Colorimetric Humidity and Solvent Recognition Based on a Cation

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Colorimetric Humidity and Solvent Recognition Based on a CationExchange Clay Mineral Incorporating Nickel(II)-Chelate Complexes Hitoshi Hosokawa, and Tomoyuki Mochida Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b03709 • Publication Date (Web): 05 Nov 2015 Downloaded from http://pubs.acs.org on November 14, 2015

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Colorimetric Humidity and Solvent Recognition Based on a Cation-Exchange Clay Mineral Incorporating Nickel(II)-Chelate Complexes

Hitoshi Hosokawa, Tomoyuki Mochida* Department of Chemistry, Graduate School of Science. Kobe University, Rokkodai, Nada, Hyogo 657-8501, Japan

ABSTRACT: Solvatochromic nickel(II) complexes with diketonato and diamine ligands were incorporated into a saponite clay by ion exchange, and their colorimetric humidity- and solvent-recognition properties were investigated. These powders exhibit color change from red to blue-green depending on humidity, and the detection range can be controlled by modifying the metal complex. The humidity response takes advantage of the humidity-dependent water content in clay and the coordination of water molecules to the metal complex in equilibrium. The addition of organic solvents to the powders causes a color change to occur, varying from red to blue-green depending on the donor number of the solvent, thereby enabling solvent recognition. In the clay, the affinity of less sterically hindered complexes to water or solvent molecules is decreased compared with that in solution because the cationic complexes interact with the anionic layers in the clay. Incorporating diethylene glycol into the materials produced thermochromic powders.

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INTRODUCTION The development of techniques for monitoring humidity and organic solvents/vapors is of great importance in diverse areas.1–5 Although such sensors are mostly based on electronic devices, colorimetric indicators provide convenient detection methods without requiring analytical instruments. Several types of colorimetric humidity indicators have been developed to date. Cobalt(II) chloride-containing materials are typical examples that exhibit color change resulting from the coordination of water molecules.6–9 The incorporation of cationic organic dyes into ion-exchange polymer Nafion10–12 or Zeolite13 generates materials with humidity-sensing properties based on the protonation-deprotonation equilibrium. Colorimetric humidity sensors based on polymers,14–18 photonic materials19–22 and graphene oxides23 have been reported to exhibit high sensitivity and fast response. The development of colorimetric solvent indicators and volatile organic compound (VOC) sensors has also attracted increasing attention in recent years.24–31 In the present study, we found that incorporating solvatochromic complexes into a clay mineral produces hybrid materials that exhibit colorimetric humidity- and solvent-recognition properties. We previously described colorimetric solvent indicator films consisting of Nafion films incorporating cationic solvatochromic nickel(II) complexes with diketonato and diamine ligands.30.31 As shown in Figure 1, the red square-planar complex in the film changes to a blue-green solvent-coordinated octahedral complex when immersed in solvents with high donor numbers.32–36 The equilibrium constant is dependent on the solvent and is responsible for the color variation. Applying the same approach, we incorporated solvatochromic complexes into synthetic saponite (Sumectone SA, abbreviated as SAP) in the present study. Saponite has a layered structure (Figure 2) and contains Na+ and Mg2+ ions between the layers, which are exchangeable with other cations.37–48 Clays incorporating cationic porphyrins and spiropyrans have been reported to exhibit solvatochromic or photochromic properties.37–45 Intriguingly, 2 ACS Paragon Plus Environment

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the reactions in clay are affected by the interlayer distances and host-guest interactions.46–48 Humidity sensing based on impedance changes in clay has also been reported.49–51 Herein, we report the preparation and properties of saponite incorporating solvatochromic complexes 1–5 ([X]-SAP, X = 1–5; Figure 3). These hybrid materials exhibit colorimetric humidity-sensing and solvent-recognition properties. The characteristic responses of the complexes in the clay are discussed in detail based on their solvent responses. Incorporating diethylene glycol (DEG) into the clay produced thermochromic materials.

Figure 1. Equilibrium between the square-planar and octahedral species with the reversible coordination of solvent molecules, which are represented by balls.

Figure 2. Schematic illustration of the layered structure of saponite. The circles in the interlayer space represent water or other solvent molecules.


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Figure 3. Structural formulas of cationic Ni(II) complexes 1–5.

EXPERIMENTAL General.

Sumecton

SA ([(Si7.20Al0.80)(Mg5.97Al0.03)·O20·(OH)4]–0.77(Na0.49Mg0.14)+0.77)38,43,44,

a

synthetic saponite (SAP), was purchased from Kunimine Industries Co., Ltd. [X]BPh4 (X = 1–5) and related salts were prepared according to the method in the literature.30–36 The preparation of [X]-Nafion (X = 2, 3, 4) was reported previously,30 and [X]-Nafion (X = 1, 5) was obtained by the same method. The humid atmospheres (11% RH, 31% RH, 52% RH, 75% RH, and 97% RH) at 25 °C were produced using saturated solutions of LiCl, Zn(NO3)2, Mg(NO3)2, NaCl, and K2SO4, respectively.52 The powders were exposed to each humidity for 1 d before analysis to ensure water absorption equilibrium. Visible-near infrared (Vis-NIR) spectra of the powder samples were recorded on a JASCO V-570 spectrophotometer equipped with an ISN-470 integrating sphere apparatus using the diffuse reflectance method, while the spectra of powders immersed in solvents were measured using quartz cells (1-mm path length). The absorption spectra of saponite were calculated from diffused reflectance spectra by applying the Kubelka-Munk theory.53

Preparation of [X]-SAP (X = 1–5). Synthetic saponite (600 mg) was added to a methanol solution of [X]BPh4 (X = 1–5, 30 mM, 30 mL) and stirred for 1 h. The resulting blue-green powder was collected by filtration and dried under vacuum for 1 h at 80 °C, producing red powders of [X]-SAP. The amounts 4 ACS Paragon Plus Environment

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of incorporated cations in [X]-SAP (X = 1–5) were 17 wt%, 20 wt%, 22 wt%, 14 wt%, and 14 wt%, respectively, as estimated from the absorption spectra of the mother liquids. The amounts of the incorporated cations vs. CEC (99.7 meq/100 g) were 52%, 65%, 63%, 53%, and 54%, respectively. These were the maximum loading amounts, and no further incorporation was possible. DEG-containing powders were prepared by adding DEG (~100 mg) to [X]-SAP (X = 2–4, 100 mg), corresponding to 17–25 molar equivalent of the incorporated metal complex.

RESULTS AND DISCUSSION Humidity response. Solvatochromic complexes 1–5 were incorporated into saponite by cation exchange in methanol, and red powders of [X]-SAP (X = 1–5) were obtained after drying under vacuum. The amounts of the incorporated cations were 52–65% vs. the cation-exchange capacity (CEC). The intercalation of the complexes into the interlayer space was also confirmed by X-ray diffraction experiments; for example, the interlayer distance was increased from 12 Å (2θ = 7.5°)54 to 15 Å (2θ = 6.2°) upon incorporation of 1. The powders exhibited reversible color changes under different humidities. Photographs taken after allowing the powders to stand for 1 day under various humidities are shown in Figure 4. The colors of [X]-SAP (X = 1–3) changed to blue-green above 30% RH, 50% RH, and 75% RH, respectively. In contrast, the color of [4]-SAP only changed at nearly 100% RH, while [5]-SAP exhibited no color change, even with 100% RH. This observation clearly shows that ligand modification successfully tuned the humidity-detection range. The origin of the color change is ascribed to the shift of the equilibrium in Figure 1 to the right because of the absorption of water into the interlayer space of the clay. The amount of water in the clay increased as the humidity increased (Figure S1, ESI). The water absorption behavior of [X]-SAP was 5 ACS Paragon Plus Environment

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slightly different from those of clay minerals without metal complexes.55 The water content under 97% RH was 19 wt%, which is ~20 molar equivalent of the incorporated metal complex. The color of the powder becomes blue-green under high humidity because of an increase of the ratio of the six-coordinated species. Vis-NIR spectra of the powders under various humidities are shown in Figures 5 and S2 (ESI). The absorption peak of the four coordinated species at approximately 490 nm (λ1, 1A1g → 1A2g)56 decreased as the humidity increased, and the peaks corresponding to the six-coordinated species at 610 nm (λ2, 3A2g → 3T2g ) and 1040 nm (λ3, 3A2g → 3T1g ) also increased.56 The peaks at λ2 and λ3 are small because of their small molar absorbance coefficients. As the affinity of the incorporated complex for water molecules decreases (1 > 2 > 3 > 4 > 5), the humidity at which the color change occurs increases. The humidity response demonstrated here takes advantage of the humidity-dependent water content in the clay and the color-change property of the solvatochromic complex in equilibrium with water, which can be tuned by ligand modification.


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Figure 4. Photographs of (a) [1]-SAP, (b) [2]-SAP, (c) [3]-SAP, (d) [4]-SAP, and (e) [5]-SAP after standing for 1 day under different humidities. The humidity ranges at which the color of the powders changes to blue-green are indicated by arrows.

Figure 5. Vis-NIR spectra of [2]-SAP under various humidities. The spectra in the low-energy region (600–1200 nm) are magnified (×3) for clarity.

Cation dependence of humidity response. The humidity responses of [X]-SAP (X = 1–5) were compared in detail based on their Vis-NIR spectra. The absorbance of the peak at λ1 versus humidity is plotted for each complex in Figure 6 and reflects the ratio of the four-coordinated species in the clay. The decrease in absorbance exhibited the following order, from largest to smallest: 5 < 4 < 3 < 2 < 1; this is consistent with the color change characteristics presented in Figure 4. The color change to blue-green can be observed by the naked eye in the humidity region at which the absorbance of the four-coordinated species is less than ~0.05 for each complex. The absorbance of [5]-SAP, which did not exhibit a color change, remained large, even under high humidity. The affinity of the complexes to water in the clay is correlated with the electronic state and steric hindrance of the complex. In Figure 6, the value for [X]-SAP (X = 1–3) decreases rapidly with increasing humidity in the low humidity region. Among these materials, 1 has the highest affinity for 7 ACS Paragon Plus Environment

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water because of the electron-withdrawing CF3 substituent in the diketonato ligand, which increases the Lewis acidity of the metal center. In contrast, 3 has the lowest affinity, primarily because of the electron-donating effect of the tBu substituents. The humidity dependence of [4]-SAP and [5]-SAP, however, decreases only slightly in the low humidity region, and the decrease becomes more prominent in the high humidity region. This observation indicates strong deviation from the equilibrium behavior shown in Figure 1, suggesting the suppression of water coordination. This phenomenon is ascribed to the interaction between these less sterically hindered complexes and the anionic layers in the clay, as discussed in the section of solvent affinity of complexes in the clay. We

also

prepared

saponite

(2,2,6,6-tetramethyl-3,5-heptanedionate):

powders

incorporating

[Ni(dipm)(tmen)]+,

complexes

with

[Ni(dipm)(Me3en)]+,

dipm and

[Ni(dipm)(BuMe3en)]+. However, these powders exhibited no color change, even under 100% RH. This behavior can likely be ascribed to their low affinity for water molecules because of the high electron-donating ability and hydrophobicity of the dipm ligand bearing tBu.32 In contrast, [3]-SAP exhibited a color change when the RH exceeded 75%, despite bearing the dipm ligand. This tendency is probably caused by the hydrophilicity of the NH groups in the diamine ligand in 3.


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Figure 6. Humidity dependence of the absorbance at λ1 (490 nm, absorption of the peak of the four-coordinated species) in the Vis-NIR spectra of [X]-SAP (X = 1 (○), 2 (△), 3 (□), 4 (◇), and 5 (●)). The dashed lines serve as guides to the eye.

Solvatochromic properties. [X]-SAP (X = 1–5) exhibited not only humidity-sensing properties but also solvent-recognition properties. The addition of solvents to the powders caused their colors to change. Photographs of the powders immersed in various solvents are shown in Figure 7. The color changed from red to blue-green as the donor number (DN)56,57 of the solvents increased, although acetonitrile caused a more significant color change than expected based on its DN. Deviation of the d–d transition on acetonitrile has been reported in related solvatochromic complexes.30,31,56 This color variation is attributable to the shift of the coordination equilibrium shown in Figure 1. In the Vis-NIR spectra, the peak of the four-coordinated species was dominant in solvents with low DN, whereas the peaks of the six-coordinated species were observed in solvents with high DN (Figures 8 and S3, ESI). The color changes of [X]-SAP also depended on the solvent’s hydrophilicity (log P, octanol–water partition coefficients58). No color change occurred in solvents with log P values exceeding ~0.3, such as diethyl ether (log P = 0.85, DN = 18.5) and THF (log P = 0.49, DN = 20.6), regardless of their DN values, probably because hydrophobic solvents do not penetrate into the hydrophilic interlayer space of saponite. The solvatochromic properties of [X]-SAP (X = 1–5) were similar, but they exhibited different responses towards particular solvents. Overall, [4]-SAP and [5]-SAP exhibited less prominent response than the other materials, as observed in the humidity response. For example, in water, [5]-SAP remained red, while the others became blue-green, as expected based on their humidity response. In acetone and EtOH, [2]-SAP, [4]-SAP, and [5]-SAP were red or orange, while the others were blue-green. The 9 ACS Paragon Plus Environment

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powders can be easily reused after drying under vacuum for 1 h at 80 °C. We reported previously that Nafion films incorporating solvatochromic complexes ([X]-Nafion) also exhibit solvent-recognition properties. The color change in [X]-SAP occurred immediately after the addition of solvents (< ~5 sec) and was much more rapid than that in [X]-Nafion (> 30 sec). This improvement is probably because of the facile penetration of solvents into the interlayer space of saponite. The solvatochromic color changes of [X]-SAP and [X]-Nafion were almost the same, except for [4]-SAP and [5]-SAP, which exhibited less prominent responses than the corresponding Nafion films. [4]-SAP remained red in acetone and EtOH, and [5]-SAP remained red in these solvents and in H2O and MeOH. The corresponding Nafion films, however, became blue-green in all of these solvents. These differences will be discussed in detail in the next section.

Figure 7. Photographs of (a) [1]-SAP, (b) [2]-SAP, (c) [3]-SAP, (d) [4]-SAP, and (e) [5]-SAP immersed in various solvents. The left-most samples are dry powders without solvents. The solvents are arranged in order of increasing the DN from left to right. The log P value of each solvent is lower than 0.3 with the exception of CH2Cl2.


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Figure 8. Vis-NIR spectra of [2]-SAP immersed in various solvents.

Solvent affinity of complexes in clay. To elucidate the effects of the clay environment on the metal complexes’ solvent affinities, we compared the solvent responses of [X]-SAP and [X]-Nafion30,31 (X = 2, 4, and 5), which commonly incorporate complexes bearing the acac ligand. For comparison, the solvent affinities of [X]BPh4 in solutions were also investigated. The affinities of 2, 4, and 5 for water and EtOH in [X]-SAP and [X]-Nafion are summarized in Figure 9, which shows the ratio of the absorbance of the peak at λ3 (absorption of the six-coordinated complex) to that at λ1 (absorption of the four-coordinated complex) in the Vis-NIR spectra. In Nafion, the affinities for H2O and EtOH both increased in the order of 2 < 4 < 5. This tendency indicates that complexes with less steric hindrance favor the coordination of solvent molecules and reflects the inherent solvent affinity of the complexes. Consistently, the same tendency was also observed in ethanol solutions of the corresponding BPh4 salts (2 < 4 < 5; Figure 9, right). In saponite, however, the order of water affinity was reversed (2 > 4 > 5), as seen in the figure, which is consistent with the humidity response mentioned above. This is because complexes with less steric hindrance exhibit stronger interactions with the anionic layers in the clay, thereby reducing the affinity for solvents. The affinity for EtOH was also low in saponite. In particular, 2 had a low affinity due to its larger steric hindrance. Indeed, columbic interactions between the cationic guest molecules and clay surface have been reported 11 ACS Paragon Plus Environment

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to be influenced by the steric hindrance of the molecules.46–48 This mechanism accounts for the lower responses of 4 and 5 in clay, as described in the previous sections.

Figure 9. Ratios of the absorbance of the peaks at λ3 and λ1 in the Vis-NIR spectra of Nafion films and saponite incorporating 2 (■), 4 (■), and 5 (□) in H2O and EtOH. The spectra of saponite were measured after standing for 24 h at 97% RH. The values for ethanol solutions of the corresponding BPh4 salts are also shown on the right.

Thermochromic properties. The addition of alcohols to [X]-Nafion produces thermochromic films.30 Therefore, we investigated the effects of adding alcohol to [X]-SAP. The blue-green powders obtained by the addition of diethylene glycol (DEG) to [X]-SAP (X = 2–4) exhibited thermochromism, and their color-change temperatures were dependent on the complex. DEG was chosen as the additive because of its high boiling point and suitable coordination ability.30,31 Photographs of the powders at various temperatures are shown in Figure 10. Their colors changed from blue-green to red as the temperature increased, and the response was reversible. The thermochromism is based on the reversible coordination of DEG to the metal complex. The temperatures at which the color change starts in [2]-SAP, 12 ACS Paragon Plus Environment

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[3]-SAP, and [4]-SAP were approximately 20 °C, 90 °C, and 70 °C, respectively. This observation suggests that the affinity of the complexes for DEG in saponite increases in the order of 2 < 4 < 3. This order is different from that of the affinity for water in saponite (4 < 3 < 2) probably because of the bulkiness of DEG, which hinders its coordination to the bulky complex 2. [1]-SAP exhibited no color change, even at 100 °C, because of strong coordination. [5]-SAP does not exhibit thermochromism because of the absence of DEG coordination. The reversibility of these powders’ thermochromic responses gradually disappeared above 100 °C because of the desorption of DEG, whereas the corresponding thermochromic Nafion films were durable up to 130 °C. In contrast to the Nafion films, relatively volatile alcohols, such as methanol and ethanol, were not suitable for the preparation of thermochromic saponite because loss of alcohol occurred, even at ambient temperature.

Figure 10. Photographs of (a) [2]-SAP, (b) [3]-SAP, and (c) [4]-SAP containing DEG taken at various temperatures.

CONCLUSION We prepared saponite clays incorporating solvatochromic cationic nickel complexes and found that they exhibit colorimetric humidity-sensing and solvent-recognition properties. The humidity response takes 13 ACS Paragon Plus Environment

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advantage of the humidity-dependent water content of clay and the solvatochromic properties of the metal complex. The detection range is tunable by ligand modification. In the clay, complexes with less steric hindrance exhibit stronger interactions with the anionic layers in the clay, thereby reducing their affinities for water and other solvent molecules. Adding DEG to the powders allowed the generation of thermochromic materials. In this study, we prepared multi-stimuli responsive materials from clay that are easily produced and reusable. Although we used only saponite here, the response characteristics of the materials may be modified by changing the cation-exchange material. These experiments, which involved the preparation of the hybrid materials, demonstration of their stimuli responses, and elucidating the underlying mechanisms, are also useful for educational purposes.

ASSOCIATED COTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:

Notes The authors declare no competing financial interest.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel/Fax: +81-78-803-5679.

ACKNOWLEDGMENTS

This work was supported financially by KAKENHI (grant number 24350073) from Japan Society of Promotion of Science. We thank Dr. Y. Funasako (Tokyo University of Science, Yamaguchi) for helpful 14 ACS Paragon Plus Environment

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discussions.

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(20) Yu, Y.; Zhang, X. M.; Ma, J. P.; Liu, Q. K.; Wang, P.; Dong, Y. B. Cu(I)-MOF: naked-eye colorimetric sensor for humidity and formaldehyde in single-crystal-to-single-crystal fashion. Chem. Commun. 2014, 50, 1444–1446. (21) Wang, Z.; Zhang, J.; Xie, J.; Li, C.; Li, Y.; Liang, S.; Tian, Z.; Wang, T.; Zhang, H.; Li, H.; Xu, W.; Yang, B. Bioinspired Water-Vapor-Responsive Organic/Inorganic Hybrid One-Dimensional Photonic Crystals with Tunable Full-Color Stop Band. Adv. Funct. Mater. 2010, 20, 3784–3790. (22) Hawkeye, M. M.; Brett, M. J. Optimized Colorimetric Photonic-Crystal Humidity Sensor Fabricated Using Glancing Angle Deposition. Adv. Funct. Mater. 2011, 21, 3652–3658. (23) Chi, H.; Liu, Y. J.; Wang, F.; He, C. Highly Sensitive and Fast Response Colorimetric Humidity Sensors Based on Graphene Oxides Film. ACS Appl. Mater. Interfaces 2015, 7, 19882–19886. (24) Rakow, N. A.; Suslick, K. S. A colorimetric sensor array for odour visualization. Nature 2000, 406, 710–713. (25) Lee, J.; Chang, H. T.; An, H.; Ahn, S.; Shim, J.; Kim, J. M. A protective layer approach to solvatochromic sensors. Nat. Commun. 2013, 4, 2461. (26) Wang, X.; Sun, X.; Hu, P. A.; Zhang, J.; Wang, L.; Feng, W.; Lei, S.; Yang, B.; Cao, W. Colorimetric Sensor Based on Self-Assembled Polydiacetylene/Graphene-Stacked Composite Film for Vapor-Phase Volatile Organic Compounds. Adv. Funct. Mater. 2013, 23, 6044–6050. (27) Ishihara, S.; Iyi, N.; Labuta, J.; Deguchi, K.; Ohki, S.; Tansyo, M.; Shimizu, T.; Yamauchi, Y.; Sahoo, P.; Naito, M.; Abe, H.; Hill, J. P.; Ariga, K. Naked-Eye Discrimination of Methanol from Ethanol Using Composite Film of Oxoporphyrinogen and Layered Double Hydroxide. ACS Appl. Mater. Interfaces 2013, 5, 5927–5930. (28) Do, J.; Huh, J.; Kim, E. Solvatochromic Fluorescence of Piperazine-Modified Bipyridazines for an Organic Solvent-Sensitive Film. Langmuir 2009, 25, 9405–9412. 17 ACS Paragon Plus Environment

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(29) Funasako, Y.; Mochida, T.; Takahashi, K.; Sakurai, T.; Ohta, H. Vapochromic Ionic Liquids from Metal–Chelate Complexes Exhibiting Reversible Changes in Color, Thermal, and Magnetic Properties. Chem. Eur. J. 2012, 18, 11929–11936. (30) Hosokawa, H.; Funasako, Y.; Mochida,T. Colorimetric Solvent Indicators Based on Nafion Membranes Incorporating Nickel(II)-Chelate Complexes. Chem. Eur. J. 2014, 20, 15014–15020. (31) Funasako, Y.; Mochida, T. Thermochromic and solvatochromic Nafion films incorporating cationic metal–chelate complexes. Chem. Commun. 2013, 49, 4688–4690. (32) Fukuda, Y. Inorganic Chromotropism, Basic Concepts and Applications of Colored Materials. Kodansha/Springer, Tokyo, 2007. (33) El-Ayaan, U.; Murata, F.; Fukuda, Y. Thermochromism and Solvatochromism in Solution. Monatsh. Chem. 2001, 132, 1279–1294. (34) Nga, N. T.; Fukuda, Y.; Sone, K. Studies on Mixed Chelates. VII. Mixed Nickel(II) Chelates Containing N,N'- or N,N'-Dialkylethylenediamines and Acetylacetone. Bull. Chem. Soc. Jpn. 1977, 50, 154–157. (35) Fukuda, Y.; Sone, K. Studies on mixed chelates—IV: Influence of the α-substituent groups in

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Characterization of clay intercalated cobalt-salen catalysts for the oxidation of p-cresol. Appl. Catal. A 2009, 370, 16–23. (48) Kohno, Y.; Kinoshita, R.; Ikoma, S.; Yoda, K.; Shibata, M.; Matsushima, R.; Tomita, Y.; Maeda, Y.; Kobayashi, K. Stabilization of natural anthocyanin by intercalation into montmorillonite. Appl. Clay Sci. 2009, 42, 519–523. (49) Su, P. G.; Chen, C. Y. Humidity sensing and electrical properties of Na- and K-montmorillonite. Sens. Actuators B 2008, 129, 380–385. (50) Gao, D.; Heimann, R. B.; Lerchner, J.; Seidel, J.; Wolf, G. Development of a novel moisture sensor based on superabsorbent poly(acrylamide)-montmorillonite composite hydrogels. J. Mater. Sci. 2001, 36, 4567–4571. (51) Tsuyumoto, I.; Iida, Y. Percolation-Type Chemical Sensor: Electrical- and Humidity-Sensing Properties of Carbon–Montmorillonite Composites. Int. J. Appl. Ceram. Technol. 2011, 8, 793–799. (52) Rocklacn, L. B. Saturated Salt Solutions for Static Control of Relative Humidity between 5° and 40°C. Anal. Chem. 1960, 32, 1375–1376. (53) Kubelka, P.; Munk, F. Ein Beitrag zur Optik Farbanstriche. Z. Tech. Phys., 1931, 12, 593–601. (54) Yui, T.; Yoshida, H.; Tachibana, H.; Tryk, D. A.; Inoue, H. Intercalation of Polyfluorinated Surfactants into Clay Minerals and the Characterization of the Hybrid Compounds. Langmuir 2002, 18, 891–896. (55) Hatch, C. D.; Wiese, J. S.; Crane, C. C.; Harris, K. J.; Kloss, H. G.; Baltrusaitis, J. Water Adsorption on Clay Minerals As a Function of Relative Humidity: Application of BET and Freundlich Adsorption Models. Langmuir 2012, 28, 1790–1803. (56) Taha, A.; Gutmann, V.; Linert, W. Spectroscopic and thermodynamic studies on solvatochromic nickel(II) complexes. Monatsh. Chem. 1991, 122, 327–339. 20 ACS Paragon Plus Environment

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Colorimetric Humidity and Solvent Recognition Based on a Cation

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