In Situ Crystallization of Al-Containing Silicate ... - ACS Publications

Article pubs.acs.org/Langmuir

In Situ Crystallization of Al-Containing Silicate Nanosheets on Monodisperse Amorphous Silica Microspheres Tomohiko Okada,*,† Mai Sueyoshi,† and Hikari M. Minamisawa§ †

Department of Chemistry and Material Engineering and §Technology Division, Faculty of Engineering, Shinshu University, Wakasato 4-17-1, Nagano 380-8553, Japan S Supporting Information *

ABSTRACT: The fine crystals of an Al-containing layered silicate, whose negative layer charge is generated by an isomorphous substitution in the tetrahedral SiO4 framework, successfully grew on monodisperse amorphous silica microspheres with diameters of 1.0 and 2.6 μm. The fine, plate-like crystals were observed to thoroughly cover the surface of the silica spheres, irrespective of their size, by the hydrothermal reactions of the silica powder in aqueous alkali solution containing Al and Mg ions in a rotating Teflon-lined autoclave. The crystal size increased when the concentration of the precursors was low. The presence of fluorine in the reaction media enlarged the crystalline phase in the direction of the layer stacking while reducing the plate size. The difference in the crystal size affected the kinetics on the hinokitiol desorption in n-hexane from the layered silicates modified with organoammonium ions. The organically modified layered silicate behaved as an exfoliated nanosheet in the nonpolar solvent. The less harmful elements in this hybrid suggest that it can be used in cosmetic and pharmaceutical applications as a drug support, without flaking off the fine layers on the microspherical substrates.

■

INTRODUCTION

The direct (in situ) crystallization of the inorganic host materials on bulk solid substrates is a way to produce mechanically improved supporting materials. When the other chemical sources are supplied from the solution media, the solid substrate has been used as a source of layered materials. As a result of the reactions at the interface, fine crystals form on the substrates while maintaining the shapes of the substrate (the so-called self-template or sacrificial template). The selection of solid substrate chemicals in a solution leads to forming various 2D nanostructural fine crystals at the interfaces such as potassium titanates−Ti spheres,20 M(= Ni, Zn)/Al layered double hydroxides (LDHs)−Al2O3 plates,21,22 M(= Mg, Zn)/Al LDH−Al2O3 fibers,23,24 and titanosilicate−silica fibers.25 Smectites, a class of layered clay minerals comprising crystalline silicate layers separated by hydrated interlayers, have long been studied as host materials for multifunctional supramolecular systems.26−28 A silicate layer (ca. 1.0 nm) is composed of two Si tetrahedral sheets and one Mg (or Al) octahedral sheet. A negative charge in the silicate layers generated by an isomorphous substitution is compensated by interlayer exchangeable alkali metal cations.29,30 Synthetic smectite-like layered silicates, such as Laponite (a synthetic hectorite supplied from Rockwood Holdings) and Sumecton

The expandable two-dimensional (2D) interlayer spaces of layered materials are useful reaction media for supramolecular guest self-assembly.1−3 The intercalation of organic guest species into the 2D interlayer spaces of inorganic layered solids has been extensively utilized to construct supramolecular inorganic−organic nanoarchitectures.4−6 Inorganic nanosheet components play an important role as scaffolds to create nanospaces that accommodate organic guest species.7−9 The spatial distribution of the organic functional units in the 2D nanospaces has been devised through changing the number, location, and size (molecular geometry). Using the nanostructural versatility of the 2D supramolecular systems allows for applications in a wide variety of fields in materials chemistry, to impart functionalities such as adsorption, separation, controlled release, catalysis, and useful photo/electrochemical reactions. Alternately, designing the shape of the supramolecular assemblies is an important task for practical use and improving their mechanical properties. For instance, optically transparent films10−12 on flat plates have been developed from exfoliated nanosheets through various techniques, including simple deposition, Langmuir−Blodgett method,13 and the layer-by-layer deposition technique.14−16 Instead of finely divided powders, the supporting materials are useful for flow systems based on solid−gas and solid−liquid reactions because they diminish the pressure drop problems.17−19 © 2015 American Chemical Society

Received: October 19, 2015 Revised: November 20, 2015 Published: December 6, 2015 13842

DOI: 10.1021/acs.langmuir.5b03874 Langmuir 2015, 31, 13842−13849

Article

Langmuir Table 1. List of Samples Used in the Present Study molar ratio of the starting mixture diameter of silica grain 2.6 μm 1.0 μm

Al 0.09

Mg 0.45

Si 3.4

urea 0.3

H2O

0 0 0.09 0

0.3 × 10

sample name 2

2.5 × 102

[email protected] [email protected] [email protected]−NaF [email protected]

to TMA) chloride was purchased from Tokyo Chemical Industry Co., Ltd. Monodisperse spherical silica powders with the grain sizes of 1.0 μm (KE-S100, Nippon Shokubai Co., Ltd.) and 2.6 μm (KE-P250, Nippon Shokubai Co., Ltd.) were used as the source of an Alcontaining layered silicate. All these chemicals were used without further purification. Commercially available smectites used in this study were natural Na−montmorillonite (Kunipia F, JCSS-3101; hereafter abbreviated to KF; (Na 0 . 5 3 Ca 0 . 0 9 ) 0 . 7 1 + [(Al 3 . 2 8 Fe 0 . 3 1 Mg 0 . 4 3 ) (Si 7 . 6 5 Al 0 . 3 5 )O20(OH)4]0.71−), and a synthetic Na−saponite (Sumecton SA, JCSS3501; hereafter abbreviated to SA; (Na0.49Ca0.14)0.77+[(Mg5.97Al0.03) (Si7.20Al0.80 )O20 (OH)4] 0.77− ). Both clay minerals, supplied by Kunimine Industries Co., Ltd., are the reference samples of the Clay Science Society of Japan. The cation exchange capacities (CECs) of KF and SA are 1.19 and 0.71 mEq/g clay, respectively. Crystal Growth of an Al-Containing Layered Silicate on Colloidal Spherical Silica. The molar ratio of Al[OCH(CH3)2]3: (CH3 COO)2 Mg:SiO2:(NH 2 )2CO in the starting mixture was 0.09:0.45:3.4:0.3. The amounts of Al and Mg sources added, relative to the amount of SiO2, were decreased by 15% from the Al:Mg:Si ratio of 0.6:3.0:3.4.44 Urea (0.18 g: 2.9 mmol), (CH3COO)2Mg·4H2O (0.94 g/4.4 mmol), and Al[OCH(CH3)2]3 (0.18 g/0.88 mmol) were dissolved in water (0.3 mol/5.0 g). NaF (0.037 g/0.88 mmol) was added to this solution, if necessary, as a mineralizer. The resulting solution was mixed with spherical silica powder (2.0 g) using an ultrasonic agitation (28 kHz) for a few minutes at room temperature. To increase the amount of water in the hydrothermal reactions to 2.5 mol, urea (2.9 mmol), (CH3COO)2Mg·4H2O (4.4 mmol), and Al[OCH(CH3)2]3 (0.88 mmol) dissolved in water (1.4 mol) was mixed with an aqueous suspension of spherical silica particles (2.0 g in 1.1 mol of water) using a mechanical homogenizer (at 4600 rpm) for 30 min at room temperature. The slurry was transferred to a Teflonlined autoclave and heated to 448 K for 72 h. The reaction temperature was determined in order to prevent coprecipitating byproducts such as LDHs in preliminary experiments; Mg/Al LDH was shown to form at a lower temperature (423 K). The X-ray powder diffraction (XRD) pattern and the scanning electron microscope (SEM) image are exemplified in Supporting Information Figure S1. The autoclave was rotated at 5−11 rpm using a hydrothermal synthesis reactor unit (Hiro Company) during the heat treatment. The slurry was then cooled in an ice bath and centrifuged (at 1400g for 20 min, g = gravitational acceleration), and then the precipitate was collected and dried at 323 K. The grain size of spherical silicas and amounts of water and NaF in the initial mixture were changed as summarized in Table 1. The sample name was designated hereafter as Silicax@Sapoy, where x and y are the diameter of the spherical silica grain (μm) and amount of water (mol) in the starting mixtures, respectively. Preparation of Organically Modified Samples. The cation exchange reactions of 2C18 bromide with commercially available smectites were performed based on a previous report.45,46 The smectites of KF and SA (1.9 g) were allowed to react, using magnetic stirring, for 1 day in a mixture of water and ethanol (10 mL, 50/50 v/ v) containing 2C18 bromide with 1.1 times each CEC of smectites. The basal spacing determined by XRD analysis was 3.6 nm (2C18−KF) and 2.2 nm (2C18−SA). The interlayer space was obtained by subtracting the thickness of the silicate layer (0.96 nm) from the observed basal spacing and found to be 2.6 nm (2C18−KF) and 1.2 nm (2C18−SA), reflecting the 2C18 conformation in the interlayer space as paraffin-

SA (a synthetic saponite supplied from Kunimine Ind. Co., Ltd.) have been investigated for use as advanced materials, because they are fine powders and contain no colored impurity.5,6,9,27,28 The isomorphous substitution in hectorite and saponite are Mg2+ to Li+ in the octahedral sheet and Si4+ to Al3+ in the tetrahedral sheet, respectively. It has been pointed out that the variations in the position and amount of the isomorphous substitution affect functions such as catalytic activity,31 adsorption capability of organic molecules,32 optical resolution,33 photoinduced redox reactions,34 and cationic dye aggregations.35−37 We reported the in situ crystallization of hectorite-like layered silicate (isomorphous Mg2+−Li+ substitution) on an amorphous silica substrate, which partly converts to the source of the hectorite-like silicate without losing the silica morphology.38,39 Using our in situ crystallization method, it is possible to avoid flakes from falling off the silica particles, even in aqueous media.38 The intercalation of a bulky cation into the fine crystals of the layered silicates expands the interlayer space, resulting in an increase in the grain size, topochemically, without any change in the shapes of the whole particles.39 Therefore, the layered silicate−silica hybrid system is useful as a hierarchically designed supporting material. It has been suggested that the isomorphous Si4+−Al3+ substitution in smectites plays a role in enhancing the adsorption efficiency of organic molecules,32 increases the efficiency of photoinduced electron transfer,34 and improves acid-catalytic activity.31 However, the in situ crystallization of Si4+−Al3+-substituted layered silicate on the silica substrate has yet to be investigated. Here, we describe the in situ crystallization of the saponite-like (Si4+−Al3+-substituted) layered silicate on amorphous silica spherical particles. In the homogeneous nucleation studies, many factors, including the nature of the starting silica, concentration, and chemical composition of the starting mixtures, are important to the crystallization.40,41 It has been reported that fluoride ions act as mineralizing agents in the hydrothermal syntheses of layered silicates.42 Thus, we report heterogeneous nucleation reactions with the changing concentration of the starting mixtures, grain size of silica spheres (diameters of 1.0, and 2.6 μm), and nature of the additives (mineralizing agent). Besides the advantage in applications derived from the Si4+−Al3+ substitution, more biocompatibility is expected because the new materials contain less harmful elements. To exhibit a possible biocompatibility, we also performed a desorption test of an antibacterial agent (hinokitiol)43 in n-hexane from hydrophobic samples obtained using a quaternary long-chain alkylammonium, for controlled release materials.

■

NaF

EXPERIMENTAL SECTION

Materials. Aluminum isopropoxide, magnesium diacetate tetrahydrate, sodium fluoride, urea, dioctadecyldimethylammonium (abbreviated as 2C18) bromide, and hinokitiol were purchased from Wako Pure Chemical Industries, Ltd. Tetramethylammonium (abbreviated 13843

DOI: 10.1021/acs.langmuir.5b03874 Langmuir 2015, 31, 13842−13849

Article

Langmuir type and pseudo-trimolecular layer arrangements, respectively, to give hydrophobic smectites.47 Cation-exchange with TMA chloride was carried out through the reaction of the smectites (0.20 g) with an aqueous solution containing TMA chloride and 5 times the CECs of the smectites. After repeated washing with water, the resulting solids were dried at 323 K. The interlayer space was 0.5 nm for each smectite, showing microporous clay-organic hybrids, as reported previously.10 Cation-exchange reactions were also conducted using a Silica@Sapo sample (0.1 g) with 2C18 bromide (10 mg) using the above-described procedure. Exfoliation Test. Silica@Sapo samples (0.1 g) were immersed in water (10 mL of Elix Milli-Q) for 1 week. After the supernatant was thoroughly removed by a pipet, the resulting solid was analyzed using a SEM. In addition, the resulting solid was allowed to react with 2C18 bromide (10 mg) by the above-described procedure for estimation of the amount of the layered silicates released from the samples. Adsorption and Desorption of Hinokitiol in n-Hexane. The organically modified samples (0.1 g) were reacted with 10 mL of an nhexane solution of hinokitiol (0.12 mM) in a glass vessel for 1 day at 25 °C. The concentration of hinokitiol remaining in the supernatant was determined by UV−vis spectrometry. Blank samples containing 10 mL of the hinokitiol solution with no adsorbent were also prepared. After the amount of adsorbed hinokitiol was recorded, the supernatant was replaced with fresh n-hexane (10 mL) to start the desorption experiment. The concentration was continuously measured using UV− vis spectrometry until the desorption equilibrium was reached (within 24 h). Equipment. XRD patterns were obtained by a Rigaku RINT 2200 V/PC diffractometer (monochromatic Cu Kα radiation), operated at 20 mA and 40 kV. Solid-state 27Al MAS−NMR spectra were measured on a Bruker ASCEND 500 spectrometer at a resonance frequency of 130.31 MHz. Thermogravimetric differential thermal analysis (TGDTA) curves were recorded on a Rigaku TG8120 instrument at a heating rate of 10 K/min using α-alumina as the standard material. Nitrogen adsorption−desorption isotherms were measured at 77 K on a Belsorp-mini (BEL Japan, Inc.). Before the adsorption experiment, the samples were heat-treated at 393 K under a reduced pressure. SEM images were captured on a Hitachi SU-8000 field emission scanning electron microscope (operated at 1 kV) equipped with an HORIBA EMAX-5770Q EDX spectrometer (accelerating voltage of 15 kV) after the osmium plasma coating of the samples. Transmission electron micrographic (TEM) studies were conducted using a JEOL JEM-2010 transmission electron microscope, whose accelerated voltage was 200 kV. Zeta potential (in 2 mM−NaCl aqueous solution, pH = 4, 6, and 9 adjusted with HCl and NaOH) was measured using an Otsuka Electronics ELSZ-1000ZS. UV−Vis spectra were recorded on a Shimadzu UV-2450PC spectrophotometer.

■

Figure 1. SEM images of (a) pristine 1.0-μm-spherical silica particles, (b) [email protected], (c) [email protected], and (d) Silica1.0@ Sapo0.3−NaF samples.

RESULTS AND DISCUSSION

Crystal Growth of a Si4+−Al3+-Substituted Layered Silicate on Colloidal Spherical Silica. The hydrothermal reactions of 1.0-μm spherical silica (Figure 1a) in an aqueous solution with Al[OCH(CH3)2]3, (CH3COO)2Mg, and urea at 448 K caused fine, plate-like particles around the silica grains, irrespective of the amount of water added in the starting mixtures, as shown in the SEM images (Figure 1b,c). Increasing the amount of water from 0.3 to 2.5 mol resulted in the formation of large and thick plate-like particles. A smaller water amount in the hydrothermal reactions was important for maintaining the spherical shape reflected from the morphology of the initial silica. When NaF was added to 0.3 mol of water in the starting mixture, the plate-like particles also formed on the silica grains (Figure 1d). The cross-sectional TEM images of the typical samples ([email protected] and [email protected]−NaF) exhibit stacked layers around the silica core (Figure 2a,b, respectively).

Figure 2. Cross-sectional TEM images of the representative samples of (a) [email protected] and (b) [email protected]−NaF. Photographs (a′) and (b′) are the magnification images. The image (c) is the crosssectional TEM image of the pristine silica with a diameter of 1.0 μm.

Without NaF, only a few laminated layers with fluttering sheets exist off the layer, showing poor crystalline growth parallel to the c-axis, with a high aspect ratio. The stacked layers readily bend and partly grow toward the outside of the silica surfaces 13844

DOI: 10.1021/acs.langmuir.5b03874 Langmuir 2015, 31, 13842−13849

Article

Langmuir because of the flexibility of the silicate layer. Thin bundle crystals can be seen as plate-like protrusions forming on the spherical surfaces. The layered structures can act as a pseudoexfoliated nanosheet, even when immobilized on the spherical silica (Figure 2a). It can be seen that the addition of NaF to the starting mixture resulted in thick bundle sheets of the silicate layers (Figure 2b). The degree of stacking of the silicate layers is also seen in the (00l) diffractions in the powder XRD patterns (Figure 3a).

been pointed out that, in alkaline medium, the Mg(OH)2 layers incorporate some Al cations inducing a positive charge to this octahedral sheet.42 Because magnesium was the main element occupying the octahedral sheet, as determined from the XRD data, we conclude that a saponite-like layered structure (the ideal formula of the silicate is M(−x+y)[(Mg6−xAlx) (Si8−yAly)O20(OH)4](−x+y), M: interlayer cations) formed in the hydrothermal reactions with alkaline medium. The ζ potential of the samples dispersed in a 2 mM NaCl aqueous solution is summarized in Table 2, together with that Table 2. ζ-Potential of Pristine Silica and Representative Samples ζ-potential [mV]

Figure 3. (a) XRD patterns and (b) solid-state spectra of the Silica1.0@Sapo samples.

27

sample name

pH 4

6

9

pristine silica [email protected] [email protected]

−17.7 −45.4 −41.9

−61.4 −48.7 −50.9

−90.1 −53.2 −45.0

of the original silica particles. The ζ potential of the pristine silica depends on the pH; −17.7 (pH 4), −61.4 (pH 6), and −90.1 mV (pH 9), showing the variable charge of silica in water. In addition, we associate that the spherical particles would disperse well in the hydrothermal condition, because the ζ potential was negatively large in alkali medium. On the other hand, the ζ potential of the present samples was in a range of −40 to −50 mV and is independent of pH in the higher region (4 to 9), indicating that a permanent charge is dominant on the surfaces. As shown in the Z-contrast image (see the Supporting Information, Figure S2), a core−shell type structure was observed in every particle of the sample. Although a smooth spherical surface was exposed on the surface of some particles (Figure 1), layers stacking parallel to the silica surface were observed in the TEM images (see Supporting Information, Figure S3). These observations strongly suggest that the spherical silica particles are thoroughly covered with saponitelike layered silicates. In the TEM images, roughness is observed at the interface between the crystal layers and the residual silica core in each sample (Figure 2a′,b′), whereas the pristine silica was smooth (Figure 2c). Hydroxyl ions that evolved through the hydrolysis of urea would yield water-soluble (Mg, Al) (OH)x in an aqueous alkali medium containing Mg2+ and Al3+ ions, Si(OH)3O− and [Al(OH)4]− supplied from the partial dissolution of the spherical silica particles, and the aqueous Mg2+/Al3+ solution. These substances are the likely sources of the nuclei. Cooling the solution caused it to become supersaturated, so that the layered silicate crystals would grow on the silica surfaces by cancelation of the supersaturation around the silica particles. The dissolution of the silica substrate did not form micro/mesopores, because specific surface area of hydrothermally treated pristine silica in the absence of Al and Mg salts (3 m2/g determined by BET plots of the N2 adsorption isotherm shown in Supporting Information Figure S4) was close to the initial silica (4 m2/g), indicating that the observed roughness at the interface (Figure 2) is derived from a part of the fine crystals, rather than eroded silica surface. The specific surface area increased after the crystal growth to 59 m2/ g for [email protected] as an example. We empirically deduce that cancellation of the supersaturation would be superiorly used for the crystal growth than nuclei formation in this reaction condition, when the

Al MAS−NMR

While the diffraction because of the (001) plane was negligible for [email protected], the addition of NaF in the starting mixture emerged the basal plane at approximately 6° (2θ Cu Kα), developing crystallites toward the c-axis. This result is in accordance with the TEM observations (Figure 2). Increasing the amount of water in the hydrothermal reactions to 2.5 mol also led to emerging the (001) plane. We deduce that the presence of fluorine and the large amount of water in the hydrothermal reactions plays a role in developing the crystallite toward the c-axis. In addition, diffraction peaks ascribed to smectites at 34.6°(2θ Cu Kα) for (201) and 60.1° (2θ) for (060) appeared in all three samples. The d value of the (060) reflection observed in each sample was 0.154 nm, meaning that magnesium ions with all three samples in octahedral positions are coordinated with six oxygen atoms (or hydroxyls) in the smectite structure.48 Hydroxide minerals including brucite Mg(OH)2 and gibbsite Al(OH)3 were not observed. Solid-state 27 Al MAS−NMR measurement has been used to identify the coordination number of Al atoms from the chemical shifts, whether in the tetrahedral (AlIV) or the octahedral (AlVI) sheets.49 In the 27Al MAS−NMR spectra of the products (Figure 3b), the signals attributed to AlIV and AlVI were observed at approximately 70 and 10 ppm, respectively. It is thought that the added Al was distributed to both the tetrahedral and octahedral sheets in the layered silicate. It has 13845

DOI: 10.1021/acs.langmuir.5b03874 Langmuir 2015, 31, 13842−13849

Article

Langmuir

morphology, as is shown in the SEM image, suggesting a possible application as an HPLC column packing material.51 Cation-Exchange Reactions of Silica1.0@Sapo Samples. A cationic surfactant (2C18) was used to examine cationexchange reactions with the interlayer-exchangeable cations in the layered silicates on the silica spheres. An exothermic peak was observed in a temperature range of 512−973 K in each DTA curve of the products, and this peak was accompanied by mass loss in the corresponding TG curve, as listed in Table 3. From the mass loss, the amount of 2C18 adsorbed was determined to be 0.10−0.14 mmol/g (Table 3). The basal spacing of the layered silicate in the [email protected] sample increased when the 2C18 exchange occurred, from 1.5 to 2.2 nm (see Supporting Information, Figure S5a). A methylene group is 0.4 nm thick, therefore, the interlayer space of 1.2 nm, determined by subtracting the thickness of the silicate layer (1.0 nm) from the observed basal spacing, indicated that 2C18 is arranged in pseudo-trimolecular layers. In such arrangement, the amount of 2C18 intercalated into smectites is estimated to be 0.60 mmol per gram of layered silicates.52 Considering the amount of 2C18 adsorbed (0.14 mmol/g) in the core−shell samples, it is thought that the portion of the saponite-like layered silicate in the [email protected] sample is 23%. We applied the same calculation to the [email protected]−NaF system: from the amount of 2C18 adsorbed (0.12 mmol/g, Table 3) and basal spacing of 2.6 nm (see Supporting Information, Figure S5b), which corresponds to 0.80 mmol of 2C18 per gram, 15% of the sample is layered silicate. The calculation of the “shell” portion was difficult in the Silica1.0@ Sapo0.3, because the diffraction peak of the (001) plane did not appear (Figures 3a, and 4c). The ideal formula of the silicate in the shell part is [(Mg6−xAlx) (Si8−yAly)O20(OH)4](−x+y). The Mg/Al molar ratio in the as-made core−shell samples was determined by an energy-dispersive X-ray (EDX) analysis and the estimated negative layer charge density (−x+y) from the XRD and TGDTA analyses, as listed in Table 3. From the molar ratio of Mg/ Al = (6 − x)/(x + y) and negative layer charge density, we estimated the distribution of Al in octahedral (Mg6−xAlx) or tetrahedral (Si8−yAly) substitutions, as summarized in Table 3. This shows that the tetrahedral Si4+−Al3+ substitution, which is a characteristic of saponite, is dominant. Exfoliation Test. We examined exfoliation and release of the silicate layers using the cation-exchange reactions with 2C18. Before the cation-exchange, the [email protected] and the [email protected]−NaF samples were immersed in water for a week, whether silicate layers were released from the hybrids or not. As a result, the platy surface was preserved after the immersion as exemplified by SEM observations (see the Supporting Information, Figure S6). However, approximately 20% of the silicate layers was released, because the amount of adsorbed 2C18 decreased to 80% in each case; the 2C18 amounts decreased to 0.08 and 0.10 mmol/g for Silica1.0@ Sapo0.3 and [email protected]−NaF, respectively (TG-DTA curves of the 2C18-exchanged forms are shown in Supporting Information Figure S7.). We assume that electrostatically interacted silicate layers, including the layer stacking, can exfoliate in water from the hybrids. Adsorption/Desorption of Hinokitiol in n-Hexane. Hinokitiol is an antibacterial agent43 and has been reported for adsorption and controlled release tests on organically modified smectites.53 As shown in Table 4, hinokitiol was adsorbed on microporous TMA-modified smectites (KF and

amount of water in the starting mixture was increased (lower concentration of the precursor) in the heterogeneous nucleation reactions. As revealed in the SEM (Figure 1) and XRD results, the crystallite size, including the stacks of the silicate layers, enlarged when the amount of added water was increased from 0.3 to 2.5 mol. On the other hand, fluorine in the hydrothermal syntheses of layered silicates has been reported to act as a mineralizing agent, and the role of fluorine in determining the local structures has been discussed.42 It has been postulated that nuclei form at an initial step of the reactions including condensation of silica monomers onto previously formed octahedral sheets. Some of these initial nuclei redissolve and recondense to form true nuclei of layered silicates.42,50 In the present heterogeneous nucleation reactions, the presence of fluorine resulted in enhanced layer stacking in the direction of the c-axis. We assume that the relative concentration of the true nuclei would be higher in the presence of fluorine ([email protected]−NaF), as a result of fluorine acting as an accelerator of the true nuclei formation during aging for 3 days, possibly leading to enhanced crystallization. A different grain size (2.6 μm) of the initial silica sphere was used to examine how the surface area affects the crystallite growth of the saponite-like silicates, where the Al:Mg:Si:urea:water molar ratio in the starting mixture was the same as the [email protected] fabrication. The SEM image of the hydrothermal product is shown in Figure 4a. Irrespective of the grain

Figure 4. (a) SEM image, (b) solid-state 27Al MAS−NMR spectrum, and (c) XRD pattern of the [email protected] sample.

size of the silica, fine, plate-like particles formed around the silica grains. Diffraction peaks ascribable to saponite were also observed in the XRD pattern (Figure 4c) of the sample. The intensity was almost the same as the [email protected] sample (Figure 3a), suggesting that the crystallite size was independent of the grain size of the pristine silica. The sample also contained AlIV located in the tetrahedral sheet, as confirmed by the 27Al MAS−NMR spectrum (Figure 4b). Using the silica size of 2.6 μm resulted in maintaining the spherical shape of the whole 13846

DOI: 10.1021/acs.langmuir.5b03874 Langmuir 2015, 31, 13842−13849

Article

Langmuir Table 3. Chemical Compositions of the Samples TG-DTA sample name [email protected] [email protected] [email protected]− NaF

XRD

mass loss [%]

amount 2C18 adsorbed [mmol/g]

d (001) [nm]

negative layer chargea (−x+y)

Mg/Al (molar)b

chemical composition of the silicate anionb

7.1 5.3 6.2

0.14 0.10 0.12

2.2 N/A 2.6

0.5 0.6

3 2

[(Mg5.3Al0.7) (Si7Al1)O20(OH)4]−0.5 [(Mg5.2Al0.8) (Si7Al1)O20(OH)4]−0.6

N/A: unknown because of the absence of (001) reflection in the XRD. aThe negative layer charge was calculated from the amount of 2C18 adsorbed [mmol/g], and Mw of layered silicate (758 g/mol), where x and y values in the ideal formula are assumed to be zero. bRaw data of EDX analysis and the calculation in detail are summarized in Supporting Information Table S1.

Table 4. Summary of the Results on the Adsorption/Desorption Tests on/from Organoclays in n-Hexanea 2C18 b

interlayer cation host samples

Q

[email protected] [email protected]−NaF SA (Sumecton SA) KF (Kunipia F)

0.37 0.16 0.66 0.58

TMA 2

a

b

R

0.98 0.96 0.95 1.0

0.53 1.2 1.3 0.74

0.998 0.997 0.989 0.995

Q

a

b

R2

0.0063 0.69 0.11

0.89 0.90

0.81 0.64

0.959 0.966

Approximate curves in Figure 5 were obtained by fitting these plots to a exponential growth equation: C/C0 = a(1 − exp(−bt)). bQ: amount of adsorbed hinokitiol from n-hexane solution [mmol/g]. a

Figure 5. Time course of the hinokitiol desorption from (a) 2C18-, and (b) TMA-modified samples in n-hexane at 298 K. Curves in these figures were superimposed to an exponential growth equation: C/C0 = a(1 − exp(−bt)).

SA) from n-hexane, while only a slight amount was adsorbed on a representative sample ([email protected]). The organophilic 2C18-exchanged samples adsorbed hinokitiol from n-hexane, as summarized in Table 4, indicating that hinokitiol prefers the phase of these organically modified samples. The NaF addition to [email protected] in the hydrothermal reactions maintained the activity for the adsorption of hinokitiol. However, [email protected] did not adsorb any hinokitiol in the reaction period (data not shown), probably due to the slow diffusion of hinokitiol in the dense, large platy crystals of the layered silicates. The desorption of hinokitiol in fresh n-hexane was subsequently examined. Figure 5 shows the relative concentration of the supernatant with a time course. It is clearly shown that the desorption equilibrium is reached within 24 h. The kinetics were evaluated by superimposing to an exponential rising curve:

C /C0 = a(1 − exp( −bt ))

(1)

where C is the concentration of hinokitiol, C0 is the equilibrium concentration, and a and b are the constants of the exponential curve. The exponential parameters are listed in Table 4. The b value directly correlates with the kinetics of the hinokitiol desorption. The desorption rate in the 2C18 system can be regarded in the order of SA > [email protected]−NaF ≫ KF > [email protected]. This trend was observed in the microporous (nonexfoliated) TMA-modified system. SA is composed of significantly smaller platy layers (ca. 20 nm),36,54 compared with that of KF (0.1−1.0 μm).36 The smaller size of the individual platy layer (a−b plane) in SA led to the high rate desorption. It can be deduced that the desorption rate was strongly affected by the size of the plates, rather than the mobility of the silicate layers (including microscopic exfoliation and macroscopic swelling in n-hexane). In the present hybrid particles, the platy layers in [email protected] are as large as 13847

DOI: 10.1021/acs.langmuir.5b03874 Langmuir 2015, 31, 13842−13849

Langmuir

■

that of KF. The fluorine medium in the hydrothermal synthesis led to the smaller platy layers as in SA. The organophilic layered silicates on the present colloidal silica spheres should behave as exfoliated layers in n-hexane; however, the layers flaking off from the silica substrates would be prevented, owing to firm immobilization via electrostatic and hydrophobic interactions between the layers, and possibly covalent bonding with the silica surfaces. A merit of using this hybrid material as a drug support is that through proper handling it can be used to resolve the practical problems in cosmetics and ointments, thereby preventing from leaving the residue of the exfoliated silicate layers on human skin. Controlled release materials with such characteristics can be provided by the careful design of the fine crystals of the layered silicates on the spherical silica by changing the starting chemical composition, especially the amount of fluorine and water.

AUTHOR INFORMATION

Corresponding Author

*Phone: +81-26-269-5414, Fax: +81-26-269-5424, E-mail:to [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by JSPS KAKENHI (Grant-in-Aid for Scientific Research, Grant 26810121), the Cosmetology Research Foundation, and by the JGC-S Scholarship Foundation.

■

REFERENCES

(1) Alberti, G.; Bein, T.; Eds.; Solid-State Supramolecular Chemistry: Two- and Three-Dimensional Inorganic Networks; Pergamon: Oxford, 1996. (2) Ruiz-Hitzky, E.; Aranda, P.; Darder, N.; Ogawa, M. Biohybrid Silicate Based Materials: Molecular vs. Block-assembling Bottom−up Processes. Chem. Soc. Rev. 2011, 40, 801−828. (3) Kuroda, K.; Shimojima, A.; Kawahara, K.; Wakabayashi, R.; Tamura, Y.; Asakura, Y.; Kitahara, M. Utilization of Alkoxysilyl Groups for the Creation of Structurally Controlled Siloxane-Based Nanomaterials. Chem. Mater. 2014, 26, 211−220. (4) Whittingham, M. S.; Jacobson, A. J.; Eds.; Intercalation Chemistry; Academic Press: New York, 1982. (5) Ogawa, M.; Kuroda, K. Photofunctions of Intercalation Compounds. Chem. Rev. 1995, 95, 399−438. (6) Okada, T.; Sohmiya, M.; Ogawa, M. Photochromic Intercalation Compounds. Struct. Bonding (Berlin, Ger.) 2015, 166, 177−211. (7) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular Sieves; Academic Press: London, 1978. (8) Mitchell, I. V., Ed.; Pillared Layered Structures: Current Trends and Applications; Elsevier: London, 1990. (9) Okada, T.; Ide, Y.; Ogawa, M. Organic-Inorganic Hybrids Based on Ultrathin Oxide Layers -Designed Nanostructures for Molecular Recognition. Chem. - Asian J. 2012, 7, 1980−1992. (10) Ogawa, M.; Takahashi, M.; Kato, C.; Kuroda, K. Oriented Microporous Film of Tetramethylammonium Pillared Saponite. J. Mater. Chem. 1994, 4, 519−523. (11) Sasai, R.; Iyi, N.; Fujita, T.; Arbeloa, L. F.; Martinez, M. V.; Takagi, K.; Itoh, H. Luminescence Properties of Rhodamine 6G Intercalated in Surfactant/Clay Hybrid Thin Solid Films. Langmuir 2004, 20, 4715−4719. (12) Ishida, Y.; Kulasekharan, R.; Shimada, T.; Takagi, S.; Ramamurthy, V. Efficient Singlet−Singlet Energy Transfer in a Novel Host−Guest Assembly Composed of an Organic Cavitand, Aromatic Molecules, and a Clay Nanosheet. Langmuir 2013, 29, 1748−1753. (13) Inukai, K.; Hotta, Y.; Taniguchi, M.; Tomura, S.; Yamagishi, A. Formation of a Clay Monolayer at an Air−Water Interface. J. Chem. Soc., Chem. Commun. 1994, 959−959. (14) Kleinfeld, E. R.; Ferguson, G. S. Stepwise Formation of MultiLayered Nanostructural Films from Macromolecular Precursors. Science 1994, 265, 370−373. (15) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. Formation of Ultrathin Multilayer and Hydrated Gel from Montmorillonite and Linear Polycations. Langmuir 1996, 12, 3038−3044. (16) Lotsch, B. V.; Ozin, G. A. Clay Bragg Stack Optical Sensors. Adv. Mater. 2008, 20, 4079−4084. (17) Yan, Y.; Bein, T. Molecular Recognition on Acoustic Wave Devices: Sorption in Chemically Anchored Zeolite Monolayers. J. Phys. Chem. 1992, 96, 9387−9393. (18) Ha, K.; Lee, J.; Lee, J.; Yoon, K. Facile Assembly of Zeolite Monolayers on Glass, Silica, Alumina, and Other Zeolites Using 3Halopropylsilyl Reagents as Covalent Linkers. Adv. Mater. 2000, 12, 1114−1117.

■

CONCLUSION We demonstrated that the fine crystals of Al-containing (saponite-like) layered silicates thoroughly covered the surfaces of monodisperse spherical amorphous silica particles (1.0 and 2.6 μm) through heterogeneous nucleation reactions. This was achieved by the hydrothermal reactions of the silica spherical particles with Al and Mg ions under alkaline conditions at 448 K in a rotating Teflon-lined autoclave. While the size of the starting spherical silica was independent of the crystal size, the concentration of the precursors of the layered silicates was shown to be important in the crystallinity, as determined from XRD, TEM, and hinokitiol adsorption/desorption. Crystal growth was superior to nucleation when the amount of water in the starting mixture was increased. Fluorine in the hydrothermal reactions played a role in developing the crystallite growth toward the c-axis and diminishing the sheet size (a−b plane) to a lower aspect ratio. We found a clue to the precise crystal design of the fine biocompatible layered silicates supported on amorphous silica substrates by varying the amounts of water and mineralizing agents (e.g., fluorine) of the precursors, for developing highly functional pharmaceutical and cosmetic uses.

■

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03874. XRD pattern and SEM image of the hydrothermal product at 423 K (Figure S1), Z-contrast image of [email protected] (Figure S2), cross-sectional TEM images of Silica1.0@Sapo samples (Figure S3), N2 adsorption−desorption isotherms of pristine silica, hydrothermally treated silica in the absence of the Mg and Al salts, [email protected] (Figure S4), change in the XRD patterns of [email protected] and Silica1.0@ Sapo0.3−NaF occurring by the cation-exchange with 2C18 (Figure S5), SEM images of the Silica1.0@Sapo sample particles after immersion in water for 1 week (Figure S6), TG-DTA curves of 2C 18 -exchanged Silica1.0@Sapo samples (Figure S7), and results of EDX analysis for representative Silica1.0@Sapo samples (Table S1) (PDF) 13848

DOI: 10.1021/acs.langmuir.5b03874 Langmuir 2015, 31, 13842−13849

Article

Langmuir

Using Monodispersed Silica Particles. J. Phys. Chem. C 2012, 116, 21864−21869. (39) Okada, T.; Suzuki, A.; Yoshido, S.; Minamisawa, M. H. Crystal Architectures of a Layered Silicate on Monodisperse Spherical Silica Particles Cause the Topochemical Expansion of the Core-Shell Particles. Microporous Mesoporous Mater. 2015, 215, 168−174. (40) Kloprogge, J. T.; Komarneni, S.; Amonette, J. E. Synthesis of Smectite Clay Minerals: A Critical Review. Clays Clay Miner. 1999, 47, 529−554. (41) Carrado, K. A.; Decarreau, A.; Petit, S.; Bergaya, F.; Lagaly, G. In Handbook of Clay Science (Developments in Clay Science, Vol. 1); Bergaya, F., Theng, B. K. G., Lagaly, G., Eds.; Elsevier: Amsterdam, 2006; pp 115−139. (42) Jaber, M.; Miehé-Brendlé, J. Synthesis, Characterization and Applications of 2:1 Phyllosilicates and Organophyllosilicates: Contribution of Fluoride to Study the Octahedral Sheet. Microporous Mesoporous Mater. 2008, 107, 121−127. (43) Inamori, Y.; Shinohara, S.; Tsujibo, H.; Okabe, T.; Morita, Y.; Sakagami, Y.; Kumeda, Y.; Ishida, N. Antimicrobial Activity and Metalloprotease Inhibition of Hinokitiol-Related Compounds, the Constituents of Thujopsis dolabrata S. and Z. hondai MAK. Biol. Pharm. Bull. 1999, 22, 990−993 and references are therein.. (44) Kloprogge, J. T.; Breukelaar, J.; Jansen, J. B. H.; Geus, J. W. Development of Ammonium-saponites from Gels with Variable Ammonium Concentration and Water Content at Low Temperatures. Clays Clay Miner. 1993, 41, 103−110. (45) Ogawa, M.; Aono, T.; Kuroda, K.; Kato, C. Photophysical Probe Study of Alkylammonium-Montmorillonites. Langmuir 1993, 9, 1529− 1533. (46) Ogawa, M.; Wada, T.; Kuroda, K. Intercalation of Pyrene into Alkylammonium-Exchanged Swelling Layered Silicates: The Effects of the Arrangements of the Interlayer Alkylammonium Ions on the States of Adsorbates. Langmuir 1995, 11, 4598−4600. (47) Lagaly, G. Characterization of Clays by Organic Compounds. Clay Miner. 1981, 16, 1−21. (48) Brindley, G. W., Brown, G., Eds. Crystal Structures of Clay Minerals and Their X-ray Identification; Mineralogical Society: London, 1980. (49) Woessner, D. E. Characterization of Clay Minerals by 27Al Nuclear Magnetic Resonance Spectroscopy. Am. Mineral. 1989, 74, 203−215. (50) Baird, T.; Cairns-Smith, A. G.; MacKenzie, D. W. An Electron Microscope Study of Magnesium Smectite Synthesis. Clay Miner. 1973, 10, 17−26. (51) Okada, T.; Kumasaki, A.; Shimizu, K.; Yamagishi, A.; Sato, H. Application of Hectorite-silica Core-shell Particles as a Packing Material for Chromatographic Resolution. J. Chromatogr. Sci., submitted for publication, 2015. (52) Ogawa, M.; Matsutomo, T.; Okada, T. Preparation of Hectoritelike Swelling Silicate with Controlled Layer Charge Density. J. Ceram. Soc. Jpn. 2008, 116, 1309−1313. (53) Ishii, R.; Imai, Y.; Wada, M.; Ebina, T.; Hanaoka, T.; Mizukami, F. Adsorption and Desorption Behaviors of Flavor Molecules into a Microporous Pillared Clay Mineral and the Application to Flavor Capsule Composites. Appl. Clay Sci. 2006, 33, 99−108. (54) Okada, T.; Kato, T.; Yamaguchi, T.; Sakai, T.; Mishima, S. Layered Clay Aerogels by a Freeze-Drying Process for a PlatinumSupported Catalyst. Ind. Eng. Chem. Res. 2013, 52, 12018−12024.

(19) Kulak, A.; Lee, Y.; Park, Y.; Yoon, K. Orientation-Controlled Monolayer Assembly of Zeolite Crystals on Glass and Mica by Covalent Linkage of Surface-Bound Epoxide and Amine Groups. Angew. Chem., Int. Ed. 2000, 39, 950−953. (20) Suzuki, S.; Teshima, K.; Kiyohara, M.; Kamikawa, H.; Yubuta, K.; Shishido, T.; Oishi, S. Growth of Ultralong Potassium Titanate Whiskers by the KCl Flux Method with Metallic Titanium Materials. CrystEngComm 2012, 14, 4176−4180. (21) Chen, H.; Zhang, F.; Fu, S.; Duan, X. Orientation-Controlled Monolayer Assembly of Zeolite Crystals on Glass and Mica by Covalent Linkage of Surface-Bound Epoxide and Amine Groups. Adv. Mater. 2006, 18, 3089−3093. (22) Zhang, F.; Zhao, L.; Chen, H.; Xu, S.; Evans, D. G.; Duan, X. Corrosion Resistance of Superhydrophobic Layered Double Hydroxide Films on Aluminum. Angew. Chem., Int. Ed. 2008, 47, 2466−2469. (23) Zhang, T.; Zhou, Y.; He, M.; Zhu, Y.; Bu, X.; Wang, Y. Biomimetic Fabrication of Hierarchically Structured LDHs/ZnO Composites for the Separation of Bovine Serum Albumin. Chem. Eng. J. 2013, 219, 278−285. (24) Zhang, T.; Zhou, Y.; Bu, X.; Xue, J.; Hu, J.; Wang, Y.; Zhang, M. Bio-inspired Fabrication of Hierarchically Porous Mg−Al Composites for Enhanced BSA Adsorption Properties. Microporous Mesoporous Mater. 2014, 188, 37−45. (25) Pérez-Carvajal, J.; Aranda, P.; Berenguer-Murcia, A.; CazorlaAmorós, D.; Coronas, J.; Ruiz-Hitzky, E. Nanoarchitectures Based on Layered Titanosilicates Supported on Glass Fibers: Application to Hydrogen Storage. Langmuir 2013, 29, 7449−7455. (26) Barrer, R. M. Shape-Selective Sorbents Based on Clay Minerals: A Review. Clays Clay Miner. 1989, 37, 385−395. (27) Takagi, S.; Shimada, T.; Ishida, Y.; Fujimura, T.; Masui, D.; Tachibana, H.; Eguchi, M.; Inoue, H. Size-Matching Effect on Inorganic Nanosheets: Control of Distance, Alignment, and Orientation of Molecular Adsorption as a Bottom-Up Methodology for Nanomaterials. Langmuir 2013, 29, 2108−2119. (28) Okada, T.; Seki, Y.; Ogawa, M. Designed Nanostructures of Clay for Controlled Adsorption of Organic Compounds. J. Nanosci. Nanotechnol. 2014, 14, 2121−2134. (29) Theng, B. K. G. The Chemistry of Clay−Organic Reactions; Adam Hilger: London, 1974. (30) Handbook of Clay Science (Developments in Clay Science, Vol. 1); Bergaya, F., Theng, B. K. G.. Lagaly, G., Eds.; Elsevier: Amsterdam, 2006. (31) Urabe, K.; Sakurai, H.; Izumi, Y. Cation-Exchange Synthetic Saponite as a ’Heat-Stable’ Acidic Clay Catalyst. J. Chem. Soc., Chem. Commun. 1988, 29, 1520−1521. (32) Okada, T.; Oguchi, J.; Yamamoto, K.; Shiono, T.; Fujita, M.; Iiyama, T. Organoclays in Water Cause Expansion That Facilitates Caffeine Adsorption. Langmuir 2015, 31, 180−187. (33) Nakamura, Y.; Yamagishi, A.; Iwamoto, T.; Koga, M. Adsorption Properties of Montmorillonite and Synthetic Saponite as Packing Materials in Liquid-Column Chromatography. Clays Clay Miner. 1988, 36, 530−536. (34) Kakegawa, N.; Kondo, T.; Ogawa, M. Variation of ElectronDonating Ability of Smectites as Probed by Photoreduction of Methyl Viologen. Langmuir 2003, 19, 3578−3582. (35) Ogawa, M.; Kawai, R.; Kuroda, K. Adsorption and Aggregation of a Cationic Cyanine Dye on Smectites. J. Phys. Chem. 1996, 100, 16218−16221. (36) Miyamoto, N.; Kawai, R.; Kuroda, K.; Ogawa, M. Adsorption and Aggregation of a Cationic Cyanine Dye on Layered Clay Minerals. Appl. Clay Sci. 2000, 16, 161−170. (37) Egawa, T.; Watanabe, H.; Fujimura, T.; Ishida, Y.; Yamato, M.; Masui, D.; Shimada, T.; Tachibana, H.; Yoshida, H.; Inoue, H.; Takagi, S. Novel Methodology To Control the Adsorption Structure of Cationic Porphyrins on the Clay Surface Using the “Size-Matching Rule. Langmuir 2011, 27, 10722−10729. (38) Okada, T.; Yoshido, S.; Miura, H.; Yamakami, T.; Sakai, T.; Mishima, S. Swellable Microsphere of a Layered Silicate Produced by 13849

DOI: 10.1021/acs.langmuir.5b03874 Langmuir 2015, 31, 13842−13849