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Kinetics of Interlayer Expansion of a Layered Silicate Driven by Caffeine Intercalation in the Water Phase Using Transmission X‑ray Diffraction Tomohiko Okada,*,† Takumi Yoshida,§ 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 § Department of Chemistry, Faculty of Science, Shinshu University, Asahi 3-1-1, Matsumoto 390-0802, Japan S Supporting Information *

ABSTRACT: The kinetics of caffeine uptake into the interlayer nanospace of silicate nanosheets modified with benzylammonium (BA) was evaluated by in situ monitoring the basal spacing in aqueous media using transmission X-ray diffraction. An interlayer spacing of 0.58 nm in water before caffeine uptake indicates a monomolecular layer of BA and a few water layers in each interlayer. The interlayer space expanded by 0.10 nm upon caffeine uptake (intercalation) and saturated even in the presence of excess caffeine. Time-course profiles of the interlayer spacing and the uptake amount after injection of caffeine into the water slurry were obtained. At the initial period, the plot for the basal spacing was located above that for the adsorbed amount, suggesting that the rate of the interlayer spacing change was faster than that to attain the adsorption equilibrium. A first-order kinetic simulation fitted to the profile also indicates that the basal spacing included a rapid expansion of 0.08 nm within a few minutes and a slow expansion of 0.02 nm over several hours. Regarding the slow component, the rate constant for the basal spacing was lower than that for the amount of caffeine adsorbed, meaning that a steady-state basal spacing is reached after the adsorption equilibrium.

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INTRODUCTION Strong specific binding of molecules onto solid surfaces is important from scientific and practical viewpoints, with uses such as selectively removing harmful compounds and recovering desired substances. The periodic structures of appropriately designed nanostructures are ideally suited to selective adsorption.1−8 Intercalation of organic species into inorganic layered solids is a method of preparing regulated nanostructures to accommodate extra molecules into their twodimensional expandable interlayer spaces9−13 because of swellable and flexible supramolecular assemblies that allow insertion of their extra guest species. The interlayer expansion of the hybrids often occurs depending on the pre-intercalated organic constituents and the supplied solvents (adsorbates): (1) infinite expansion (delamination14,15 and exfoliation16,17) to form a dispersion of nanosheets, (2) lyotropic liquid crystallinity that is stabilized by a high aspect (lateral lengthto-thickness) ratio of the inorganic nanosheets (nanosheet colloids),18−20 and (3) a slight expansion (up to ∼1 nm) while maintaining the layered structures of their intercalation compounds.21,22 In such inorganic-based soft matter systems, regulation of swelling (as in the case of 3) should be of importance in the recognition of a specific molecule, which can be achieved by selecting layered solids with appropriate layer © XXXX American Chemical Society

charge density and pre-intercalated organic constituents for tuning the interlayer nanospace by spatially controlling the number, size, and geometry. The basal spacing (or interlayer space) measured by X-ray diffraction (XRD) is often used to discuss how does the interlayer nanostructures reflect the recognition phenomena. Change in the basal spacing has been pursued using the data before uptake of molecules into the interlayer space and after attaining adsorption equilibrium.13,23−26 Time-course profiles both of the uptake amount and the basal spacing should also be of importance in fundamental understanding of the intercalation; however, few studies are found in the literature on combining in situ monitoring the structural evolution with adsorption data. The smectite group of layered clay minerals has been extensively studied as a class of expandable layered inorganic solids27−29 because they comprise ultrathin (ca. 1.0 nm) crystalline silicate layers and hydrated exchangeable cations. Cation-exchange reactions of the interlayer cations with a smallsize organoammonium (e.g., tetramethylammonium) create nanospaces in the interlayer space,1,9 which can accommodate Received: April 5, 2017 Revised: June 22, 2017 Published: June 23, 2017 A

DOI: 10.1021/acs.jpcb.7b03200 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

[Si7.65Al0.35]O20(OH)4)0.71−) was supplied by Kunimine Ind. Co., Ltd., reference clay sample of the Clay Science Society of Japan. The cation-exchange capacity (CEC) is 1.19 meq/g clay. Benzylamine hydrochloride (abbreviated as BA−HCl) was purchased from Merck Schuchardt OHG (Germany) and was used as received. Caffeine (C8H10N4O2, Mw = 194.2 g/mol, Wako Chemical Industries, Ltd.) was used without further purification. Preparation of BA−KF Intercalation Compound. The BA−KF intercalation compound was prepared by cationexchange reactions based on a previous report.26 KF powder (2.0 g) was dispersed in BA−HCl aqueous solution (40 mL) for 1 day at room temperature. BA−HCl was added in a quantity equal to the CEC. The resulting solid was washed repeatedly with deionized water until a negative AgNO3 test was obtained. A solid product was then collected by centrifugation (1400g, 15 min) and dried at 120 °C for 3 h to obtain the BA−KF intercalation compound. The amount of pre-intercalated BA in KF, determined by carbon content (5.1 mass %, Yanaco CHN corder MT−5), was 1.2 meq/g clay, which is close to the CEC of KF, confirming the ion exchange. Adsorption Isotherm of Caffeine from the Aqueous Solution. A powder of the BA−KF intercalation compound (0.019 g) was immersed into 100 mL of aqueous caffeine solution (0.05−21 mM) in a polypropylene tube sealed with a polypropylene cap for 8 h at 298 K. To estimate the adsorption of caffeine to the vessel, blank samples containing 100 mL of aqueous caffeine solution with no adsorbents were also prepared. The concentration of caffeine remaining in the supernatant was determined using a UV−vis spectrophotometer (Shimadzu UV−2550, λmax = 273 nm). Transmission XRD Measurement. A 1 mm thick slitshaped cell comprising an acrylonitrile−butadiene−styrene copolymer resin with a pair of polyethylene terephthalate films (t = 50 μm) as the X-ray window was used for the in situ X-ray measurements of aqueous dispersions. The XRD measurements were performed at 298 K by a Rigaku RINT Ultima III with transmission geometry using an angledispersion diffractometer and a parallel Cu Kα radiation beam, operated at 30 mA, 40 kV. The XRD profile was recorded in the 2θ range 3.0−7.5° (at a scan rate of 5.0°/min) with 10 accumulations. The background scattering of a blank (the X-ray window substance) was subtracted from the profile of each sample before data analysis was carried out.44 An equilibrium test was performed to exhibit the dependence of the initial concentration of an aqueous caffeine solution on the interlayer expansion of BA−KF upon caffeine intercalation after aging for 8 h at 298 K. An aqueous dispersion (30 μL) containing 0.019 g BA−KF was packed into the cell, followed by adding an aqueous caffeine solution (70 μL) to adjust the concentration in the range from 0 to 111 mM. A kinetics test was performed to display the time-course change (for 8 h) in the interlayer expansion of BA−KF upon caffeine intercalation at 298 K. An aqueous dispersion of BA−KF powder (0.019 g) in deionized water (30 μL) was packed into the cell and aged at 298 K for several hours. Then, an aqueous caffeine solution (70 μL) was injected into the cell. The amount of caffeine added into the cell was 11 μmol. The time-course change in the adsorbed amount was obtained using a UV−vis spectrophotometer by monitoring the concentration of a 100 mL aqueous caffeine solution containing the same amount of caffeine (11 μmol) after addition of 0.019 g BA−KF.

nonionic organic molecules by size exclusion effect in the interlayer space without interlayer expansion.10,13,23,24,30−33 On the other hand, the expansion may occur upon the intercalation of nonionic organic compounds, when the organic cations adopt a planar geometry.25,34−37 In the water phase, such structural flexibility is potentially adaptable for the uptake of a target molecule; intercalation of water molecules through hydrogen bonding expands the interlayer spaces and thus possibly creates nanostructures to accommodate a target. Selective removal of caffeine (Scheme 1a) in tea has attracted Scheme 1. Molecular Structures of (a) Caffeine and (b) BA

attention especially in the beverage industry because of diminishing their negative effects on health, such as sleep disorder,38 elevation of blood pressure,39 and risk of fetal growth disorder.40 Smectites were found to strongly interact with caffeine;26,41,42 therefore, we have devoted the surface chemistry of caffeine-smectites. We reported that a benzylammonium (BA)-modified (Scheme 1b) smectite was found to be the most effective adsorbent for the caffeine adsorption in aqueous solution among commercially available organoclays containing organoammonium ions as quaternary long-chain alkylammonium and BA.26 Transmission XRD with an angledispersion diffractometer43,44 and small-angle X-ray scattering have been recently adopted to investigate the interlayer expansion of organically functionalized smectites in water26,45 and in aqueous solutions of caffeine.26 BA-exchanged smectites swelled in liquid water by ∼0.2 nm and expanded further by 0.1 nm in an aqueous caffeine solution. It has been assumed that the interlayer expansion occurring by cointercalation of water molecules facilitates the caffeine intercalation even in excess amounts of water.26 The chemical and dynamic processes of the intercalation behavior are worth investigating to understand the molecular recognition mechanisms. Here, we report the dynamics of the interlayer expansion caused by intercalation of caffeine molecules into an expandable BA−smectite intercalation compound in water using the transmission XRD with an angle-dispersion diffractometer. A natural montmorillonite, known as Kunipia F (abbreviated as KF), was used as the host material, which possesses a high aspect ratio and a large number of silicate layers stacked along the direction of the c axis. The high crystallinity is advantageous to kinetically trace the structural changes using our transmission XRD because the basal plane can be recorded in as little as a few minutes. The dynamic behavior of the caffeine intercalation into the BA−KF intercalation compound in the aqueous phase is, for the first time, discussed by combining time-course plots of the amount of adsorbed caffeine with the basal spacing.

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EXPERIMENTAL SECTION Reagents and Materials. Natural Na-montmorillonite (KF, JCSS−3101; (Na 0.53 Ca 0.09 ) 0.71+ ([Al 3.28 Fe 0.31 Mg 0.43 ]B


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RESULTS AND DISCUSSION Adsorption of Caffeine from Aqueous Solutions. Figure 1 shows the adsorption isotherm of caffeine adsorbed

strong adsorbate−adsorbent interactions. According to the literature,26,41,42 the raw KF adsorbed caffeine from aqueous solutions to yield a type-L adsorption isotherm. Caffeine molecules thus likely interact with the siloxane surface of the silicate layer (partial donation of electron pairs from the siloxane oxygens to the antibonding π orbitals of caffeine).26 Pre-intercalation of BA into KF increased the affinity between the clay and caffeine, increasing the adsorption capacity of caffeine relative to raw KF. Nanostructural Change of BA−KF in Aqueous Caffeine Solution. The basal spacing of BA−KF was determined by the powder XRD pattern (Figure 2a) to be

Figure 1. Adsorption isotherm of caffeine in aqueous solution adsorbed to BA−KF. The fitting curve indicates the Langmuir− Freundlich mixed model (eq 3) with weighing coefficient α = 0.2.

to the BA−KF intercalation compound from aqueous solutions. The adsorption yields a type-L isotherm, according to the Giles46 classification. The isotherm is modeled by the Freundlich equation,47 given by W = KFCe1/ n

(1) 1/n

where KF ((L/mmol) ) and n are the constants pertaining to the maximum adsorbed amount and shape of the isotherm (empirical adsorption characteristics), respectively (Table 1).

Figure 2. Changes in the XRD patterns of BA−KF during caffeine adsorption. The powder diffraction pattern was recorded after being dried at 393 K for 3 h under vacuum (a). The patterns of the aqueous dispersions were measured by a transmission technique in the absence (b) and presence (c) of caffeine.

Table 1. Langmuir and Freundlich Parameters of Caffeine Adsorption to BA−KF Langmuir

1.42 nm. The interlayer space was calculated by subtracting the thickness of the silicate layer (0.96 nm) from the observed basal spacing to be 0.46 nm, indicating a monomolecular layer of BA, whose arrangement is inferred from the interlayer space and the thickness of BA (ca. 0.3 nm). The XRD pattern of an aqueous dispersion of BA−KF in the absence of caffeine is shown in Figure 2b. Water immersion in the absence of caffeine increased the basal spacing from 1.42 to 1.54 nm. This result indicates that water molecules intercalated into BA−KF to expand the interlayer space. Injection of caffeine (11 μmol) into the aqueous dispersion (0.019 g of BA−KF in 100 μL) increased the basal spacing by 0.1 nm (Figure 2c). The variations in the basal spacing of BA−KF at different amounts of caffeine adsorbed and different concentrations of aqueous caffeine solutions are displayed in Figure 3. Because the basal spacing linearly increased to 1.63 nm in the plots with increasing the adsorbed amount of caffeine (Figure 3a), the intercalation of caffeine plays a dominant role in the interlayer expansion. From 10 mg/g to the saturated amount of 13 mg/g, the expansion became less pronounced with increasing amounts of adsorbed caffeine, indicating rearrangement of the intercalated caffeine and BA. Regarding the initial concentration (Figure 3b), the basal spacing linearly increased to ca. 50 mM. Although an excess amount of caffeine is present in the range more than 50 mM, the basal spacing saturated to be 1.64 nm.

Freundlich 1/n

Wm [mmol/g]

KL [L/g]

n

KF [(L/mmol) ]

0.78

1.1

2.3

0.34

W and Ce denote the adsorbed amounts of caffeine and equilibrium concentrations, respectively. Because the Freundlich affinity index (n) exceeds 1, the curvature of the isotherm is convex upward (Figure 1). The adsorption isotherm was also fitted to the Langmuir equation,48 given by Ce/W = (1/KLWm) + (1/Wm)Ce

(2)

where Wm (mmol/g) and KL (L/g) are the constants related to the maximum adsorbed amount and binding energy, respectively. The Langmuir parameters derived from the adsorption isotherm are presented in Table 1. To include a higher-concentration region, a combination of Langmuir and Freundlich equations49 is useful to consider the adsorption model, given by Wmix = αWL + (1 − α)WF ,

(0 ≤ α ≤ 1)

(3)

where α and Wmix (mmol/g) are the weighting coefficient and the adsorption capacity, respectively. Good fitting was obtained, as shown in Figure 1; α of 0.2 means that the Freundlich model would govern the caffeine adsorption, which is reflected from C


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Figure 3. Plots of the basal spacing of BA−KF vs (a) the amount of adsorbed caffeine and (b) the initial concentration of the aqueous caffeine solution.

Hence, the adsorption of caffeine to BA−KF in aqueous media causes a slight expansion with binding of guest species by silicate nanosheets. It has been pointed out that the interlayer expansion (swelling) occurs as a result of a balance between the interlayer cohesive force (attractive force including van der Waals or electrostatic interaction) and the force required to swell.14 The latter force includes an attractive one between interlayer cations and organic molecules. The interlayer expansion occurred in the present system owing to the intercalation of larger amount of caffeine into BA−KF as a result of multipoint caffeine interactions with the interlayer siloxane surface and BA.26 Figure 4 shows the time-course profiles of the basal spacing of BA−KF and the amount of adsorbed caffeine after injection of caffeine into the water slurry of BA−KF. Good reproducibility was obtained in recording the basal spacing (S.D. was within 0.01 nm for each plot: see Supporting Information, Figure S1). The basal spacing (1.54 nm) immediately increased to 1.61 nm after the injection (within

a few minutes). The plot for the basal spacing is located above that for the adsorbed amount, suggesting that at the initial period the rate of the basal spacing change was faster than that to attain the adsorption equilibrium. During the reactions, there was only a slight change in the half-width of the 001 reflection (0.45 ± 0.03°: see Figure S2), indicating the single-phase XRD pattern before and during the caffeine intercalation. Although it can be visually determined from the plot that the adsorption equilibrium was reached within 4 h, a strict plateau in the basal spacing change cannot be observed. Hence, we assume that the change in the basal spacing includes both rapid and slow components. To compare the rate constants of the observed changes between the basal spacing and the adsorbed amount, first-order relaxation has been adopted as a kinetic model to the timecourse plots in Figure 4. Because the change of the basal spacing includes rapid (drapid) and slow (dslow) components, the time-course profile (dmodel: eq 4-1) has been fitted with the weighted sum of two of first-order equations (eqs 4-2 and 4-3) with a weighting coefficient (a), given by dmodel = aΔdrapid + (1 − a)Δdslow + deq

(0 ≤ a ≤ 1) (4-1)

Δdrapid = (d0 − deq) exp( −kd 0t )

(4-2)

Δdslow = (d0 − deq) exp( −kd1t )

(4-3)

where d0 and deq are the basal spacings [nm] recorded before caffeine injection and after reaching the adsorption equilibrium, respectively. The rate constants (kd0 and kd1) of each first-order plot and the weighting coefficient are listed in Table 2. Because weighting coefficient a resulted in 0.79, the basal spacing just after caffeine injection (d1) was estimated to be 1.62 nm, Table 2. Constants of the DE Kinetic Models of the Basal Spacing Change (Equations 4-1−4-3) and the Amount of Adsorbed Caffeine (Equations 5-1−5-3) basal spacing

Figure 4. Time-course profiles of the basal spacing of BA−KF (red) and the amount of adsorbed caffeine (blue) after injection of caffeine into the water slurry of BA−KF in the XRD cell. D

adsorbed amount

a

kd0 [h−1]

kd1 [h−1]

b

kW0 [h−1]

kW1 [h−1]

0.79

18.9

0.19

0.54

29.1

0.98


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caffeine volume (0.17 nm3/molecule) by the maximum adsorbed amount of caffeine (0.78 mmol/g) to be 0.079 cm3/g (Table 3). The volume of caffeine in the interlayer space was smaller than the calculated pore volume (43%, Table 3), indicating that the interlayer micropores were occupied by both water and caffeine molecules. The volume of adsorbed caffeine (0.079 cm3/g) exceeded the increase in pore volume (0.18 − 0.15 = 0.03 cm3/g), suggesting that water molecules in the interlayer space were exchanged with caffeine molecules to be intercalated. The empirical and simulation results are used to discuss the structural change of BA−KF. In the presence of water, the interlayer space expanded by 0.1 nm to be 1.54 nm (Scheme 2a) from the dried state. After caffeine injection into the water phase, the adsorption of caffeine occurred, followed by

indicating that the change in the basal spacing includes fast (ca. 0.08 nm) and slow (ca. 0.02 nm) components. Likewise, the profile regarding the amount of adsorbed caffeine has also been fitted to the double exponential (DE) kinetic model with a weighting coefficient (b) (eq 5-1), and has been divided into rapid and slow components (eqs 5-2 and 5-3), given by Wmodel = bΔWrapid + (1 − b)ΔWslow + Weq

(0 ≤ b ≤ 1) (5-1)

ΔWrapid = −Weq exp( −kW 0t )

(5-2)

ΔWslow = −Weq exp( −kW 1t )

(5-3)

where Weq is the amount [mg/g] of caffeine adsorbed at the adsorption equilibrium (12 mg/g). The rate constants (kW0 and kW1) and weighting coefficient b are also shown in Table 2. From weighting coefficient b (0.54), the amount of adsorbed caffeine just after injection (W1) can be calculated to 6.5 mg/g, corresponding to the fast (6.5 mg/g) and slow (5.5 mg/g) components. In both changes of d and W, rate constants k of rapid and slow components are completely different. In the early period, rapid process contribution for the basal spacing (a = 0.79) is larger compared with the uptake of caffeine (b = 0.54). It is suggested that the interlayer expansion is considered to occur more rapidly than the uptake of caffeine. Regarding the slow component of the rate constant, the value for the basal spacing (kd1) is lower than that for the amount adsorbed (kW1), meaning that the basal spacing change would reach a steady state after the adsorption equilibrium of caffeine is reached. Molecular Geometrical Estimates. Because the interlayer micropore size should change upon intercalation of water and caffeine, the micropore volume created by the BA cations and silicate layers was calculated from the microscopic geometry of BA−KF.39 Relevant parameters (Table 3) are the interlayer

Scheme 2. Schematic Representations of Changes in Interlayer Structures of the BA−KF Intercalation Compound in Watera

Table 3. Geometric Estimates of Caffeine Intercalation into BA−KF interlayer space (Δd)a [nm]

pore volumeb [cm3/g]

in water

in aq. caffeine soln.

in water

in aq. caffeine soln. (A)

volume of adsorbed caffeine (B)c [cm3/g]

B/A [%]

0.58

0.68

0.15

0.18

0.079

43

a

Calculated by subtracting the thickness of the silicate layer (0.96 nm) from the observed basal spacing [nm]. bPore volume [cm3/g] is calculated as (SΔd/2) − (NAWBAVBA), where S, NA, WBA, Δd, and VBA are the ideal surface area of montmorillonite (7.5 × 1020 nm2/g), Avogadro’s number [1/mmol], amount of adsorbed BA (1.2 mmol/g), the interlayer space of the sample in aqueous caffeine solutions (0.68 nm), and van der Waals molecular volume of BA (0.10 nm3, obtained by a WinMostar MOPAC simulation). cTo obtain the volume, the maximum amount of adsorbed caffeine (0.78 mmol/g: Langmuir constant) was multiplied by the van der Waals molecular volume of caffeine (0.17 nm3), obtained by a WinMostar MOPAC simulation.

space (Δd), amount of pre-intercalated BA, volume of BA (the van der Waals volume of 0.10 nm3), and the ideal surface area of KF (750 m2/g).24 The calculated pore volume was 0.15 cm3/ g. When BA−KF is immersed in water, the micropores may become filled with water molecules. After increase in the Δd value of BA−KF at the adsorption equilibrium, the pore volume was estimated in the presence of caffeine to be 0.18 cm3/g. The volume of adsorbed caffeine was calculated by multiplying the

a

(a) In the absence of caffeine, (b) after injection of caffeine into the water phase, (c) progression of caffeine intercalation, and (d) completion of caffeine intercalation. E



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initiation of intercalation from the rims of the silicate layers to partly expand the interlayer space by 0.08 nm (Scheme 2b). The intercalation of caffeine molecules then progressed in all interlayers (Scheme 2c). An immediate increase in the basal spacing possibly occurred at this stage. Because diffusion of caffeine is slower than the increase in the basal spacing, we assume that caffeine was not fully spread (or there was room to accommodate caffeine molecules) to the interlayer space at this stage. This period could be very short and is estimated to be less than 5 min. As discussed above, water molecules may partially deintercalate and may be excluded by exchanging with caffeine molecules through caffeine intercalation. Further intercalation of caffeine proceeded to expand the interlayer space gradually from 1.62 to 1.63 nm (Scheme 2d) during the reaction period of 3 h, and the adsorption equilibrium was thus attained at 3 h. The crystallite size should also be a rate-determining factor for stabilizing such a hybrid structure. We have also studied the adsorptive properties using a smaller crystalline smectite (smaller aspect ratio and stacking), a synthetic saponite (Sumecton SA); unfortunately, negligible change in the basal spacing was observed upon caffeine adsorption due to a larger pore volume in the interlayer space compared to that in BA− KF.26

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +81-26-269-5414. Fax: +81-26-269-5424 (T.O.). *E-mail: [email protected] (T.I.). ORCID

Tomohiko Okada: 0000-0002-9361-7004 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI (Grant-in-Aid for Scientific Research), Grant numbers of 15H03767 (T.I.) and 26810121 (T.O.), and by Mukai Science and Technology Foundation (T.O.).

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ABBREVIATIONS BA, benzylammonium; KF, a natural montmorillonite (Kunipia F); XRD, X-ray diffraction; CEC, cation-exchange capacity; DE, double exponential

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REFERENCES

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CONCLUSIONS This study investigates the kinetics of the intercalation behavior of an organic molecule in an organically modified layered silicate (BA−KF) in aqueous media, which was obtained by cation-exchange reactions between BA and the interlayer Na+. Caffeine molecules were shown to be effective at causing interlayer expansion owing to their efficient uptake through cooperative interaction with silicate layers and BA. To our knowledge, this is the first example of applying the transmission XRD technique to the dynamics of interlayer expansion upon intercalation of organic molecules in such a soft matter system. The basal spacing of BA−KF (1.54 nm) in water expanded to 1.64 nm upon caffeine molecule intercalation, which was saturated even when an excess amount of caffeine was present in the aqueous solution. The interlayer expansion (0.10 nm) includes rapid (0.08 nm increase within a few minutes) and slow (0.02 nm increase over a few hours) components from the time-course profile of the basal spacing obtained by transmission XRD. A first-order kinetic simulation (eqs 4-1−4-3) fitted to the time-course profile also indicates that the interlayer expansion includes rapid (kd0) and slow (kd1) components. In the early period, the rapid process contribution for the basal spacing, a in eq 4-1, was larger compared to that for the adsorbed amount of caffeine (b in eq 5-1), suggesting that the interlayer expansion is considered to occur more rapidly than the uptake of caffeine. The rate constant for the basal spacing in the slow component (kd1) was lower than that for the amount of adsorbed caffeine (kW1), indicating that a steady-state basal spacing is attained after the adsorption equilibrium.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.7b03200. Time-course profiles of the basal spacing (Figure S1) and of the half-width of the 001 reflection peak (Figure S2) for BA−KF after injection of caffeine (PDF) F


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