Langmuir 2003, 19, 8057-8064
8057
Liquid Crystalline Colloidal System Obtained by Mixing Niobate and Aluminosilicate Nanosheets: A Spectroscopic Study Using a Probe Dye Nobuyoshi Miyamoto† and Teruyuki Nakato*,†,‡ PRESTO, Japan Science and Technology Corporation, 4-1-8 Hon-cho, Kawaguchi-shi, Saitama 332-0012, Japan, and Laboratory of Environmentally Benign Materials Chemistry, Faculty of Agriculture, Tokyo University of Agriculture and Technology, 3-5-8 Saiwai-cho, Fuchu-shi, Tokyo 183-8509, Japan Received November 13, 2002. In Final Form: May 10, 2003 A novel colloidal system composed of two kinds of inorganic oxide nanosheets dispersed in water was prepared by mixing exfoliated layered niobate K4Nb6O17 and montmorillonite clay. According to the observation of the colloids under crossed polarizers, the obtained niobate-clay colloids were liquid crystalline with apparently homogeneous macroscopic ordering, similar to the previously reported single-component colloids of the exfoliated niobate layers. X-ray diffraction measurements of the dried colloids indicated that the niobate-clay colloids were regarded as mixtures of the niobate and clay microphases: the colloids are microscopically demixed. Spectroscopic investigations using a cationic cyanine dye as a probe showed that the dye was selectively adsorbed on the clay platelets; the results indicated mass transfer of the dye molecules from the niobate nanosheets to clay platelets, and the transfer was accompanied by alterations of the aggregation state of the probe dye. The microenvironments provided by the clay nanosheets were the same as those given by the single-component clay colloids. Polarized spectra of the dye-adsorbed niobate-clay colloids demonstrated that the clay nanosheets were present in the isotropic state among the niobate nanosheets, which were orientationally ordered to give liquid crystallinity of the colloids.
Introduction Colloids consisting of anisotropic particles show peculiar properties such as liquid crystallinity and unusual rheological properties due to anisotropic interactions between the particles,1 in contrast to those with spherical particles. Colloids of inorganic platelets with thicknesses of around 1 nm, called “nanosheet colloids” hereafter, can be obtained by exfoliation of various layered inorganic crystals. Whereas this type of colloid is regarded as a fascinating class of such anisotropic colloidal systems, the only wellstudied system is colloids of layered clay minerals exemplified by montmorillonite, which are infinitely swelled in water through hydration of interlayer exchangeable cations (mainly Na+) to give exfoliated aluminosilicate nanosheets. The clay nanosheet colloids have been found to show various peculiar properties such as thixotropy2 and flow-birefringence.3 Other layered solids investigated as nanosheet colloids are V2O54-6 and very recently reported H3Sb3P2O14.7 We have recently found a colloidally dispersed system of layered niobate K4Nb6O17, whose structure is con* To whom correspondence should be addressed. E-mail: tnakat@ cc.tuat.ac.jp. † PRESTO, Japan Science and Technology Corporation. ‡ Laboratory of Environmentally Benign Materials Chemistry, Faculty of Agriculture, Tokyo University of Agriculture and Technology. (1) Onsager, L. Ann. N.Y. Acad. Sci. 1949, 51, 627-659. (2) van Olphen, H. In An Introduction to Clay Colloid Chemistry, 2nd ed.; Wiley-Interscience: New York, 1977. (3) Langmuir, I. J. Chem. Phys. 1938, 6, 873-896. (4) Pelletier, O.; Bourgaux, C.; Diat, O.; Davidson, P.; Livage, J. Eur. Phys. J. B 1999, 12, 541-546. (5) Desvaux, H.; Gabriel, J.-C. P.; Berthault, P.; Camerel, F. Angew. Chem., Int. Ed. 2001, 40, 373-375. (6) Lamarque-Forget, S.; Pelletier, O.; Dozov, I.; Davidson, P.; Martinot-Lagarde, P.; Livage, J. Adv. Mater. 2000, 12, 1267-1270. (7) Gabriel, J.-C. P.; Camerel, F.; Lemaire, B. J.; Desvaux, H.; Davidson, P.; Batail, P. Nature 2001, 413, 504-508.
structed by Nb6O174- niobate layers and interlayer K+ ions, as a unique end member of the nanosheet colloids.8,9 This material is exfoliated through exchange of interlayer K+ ions for alkylammonium ions and subsequent dispersion in water. The niobate nanosheet colloids exhibit unusual properties of stable liquid crystallinity accompanied by macroscopic (on the centimeter scale) orientation8 and pH-induced sol-gel transition.9 These facts prompt us to utilize the niobate nanosheet colloids as novel soft materials which cannot be prepared from other inorganic colloidal systems. As a method of such utilization, we expect that the niobate nanosheet colloids can be used as “soft matrixes” of cationic functional molecules since the negatively charged Nb6O174- layers have the capability of adsorbing cationic species.8,10,11 For this purpose, ordering of the soft cavities provided by the niobate nanosheets, that is, liquid crystallinity of the nanosheets, should play an important role in constructing hybridized systems consisting of colloidal nanosheets. Addition of other anisotropic inorganic nanoparticles to the niobate nanosheet colloids can influence both the structure, from molecular arrangement to macroscopic ordering, and properties of the colloidal systems. However, to our knowledge, such attempts have not been reported. Here, we report a colloidal system of inorganic nanosheets obtained by mixing two different kinds of nanosheets, Nb6O174- and montmorillonite clay. While the niobate nanosheets are characterized by rather large lateral size (
