Article pubs.acs.org/Langmuir
Clay−Alcohol−Water Dispersions: Anomalous Viscosity Changes Due to Network Formation of Clay Nanosheets Induced by Alcohol Clustering Yuji Kimura and Kazutoshi Haraguchi* Department of Applied Molecular Chemistry, College of Industrial Technology, Nihon University, 1-2-1 Izumi-cho, Narashino, Chiba 275-8575, Japan S Supporting Information *
ABSTRACT: Clay−alcohol−water ternary dispersions were compared with alcohol−water binary mixtures in terms of viscosity and optical absorbance. Aqueous clay dispersions to which lower alcohols (ethanol, 1-propanol, 2-propanol, and tert-butanol) were added exhibited significant viscosity anomalies (maxima) when the alcohol content was 30−55 wt %, as well as optical absorbance anomalies (maxima). The maximum viscosity (ηmax) depended strongly on the clay content and varied between 300 and 8000 mPa·s, making it remarkably high compared with the viscosity anomalies (2 mPa·s) observed in alcohol−water binary mixtures. The alcohol content at ηmax decreased as the hydrophobicity of the alcohol increased. The ternary dispersions with viscosity anomalies exhibited thixotropic behaviors. The effects of other hydrophilic solvents (glycols) and other kinds of clays were also clarified. Based on these findings and the average particle size changes, the viscosity anomalies in the ternary dispersions were explained by alcohol-clustering-induced network formation of the clay nanosheets. It was estimated that 0.9, 1.7, and 2.5 H2O molecules per alcohol molecule were required to stabilize the ethanol, 2-propanol, and tert-butanol, respectively, in the clay− alcohol−water dispersions.
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INTRODUCTION Clay minerals belonging to the smectite group, such as hectorite, montmorillonite, and saponite, swell in water and gradually cleave into discrete disc-like nanosheets. The exfoliated clay nanosheets have thicknesses of 1 nm and diameters of several tens to hundreds of nanometers depending on the type of clay, e.g., 25−30 nm for synthetic hectorite1−7 and 300−1,000 nm for natural montmorillonite.8,9 Figure 1a depicts the layered 2:1 crystal structure of hectorite and the exfoliating state in water. To determine the structures and properties of aqueous clay dispersions, they have been extensively studied using dynamic light scattering,1,2,10,11 small-angle neutron scattering,1,3 scanning electron microscopy,12 and viscosity measurements.4,13,14 Because clay nanosheets consist of nanoscale platelets with high aspect ratios and have negative surface charges due to the partial isomorphic substitution (Mg2+ → Li+) in their central octahedral MgO layers, they can form so-called house-of-cards structures in water (Figure 1a(iv)) when the clay concentration (Cclay) is high.15,16 Clays also exhibit unique phenomena such as ion exchange, adsorption, thickening, stabilization (Pickering effect), and intercalation and catalytic abilities.17,18 Thus, clays have been widely used in civil engineering, architecture, agriculture, food, medical and health care, and so on.19 In addition, clays have recently become recognized as promising © XXXX American Chemical Society
nanomaterials for preparing advanced nanocomposites with characteristics such as the ability to act as gas barriers,20 incombustibility,21 selective adsorption,22,23 controllable mesoporosity,24,25 and desirable mechanical properties.26,27 In particular, exfoliated clay (synthetic hectorite) has made it possible to develop superhydrogels28 and novel soft nanocomposites29 with excellent mechanical properties30 and numerous new capabilities31,32 because of their unique polymer−clay network structures. In general, water and primary alcohols (e.g., methanol, ethanol, and propanol) have been mixed uniformly in arbitrary ratios on the macroscopic level. These water−alcohol binary mixtures have been important scientific targets due to their unique properties, such as anomalous enthalpies and entropies of mixing,33−36 mean molar volumes,37 self-diffusion constants,38 and viscosities39,40 in specific alcohol content ranges, although their viscosity anomalies were observed to be about 2 mPa·s at most regardless of the kind of alcohol.40 These anomalies have been explained by microinhomogeneities caused by the molecular clustering or hydrophobic interactions of the alcohols in such binary mixtures. These microReceived: March 8, 2017 Revised: April 18, 2017
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DOI: 10.1021/acs.langmuir.7b00764 Langmuir XXXX, XXX, XXX−XXX
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Figure 1. (a) Schematic representations of the crystal structure of clay (hectorite) and the exfoliation states of clay nanosheets in water. (b) Clay aqueous dispersions with Cclay = 2.0 and 5.0 wt %. Industries Co., Ltd., Tokyo, Japan), were also utilized. In addition, spherical, 30 nm-diameter SiO2 nanoparticles (SNOWTEX: Nissan Chemical Industry Co., Ltd.) were employed as a control. The Laponite XLG and Laponite XL-21 were purified by washing and freeze-drying. The other substances were used without purification. The water that was utilized in all of the experiments was ultrapure water supplied by a Puric-Mx system (Organo Co., Japan). As organic solvents, special grades of methanol (MeOH), ethanol (EtOH), 1propanol (n-PrOH), 2-propanol (iso-PrOH), tertiary-buthanol (tBtOH), ethylene glycol (EG), propylene glycol (PG), 1,3-propanediol (PD), and glycerol (G) purchased from Wako Pure Chemical (Tokyo, Japan) were employed (Supporting Information, Table S1). Preparation of Binary and Ternary Dispersions. Aqueous clay dispersions with Cclay = 0.5−5 wt % were prepared by carefully mixing clay and water and stirring for 24 h at ambient temperature. Clay− solvent−water ternary dispersions with different solvent contents were prepared by adding various kinds of solvents (e.g., lower alcohols and glycols) dropwise to a homogeneous aqueous clay dispersion with an initial clay concentration (Cclayinit) of 0.5−5 wt % while stirring at ambient temperature (20 ± 1 °C). In most cases, five kinds of lower alcohols (methanol, ethanol, 1-propanol, 2-propanol, and tert-butanol) were used as solvents and Cclayinit was 2 wt %. Each of the clay− ethanol−water dispersions with Cclay fixed at 1 wt % was individually prepared by mixing a different amount of alcohol with an aqueous clay dispersion with the prescribed Cclay. Measurements. The viscosities of the clay, binary, and ternary dispersions were measured using a sine-wave vibro-viscometer (frequency = 30 Hz) (SV-10: A&D Co., Japan) at 20 °C. In each case, the temperature and viscosity were recorded simultaneously. The viscosity was measured for 15 min for each of the dispersions and mixtures, unless otherwise noted. The effects of stirring on the viscosities of the aqueous clay and clay−ethanol−water dispersions
inhomogeneous structures have been investigated by performing mass spectrometry,39−41 small- and large-angle X-ray scattering,40−43 neutron scattering,44−46 nuclear magnetic resonance,47 dielectric measurements,48 and femtosecond fluorescence upconversion technique,49 as well as molecular dynamics simulations.50−53 In addition, water−alcohol mixtures have been of interest due to their abilities to affect the denaturation and structural transformation of proteins54−57 and polymer chains.58 To date, no reports have been published on ternary systems consisting of clay nanosheets and water−alcohol mixtures. However, we expected clay nanosheets to exhibit exceptional behavior or activity in water−alcohol mixtures because of their unique characteristics in aqueous media, such as their nanoscale disc-like shapes with high aspect ratios, mildly charged surfaces, formation of super- (house-of-card) structures, and stabilization and intercalation of solutes. In the present study, we investigated clay−alcohol−water ternary dispersions, which consisted of exfoliated clay (synthetic hectorite) nanosheets in alcohol−water mixtures. The changes in viscosity, optical absorbance, and light scattering were examined for ternary dispersions with various compositions.
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EXPERIMENTAL SECTION
Materials. A synthetic hectorite, Laponite XLG (Rockwood, Ltd., UK; [Mg5.34Li0.66Si8O20(OH)4]Na0.66: cation exchange capacity = 104 mequiv/100 g), supplied by Wilbur-Ellis Co. (Tokyo, Japan), was used as a type of inorganic clay. A fluorinated synthetic hectorite, Laponite XL-21 (Rockwood, Ltd., UK), supplied by Wilbur-Ellis Co. (Tokyo, Japan), and a natural montmorillonite, Kunipia F (Kunimine B
DOI: 10.1021/acs.langmuir.7b00764 Langmuir XXXX, XXX, XXX−XXX
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Langmuir were conducted by repeatedly measuring their viscosities for 5 min (in static conditions), followed by stirring for 1 min. The optical transmittances and absorbances were measured at 600 nm using a UV−vis spectrophotometer (U-1800: Hitachi Ltd., Japan). Each dispersion was contained in a quartz cell (10 mm × 10 mm × 30 mm) with a cap and held in the sample holder at 20 °C. The average sizes of the particles in clay−ethanol−water dispersions with different ethanol contents were measured using a dynamic light-scattering apparatus (Zetasizer Ver.7.10: Malvern Ltd., UK).
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RESULTS AND DISCUSSION Properties of Clay−Water Dispersions. Typical aqueous clay dispersions of synthetic hectorite (Laponite XLG) with Cclay = 2.0 and 5.0 wt % are depicted in Figure 1b. The viscosity changes with time are presented in Figure 2a for dispersions with Cclay = 1−5 wt %, and the relationship between the viscosity and Cclay is summarized in Figure 2b. The viscosities of the dispersions significantly depend on Cclay and the retention time, although all of the dispersions are transparent, regardless of Cclay. The aqueous clay dispersions with high Cclay (≥2.7 wt %) display thixotropic behaviors (Figure 2c(ii)), where the viscosity increases with time and suddenly decreases upon stirring. This behavior is attributed to the reversible formation (and destruction) of the house-of-cards structure of the clay nanosheets (Figure 1a(iii) and a(iv)). On the other hand, the viscosity only marginally changes during retention and stirring when Cclay is low (e.g., 1, 2, and 2.5 wt %) (Figure 2a and c(i)). Thus, the aqueous clay dispersions with low Cclay (≤2 wt %) exhibit very low viscosities (≤2 mPa·s) and high optical transmittances (≥97%) regardless of the retention and stirring conditions, because the exfoliated clay nanosheets are uniformly and stably dispersed in water. Properties of Clay−Alcohol−Water Dispersions. Clay− alcohol−water ternary dispersions obtained by adding five kinds of lower alcohols (methanol, ethanol, 1-propanol, 2propanol, and tert-butanol) to a homogeneous aqueous clay dispersion with an initial clay concentration (Cclayinit) of 2 wt % were examined. The properties of the solvents, including the alcohols, that were used in the present study are summarized in the Supporting Information (Table S1). The changes in Cclay, viscosity, and optical absorbance at 600 nm (A600) are presented as functions of alcohol content (Calc) in Figure 3a, b, and c, respectively. Of course, Cclay linearly decreases with the amount of alcohol added (Figure 3a), e.g., Cclay = 1 wt % when Calc = 50 wt %. In Figure 3b, the viscosities of the clay− alcohol−water dispersions exhibit unique changes that depend on the kind of alcohol and Calc. For instance, the viscosity of the clay−ethanol−water dispersion changes marginally until the ethanol content (CEtOH) reaches 40 wt %, but it steeply increases between CEtOH = 42 and 55 wt %. Then, after reaching its maximum (ηmax = 312 mPa·s) at CEtOH = 55 wt %, the viscosity steeply decreases upon further addition of ethanol. Finally, the viscosity reaches low values ( 55 wt % because of the destruction of the clay network. The microscopic structure of a dispersion with high CEtOH (e.g., 65 wt %) is shown in Figure 9d. In a dispersion with CEtOH ≥ 70 wt %, A600 decreases due to further clay
(CEtOH = 0 wt %), where the clay is completely exfoliated into a unit crystal layer (a 1 nm-thick, 30 nm-diameter nanosheet). The number of clay nanosheets in the same volume of the dispersion is estimated to be 28.3 based on the true density of clay (2.65 g/cm3) and its formula, [Mg5.34Li0.66Si8O20(OH)4]Na0.66. In water, clay nanosheets are sufficiently hydrated and uniformly dispersed. Consequently, the aqueous clay dispersion with Cclay = 2 wt % exhibited a very low viscosity (2.06 mPa·s) and high transparency (A600 = 0.015). Figure 9b shows the microscopic structure of a clay− ethanol−water dispersion with CEtOH = 30 wt %. Ethanol (and the other lower alcohols used in the present study) is a poor solvent for clay, as proven by the fact that clay is not dissolved, nor even swollen, in pure ethanol. Therefore, because ethanol molecules form clusters by self-association in ethanol−water mixtures,39,40,43 clay nanosheets exist in the water region in the ternary system, where the exfoliation and uniform dispersion are maintained. On the other hand, the self-associated ethanol molecules (clusters) are stabilized by hydrogen bonding with the surrounding water at their interfaces. Thus, some of the water molecules exclusively serve to stabilize the ethanol clusters. In other words, the water in clay−ethanol−water dispersions consists of H2O(EtOH) molecules that hydrate and stabilize the ethanol clusters and H2O(clay) molecules that hydrate and disperse the clay nanosheets. In Figure 9b, the ethanol clusters are dispersed in the clay-in-water matrix, so the low viscosity and high transparency are maintained. In addition, the viscosity is not measurably changed by the stirring and holding processes. Figure 9c presents the microscopic structure of a dispersion with CEtOH = 55 wt %. In this case, the dispersion may also consist of ethanol clusters and clay-in-water regions; otherwise, the clay platelets should aggregate via direct contact between the clay and the ethanol (which is a poor solvent). In the clayin-water regions, the network formation of clay nanosheets is accelerated because the amount of H2O(clay) decreases, so the effective value of Cclay increases as the amount of H2O(EtOH) increases. On the other hand, the transparency of the dispersion remains high, which indicates that the clay is still molecularly exfoliated (not aggregated) and that the increase in the quantity of ethanol clusters does not affect the light scattering. H2O(clay) and H2O(EtOH) could be derived from the experimental data as follows. In Figure 3, the viscosity increases above CEtOH = 42 wt %, whereas in Figure 2b, it increases above Cclay = 2.65 wt %. Therefore, the effective value of Cclay in the clay−ethanol−water dispersion (CEtOH = 42 wt %) is expected to be 2.65 wt % because some of the water (H2O(clay)) only serves to hydrate and disperse the clay. Based on this assumption, the G
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Langmuir aggregation. The microscopic structures depicted in Figure 9a− d are consistent with the results of the dynamic light scattering experiments. That is, for the dispersions illustrated in Figure 9b and c, dave was observed to be 10 nm, as it was for the dispersion in Figure 9a, which indicates that the exfoliated clay nanosheets were uniformly dispersed in each case. For the dispersion in Figure 9d, dave was found to be more than 1000 nm and to increase with increasing CEtOH due to further clay aggregation. Thus, the large viscosity increase that appeared with increasing Calc in the ternary dispersions (Figure 3b) was explained by a new phenomenon (alcohol-clustering-induced network formation of clay nanosheets). Then, the dispersion became a gel in a specific high range of Calc, e.g., the clay− ethanol−water dispersion with CEtOH = 55 wt % was a gel (Figure 3e(E2)). The viscosity steeply decreased as CEtOH increased further, and the dispersion became a turbid sol, because the clay nanosheet network was destroyed due to the lack of sufficient water to hydrate the nanosheets above CEtOH(ηmax). Consequently, the ternary dispersion exhibited an anomalous viscosity maximum. Concerning the gelation point, no significant connected component existed below 42 wt % (CEtOH), because the viscosity did not increase at all. We noticed that slightly above 42 wt %, the viscosity began to increase gradually and then increased steeply above 50 wt % (Figure 3b). Therefore, the percolation threshold corresponding to the formation of long-range clay nanosheet connectivity may exist at around 50 wt %. The apparent gelation point (Calc(gel)) defined by the steep increase of η to 100 mP·s was determined to be 51, 43, and 38 wt % for clay−EtOH, isoPrOH, and t-BuOH−water dispersions, respectively (Table 1). The important question to address at this point was whether there were transition points or whether the variations occurred gradually during the gelation and aggregation processes. As the viscosity changed continuously (Figure 3b), no well-defined sol−gel (or gel−sol) transition at which the viscosity jumped to a discrete value seemed to exist. However, as quite rapid increases and decreases in viscosity and abrupt changes in particle size (DLS) were observed when CEtOH was changed only slightly, more precise investigation is necessary to determine the answer to the aforementioned question, e.g., by using time-related measurements such as dynamic viscoelastic and small-angle neutron- or X-ray scattering measurements. Effects of the Other Kinds of Hydrophilic Solvents and Clay. Next, the effects of the other hydrophilic solvents on the properties of the clay−solvent−water ternary dispersions were investigated. Figure 10a presents the viscosity changes observed upon adding various kinds of glycols to an aqueous clay dispersion with Cclayinit = 2 wt %. Four kinds of glycols, ethylene glycol (EG), propylene glycol (PG), 1,3-propanediol (PD), and glycerol (G) are miscible with water at any mixing ratio, but they are poor solvents for clay, similarly to the lower alcohols used in the present study. In Figure 10a, the viscosity gradually and monotonically increases with increasing EG, PG, and PD content, and no viscosity anomalies are observable. The results are similar for the clay−glycerol−water system, although the viscosity rather rapidly increases when the G content is high (>50 wt %), due to the high viscosity of pure glycerol (1023 mPa·s). Thus, all of the aqueous clay dispersions with glycols exhibit normal viscosity changes that are almost proportional to the glycol content and different from those of the dispersions with alcohols with the same carbon numbers (e.g., EG vs
Figure 10. (a) Effects of using different kinds of hydrophilic solvents (glycols) on the variations of viscosity with Csolvent for clay−solvent− water dispersions with Cclayinit = 2 wt %. The dotted lines depict the data for lower alcohol solvents. (b) Effects of using different kinds of clays or silica nanoparticles on the variations of viscosity with CEtOH for clay (silica)−ethanol−water dispersions with Cclayinit (CSiO2init) = 2 wt %.
ethanol, PD vs 1-propanol, PG vs 2-propanol), as indicated by the dotted lines in Figure 10a. Also, high transparency (A600 ≤ 0.01) was maintained in all of the clay−glycol−water dispersions regardless of the kind of glycol and the glycol content (Figure S3). Thus, the presence of clay nanosheets (Cclayinit = 2 wt %) in glycol−water mixtures negligibly affects their viscosities. This finding indicates that exfoliated clay nanosheets are simply well dispersed without causing any microinhomogeneities in glycol−water mixtures. The remarkable differences between the two types of hydrophilic solvents (lower alcohols and glycols) are attributed to the differences between their hydrophobicities/hydrophilicities. Because the glycols contained two or three hydroxyl groups, they did not exhibit clustering in the glycol−water mixtures. In contrast, the lower alcohols caused clustering in the alcohol−water mixtures because of their relatively high hydrophobicities. The viscosity increases (2 mPa·s) in the alcohol−water mixtures, which were also confirmed in the present study (Figure 3d inset), were remarkably amplified by the presence of clay nanosheets. In conclusion, only solvents with both sufficient hydrophilicities to be miscible with water and adequate hydrophobicities to enable them to form clusters by self-association in water can cause viscosity anomalies in clay−solvent−water ternary dispersions. The higher the hydrophobicity of the solvent, the smaller the solvent content that causes viscosity anomalies. Figure 10b illustrates the effects of the kind of clay on the viscosity at various CEtOH values for clay−ethanol−water dispersions with Cclayinit = 2 wt %. Among the dispersions H
DOI: 10.1021/acs.langmuir.7b00764 Langmuir XXXX, XXX, XXX−XXX
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destroyed above the effective Cclay value (=3.6 wt %) yielding ηmax. The results of this study will facilitate the development of more rational approaches to solution structure and physicochemical parameter analysis and will serve as a bridge between the disciplines of clay systems and solution properties. Furthermore, from a practical point of view, versions of the present high-viscosity system (e.g., water−ethanol and water− 2-propanol gels) might be useful in fields such as analysis, offset printing, cosmetics, and medical care and examination.
containing the three investigated kinds of clays, the dispersion containing Kunipia F, which has a large crystal size, exhibits a small change in viscosity (ηmax = 80 mPa·s) and low transparency even at low CEtOH (Aabs = 1.93 at CEtOH = 30 wt %). These characteristics are attributed to the insufficient exfoliation of the Kunipia F, which would have limited the microscopic structure formation. On the other hand, the dispersion containing Laponite XL-21, in which the OH groups of XLG are replaced with F, displays viscosity anomalies (ηmax = 365 mPa·s) larger than those of the dispersion containing Laponite XLG. Between these two dispersions, the dispersion containing Laponite XL-21 exhibits lower CEtOH values at the points at which the viscosity starts to increase and is maximized. These characteristics are attributed to the fact that more water is required to hydrate fluorinated Laponite XL-21 than Laponite XLG. Thus, the anomalous increase in viscosity occurred at a lower value of CEtOH. The effect of the other inorganic nanoparticles (spherical, 30 nm-diameter SiO2 nanoparticles) was also examined with CSiO2init = 2 wt %. However, the ternary silica−alcohol−water dispersions did not exhibit any viscosity anomalies, regardless of CEtOH (Figure 10b).
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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.langmuir.7b00764. Chemical structures and properties of the alcohol and glycol solvents; changes in viscosity upon the addition of water to aqueous clay dispersions (Cclayinit = 2.0−5.0 wt %); changes in A600 upon the addition of ethanol to aqueous clay dispersions (Cclayinit = 0.5−5.0 wt %); effects of different kinds of hydrophilic solvents (glycols) on the variations of A600 for clay−solvent−water dispersions (Cclayinit = 2 wt %) (PDF)
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CONCLUSION
Clay−solvent−water ternary dispersions were investigated in terms of viscosity, optical absorbance, and dynamic light scattering, particularly focusing on lower alcohols as the solvents and synthetic hectorites as the clays. In the aqueous clay dispersions, Laponite XLG was completely exfoliated into nanosheets and uniformly dispersed. Upon the addition of lower alcohols (CnH2n+1OH: n = 1−4) into the aqueous clay dispersions (Cclayinit = 2 wt %), anomalous viscosity increases (maxima) were observed, followed by decreases at higher Calc values. ηmax was about 300 mPa·s for all of the alcohols except methanol. ηmax appeared at an intermediate value of Calc (30− 55 wt %) and decreased as the hydrophobicity of the alcohol increased. The ternary dispersions with anomalous viscosities exhibited thixotropic behaviors upon repeated stirring and holding. With increasing Cclayinit, ηmax increased significantly and reached 8200 mPa·s at Cclayinit = 5 wt %. A600 did not change until Calc(ηmax) and subsequently exhibited a steep increase and a maximum upon further addition of alcohol. The same anomalous viscosity and A600 behaviors were also observed for different kinds of clays, although the addition of other hydrophilic solvents (glycols) with two or three hydroxyl groups or the use of spherical silica nanoparticles (instead of clay) did not cause any anomalies. Thus, the viscosity anomalies observed in the alcohol−water mixtures (2 mPa·s) were amplified by 150−1500 times due to the presence of the clay nanosheets. Based on these findings, as well as the average particle sizes measured by light scattering, these phenomena were explained by network formation of the clay nanosheets induced by alcohol clustering in the ternary dispersions. It was estimated that the ethanol clusters in each ternary dispersion were hydrated by a large amount of water, whose weight was equal to a third of the weight of ethanol, so that 9.2 H2O molecules were needed to hydrate 10 ethanol molecules. Also, the number of H2O molecules required to stabilize one alcohol molecule increased from 0.92 (ethanol) to 1.67 (2-propanol) and 2.52 (tert-butanol), which is consistent with the increase in the hydrophobicities of these alcohols. Further, the networks of clay nanosheets formed in the ternary dispersions were
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Kazutoshi Haraguchi: 0000-0003-0919-3024 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI (Grant Number JP15H03870). REFERENCES
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DOI: 10.1021/acs.langmuir.7b00764 Langmuir XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.langmuir.7b00764 Langmuir XXXX, XXX, XXX−XXX
