Dehydroindigo ... - ACS Publications

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J. Phys. Chem. C 2009, 113, 12118–12131

Evidence of Topological Indigo/Dehydroindigo Isomers in Maya Blue-Like Complexes Prepared from Palygorskite and Sepiolite Antonio Domeńech,*,† Marıá Teresa Domeńech-Carbo´,‡ Manuel Sańchez del Rıó,§ Sara Goberna,† and Enrique Lima| Departament de Quı´mica Analı´tica, UniVersitat de Vale`ncia, Dr. Moliner, 50, 46100 Burjassot, Vale`ncia, Spain, Institut de Restauracio´ del Patrimoni, UniVersitat Politećnica de Vale`ncia, Camı´ de Vera 14, 46022, Vale`ncia, Spain, European Synchroton Radiation Facility, BP220, F-38043 Grenoble Cedex, France, Instituto de InVestigaciones en Materiales, UniVersidad Nacional Autońoma de Me´xico, Circuito Exterior, Ciudad UniVersitaria, 04510 Me´xico D.F., Mexico ReceiVed: January 23, 2009; ReVised Manuscript ReceiVed: April 21, 2009

Association of indigo to palygorskite and sepiolite phyllosilicates, forming Maya Blue-like systems, is studied by means of electron microscopy, solid-state multinuclear magnetic resonance (1H-13C CP MAS, 27Al, 29Si NMR), visible and infrared spectroscopies (ATR-FTIR, VIS), and solid state electrochemistry. Combination of such techniques suggest that Maya Blue must be viewed as a complex polyfunctional organic-inorganic hybrid material in which different topological isomers of indigo and dehydroindigo molecules, distributed in the surface of the palygorskite framework and in the clay channels, as suggested by size-excluding electrochemical experiments, are involved. Such isomers can tentatively be assigned to different dye-clay interactions involving hydrogen bonding with interaction with Mg2+ and Al3+ ions mediated by structural water and interaction of dye molecules with silanol units of the clay. 1. Introduction Maya Blue (MB), a pigment widely used in wall paintings, pottery, and sculptures by the ancient Mayas and other peoples in Mesoamerica since pre-Columbian times, has attracted considerable attention because of its peculiar palette, ranging from a bright turquoise to a dark greenish blue, and its enormous resistance to the attack of acids, alkalis, organic reagents, and biodegradation. This attention has been considerably reinforced in recent years because of the characteristics of MB as a nanostructured, hybrid organic-inorganic material.1 The existence of the pigment was first reported by Merwin,2 Gettens and Stout introducing the term Maya Blue for its description.3 In the 1960s, Shepard4 introduced the idea of MB resulting from embedding a dye to certain clays in the Yucatan, whereas Gettens5 systematized acid attack tests for identifying MB, and Van Olphen6 prepared pigmenting materials with properties close to that of MB by crushing and heating indigo with palygorskite and sepiolite. Arnold first discovered the use of palygorskite in the Yucatan and that the contemporary Maya were using it for pottery temper and for medicinal purposes. He studied several locations in the Yucatań peninsula where palygorskite has been found, some of them probably exploited by the ancient Mayas.7-10 Although several authors first considered MB as a blue form of the mineral palygorskite5 or montmorillonite11 as a possible source of MB, the validity of the indigo-palygorskite model is now commonly recognized.4,6,12-14 Accordingly, MB results from the attachment of indigo, a blue dye extracted from leaves of an˜il or xiuquitlitl (Indigofera suffruticosa and other species), to the clay matrix of palygorskite, a fibrous phyllosilicate. * Corresponding author. E-mail: [email protected]. † Universitat de Vale`ncia. ‡ Universitat Politećnica de Vale`ncia. § European Synchroton Radiation Facility. | Universidad Nacional Autońoma de Me´xico.

However, the nature of the indigo-palygorskite association and the reasons for its hue and stability have become controversial.15-33 Indigo (or indigotin, 3H-indol-3-one, 2-(1,3-dihydro-3-oxo2H-indol-2-ylidene)-1,2-dihydro), is constituted by quasiplanar molecules of approximate dimensions 4.8 × 12 Å.24 The size of the indigo molecules is small enough for entering into the channels of palygorskite, a fibrous clay, whose structure was first reported by Bradley in 194034 and later refined by several authors.35-39 Palygorskite, with ideal composition Si8(Mg2Al2)O20(OH)2(H2O)4 · 4H2O, can be described as a mixture of two polymorfs: monoclinic and orthorhombic. The structure of palygorskite can be described as a continuous set of layers, each one formed by an octahedral sheet surrounded by two tetrahedral ones. The tetrahedra in each sheet present a periodic inversion of the apical oxygen, resulting in a discontinuous octahedral sheet. The tetrahedral and octahedral mesh gives rise to a series of rectangular tunnels of dimensions 6.4 × 3.7 Å, which are filled by weakly bound, nonstructural (zeolitic) water. Magnesium and aluminum cations complete their coordination with tightly bound water molecules (structural water). Sepiolite, a second fibrous aluminosilicate of ideal composition Si6Mg4O15(OH)2 · 6H2O, has also been used for preparing MB-type specimens. In this clay, all octahedral positions are occupied by Mg2+ ions, the structure defining channels of 10.6 × 3.7 Å dimensions. MB-type samples prepared from sepiolite result, however, less stable to acidic attack than those prepared with palygorskite.18,21,23,25,26 Different approaches have been proposed for explaining the nature of the indigo-palygropskite association. The indigo molecules could anchor into the channels of palygorskite4,12 or could be adsorbed onto the external surface of the clay.6 Recently, Hubbard et al.18 proposed that MB involves formation of hydrogen bonds between the carbonyl and amino groups of indigo with edge silanol units of the clay, thus blocking the

10.1021/jp900711k CCC: $40.75  2009 American Chemical Society Published on Web 06/12/2009

Maya Blue-Like Complexes entrance of indigo molecules into the palygorskite channels. Polette et al.17 and Chiari et al.19,22,25 attributed the stability of the indigo-palygorskite association to the formation of hydrogen bonds between CdO of indigo molecules and structural water molecules. Molecular modeling suggests that indigo molecules become highly disordered in the palygorskite framework.20 Further spectroscopic data, however, has suggested that hydrogen bonding between structural water molecules and N-H units of indigo occurs.25 Molecular modeling from Fois et al.20 has been used to propose that, in addition to hydrogen bonding, van der Waals interactions and a direct interaction between indigo and clay octahedral cations, not mediated by structural water, could also play an important role in anchoring indigo molecules. Spectral data suggesting the interaction of dye molecules with Al3+ ions have been recently provided.30-32 In their most recent proposal, Giustetto et al.33 suggest that indigo attachment to palygorskite is restricted to the grooves (half channels cut along their axis) present in the exterior crystal surfaces: however, this hypothesis appears to be contradicted by data from Yasarawan and van Duijneveldt for sepioliteindigo specimens.40 These last authors also consider the possibility of a head-to-tail interaction of indigo molecules in the channels of the clay. In this context, the voltammetry of microparticles, a solidstate electrochemical methodology developed by Scholz et al.,41-43 was applied by Domeńech et al. for studying Maya Blue samples and synthetic specimens.44-49 Application of this methodology, supported by infrared (ATR-FTIR) and visible spectroscopies and microscopical analysis (SEM/EDX, TEM, AFM) provided evidence for the presence of dehydroindigo, the oxidized form of indigo, in Maya Blue samples, thus introducing a new factor for explaining the peculiar hue of the pigment.44 On the basis of spectroscopy data, Vandenabeele et al.,50 suggested that other indigoid compounds may eventually be present in genuine MB samples.48,49 Thermochemical data calculated from electrochemical measurements indicated that a palygorskite-indigo complex comes spontaneously from the palygorskite-indigo association under the thermal treatment usually employed for preparing MB.44,45 Further analysis of archeological MB samples has suggested that the ancient Mayas could have employed different preparation procedures.44,46-49 It should be noted that there is no deposit of any direct historical source relating the mode of preparation of Maya Blue. Proposed methods for preparing synthetic analogues to Maya Blue are based in those initially proposed by Val Olphen:6 (i) by embedding palygorskite with a solution of indoxylacetate, (ii) vat dyeing, and (iii) dry crushing indigo with powdered palygorskite. This last method is the most extensively used in recent literature and involves mixing and grinding a mixture of palygorskite and indigo.8,12,14,15,17,18,20 The vat dyeing method involves the soaking of indigo (or possibly the maceration of Indigofera leaves) in a water suspension of palygorskite.15,17,20 In several approaches, an organic solvent is used.19,23 It is in general assumed that a heating step is required for removing zeolitic and, at least partly, structural water, for obtaining acidresistant pigments. Accordingly, after the ingredients have been combined and dried, a heating process with variable duration (from few minutes to hours or days) and a temperature range (90-100 °C, 190 °C, or even 250-300 °C) is applied.18,20,21,23,25,30,31,39 It is possible, however, to prepare acid-resistant Maya Blue-type pigments without heating.18,26 Preparation of Maya Blue pigments from indigo and sepiolite18,21,23,25,26 and using other dyes21,26 have been also reported. More recently, Arnold et al.51 have proposed

J. Phys. Chem. C, Vol. 113, No. 28, 2009 12119 that burning incense was one way in which the ancient Mayas made MB in the context of ritual ceremonies. Our more recent results, using the voltammetry of microparticles approach, suggested that different topological redox isomers of indigo/dehydroindigo molecules are attached to the palygorskite framework.48 The term topological redox isomers was coined by Turro and Garcıá-Garibay52 and Bessel and Rolison,53 in the study of zeolite-associated species and refers to the location of guest molecules in different sites in the aluminosilicate framework. Presumably, different locations of guest dye molecules in the inorganic host correspond to the existence of different generic attachment types (adsorptive or bond-forming) as well as different coordinative arrangements (hydrogen bonds with water molecules, direct interaction with silanol units, direct interaction with Al3+- and Mg2+-containing ions). In the current work, electrochemical (cyclic and square wave voltammetries, chronoamperometry), spectroscopic (attenuated total reflectance Fourier transform infrared spectrocopy, visible spectroscopy, multinuclear magnetic resonance (1H-13C CP MAS, 27Al, 29Si NMR)) and transmission electron microscopy (TEM) techniques have been applied for studying the attachment of indigo to palygorskite and sepiolite forming MB-type specimens. These specimens were prepared by crushing palygorskite and sepiolite with indigo powder and submitted to different thermal treatments. Electrochemical experiments were conducted upon immersion of MB-type probes in aqueous sodium acetate buffer. Complementary experiments were performed in contact with DMSO and MeCN containing Hex4NPF6, Bu4NPF6, Et4NClO4, and LiClO4 as supporting electrolytes to assess the existence of size-excluding effects characterizing the attachment of dye molecules to the internal regions of the clay crystals. To obtain information on the nature of the dye-clay attachment, complementary experiments have been performed in samples prepared from different dye-support pairs. These include indigo plus alumina, to test possible direct dye-Al3+ interactions, and indigo plus montmorillonite, the last a laminar aluminosilicate. For testing possible silanol-dye interactions, indigo plus silica and indigo plus MCM-41 specimens were studied. Additional experiments were performed by attaching alizarin, an anthraquinonic dye with molecular dimensions similar to those of indigo, to palygorskite. Here, hydrogen bonding between the dye and the clay is possible. Finally, complementary experiments were performed for indigo plus palygorskite specimens immersed into solutions containing Al3+ and Mg2+ ions for testing the possibility of direct dye-metal ion interactions. As a result, (1) MB can be viewed as a complex polyfunctional organic-inorganic hybrid material in which different topological isomers of indigo and dehydroindigo molecules are distributed in the inorganic support; (ii) contrary to the implicitly extended idea that one dominant dye-clay type of interaction exists in MB,17,19,22,25,30-33 such isomers can tentatively be assigned to different types of clay-dye interactions; (iii) electrochemical characterization of the existence of “internal” isomers can be performed; and (iv) combining chronoamperometric and chronocoulometric data provides an estimate of the variation with the depth of the different topological isomers. 2. Experimental Section Palygorskite was collected from the Yucatań site of Ticul;54 sepiolite from Yunclillos (Spain) was supplied by Tolsa. Two series of MB-type specimens were prepared by finely grinding

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and mixing 1.0% (w/w) of synthetic indigo (Fluka) with each one of the clays in an agate mortar and pestle for 60 min. The resulting specimens were separated into three aliquots and submitted to heating at 130 and 180 °C for 24 h in furnace. The untreated indigo-palygorskite and indigo-sepiolite specimens will be labeled as MB@PL and MB@SP, respectively. Blanks for palygorskite and sepiolite submitted to identical crushing and crushing plus thermal treatment in the absence of indigo were also prepared. The heated MB-type specimens will be labeled as MB@PL130, MB@PL180, MB@SP130, and MB@ SP180, respectively. All specimens were suspended in DMSO and ultrasonicated for 15 min. After filtration, the resulting powders were repeatedly rinsed with acetone until the entire disappearance of blue traces in the liquid phase, and dried in air. A third series consisted of indigo-alizarin specimens prepared from alizarin (Aldrich) and palygorskite using identical dosage, preparation, and thermal treatments as for the MB-like specimens. The corresponding alizarin pigments evolved from purple (AL@PL) to purple-brown (AL@PL130 and AL@PL180) under heating. Additional data were taken for indigo plus montmorillonite (IN@MT), indigo plus alumina (IN@AL), indigo plus silica (IN@SI), and indigo plus MCM-41 (IN@MCM) specimens prepared similarly. Montmorillonite (MT) was obtained through the Source Clays Repository of the Clay Mineral Society, whereas silica and alumina were Merck reagents. MCM-41 was prepared by hydrothermal crystallization, as described in the literature.55 For modified electrode preparation, ∼0.5 mg of the samples were thoroughly powdered in an agate mortar and pestle and extended, forming a spot of finely distributed material. The lower end of the graphite electrode was pressed over that spot of sample to obtain a sample-modified surface. Voltammetry of microparticle experiments was performed at sample-modified paraffin-impregnated graphite electrodes (PIGEs) using a CH I420 instrument. A standard three-electrode arrangement was used with a platinum auxiliary electrode and a AgCl (3 M NaCl)/Ag reference electrode, separated from the bulk solution by a salt bridge, in a cell thermostatted at 298 K. Experiments in aqueous media were performed with 0.50 M acetic acid/sodium acetate solutions at pH 4.75, eventually adding MgCl2 · 6H2O (Panreac) and Al3(NO3)3 · 9H2O (Panreac) in 0.01-0.10 M concentrations to the electrolyte. Complementary experiments were performed upon immersion of samplemodified PIGEs into dry DMSO and MeCN solutions using using Hex4NPF6 (Fluka), Bu4NPF6 (Fluka), Et4NClO4 (Acros), and LiClO4 (Aldrich) as supporting electrolytes. TEM images were obtained with a Philips CM10 transmission electron microscope equipped with a Keen view camera: a soft imaging system operating voltage 100 kV was used. Samples were prepared by grinding a few micrograms in an agate mortar and then dispersing them with the help of an ultrasons bath in dichloroethane. A drop of the dispersion was poured onto TEM grids pretreated with a polymer film layer with holes to improve the quality of the images. Visible spectra were obtained with a Shimadzu UV-2101 PC. ATR-FTIR spectra of indigo-modified electrodes were obtained with a Perkin-Elmer BX Spectrum Fourier transform infrared spectrometer. Number of coadded scans, 64; resolution, 4 cm-1. Solid-state NMR spectra were acquired under MAS conditions using an ASX 300 Bruker spectrometer with a magnetic field strength of 7.05 T, corresponding to a 27Al Larmor frequency of 7.8.3 MHz. Short, single pulses π/12 were used with a recycle time of 0.5 s. The 27Al chemical shift was referenced using a 1 M Al(NO3)3 aqueous solution as the

Domeńech et al.

Figure 1. TEM images of samples: (a) MB@PL180 and (b) MB@SP180.

external standard. 29Si MAS NMR spectra were obtained operating the spectrometer at a resonance frequency of 59.59 MHz with a recycling time of 8 s and a pulse time of 3 µs. The spinning frequency was 5 kHz, and tetramethylsilane (TMS) was used as a reference. 13 C CP MAS NMR spectra were obtained at a frequency of 75.422 MHz using a 4 mm cross-polarization (CP) MAS probe spinning at a rate of 5 kHz. Typical 13C CP MAS NMR conditions for the 1H-13C polarization experiment used a π/2 pulse of 4 µs, contact time of 1 ms, delay time of 5 s, and 20 000 scans. Chemical shifts were referenced to a solid shift at 38.2 ppm of adamantane, relative to TMS. 3. Results and Discussion TEM Examination of Samples. The prepared MB specimens appeared as homogeneous materials with a blue-greenish hue. TEM images of samples and blanks of the clays show agglomerates of elongated crystals of 100-30 nm size, as shown in Figure 1. Contrary to what was observed in ref 40, no traces of very characteristic irregular indigo crystals were detected in any sample, thus denoting that the rinsing extraction process used for sample preparation managed to completely eliminate the amount of free indigo not associated with the clay matrix. On comparing TEM images for crystals of palygorskite and sepiolite and those for pigment specimens prepared from such

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Figure 3. Variation with the absorbance of MB@PL of the absorbance of (a) MB@SP and (b) MB@PL180 at the same wavelength (absorbance data 25 nm spaced).

Figure 2. VIS spectra of samples: (a) MB@PL, (b) MB@PL180, (c) MB@SP, and (d) MB@SP180.

clays, one may find several significant differences: (i) the fine fibrous structure appearing in sepiolite and palygorskite crystals becomes ill-defined; (ii) in most crystals in samples subjected to crushing plus heating, the surface becomes corrugated and exhibits pores of variable size. Visible Spectrum of MB Samples. Figure 2 compares visible (350-800 nm) spectra of (a) MB@PL, (b) MB@PL180, (c)

MB@SP, and, (d) MB@SP180. The reference spectra for these compounds are taken from those theoretical and experimental in ethanol solution provided by Klessinger and Lu¨ttke56 and Perpe`te et al.57 The spectra of MB@PL and MB@SP feature a unique band, with its absorption maxima located at λmax ) 658 and 635 nm, respectively. Such spectra are similar to that of indigo in organic solvents, for which the absorption maximum is recorded at 605 nm.56-58 For samples MB@PL130, MB@SP130, and especially MB@PL180 and MB@SP180, a new absorption band appears in the 400-500 nm region with λmax between 460 and 475 nm, and the indigo band at ∼650 nm becomes wider. This spectral response can be described as a result of the superposition of the absorption due to dehydroindigo, whose spectrum consists of a unique band with λmax ) 442 nm in ethanolic solution56 and to that of indigo, in agreement with previous results44,46,49 and literature data21,59 for genuine MB samples from different archeological sites. Figure 3 shows the absorbance of (a) MB@SP and (b) MB@PL180 plotted as a function of the absorbance for MB@PL at the same wavelength (absorbance data 25 nm spaced). In the case of MB@SP and MB@PL samples, the corresponding twoabsorbance graph produces two linear regions of identical slope, just as expected for spectra showing the same one-band profile (see Figures 3a). As described in detail elsewhere,49 in the case of systems containing variable amounts of two components, each one producing a different one-peak absorption spectrum, the two-absorbance graph should ideally be formed by two different linear regions, each one corresponding to the wavelength region where the absorbance of each one of the components prevails over the other. This situation is close to that depicted in Figure 3b, corresponding to the plot of the absorbance of MB@PL180

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Figure 4. Spectral profile resulting from the subtraction of the spectrum of MB@PL180 from that of MB@PL: (a) point-to-point subtraction and (b) averaged curve. Averaged spectra for indigo and dehydroindigo are also presented.

vs that for MB@PL. Identical results were obtained plotting the absorbance of MB@SP180 vs that of MB@SP. First of all, the close similarity between the spectral profiles obtained for palygorskite- and sepiolite-based specimens clearly suggests that the exclusive attribution to a specific dye-Al3+ interaction as responsible for the structure of MB30-32 needs to be revised. Yasarawan and van Duijneveldt,40 reported two overlapping bands at 650 and 710 nm in toluene suspensions (concentrations

Domeńech et al. up to 0.16 wt %) of indigo-doped sepiolite with an indigo concentration of 4.5 wt %. These authors do not report dehydroindigo bands, but their reported spectra are confined to the 500-800 nm wavelength range.40 The band at 710 nm was assumed to denote the existence of indigo dimers, an effect studied in detail by Miliani et al.60 for indigo solutions in CHCl3. Although indigo molecules located in clay channels are expected to associate in a different way from in solution, a certain headon-tail interaction could be possible in both cases. This association should result in a red shift of absorption bands, as studied by Kasha.61 In contrast to these results, spectra of MBtype specimens in Figure 2 exhibit only a weak absorption at 710 nm. This band, however, increases on increasing the temperature of thermal treatment, as can be seen using subtractive spectra (vide infra). Using the value for peak absorptivity per mass unit of indigo in indigo-doped sepiolite calculated by Yasarawan and van Duijneveldt,40 for the peak at 650 nm (51 (wt %)-1 cm-1), our absorbance values allow us to estimate indigo concentrations ranging from 0.40 (for MB@PL and MB@SP) to 0.25 wt % (for MB@PL180 and MB@SP180), in agreement with electrochemical data (vide infra). The difference spectrum of MB@PL minus MB@PL180 (see Figure 4a) apparently produced several bands. Smoothing the subtracted spectrum resulted in the profile depicted in Figure 4b, where “difference bands” at 475, 510, 540, 570, 615, 645, and 690 nm appear. Several bands, marked by vertical dotted lines, correspond to absorbance maxima of indigo or dehydroindigo and are simply the result of the different concentrations of such components in MB@PL and MB@PL180 samples. Interestingly, a clear absorption difference was recorded at 690 nm, close to the dimer band at 710 nm described in the literature.40 Other difference bands, however, correspond to different wavelengths (marked by point lines in Figure 4b) and should be attributed to other absorbing species. Remarkably, difference bands at identical wavelengths were obtained by subtracting the spectrum of MB@SP from that of MB@SP180. These spectral features can be attributed to (i) the occurrence of additional absorbing compounds (apart from indigo and dehydroindigo) in the sample, (ii) the effect of different palygorskite-dye associations (i.e., different topological dye isomers), and eventually, (iii) the appearance of dye-dye associations. Regarding the first hypothesis, one should expect the presence of additional compounds in genuine MB samples containing natural indigo, which is always related to other products, such as indirubin. This can be confirmed by the appearance of a voltammetric signal at -470 mV47 and an absorption band at λmax ) 550 nm.48,58 The spectra analyzed here (vide infra) do not present any signature of this type, probably because of the use of pure synthetic indigo so that it appears more reasonable to discuss the absorbance splitting in subtracted spectra (Figure 4) on the basis of the hypotheses ii and iii. Moreover, it should be noted that the main band for indigo in MB@PL and MB@SP (λmax ) 660 nm) is shifted with respect to the typical band of indigo in solution (λmax ) 605 nm). This is attributable to the bathochromic effect produced by the strong interaction between indigo molecules and the palygorskite matrix, as discussed by Reinen et al.21 A similar effect should be responsible for the displacement of the main dehydroindigo band from a typical value in solution (λmax ) 440 nm) to 460-475 nm. Accordingly, the presence of multiple bands in the difference spectra of Figure 4 could be taken as indicative of the coexistence of different topological isomers of indigo and dehydroindigo, each one



Figure 5. ATR-FTIR spectra for (a) MB@PL, (b) MB@PL130, and (c) MB@PL180.

having a different attachment to the palygorskite or sepiolite lattice. The idea of the coexistence of different types of association between the indigo (or dehydrodndigo) and the clay lattice was proposed previously.48 It is known that indigo interacts with several clays, including lamellar clays such as montmorillonite, which is demonstrated by the fact that their Raman spectra are similar to that of MB.28 The coexistence of multiple interactions could also be the reason why many different bonding mechanisms have been proposed in the literature for indigo-palygorskite and indigo-sepiolite associations. However, it is crucial to identify which interactions are responsible for the great stability of MB. These interactions should account for irreversible attachment processes. Therefore, it is extremely important for their analysis to eliminate the part of the material that did not interact, or interacted very loosely, by an energetic washing, as performed here. ATR-FTIR Data. Infrared spectra of current MB@PL and MB@SP samples reproduce features reported for palygorskite25,28,30 and sepiolite-based28,29 synthetic specimens and Maya Blue samples.25,28,43,44,49 ATR-FTIR spectra for (from upper to below) MB@PL, MB@PL130, and MB@SP180 are depicted in Figure 5. The loss of water in heated samples is denoted by the decrease in bands at 3100-3300 cm-1 and the variation in the profile of the O-H band around 1660 cm-1, corresponding to the clay’s zeolitic water. It should be noted that, according to literature data, the loss of zeolitic water occurs at temperatures above 230 °C so that it is reasonable to expect that significant amounts of zeolitic water persist in the studied MB-type specimens.18,25,33 The distinction between zeolitic and structural water, however, becomes uneasy due to the complicated profile of the O-H absorption bands in the concerned spectral window and the superposition of bands attributable to dye molecules in MB-type samples, as can be seen in Figure 6, where spectra restricted to the 1400-1800 cm-1 wavenumber region for PL180, indigo, IN@MT, MB@PL, and MB@PL180 are shown. The bands corresponding to ν(CdO) and ν(CdC) vibrations are not easily distinguished, and appear in the 1600-1750 cm-1 zone. These bands are expected to be modified from those of indigo when the indigo-clay complexation is produced, and they are overlapped with a broadband centered around 1660 cm-1,

corresponding to the clay’s zeolitic water, which is important also in pigment samples, thus indicating the recognized persistence of zeolitic water in the pigments.18,25,33 The band at ∼1725 cm-1 is attributable to dehydroindigo,44 following the assignment from Klessinger and Lu¨ttke.56 Essentially identical results were obtained for indigo-sepiolite samples. Remarkably, the spectra for MB@PL and MB@PL180 in the 1800-1400 cm-1 window become clearly different from the spectrum of indigo and the spectrum of indigo attached to laminar clays, such as montmorillonite (see Figure 6). Thus, spectra for indigo and IN@MT exhibit almost identical bands at 1460 and 1480 cm-1. In sharp contrast, spectra for MB@PL and MB@PL180 exhibit a significantly different profile, with bands at 1792, 1772, 1750, 1734, 1717, 1700, 1696, 1684, 1667, 1653, 1648, 1636, 1623, 1616, 1576, 1569, 1558, 1540, 1521, 1507, 1489, 1472, 1457, 1436, and 1418 cm-1. At the expense of concrete band assignment, the observed spectral profile can, in principle, be attributed to the presence of different dye molecules with different associations with the clay matrix; that is, to indigo and dehydroindigo topological isomer molecules occupying different positions in the clay framework. The above spectral features differ from those obtained for alizarin and AL@PL specimens, as can be seen in Figure 7, where ATR-FTIR spectra of the dye and AL@PL are shown. Remarkably, the spectrum of AL@PL is similar to that resulting from the superposition of the individual spectra for the pristine dye and palygorskite. This situation is similar to that observed for indigo and IN@MT (see Figure 6) but clearly differs from spectral data for indigo and MB@PL180 (see Figure 6). In particular, multiple peak features observed in the 1600-1800 cm-1 wavenumber region for MB@PL180 are absent in IN@MT, AL@PL, and AL@PL180. In all these last systems, only “external” adsorption of dye molecules occur and the spectra of these materials become essentially identical to that of the pristine dye. In contrast, genuine MB-type specimens prepared from indigo and palygorskite or sepiolite exhibit significant peak splitting, thus suggesting that different dye molecules/different coordinative arrangements should exist. All these observations can be considered as consistent with the idea that different

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Domeńech et al. Figure 8). The close similarity among all the spectra suggests that there are no significant modifications in the environment of the Al3+ centers upon attachment of indigo to the palygorskite matrix. Of course, water coordinated to aluminum could leave the coordination sphere, and then dye interacts directly with aluminum centers, thus preserving the observed octahedral environment of aluminum. However, note that such a fact should modify the broadness of NMR signal because of the high electronic density of dye molecule. This is not the case, and then it seems that a direct Al-dye interaction does not occur. The indirect examination of the aluminum close to dye is preferential because of the polarization transfer from the excited protons of the indigo molecule to the neighboring aluminum throughout heteronuclear dipolar interaction. 27Al and 1H can interact through dipolar coupling according to the Hamiltonian operator as follows:

H)-

Figure 6. ATR-FTIR spectra in the 1400-1800 cm-1 wavenumber region for (a) PL180, (b) indigo, (c) IN@MT, (d) MB@PL, and (e) MB@PL180. Spectra vertically translated for clarity.

topological isomers of indigo exist with regard to the dye attachment to palygorskite and sepiolite frameworks. NMR Data. Figure 8 shows 27Al MAS NMR spectra for (a) palygorskite, (b) MB@PL, and (c) MB@PL180 accompanied by (d) the 1H-27Al CP/MAS spectrum for MB@PL180. There are no changes in the Al NMR signal. Spectra are composed of an intense peak close to 4 ppm, which is assigned to aluminum 6-fold coordinated in an oxygen environment. A very low intense peak close to 60 ppm was identified, revealing a minimum amount of aluminum 4-fold coordinated (see inset in

γHγAlh 2πrHAl3

(3 cos2 Θ - 1)IAlIH

(1)

where γH and γAl are the gyromagnetic constants of Al and H nuclei, respectively, h is the Planck constant, IH and IAl are the corresponding nuclear spin, and Θ is the angle between the internuclear vector and the external magnetic field. It is important to note that dipolar interaction depends on the distance (rHAl) between the Al and H nuclei. These results suggest that, in our clay, aluminum is structurally stable, and they are not exposed to interact with the dye because they are well shielded by six oxygen neighbors. These features act in contravention of the hypothesis of Chianelli et al.30-32 attributing to the Al3+-dye direct coordination a capital role in the indigo-clay association. However, note that differences could be present in the clay used by these authors and those used here. It has been previously reported that interaction between aluminum and organic molecules takes place advantageously when unsaturated aluminum centers (e.g. aluminum 4-fold and 5-fold coordinated) are present.62 Contrary to what happens for aluminum resonances, Figure 9 reveals that 29Si NMR signals are shifted to stronger fields and became broader as a consequence of dye incorporation to palygorskite. It seems that electronic density increases close to (SiO4)4- tetrahedral units, or in other words, indigo could be adsorbed close to silicon atoms, maybe interacting through Si-OH groups of clay and polar groups or π bonds of the dye. In addition, the interaction of the indigo with silicon could come from the external attachment of the indigo to the palygorskite surface via terminal silanols. This bonding is not crucial for determining the stability of MB-like materials, and it has been seen to exist in indigo attached to laminar clays, also producing Raman features similar to those for MB.28 13 C CP MAS NMR spectra of indigo, MB@PL, and MB@PL180 are shown in Figure 10. Comparison of the spectrum of MB@PL180 and the 13C NMR simulated spectra of indigo and dehydroindigo reveal some significant correlations, as can be seen in Table 1. The majority of peaks can be attributed to indigo signals suffering peak splitting. However, signals at 174.3, 168.9, and 166.5 ppm can be attributed only to dehydroindigo signals (or indigo signals but undergoing a considerably high shift). Several groups of signals (152.2 and 150.5, 137.2, 135.2 and 132.5 ppm, and 123.8 and 122.2 ppm) can be attributed to indigo and dehydroindigo. The most reasonable conclusion from such data is that, confirming previous results,44 dehydroindigo accompanies indigo in palygorskite.



Figure 7. ATR-FTIR spectra for (a) alizarin and (b) AL@PL in the 1400 to 1800 cm-1 wavenumber region.

Figure 9. 29Si HPDEC MAS/NMR for (a) palygorskite and (b) MB@PL180.

Figure 8. 27Al MAS/NMR spectrum for (a) palygorskite, (b) MB@PL, (c) MB@PL180, and (d) 1H-27Al CP/MAS for MB@PL180. The asterisk (*) indicates spinning side bands (10 kHz). The nset displays the spectra in the scale where commonly aluminum 4-fold resonances are present.

The high peak splitting observed for palygorskite-indigo specimens was not observed for crushed sepiolite-indigo ones,18 for which only few peaks exhibited peak splitting. Hubbard et al.18 interpreted peak splitting in terms of the presence of two species of indigo, one being unaffected pure indigo and the other representing an altered state of the indigo molecules. Following this line of reasoning, the remarkable peak splitting observed in the spectrum of MB@PL180 suggests that indigo molecules are submitted to significant interactions with the palygorskite framework, resulting in the appearance of different topological isomers, but also suggests that possibly, indigo-indigo interactions occur for palygorskite-associated species. Solid State Voltammetry. Figure 11 shows the SQWV response of (a) MB@PL, (b) MB@PL130, (c) MB@PL180, (d)

Figure 10. 1H-13C CP/MAS NMR for (a) indigo, (b) MB@PL180 with a 6% wt indigo, (c) MB@PL, and (d) MB@PL180.

MB@SP, (e) MB@SP130, and (f) MB@SP180 attached to PIGEs in contact with acetate buffer upon scanning the potential from -0.75 V in the positive direction. For MB@PL and MB@SP, well-defined peaks at +0.45 (I) and -0.28 V (II) vs AgCl/Ag appear.

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TABLE 1: 1H-13C CP MAS NMR Data for MB@PL180 Detailing Possible Correlations and Peak Shifts with the Corresponding Spectra for Indigo (5 kHz) and Simulated Dehydroindigo Spectruma δ (ppm) indigo (5 kHz) 188.5

152.8 134.6

124.5 120.8 113.5 a

δ (ppm) MB@PL180 194.6 192.6 191.1 174.3 168.9 166.5 152.2 150.5 137.2 135.2 132.5 126.1 124.5 123.8 122.2 115.5 113.7

possible C atom assignment

δ (MB@PL180)-δ (indigo) (ppm)

1

4 6 and 8

2 7 3 and 5

possible assignment dehydroindigo (simulated)

δ (MB@PL180)-δ (dehydroindigo) (ppm)

1 and 2 (163.0 and 162.4)

11.3 5.9 3.5 2.2 0.3 1.3 -0.7 -3.4 -1.7

6.1 4.1 2.6

-0.6 -2.3 2.6 0.6 -2.1 0.0 3.0 1.4 2.0 0.2

4 (150.2) 3 (130.2) 6 (131.2) 8 (135.9) 7 (127.8) 5 (122.8)

1.0 0.6

Peak taken for estimate peak shifts is underlined.

Figure 12. SQWVs for (a) AL@PL and (b) AL@PL180 attached to PIGEs in contact with 0.50 M HAc/NaAc aqueous buffer (pH 4.75). Potential scan initiated at -0.75 V in the positive direction. Potential step increment, 4 mV; square wave amplitude, 25 mV; frequency, 5 Hz.

Figure 11. SQWVs for (a) MB@PL, (b) MB@PL130, (c) MB@PL180, (d) MB@SP, (e) MB@SP130, (f) MB@SP180 attached to PIGEs immersed into 0.50 M HAc/NaAc aqueous buffer (pH 4.75). Potential scan initiated at -0.75 V in the positive direction; potential step increment, 4 mV; square wave amplitude, 25 mV; frequency, 5 Hz.

The peak potential for processes I and II remained essentially frequency-independent in the 2-200 Hz frequency range, suggesting that both electrochemical processes behave almost reversibly. For MB@PL130 and MB@SP130, peak I is accompanied by secondary peaks at +0.55 and +0.61 V, whereas the width of peak II increases. For MB@PL180 and MB@SP180, signal I appears as three overlapping peaks at +0.47 (Ia), +0.55 (Ib), and +0.61 V (Ic).

The mean concentration of indigo in MB-type samples can be estimated by comparing the peak current for process I for MB@PL and MB@SP with that for heated samples in lowfrequency SQWVs. For MB@PL180 and MB@SP180, the average concentrations are between 0.25 and 0.35 wt %, in agreement with visible spectra data. Similar features were obtained for indigo-sepiolite specimens. However, no comparable variation of the electrochemical response was obtained for alizarin-palygorskite specimens. As can be seen in Figure 12, the voltammetric response of AL@PL, AL@PL130, and AL@PL180 was essentially identical, with peaks at +0.40 (III) and -0.47 V (IV), which correspond to the oxidation of o-diphenol and the reduction of the quinone group of alizarin, respectively.64 Additionally, it should be noted that peak splitting observed for heated MB-like specimens was observed in neither the electrochemistry of immobilized mi-



croparticles of pristine pigments63,64 nor in indigo associated with laminar clays such kaolinite and montmorillonite.48 All these features clearly suggest that peak splitting can be attributed to a specific indigo-palygorskite and indigo-sepiolite interaction that differs for a merely external adsorption, as occurring for alizarin-sepiolite, alizarin-palygorskite, indigo-montmorillonite, and indigo-kaolinite specimens. This is in agreement with the sitedependent photochemical reactivity52,65,66 and electrochemistry53,66-68 of electroactive species anchored to zeolites. Accordingly, processes I and II can be represented, respectively, as the oxidation of indigo (H2IN) to dehydroindigo (IN) and the reduction of indigo to leucoindigo (H4IN), as:

{H2IN} f {IN} + 2H+(aq) + 2e-

(2)

{H2IN} + 2H+(aq) + 2e- f {H4IN}

(3)

where { } represents clay-associated species. Charge conservation is ensured in such processes by the coupled issue/entrance of protons and electrons in the solid. Interestingly, upon repetitive scanning the potential in MB@PL180 and MB@SP180 samples, the peak Ic at +0.61 V vanishes, whereas the peaks at Ia and Ib remain essentially unchanged, with only a slow peak current decay in successive scans. This is just one of the conditions characterizing the electrochemistry of species strongly attached to microporous aluminosilicates,53 thus suggesting that the signals Ia at +0.45 and Ib at +0.55 V can be attributed to a strongly palygorskiteor sepiolite-associated species. In contrast, the peak Ic at +0.61 V can be assigned to a species confined to the surface of the particles, which is rapidly exhausted upon repetitive potential scans. The above considerations are confirmed by experiments performed upon immersion of sample-modified PIGEs into MeCN solutions containing Hex4N+, Bu4N+, and Et4N+ electrolyte ions. Here, the most significant results were obtained for the indigo-to-leucoindigo reduction process already described.45 Pertinent data are shown in Figure 13, where SQWVs of MB@PL180-modified electrodes immersed into Hex4NPF6/ MeCN, Bu4NPF6/MeCN, and Et4NClO4/MeCN are shown. Identical results were obtained for MB@SP180. Here, in the presence of bulky Hex4N+and Bu4N+ cations, only one peak at -0.76 V is recorded (Figures 13a,b), attributable to the reduction of external indigo to its corresponding radical anion, as described by Bond et al.63 for indigo in solution. This process can be represented as

H2IN(ads) + e- f H2IN · -(ads)

(4)

where (ads) denotes adsorbed species. Only in the presence of Et4N+ electrolyte, a cation able to efficiently enter into the palygorskite channel system, is an additional peak at -0.54 V (see Figure 13c) recorded. This peak can unambiguously be assigned to the reduction of palygorskite-associated species,

{H2IN} + Et4N+ + e- f {H2IN · -Et4N+}

(5)

involving the ingress of electrolyte Et4N+ cations into the clay channels system. Electrochemistry of the Indigo-Clay Interaction. To assign specific clay-indigo interactions to the different topo-

Figure 13. SQWVs of MB@PL180-modified electrodes immersed into Hex4NPF6/MeCN, Bu4NPF6/MeCN, and Et4NClO4/MeCN, all at a concentration of 0.10 M. Potential scan initiated at 0.0 V in the negative direction. Potential step increment, 4 mV; square wave amplitude, 25 mV; frequency, 5 Hz.

logical isomers previously studied, the voltammetric response of MB-type specimens was compared with that of different indigo support pairs and that of indigo microparticles in contact with solutions containing Mg2+ and Al3+ ions. Peak potentials were taken as average values from five independent measurements. Maximum deviations of (5 mV were obtained in all cases, thus suggesting that although small, consistent peak potential variations were obtained from one system to another. Figure 14 compares SQWVs for microparticulate deposits of indigo in contact with aqueous sodium acetate buffer (a) before, and (b) after addition of Mg2+- and Al3+-containing salts and, additionally, with that for IN@AL. The oxidation of indigo (peak I) in acetate buffer occurs at a potential of +0.46 V, slightly more positive than peak I recorded for unheated MBtype specimens previously described (+0.45 V) (see Figure 14a). In the presence of Mg2+ or Al3+ ions in the electrolyte, however, peaks I and II become positively shifted 5-30 mV, the peak potential shift increasing on increasing the concentration of metal ion from 0.01 to 0.10 M until a value of +0.50 V is achieved. For large concentrations of the metal ion, an additional peak at +0.33 V (V) was recorded, as can be seen in Figure 14b. Interestingly, in SQWVs for IN@AL in contact with acetate buffer, the peak V becomes clearly marked, whereas peaks I and II appear at identical potentials as for indigo microparticles (see Figure 14c). Voltammograms for indigo plus silica, montmorillonite, and MCM-41 (Figure 14d) were found to be close to that of indigo microparticles, the peak potential of I being slightly displaced toward +0.43 V. The detail of peak potential shifts observed for peak I can be seen in Figure 15. The appearance of two different features in the voltammetry of indigo microparticles in the presence of Mg2+ and Al3+ ions can be rationalized upon assuming the existence of two different interactions between such metal ions and indigo molecules. By the first token, the positive potential shift can be attributed to the formation of water-mediated surface complexes acting as an energy barrier, which increases the difficulty in oxidizing indigo to dehydroindigo (process I). The formation of such

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(Mn+){H2IN} f (Mn+){IN} + 2H+(aq) + 2e-

(7)

The appearance of a new signal (peak V) at +0.33 V, however, can be interpreted as due to the formation of direct metal ion-indigo surface complexes partially breaking the hydrogen bond system lying surface indigo molecules with those located in the crystal bulk. The overall oxidation process could be described as

{H2IN} + (2/n)Mn+(aq) f {(M)2/nIN} + 2H+(aq)

(8) {(M)2/nIN} f {IN} + (2/n)Mn+(aq) + 2e-

(9)

The electrochemical oxidation of the metal ion-indigo surface adduct is facilitated, with respect to the oxidation of pristine indigo crystals, as a result of the breaking of the strong H-bond system of indigo due to the ingress of Mn+ ions. As far as the oxidized form of indigo, dehydroindigo, does not show a hydrogen bond system comparable to that of indigo, and one can expect that the metal ion-dehydroindigo adduct is thermodynamically favored with respect to the metal ion-indigo adduct. Consistently with the foregoing set of considerations, the voltammetry of indigo attached to alumina (IN@AL), where only a “direct” Al3+-indigo interaction is possible, exhibits the peak V, attributed to that strong Al3+-indigo interaction, but the main oxidation peak I becomes unchanged with respect to the peak of indigo. Remarkably, peaks Ia, Ib, and Ic recorded for MB@PL and MB@SP specimens exhibit peak potential shifts with respect to the peak I for indigo microparticles, but the peak at +0.33 V is absent in such MB-type specimens. These features suggest that, in agreement with NMR data, interaction of Mg2+ and Al3+ ions with indigo molecules should be (at least mainly) mediated by hydrogen bond formation with structural water. Accordingly, the isomer responsible for the Ic peak at +0.61 V can be described as an “external” indigo molecule attached to the surface of palygorskite crystals and eventually placed in the channel grooves. Isomers Ia and Ib can tentatively be

Figure 14. SQWVs for indigo microparticles immersed into (a) 0.50 M HAc/NaAc (pH 4.75) or (b) 0.50 M HAc/NaAc plus 0.10 M MgCl2 · 6H2O (pH 4.75). (c) Voltammogram for IN@AL in contact with 0.50 M HAc/NaAc (pH 4.75) and (d) voltammogram for IN@MCM in contact with 0.50 M HAc/NaAc (pH 4.75). Potential scan initiated at -0.75 V in the positive direction; potential step increment, 4 mV; square wave amplitude, 25 mV; frequency, 5 Hz.

complexes and their subsequent electrochemical oxidation could be represented as (Mn+ ) Mg2+, Al3+)

{H2IN} + Mn+(aq) f (Mn+){H2IN}

(6)

Figure 15. Detail of peak I region in SQWVs for indigo microparticles immersed into (a) 0.50 M HAc/NaAc (pH 4.75), (b) 0.50 M HAc/ NaAc plus 0.10 M MgCl2 · 6H2O (pH 4.75), and (c) IN@MCM in contact with 0.50 M HAc/NaAc (pH 4.75). Potential scan initiated at -0.75 V in the positive direction; potential step increment, 4 mV; square wave amplitude, 25 mV; frequency, 5 Hz.



Figure 16. Variation of charge with time in chronocoulometric experiments performed at MB@P180-modified electrodes immersed into 0.50 M HAc/NaAc aqueous buffer (pH 4.75). Charges at applied potentials of +0.70 V (solid rhombs), +0.60 V (solid squares) and +0.475 V (solid triangles). Differences between charges at +0.70 and +0.60 V (squares) and +0.60 and +0.475 V (rhombs) are also shown.

described as “internal” isomers, placed in the palygorskite channels and lying, respectively, and Mg2+ and Al3+ ions through hydrogen bonds mediated by structural water molecules (positive peak potential shift with respect to indigo microparticles, isomer Ib) and attached to silanol groups (negative peak potential shift with respect to indigo microparticles, isomer Ia). Such attributions, however, must be taken as merely tentative so that a direct interaction between indigo (and dehydroindigo) molecules with Mg2+ and Al3+ ions, nonmediated by structural water, cannot be discarded. Spectral and electrochemical data presented here suggest that similar attachments of dye molecules occur to both palygorskite and sepiolite clays. The difference between the resistance to acidic attack (larger for palygorskite than for sepiolite) can be explained by the fact that the channels in sepiolite (10.6 × 3.7 Å) are larger than the palygorskite channels (6.4 × 3.7 Å), thus making the internal surfaces of sepiolite more accessible to aggressive agents than those of palygorskite, as discussed by Sua´rez-Barrios et al.74 and Sańchez del Rıó et al.26 Concentation Profiles and Dehydroindigo/Indigo Relationship. To estimate the variation of indigo concentration with depth for the different topological isomers, chronocoulometric experiments were performed by applying different potential steps. The idea, based on the model developed by Lovric, Oldham, Scholz, and co-workers69-72 on the electrochemistry of ion-insertion solids, is that the charge passed at a time t under the application of a given constant potential, under diffusioncontrolled conditions, should be representative of the total number of electroactive molecules reached by the advance of the diffusion layer through the crystal. In short, it should be noted that solid state voltammetric experiments under diffusive conditions involves the advance of the diffusion layer through the crystals. As previously described,48 this means that at short times, the response will be representative of the composition of the solid in the more external layers of the crystals, whereas at longer times, the composition of more depth regions will also be influential on the electrochemical response. To perform the CC experiment under diffusive conditions, a potential of 100-150 mV more positive than the corresponding voltammetric peak for oxidation processes is necessary.

Figure 17. Variation with the apparent depth, xapp () (Det)1/2), of (a) estimated amount of topological isomers Ia (squares), Ib (triangles), and Ic (rhombs) in MB@PL180 and (b) estimated molar fraction of dehydroindigo for MB@PL180 (rhombs) and MB@SL180 (squares). From CC and CV data, respectively.

To separate the contributions of the different topological isomers, CC experiments at different applied potentials were performed. Roughly, one can expect that under application of a potential of +0.475 V, isomer Ia would be oxidized under diffusive conditions, where isomers Ib and Ic should remain unoxidized. Application of a potential of +0.60 V should oxidize conjointly isomers Ia and Ib under diffusive conditions, whereas application of a potential of +0.70 V should produce the reduction of all three hypothesized Ia, Ib, and Ic isomers. Then, subtracting the charges measured at a time t for each one of the applied potentials, qE(t), one can separate the contributions for the different topological isomers (qa(t), qb(t), qc(t)); i.e., the (q0.70(t) - q0.60(t)) difference should be representative of the number of accessible molecules of the isomer Ic, and the (q0.60(t) - q0.475(t)) difference should correspond to the Ib isomer, the value of q0.475(t) yielding directly the contribution of isomer Ia. The corresponding data for MB@PL180 are shown in Figure 16. Consistent with the foregoing set of considerations, the

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charge passed for isomer Ic, qc(t) () q0.70(t) - q0.60(t)), decreases rapidly with time, thus suggesting (vide infra) that this species is exhausted on increasing depth. The above variations of charge passed with time can be assimilated to variations of concentration with depth by taking an apparent depth, xapp, calculated as the advance of the diffusion layer. As far as electron diffusion is rate-determining,45,48 xapp can be approached by (Det)1/2, where De represents the coefficient of diffusion for electrons. This was reevaluated from the previously calculated effective concentration of indigo (5 × 10-4 mol/cm3), the density of the clays (∼2.3 g/cm3), and the average size of the particles (200 × 25 × 25 nm) estimated from TEM measurements. For sample deposits of 0.5 mg, these parameter values lead to N ) 1.74 × 1012 particles. Values for the coefficients of diffusion of electrons (De) and protons (DH) of 9.8 × 10-12 and 6.5 × 10-9 cm2/s, respectively, were calculated from chronoamperometric data, as already described.48 The calculated variations of concentration (in arbitrary units) with depth are depicted in Figure 17a, assuming proportions of 40%, 10%, and 50%, for isomers Ia, Ib, and Ic, respectively, in MB@PL180. These proportions are consistent with the relative peak current values in SQWVs in Figure 11. The variation of the dehydroindigo/indigo molar ratio with the depth was estimated by using a modification48 of the method devised by Scholz and Hermes73 for determining the composition of a system containing a reversibly reducible/oxidizable electroactive species in two oxidation states. Using CVs at different potential scan rates, an estimate of the variation of the molar fraction of indigo, RDI, with the depth in palygorskite or sepiolite crystals can be made. The resulting RDI vs xapp plots for MB@P180 and MB@SP180 are shown in Figure 17b. In these last results, however, no discrimination between the different topological isomers was made. Conclusions Combination of voltammetry of microparticles with electron microscopy, multinuclear magnetic resonance, and visible and infrared spectroscopies provides experimental data supporting the idea that indigo molecules attach the clay matrix in a similar way for palygorskite or sepiolite when forming Maya Bluelike materials. Different topological isomers displaying differentiated electrochemical and spectral responses can be discerned, all distributed in a relatively narrow region near the surface of the clay crystals, but not limited to the external region, as denoted by size-excluding electrochemical experiments. Reported data suggest that a head-to-tail interaction among indigo molecules within palygorskite may exist. Dehydroindigo accompanies indigo in sepiolite- and palygorskite-based specimens, its distribution also varying with the depth within the clay particles, depending on the thermal treatment used for sample preparation, the maximum dehydroindigo/indigo relationship being estimated as ∼20%. Each one of the different topological isomers is probably associated with a particular coordinative arrangement, mainly involving coordination with Al3+ and Mg2+ ions mediated by structural water and bonding with silanol groups of the clay. These results provide a more complex view of Maya Blue as a polyfunctional organic-inorganic hybrid system in which the guest molecules are distributed in different sites in the palygorskite (or sepiolite) host network. Acknowledgment. Financial support is gratefully acknowledged from the Generalitat Valenciana GV04B/197 and GV04B/ 441 I+D+I Projects and the MEC projects CTQ2004-06754-

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