Layered double hydroxide hybrids with dicetylphosphate

Layered double hydroxide hybrids with dicetylphosphate

Journal of Colloid and Interface Science 291 (2005) 218–222 Layered double hydroxide hybrids with dicetylphosphate Toshi...

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Journal of Colloid and Interface Science 291 (2005) 218–222

Layered double hydroxide hybrids with dicetylphosphate Toshio Itoh a , Tetsuya Shichi b , Tatsuto Yui a,c , Katsuhiko Takagi a,c,∗ a Department of Crystalline Materials Science, Graduate School of Engineering, Nagoya University, Chikusa, Nagoya 464-8603, Japan b Central Japan Railway Company, Komaki, Aichi 485-0801, Japan c CREST, Japan Science and Technology (JST)

Received 8 March 2005; accepted 28 April 2005 Available online 1 July 2005

Abstract Dicetyl phosphate (DCP) ions incorporated into layered double hydroxide (LDH) clays to form a DCP/LDH hybrid were prepared and structurally characterized by X-ray diffraction analysis, scanning electron microscopy, thermogravimetry, and differential thermal analyses. DCP was concluded not only to form well-aligned bilayer structures along the vertical axis of the LDH layers but also to be arranged in a distorted hexagonal packing orientation in the lateral planes within the DCP/LDH hybrid interlayers.  2005 Elsevier Inc. All rights reserved. Keywords: Dicetyl phosphate; Clay; Layered double hydroxide; Hydrophobic interaction

1. Introduction Organic ions accommodated in the interlayer spaces of inorganic layered hosts show characteristic and topologically aligned conformations [1–7]. Among various layered materials, clay minerals, which have an anisotropic structure, are capable of accommodating organic guests into their interlayers by cationic or anionic exchange to form spatially organized “organic guest–clay host” hybrids. Such hybrids have great potential for the development of efficient photofunctional materials, particularly as stable and translucent thin films on solid surfaces [8–17]. There have been many reports of various kinds of hybrid materials with cationic layered clays, e.g., montmorillonite, but only a few structural studies of hybrid materials with anionic layered clays such as layered double hydroxide (LDH) [18–31]. LDH, which is referred to by the chemical formula Mg4.5 Al2 (OH)13 Cl2 ·3.5H2 O, possesses an anion exchangeable capacity (AEC) of 3.5 meq g−1 . It is capable of accommodating higher aliphatic acids within its interlayers not only by anion exchange with exchangeable ions but also * Corresponding author. Fax: +81 52 789 3338.

E-mail address: [email protected] (K. Takagi). 0021-9797/$ – see front matter  2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.04.102

by the hydrophobic intercalation of the guests themselves. For instance, the LDH interlayers can accommodate excess amounts of stearate ions over the anion exchangeable capacity (AEC) to form a closely packed bilayer structure so that the intercalated stearate ions are present independent of the AEC of the LDH [32,33]. On the basis of our previous study, LDH showed good promise for the anisotropical accommodation of two long alkyl-chained anionic surfactants, e.g., dicetylphosphate (DCP). DCP bilayer vesicles synthesized by making use of their self-assembling properties can provide interesting reaction fields since their bilayers show anisotropic characteristics. Moreover, the cis–trans photoisomerization of the stilbene-type chromophores inside the vesicles exhibit unique photoresponsive behavior [34–38]. A clarification of the conformational structure of such hybrids incorporating molecularly assembling DCP within LDH clay interlayers is vital for an understanding of their interactions with organic chromophores for nanoscale organic–inorganic hybrid systems as well as for novel photochemical applications not only in vesicle matrices but also in solid state systems. In the present work, we report on a structural investigation of DCP/LDH hybrids with spatially packed bilayers composed of DCP ions within the LDH interlayers, using

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X-ray diffraction, scanning electron microscopy, thermogravimetry, and differential thermal analyses.


were dispersed in 5 ml H2 O and vigorously stirred magnetically at room temperature for 3 h, filtrated, and dried for 3 h in vacuo to obtain a rehydrated powder sample.

2. Materials and methods 3. Results and discussions 2.1. Synthesis of the dicetylphosphate/LDH hybrid DCP sodium salt was obtained by the addition of an equimolar amount of sodium methoxide in methanol, condensation by solvent evaporation, and then drying in vacuo for 3 h. An aqueous suspension of the DCP sodium salt (5 mmol dm−3 ) was prepared as a stock solution. Forty-eight milliliters of the suspension including an amount of DCP corresponding to 150% AEC was added to 45.7 mg of LDH, and the solution was stirred magnetically for 1 day at 5 ◦ C. The resulting hybrid precipitate was filtrated and washed with ethanol and then dried in vacuo for 3 h to yield the DCP/LDH hybrid white powder samples. 2.2. X-ray diffraction analysis of the DCP/LDH hybrid Powder X-ray diffraction analysis (XRD) of the DCP/ LDH hybrid was carried out with a Rigaku RINT-2100 XRD apparatus operating at 40 kV and 40 mA and equipped with a Rigaku CN2173C3 goniometer set at 1.54 Å with Ni-filtered CuKα radiation and a Rigaku PTC-20A temperature controller. The XRD patterns were measured in the 2θ/θ mode within a 2θ range of 1.5◦ –10◦ and a scan rate of 1◦ min−1 . The interlayer distance was calculated by the averaged d value measured by a series of reflection peaks assigned to the bilayer DCP/LDH hybrid. The morphology of the DCP/LDH hybrid cast film was investigated with a JEOL JSM-6300 SEM apparatus. Twodimensional XRD diffraction analysis of the oriented hybrid film was carried out with a Rigaku R-AXIS IV XRD apparatus operating at 60 kV and 40 mA, and equipped with Ni-filtered CuKα radiation using a Rigaku imaging plate detector, as detailed in the supporting information section.

3.1. The interlayer structure of the DCP/LDH hybrid in the vertical direction Fig. 1a shows a series of (00l) XRD diffraction peaks for the DCP/LDH hybrids, which indicate the disappearance of the interlayer spaces in the original LDH upon the intercalation of DCP. The bilayer structure for the DCP ions within the LDH interlayers was formed on the basis of the interlayer distances, their molecular length of 25.3 Å, and the LDH framework thickness of 4.8 Å. Fig. 1, curves b–f, show the XRD patterns of the DCP/LDH upon dehydration with heat treatment at 80 ◦ C and subsequent rehydration by the addition of water. The d values of the dehydrated hybrids were reduced by 3.2 Å; however, rehydration of the sample was able to restore the original layer distance. The TG and DTA patterns for the DCP/LDH hybrid are shown in Fig. 2. An endothermic weight loss of 5% at around 70 ◦ C was attributed to the release of the adsorbed water on the surface of the LDH framework. The DCP/LDH hybrid was observed to include water layers under the DCP polar phosphate groups on the LDH surfaces within the interlayer of the hybrid. The results of XRD and TG analyses were in good agreement with the reversible interlayer expansion and shrinkage observed during the dehydration and rehydration process. The hybrid was then decomposed fur-

2.3. Thermogravimetry and differential thermal analyses of the DCP/LDH hybrid The thermogravimetry and differential thermal analyses of the DCP/LDH hybrid were carried out with a Seiko Instruments DSC 6200 differential scanning calorimeter. About 3 mg of the hybrid powder was placed in a platinum vessel and heated at a rate of 5 ◦ C min−1 up to 600 ◦ C under aerated atmosphere. 2.4. Dehydration and rehydration of the DCP/LDH hybrid The DCP/LDH hybrid prepared was heated up to 80 ◦ C for 1 h by drying on a Peltier device with a direct current power source. The resulting dehydrated hybrids (30 mg)

Fig. 1. The morphological changes seen by monitoring the temperaturedependent XRD patterns: (a) room temperature; (b) 50 ◦ C; (c) 60 ◦ C; (d) 70 ◦ C; (e) 80 ◦ C; and (f) the rehydrated samples after heating at 80 ◦ C.


T. Itoh et al. / Journal of Colloid and Interface Science 291 (2005) 218–222

Fig. 3. SEM image of the uniformly oriented DCP/LDH cast film from the edge. Fig. 2. TG and DTA analyses patterns of the DCP/LDH hybrid in air.

ther by heat treatment at 250 ◦ C, accompanied by a weight loss of ca. 15%. The nonintercalated LDH involving CO2− 3 in the interlayers was shown to release interlayer water upon heating at 100–250 ◦ C, while both the structural water and CO2 were desorbed from the interlayer at a temperature range of 200–400 ◦ C, inducing the collapse of the layered structure [26,30]. An additional weight loss of ca. 55% at over 300 ◦ C may be attributed to the exothermic thermal decomposition of the DCP molecules themselves and resulted in a total 70% weight loss. The molar ratio of the adsorbed water and DCP content was ca. 2.6:1 using the weight loss observed by TG analysis. From the estimated molar ratio, ca. 19 Å2 was determined for the occupied area of the unit water molecule, which was estimated on the basis of 50.6 Å2 , the surface area of the unit ionic site covered by a DCP molecule. Moreover, the value is comparable with that for the stearate ions/LDH hybrid reported in previous literature [32]. It could be concluded from the estimated value that the water molecules are aggregated with high density. 3.2. The spatially packed structure of DCP assembled into the LDH hybrid in the lateral direction A regularly aligned thin film of the DCP/LDH hybrid was prepared on a glass tube by casting the suspension of the hybrids in n-heptanol and then drying at 50 ◦ C. An SEM image of the DCP/LDH cast film on the glass plate indicates that the hybrid powders are oriented in a regular alignment along the face of the glass plate, as shown in Fig. 3. Fig. 4 shows the imaging plate (IP) of the DCP/LDH cast film by twodimensional XRD diffraction analysis, as depicted in Fig. 5. A series of (00l) basal spacing peaks could be observed in the vertical direction, while additional peaks that appeared could be assigned to the regular alignment of the DCP/LDH hybrid in the lateral direction. These lateral diffraction peaks can be understood as follows: (1) the DCP/LDH hybrid film was confirmed to be cast in a uniform orientation on the sub-

Fig. 4. IP image of the DCP/LDH hybrid film.

strate; (2) the lateral peaks were derived from the diffraction peaks different from the basal spacing peaks; and (3) the LDH frameworks did not exhibit any definite peaks in the range of the XRD patterns observed. The additional peaks were similar to the packing structure of the head groups for stearate ions in LDH hybrid films [33]. It could therefore be concluded from these three peaks that the methylene chains of DCP form a regular packing alignment with a distorted hexagonal structure in the lateral direction within the LDH interlayers. 3.3. The spatial model for the DCP/LDH hybrid A three-dimensional morphological structure for the bilayer DCP/LDH hybrid could be drawn up on the basis of these various investigations, as shown in Fig. 6. The structure of the hybrid was in good agreement with the XRD signals along the c-axis, as can be seen in Fig. 6a. The hybrid

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Fig. 5. Two-dimensional XRD analysis of the DCP/LDH oriented film cast on a glass plate (a) in the vertical direction and (b) in the lateral direction. Arrows show the incident and diffracted X-rays.

4. Conclusions In the present work, a structural characterization of a bilayer DCP in a LDH hybrid was observed by XRD, TGDTA, and SEM investigations. A series of (00l) XRD peaks revealed a regular alignment of the guest DCP ions as bilayer aggregates within the LDH interlayers. It was also found that dehydration and rehydration of the water molecules could induce a reversible formation of water molecule layers between the LDH layers and the DCP guests within the hybrid. Moreover, two-dimensional XRD analysis of the DCP/LDH cast film revealed a distorted hexagonal packing arrangement for the methylene chains of DCP within the LDH interlayers. Acknowledgments This work was partly supported by a Grant-in Aid for Scientific Research on Priority Areas (417) and the 21st Century COE Program “Nature-Guided Materials Processing” of the Ministry of Education, Science, Culture, and Sports, Science and Technology (MEXT) of Japan. We would like to express our appreciation for their kind support. References

Fig. 6. Morphological illustration of the spatially packed DCP/LDH hybrid: (a) side view and (b) top view of the bilayer DCP/LDH hybrid.

lattice constants were also determined by two-dimensional XRD analysis, as shown in Fig. 6b. The most plausible threedimensional structure for the DCP/LDH hybrid has been depicted in Fig. 6.

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