Spectroscopic Study of Alizarin and Alizarin Red Adsorbed on Anodic Aluminum Oxide Films

Trans. Mat. Res. Soc. Japan 43[3] 197-200 (2018)

Hiroe Fujikawa, Shohei Yamaguchi and Kazunori Matsui* Department of Applied Chemistry, Graduate School of Engineering, Kanto Gakuin University, Yokohama, Kanagawa, 236-8501, Japan

* Corresponding author: e-mail: [email protected]

Abstract The adsorption of alizarin (AZ) and alizarin red (AZR) on anodic aluminum oxide (AAO) plates was studied by spectroscopic methods. AAO plates were prepared by anodization of aluminum plates in a 1.8 M sulfuric acid solution. Adsorption studies were carried out by immersing the AAO plates in ethanol solutions of AZ and AZR at different pH values, whereby the color of the solutions changed according to the pH due to protonation/deprotonation. The absorption spectra of AZ in solution and that adsorbed on AAO were different, indicating the formation of AZ-Al3+ complexes with 1:1 stoichiometry by coordination of AZ to the surface of AAO. Similar spectra were obtained for AZR adsorbed on AAO, confirming that AZR is also adsorbed on the surface by coordination and not via electrostatic interactions between the sulfonate group of AZR and the AAO surface. Based on the spectral features of the AZ-adsorbed AAO plates, the possibility of their application in pH sensing is proposed. Keywords: Alizarin, Alizarin red, UV-vis spectroscopy, Anodic aluminum oxide, Dyeing

1. INTRODUCTION equilibrium anthraquinoid structures similar to those of Anodic aluminum oxide (AAO), which is formed by alizarin depending on the pH [10]: a neutral anthraquinoid - anodic oxidation of aluminum, has long received structure with a sulfonate group (I-SO3 ), a monoanionic - - considerable attention for practical applications such as structure (II-SO3 ), and a dianionic form (III-SO3 ). AZR is - electronic components, cooking gadgets, vehicles soluble in water due to its SO3 group and is typically used (including space and aerospace vehicles), and architectural for determination of metal ions in water as a chelating materials [1,2]. Moreover, due to its high porosity, AAO agent. AZR complexes with Al3+ ions have been reported has been used as an adhesive base for coloring aluminum to present Al3+:AZR ratios of 1:2 [10], 1:2 and 1:1 [11], surface. AAO dyeing is conveniently performed by 1:2 [12], and 1:3 [13]. Therefore, the reported structures of dipping AAO films in a bath containing an organic dye or the complexes formed by AZ and AZR with Al3+ are not pigment. The mechanism of dye adsorption to the porous uniform but cover a wide range of stoichiometries. surface is mainly attributed to electrostatic interactions In recent years, the usage of AAO has received much between the positive charge of the AAO surface, e.g., attention as a platform for light-emitting devices, + Al(H2O)4OH2 , and the negatively charged dye molecule, biosensors, medical equipment, and so on [2,14,15]. - e.g., molecules with a sulfonate group (SO3 ) [1,2]. Another interesting application involves the use of Alizarin (AZ) has been prominently used throughout dye-modified AAO for simple, portable, inexpensive, and history as a red dye to color textiles. AZ has three reusable pH sensors [16]. equilibrium structures depending on the pH of the solution, In this report, we present the spectroscopic features of as shown in Scheme 1 [3–5]. At pH < 5.2, most alizarin AZ and AZR adsorbed on AAO plates and the pH molecules exist in a yellow-colored undissociated form (I). dependence of the adsorption of AZ on AAO from the At pH 6.8 to 10.1, deprotonation occurs, resulting in the viewpoint of pH sensing applications. red monoanion structure (II), as depicted in Scheme 1. This structure has been calculated to be the most stable 2. EXPERIMENTAL among all the possible monoanionic forms [5]. The second 2.1 Preparation of AAO plates deprotonation leads to a bluish violet dianion (III) at pH > Aluminum plates (99.59% purity from Nilaco Co. 12.1. with dimensions 20×13×0.3 mm3) were washed in a AZ is also known to form stable complexes with Al3+, 3 wt% weak alkaline degreaser solution at 60 °C known as “lake pigments”. A wide range of Al3+:AZ for 5 min. The plate was anodized in a 1.8 M stoichiometries have been reported for these complexes sulfuric acid solution at 3 A/dm2 and 25 °C for 30 depending on the preparation and/or measurement min to obtain AAO. conditions, e.g., 1:3 or 1:1 [3], 1:2 [4,6,7,8], 1:1 [5], and 1:1 or 2:1 [9]. 2.2. Dyeing of AAO plates and pH effects Another famous analogous dye is alizarin red (AZR), The obtained AAO plates were immersed in a 0.1 mM shown in Scheme 1 (structure (IV)). AZR also takes ethanol (EtOH) solution of AZ or AZR sodium salt (both

197 198 Spectroscopic Study of Alizarin and Alizarin Red Adsorbed on Anodic Aluminum Oxide Films

from Wako Pure Chemical Ind.) at 25 °C for 10 min at indicate that AZ is adsorbed on the AAO surface 3+ different pH values, as adjusted with H2SO4 or NaOH. forming a complex with Al , whose structure is AAO plates, which were immersed in a 0.2 mM EtOH proposed in Scheme 2 (structure (V)) [5]. In the solution of AZ at 60 °C for 10 min, were further immersed case of pH 0.6, it is reasonable to conclude that in H2SO4 or NaOH aqueous solutions at different pH for 1 neutral form (I) is not adsorbed on the AAO min at 25 °C to monitor the color changes. surface.

2.3. Spectra measurements The absorption and diffuse reflectance spectra (analyzed with the Kubelka–Munk function) of the dyed-AAO plates were recorded on a UV-vis spectrophotometer (JASCO V-570).

(I) (II) Fig. 1. Absorption spectra of AZ in EtOH solutions at different pH values: (a) 0.6, (b) 7.4, (c) 10.3, and (d) 13.4.

(III) (IV)

Scheme 1. Different molecular forms of AZ (I–III) and AZR (IV) [3]: (I) undissociated form, (II) monoanion form, (III) dianion form, and (IV) sodium salt.

3. RESULTS AND DISCUSSION 3.1. Absorption spectra of AZ and AZR in solutions and adsorbed on AAO plates Figure 1 shows the absorption spectra of AZ in EtOH solutions at different pH values (0.6, 7.4, 10.3, and 13.4). The absorption spectrum at pH 0.6 Fig. 2. Absorption spectra of immersed AAO plates shows a broad peak at 432 nm assigned to the in AZ/EtOH solutions at different pH values: neutral species (I) [3,6]. The intensity of the (a) 0.6, (b) 7.4, (c) 10.3, and (d) 13.4. absorption band at 432 nm decreases at pH 7.4, while a weak but broad shoulder appears at around 540 nm, suggesting the gradual formation of monoanionic species (II) from structure (I). At pH 10.3, the spectrum only exhibits the 524-nm band, indicating that structure (II) has become the main species in solution. Under strong basic conditions (pH 13.4), new bands are observed at 574 and 618 nm as well as 542 nm, indicating the presence of both monoanionic (II) and dianionic (III) species. These spectral changes are in good agreement with those reported in the previous literatures [3–5]. Figure 2 shows the diffuse reflectance spectra of (V) (VI) immersed AAO plates in the above AZ solutions at pH (0.6, 7.4, 10.3, and 13.4). In contrast to the Scheme 2. Schematic structures for the (V) spectra of the solutions, a peak at 486 nm is monoanionic and (VI) dianionic species of AZ observed in all cases except at pH 0.6. This peak adsorbed on the AAO surface. essentially corresponding to an Al3+:AZ lake complex with 1:1 stoichiometry [3,5,9]. The results Hiroe Fujikawa et al. Trans. Mat. Res. Soc. Japan 43[3] 197-200 (2018) 199 from Wako Pure Chemical Ind.) at 25 °C for 10 min at indicate that AZ is adsorbed on the AAO surface Figure 3 shows the absorption spectra of AZR in 3.2. pH sensitivity of AZ adsorbed on AAO 3+ different pH values, as adjusted with H2SO4 or NaOH. forming a complex with Al , whose structure is EtOH solutions at different pH values. The In order to study the feasibility of AZ-adsorbed AAO plates, which were immersed in a 0.2 mM EtOH proposed in Scheme 2 (structure (V)) [5]. In the absorption spectrum at pH 0.7 shows a broad peak AAO for pH sensing applications, the changes in - solution of AZ at 60 °C for 10 min, were further immersed case of pH 0.6, it is reasonable to conclude that at 426 nm assigned to the structure of (I-SO3 ). The the spectral features of a dyed-AAO plate were in H2SO4 or NaOH aqueous solutions at different pH for 1 neutral form (I) is not adsorbed on the AAO absorption spectrum at pH 6.9 exhibits a weak monitored after dip in solutions at different pH min at 25 °C to monitor the color changes. surface. shoulder band at around 535 nm due to the gradual values. - formation of species (II-SO3 ) in addition to Figure 5 shows examples of the coloration for - 2.3. Spectra measurements (I-SO3 ), as discussed above for AZ. At pH 10.4, AZ-dyed AAO surface after dip in acidic or basic The absorption and diffuse reflectance spectra the spectrum exhibits a more intense band at 535 aqueous solutions. Color of an as-dyed AAO plate - (analyzed with the Kubelka–Munk function) of the nm, indicating that (II-SO3 ) is the main species in changed from orange to yellow or pink colors, dyed-AAO plates were recorded on a UV-vis solution. Finally, under strong alkaline conditions, when the AAO plate dipped in an acidic solution at spectrophotometer (JASCO V-570). new bands are observed at around 550 and 607 nm, pH 0.1 or basic solution at pH 13.0, respectively. - indicating the formation of (III-SO3 ). These spectral changes are essentially equivalent to those observed in the case of AZ.

(I) (II) Fig. 1. Absorption spectra of AZ in EtOH solutions (a) (b) (c) at different pH values: (a) 0.6, (b) 7.4, (c) 10.3, and (d) 13.4. Fig. 5. Photographs of coloration for the AZ-dyed AAO plates: (a) as-dipped, (b) dipped in an acidic solution at pH 0.1, and (c) dipped in a basic solution at pH 13.0.

(III) (IV) Figure 6 shows the spectra of an AZ-adsorbed AAO plate immersed in acidic solutions. In general, Scheme 1. Different molecular forms of AZ (I–III) the intensity of the absorbance at 486 nm gradually Fig. 3. Absorption spectra of AZR in EtOH solutions at and AZR (IV) [3]: (I) undissociated form, (II) decreases with a decrease in the pH, and the peak different pH values: (a) 0.7, (b) 6.9, (c) 10.4, and (d) 13.5. monoanion form, (III) dianion form, and (IV) sodium finally shifted to 430 nm at pH 0.1 with about salt. one-fifth of the initial absorbance intensity. After Figure 4 shows the diffuse reflectance spectra of the plate is washed with distilled water, the AAO plates immersed in the above solutions. A blue-shifted peak returns to the initial position at 3. RESULTS AND DISCUSSION peak is observed at 478 nm except in the case of 486 nm, albeit with one-third the original intensity. 3.1. Absorption spectra of AZ and AZR in solutions pH 0.7, indicating the formation of an AZR These spectral changes are attributed to the and adsorbed on AAO plates complex on the AAO surface with a similar protonation of species (V), which shifts the Figure 1 shows the absorption spectra of AZ in structure to that suggested in Scheme 2 (V) for AZ. equilibrium toward structure (I), resulting in a EtOH solutions at different pH values (0.6, 7.4, These results also implie that the possible reduction of the amount of complex (V) and the - 10.3, and 13.4). The absorption spectrum at pH 0.6 electrostatic interactions between the SO3 group of weakening of the coordination to the AAO surface. Fig. 2. Absorption spectra of immersed AAO plates shows a broad peak at 432 nm assigned to the AZR and the AAO surface hardly contribute to the in AZ/EtOH solutions at different pH values: neutral species (I) [3,6]. The intensity of the surface adsorption. (a) 0.6, (b) 7.4, (c) 10.3, and (d) 13.4. absorption band at 432 nm decreases at pH 7.4, while a weak but broad shoulder appears at around 540 nm, suggesting the gradual formation of monoanionic species (II) from structure (I). At pH 10.3, the spectrum only exhibits the 524-nm band, indicating that structure (II) has become the main species in solution. Under strong basic conditions (pH 13.4), new bands are observed at 574 and 618 nm as well as 542 nm, indicating the presence of both monoanionic (II) and dianionic (III) species. These spectral changes are in good agreement with those reported in the previous literatures [3–5]. Figure 2 shows the diffuse reflectance spectra of (V) (VI) immersed AAO plates in the above AZ solutions at pH (0.6, 7.4, 10.3, and 13.4). In contrast to the Scheme 2. Schematic structures for the (V) Fig. 6. Absorption spectra of AZ adsorbed on an spectra of the solutions, a peak at 486 nm is monoanionic and (VI) dianionic species of AZ AAO plate and followingly immersed in solutions observed in all cases except at pH 0.6. This peak adsorbed on the AAO surface. 3+ Fig. 4. Absorption spectra of immersed AAO plates at different pH values: (a) as-prepared AZ-dyed essentially corresponding to an Al :AZ lake in AZR/EtOH solutions at different pH values: AAO plate, (b) 5.7, (c) 1.9, (d) 1.4, (e) 0.1, and complex with 1:1 stoichiometry [3,5,9]. The results (a) 0.7, (b) 6.9, (c) 10.4, and (d) 13.5. (f) washed with distilled water. 200 Spectroscopic Study of Alizarin and Alizarin Red Adsorbed on Anodic Aluminum Oxide Films

Figure 7 shows the spectra of an AZ-adsorbed [8] V. Ya. Fain, B. E. Zaitsev, and M. A. Ryabov, Russ. AAO plate immersed in solutions with increasing J. Coord. Chem., 30, 365–370 (2004). pH. The spectra exhibit a red-shift from 486 to 498 [9] S. S-L.-Fat. And J.-P. Cornard, Polyhedron, 30, Dynamics and Aggregate Structuring of Water Molecules in Edible Oil nm, where the intensity of the absorbance generally 2326–2332 (2011). Analyzed by Dielectric Spectroscopy increases with the pH. In contrast to the case of [10] D. A. Rowley and J. C. Cooper, Inorg. Chim. Acta, acidic solutions, the peak position returns to the 147, 257–259 (1988). initial wavelength, with a slight increase in the [11] A. Safavi, H. Abdollahi, and R. Mirzajani, absorbance intensity. It is thus plausible to Spectrochim. Acta A, 63, 196–199 (2006). 1* 1 2 2 2 3 consider that further deprotonation occurs and [12] R. S. Sathish, M. R. Kumar, G. N. Rao. K. A. Kumar, K. Shoji , T. Saito , R. Kita , N. Shinyashiki , S. Yagihara , M. Fukuzaki , makes a shift from complex (V) to (VI) with a and C. Janardhana, Spectrochim Acta A, 66, T. Ohzono4, S. Nishimura4, M. Hayashi5 and H. Tanaka5 stronger coordination binding. The incomplete 457–461 (2007). 1 Graduate School of Science, Tokai University, 4-1-1 Kitakaname, Hiratsuka-shi, Kanagawa 259-1292, Japan recovery of the original absorbance intensity after [13] S. M. Supian, T. L. Ling, L. Y. Heng, and K. F. 2 Department of Physics, Tokai University, 4-1-1 Kitakaname, Hiratsuka-shi, Kanagawa 259-1292, Japan washing with distilled water is attributable to the Chong, Analytical Methods, 5, 2602–2609 (2013). 3 Liberal Arts Education Center, Kumamoto Campus, Tokai University, - presence of residual OH . [14] A. V. Kukhta, G. G. Gorokh, E. E. Kolesnik, A. I. 9-1-1 Toroku, Higashi-ku, Kumamoto-shi, Kumamoto 862-8652, Japan Mitkovets, M. I. Taoubi, Y. A. Kochin, and A. M. 4 Dynamic Functional Material Group, Research Institute of Sustainable Chemistry, Mozalev, Surf. Sci., 507, 593–597 (2002). National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba-shi, Ibaraki 305-8565, Japan [15] A. Santos, T. Kumeria, D. Losic, Materials, 7, 5 evertron Inc., 2-3-25 Shiba, Minato-ku, Tokyo 105-0014, Japan 343–351 (2014). * Corresponding author: Fax: 0463-58-9543, and/or e-mail: [email protected]

(Received March 15, 2018; Accepted April 15, 2018;

Published Online June 1, 2018)

Dielectric measurements for 10wt % W/O emulsion samples prepared with various surfactant concentrations between 0 and 10wt % were carried out in the frequency range of 40Hz up to 20GHz at room temperature during 2 hours at 15-minute intervals. Three distinct relaxation processes such as an electrode polarization in the kHz region, an interfacial polarization originated from ion dynamics on the interface of water and oil in MHz region, and an orientation polarization of oil molecules in the 100MHz region were observed. The relaxation time and relaxation strength of the interfacial polarization Fig. 7. Absorption spectra of AZ adsorbed on an increased with time. These time dependences of the relaxation time and strength become AAO plate and followingly immersed in solutions more moderate with increasing surfactant concentration. In addition, these time at different pH values: (a) as-prepared AZ-dyed dependences also suggest increasing water-droplet size and reflect stability of the AAO, (b) 8.0, (c) 12.1, (d) 13.0, and (e) washed dispersion system. In this study, all measurements were carried out with a cell type with distilled water. electrode, in which the water content was considered to be larger at the bottom of electrode, and the ε∞ value can be an indicator of the water content. The 1% surfactant W/O emulsion 4. CONCLUSIONS samples did not show an increase in the water content, but the relaxation time for the AZ and AZR dyes were immobilized on the surface interfacial polarization increased much. This is an evidence of increasing water-droplet size, of AAO plates by immersing the AAO plates in the because the relaxation time of interfacial polarization is considered to reflect spatial scale ethanol dye solutions. These dyes were adsorbed on the AAO surface via a chelating reaction involving of interface of water droplets. These results indicate a change in the W/O emulsion structure the dyes and Al3+. The AZ-dyed AAO sample with time by dielectric spectroscopy. showed color changes when exposed to aqueous Key words: interfacial polarization, dielectric spectroscopy, W/O emulsion, disperse system solutions at different pH values, demonstrating the potential use of these plates for pH sensing 1. INTRODUCTION yet in all details of changes in unstable structures and applications. Oil is indispensable in our life and used in various aggregations with the elapsed time. fields, for example, as edible and machine oils. It is well These studies on emulsions should be help to REFERENCES known that oil changes its chemical structure by an aging understand dispersion systems like food stuffs and living [1] J. W. Diggle, T. C. Downie, and C. W. 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