Organoclays in Water Cause Expansion That Facilitates Caffeine

Dec 6, 2014 - In contrast, when the adsorbents were BA–KFs, caffeine adsorption (0.68–0.74 mmol/g) increased the Δd by 0.1 nm. The pore size dete...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/Langmuir

Organoclays in Water Cause Expansion That Facilitates Caffeine Adsorption Tomohiko Okada,*,† Junpei Oguchi,† Ken-ichiro Yamamoto,§ Takashi Shiono,§ Masahiko Fujita,∥ and Taku Iiyama‡,∥ †

Department of Chemistry and Material Engineering, Faculty of Engineering, and ‡Center for Energy and Environmental Science, Shinshu University, Wakasato 4-17-1, Nagano 380-8553, Japan § Kirin Company, Ltd., Namamugi 1-17-1, Tsurumi-ku, Yokohama 230-8628, Japan ∥ Department of Chemistry, Faculty of Science, Shinshu University, Asahi 3-1-1, Matsumoto 390-0802, Japan S Supporting Information *

ABSTRACT: This study investigates the adsorption of caffeine in water on organically modified clays (a natural montmorillonite and synthetic saponite, which are smectite group of layered clay minerals). The organoclays were prepared by cation-exchange reactions of benzylammonium and neostigmine with interlayer exchangeable cations in the clay minerals. Although less caffeine was uptaken on neostigmine-modified clays than on raw clay minerals, uptake was increased by adding benzylammonium to the clays. The adsorption equilibrium constant was considerably higher on benzylammonium-modified saponite (containing small quantities of intercalated benzylammonium) than on its montmorillonite counterpart. These observations suggest that decreasing the size and number of intercalated cations enlarges the siloxane surface area available for caffeine adsorption. When the benzylammonium−smectite powders were immersed in water, the intercalated water molecules expanded the interlayer space. Addition of caffeine to the aqueous dispersion further expanded the benzylammonium−montmorillonite system but showed no effect on benzylammonium−saponite. We assume that intercalated water molecules were exchanged with caffeine molecules. By intercalating benzylammonium into smectites, we have potentially created an adaptable two-dimensional nanospace that sequesters caffeine from aqueous media.

■

INTRODUCTION

for these systems in selective adsorption, separation, and catalysis.12 Among the expandable layered inorganic solids, the smectite group of layered clay minerals has been most extensively studied.14,15 Smectites are composed of ultrathin (ca. 1.0 nm) crystalline silicate layers separated by hydrated interlayers.15,16 The cations in the interlayer spaces that compensate the negatively charged silicate layers are readily exchanged with various organic cations. Cation-exchange reactions with a relatively small size of organoammonium cations, such as tetramethylammonium and trimethylphenylammonium ions, create nanospaces surrounded by cation−silicate layer structures. This technique has been used to produce inorganic−organic microporous clays.12,17−19 In nanospace engineering, the structure of the pillaring agents and charge density of the smectites can be varied to control the adsorption of nonionic aromatic compounds.20−26 However, although the relationship between structure and absorptive properties affects the adsorption behavior of organic molecules in organoclay

Adsorption of specific molecular species onto solid surfaces is currently exploited in a wide range of scientific and practical applications such as removing toxic compounds and recovering desired substances. The periodic structures of appropriately designed nanostructures are ideally suited for selective adsorption, and well-established microporous and mesoporous solids (e.g., zeolites and nanoporous silicas) have been extensively researched for this purpose.1−5 Another class of nanostructured materials with regularly arranged organic moieties is porous inorganic−organic hybrid solids (e.g., metal−organic frameworks: MOFs and porous coordination polymers: PCPs). In these compounds, the desired organic molecules are concentrated by interaction with moieties.6−8 On the other hand, inorganic ultrathin layers act as useful nanospace scaffolds because they incorporate organic moieties as so-called “pillars” into their two-dimensional expandable interlayer spaces.9−12 In such inorganic−organic hybrid systems, the nanospace can be tuned by spatially controlling the number and size (molecular structure) of the organic moieties, which affects their spatial distribution.12,13 Nanostructural versatility encourages us to seek further applications © 2014 American Chemical Society

Received: September 17, 2014 Revised: December 1, 2014 Published: December 6, 2014 180

dx.doi.org/10.1021/la503708t | Langmuir 2015, 31, 180−187

Langmuir

Article

further purification. Kunifil-D36, Kunifil-HY, and Kunifil-B1 are commercially available organoclays based on KF (supplied by Kunimine Ind. Co., Ltd.), in which contains 2Cn (Scheme 1c), CnOH (Scheme 1d), and BA (Scheme 1e), respectively. Preparation of Adsorbents. BA−smectite intercalation compounds were prepared by cation exchange reactions as follows. Smectite powder (2.0 g) was dispersed in BA−HCl aqueous solution (40 mL) for 1 day at room temperature. BA−HCl was added at quantities of one or one-half the CECs. The resulting solids were repeatedly washed with deionized water until a negative AgNO3 test was obtained. The solid products were then collected by centrifugation (1400g, 15 min) and dried at 50 °C to obtain the intercalation compounds BA−KF (x) or BA−SA (x), where x is the amount of BA adsorbed to the smectites (mmol/g). CONH−smectite intercalation compounds were prepared based on a previous report.28 Briefly, the smectites (2.0 g) were reacted with an aqueous solution containing twice the CEC of CONH to proceed quantitative cation-exchange reactions. After repeated washing with deionized water and centrifugation, the resulting solids were dried at 50 °C. The amount of adsorbed CONH, obtained from mass losses of CONH in thermogravimetric (TG) curves, was 1.2 mmol/g for CONH−KF and 0.76 mmol/g for CONH−SA. These values, which are close to the CECs of smectites, quantify the ion exchange. The basal spacings determined by XRD patterns (Figure S2 in Supporting Information) were 1.73 nm (CONH−KF) and 1.51 nm (CONH− SA). The interlayer space (Δd) was obtained by subtracting the thickness of the silicate layer (0.96 nm) from the observed basal spacings to be 0.77 nm (CONH−KF) and 0.55 nm (CONH−SA), reflecting the microporous structures (organically pillared clay).28 Adsorption of Caffeine from Aqueous Solution. Adsorbents (0.05 g) were reacted with 25 mL of aqueous caffeine solution (0.051−2.1 mM) in a polypropylene tube closed with a polypropylene cap for 1 day at 25 °C. The adsorption equilibrium reached within 24 h, judging from a time course experiment as exemplified in Figure S1 of the Supporting Information. To estimate the adsorption of caffeine to the vessel, blank samples containing 25 mL of aqueous caffeine solution with no adsorbents were also prepared. After centrifugation (1400g, 15 min), the sample was filtered (Merck Millipore, Millex-GP Filter 0.22 μm, PES) to separate the adsorbents from the supernatants. The concentration of caffeine remaining in the supernatant was determined by HPLC. Transmission-Based XRD Measurement. Adsorbents (0.05 g) were mixed with 25 mL of aqueous solution of caffeine (2.1 mM) or water in a polypropylene tube closed with a polypropylene cap for 1 day at 25 °C. A 1 mm thick slit-shaped cell consisting acrylonitrile− butadiene−styrene copolymer (ABS) resin with pair of poly(ethylene terephthalate) films (t = 50 μm) as X-ray window was used for in situ X-ray measurements of aqueous dispersions. Without centrifugation, the aqueous dispersion was packed into the cell. The X-ray diffraction (XRD) were measured at 298 K by a Rigaku RINT Ultima III with transmission geometry using angle-dispersion diffractometer and parallel Cu Kα radiation beam, operated at 40 mA and 40 kV. Equipment. Powder XRD patterns were obtained by a Rigaku RINT 2200 V/PC diffractometer (monochromatic Cu Kα radiation), operated at 20 mA and 40 kV. Thermogravimetric−differential thermal analysis (TG−DTA) curves were recorded on a Rigaku TG8120 instrument at a heating rate of 10 °C/min with α-alumina as the standard material. HPLC was performed on a JASCO LC-2000 Plus equipped with a UV−vis detector (λ = 275 nm) by using an octadecylsilane column (Mightysil RP-18) at 40 °C. CHN analysis was performed by a Yanaco CHN corder MT-5.

series, the nanostructures of the adsorbents in aqueous media have not been reported. We expect that species occluded in the interlayer space will be rearranged in the presence of both adsorbates and solvents, potentially enhancing the adsorption (intercalation) of the target molecules. For example, irradiating interlayered cationic azobenzene with UV light encourages phenol intercalation.27 Molecular environmental changes and external triggers present as important means of controlling the adsorptive properties of organic molecules, since they realize expandable and/or adaptable interlayer spaces. Here, we report the adsorption and intercalation behaviors of caffeine molecules (Scheme 1a) on expandable inorganic− Scheme 1. Molecular Structures of (a) Caffeine, (b) Benzylammonium (BA), (c) Dialkyldimethylammonium (2Cn), (d) Alkylbis(2-hydroxyethyl)methylammonium (CnOH), and (e) Neostigmine (CONH)

organic hybrid smectites in water. In our preliminary experiments, a benzylammonium (BA; Scheme 1b)-modified smectite was found to be the most effective adsorbent for the caffeine adsorption in aqueous solution among commercially available organoclays containing organoammonium ions as dialkyldimethylammonium (2Cn; Scheme 1c), alkylbis(2hydroxyethyl)methylammonium (CnOH; Scheme 1d), and BA. Thus, we have paid attention to organic cations containing aromatic ring for using organically modified smectites (a natural montmorillonite and synthetic saponite) through cationexchange reactions with BA and neostigmine (CONH, which has been reported for effective adsorption of phenylphenol;28 Scheme 1e). Transmission X-ray diffraction (XRD) using an angle-dispersion diffractometer29 was used for pursuing the nanostructural change in the aromatic ammonium−smectites upon the adsorption in aqueous media. We will discuss a role of the nanostructures, which correlate the adsorption behavior of caffeine.

■

EXPERIMENTAL SECTION

Reagents and Materials. The clays used in this study were natural Na−montmorillonite (Kunipia F, JCSS-3101; hereafter abbreviated as KF; (Na0.53Ca0.09)0.71+[Al3.28Fe0.31Mg0.43](Si7.65Al0.35)O20(OH)4)0.71−) and a synthetic Na−saponite (Sumecton SA, JCSS3501; hereafter abbreviated as SA; (Na0.49Ca0.14)0.77+[Mg5.97Al0.03](Si7.20Al0.80)O20(OH)4)0.77−). Both clays, supplied by Kunimine Ind. Co., Ltd., are reference clay samples of the Clay Science Society of Japan. The cation exchange capacities (CECs) of KF and SA are 1.19 and 0.71 mequiv/g clay, respectively. Benzylamine hydrochloride (abbreviated as BA-HCl) and neostigmine bromide (CH3)3N+(C6H4)CO2N(CH3)2Br− (abbreviated as CONH-Br−) were purchased from Merck Schuchardt OHG (Germany) and Wako Chemical Industries, Ltd. (Japan), respectively, and used as received. Caffeine (C8H10N4O2, Mw = 194.2 g/mol, Wako Chemical Industries, Ltd.) was used without

■

RESULTS AND DISCUSSION Preparation of BA−Smectite Intercalation Compounds. In the DTA curves of the products obtained by reacting smectites with BA−HCl, exothermic peaks were observed in the approximate temperature range 200−600 °C. These peaks accompanied the mass lost by oxidative decomposition of BA in the corresponding TG curve (TG− 181

dx.doi.org/10.1021/la503708t | Langmuir 2015, 31, 180−187

Langmuir

Article

Table 1. Characteristics of BA-Intercalated Smectites chemical composition sample

C [mass %]

N [mass %]

amount adsorbed BA [mmol/g]

basal spacing [nm]

interlayer space (Δd) [nm]

BA−SA (0.86) BA−SA (0.54) BA−KF (1.2) BA−KF (0.83)

4.0 2.6 5.1 3.7

0.77 0.54 0.94 1.0

0.86 0.54 1.2 0.83

1.31 1.30 1.41 1.32

0.35 0.34 0.45 0.36

Figure 1. XRD patterns of (a) BA−KF and (b) BA−SA intercalation compounds.

DTA curves of these products are shown in the Supporting Information, Figure S3). The amounts of adsorbed BA, determined from the C content in the CHN analysis, are listed in Table 1. When BA−HCl was added at an amount equaling CECs (1.19 mequiv/g KF and 0.71 mequiv/g SA), a similar amount was adsorbed (1.2 mmol/g KF and 0.86 mmol/g SA). The larger amount of BA adsorbed on SA than either CEC or the amount taken with reference to CEC may be due to surface adsorption of BA on fine particles of SA, in addition to the cation exchange. When the amount of added BA−HCl was reduced to half the CEC, the adsorbed amount on KF and SA decreased to 0.83 and 0.54 mmol/g, respectively. Figure 1 shows XRD patterns of the products and original clays. When reacted with BA−HCl, the basal spacing of SA was slightly increased from 1.26 nm to 1.30 or 1.31 nm, whereas that of KF was increased from 1.23 to 1.32 nm (0.83 mmol/g) or 1.41 nm (1.2 mmol/g) (Table 1). The interlayer space (Δd), obtained by subtracting the thickness of the silicate layer (0.96 nm) from the observed basal spacing, is also listed in Table 1. On the basis of the interlayer space (Δd) and BA thickness (ca. 0.3 nm), we consider that the adsorbed BA molecules arrange as a monomolecular layer with their molecular planes parallel to the silicate layers. When 1.2 mmol/g BA was adsorbed on KF, the interlayer space was 0.45 nm, indicating that the BA forms an interdigitated monolayer with its molecular long axis inclined to the silicate layer. Although spatial distribuition of BA in plane is not clear at present, the KF interlayer possibly does not segregate BA from exchangeable Na ions, judging from symmetric shape of the (001) diffraction peak. Therefore, it appears that the amount of adsorbed BA can be varied to yield intercalation compounds with different spatial distributions of BA. Adsorption of Caffeine from Aqueous Solutions. We first examined adsorption of caffeine on commercially available organoclays of KF modified with 2Cn, CnOH, and BA from aqueous solution. The adsorbed amount of caffeine is listed in

Table S1 of the Supporting Information. The adsorption on BA-modified KF (Kunifil B1) was superior to raw KF, while the amount on the hydrophobic organoclays containing 2Cn and CnOH was smaller compared with raw KF and the BA counterpart. Thus, we investigated adsorptive properties of smectites (KF and SA) modified with the organoammoniumcontaining aromatic ring. Figure 2 shows the adsorption isotherms of caffeine adsorbed to the BA-intercalated smectites from aqueous solutions. The isotherms of adsorption to CONH-modified smectites and raw clays are also shown. Adsorption to CONH−KF yields a type S isotherm as shown in the magnification profile of Figure 2a while other isotherms to the organoclays are type L, according to the Giles30 classification. In Table 2, the efficiency of caffeine uptake at a lower concentration (0.26 mM) is summarized. It is clearly revealed that BA-modified smectites are effective for the adsorption because of the efficiency of caffeine uptake more than 96%. The adsorption isotherms were fitted to the Langmuir equation,31 given by Ce/Q = (1/KLQ m) + (1/Q m)Ce

(1)

where Qm and KL are constants related to the maximum adsorbed amount and binding energy, respectively. Q and Ce denote the adsorbed amounts of caffeine and equilibrium concentrations, respectively. The Langmuir parameters derived from the adsorption isotherms are presented in Table 2. Jaynes and Boyd21 reported that the Langmuir model adequately describes adsorption to organically pillared clays, provided that the r2 value (correlation coefficient of the Langmuir equation) exceeds 0.89. Since the correlation coefficients (r2) of the Langmuir plots of the isotherms in this study are greater than 0.90, the adsorption to organoclays is empirically consistent with the Langmuir model. Both the raw smectites adsorbed caffeine from aqueous solution, indicating that adsorption likely occurs by interactions with the siloxane surface of the silicate layer.21 CONH−KF 182

dx.doi.org/10.1021/la503708t | Langmuir 2015, 31, 180−187

Langmuir

Article

Figure 2. Adsorption isotherms of caffeine in aqueous solution adsorbed to organoclays and raw smectites: (a) KF and (b) SA. Initial concentration of aqueous caffeine solutions was increased in the order of 0.051, 0.26, 0.51, 1.0, and 2.1 mM.

lower concentration. The Langmuir adsorption constant (KL = 1.6 × 102 L/g) was especially large in BA−SA (0.54). Nanostructural Change of Organoclays in Water and Aqueous Caffeine Solution. XRD patterns of aqueous dispersions of organoclays in the absence or presence of caffeine are shown in Figure 3. The powder XRD patterns before caffeine adsorption are also shown. The interlayer spaces (Δd) of the organoclays obtained from the basal spacings in water are listed in Table 3. Water immersion increased the Δd of the organoclays, even in the absence of caffeine. This result indicates that water molecules were intercalated into the organoclays, with consequent changes in the orientation of the preintercalated CONH and BA cations (swelling). Since CONH is larger than BA, the Δd of the CONH-modified clays (0.80 nm) is larger than that of their BA counterparts (0.64−0.68 nm). In water, these cations should arrange in one of two ways: as interdigitated monolayers with the molecular long axis inclined to the silicate layer (Scheme 2a) or as bimolecular layers with molecular planes parallel to the silicate layers (Scheme 2b). These arrangements are inferred from the Δd, thickness of BA (ca. 0.3 nm), and size of the methylene groups in CONH (ca. 0.4 nm). Once the water molecules have been intercalated, the nanostructures could become stabilized

Table 2. Langmuir Parameters and Efficiency of Caffeine Adsorption Langmuir parameters

sample BA−SA (0.86) BA−SA (0.54) BA−KF (1.2) BA−KF (0.83) CONH−KF CONH−SA KF SA

Qm [mmol/g]

KL [10 L/g]

0.69 0.50 0.68 0.74

6.9 16 1.2 0.70

0.27 0.54 0.61

1.5 0.33 0.79

r

2

0.986 0.995 0.966 0.901 0.133 0.991 0.649 0.816

efficiency of uptake of caffeine at a lower conc (0.26 mM) [%] 99 99 96 97 2 59 38 53

does not adsorb caffeine because its surface is occupied by CONH ions. The preintercalation of BA into KF increased the affinity between the clay and caffeine and increased the adsorption capacity of caffeine relative to raw KF. The amount of adsorbed caffeine was independent of the amount of preintercalated BA. In the SA intercalated with BA, the clay surface was modified to allow effective caffeine adsorption at a 183

dx.doi.org/10.1021/la503708t | Langmuir 2015, 31, 180−187

Langmuir

Article

Figure 3. Changes in the XRD patterns of caffeine-adsorbing organoclays used in this study. The patterns of the aqueous dispersions were measured by a transmission technique in the presence and absence of caffeine (2.1 mM). The powder diffractions were recorded using a conventional method at room temperature with relative humidity ca. 50%.

adsorbed caffeine exceeded the increase in pore volume due to the contacting aqueous caffeine solution, suggesting that water molecules in the interlayer space were exchanged with adsorbed caffeine molecules. The Δd of CONH−KF (0.80 nm) was unaltered by addition of caffeine to water, suggesting a molecular sieving effect (negligible adsorption of caffeine) as displayed in the isotherm of Figure 2. Similarly, in the CONH−SA system, the Δd (0.80 nm) was close to that of CONH−SA immersed in water alone. The intercalated CONH in SA can be regarded as a pillar in the inorganic−organic porous clay that exchanges some of its interlayer water molecules with caffeine. Even when intercalated in large amounts (0.69 mmol/g), caffeine did not expand the interlayer space of BA−SA (0.86). Intercalation of 0.50 mmol/g of caffeine increased the Δd of BA−SA (0.54) by only 0.04 nm. In contrast, when the adsorbents were BA−KFs, caffeine adsorption (0.68−0.74 mmol/g) increased the Δd by 0.1 nm. The pore size determined by the interlayer space (vertical direction) and/or the spatial density of BA and CONH (lateral direction) may be concerned for the observed additional expansion after the caffeine adsorption. Upon the caffeine adsorption, IR spectral change of caffeine ascribed to primidine and imidazole rings was observed. FT-IR absorption spectra of caffeine molecule in CONH−SA and BA−SA (0.86) (see Figure S4 in Supporting Information)

by interactions between the aromatic rings of the cations (π−π interactions) and by electrostatic interactions between the organic cations and negative silicate layers. The micropore volumes created by the CONH/BA cations and silicate layers were calculated from the microscopic geometry of the organoclays.24,32 Relevant parameters are the interlayer space (Δd), amount of immobilized cation, and the ideal surface area of smectite (750 and 690 m2/g for KF and SA,24 respectively). The results are summarized in Table 3. The pore volume ranges from 0.15 to 0.20 cm3/g depending on the amount and volume of the cations used (the van der Waals volumes of BA and CONH are 0.10 and 0.21 nm3, respectively). When the organoclays are immersed in water, their micropores may become filled with water molecules. As listed in Table 3, the Δd values of organoclays immersed in aqueous caffeine solutions (2.1 mM) were also obtained by the XRD transmission technique, and the calculated pore volume in the presence of caffeine is also listed in this table. The volume of adsorbed caffeine, calculated by multiplying the caffeine volume (0.17 nm3/molecule) by the maximum adsorbed amount of caffeine in Figure 2, ranged from 0.024 to 0.067 cm3/g (Table 3). In all cases, the volume of caffeine in the interlayer spaces was smaller than the calculated pore volume, indicating that the interlayer micropores were occupied by both water and caffeine molecules (Table 3). The volume of 184

dx.doi.org/10.1021/la503708t | Langmuir 2015, 31, 180−187

Langmuir

Article

We deduced that since BA is a smaller pillaring agent than CONH, more sites are available in the micropores of the interlayer space for caffeine adsorption. Figure 4 plots the

Table 3. Geometric Estimates of Caffeine Adsorption to Organoclays interlayer space (Δd) [nm]

pore volumea [cm3/g]

sample

in water

in aq caffeine solnb

in water

in aq caffeine solnb (A)

BA−SA (0.86) BA−SA (0.54) BA−KF (1.2) BA−KF (0.83) CONH−SA CONH−KF

0.68 0.67 0.64 0.64 0.80 0.80

0.68 0.71 0.74 0.74 0.80 0.80

0.18 0.20 0.17 0.19 0.18 0.15

0.18 0.21 0.20 0.23 0.18 0.15

volume of adsorbed caffeine (B)c [cm3/g] 0.067 0.051 0.052 0.044 0.024 N/A

B/A [%] 37 24 26 19 13

Pore volume [cm3/g] is calculated as (SΔd/2) − (NAQcationVcation), where S, NA, Qcation, Δd, and Vcation are the ideal surface area of smectite [nm2/g], Avogadro’s number [1/mmol], amount of adsorbed cation [mmol/g], the interlayer space of the sample in aqueous caffeine solutions [nm], and molecular volume of the cation [nm3], respectively. The ideal surface areas (S) of KF and SA are 7.5 × 1020 and 6.8 × 1020 nm2/g, respectively. Vcation is assumed as the van der Waals volume; 0.10 nm3 for BA and 0.21 nm3 for CONH, obtained by MOPAC simulation (WinMostar). bInitial concentration of the aqueous caffeine solution is 2.1 mM. cTo obtain the volume, the maximum amount of adsorbed caffeine in Figure 2 was multiplied by the van der Waals molecular volume of caffeine (0.17 nm3), obtained by a WinMostar MOPAC simulation. a

Figure 4. Plots of Langmuir constant (KL) versus the amount of caffeine adsorbed to BA combined with SA (circles) and KF (squares).

relationship between the Langmuir adsorption constant (KL, Table 2) and the amount of adsorbed BA. In the KF system, the KL value increases as more BA is adsorbed, suggesting that adsorption occurs by interactions between the BA and caffeine. On the other hand, the KL of BA−SA (0.54) is the maximum value observed among the test adsorbents. Caffeine may also interact with the siloxane surface of SA in the micropores formed by BA. The lower KL of BA−KF may also be attributable to the close packing of BA, evidenced by the 0.10 nm increase in Δd upon caffeine adsorption. The crystalline sizes (along the stacking direction of layers) obtained by applying the Debye−Scherrer equation to the (001) diffractions in XRD data (Figure 3) indicated larger sizes of the KF-based intercalation compounds in aqueous dispersions than those of the SA analogues (detailed results are shown in Table S2 of the Supporting Information). Because the adsorption equilibrium has already reached for both organically modified KF and SA (see Figure S1 in Supporting Information) in the present reaction period, we deduce that the difference in the crystalline size is a minor contribution for the observed differences of the adsorption characteristics depending on the nature of the used smectites. The 0.1 nm increase in Δd induced by caffeine adsorption on BA−KF may derive from changes in the inclination angle of BA as shown in Figure 3 and/or Table 3. On the other hand, caffeine adsorption only slightly increased the Δd of BA−SA. The powder XRD patterns of BA−SA (0.86) before and after caffeine adsorption with no excess water are shown in Figure 5. Although caffeine intercalation with 0.66 mmol/g increased the basal spacing from 1.31 to 1.41 nm, the basal spacing in aqueous caffeine solution (2.1 mM, the adsorbed amount: 0.66 mmol/g) was increased to 1.64 nm. The reduction in the former case is probably due to deintercalation of water by drying. The basal spacing of 1.41 nm (or Δd = 0.45 nm) in the absence of excess water indicates that BA is covered by an interdigitated monolayer, as shown in Scheme 2a. This structural change implies an expandable interlayer space of the BA-modified clays. Moreover, the inclination angle of BA alters when both water and caffeine are intercalated. Therefore, the arrangement shown in Scheme 2a is a plausible nanostructure of BA-modified smectites in aqueous dispersion.

Scheme 2. Schematic Drawing of Possible Arrangement of BA−Smectite Intercalation Compounds (For Example, BAKF (1.2)) in Water, (a) Interdigitated Monolayer Coverage with the Molecular Axis Inclined to the Silicate Layers, and (b) Bilayer Arrangement with Their Molecular Planes Parallel to the Silicates

exhibit the bands due to stretching vibrations of the aromatic rings33 shifted to higher frequency from 1548 to 1558 cm−1 in these adsorbents, indicating partial donation of electron pairs from the siloxane−oxygens to antibonding π orbitals of caffeine. The adsorption of aromatic molecules from aqueous solution onto organically pillared smectites containing aromatic rings has been previously reported.21,23−26,28,34,35 The intercalation of cationic pillaring agents alters the surface properties of smectites, enhancing the adsorption of organic molecules. The adsorption capacity of phenylphenol is higher in CONH−SA than in CONH−KF because of the lower coverage of CONH on SA.28 In the present study, the preintercalation of CONH into KF and SA reduced the adsorption capacity of caffeine. This result can be explained by the reduced surface area of the siloxane, which is partially covered by CONH. 185

dx.doi.org/10.1021/la503708t | Langmuir 2015, 31, 180−187

Langmuir

Article

the FT-IR spectra of CONH-SA and BA−SA (0.86) upon the adsorption of caffeine (Figure S4).This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Phone +81-26-269-5414, Fax +81-26-269-5424, e-mail [email protected] (T.O.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS T.O. acknowledges JSPS KAKENHI (Grant-in-Aid for Scientific Research, Young Scientists (B), Grant 26810121).

■

Figure 5. XRD patterns of powder BA−SA (0.86) samples in the absence and presence of caffeine and of aqueous BA−SA (0.86) dispersion containing caffeine (2.1 mM).

We also consider that in this arrangement, the molecular planes of BA may readily interact with the surfaces of the caffeine molecules. Thus, three main reasons are proposed for the enhanced ability of BA-modified smectites to sequester caffeine molecules from aqueous solution: (1) BA creates nanospaces that readily accommodate caffeine molecules, (2) caffeine interacts with the exposed siloxane surface of the interlayer, and (3) BA interacts with the caffeine adsorbates. Moreover, in the aqueous phase, the interlayer is expanded (swelled) by the intercalating water molecules, increasing the available nanospace for caffeine adsorption.

■

CONCLUSIONS This study investigated the adsorption of caffeine over waterdispersed BA- and CONH-modified smectites (KF and SA). The CONH−KF intercalation compound did not effectively adsorb caffeine from aqueous solution as the CONH molecules were large and plentiful in the interlayer space, showing a possible size exclusion effect. Cation exchange between the smectites and BA manifests as increase in the Langmuir equilibrium constants that in turn witnessed an enhanced adsorbed amount of caffeine. Among the tested adsorbents, BA−SA containing 0.54 mmol/g of BA is the most efficient adsorbent at lower concentrations of caffeine solution. The interlayer space of the BA−SA intercalation compound expanded in water, creating nanospaces suitable for accommodating caffeine molecules. Besides interacting with BA, the siloxane in the silicate layers plays an important role in caffeine adsorption. The cooperative effects on caffeine adsorption were triggered by the intercalation of water molecules.

■

REFERENCES

(1) Photochemistry in Organized & Constrained Media; Ramamurthy, V., Ed.; VCH Publishers Inc.: New York, 1991. (2) Thomas, J. K. Physical Aspects of Photochemistry and Radiation Chemistry of Molecules Adsorbed on Silica, γ-Alumina, Zeolites, and Clays. Chem. Rev. 1993, 93, 301−320. (3) Stein, A. Advances in Microporous and Mesoporous Solids Highlights of Recent Progress. Adv. Mater. 2003, 15, 763−775. (4) Ruiz-Hitzky, E.; Aranda, P.; Darder, N.; Ogawa, M. Hybrid and Biohybrid Silicate Based Materials: Molecular vs. Block-Assembling Bottom-Up Processes. Chem. Soc. Rev. 2011, 40, 801−828. (5) Ariga, K.; Vinu, A.; Yamauchi, Y.; Li, Q.; Hill, J. P. Nanoarchitectonics for Mesoporous Materials. Bull. Chem. Soc. Jpn. 2012, 85, 1−32. (6) Yaghi, O. M.; Li, H.; Davis, C.; Richardson, D.; Groy, T. L. Synthetic Strategies, Structure Patterns, and Emerging Properties in the Chemistry of Modular Porous Solids. Acc. Chem. Res. 1998, 31, 474−484. (7) Kitagawa, S.; Kitaura, R.; Noro, S. Functional Porous Coordination Polymers. Angew. Chem., Int. Ed. 2004, 43, 2334−2375. (8) Fujita, S.; Inagaki, S. Self-Organization of Organosilica Solids with Molecular-Scale and Mesoscale Periodicities. Chem. Mater. 2008, 20, 891−908. (9) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular Sieves; Academic Press: London, 1978. (10) Ogawa, M.; Kuroda, K. Photofunctions of Intercalation Compounds. Chem. Rev. 1995, 95, 399−438. (11) Handbook of Layered Materials; Auerbach, S. M., Carrado, K. A., Dutta, P. K., Eds.; Marcel Dekker: New York, 2004. (12) 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. (13) 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. (14) Theng, B. K. G. The Chemistry of Clay-Organic Reactions; Adam Hilger: London, 1974. (15) Handbook of Clay Science (Developments in Clay Science, Vol. 1); Bergaya, F., Theng, B. K. G., Lagaly, G., Eds.; Elsevier: Amsterdam, 2006. (16) van Olphen, H. An Introduction to Clay Colloid Chemistry, 2nd ed.; Wiley-Interscience: New York, 1977. (17) Barrer, R. M. Shape-Selective Sorbents Based on Clay Minerals: A Review. Clays Clay Miner. 1989, 37, 385−395. (18) Xu, S.; Sheng, G.; Boyd, S. A. Use of Organiclays in Pollution Abatement. Adv. Agron. 1997, 59, 25−62. (19) Okada, T.; Seki, Y.; Ogawa, M. Designed Nanostructures of Clay for Controlled Adsorption of Organic Compounds. J. Nanosci. Nanotechnol. 2014, 14, 2135−2147.

ASSOCIATED CONTENT

S Supporting Information *

Time course of the adsorbed amount of caffeine from aqueous solution on organoclays (Figure S1), XRD patterns of CONH− smectite intercalation compounds (Figure S2), TG-DTA curves of BA−smectite intercalation compounds (Figure S3), amount adsorbed caffeine and efficiency of the uptake on commercially available organoclays (Table S1), crystalline sizes of organoclays fabricated in the present study (Table S2), and change in 186

dx.doi.org/10.1021/la503708t | Langmuir 2015, 31, 180−187

Langmuir

Article

(20) Mortland, M. M.; Shaobai, S.; Boyd, S. A. Clay-Organic Complexes as Adsorbents for Phenol and Chlorophenols. Clays Clay Miner. 1986, 34, 581−587. (21) Jaynes, W. F.; Boyd, S. A. Hydrophobicity of Siloxane Surfaces in Smectites as Revealed by Aromatic Hydrocarbon Adsorption from Water. Clays Clay Miner. 1991, 39, 428−436. (22) Lawrence, M. A. M.; Kukkadapu, R. K.; Boyd, S. A. Adsorption of Phenol and Chlorophenols from Aqueous Solution by Tetramethylammonium- and Tetramethylphosphonium-Exchanged Montmorillonite. Appl. Clay Sci. 1998, 13, 13−20. (23) Okada, T.; Ogawa, M. 1,1′-Dimethyl-4,4′-bipyridiniumSmectites As a Novel Adsorbent of Phenols from Water through Charge-Transfer Interactions. Chem. Commun. 2003, 1378−1379. (24) Okada, T.; Morita, T.; Ogawa, M. Tris(2,2′-bipyridine)ruthenium(II)-Clays as Adsorbents for Phenol and Chlorinated Phenols from Aqueous Solution. Appl. Clay Sci. 2005, 29, 45−54. (25) Okada, T.; Matsutomo, T.; Ogawa, M. Nanospace Engineering in Methylviologen Modified Hectorite-like Layered Silicates with Varied Layer Charge Density for the Adsorbents Design. J. Phys. Chem. C 2010, 114, 539−545. (26) 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. (27) Okada, T.; Watanabe, Y.; Ogawa, M. Photocontrol of the Adsorption Behavior of Phenol for Azobenzene-Montmorillonite Intercalation Compound. Chem. Commun. 2004, 320−321. (28) Seki, Y.; Ogawa, M. The Removal of 2-Phenylphenol from Aqueous Solution by Adsorption onto Organoclays. Bull. Chem. Soc. Jpn. 2010, 83, 712−715. (29) Futamura, R.; Iiyama, T.; Hamasaki, A.; Ozeki, S. Small- and Large-Angle X-ray Scattering Studies of Nanometer-Order Sulfuric Acid Solution in Carbon Micropores. Chem. Lett. 2012, 41, 159−161. (30) Giles, C. H.; Smith, D.; Huitson, A. Studies in Adsorption: Part Xl. A System of Classification of Solution Adsorption Isotherms, and Its Use in Diagnosis of Adsorption Mechanisms and in Measurement of Specific Surface Areas of Solids. J. Chem. Soc. 1960, 111, 3973− 3993. (31) Langmuir, I. The Adsorption of Gases on Plane Surfaces of Glass, Mica and Platinum. J. Am. Chem. Soc. 1918, 40, 1361−1402. (32) Ogawa, M.; Takahashi, M.; Kato, C.; Kuroda, K. Oriented Microporous Film of Tetramethylammonium Pillared Saponite. J. Mater. Chem. 1994, 4, 519−523. (33) Kesimli, B.; Topacli, A.; Topacli, C. An Interaction of Caffeine and Sulfamethoxazole: Studied by IR Spectroscopy and PM3 Method. J. Mol. Struct. 2003, 645, 199−204. (34) Stevens, J. J.; Anderson, S. J.; Boyd, S. A. FTIR Study of Competitive Water-Arene Sorption on TMA- and TMPA-Montmorillonites. Clays Clay Miner. 1996, 44, 88−95. (35) Jaynes, W. F.; Vance, G. F. Sorption of Benzene, Toluene, Ethylbenzene and Xylene (BTEX) Compounds by Hectorite Clays Exchanged with Aromatic Organic Cations. Clays Clay Miner. 1999, 47, 358−363.

187

dx.doi.org/10.1021/la503708t | Langmuir 2015, 31, 180−187