brucite, (b) brucite with xylitol, and (c) brucite with D-sorbitol derived by the melt intercalation.
ibrated with respect to the C1s line at 284.5 eV (C–C), and a nonlinear least-squares curve-fitting program was employed with a Gaussian– Lorentzian (Voigt) function. Energy differences between theoretical and measured C1s lines were generally less than ±4 eV. 3. Results and discussion 3.1. Structural characteristics of XYL-BR and SOR-BR Fig. 1A(a) shows the XRD pattern of the brucite starting material. The profile shows a d value of 0.48 nm corresponding to the (0 0 1) reflection [7]. After the melt intercalation reaction with XYL, the peak corresponding to the brucite phase (0.48 nm) nearly disappeared with the concomitant appearance of a new peak at 0.87 nm [Fig. 1A(b)]. Faint peaks at 2h values of 18.4° arose from the unexpanded phases of brucite. No peak due to the MgO phase was generated. These results indicate the occurrence of interlayer expansions of brucite layers resulting from the incorporation of XYL molecules. In the case of SOR, the relatively broad peak observed at 1.09 nm is a newly constructed phase with the disappearance of the brucite peak (0.48 nm) [Fig. 1A(c)]. By subtracting the thickness of the brucite layers (0.48 nm) from the observed d values [7], the interlayer spacing (Dd) values of the
XYL-BR and SOR-BR hybrids were calculated to be 0.39 and 0.61 nm, respectively. In comparison, the observed Dd values for an ethylene glycol-brucite hybrid and a glycerol-brucite hybrid are 0.35 and 0.81 nm, respectively [6]. For smectite clays, which are easily expandable, the interlayer expansion is reported to be 0.43 nm for both ethylene glycol and glycerol. Thus, the present results support the formation of monolayer organic complexes for XYL [8,9]. The molecules may possess a flattened monolayer arrangement between the BR layers. In contrast to the XYL-BR hybrid, the Dd value for the SOR-BR hybrid (0.61 nm) was much larger, but was still too small for a bilayer arrangement (Dd 0.8 nm). These results suggest that SOR molecules formed a monolayer with the molecular long axes inclined toward the BR layer or a pseudo-bilayer arrangement. Fig. 1B shows the XRD pattern of the sample treated with an aqueous solvent. No new peak other than the d001 peak of the initial brucite appeared. Thus, no intercalation of sugar alcohol molecules occurred under these conditions. Fig. 2 shows SEM images of the starting BR and the XYL-BR and SOR-BR hybrid powders. The starting BR is composed of planar particles that vary in size from about 100 nm to larger than 500 nm [Fig. 2(a)], while the BR treated with XYL is composed of aggregates of several lm, which consist of BR platelets held together by XYL molecules [Fig. 2(b)]. In Fig. 2(c), one can see that SOR-BR particles
K. Morimoto et al. / Journal of Colloid and Interface Science 368 (2012) 578–583
581
Fig. 5. (A) TG curves, (B) DTG profiles, and (C) DTA profiles under a nitrogen flow at a uniform heating rate of 10 °C/min for (a) XYL reagent and (b) SOR reagent.
form aggregates of different sizes varying from a fraction of a micrometer up to ten micrometers in both length and width. The surfaces of BR layers are coated completely by SOR molecules. 3.2. Hybridization mechanisms of sugar alcohols into BR interlayers Fig. 3 shows the FTIR absorption spectra of starting BR, BR treated with XYL, and BR treated with SOR. Starting BR exhibits a sharp absorption peak at 3694 cm1, which is due to the stretching frequency of internal hydroxyl groups [Fig. 3A(a)] [10]. The absorption band observed at 409 cm1 is associated with the Mg–O lattice vibration. The spectra of the XYL- and SOR-BR hybrids differed from the starting BR, but were similar to each other [Figs. 3A(b) and A(c)]. The stretching vibration of internal hydroxyl groups at 3694 cm1 completely disappeared in both samples. The doublet at ca. 2800–2900 cm1 in the spectra corresponds to the asymmetric and symmetric stretching vibrations of C–H bonds in the sugar alcohols. The O–H stretching vibration of the combination of sugar alcohols and expanded brucite appeared in the range 3000–3600 cm1 [11]. The absorption band of the Mg–O lattice vibration also was detected at approximately 410 cm1. Furthermore, characteristic peaks associated with metallic alkoxides were identified near 830 and 1070 cm1 in the spectra of the XYL-BR and SOR-BR hybrids [12,13]. These results suggest the partial formation
of Mg–O–C linkages through dehydroxylation [6]. These results along with the XRD analyses confirmed that both XYL and SOR molecules infiltrated into the brucite interlayers and broke the hydroxyl hydrogen bonds at temperatures above their melting points (under nonaqueous conditions). For the hydrothermally treated samples, no significant change was found in the absorption spectra, with the internal O–H band remaining at 3694 cm1 (Fig. 3B). Fig. 4 shows the TG, DTG, and DTA profiles of starting BR and the XYL-BR and SOR-BR hybrids. The weight decrease process at around 380 °C for BR was endothermic and is attributed to the dehydration of structural water [Fig. 4C(a)]. The total weight loss of BR, subjected to temperatures of up to 600 °C, was estimated to be 32% [Fig. 4A(a)], which lead to the transition from Mg(OH)2 to MgO. On the other hand, the hybrids consisting of BR and sugar alcohol decomposed through several successive processes at around 70 °C, 280 °C, 400– 450 °C [Fig. 4B(b) and B(c)]. The first process was associated with an endothermic response in DTA and is attributed to the desorption of physisorbed water molecules [Fig. 4C(b) and C(c)]. The next two processes were highly exothermic, suggesting thermal degradation of the sugar alcohol moiety took place at these temperatures, while BR and sugar alcohol reagents showed only endothermic peaks [Figs. 4C(a) and 5C]. Total weight losses of the XYL-BR and SOR-BR hybrids, up to 600 °C, were estimated to be 58% and 66%, respectively [Fig. 4A(b) and A(c)]. The weight loss peak maximum in DTG profiles
582
K. Morimoto et al. / Journal of Colloid and Interface Science 368 (2012) 578–583
Fig. 6. XPS narrow spectra of (a) starting BR, (b) XYL-BR, and (c) SOR-BR: (A) XPS spectrum of C1s, (B) XPS spectrum of O1s, and (C) XPS spectrum of Mg2s.
of XYL-BR and SOL-BR hybrids occurred near 389 and 411 °C, respectively (see Fig. 4B). Importantly, these temperatures were higher than those of the starting brucite and the XYL (322 °C) and SOR (349 °C) reagents (Fig. 5B). Fig. 6 shows the XPS C1s, O1s, and Mg2s narrow spectra of starting BR and the XYL-BR and SOR-BR hybrids. In the scans of XYL and SOR, the binding energy of C1s photoelectrons from the C–C species was strongly observed near 284.5 eV (not shown). The C1s region scans of XYL-BR and SOR-BR were resolved into three component peaks. In addition to the peak from the hydrocarbon environment at 284.5 eV, the peaks at 285.2 eV for XYL-BR and at 285.6 eV for SOR-BR were assigned to a carbon singly bound to oxygen formed by the dehydroxylation reaction, and the shoulders on the higher binding energy side at 287.6 eV for XYL-BR and at 288.2 eV for SOR-BR were assigned to a carbon doubly bonded to oxygen [shown in Fig. 6A(b) and A(c)] [14–19]. The O1s peaks in the scan of BR (529.2 and 531.0 eV, which are labeled as 1 and 2, respectively) were attributed to the presence of oxide O–Mg and hydroxide HO–Mg, respectively [Fig. 6B(a)] [17–19]. In comparison, the main peaks from the O1s scans of the intercalated XYL-BR and SOR-BR were due to the carbonyl and oxide species from sugar alcohols and brucite, respectively [Figs. 6B(b) and B(c)]. Upon intercalation of the sugar alcohols, significant changes were observed for their relative full width at half maximum (FWHM). Especially remarkable was the increase of the FWHM of the O1s peaks for the hybrid sample (e.g., XYL-BR, 2.5 eV instead of 1.7 eV for the XYL; SOR-BR, 2.25 eV instead of 1.6 eV for the SOR). Fig. 6C shows narrow spectra of the Mg2s. The sugar alcohol treated samples show higher Mg2s binding energies (89 eV for XYL-BR and 88.4 eV for SOL-BR) than the starting BR (87.4 eV). The slightly increased binding energy of the magnesium core lines could be due to reduced shielding on the Mg core. Both behaviors, the broadening of the O1s peak and the Mg2s peak shift observed in the hybrids, suggest a possible formation of an Mg–O–C linkage to the MgOH surface. Table 1 shows the atomic concentration results from the XPS scans. The XYL-BR and SOR-BR samples clearly have increased
Table 1 The atomic concentrations of the surface of samples using XPS (atomic%). Sample
O1s
C1s
Mg2s
XYL-BR SOR-BR BR XYL SOR
44.3 50.0 56.8 47.0 48.6
34.1 22.7
21.6 27.3 43.2
53.0 51.4
Table 2 Changes in oxygen ratios of each sample as a function of degree of grafting. The values for XYL-BR, SOR-BR, and BR were normalized according to Mg content, and then the values for XYL and SOR were normalized according to the C content of the XYL-BR and SOR-BR samples, respectively. Sample
C
O
Mg
XYL-BR SOR-BR BR XYL SOR
1.6 0.8
2.1 1.8 1.3 1.5 0.8
1.0 1.0 1.0
1.6 0.8
carbon and reduced oxygen content at the surface compared to BR. To clarify changes in the oxygen ratios for each sample as a function of degree of grafting, the values for XYL-BR, SOR-BR, and BR were normalized according to magnesium content, and then the values for XYL and SOR were normalized according to the carbon content of the XYL-BR and SOR-BR samples, respectively (Table 2). The SOR-BR indicates half the carbon ratio than the XYL-BR. This may be because the physically adsorbed SOR was removed by argon etching before the XPS measurement. This means that the XYL molecules are efficiently incorporated into the BR interlayers. Furthermore, it is very interesting to compare the O atomic ratios of the hybrid materials (XYL-BR and SOR-BR) and the values of the O atomic ratio of BR plus that of the sugar alcohols. The oxygen levels for the hybrids were low compared to the mixture of BR and sugar alcohols: 2.1 for XYL-BR hybrid versus 2.8 for mixture of BR and XYL, and 1.8 for SOR-BR versus 2.1 for mixture of BR and SOR. These results also strongly suggest that the BR and sugar alcohols had not simply mixed, but that a covalent linkage between the sugar alcohols and brucite surface had been formed through Mg–O–C bonds. This is consistent with the exothermic DTA profiles of the hybrids [curves (b) and (c) in Fig. 4C], suggesting BR incorporated the sugar alcohols through dehydration. During the melt-mixed process, the formation of the brucite-sugar alcohol derivatives (covalent linkage) must involve the displacement of hydroxyl groups in the coordination sites of magnesium into the alkoxyl group. Although this is the reverse reaction of alkoxide hydrolysis, the alkoxides and hydroxides are in equilibrium and it is known that some alkoxides can be prepared by continuous removal of water from the reaction system [20,21]:
MðOHÞn þ nROH MðORÞn þ nH2 O
ð1Þ
where M represents alkali or alkali earth metal, and R represents an alkyl group CnH2n+1. However, when the hydrothermal process is
K. Morimoto et al. / Journal of Colloid and Interface Science 368 (2012) 578–583
583
utilized, the addition of water shifts the equilibrium composition to the left-hand side of the reaction (1). Therefore, by hydrothermal treatment, sugar alcohol molecules are not incorporated into the brucite interlayer region.
diverse drugs as well as to applications in topical therapies, such as food, drugs, cosmetics, and medical applications.
4. Conclusions
This work was supported by a Grant-in-Aid for Scientific Research category C (23550235) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government.
In conclusion, the successful incorporation of sugar alcohols (XYL and SOR) into BR was achieved via a melt intercalation procedure within 24 h at 175 °C. The XRD and FTIR analyses confirmed that the melted sugar alcohol infiltrated into the interlayers with a breaking of the hydroxyl bonds in brucite and also that the hydrothermal treatment of brucite with sugar alcohols had no effect. It was observed that, during this study, hydroxide and alcohol were in equilibrium with alkoxide and water, similar to a previous work [6]. However, in this study, the equilibrium was observed using, for the first time, solid sugar alcohols. Because of this equilibrium, alkoxide is generated from the hydroxide under glycothermal conditions. Glycothermal reaction between the BR and sugar alcohols is a convenient route for the synthesis of organic–inorganic hybrids, avoiding the effect of water. This is significant because there are many organic chemicals which have poor water solubility that are used in biofunctional substances, such as drugs, enzymes, cosmetic chemicals. While many wet colloidal processes use inorganic particles, this melt process could be used to produce new types of hybrid that may lead to innovative technology in many fields. To give a specific example, the combination of sugar alcohols and/or other biomolecules produces new matrices for DDS that can be used for direct encapsulation of drugs and to develop bionanocomposite materials with drug-intercalated brucite systems. In comparison with other DDS, the system proposed here has the advantages of easy processing and the use of abundant, inexpensive, and safe biomolecules. The possibility of using brucite may allow the extension of this approach to the immobilization of
Acknowledgments
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
M.I. Carretero, M. Pozo, Appl. Clay Sci. 47 (2010) 171. Y. Dong, K. He, B. Zhao, Y. Yin, L. Yin, A. Zhang, Catal. Commun. 8 (2007) 1599. G.J. Simandl, S. Paradis, M. Irvine, Geosci. Can. 34 (2007) 57. E.M. Opiso, T. Sato, K. Morimoto, A. Asai, S. Anraku, C. Numako, T. Yoneda, Miner. Eng. 23 (2010) 230. F. Wypych, W.H. Schreiner, R. Marangoni, J. Colloids Interface Sci. 253 (2002) 180. M. Inoue, H. Kominami, Y. Kondo, T. Inui, Chem. Mater. 9 (1997) 1614. M. Catti, G. Ferraris, S. Hull, A. Pavese, Phys. Chem. Miner. 22 (1995) 200. R.W. Hoffmann, G.W. Brindley, Am. Mineral. 46 (1961) 450. T. Sato, T. Watanabe, R. Otsuka, Clay. Clay. Miner. 40 (1992) 103. R.L. Frost, J.T. Kloprogge, Spectrochim. Acta Part A 55 (1999) 2195. E. Libowitzky, Monatshefte für Chemie 130 (1999) 1047. P.F. Fuls, L. Rodrique, J.J. Fripiat, Clays Clay Miner. 18 (1970) 53. H.S. Jung, J.-K. Lee, J.-Y. Kim, K.S. Hong, J. Colloids Interface Sci. 259 (2003) 127. Z.-Z. Jiang, Z.-B. Wang, D.-M. Gu, E.S. Smotkin, Chem. Commun. 46 (2010) 6998. S. Fleutot, J.C. Dupin, I. Baraille, C. Forano, G. Renaudin, F. Leroux, D. Gonbeau, H. Martinez, Phys. Chem. Chem. Phys. 11 (2009) 3554. J.L. Figueiredo, M.F.R. Pereira, M.M.A. Freitas, J.J.M. Órfão, Carbon 37 (1999) 1379. J.I. Di Cosimo, V.K. Díez, M. Xu, E. Iglesia, C.R. Apesteguía, J. Catal. 178 (1998) 499. J.D. Farr, M.P. Neua, R.K. Schulze, B.D. Honeyman, J. Alloy. Comp. 444–445 (2007) 533. D.K. Aswal, K.P. Muthe, S. Tawde, S. Chodhury, N. Bagkar, A. Singh, S.K. Gupta, J.V. Yakhmi, J. Crystal Growth 236 (2002) 661. A.Y. Platonov, A.V. Kurzin, A.N. Evdokimov, J. Solution. Chem. 39 (2010) 335. D.C. Bradley, Prog. Inorg. Chem. 12 (1960) 303.
Contents lists available at SciVerse ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Hybridization of sugar alcohols into brucite interlayers via a melt intercalation process Kazuya Morimoto a, Kenji Tamura b,⇑, Tamao Hatta c, Seiko Nemoto c, Takuya Echigo b, Jinhua Ye b, Hirohisa Yamada b a b c
Graduate School of Science and Engineering, Ehime University, 2-5, Bunkyo-cho, Matsuyama 790-8577, Japan National Institute for Materials Science (NIMS), Environmental Remediation Materials Unit, 1-1 Namiki, Tsukuba 305-0044, Japan Japan International Research Center for Agricultural Sciences (JIRCAS), 1-1, Ohwashi, Tsukuba 305-8686, Japan
a r t i c l e
i n f o
Article history: Received 10 August 2011 Accepted 19 November 2011 Available online 1 December 2011 Keywords: Brucite Xylitol D-sorbitol Intercalation
a b s t r a c t We report the preparation of organic-brucite (BR) hybrids using harmless sugar alcohols (xylitol, XYL, and sorbitol, SOR). Since XYL and SOR are solid materials at room temperature, the hybridization was investigated by comparing two separate methods, hydrothermal treatment and melt mixing. BR-sugar alcohol hybrids were successfully prepared by a melt intercalation method at 175 °C. X-ray diffraction and Fourier transform infrared spectroscopy analyses indicated that organic molecules were intercalated into the brucite layers, overcoming the barrier of hydroxyl bonds between the BR layers. Moreover, X-ray photoelectron spectroscopy and thermal analyses showed that the intercalated materials at 175 °C resulted in the formation of covalent Mg–O–C bond linkages on the interlayer surface of BR. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Brucite, BR [magnesium hydroxide, Mg(OH)2] is a hydrous layer mineral, which has been used as a non-toxic industrial material in applications such as pharmaceutical substances, cosmetic products [1], catalysts [2], flame retardants [3], and inorganic absorbents [4]. Despite its many practical applications, there have been remarkably few attempts to construct hybrids of brucite using host–guest complexation. When brucite is organically modified as a nano-hybrid material, its applicability becomes broader. For example, in terms of ‘‘harmless’’ materials, the possibility of using an intercalation compound based on brucite (an aluminum-free biomedical material) as a molecular container for drug delivery systems (DDS) is very attractive. However, it is difficult to expand each layer by host–guest intercalation reactions because the octahedral layers of brucite carry no electrical charge, instead being held together tightly through hydrogen bonding. Nevertheless, a few reports based on the glycothermal reaction of layered hydroxides can be found. One approach for preparing organic-brucite hybrids is through the formation of covalent linkages on the hydroxyl groups of a layer surface. The incorporation of glycols such as ethylene glycol and glycerol at higher temperatures has been reported [5]. In another report, the formation of an ethylene glycol–gibbsite [Al(OH)3] intercalation compound was studied [6]. The reaction of gibbsite (one of the polymorphs of aluminum ⇑ Corresponding author. Fax: +81 29 860 4667. E-mail address: [email protected] (K. Tamura). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.11.048
hydroxide) in ethylene glycol at high temperatures yielded a glycol derivative of boehmite. In each of these cases, only small molecular polyols such as ethylene glycol and glycerol have been used to form the intercalation compounds. XYL and SOR, which belong to the sugar family, are not hazardous materials according to the Regulation of Hazardous Substances and are found in numerous food products. If these sugar alcohols could be fixed into brucite interlayers, the resultant hybrids would have applications as harmless materials in controlling the stability of an intercalant and in sustained release applications (properties required for DDS). The purposes of this study are to find a preparative methodology for sugar alcohol-brucite hybrids and to characterize them in detail. In this new effort, XYL and SOR were used as the nontoxic intercalants. This paper reports the intercalation and interlamellar grafting of these polyols. The intercalation was performed directly in the melt of the sugar alcohol at temperatures above its melting point.
2. Experimental 2.1. Materials preparation BR reagent (Rare Metallic Co., Ltd.) was used as a starting material. XYL (melting point: 92–96 °C) and SOR (melting point: 97– 100 °C) were used as sources of sugar alcohols and were purchased from Wako Pure Chemicals (Japan). Each sugar alcohol powder (2 g) was melted at 175 °C in a glass vial, and then mixed well with
K. Morimoto et al. / Journal of Colloid and Interface Science 368 (2012) 578–583
579
Fig. 1. Powder XRD patterns of brucite with sugar alcohols prepared by (A) the melt intercalation method and (B) hydrothermal treatment. Profiles of: (a) starting brucite, (b) xylitol supplemented brucite, and (c) D-sorbitol-supplemented brucite.
brucite powder (0.1 g). The mixtures were held at 175 °C in a thermostatic oven for 24 h under atmospheric conditions. The products obtained were washed with methanol and centrifuged. No removal of sugar alcohols was observed through this washing treatment. Thereafter, the solid products were dried using a vacuum freezedrier. The samples are denoted XYL-BR and SOR-BR. For comparison, samples treated with an aqueous solvent were prepared. For that purpose, 4 g of a sugar alcohol was dissolved in 4 mL distilled water. The solution was mixed with 0.1 g of BR. The suspension was hydrothermally aged using a TeflonÒ-lined stainless steel crucible at 175 °C for 24 h in a thermostatic oven. After treatment, the solid product obtained was isolated by centrifugation and washed with methanol.
2.2. Characterization The samples were subjected to X-ray diffraction (XRD; Ultima IV, Rigaku) measurements using Cu Ka radiation (k = 0.15406 nm) under conditions of 40 kV, 30 mA, and 2°/min scanning. The morphology of the sugar alcohol-brucite samples was observed using an SEM (JSM-6700FT, Jeol) at 7 kV. Fourier transform infrared spectroscopy (FTIR) spectra were obtained with an ATR–FTIR spectrometer (IR Affinity, Shimadzu) on a diamond crystal from 400 to 4000 cm1. Thermogravimetric-differential thermal analysis (TG-DTA) was performed using a Thermoplus TG8120 thermogravimetric analyzer
Fig. 3. The ATR–FTIR spectra of brucite with sugar alcohols prepared by (A) the melt intercalation method and (B) hydrothermal treatment: (a) starting brucite, (b) brucite with xylitol, and (c) brucite with D-sorbitol.
from Rigaku under a nitrogen flow at a uniform heating rate of 10 °C/min up to 600 °C. X-ray photoelectron spectroscopy (XPS) was conducted to further investigate the electronic state of the starting material and the intercalated samples prepared by the melt intercalation method. The XPS spectra were collected using a VG-Scienta ESCA-300 (Sweden) with a monochromatized Al Ka X-ray source (hv = 1486.6 eV) at a power of 1.0 kW and a base pressure of 7.3 108 Pa in the analytical chamber. The analysis chamber was equipped with a sputter gun, in which Ar gas was used to sputter clean the sample [Mg(OH)2] surface. Electrostatic charging due to the poor electrical conductivity of sugar alcohol samples was minimized by using a flood gun. Using a takeoff angle of 90°, survey scans were performed to identify the C, O, and Mg elements. Narrow region XPS spectra of the C1s and O1s were acquired with an analyzer pass energy of 75 eV, a step energy of 0.02 eV, a time per step of 0.5 s, a slit of 0.3 mm, with three sweeps, and a flood gun of 4 eV. Electron binding energies were cal-
Fig. 2. SEM images of (a) BR, (b) XYL-BR, and (c) SOR-BR powders.
580
K. Morimoto et al. / Journal of Colloid and Interface Science 368 (2012) 578–583
Fig. 4. (A) TG curves, (B) DTA profiles, and (C) DTG profiles under a nitrogen flow at a uniform heating rate of 10 °C/min for (a) starting brucite, (b) brucite with xylitol, and (c) brucite with D-sorbitol derived by the melt intercalation.
ibrated with respect to the C1s line at 284.5 eV (C–C), and a nonlinear least-squares curve-fitting program was employed with a Gaussian– Lorentzian (Voigt) function. Energy differences between theoretical and measured C1s lines were generally less than ±4 eV. 3. Results and discussion 3.1. Structural characteristics of XYL-BR and SOR-BR Fig. 1A(a) shows the XRD pattern of the brucite starting material. The profile shows a d value of 0.48 nm corresponding to the (0 0 1) reflection [7]. After the melt intercalation reaction with XYL, the peak corresponding to the brucite phase (0.48 nm) nearly disappeared with the concomitant appearance of a new peak at 0.87 nm [Fig. 1A(b)]. Faint peaks at 2h values of 18.4° arose from the unexpanded phases of brucite. No peak due to the MgO phase was generated. These results indicate the occurrence of interlayer expansions of brucite layers resulting from the incorporation of XYL molecules. In the case of SOR, the relatively broad peak observed at 1.09 nm is a newly constructed phase with the disappearance of the brucite peak (0.48 nm) [Fig. 1A(c)]. By subtracting the thickness of the brucite layers (0.48 nm) from the observed d values [7], the interlayer spacing (Dd) values of the
XYL-BR and SOR-BR hybrids were calculated to be 0.39 and 0.61 nm, respectively. In comparison, the observed Dd values for an ethylene glycol-brucite hybrid and a glycerol-brucite hybrid are 0.35 and 0.81 nm, respectively [6]. For smectite clays, which are easily expandable, the interlayer expansion is reported to be 0.43 nm for both ethylene glycol and glycerol. Thus, the present results support the formation of monolayer organic complexes for XYL [8,9]. The molecules may possess a flattened monolayer arrangement between the BR layers. In contrast to the XYL-BR hybrid, the Dd value for the SOR-BR hybrid (0.61 nm) was much larger, but was still too small for a bilayer arrangement (Dd 0.8 nm). These results suggest that SOR molecules formed a monolayer with the molecular long axes inclined toward the BR layer or a pseudo-bilayer arrangement. Fig. 1B shows the XRD pattern of the sample treated with an aqueous solvent. No new peak other than the d001 peak of the initial brucite appeared. Thus, no intercalation of sugar alcohol molecules occurred under these conditions. Fig. 2 shows SEM images of the starting BR and the XYL-BR and SOR-BR hybrid powders. The starting BR is composed of planar particles that vary in size from about 100 nm to larger than 500 nm [Fig. 2(a)], while the BR treated with XYL is composed of aggregates of several lm, which consist of BR platelets held together by XYL molecules [Fig. 2(b)]. In Fig. 2(c), one can see that SOR-BR particles
K. Morimoto et al. / Journal of Colloid and Interface Science 368 (2012) 578–583
581
Fig. 5. (A) TG curves, (B) DTG profiles, and (C) DTA profiles under a nitrogen flow at a uniform heating rate of 10 °C/min for (a) XYL reagent and (b) SOR reagent.
form aggregates of different sizes varying from a fraction of a micrometer up to ten micrometers in both length and width. The surfaces of BR layers are coated completely by SOR molecules. 3.2. Hybridization mechanisms of sugar alcohols into BR interlayers Fig. 3 shows the FTIR absorption spectra of starting BR, BR treated with XYL, and BR treated with SOR. Starting BR exhibits a sharp absorption peak at 3694 cm1, which is due to the stretching frequency of internal hydroxyl groups [Fig. 3A(a)] [10]. The absorption band observed at 409 cm1 is associated with the Mg–O lattice vibration. The spectra of the XYL- and SOR-BR hybrids differed from the starting BR, but were similar to each other [Figs. 3A(b) and A(c)]. The stretching vibration of internal hydroxyl groups at 3694 cm1 completely disappeared in both samples. The doublet at ca. 2800–2900 cm1 in the spectra corresponds to the asymmetric and symmetric stretching vibrations of C–H bonds in the sugar alcohols. The O–H stretching vibration of the combination of sugar alcohols and expanded brucite appeared in the range 3000–3600 cm1 [11]. The absorption band of the Mg–O lattice vibration also was detected at approximately 410 cm1. Furthermore, characteristic peaks associated with metallic alkoxides were identified near 830 and 1070 cm1 in the spectra of the XYL-BR and SOR-BR hybrids [12,13]. These results suggest the partial formation
of Mg–O–C linkages through dehydroxylation [6]. These results along with the XRD analyses confirmed that both XYL and SOR molecules infiltrated into the brucite interlayers and broke the hydroxyl hydrogen bonds at temperatures above their melting points (under nonaqueous conditions). For the hydrothermally treated samples, no significant change was found in the absorption spectra, with the internal O–H band remaining at 3694 cm1 (Fig. 3B). Fig. 4 shows the TG, DTG, and DTA profiles of starting BR and the XYL-BR and SOR-BR hybrids. The weight decrease process at around 380 °C for BR was endothermic and is attributed to the dehydration of structural water [Fig. 4C(a)]. The total weight loss of BR, subjected to temperatures of up to 600 °C, was estimated to be 32% [Fig. 4A(a)], which lead to the transition from Mg(OH)2 to MgO. On the other hand, the hybrids consisting of BR and sugar alcohol decomposed through several successive processes at around 70 °C, 280 °C, 400– 450 °C [Fig. 4B(b) and B(c)]. The first process was associated with an endothermic response in DTA and is attributed to the desorption of physisorbed water molecules [Fig. 4C(b) and C(c)]. The next two processes were highly exothermic, suggesting thermal degradation of the sugar alcohol moiety took place at these temperatures, while BR and sugar alcohol reagents showed only endothermic peaks [Figs. 4C(a) and 5C]. Total weight losses of the XYL-BR and SOR-BR hybrids, up to 600 °C, were estimated to be 58% and 66%, respectively [Fig. 4A(b) and A(c)]. The weight loss peak maximum in DTG profiles
582
K. Morimoto et al. / Journal of Colloid and Interface Science 368 (2012) 578–583
Fig. 6. XPS narrow spectra of (a) starting BR, (b) XYL-BR, and (c) SOR-BR: (A) XPS spectrum of C1s, (B) XPS spectrum of O1s, and (C) XPS spectrum of Mg2s.
of XYL-BR and SOL-BR hybrids occurred near 389 and 411 °C, respectively (see Fig. 4B). Importantly, these temperatures were higher than those of the starting brucite and the XYL (322 °C) and SOR (349 °C) reagents (Fig. 5B). Fig. 6 shows the XPS C1s, O1s, and Mg2s narrow spectra of starting BR and the XYL-BR and SOR-BR hybrids. In the scans of XYL and SOR, the binding energy of C1s photoelectrons from the C–C species was strongly observed near 284.5 eV (not shown). The C1s region scans of XYL-BR and SOR-BR were resolved into three component peaks. In addition to the peak from the hydrocarbon environment at 284.5 eV, the peaks at 285.2 eV for XYL-BR and at 285.6 eV for SOR-BR were assigned to a carbon singly bound to oxygen formed by the dehydroxylation reaction, and the shoulders on the higher binding energy side at 287.6 eV for XYL-BR and at 288.2 eV for SOR-BR were assigned to a carbon doubly bonded to oxygen [shown in Fig. 6A(b) and A(c)] [14–19]. The O1s peaks in the scan of BR (529.2 and 531.0 eV, which are labeled as 1 and 2, respectively) were attributed to the presence of oxide O–Mg and hydroxide HO–Mg, respectively [Fig. 6B(a)] [17–19]. In comparison, the main peaks from the O1s scans of the intercalated XYL-BR and SOR-BR were due to the carbonyl and oxide species from sugar alcohols and brucite, respectively [Figs. 6B(b) and B(c)]. Upon intercalation of the sugar alcohols, significant changes were observed for their relative full width at half maximum (FWHM). Especially remarkable was the increase of the FWHM of the O1s peaks for the hybrid sample (e.g., XYL-BR, 2.5 eV instead of 1.7 eV for the XYL; SOR-BR, 2.25 eV instead of 1.6 eV for the SOR). Fig. 6C shows narrow spectra of the Mg2s. The sugar alcohol treated samples show higher Mg2s binding energies (89 eV for XYL-BR and 88.4 eV for SOL-BR) than the starting BR (87.4 eV). The slightly increased binding energy of the magnesium core lines could be due to reduced shielding on the Mg core. Both behaviors, the broadening of the O1s peak and the Mg2s peak shift observed in the hybrids, suggest a possible formation of an Mg–O–C linkage to the MgOH surface. Table 1 shows the atomic concentration results from the XPS scans. The XYL-BR and SOR-BR samples clearly have increased
Table 1 The atomic concentrations of the surface of samples using XPS (atomic%). Sample
O1s
C1s
Mg2s
XYL-BR SOR-BR BR XYL SOR
44.3 50.0 56.8 47.0 48.6
34.1 22.7
21.6 27.3 43.2
53.0 51.4
Table 2 Changes in oxygen ratios of each sample as a function of degree of grafting. The values for XYL-BR, SOR-BR, and BR were normalized according to Mg content, and then the values for XYL and SOR were normalized according to the C content of the XYL-BR and SOR-BR samples, respectively. Sample
C
O
Mg
XYL-BR SOR-BR BR XYL SOR
1.6 0.8
2.1 1.8 1.3 1.5 0.8
1.0 1.0 1.0
1.6 0.8
carbon and reduced oxygen content at the surface compared to BR. To clarify changes in the oxygen ratios for each sample as a function of degree of grafting, the values for XYL-BR, SOR-BR, and BR were normalized according to magnesium content, and then the values for XYL and SOR were normalized according to the carbon content of the XYL-BR and SOR-BR samples, respectively (Table 2). The SOR-BR indicates half the carbon ratio than the XYL-BR. This may be because the physically adsorbed SOR was removed by argon etching before the XPS measurement. This means that the XYL molecules are efficiently incorporated into the BR interlayers. Furthermore, it is very interesting to compare the O atomic ratios of the hybrid materials (XYL-BR and SOR-BR) and the values of the O atomic ratio of BR plus that of the sugar alcohols. The oxygen levels for the hybrids were low compared to the mixture of BR and sugar alcohols: 2.1 for XYL-BR hybrid versus 2.8 for mixture of BR and XYL, and 1.8 for SOR-BR versus 2.1 for mixture of BR and SOR. These results also strongly suggest that the BR and sugar alcohols had not simply mixed, but that a covalent linkage between the sugar alcohols and brucite surface had been formed through Mg–O–C bonds. This is consistent with the exothermic DTA profiles of the hybrids [curves (b) and (c) in Fig. 4C], suggesting BR incorporated the sugar alcohols through dehydration. During the melt-mixed process, the formation of the brucite-sugar alcohol derivatives (covalent linkage) must involve the displacement of hydroxyl groups in the coordination sites of magnesium into the alkoxyl group. Although this is the reverse reaction of alkoxide hydrolysis, the alkoxides and hydroxides are in equilibrium and it is known that some alkoxides can be prepared by continuous removal of water from the reaction system [20,21]:
MðOHÞn þ nROH MðORÞn þ nH2 O
ð1Þ
where M represents alkali or alkali earth metal, and R represents an alkyl group CnH2n+1. However, when the hydrothermal process is
K. Morimoto et al. / Journal of Colloid and Interface Science 368 (2012) 578–583
583
utilized, the addition of water shifts the equilibrium composition to the left-hand side of the reaction (1). Therefore, by hydrothermal treatment, sugar alcohol molecules are not incorporated into the brucite interlayer region.
diverse drugs as well as to applications in topical therapies, such as food, drugs, cosmetics, and medical applications.
4. Conclusions
This work was supported by a Grant-in-Aid for Scientific Research category C (23550235) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government.
In conclusion, the successful incorporation of sugar alcohols (XYL and SOR) into BR was achieved via a melt intercalation procedure within 24 h at 175 °C. The XRD and FTIR analyses confirmed that the melted sugar alcohol infiltrated into the interlayers with a breaking of the hydroxyl bonds in brucite and also that the hydrothermal treatment of brucite with sugar alcohols had no effect. It was observed that, during this study, hydroxide and alcohol were in equilibrium with alkoxide and water, similar to a previous work [6]. However, in this study, the equilibrium was observed using, for the first time, solid sugar alcohols. Because of this equilibrium, alkoxide is generated from the hydroxide under glycothermal conditions. Glycothermal reaction between the BR and sugar alcohols is a convenient route for the synthesis of organic–inorganic hybrids, avoiding the effect of water. This is significant because there are many organic chemicals which have poor water solubility that are used in biofunctional substances, such as drugs, enzymes, cosmetic chemicals. While many wet colloidal processes use inorganic particles, this melt process could be used to produce new types of hybrid that may lead to innovative technology in many fields. To give a specific example, the combination of sugar alcohols and/or other biomolecules produces new matrices for DDS that can be used for direct encapsulation of drugs and to develop bionanocomposite materials with drug-intercalated brucite systems. In comparison with other DDS, the system proposed here has the advantages of easy processing and the use of abundant, inexpensive, and safe biomolecules. The possibility of using brucite may allow the extension of this approach to the immobilization of
Acknowledgments
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
M.I. Carretero, M. Pozo, Appl. Clay Sci. 47 (2010) 171. Y. Dong, K. He, B. Zhao, Y. Yin, L. Yin, A. Zhang, Catal. Commun. 8 (2007) 1599. G.J. Simandl, S. Paradis, M. Irvine, Geosci. Can. 34 (2007) 57. E.M. Opiso, T. Sato, K. Morimoto, A. Asai, S. Anraku, C. Numako, T. Yoneda, Miner. Eng. 23 (2010) 230. F. Wypych, W.H. Schreiner, R. Marangoni, J. Colloids Interface Sci. 253 (2002) 180. M. Inoue, H. Kominami, Y. Kondo, T. Inui, Chem. Mater. 9 (1997) 1614. M. Catti, G. Ferraris, S. Hull, A. Pavese, Phys. Chem. Miner. 22 (1995) 200. R.W. Hoffmann, G.W. Brindley, Am. Mineral. 46 (1961) 450. T. Sato, T. Watanabe, R. Otsuka, Clay. Clay. Miner. 40 (1992) 103. R.L. Frost, J.T. Kloprogge, Spectrochim. Acta Part A 55 (1999) 2195. E. Libowitzky, Monatshefte für Chemie 130 (1999) 1047. P.F. Fuls, L. Rodrique, J.J. Fripiat, Clays Clay Miner. 18 (1970) 53. H.S. Jung, J.-K. Lee, J.-Y. Kim, K.S. Hong, J. Colloids Interface Sci. 259 (2003) 127. Z.-Z. Jiang, Z.-B. Wang, D.-M. Gu, E.S. Smotkin, Chem. Commun. 46 (2010) 6998. S. Fleutot, J.C. Dupin, I. Baraille, C. Forano, G. Renaudin, F. Leroux, D. Gonbeau, H. Martinez, Phys. Chem. Chem. Phys. 11 (2009) 3554. J.L. Figueiredo, M.F.R. Pereira, M.M.A. Freitas, J.J.M. Órfão, Carbon 37 (1999) 1379. J.I. Di Cosimo, V.K. Díez, M. Xu, E. Iglesia, C.R. Apesteguía, J. Catal. 178 (1998) 499. J.D. Farr, M.P. Neua, R.K. Schulze, B.D. Honeyman, J. Alloy. Comp. 444–445 (2007) 533. D.K. Aswal, K.P. Muthe, S. Tawde, S. Chodhury, N. Bagkar, A. Singh, S.K. Gupta, J.V. Yakhmi, J. Crystal Growth 236 (2002) 661. A.Y. Platonov, A.V. Kurzin, A.N. Evdokimov, J. Solution. Chem. 39 (2010) 335. D.C. Bradley, Prog. Inorg. Chem. 12 (1960) 303.