Effect of the layer charge of clay minerals on optical properties of organic dyes. A review

Applied Clay Science 34 (2006) 58 – 73 www.elsevier.com/locate/clay

Effect of the layer charge of clay minerals on optical properties of organic dyes. A review Juraj Bujdák ⁎ Institute of Inorganic Chemistry, Slovak Academy of Sciences, SK-845 36 Bratislava, Slovak Republic Received 15 August 2005; accepted 17 February 2006 Available online 8 September 2006

Abstract Interactions between clay minerals and cationic organic dyes cause significant changes in the optical, spectral and chemical properties of the chromophores. These changes are due to the formation of supramolecular assemblies of dye cations, called molecular aggregates. Numerous experiments indicate that dye molecular aggregation is sensitively controlled by the layer charge of a clay mineral. Interpretations are based on a detailed analysis of papers investigating reactions of dyes with clay minerals but also considering the reactions with other inorganic solid materials and templates. Older papers dealing with the subject and alternative interpretations of the phenomenon are analysed and critically reviewed. Significance for clay science, material sciences, nanotechnology and potential industrial applications are discussed. © 2006 Elsevier B.V. All rights reserved. Keywords: Clay minerals; Layer charge; Molecular aggregation; Organic dyes; Electron spectroscopy

1. Introduction Interest in nanomaterials is rapidly increasing due to their unique properties. Supramolecular assemblies are one of the most interesting types of nano-sized species (Vos et al., 2004). Supramolecular chemistry is based on spontaneous self-assembly of molecules. The selfassembly is based on the stereochemistry of the molecular components including the distribution of active groups. The molecules (ions) in supramolecular systems are held together by non-covalent bonds, mostly due to intermolecular van der Waals’ forces, ionic interactions and H-bonds. Some organic dyes form molecular aggregates that self-assemble into supramolecular systems (Kuhn, ⁎ Tel.: +421 25941 0459; fax: +421 25941 0444. E-mail address: [email protected]. 0169-1317/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2006.02.011

2000). Dye molecular aggregation may proceed to a limited extent in concentrated solutions or to greater extents in the presence of polyelectrolytes and at interfaces. However, the extent of formation and the “quality” of dye molecular aggregates formed at clay mineral surfaces are unique. The very interesting phenomena dealing with molecular dye aggregation on clay surfaces have been described for almost 50 years (Bergman and O'Konski, 1963). However, the close relationship between dye molecular aggregation and layer charge of clay minerals has been only recently discovered (Bujdák and Komadel, 1997). This paper focuses on the basic phenomenon of dye molecular aggregation on clay surfaces, summarising novel findings of dye/clay interactions, critically reviews literature, and addresses the significance of the phenomenon for clay science and other scientific fields.

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2. Dye molecular aggregates Organic dyes may change colour and other optical and chemical properties in relation to their chemical environment. Therefore, dyes have often been used as probes or sensors in various scientific fields. The idea of using dyes to help identify clay minerals or to probe their properties first appeared several decades ago (Yariv, 2002, and references therein). This paper deals with the phenomenon related to dye molecular aggregation on the surfaces of clay minerals. The molecular aggregation is due to hydrophobic interactions, which is a general tendency of non-polar molecules to associate physically in aqueous solutions. The presence of non-polar molecules in water disrupts the hydrogen bond network of water molecules, resulting in a loss of translational and rotational degrees of freedom of the solvent within the hydrophobic hydration shell. Such a mechanism is generally referred to as a loss of entropy of the system. Absorption spectra of dye molecular aggregates usually show large differences when compared to the individual molecules. Optical properties of the dye molecular aggregates are explained by the molecular exciton1 model (Fig. 1). The model considers electrostatic interactions between transition moments of individual dye molecules, called an exciton coupling. Exciton coupling leads to the splitting of the excited electronic state of the molecule (Fig. 1, right), the magnitude of which depends on distances between interacting transition dipoles and their arrangement in space. Two excited energy levels are the result of electrostatic attraction and repulsion between the transition moments (Antonov et al., 1999). There are two main types of dye molecular aggregates, formally and originally assigned according to their optical properties, but closely related to their structure and the type of intermolecular association. Haggregates are based on a sandwich-type intermolecular association (Fig. 1, upper). Less frequent J-aggregates (Fig. 1, lower) are formed by head-to-tail intermolecular interactions (Kobayashi, 1996). In the H-aggregates, coupled transition moments could be oriented in either the parallel or anti-parallel fashion. The anti-parallel orientation (Ψ−) of an H-dimer has a lower energy state, due to the electrostatic attraction between the transition moments. In the J-aggregates, anti-parallel moments (Ψ−) result in a higher energy due to electric repulsion. Resulting dipole for the anti-parallel arrangements of the transition moments is always zero; and therefore, the 1

Exciton is an electrically neutral excited state, often regarded as a bound state of an electron and a hole.

Fig. 1. Energy graphs for the dye dimers of H- and J-type types according to a molecular exciton theory. Ψ+–directions of the transition moments are parallel, and the transition is non-zero. Ψ−– directions of the transition moments are anti-parallel, and the transition is zero.

transitions to such states are symmetry forbidden. Only the transitions to the states with parallel orientations (Ψ+) are allowed, which is to the lower energy state for the J-aggregates and to the higher energy state for the Haggregates. The perfect H- and J-aggregates are just two ideal states of all possible forms and variations of molecular assemblies. Many dyes may form such assemblies, which include the structural features of both the H- and J-aggregates. In such cases, the molecular aggregates absorb light with energies corresponding to both the higher and lower energy states. The assignments of the bands absorbing at the low- and high-energy transitions indicate the structural features of the dye supramolecular assemblies. Unfortunately, there are serious problems identifying molecular assemblies’ structures from the spectrum when several types of assemblies are present in a mixture.

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2.1. Layer charge of clay minerals Dispersions of clay minerals contain layered particles or their crystallites composed of many layers. The individual layers have a negative surface charge, which is the sum of variable and permanent charges. The variable charge occurs at the edges of clay layers dominantly at hydroxyl groups, which exhibit acid/base properties. The permanent charge is due to the presence of non-equivalent, isomorphic substitutions of the central atoms within the octahedral and/or tetrahedral sheets. For smectites, the value of the permanent charge is always significantly higher than that of the variable charge. The term “layer charge” in the text below denotes the permanent charge. Exchangeable cations balance the negative surface charge of clay particles. Attractive electrostatic forces decrease with the distance between oppositely charged sites. Therefore, the distances between negatively charged sites and positively charged cations are as short as possible, considering opposing forces, which are influenced by cationic size and shape, hydration energy, osmotic potential, etc. Consequently, one could expect the distribution of the cations on a clay particle to be on average very similar to the distribution of the layer charge. Since inorganic cations are small and form strong complexes with water molecules, the effect of charge density on the properties of inorganic cations is negligible and has been observed only rarely (Clementz et al., 1974). However, surface charge density significantly affects the distribution of large organic cations at the mineral surface and the molecular arrangement and orientation that the cations may take. Alternatively, measuring the distribution of the large organic cations can provide valuable information on the negative layer charge. This is the basis of the alkylammonium method for determining layer charge (Lagaly, 1994): From measured basal spacings of alkylammonium–clay complexes, one may predict the structure and distribution of the interlayer alkylammonium cations and calculate the distribution of layer charge of the clay. Additionally, choosing the mineral of a specific layer charge may be very useful for the preparation of materials with specific properties (Boyd and Jaynes, 1994; Komadel et al., 2005). 3. Dye aggregation in clay dispersions: types of dye molecular assemblies The interaction of organic dyes with clay minerals has been studied extensively for decades and reviewed (e.g., Ogawa and Kuroda, 1995; Shichi and Takagi, 2000; Yariv, 2002). Methylene blue (MB) is the cationic dye

(Fig. 2), which has been used most frequently in studying spectral properties of dye–clay dispersions. Adsorption of MB has been applied for determinations of bentonite composition, cation exchange capacity (CEC) and the specific surface area of clay minerals (Kahr and Madsen, 1995). However, either these methods do not give reliable results or their applications are too limited with respect to sample type, properties, purity and compositions. Bergman and O'Konski (1963) were the first to observe molecular aggregation of MB in montmorillonite dispersions by visible spectroscopy. Dilute aqueous solutions of MB absorb light at 665 nm. Lower absorption occurs also at about 605 nm, generally as a weak shoulder, attributed to a vibronic component of the 0–1 transition (Fig. 3c). Addition of a clay mineral always causes significant changes in the dye spectrum, including disappearance of the main band and the appearance of new bands, most frequently at lower wavelengths (Fig. 3a). The bands at around 600 nm and 570 nm have been assigned to H-dimers and larger size H-aggregates, respectively (Bergman and O'Konski, 1963). Considering exciton theory, the band's position and shape depend on the number and relative arrangement of interacting molecules and their transition moments in the molecular aggregate. Therefore, energies of observed transitions are variable and may deviate from the values mentioned above. The band at higher wavelengths (about 760 nm) appears less frequently (Fig. 3b), but has been assigned to either J-aggregates (Bergman and O'Konski, 1963) or an acidic form composed from protonated bivalent MB cations (Cenens and Schoonheydt, 1988; Schoonheydt and Heughebaert, 1992). There have been several arguments for both assignments, and there is no agreement on the interpretations of this band. The main argument for the protonated bivalent MB cations is the band position, which is similar to that observed for strongly acidic aqueous solutions of MB. There are, however, important facts and arguments, which do not support the existence of an acidic form of MB in dilute clay dispersions: 1. MB is a very weak base and reacts only in the solutions of strong acids to yield low amounts of protonated cations (Czímerová et al., 2004a). The pKa value for MBH2+ cation is <1, approaching 0 (Ohno et al., 1979), which indicates that MBH2+ is a strong acid. The band at 760 nm is commonly observed for MB/clay dispersions with neutral pH.2 2 Concentrations of dye in dye/clay dispersions are generally very dilute (around 10− 5–10− 6mol dm− 3). Similarly, the concentrations of clay in the dispersions are also very low, e.g., 0.1%.

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Fig. 2. Structural formulas of selected dyes’ cations. MB–methylene blue, Th–thionine, Py–pyronin Y, AO–acridine orange, Ox4–Oxazine 4, CV– crystal violet, MG–malachite green, Rh6G–rhodamine 6G, PIC–pseudoisocyanine, DEC–1,1 diethyl-4,4-cyanine.

The band at about 760 nm also appears under basic conditions, such as in dispersions of laponite (Cenens and Schoonheydt, 1988). A relationship between the occurrence of the band and clay surface acidity has never been reported. 2. Acidic exchangeable cations do not significantly contribute to the formation of MB species, which absorb light at higher wavelengths (Czímerová et al., 2004a). On the other hand, the low-energy band unexpectedly appears in the dispersions of smectites saturated with K+ , NH4+ , Cs + and Rb+ cations

(Czímerová et al., 2004a; Schoonheydt and Heughebaert, 1992). These cations are pH neutral due to their low polarizing power, have low hydration energies and strong fixation to the basal surface of clay minerals (Sutton and Sposito, 2002). The strong electrostatic association of the cations and clay surface may affect ion exchange reactions with MB. Large size MB H-aggregates with a perfect sandwich-type stacking may not be able to efficiently exchange inorganic cations, which are strongly attached to the clay surface. It may lead to the

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accepted, because it does not explain or is contradictory to the following experimental observations:

Fig. 3. Visible spectra of methylene blue in Kunipia montmorillonite dispersion and aqueous solution. The spectra of the dispersion measured 1 min (a) and 24 h (b) after mixing the dye solution with the clay dispersion are compared with the MB solution (c). The dye concentration is 2.5 μmol/l and a ratio n(dye)/m(clay)=0.05 mmol/g.

destabilisation of ordered molecular assemblies (such as H-aggregates) and to the formation of molecular aggregates with a non-perfect stacking. Such disordered molecular aggregates would absorb light at both the lower and higher wavelengths, hence, having the features of both the H- and J-aggregates (Czímerová et al., 2004a). 3.1. Theories on reaction mechanism Dye molecular aggregation often takes place when the clay is undersaturated with dye cations (i.e., low dye/clay mineral ratios). For example, MB forms mostly H-aggregates in Kunipia montmorillonite (Fig. 3), although the amount of dye is only 0.05 mmol/g, which is below 5% of the CEC. Under such conditions, if the dye cations were homogeneously distributed on the clay surface, molecular aggregation would be unlikely to occur. The charge-transfer reaction hypothesis has been discussed in several papers of Yariv et al. and has been described in detail recently (Yariv, 2002). According to the hypothesis, a charge-transfer reaction between the clay surface and the dye is essential and specific for clay minerals with highly basic basal oxygen atoms. The charge-transfer reaction theory, however, has not been widely

1. The role of molecular aggregation for spectral changes of dyes was confirmed by independent methods (Kobayashi, 1996). Some cationic dyes change their spectra and react in a similar way in various chemically different reaction systems, yet yield very similar spectral characteristics. For example, the H-aggregates of MB with similar spectral characteristics were reported for solutions with anionic polymers (Kugel, 1993; Bhattacharyya and Bhattacharya, 1996), in nucleic acids (Huang et al., 1999) and other biomolecules (e.g., Gabrielli et al., 2004), in colloids (e.g., Soedjak, 1994), interfaces (e.g., Ohline et al., 2001, Li and Zare, 2005, Fujita et al., 2005), at surfaces of various electrodes (e.g., Matsuda et al., 2003), clays (e.g., Bergman and O'Konski, 1963) and solutions (e.g., Patil et al., 2000), etc. The theory of a charge-transfer reaction has never been suggested as an explanation for similar phenomena involving non-clay reaction templates. 2. The highest basicity of basal oxygen atoms is expected for clay minerals with the substitutions in tetrahedral sheets (Bleam, 1990). Therefore, according to the charge-transfer reaction hypothesis, the highest metachromasy 3 would be expected for tetrahedrally charged smectites. However, low metachromasy was observed for some saponites, which are tetrahedrally charged but have low charge densities (Czímerová et al., 2004b). Evidence supporting molecular aggregation as the cause of spectral changes for MB-type dyes can be deduced from the spectroscopic study of a competitive adsorption of thioflavin T, proflavine and acridine yellow (Breen and Rock, 1994) and MB and acridine orange (Coine et al., 1998). Comparison of the spectra for systems with two dyes with those containing a single dye showed the non-additivity of the spectra, which is consistent with the formation of mixed molecular aggregates.

3

The property of the dyes that enable them to form molecular aggregates with different spectral and optical properties (change of colour) has been termed metachromasia or metachromasy. Originally, the term metachromasia was used for the phenomenon observed in clinical medicine related to the changes of dye colour when the dye was adsorbed on a tissue or cells. Although this term is not in frequent use now, it is used in this paper to name the phenomenon of dye colour changes on adsorption, regardless of the mechanism.

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The existence of molecular aggregates under the conditions of low dye/clay ratios can be explained in terms of the reaction mechanism. The reaction of dye molecular aggregation and adsorption, proceeding almost instantaneously, are completed before the mixing of a dye solution with a clay mineral dispersion is finished. Consequently, the clay particles are not covered homogeneously with dye cations. Instead, the surface of a clay is covered by islands of dye molecular aggregates, in similar fashion as has been directly proven for cyanine dye/mica films by atomic force microscopy (Ono et al., 1999). Fast adsorption of dye cations and their initial inhomogeneous distribution is supported by further observations. MB adsorption on clay includes two qualitatively different processes. Firstly, dye aggregation proceeds almost instantaneously in the vicinity of clay colloid particles, namely, within the electric double layer. Secondly, after an almost instantaneous molecular aggregation, decomposition of the dye molecular aggregates starts (e.g., Bujdák et al., 1998, 2002a). Dye aggregate decomposition proceeds slowly for hours or days, after or during the adsorption of previously formed aggregates reached the clay basal surfaces (Bujdák et al., 2002a). For the case of MB, the decomposition of initially formed molecular aggregates is observed mainly for clays with low layer charge. At the surfaces of high charge densities, the H-aggregates are formed and stabilised against decomposition. The existence of two processes of dye adsorption is partially supported by the interpretations of Jacobs and Schoonheydt (2001). They describe dye/clay interaction as a competitive adsorption of water molecules and dye cations, leading to the presence of the dye either at basal surface or in zones of electric double layer, where dye ions are surrounded by water molecules and not in direct contact with clay surfaces. In the case of smectites with tetrahedral charge, direct association between the dye cations and basal oxygen atoms preferentially takes place (Jacobs and Schoonheydt, 2001). This association may partially break the molecular aggregation, which has also been observed elsewhere (Bujdák et al., 1998). Furthermore, the existence of the two sites of dye adsorption is supported by two types of dye monomers, which are frequently observed coexisting in some reaction systems (e.g., Jacobs and Schoonheydt, 2001). Observed absorption bands at 670 and 655 nm were assigned to MB monomers interacting with the surface oxygen atoms and monomers in the electric double layer, respectively. A secondary or indirect influence of clay layer charge on dye aggregation on clay surfaces has also been

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postulated (Gessner et al., 1994; Coine et al., 1998, 2000; Neumann et al., 1996). According to this hypothesis, the magnitude of the layer charge affects colloid properties of clay dispersions, namely, the association between clay particles. The different colloid properties should lead to variations in the amount of external clay surface area accessible for any initial dye adsorption. This assumption is based on the fact that smectites of high layer charge form larger quasicrystals4 (Gessner et al., 1994). Lower surface area would lead to higher average concentration of dye, which would promote dye aggregation (Gessner et al., 1994). However, there are several aspects and observations that do not support this hypothesis: 1. The presence of tetrahedral charge may suppress swelling of expandable clay minerals much more than the magnitude of the octahedral charge (Güven, 1992). No influence of the tetrahedral charge on enhanced dye aggregation has been observed (e.g., Bujdák et al., 1998; Czímerová et al., 2004b). 2. Slight changes in clay layer charge (CEC reduction < 10% of initial CEC) induced by Li+ mild thermal treatment would not affect the colloid properties and accessible surface area to the extent assumed by Gessner et al. (1994). However, these small changes in layer charge always led to significant changes of MB aggregation (e.g., Bujdák and Komadel, 1997). Furthermore, variation of the dye/clay ratio leads in many cases to only negligible changes of the dye aggregation (Bujdák et al., 2002a; Czímerová et al., 2004b). 3. Reduced charge montmorillonites (RCMs) with very low charge (< 50% of initial CEC) and containing non-swelling or mixed phase swelling/non-swelling interlayer spaces exhibit significantly reduced surface areas (Komadel et al., 1996). Hence, enhanced aggregation of the dye on non-swelling RCMs would be expected according to the hypothesis of Gessner et al. (1994). However, the dye aggregation is not commonly observed in dispersions of such specimens (Bujdák and Komadel, 1997; Bujdák et al., 2001; Su and Shen, 2005). 4. Dye molecular orientation Dye molecular aggregation on flat surfaces of various chemical types has been studied for decades. The structure and molecular orientation in molecular 4

Quasicrystals are packets of single platelets (fundamental layer particles), which commonly occur in clay suspensions.

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aggregates at various surfaces and interfaces have been determined using modern physical methods, such as measurements of second harmonic generation, applying linearly polarized spectroscopies, atomic force microscopy, etc. However, the orientation of dye molecules at dye/clay mineral interfaces has only recently been clearly identified (e.g., Fujita et al., 1997; Sasai et al., 2000; Yamaoka and Sasai, 2000; Yamaoka et al., 2000). The assumption that dye molecules are arranged on clay surface exclusively in parallel fashion was common in earlier papers (e.g., Margulies and Rozen, 1986; Chernia et al., 1994). On the basis of this assumption, some scientists disputed the dye aggregation on clay basal surface. The molecular aggregation was assumed to take place only on the “external surface”, although the state of clay particles in dispersions was not analysed and the term “external surface” was not well defined in these studies. The hypothesis that MB has a parallel orientation came from measured basal spacings of dye/ clay intercalates (Hang and Brindley, 1970). For example, the basal spacing of 1.8 nm determined for MB/montmorillonite was interpreted as two layers of MB cations lying flat in the interlayer spaces (Yariv and Lurie, 1971). The existence of other, e.g., tilted or perpendicular, orientations was not considered, although it had been hypothesized in the first papers dealing with this subject (Hang and Brindley, 1970; Brindley and Thompson, 1970). The formation of large-size molecular aggregates at flat surfaces can be achieved by tilted and, in some

cases, an almost perpendicular orientation of the molecules (Kobayashi, 1996). Only the tilted orientation is suitable for the formation of large molecular aggregates on surfaces and, simultaneously, providing an electrostatic interaction between the negatively charged surface and each adsorbed dye cation. Several papers have been published on the molecular orientation of MB adsorbed on various kinds of non-clay surfaces (Table 1). The tilted orientation has been proven using direct methods. The increase of the orientation angle is always associated with the formation of molecular aggregates (e.g., Fujita et al., 2005). Exclusively parallel molecular orientation of MB was disputed after the molecular orientation of this dye on the mica surface was reported (Hähner et al., 1996). X-ray photoelectron spectroscopy and nearedge X-ray absorption fine structure spectroscopy, both performed using polarised light, provided direct evidence that the MB cations were tilted at an angle of about 65–70° with respect to the mica surface plane. The high charge density on mica surfaces could contribute to the high density of adsorbed dye cations and, thus, increase their average orientation angle. A similar model for MB molecular orientation was adopted in later studies on MB/clay solids (Bujdák et al., 2003a) and clay Langmuir–Blodgett films (Umemura, 2003). A tilted orientation (42–52°) of acridine dyes on clay surfaces was proven using linearly polarised spectroscopy (Yamagishi and Soma, 1981).

Table 1 Summary of reported molecular orientations of MB cations on various surfaces Substrate

Orientation

Method

Literature

Mica Mica, clay surface Glass films Glass/solution interface Glass Quartz Fused silica Silica, modified silica Pt electrode Ag electrode Metal (steel) Graphite Graphite H2Ti4O9 TaS2 K4Nb6O17

T P, T T P, T T T P, T P, T T P, T T T T T P, T T

XRS Theoretical studies SHG Internal reflection spectroscopy polarized ATR spectroscopy Polarized UV–Vis spectroscopy Evanescent-wave cavity ring-down spectroscopy Total reflection spectroscopy with a slab optical waveguide SHG Surface-enhanced Raman spectroscopy Polarized IR spectroscopy Reflectance spectra UV–Vis reflectance XRD Theoretical studies Polarized absorption spectra

Hähner et al., 1996 Yu et al., 2000 Higgins et al., 1991 Hinoue et al., 1993 Kobayashi et al., 1988 Kobayashi et al., 2001; Fujita et al., 2005 Li and Zare, 2005 Tsunoda et al., 2003, 2005 Campbell et al., 1990 Chen et al., 2000 Imamura et al., 2002 Lezna et al., 1991, 1995 Sagara et al., 1996 Nakato et al., 1992 Čapková et al., 2002 Kaito et al., 2003

T–tilted orientation: including perpendicular, edge-on and nearly perpendicular orientation. P, T–the species with parallel orientation of dye cations with respect to the surface together with the ones of tilted or perpendicular orientation. XRS–X-ray spectroscopy; XRD–X-ray diffraction, UV–Vis–ultraviolet–visible; SHG–second harmonic generation; ATR–attenuated total reflectance; PM-FTIR–polarization modulation–Fourier transform infrared; IR–infrared.

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The parallel orientation was claimed even for the dyes, whose molecular shape is not perfectly flat, namely, triarylmethane dyes with the cations of a propeller-like molecular shape (Chernia et al., 1994). Later, Fischer et al. (1998) studied in detail the molecular orientation of two triphenylmethane dyes, crystal violet (CV) and malachite green (MG) (Fig. 2) on muscovite mica crystals using near-edge X-ray absorption fine structure and X-ray photoelectron spectroscopies. A significant tilt angle with respect to the surface was found for all investigated species; a flat orientation was effectively ruled out. Similar models were proposed for the dye orientation in dispersed montmorillonite systems with adsorbed CV and MG (Yamaoka and Sasai, 2000). Later, numerous works reported almost perpendicular orientations of various cationic dyes on clay surfaces. Dyes included rhodamines (Sasai et al., 2002; Chen et al., 2002; Iyi et al., 2002; Bujdák and Iyi, 2005) and others (Sasai et al., 2000; Kaneko et al., 2004). An almost perpendicular orientation was observed mainly for assemblies with large H-aggregates (i.e., the species absorbing light at the lowest wavelengths). Models including a tilted orientation of the dye molecules on clay surface were further supported using the sensitive method of polarised fluorescence (López Arbeloa and Martínez Martínez, 2006a,b; Martínez Martínez et al., 2006).

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Fig. 4. Visible spectra of methylene blue (a) and 1,1-diethyl-4,4cyanine (b) in the dispersions of reduced charge montmorillonites. The arrows indicate the trends of the spectral changes with increasing layer charge.

5. Methylene blue molecular aggregation Molecular aggregation of MB proceeds in concentrated aqueous solutions, although the fraction of the molecular aggregates is low and the size of the aggregates is limited to dimers and trimers (Antonov et al., 1999; Fujita et al., 2005). The positive charge in the cations contributes to repulsive intermolecular forces and prevents the formation of large molecular assemblies in solutions. The aggregation at solid surfaces with a negative surface charge is much larger. The attractive electrostatic forces between the negatively charged surface and the dye cations contribute to the higher concentration of the dye at the interface. The distribution of the layer charge affects the distribution of the cations on the clay surface. This relationship was observed and successfully interpreted for the first time for the case of MB aggregation in dispersions of reduced charge montmorillonites (Bujdák and Komadel, 1997). The layer charge of RCMs sensitively controlled the spectral properties of MB, and the trends were the same as observed with another series of RCMs shown in Fig. 4a. High layer charge density of the smectite promoted MB aggregation. Low charge probably induced large

distances between neighbouring dye cations adsorbed on clay surface and suppressed dye aggregation. Although MB spectra changed with time, similar trends are observed under other conditions. The sensitivity of the spectra to the charge of mineral template is very high. For example, the band of H-aggregates at 570 nm in the spectra of fresh dispersions significantly decreased in favour of those assigned to monomers (670 nm) and J-aggregates and/or acidified cations (760 nm) with decreasing CEC from 1.07 to 0.88 mmol/ g. Further decrease of the CEC to 0.71 mmol/g led to a substantial change of the spectrum indicating a dominant formation of the isolated dye cations (670 nm) (Bujdák and Komadel, 1997). Bujdák et al. (1998) also applied visible spectroscopy of MB for the characterization of the layer charge of four montmorillonites (SAz-1, USA; Horné Dunajovice, Slovakia; Jelšový Potok, Slovakia; M40A, USA), iron-rich beidellite Stebno (Czech Republic), and nontronite SWa-1 (USA) and the specimens prepared by an acid treatment of these minerals. MB spectra for the dispersions of these clays were compared with the

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data of cation exchange capacities and the characteristics of the larger charge obtained by the alkylammonium method (Janek et al., 1997). Also, for these samples, the trends in the MB spectra sensitively reflected the layer charge of the samples. A significant charge reduction was observed upon the acid treatment for all the samples. Sensitivity of the method to detect charge reduction was confirmed recently for samples prepared by heating ammonium-saturated smectites (Jankovič, 2006) and acid-activated bentonites (Madejová et al., in preparation). The method of MB spectra was successfully used for the analysis of the charge reduction of smectites upon Li+ fixation (Bujdák et al., 2001, 2002a; Czímerová et al., 2003; Czímerová et al., 2006-this issue; Su and Shen, 2005). Bujdák et al. (2001) studied charge reduction in thermally treated Li + -smectites using three montmorillonites, beidellite and nontronite. MB spectra were used to detect the charge in layer change with the temperature of thermal treatment. The extent of the charge reduction was related to the magnitude of the octahedral charge. In another study, a series of RCMs prepared from SAz-1 was compared with Nanocor (N) and Cloisite (Southern Clay Products, USA) (SCP) montmorillonites, which represented high- and low-charge smectites (Bujdák et al., 2002a). In addition to N and SCP montmorillonites, two RCM samples were selected in order to have two pairings, i.e., two high- and two lowcharge samples. These four samples were characterised in detail using UV–Vis spectroscopy of the MB complexes. Although the two samples in each pair were of different origin, their dispersions with MB exhibited very similar spectral properties for various dye/clay ratios. On the other hand, RCMs, which were of the same origin, and with an almost initial identical structure and crystallinity, gave significantly different spectra with MB. Hydrophobization of the clay particle edges via the grafting with alkylsilanes did not change MB spectra significantly, although the colloid properties of the materials were strongly affected by the treatment (Bujdák et al., 2002a). Since then, further series of RCMs were tested using the spectra of MB (e.g., Czímerová et al., 2003; Su and Shen, 2005). The trends of spectral changes were very similar to those described above and shown in Fig. 4a. 6. Dyes with the structure similar to methylene blue The metachromasy of various cationic dyes in clay dispersions has been observed and seems to be a general phenomenon. For example, metachromasy has

been observed for thionine (Sunwar and Bose, 1990; Czímerová et al., 2003), acridine orange and other acridine dyes (Yariv, 2002 and references therein; Chattopadhyay and Traina, 1999), acridine dyes thioflavine, proflavine (Breen and Rock, 1994; Schoonheydt et al., 1986), and oxazine dyes (Iyi et al., 2002), all of which are cations of similar shape and size to MB. Sunwar and Bose (1990) proved larger metachromasy of thionine (Th) (Fig. 2) and tetraethylthionine in the dispersions of Na+-vermiculite than in the presence of Na+ montmorillonite. This fact was attributed to the higher charge density of vermiculite. The presence of a low-charge laponite did not induce significant metachromasy (Sunwar and Bose, 1990). Low molecular aggregation of Py in the dispersions of low charge laponite (CEC = 0.4 mmol/g) was observed. Greater metachromasy of this dye was observed for a Na+-montmorillonite with a higher layer charge (CEC =1 mmol/g) under similar conditions (Grauer et al., 1987). Due to the structural difference between Th and MB cations, some new features were observed in the spectra of Th/clay dispersions (Czímerová et al., 2003, 2004b). In addition to the molecular aggregation, reactions related to the acidic properties of the nonfunctionalized amino groups (–NH2) in the Th dye cations need to be considered. These groups are more reactive than –N(CH3)2 in the MB ion (Fig. 2) and are probably the sites for the oxidative formation of cation radicals. Similar reactions commonly occur for aromatic amines adsorbed on clay minerals (e.g., Yariv, 2002, and references therein). These reactions proceed mainly in the dispersions of high-charge clay minerals (Czímerová et al., 2003, 2004b), which is in principle the same feature as observed for other reactive dyes (see below). Spectra of acridine orange (AO) (Fig. 2) were studied in detail for the dispersions with various clay minerals (Garfinkel-Shwenky and Yariv, 1997a,b, 1999). In the latter work, the authors observed decreasing metachromasy in the order of beidellite, vermiculite, montmorillonite, saponite and laponite. Unfortunately, no CEC data for the used minerals are given. The trend was interpreted in terms of mineral surface oxygen basicity (Garfinkel-Shwenky and Yariv, 1997b); however, this could also be assigned to decreasing layer charge density, which was concluded in another study (Chattopadhyay and Traina, 2000). More recently, Bujdák and Iyi (2002) reported the relationship between molecular aggregation of AO and the layer charge of RCMs in dispersions. The amount of the aggregates increased

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with increasing layer charge of clay mineral templates in a similar way, as it had been observed for MB spectra. 7. Other types of dyes 7.1. Triarylmethane and rhodamine dyes Several papers on the interaction between triarylmethane cationic dyes and clay minerals have been published (e.g., Chernia et al., 1994). The effect of layer charge densities of RCMs on the spectral properties of the dyes, CV, MG and brilliant green, was investigated (Bujdák and Iyi, 2002). The general trends were the same as those observed for the phenazine dyes. Forms of the triarylmethane dyes aggregates were not as well resolved in the spectra as those for MB/clay dispersions. The differences induced by varying the layer charge increased with the ageing of the dispersions and were strongly dependent on the structure of the dye cations (Bujdák and Iyi, 2002). Rhodamine (Rh) dyes (Fig. 2) are very important for laser technology due to their luminescent properties and high emission yields. Interaction of clay minerals with Rh dyes has been studied extensively (e.g., López Arbeloa et al., 1995, 1998, 2002; Čapková et al., 2004; Klika et al., 2004; Pospíšil et al., 2003; Tapia Estévez et al., 1995). The results of Grauer et al. (1984) indicate the effect of layer charge on the formation of molecular aggregates of Rh 6G. Na+-montmorillonite (CEC=0.97 mmol/g) and Na+-laponite (CEC = 0.71 mmol/g) with medium and low charge densities were used. The effects of the clay minerals on absorption, excitation and emission spectra of this dye were compared. The presence of montmorillonite induced formation of molecular aggregates of an H-type, which absorbed light at higher energies. On the other hand, absorption and fluorescence spectra of Rh 6G/laponite dispersions were similar to that of the dye solution (Grauer et al., 1984). The effect of the layer charge on the aggregation of Rh 6G was directly studied recently (Bujdák et al., 2003b, 2004): The amount of Haggregates increased with the layer charge. Some new phenomena occurring for Rh-clay dispersion systems were reported for the first time by Bujdák et al. (2004). The H-aggregates of Rh 6G are formed slowly in clay mineral dispersions and their amount increases with the reaction time, opposite to that observed for MB-clay systems. In addition to the reaction rates of dye molecular re-arrangement and redistribution, the reaction pathways and directions, indicated by the changes in the spectra, were specific to the layer charge of a clay template (Bujdák et al., 2004; Bujdák and Iyi, 2005).

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7.2. Cyanine dyes Although research on molecular assemblies of cyanine dyes are among the hottest topics in material sciences today (Kobayashi, 1996), the interaction between clay minerals and cyanine dyes has not been extensively studied. The formation of cyanine dye molecular assemblies on mica surfaces has been investigated to a greater extent and the results are useful in interpreting interactions of cyanine dye on clay mineral surfaces. Pseudoisocyanine (PIC) dye (Fig. 2) forms nanometer-scale leaf-like islands of Jaggregates on a mica surface, which was observed using atomic force microscopy (Ono et al., 1999). Polarization absorption measurements revealed anisotropy with respect to the lattice of a mica substrate, which is the result of the polar interactions and stereochemistry of charged groups of the dye cations and the shape of ditrigonal cavities on the mica surface (Ono et al., 1999, 2001; Karthaus and Kawatani, 2003; Yao et al., 2004). The relationship between aggregation of PIC dye and layer charge was observed for dispersions and films of saponite and montmorillonite with CECs of 0.71 and 1.19 mmol/g, respectively (Ogawa et al., 1996). Similar results were achieved, when the experiments were extended to montmorillonite, tetrasilicic mica (CEC = 1.00 mmol/g), saponite and hectorites (CECs = 1.00 and 0.73 mmol/g) (Miyamoto et al., 2000). In contrast to the higher charge montmorillonite and synthetic mica, the hectorites and the saponite were not able to induce the formation of the molecular aggregates of the dye. Although this difference was attributed to the different morphology of clay particles (Miyamoto et al., 2000), an alternative interpretation based on the effect of the layer charge must be considered. Direct evidences of the layer charge effect on cyanine dyes aggregation were achieved in the experiments, which included RCMs together with a series of expandable clay minerals of variable charge, namely, synthetic fluorohectorite and Li+-teaniolite, Nanocor, Kunipia F and Na+-cloisite (Southern Clay Products) montmorillonites, and SapCa-1 Na+-saponite, with the CECs 1.50, 1.50, 1.30, 1.01, 0.95 and 0.91 mmol/g, respectively (Bujdák et al., 2002b,c). The highest amounts of the PIC Jaggregates were formed in dispersions with a medium layer charge clay (Bujdák et al., 2002b). Low-charge minerals, Na+-cloisite and Na+-saponite, suppressed dye molecular aggregation. In contrast, the presence of the minerals with high layer charge densities induced fast spectral bleaching, which indicates irreversible decomposition of the PIC. This phenomenon was

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attributed to both the large size and high reactivity of the dye cations. Due to their large size, PIC cations probably could not adopt a suitable arrangement to fit the distribution of charge sites on the clay surface. Molecular tension led in some cases (medium charge clays) to a reversible bleaching, which was attributed to changes of molecular conformation. However, at the surfaces of high charge (especially those of synthetic fluorohectorite and Li+-teaniolite), the molecular tension probably led to a significant destabilization of the molecule and decomposition at its molecular centre (methine group, –CH=, see Fig. 2). Products of the decomposition absorbed UV but not visible light, which proves the breakdown of a conjugated system of the chromophore. The trends were supported with the experiments using two RCM series (Bujdák et al., 2002b). The effect of the layer charge on stability of 1,1′-diethyl-4,4′-cyanine (DEC) (Fig. 2), the dye isomeric to PIC, was studied as well (Bujdák et al., 2002c). DEC is less stable in aqueous solutions than PIC, but its reactivity in relation to the clay layer charge is similar. DEC, which does not form Jaggregates (but only the H-aggregates), is stabilized on the surfaces of low-charge clay minerals, but decomposes in systems with high-charge clays. Fig. 4b shows the spectra of 1,1′-diethyl-4,4′-cyanine in the dispersions of RCMs, showing the effect of layer charge on dye decomposition. Decomposition occurred even in systems that included surfactant micelles, which otherwise efficiently stabilise the dye in the aqueous solutions in absence of clay (Bujdák et al., 2002c). Spectra of cyanine dyes of more complicated molecular structures, namely, 1,1′-diethyl-4,4′-carbocyanine and 1,1′-diethyl-4,4′-dicarbocyanine in clay dispersions were investigated as well (Bujdák et al., 2003c). Due to the presence of more methine groups, the formation of more conformers and several decomposition pathways are expected. The formation of numerous types of molecular aggregates is a consequence of the more complicated structure and large size of dye cations. Therefore, the interpretation of the spectra is very complicated. Nevertheless, the effect of layer charge was obvious for the spectra of these dyes. 7.3. Porphyrins and related dyes The effect of the layer charge of clay minerals on adsorption of cationic porphyrins and related dyes has been studied. The porphyrin dyes are very different from other dyes, due to their specific molecular structure, size and symmetries, and more complicated spectra. Japanese scientists introduced a new theory based on “size-

matching rules,” which were applied for the preparation of dye/clay hybrid materials with porphyrin dye cations densely adsorbed in a flat arrangement on clay mineral surfaces but without the formation of molecular aggregates (Takagi et al., 2002). The rules help to choose a suitable combination of a clay mineral and a dye, based on the layer charge of clay mineral and the size and charge of the dye cation. The validity of the theory was confirmed in detailed study testing the adsorption of various di-, tri-, and tetra-cationic porphyrin dyes on a synthetic saponite (Eguchi et al., 2004). In another study, the adsorption of a tetra-cationic Co(II) porphyrin dye on K10 and KSF montmorillonites and synthetic fluorohectorites was performed (Dias et al., 2000). Visible and Raman spectra of the dye adsorbed onto fluorohectorite surfaces indicated molecular aggregation. This is an interesting finding because fluorohectorite's charge density (CEC = 1.50 mmol/g) is significantly higher than those of any natural smectite. A tilted dye molecular orientation was observed for the dye/fluorohectorite system, which is another indication of the dye intermolecular association (Dias et al., 2000). A non-flat orientation of the tetra-cationic porphyrin dye on clay mineral surfaces is rather exceptional and probably related to the high surface charge of the inorganic template. For example, a highly tilted orientation for a tetra-cationic porphyrin dye was also observed for layered TiNbO5, which is a high surface charge density material (Tong et al., 2002). Similar trends were observed for adsorption of a negatively charged tetra-anionic porphyrin dye on hydrotalcites, which have a positive surface charge (e.g., Barbosa et al., 2002; Tong et al., 2003). 8. Dye spectra as a tool for clay science The dye spectra provide qualitative insight into the amounts of the species formed. The amount of molecular aggregates increases with increasing layer charge. In principle, this can be very useful for the estimation of the layer charge of clay minerals, mainly of smectites. The advantage of the method is its simplicity and high sensitivity, especially in the comparisons of the layer charge of specimens with similar structure or for detecting layer charge changes induced by chemical or physical treatments. Moreover, one is able to visually observe some trends and changes without using spectroscopy equipment as a change of colour. A quantitative analytical method for clay layer charge characterisation based on the measurements of dye spectra has not been developed. Unfortunately, the

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identification and quantification of all dye species formed at the clay surface is essential. The deconvolution of spectra to individual bands has mostly no physical meaning. The spectrum of an individual component depends on many parameters. For example, the spectrum of dye molecular aggregate depends on its structure, number of the molecular units in the aggregate, angles between transition moments, chemical environment, etc. The spectrum of a single component can be composed from several bands including those of forbidden transitions and vibronic components (Evans et al., 1993). Theoretically, the spectrum of an intermediate aggregate, which bears the properties of both the H- and J-aggregates, can be the same or very similar to the spectrum of mixed species of perfect sandwich and head-to-tail stackings (Eisfeld and Briggs, 2002). Therefore, there is a serious problem to quantitatively analyse the dye molecular aggregation, even in much simpler systems, such as dye solutions (Antonov et al., 1999). One possibility to overcome these problems would be the analysis of a large set of spectra and applying suitable statistical methods, such as principal component analysis. 9. Dye/clay systems in modern sciences and future applications trends Controlling the dye properties using a clay template with optimal or suitable properties (e.g., layer charge) is a challenge for the development of new hybrid dye/clay materials. This chapter summarises some aspects, novel observations and phenomena related to interactions between clays and organic dyes, and further possible directions for research. Dye adsorption on clays could be important for the treatment of chemical wastes containing dyes and other chemical pollutants, which has been described in numerous studies. Photodecomposition of some dyes on clay surfaces have already been reported (e.g., Feng et al., 2003). Furthermore, dye/clay systems are also efficient to adsorb other organic pollutants (Borisover et al., 2001) and, in some cases, to efficiently catalyze their photodecomposition (Sasai et al., 2003). The importance of the choice of an appropriate clay specimen, i.e., the influence of its layer charge in clay-mediated dye decomposition, has already been indicated (Bujdák et al., 2002b,c). Investigations in the future could be directed towards the optimisation of treatments for chemical wastes using suitable clay templates. Langmuir–Blodgett (LB) films are one of the most interesting hybrid nanomaterials with monomolecular

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layers. Such materials have unique properties due to a highly controlled molecular structure and arrangement. To date, only a few clay/dye-based LB films have been described (e.g., Umemura 2003, Ras et al., 2004a,b). The role of clay layer charge controlling the structure of LB films has not been studied systematically. Many perspective applications of dye/clay materials relate to their optical properties. Non-linear optical properties and second harmonic generation for MB/ clay or MB/clay/polymer films were reported by Van Duffel et al. (2001). Enhanced luminescence has been reported for materials containing Rh dyes in alkylammonium-modified clays (Sasai et al., 2004). Clay systems with reactive dyes can be applied as rewritable recording media (Ito and Fukunishi, 1997). Further applications include electron-transfer devices (Shichi and Takagi, 2000), components in copy paper (Caine et al., 2002) or in photographic materials (Liou and Wang, 2000). However, the effects of clay template properties on these phenomena have not been investigated. Redox properties of organic dyes are strongly affected by their state and closely related to the overall chemistry and optical properties of these compounds. Dye/clay hybrid materials have been used as indicators and sensors (e.g., Shan et al., 2003; Sumitani et al.; 2004; Maupin et al., 2004). The redox properties of organic dyes are used to catalyze various organic reactions. Methylene blue bound to clays exhibits highly selective catalytic properties in photo-oxidation reactions (Madhavan and Pitchumani, 2002). Dyeinduced photoactivation was applied in the preparation of polymer/clay nanocomposite materials via “in situ” polymerization reactions (Paczkowska et al., 2004). Light-induced hydrogen gas formation was observed in the reactions of chlorophyll adsorbed on clay (Itoh et al., 1998). The efficiency of organic dyes for the above mentioned reactions and phenomena will likely depend on their aggregation state, which could be controlled by layer charge of a clay template. Combinations of inorganic and organic components in the hybrid materials enable nearly unlimited possibilities for varying the environment of a chromophore, thus sensitively tuning its optical behaviour. When dyes are embedded in a clay mineral matrix, they may exhibit better stability against photobleaching, chemical degradation, and thermal decomposition. Interactions on clay mineral surfaces may allow the creation of a local anisotropy or other adjustable optical properties, which could be applied in nanoscale devices of optical memory systems, optical switching systems, amplifiers, energy storage devices and solar cells.

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Acknowledgements The financial support of the Slovak Grant Agency VEGA (Grant 2/3102) is acknowledged. Our research in the area of dye/clay mineral interactions has been supported over the years by the Slovak Grand Agency VEGA and the Japan Society for the Promotion of Science. The outstanding efforts of N. Iyi (National Institute for Materials Science, Tsukuba, Japan), A. Czímerová (Institute of Inorganic Chemistry, Slovak Academy of Sciences) and my other co-workers in this research area are gratefully acknowledged. For the critical reading of the manuscript, comments and suggestions of improvement, I thank D. Laird and referees. References Antonov, L., Gergov, G., Petrov, V., Kubista, M., Nygren, J., 1999. UV–Vis spectroscopic and chemometric study on the aggregation of ionic dyes in water. Talanta 49, 99–106. Barbosa, C.A.S., Ferreira, A.M.D.C., Constantino, V.R.L., Coelho, A. C.V., 2002. Preparation and characterization of Cu(II) phthalocyanine tetrasulfonate intercalated and supported on layered double hydroxides. J. Incl. Phenom. Macrocycl. Chem. 42, 15–23. Bergman, K., O'Konski, C.T., 1963. A spectroscopic study of methylene blue monomer, dimer and complexes with montmorillonite. J. Phys. Chem. 67, 2169–2177. Bhattacharyya, A.K., Bhattacharya, G., 1996. Chromotropic behaviours of Chalta (Dillenia indica) fruit mucilage polysaccharide towards some cationic dyes. J. Indian Chem. Soc. 73, 463–466. Bleam, W.F., 1990. Electrostatic potential at the basal (001) surface of talc and pyrophyllite as related to tetrahedral sheet distortions. Clays Clay Miner. 38, 522–526. Borisover, M., Graber, E.R., Bercovich, F., Gerstl, Z., 2001. Suitability of dye–clay complexes for removal of non-ionic organic compounds from aqueous solutions. Chemosphere 44, 1033–1040. Boyd, S.A., Jaynes, W.F., 1994. Role of layer charge in organic contaminant sorption by organo-clays. In: Mermut, A.R. (Ed.), Layer Charge Characteristics of 2:1 Silicate Clay Minerals. CMS Workshop Lectures Volume 6. The Clay Minerals Society, Boulder, USA, pp. 48–77. Breen, C., Rock, B., 1994. The competitive adsorption of methylene blue on to montmorillonite from binary solution with thioflavin T, proflavine and acridine yellow, steady state and dynamic studies. Clay Miner. 29, 179–189. Brindley, G.W., Thompson, T.D., 1970. Methylene blue absorption by montmorillonites. Determinations of surface areas and exchange capacities with different initial cation saturations (Clay-organic studies XIX). Isr. J. Chem. 8, 409–415. Bujdák, J., Iyi, N., 2002. Visible spectroscopy of cationic dyes in dispersions with reduced-charge montmorillonites. Clays Clay Miner. 50, 446–454. Bujdák, J., Iyi, N., 2005. Molecular orientation of rhodamine dyes on surfaces of layered silicates. J. Phys. Chem., B 109, 4608–4615. Bujdák, J., Komadel, P., 1997. Interaction of methylene blue with reduced charge montmorillonite. J. Phys. Chem., B 101, 9065–9068.

Bujdák, J., Janek, M., Madejová, J., Komadel, P., 1998. Influence of the layer charge density of smectites on the interaction with methylene blue. J. Chem. Soc., Faraday Trans. 94, 3487–3492. Bujdák, J., Janek, M., Madejová, J., Komadel, P., 2001. Methylene blue interactions with reduced-charge smectites. Clays Clay Miner. 49, 244–254. Bujdák, J., Iyi, N., Fujita, T., 2002a. The aggregation of methylene blue in montmorillonite dispersions. Clay Miner. 37, 121–133. Bujdák, J., Iyi, N., Hrobáriková, J., Fujita, T., 2002b. Aggregation and decomposition of a pseudoisocyanine dye in dispersions of layered silicates. J. Colloid Interface Sci. 247, 494–503. Bujdák, J., Iyi, N., Fujita, T., 2002c. Aggregation and stability of 1,1′-diethyl-4,4′-cyanine dye on the surface of layered silicates with different charge densities. Colloids Surf., A 207, 207–214. Bujdák, J., Iyi, N., Kaneko, Y., Sasai, R., 2003a. Molecular orientation of methylene blue cations adsorbed on clay surfaces. Clay Miner. 38, 561–572. Bujdák, J., Iyi, N., Kaneko, Y., Czímerová, A., Sasai, R., 2003b. Molecular arrangement of rhodamine 6G cations in the films of layered silicates: the effect of the layer charge. Phys. Chem. Chem. Phys. 5, 4680–4685. Bujdák, J., lyi, N., Fujita, T., 2003c. Control of optical properties of adsorbed cyanine dyes via negative charge distribution on layered silicates. Solid State Phenom. 90–91, 463–468. Bujdák, J., Iyi, N., Sasai, R., 2004. Spectral properties, formation of dye molecular aggregates, and reactions in rhodamine 6G/layered silicate dispersions. J. Phys. Chem., B 108, 4470–4477. Caine, M.A., McCabe, R.W., Wang, L.C., Brown, R.G., Hepworth, J.D., 2002. The inhibition of triphenylmethane primary dye fading in carbonless copying paper systems by singlet oxygen quenching bis(dithiocarbamato)nickel(II) complexes. Dyes Pigm. 52, 55–65. Campbell, D.J., Higgins, D.A., Corn, R.M., 1990. Molecular second harmonic generation studies of methylene blue chemisorbed onto a sulphur-modified polycrystalline platinum electrode. J. Phys. Chem. 94, 3681–3689. Čapková, P., Pospíšil, M., Lerf, A., 2002. Molecular simulations in structure analysis of tantalum sulfide intercalated with methylene blue. Solid State Sci. 4, 671–676. Čapková, P., Malý, P., Pospíšil, M., Klika, Z., Weissmannová, H., Weiss, Z., 2004. Effect of surface and interlayer structure on the fluorescence of rhodamine B-montmorillonite: modelling and experiment. J. Colloid Interface Sci. 277, 128–137. Cenens, J., Schoonheydt, R.A., 1988. Visible spectroscopy of methylene blue on hectorite, Laponite B and Barasym in aqueous suspensions. Clays Clay Miner. 36, 214–224. Chattopadhyay, S., Traina, S.J., 1999. Spectroscopic study of sorption of nitrogen heterocyclic compounds on phyllosilicates. Langmuir 15, 1634–1639. Chattopadhyay, S., Traina, S.J., 2000. Sorptive characteristics of acridine-surfactant-phyllosilicate systems. J. Colloid Interface Sci. 225, 307–316. Chen, W.X., Xu, Q.H., Jiang, H., Xu, Z.D., 2000. Study on adsorption states by surface-enhanced Raman spectroscopy. Spectrosc. Lett. 33, 69–81. Chen, G.M., Iyi, N.B., Sasai, R., Fujita, T., Kitamura, K., 2002. Intercalation of rhodamine 6G and oxazine 4 into oriented clay films and their alignment. J. Mater. Res. 17, 1035–1040. Chernia, Z., Gill, D., Yariv, S., 1994. Electric dichroism. The effect of dialysis on the color of crystal violet adsorbed to montmorillonite. Langmuir 10, 3988–3993.

J. Bujdák / Applied Clay Science 34 (2006) 58–73 Clementz, D.M., Mortland, M.M., Pinnavaia, T.J., 1974. Properties of reduced charge montmorillonites: hydrated Cu (II) ions as a spectroscopic probe. Clays Clay Miner. 22, 49–57. Coine, A.P.P., Neumann, M.G., Gessner, F., 1998. Time-dependent spectrophotometric study of the interaction of basic dyes with clays. J. Colloid Interface Sci. 198, 106–112. Coine, A.P.P., Schmitt, C.C., Neumann, M.G., Gessner, F., 2000. The effect of added salt on the aggregation of clay particles. J. Colloid Interface Sci. 226, 205–209. Czímerová, A., Bujdák, J., Gáplovský, A., 2003. Reduction of the negative charge of layered silicates probed by cationic azine dyes. Solid State Phenom. 90–91, 469–474. Czímerová, A., Jankovič, L., Bujdák, J., 2004a. Effect of the exchangeable cations on the spectral properties of methylene blue in clay dispersions. J. Colloid Interface Sci. 274, 126–132. Czímerová, A., Bujdák, J., Gáplovský, A., 2004b. The aggregation of thionine and methylene blue dye in smectite dispersion. Colloids Surf., A 243, 89–96. Czímerová, A., Bujdák, A., Dohmann, R., 2006-this issue. Traditional and novel methods for estimating the layer charge of smectites Appl. Clay Sci. 34, 2–13. Dias, P.M., De Faria, D.L.A., Constantino, V.R.L., 2000. Spectroscopic studies on the interaction of tetramethylpyridylporphyrins and cationic clays. J. Incl. Phenom. Macrocycl. Chem. 38, 251–266. Eguchi, M., Takagi, S., Tachibana, H., Inoue, H., 2004. The ‘size matching rule’ in di-, tri-, and tetra-cationic charged porphyrin/ synthetic clay complexes: effect of the inter-charge distance and the number of charged sites. J. Phys. Chem. Solids 65, 403–407. Eisfeld, A., Briggs, J.S., 2002. The J-band of organic dyes: line shape and coherence length. Chem. Phys. 281, 61–70. Evans, C.E., Song, Q., Bohn, P.W., 1993. Influence of molecular orientation and proximity on spectroscopic line shape in organic monolayers. J. Phys. Chem. 97, 12302–12308. Feng, J.Y., Hu, X.J., Yue, P.L., Zhu, H.Y., Lu, G.Q., 2003. A novel laponite clay-based Fe nanocomposite and its photo-catalytic activity in photo-assisted degradation of Orange II. Chem. Eng. Sci. 58, 679–685. Fischer, D., Caseri, W.R., Hähner, G., 1998. Orientation and electronic structure of ion exchanged dye molecules on mica. An X/ray absorption study. J. Colloid Interface Sci. 198, 337–346. Fujita, T., Iyi, N., Kosugi, T., Ando, A., Deguchi, T., Sota, T., 1997. Intercalation characteristics of rhodamine 6G in fluor-taeniolite: orientation in the gallery. Clays Clay Miner. 45, 77–84. Fujita, K., Taniguchi, K., Ohno, H., 2005. Dynamic analysis of aggregation of methylene blue with polarized optical waveguide spectroscopy. Talanta 65, 1066–1070. Gabrielli, D., Belisle, E., Severino, D., Kowaltowski, A.J., Baptista, M.S., 2004. Binding, aggregation and photochemical properties of methylene blue in mitochondrial suspensions. Photochem. Photobiol. 79, 227–232. Garfinkel-Shwenky, D., Yariv, S., 1997a. Metachromasy in clay-dye systems: the adsorption of acridine orange by Na-saponite. Clay Miner. 32, 653–663. Garfinkel-Shwenky, D., Yariv, S., 1997b. The determination of surface basicity of the oxygen planes of expanding clay minerals by acridine orange. J. Colloid Interface Sci. 188, 168–175. Garfinkel-Shwenky, D., Yariv, S., 1999. Metachromasy in clay-dye systems: the adsorption of acridine orange by Na-beidellite. Clay Miner. 34, 459–467. Gessner, F., Schmitt, C.C., Neumann, M.G., 1994. Time-dependent spectroscopic study of the interaction of basic dyes with clays: 1.

71

Methylene blue and neutral red on montmorillonite and hectorite. Langmuir 10, 3749–3753. Grauer, Z., Avnir, D., Yariv, S., 1984. Adsorption characteristics of Rhodamine 6G on montmorillonite and Laponite, elucidated from electronic absorption and emission spectra. Can. J. Chem. 62, 1889–1894. Grauer, Z., Grauer, G.L., Avnir, D., Yariv, S., 1987. Metachromasy in clay minerals—sorption of pyronin-Y by montmorillonite and laponite. J. Chem. Soc., Faraday Trans., I 83, 1685–1701. Güven, N., 1992. Molecular aspects of clay/water interactions. In: Güven, N., Pallastro, R. (Eds.), Clay-Water Interface and Its Rheological Implications. CMS Workshop Lectures, vol. 4. The Clay Minerals Society, Colorado, pp. 1–80. Hähner, G., Marti, A., Spencer, N.D., Caseri, W.R., 1996. Orientation and electronic structure of methylene blue on mica: a near edge Xray absorption structure spectroscopic study. J. Chem. Phys. 104, 7749–7757. Hang, P.T., Brindley, G.W., 1970. Methylene blue adsorption by clay minerals. Determination of surface areas and cation exchange capacities (Clay-organic studies XVIII). Clays Clay Miner., 18, 203–212. Higgins, D.A., Byerly, S.K., Abrams, M.B., Corn, R.M., 1991. Second harmonic generation studies of methylene blue orientation at silica surfaces. J. Phys. Chem. 95, 6984–6990. Hinoue, T., Imamura, G., Yokoyama, Y., 1993. Study of the adsorption layer at the glass-dye solution interface by variable incidence-angle internal-reflection spectrometry. Bull. Chem. Soc. Jpn. 66, 3680–3685. Huang, C.Z., Li, Y.F., Deng, S.Y., Liu, S.R., 1999. Assembly of methylene blue on nucleic acid template as studied by resonance light-scattering technique and determination of nucleic acids of nanogram. Bull. Chem. Soc. Jpn. 72, 1501–1508. Imamura, K., Ikeda, E., Nagayasu, T., Sakiyama, T., Nakanishi, K., 2002. Adsorption behavior of methylene blue and its congeners on a stainless steel surface. J. Colloid Interface Sci. 245, 50–57. Ito, K., Fukunishi, K., 1997. Alternate intercalation of fluoran dye and tetra-n-decylammonium ion induced by electrolysis in acetoneclay suspension. Chem. Lett. 4, 357–358. Itoh, T., Ishii, A., Kodera, Y., Matsushima, A., Hiroto, M., Nishimura, H., Tsuzuki, T., Kamachi, T., Okura, I., Inada, Y., 1998. Photostable chlorophyll a conjugated with poly(vinylpyrrolidone)-smectite catalyzes photoreduction and hydrogen gas evolution by visible light. Bioconj. Chem. 9, 409–412. Iyi, N., Sasai, R., Fujita, T., Deguchi, T., Sota, T., Arbeloa, F.L., Kitamura, K., 2002. Orientation and aggregation of cationic laser dyes in a fluoromica: polarized spectrometry studies. Appl. Clay Sci. 22, 125–136. Jacobs, K.Y., Schoonheydt, R.A., 2001. Time dependence of the spectra of methylene blue-clay mineral suspensions. Langmuir 17, 5150–5155. Janek, M., Komadel, P., Lagaly, G., 1997. Effect of autotransformation on the layer charge of smectites determined by the alkylammonium method. Clay Miner. 32, 623–632. Jankovič, L., 2006. unpublished results. Kahr, G., Madsen, F.T., 1995. Determination of the cation exchange capacity and the surface area of bentonite, illite and kaolinite by methylene blue adsorption. Appl. Clay Sci., B 327–336. Kaito, R., Kuroda, K., Ogawa, M., 2003. Unidirectional orientation of methylene blue intercalated in K4Nb6O17 single crystal. J. Phys. Chem., B 107, 4043–4047. Kaneko, Y., Iyi, N., Bujdák, J., Sasai, R., Fujita, T., 2004. Effect of layer charge density on orientation and aggregation of a cationic

72

J. Bujdák / Applied Clay Science 34 (2006) 58–73

laser dye incorporated in the interlayer space of montmorillonites. J. Colloid Interface Sci. 269, 22–25. Karthaus, O., Kawatani, Y., 2003. Self-assembly and aggregation control of cyanine dyes by adsorption onto mesoscopic mica flakes. Jpn. J. Appl. Phys., Part 1 42, 127–131. Klika, Z., Weissmannová, H., Čapková, P., Pospíšil, M., 2004. The rhodamine B intercalation of montmorillonite. J. Colloid Interface Sci. 275, 243–250. Kobayashi, T., 1996. J-Aggregates. World Scientific. Singapore. Kobayashi, M., Tokunaga, H., Okubo, J., Hoshi, T., Tanizaki, Y., 1988. Measurements of the polarized ATR spectra for adsorbed dye layers by using a stack of thin glass plates. Bull. Chem. Soc. Jpn. 61, 4171–4176. Kobayashi, H., Takahashi, M., Kotani, M., 2001. Spontaneous formation of an ordered structure during dip-coating of methylene blue on fused quartz. Chem. Phys. Lett. 349, 376–382. Komadel, P., Bujdák, J., Madejová, J., Šucha, V., Elsass, F., 1996. Effect of non-swelling layers on the dissolution of reduced-charge montmorillonite in hydrochloric acid. Clay Miner. 31, 333–345. Komadel, P., Madejová, J., Bujdák, J., 2005. Preparation and properties of reduced-charge smectites—a review. Clays Clay Miner. 53, 313–334. Kugel, R.W., 1993. Metachromasy—The interactions between dyes and polyelectrolytes in aqueous solutions. Adv. Chem. 236, 507–533. Kuhn, H., 2000. π-Electron systems—building blocks of supramolecular machines. Colloids Surf., A 171, 3–12. Lagaly, G., 1994. Layer charge determination by alkylammonium ions. In: Mermut, A.R. (Ed.), Layer Charge Characteristics of 2:1 Silicate Clay Minerals. CMS Workshop Lectures, vol. 6. The Clay Minerals Society, Boulder, USA, pp. 2–46. Lezna, R.O., Detacconi, N.R., Hahn, F., Arvia, A.J., 1991. In situ spectroelectrochemistry of adsorbed methylene blue on a sulphurmodified gold electrode. J. Electroanal. Chem. 306, 259–269. Lezna, R.O., Juanto, S., Zagal, J.H., 1995. Electrochromism of methylene blue absorbed on the basal plane of graphite. J. Electroanal. Chem. 389, 197–200. Li, F.P., Zare, R.N., 2005. Molecular orientation study of methylene blue at an air/fused-silica interface using evanescent-wave cavity ring-down spectroscopy J. Phys. Chem., B 109, 3330–3333. Liou, Y.W., Wang, C.M., 2000. Reusable photographic films based on iron-containing clay minerals. Chem. Commun. 1, 27–28. López Arbeloa, F., Martínez Martínez, V., 2006a. New Polarization fluorescence method to evaluate the orientation of adsorbed molecules in uniaxial 2D layered materials. J. Photochem. Photobiol. A, Chem. 181, 44–49. López Arbeloa, F., Martínez Martínez, V., 2006b. Orientation of adsorbed dyes in the interlayer space of clays. Fluorescence polarization of rhodamine 6G in laponite films. Chem. Mater. 81, 1407–1416. López Arbeloa, F., Tapia Estévez, M.J., López Arbeloa, T., López Arbeloa, I., 1995. Adsorption of rhodamine 6G on saponite—a comparative study with other rhodamine 6G–smectite aqueous suspensions. Langmuir 11, 3211–3217. López Arbeloa, F., Herrán Martínez, J.M., López Arbeloa, T., López Arbeloa, I., 1998. The hydrophobic effect on the adsorption of rhodamines in aqueous suspensions of smectites. The rhodamine 3B laponite B system. Langmuir 14, 4566–4573. López Arbeloa, F., Chaudhuri, R., López Arbeloa, T., López Arbeloa, I., 2002. Aggregation of rhodamine 3B adsorbed in Wyoming montmorillonite aqueous suspensions. J. Colloid Interface Sci. 246, 281–287.

Madejová, J., Komadel, P., Bujdák, J., Andrejkovičová, S., Hrachová, J., Čeklovský, A., Valúchová, J., in preparation. Madhavan, D., Pitchumani, K., 2002. Photoreactions in clay media: singlet oxygen oxidation of electron-rich substrates mediated by clay-bound dyes. J. Photochem. Photobiol., A Chem. 153, 205–210. Margulies, L., Rozen, H., 1986. Adsorption of methyl green on montmorillonite. J. Mol. Struct. 141, 219–226. Martínez Martínez, V., Salleres, S., Bañuelos, J., López Arbeloa, F., 2006. Application of fluorescence with polarized light to evaluate the orientation of dyes adsorbed in layered materials. J. Fluoresc. 16, 233–240. Matsuda, N., Santos, J.H., Takatsu, A., Kato, K., 2003. In situ observation of absorption spectra and adsorbed species of methylene blue on indium-tin-oxide electrode by slab optical waveguide spectroscopy. Thin Solid Films 445, 313–316. Maupin, P.H., Gilman, J.W., Harris, R.H., Bellayer, S., Bur, A.J., Roth, S. C., Murariu, M., Morgan, A.B., Harris, J.D., 2004. Optical probes for monitoring intercalation and exfoliation in melt-processed polymer nanocomposites. Macromol. Rapid Commun. 25, 788–792. Miyamoto, N., Kawai, R., Kuroda, K., Ogawa, M., 2000. Adsorption and aggregation of a cationic cyanine dye on layered clay minerals. Appl. Clay Sci. 16, 161–170. Nakato, T., Iwata, Y., Kuroda, K., Kato, C., 1992. Preparation of an intercalation compound of layered titanic acid H2Ti4O9 with methylene blue. J. Incl. Phenom. Mol. Recognit. Chem. 13, 249–256. Neumann, M.G., Schmitt, C.C., Gessner, F., 1996. Time-dependent spectroscopic study of the interaction of basic dyes with clays II. Thionine on natural and synthetic montmorillonites and hectorites. J. Colloid Interface Sci. 177, 495–501. Ogawa, M., Kuroda, K., 1995. Photofunctions of intercalation compounds. Chem. Rev. 95, 399–438. Ogawa, M., Kawai, R., Kuroda, K., 1996. Adsorption and aggregation of a cationic cyanine dye on smectites. J. Phys. Chem. 100, 16218–16221. Ohline, S.M., Lee, S., Williams, S., Chang, C., 2001. Quantification of methylene blue aggregation on a fused silica surface and resolution of individual absorbance spectra. Chem. Phys. Lett. 346, 9–15. Ohno, T., Osif, T.L., Lichtin, N.N., 1979. A previously unreported intense absorption band and the pKa of protonated triplet methylene blue. Photochem. Photobiol. 30, 541–546. Ono, S.S., Yao, H., Matsuoka, O., Kawabata, R., Kitamura, N., Yamamoto, S., 1999. Anisotropic growth of J aggregates of pseudoisocyanine dye at a mica/solution interface revealed by AFM and polarization absorption measurements. J. Phys. Chem., B 103, 6909–6921. Ono, S.S., Yamamoto, S., Yao, H., Matsuoka, O., Kitamura, N., 2001. Morphological control of the supramolecular pseudoisocyanine Jaggregates by the functions of a mica/solution interface. Appl. Surf. Sci. 177, 189–196. Paczkowska, B., Strzelec, S., Jedrzejewska, B., Linden, L.A., Paczkowski, J., 2004. Photochemical preparation of polymerclay composites. Appl. Clay Sci. 25, 221–227. Patil, K., Pawar, R., Talap, P., 2000. Self-aggregation of methylene blue in aqueous medium and aqueous solutions of Bu4NBr and urea. Phys. Chem. Chem. Phys. 2, 4313–4317. Pospíšil, M., Čapková, P., Weissmannová, H., Klika, Z., Trchová, M., Chmielová, M., Weiss, Z., 2003. Structure analysis of montmorillonite intercalated with rhodamine B: modelling and experiment. J. Mol. Model. 9, 39–46. Ras, R.H.A., Nemeth, J., Johnston, C.T., Dekany, I., Schoonheydt, R. A., 2004a. Orientation and conformation of octadecyl rhodamine B

J. Bujdák / Applied Clay Science 34 (2006) 58–73 in hybrid Langmuir–Blodgett monolayers containing clay minerals. Phys. Chem. Chem. Phys. 6, 5347–5352. Ras, R.H.A., Németh, J., Johnston, C.T., DiMasi, E., Dékány, I., Schoonheydt, R.A., 2004b. Hybrid Langmuir–Blodgett monolayers containing clay minerals: effect of clay concentration and surface charge density on the film formation. Phys. Chem. Chem. Phys. 6, 4174–4184. Sagara, T., Nomaguchi, H., Nakashima, N., 1996. Surface anisotropic electroreflectance response at an edge-plane pyrolytic graphite electrode. J. Phys. Chem. 100, 6393–6396. Sasai, R., Shichi, T., Gekko, K., Takagi, K., 2000. Continuously changing the conformational dependence of saponite hybrid materials on the intercalation degree: electric linear dichroism of stilbazolium derivatives intercalated in saponite clay. Bull. Chem. Soc. Jpn. 73, 1925–1931. Sasai, R., Fujita, T., Iyi, N., Itoh, H., Takagi, K., 2002. Aggregated structures of rhodamine 6G intercalated in a fluor-taeniolite thin film. Langmuir 18, 6578–6583. Sasai, R., Sugiyama, D., Takahashi, S., Tong, Z.W., Shichi, T., Itoh, H., Takagi, K., 2003. The removal and photodecomposition of nnonylphenol using hydrophobic clay incorporated with copperphthalocyanine in aqueous media. J. Photochem. Photobiol., A Chem. 155, 223–229. Sasai, R., Iyi, N., Fujita, T., Arbeloa, F.L., Martinez, V.M., Takagi, K., Itoh, H., 2004. Luminescence properties of rhodamine 6G intercalated in surfactant/clay hybrid Thin Solid Films. Langmuir 20, 4715–4719. Schoonheydt, R.A., Heughebaert, L., 1992. Clay adsorbed dyes: methylene blue on laponite. Clay Miner. 27, 91–100. Schoonheydt, R.A., Cenens, J., DeSchriever, F.C., 1986. Spectroscopy of proflavine adsorbed on clays. J. Chem. Soc., Faraday Trans. I 82, 281–289. Shan, D., Mousty, C., Cosnier, S., Mu, S.L., 2003. A new polyphenol oxidase biosensor mediated by azure B in laponite clay matrix. Electroanal. 15, 1506–1512. Shichi, T., Takagi, K., 2000. Clay minerals as photochemical reaction fields. J. Photochem. Photobiol., C—Photochem. Rev. C 1, 113–130. Soedjak, H.S., 1994. Colorimetric determination of carrageenans and other anionic hydrocolloids with methylene blue. Anal. Chem. 66, 4514–4518. Su, C.C., Shen, Y.H., 2005. Preparation and dispersive behaviors of reduced charge smectite Colloids Surf., A 259, 173–177. Sumitani, M., Takagi, S., Tanamura, Y., Inoue, H., 2004. Oxygen indicator composed of an organic/inorganic hybrid compound of methylene blue, reductant, surfactant and saponite. Anal. Sci. 20, 1153–1157. Sunwar, C.B., Bose, H., 1990. Effect of clay minerals on the visible spectra of thiazine dyes. J. Colloid Interface Sci. 136, 54–60. Sutton, R., Sposito, G., 2002. Animated molecular dynamics simulations of hydrated cesium-smectite interlayers. Geochem. Trans. 3, 73–80. Takagi, S., Shimada, T., Eguchi, M., Yui, T., Yoshida, H., Tryk, D.A., Inoue, H., 2002. High-density adsorption of cationic porphyrins on clay layer surfaces without aggregation. The size-matching effect. Langmuir 18, 2265–2272.

73

Tapia Estévez, M.J., López Arbeloa, F., López Arbeloa, T., López Arbeloa, I., 1995. Characterization of rhodamine 6G adsorbed onto hectorite by electron spectroscopy. J. Colloid Interface Sci. 171, 439–445. Tong, Z.W., Shichi, T., Kasuga, Y., Takagi, K., 2002. The synthesis of two types of layered niobate hybrid materials by the selective intercalation of ionic porphyrin. Chem. Lett. 1206–1207. Tong, Z.W., Shichi, T., Zhang, G.Z., Takagi, K., 2003. The intercalation of metalloporphyrin complex anions into layered double hydroxides. Res. Chem. Intermed. 29, 335–341. Tsunoda, K.I., Umemura, T., Ueno, H., Okuno, E., Akaiwa, H., 2003. Adsorption of methylene blue onto silylated silica surfaces, studied using visible attenuated total reflection spectroscopy with a slab optical waveguide. Appl. Clay Sci. 57, 1273–1277. Tsunoda, K.I., Kasuya, Y., Umemura, T., Odake, T., 2005. A subsecond, time-resolved, linear dichroism measurement system for visible attenuated total reflection spectroscopy with a slab optical waveguide. Talanta 65, 1097–1101. Umemura, Y., 2003. Preparation of methylene blue-clay hybrid films by a modified Langmuir–Blodgett method and molecular orientation of methylene blue in the film. Nendo Kagaku 42, 218–222. Van Duffel, B., Verbiest, T., Van Elshocht, S., Persoons, A., De Schryver, F.C., Schoonheydt, R.A., 2001. Fuzzy assembly and second harmonic generation of clay/polymer/dye monolayer films. Langmuir 17, 1243–1249. Vos, J.G., Forster, R.J., Keyes, T.E., 2004. Interfacial Supramolecular Assemblies. John Wiley & Sons Inc., New York. Yamagishi, A., Soma, M., 1981. Aliphatic tail effects on adsorption of acridine orange cation on a colloidal surface of montmorillonite. J. Phys. Chem. 85, 3090–3092. Yamaoka, K., Sasai, R., 2000. Pulsed electric linear dichroism of triphenylmethane dyes adsorbed on montmorillonite K10 in aqueous medial. J. Colloid Interface Sci. 225, 82–93. Yamaoka, K., Sasai, R., Takata, N., 2000. Electric linear dichroism. A powerful method for the ionic chromophore-colloid system as exemplified by dye and montmorillonite suspensions. Colloids Surf., A 175, 23–39. Yao, H., Ou, Z.M., Morita, Y., Kimura, K., 2004. Dynamic morphology of mesoscopic pseudoisocyanine J aggregates on mica induced by humidity treatments. Colloids Surf., A 236, 31–37. Yariv, S., 2002. Staining of clay minerals and visible absorption spectroscopy of dye–clay complexes. In: Yariv, S., Cross, H. (Eds.), Organo-Clay Complexes and Interactions. Marcel Dekker, Inc., New York, p. 463. Yariv, S., Lurie, D., 1971. Metachromasy in clay minerals: Part I. Sorption of methylene-blue by montmorillonite. Isr. J. Chem. 9, 537–552. Yu, C.H., Newton, S.Q., Norman, M.A., Miller, D.M., Schafer, L., Teppen, B.J., 2000. Molecular dynamics simulations of the adsorption of methylene blue at clay mineral surfaces. Clays Clay Miner. 48, 665–681.