NA2COLOR - Create!
Hybrid Clay/Dye Natural Nanopigment (NA2COLOR): A new Class of Colorant for new Times Hybrid Clay/Dye Natural Nanopigment (NA2COLOR): ErnestoaBaena-Murillo and Francisco Martínez-Verdú new class of colorant for new times
University of Alicante, Spain
Ernesto Baena-Murillo and Francisco Martínez-Verdú University of Alicante ABSTRACT An overview of nanopigments main concepts, definitions and characteristics is made, as well as the factors in its synthesis. It is shown the research perspectives in natural colorants based nanopigments at Colour and Vision Group at University of Alicante, Spain. 1. INTRODUCTION The continuing effort to obtain products with improved performance and to achieve an specific functionality or selectivity has increased the research and development of new materials. The last decade in particular, the domain of nanostructured materials is attracting more and more researchers, both academic and industrial, due to the interesting role played by particle size that, in some cases, is comparable to the particle chemical composition or structure, adding another flexible parameter for designing and controlling their behavior. Thus, controlled and reproducible synthesis methods are a major prerequisite for success in this rapidly evolving field [1]. Such is the case of nanopigments, which emerged as a novel type of pigment, with application mainly in polymeric materials, both for coloring the polymer matrix as to obtain coatings, however, its applications are growing faster and now it has been used in various types of substrates such as wood, paper, latex, glass, metals and textiles. 286
Besides, an important driving force in nanopigment development is due to greater awareness about negative environmental and toxic effects of heavy metal based pigments [2]. 2. METHODS AND MATERIALS
2.1 Clays Clay minerals belong to the phyllosilicate group (from the Greek “phyllon”: leaf, and from the Latin “silic”: flint), they are compounds with a structure in which silicate tetrahedrons (a central silicon atom surrounded by four oxygen atoms at the corners of a tetrahedron) are arranged in sheets. As a distinctive feature, particles are very small, a few micrometers maximum). While the number of their species is relatively small, clay minerals exhibit a great diversity in their chemical compositions, manly, due to the different metallic elements substituting the silicon in tetrahedral positions [3]. Smectite-type clay minerals are the most used substrates to form molecular aggregates. Smectites possess a layered structure due to the condensation space of an octahedral Al2O3 or MgO planar sheet (O) between two planar tetrahedral SiO2 sheets (T), the so-called TOT layer (figure 1).
chemical union (mostly due to intermolecular van der Waals forces, ionic interactions and H-bonds [8]) but through complete enclosure of one set of molecules in a suitable structure formed by another [9].
Figure 1. Layered structure of smectite clay minerals
TOT platelets can achieve a negative charge due to isomorphic substitutions of tetrahedral Si4+ (tetrahedral substitution) and/or octahedral Al3+ or Mg2+ (octahedral substitution) by other metals with lower valence. The negative charges are compensated by inorganic exchangeable cations which are located on the surface of TOT layer. These exchanged cations induce the stacking of TOT layers giving rise to the tactoidal structure of clay platelets, characterized by the interlayer space. Cation exchange capacity (CEC) is a measure of the cations, which balance the negative charge sites of the clay, and it is usually expressed as the retained excess cation quantity divided by the sample mass. Layer charge may also be estimated from stoichiometric coefficients in structural formulae. Smectite-type clays have a CEC in the 30–130 meq/100 g range and this characteristic makes them ideal systems to accommodate cationic or polar organic materials because of the high capacity of swelling and bonding [4, 5].
2.2. Molecular aggregates Molecular aggregates or supramolecular structures are commonly defined as compounds in which two or more components are associated without ordinary
Dye molecular aggregation in clay mineral colloids was reported for the first time by Bergman and O’Konski [6]. Significant properties of clay minerals is the adsorption of organic compounds in the form of arranged supramolecular assemblies, clusters, and self-organized molecular aggregates with specific chemical and anisotropic properties. Some of organic dyes significantly change their color upon the adsorption at solid/liquid interphases. The phenomenon, historically named metachromasy, has been frequently observed in the systems containing organic dyes of flat molecules or chromophoric groups [7]. The extent and types of formed molecular aggregates influence dramatically the optical properties of chromophores [11]. As it is shown in figure 2, considering the head and tail groups of discrete molecules or monomers, the aggregates or dimers can adopt different configurations with specific excitation energies (figure 2) [12].
Figure 2. Exciton splitting of the electronic excited states of dimers with different geometries (radiative deactivation: straight arrows, and non-radiative deactivation wavy arrows).
While the amount of dye aggregates is proportional to clay cationic exchange capacity [13], molecular
287
aggregation is mainly controlled by the concentration of the dye, pH and z-potential of the dispersion media, molecular length and dipole moment of the dye and separation between silicate layers in the clay [14].
2.3. Nanopigments synthesis and properties The first step is to condition the clay nanoparticles, ie, breaking the agglomerates which naturally forms due to its electrostatic potential, and try to increase the separation between plates depending on the size of the dye molecule to be used. This takes advantage of the absorptive capacity of the clays, particularly the clay is dispersed in deionized water producing a swelling between layers. As a result there will be appropriate contact with organic dye molecules, and a decreased of ionic strength between interlaminar cations and silicate layers. Then, the selected dye is also dissolved in deionized water and is added to the dispersion of clay. At this point the reaction takes place with ion exchange between interlaminar cations of the clay and organic dye molecules. Generally, the organic dyes used are those containing caionic groups. The amount of dye used depends on the ion exchange capacity of the clay and the desired intensity of colour [15], then concentration of dye and layer charge of clay are fundamental parameters in the optical properties of nanopigments. By visible absorption spectroscopy the kind and formation extent of molecular aggregates can be characterized (figure 4) and then, effective nanopigment formation or supramolecular dye aggregation is verified with the appearance of absorption bands due to H or J-type aggregates (curves a and b) different to those of dye disperse molecules (curve c). However, it is a challenge to obtain stable nanopigment dispersions that can retain the aggregates structure in a certain time before its application [17].
288
Figure 4. Visible spectra of methylene blue in montmorillonite dispersions and aqueous solution, measured 1 min (a) and 24 h (b) after synthesis and methylene blue solution in low concentration (c) [18].
Besides, as formed aggregates have particular structure and confinement, they will have different enthalpy formation energies and thus thermodynamic properties that can be estimated under thermal analysis tests (figure 5). Pure laponite (curve a) presents water loss and its quite stable over a wide range of temperatures. Nanopigment preparations increasing dye concentration form H and J-type aggregates (curve b and c, respectively). Water loss is less significant due to the replacement of interlaminar adsorbed water by dye aggregates. H-type aggregates are not thermally stable in any temeperature range, but its decomposition is gradual. J-type aggregates are stable below 250 ºC and at higher temperatures its decomposition is faster and more exothermic.
Figure 5. Thermal analysis of rhodamine dye in a laponite clay film: normalized TG curves (dotted line) and DTA curves (solid line). a)Pure laponite, b)laponite with rhodamine 6G 1x10-4 M, and c)laponite with rhodamine 6G 8x10-4 M [19]
Finally, whilst dyes generally have superior colour brilliance than pigments, pigments remain the dominant colorants due to their superior colour-fastness properties [30], then, hybrid clay/dye nanopigments combines thus organic/inorganic, dispersive/soluble properties.
textile fibers and natural colorants, if they are going to play a more important role than at present in the manufacture of the clothes and textiles of every kind, that are needed by present consumers and will be needed in even greater quantities by the expanding world population of tomorrow.
3. “GREEN” NANOPIGMENTS
Thus, the project objective is to develop, improve and implement nanotechnologies leading to more sustainable and environmental friendly commercial pigment products based on coloration of nanoclays with a variety of natural dyes (based on its reactivity, functionality and the completeness of colour gamut.) with an aim to improve performance properties.
Dyes and pigments are essential ingredients for textile, printing inks, paint and coatings, and plastics industries. World demand for dyes and organic pigments is forecast to increase 3,9% per year to $16,2 billion in 2013. In volume terms, demand will grow 3,5% annually to 2,3 million metric tons [32]. Increasing worldwide awareness of the pollution resulting from the production and use of some synthetic colorants and the increasing concerns in terms of carbon emission and gradual depletion of world crude oil reserve, has led to a significant revival of interest in natural colorants in the last years and is inspiring projects for sustainable and environmentally friendly development of their production [20]. Recent years have seen an unprecedented number of attempts in the development of pigments based on clays. The advantages of nanopigment, in terms of both mechanical and optical properties, are well understood and documented [21-29]. Natural dyeing symbolizes craft practices which reflect an harmonious and sustainable relationship with the ecosystem and the local plant reservoir. But, on the other hand, inconsiderate use of natural dyes at industrial scale could contribute to the reduction of biodiversity [20]. Therefore, a massive effort of interdisciplinary study is needed in order to optimize the production of
The factors for molecular natural dye aggregation between clay layers, its stability or performance under different conditions and thermodynamics and kinetic of aggregation will be key issues to estimate the potential impact of those coloration alternatives. 4. CONCLUSIONS In recent years there have been significant advances in the design, synthesis and preliminary characterization in clay and dye based nanopigments. Our initial results confirm that a vast field of study on nanopigments and their industrial applications lies ahead for the coming years. It is quite likely that in some color formulas the use of nanopigments significantly reduces the incorporation of other additives needed with conventional dyes, providing better weather resistance and increasing mechanical properties to the substrate. Moreover, there are great expectations about a whole field of study on the direct use of natural dyes (based mainly in plants and microorganisms) in the design and
289
synthesis of nanopigments, combining the best of indigenous traditions (American, Indian, etc) with the best of recent advances in nanomaterials science and technology of color. 5. ACKNOWLEDGMENTS This research is supported by the Spanish Ministry of Science and Innovation by the grant number DPI2008-06455-C02-02. 6. REFERENCES [1] P. Knauth and J. Schoonman, Nanostructured Materials: Selected Synthesis Methods, Properties and Applications, New York: Kluwer Academic Publishers, 2004. [2] S. Raha, I. Ivanov, N.H. Quazi, and S.N. Bhattacharya, "Photostability of rhodamine-B/montmorillonite nanopigments in polypropylene matrix," Applied Clay Science , vol. 42, 2009, pp. 661-666. [3] A. Meunier, Clays, Berlin: Springer-Verlag Berlin Heidelberg, 2005. [4] F. López Arbeloa, V. Martínez, T. Arbeloa López, and I. López Arbeloa, "Photoresponse and anisotropy of rhodamine dye intercalated in ordered clay layered films," Journal of Photochemistry and Photobiology C , vol. 8, 2007, pp. 85-108. [5] A. Czímerová, J. Bujdák, and R. Dohrmann, "Traditional and novel methods for estimating the layer charge of smectites," Applied Clay Science, vol. 34, 2006, pp. 2 - 13. [6] J. Bujdák and N. Iyi, "Optical properties of molecular aggregates of oxazine dyes in dispersions of clay minerals," Applied Clay Science, 2009, pp. 157-165. [7] J. Bujdák and N. Iyi, "Molecular Orientation of Rhodamine Dyes on Surfaces of Layered Silicates," J. Phys. Chem. B, 2005, pp. 4608-4615. [8] J. Bujdák, "Effect of the layer charge of clay minerals on optical properties of organic dyes. A review," Applied Clay Science, vol. 34, 2006, pp. 58-73. [9] S. Yang and P. Sheng, Physics and Chemistry of Nanostructured Materials, Philadelphia: Taylor & Francis Inc, 2000. [10] J. Bujdák, A. Czímerová, and N. Iyi, "Structure of cationic dyes assemblies intercalated in the films of montmorillonite," Thin Solid Films, vol. 517, 2008, pp. 793-799. [11] F. López Arbeloa, J. Bañuelos Prieto, T. Arbeloa López, and I. López Arbeloa, "Characterization of Rhodamine 6G Aggregates Intercalated in Solid Thin Films of Laponite Clay. 1. Absorption Spectroscopy," J. Phys. Chem. B, 2004, pp. 2003020037. [12] N. Iyi, T. Fujita, C.V. Yelamaggad, and F. López Arbeloa, "Intercalation of cationic azobenzene derivatives in a synthetic mica and their photoresponse," Applied Clay Science, 2001, pp. 47-58. [13] F. López Arbeloa, S. Salleres, V. Martínez, C. Corcóstegui, and I. López Arbeloa, "Effect of surfactant C12TMA molecules on the
290
self-association of R6G dye in thin films of laponite clay,"
Materials Chemistry and Physics , vol. 116, 2009, pp. 550-556. [14] a. Gürses, C. Doğar, M. Yalçin, M. Açikyildiz, R. Bayrak, and S. Karaca, "The adsorption kinetics of the cationic dye, methylene blue, onto clay.," Journal of hazardous materials, vol. 131, 2006, pp. 217-28. [15] J. Bujdák, F. López Arbeloa, and N. Iyi, "Spectral Properties of Rhodamine 3B Adsorbed on the Surface of Montmorillonites with Variable Layer Charge," Langmuir, 2007, pp. 1851-1859. [16] S. Salleres and T. Arbeloa, "Adsorption of fluorescent R6G dye into organophilic C12TMA laponite lms," Journal of Colloid and Interface Science, vol. 321, 2008, pp. 212-219. [17] E. Baez, N. Quazi, I. Ivanov, and S.N. Bhattacharya, "Stability study of nanopigment dispersions," Advanced Powder Technology, vol. 20, 2009, pp. 267-272. [18] J. Bujdák, K. Lang, and et. al., "Clay Mineral Particles As Efficient Carriers of Methylene Blue Used for Antimicrobial Treatment," Environmental Science & Technology , vol. 43, 2009, pp. 6202-6207. [19] F. López Arbeloa and V. Martínez, "Orientation of Adsorbed Dyes in the Interlayer Space of Clays. 2 Fluorescence Polarization of Rhodamine 6G in Laponite Films," Clay Minerals, 2006, pp. 1407-1416. [20] M.E. Davila and E. Portillo, "INTERNATIONAL SYMPOSIUM / WORKSHOP ON NATURAL DYES," Reflections on the actual use of natural dyes , Hyderabad, India: UNESCO, 2006. [21] P.M. Ajayan, L.S. Schadler and P.V. Braun,Nanocomposite Science and Technology, Weinheim, WILEY-VCH, 2003. [22] Y.-W. Mai and Z.-Z. Yu, Polymer Nanocomposites, Woodhead Publishing Ltd. and CRC Press LLC, London, 2006. [23] Q. Zeng, Fundamental studies of organoclays and polymer nanocomposites, University of New South Wales, 2004. [24] H. Fischer, Polymer nanocomposites: from fundamental research to specific applications, Mater. Sci. Eng. C, 23, 763–72, 2003. [25] G. Buxbaum and G. Pfaff, Industrial Inorganic Pigments, Weinheim, Wiley-VCH, 2005. [26] K. Hunger, Industrial Dyes: Chemistry, Properties and Applications, Weinheim, Wiley-VCH, 2003. [27] W. Herbst and K. Hunger, Industrial Organic Pigments, 3rd ed., Weinheim, Wiley-VCH, 2004. [28] P. Bamfield, Chromic Phenomena: Technological Applications of Colour Chemistry, Cambridge, The Royal Society of Chemistry, London, 2001. [29] S-H. Kim, Functional Dyes, Elsevier, Amsterdam, 2006. [30] E.B. Faulkner, R.J. Schwartz, High Performance Pigments, 2nd ed. Weinheim, Wiley-VCH, 2009. [31] M.E. Davila and E. Portillo, "INTERNATIONAL SYMPOSIUM / WORKSHOP ON NATURAL DYES," Reflections on the actual use of natural dyes, Hyderabad, India: UNESCO, 2006. [32] World Dyes & Organic Pigments, The Freedonia Group Inc., May 2009.
University of Alicante, Spain
Ernesto Baena-Murillo and Francisco Martínez-Verdú University of Alicante ABSTRACT An overview of nanopigments main concepts, definitions and characteristics is made, as well as the factors in its synthesis. It is shown the research perspectives in natural colorants based nanopigments at Colour and Vision Group at University of Alicante, Spain. 1. INTRODUCTION The continuing effort to obtain products with improved performance and to achieve an specific functionality or selectivity has increased the research and development of new materials. The last decade in particular, the domain of nanostructured materials is attracting more and more researchers, both academic and industrial, due to the interesting role played by particle size that, in some cases, is comparable to the particle chemical composition or structure, adding another flexible parameter for designing and controlling their behavior. Thus, controlled and reproducible synthesis methods are a major prerequisite for success in this rapidly evolving field [1]. Such is the case of nanopigments, which emerged as a novel type of pigment, with application mainly in polymeric materials, both for coloring the polymer matrix as to obtain coatings, however, its applications are growing faster and now it has been used in various types of substrates such as wood, paper, latex, glass, metals and textiles. 286
Besides, an important driving force in nanopigment development is due to greater awareness about negative environmental and toxic effects of heavy metal based pigments [2]. 2. METHODS AND MATERIALS
2.1 Clays Clay minerals belong to the phyllosilicate group (from the Greek “phyllon”: leaf, and from the Latin “silic”: flint), they are compounds with a structure in which silicate tetrahedrons (a central silicon atom surrounded by four oxygen atoms at the corners of a tetrahedron) are arranged in sheets. As a distinctive feature, particles are very small, a few micrometers maximum). While the number of their species is relatively small, clay minerals exhibit a great diversity in their chemical compositions, manly, due to the different metallic elements substituting the silicon in tetrahedral positions [3]. Smectite-type clay minerals are the most used substrates to form molecular aggregates. Smectites possess a layered structure due to the condensation space of an octahedral Al2O3 or MgO planar sheet (O) between two planar tetrahedral SiO2 sheets (T), the so-called TOT layer (figure 1).
chemical union (mostly due to intermolecular van der Waals forces, ionic interactions and H-bonds [8]) but through complete enclosure of one set of molecules in a suitable structure formed by another [9].
Figure 1. Layered structure of smectite clay minerals
TOT platelets can achieve a negative charge due to isomorphic substitutions of tetrahedral Si4+ (tetrahedral substitution) and/or octahedral Al3+ or Mg2+ (octahedral substitution) by other metals with lower valence. The negative charges are compensated by inorganic exchangeable cations which are located on the surface of TOT layer. These exchanged cations induce the stacking of TOT layers giving rise to the tactoidal structure of clay platelets, characterized by the interlayer space. Cation exchange capacity (CEC) is a measure of the cations, which balance the negative charge sites of the clay, and it is usually expressed as the retained excess cation quantity divided by the sample mass. Layer charge may also be estimated from stoichiometric coefficients in structural formulae. Smectite-type clays have a CEC in the 30–130 meq/100 g range and this characteristic makes them ideal systems to accommodate cationic or polar organic materials because of the high capacity of swelling and bonding [4, 5].
2.2. Molecular aggregates Molecular aggregates or supramolecular structures are commonly defined as compounds in which two or more components are associated without ordinary
Dye molecular aggregation in clay mineral colloids was reported for the first time by Bergman and O’Konski [6]. Significant properties of clay minerals is the adsorption of organic compounds in the form of arranged supramolecular assemblies, clusters, and self-organized molecular aggregates with specific chemical and anisotropic properties. Some of organic dyes significantly change their color upon the adsorption at solid/liquid interphases. The phenomenon, historically named metachromasy, has been frequently observed in the systems containing organic dyes of flat molecules or chromophoric groups [7]. The extent and types of formed molecular aggregates influence dramatically the optical properties of chromophores [11]. As it is shown in figure 2, considering the head and tail groups of discrete molecules or monomers, the aggregates or dimers can adopt different configurations with specific excitation energies (figure 2) [12].
Figure 2. Exciton splitting of the electronic excited states of dimers with different geometries (radiative deactivation: straight arrows, and non-radiative deactivation wavy arrows).
While the amount of dye aggregates is proportional to clay cationic exchange capacity [13], molecular
287
aggregation is mainly controlled by the concentration of the dye, pH and z-potential of the dispersion media, molecular length and dipole moment of the dye and separation between silicate layers in the clay [14].
2.3. Nanopigments synthesis and properties The first step is to condition the clay nanoparticles, ie, breaking the agglomerates which naturally forms due to its electrostatic potential, and try to increase the separation between plates depending on the size of the dye molecule to be used. This takes advantage of the absorptive capacity of the clays, particularly the clay is dispersed in deionized water producing a swelling between layers. As a result there will be appropriate contact with organic dye molecules, and a decreased of ionic strength between interlaminar cations and silicate layers. Then, the selected dye is also dissolved in deionized water and is added to the dispersion of clay. At this point the reaction takes place with ion exchange between interlaminar cations of the clay and organic dye molecules. Generally, the organic dyes used are those containing caionic groups. The amount of dye used depends on the ion exchange capacity of the clay and the desired intensity of colour [15], then concentration of dye and layer charge of clay are fundamental parameters in the optical properties of nanopigments. By visible absorption spectroscopy the kind and formation extent of molecular aggregates can be characterized (figure 4) and then, effective nanopigment formation or supramolecular dye aggregation is verified with the appearance of absorption bands due to H or J-type aggregates (curves a and b) different to those of dye disperse molecules (curve c). However, it is a challenge to obtain stable nanopigment dispersions that can retain the aggregates structure in a certain time before its application [17].
288
Figure 4. Visible spectra of methylene blue in montmorillonite dispersions and aqueous solution, measured 1 min (a) and 24 h (b) after synthesis and methylene blue solution in low concentration (c) [18].
Besides, as formed aggregates have particular structure and confinement, they will have different enthalpy formation energies and thus thermodynamic properties that can be estimated under thermal analysis tests (figure 5). Pure laponite (curve a) presents water loss and its quite stable over a wide range of temperatures. Nanopigment preparations increasing dye concentration form H and J-type aggregates (curve b and c, respectively). Water loss is less significant due to the replacement of interlaminar adsorbed water by dye aggregates. H-type aggregates are not thermally stable in any temeperature range, but its decomposition is gradual. J-type aggregates are stable below 250 ºC and at higher temperatures its decomposition is faster and more exothermic.
Figure 5. Thermal analysis of rhodamine dye in a laponite clay film: normalized TG curves (dotted line) and DTA curves (solid line). a)Pure laponite, b)laponite with rhodamine 6G 1x10-4 M, and c)laponite with rhodamine 6G 8x10-4 M [19]
Finally, whilst dyes generally have superior colour brilliance than pigments, pigments remain the dominant colorants due to their superior colour-fastness properties [30], then, hybrid clay/dye nanopigments combines thus organic/inorganic, dispersive/soluble properties.
textile fibers and natural colorants, if they are going to play a more important role than at present in the manufacture of the clothes and textiles of every kind, that are needed by present consumers and will be needed in even greater quantities by the expanding world population of tomorrow.
3. “GREEN” NANOPIGMENTS
Thus, the project objective is to develop, improve and implement nanotechnologies leading to more sustainable and environmental friendly commercial pigment products based on coloration of nanoclays with a variety of natural dyes (based on its reactivity, functionality and the completeness of colour gamut.) with an aim to improve performance properties.
Dyes and pigments are essential ingredients for textile, printing inks, paint and coatings, and plastics industries. World demand for dyes and organic pigments is forecast to increase 3,9% per year to $16,2 billion in 2013. In volume terms, demand will grow 3,5% annually to 2,3 million metric tons [32]. Increasing worldwide awareness of the pollution resulting from the production and use of some synthetic colorants and the increasing concerns in terms of carbon emission and gradual depletion of world crude oil reserve, has led to a significant revival of interest in natural colorants in the last years and is inspiring projects for sustainable and environmentally friendly development of their production [20]. Recent years have seen an unprecedented number of attempts in the development of pigments based on clays. The advantages of nanopigment, in terms of both mechanical and optical properties, are well understood and documented [21-29]. Natural dyeing symbolizes craft practices which reflect an harmonious and sustainable relationship with the ecosystem and the local plant reservoir. But, on the other hand, inconsiderate use of natural dyes at industrial scale could contribute to the reduction of biodiversity [20]. Therefore, a massive effort of interdisciplinary study is needed in order to optimize the production of
The factors for molecular natural dye aggregation between clay layers, its stability or performance under different conditions and thermodynamics and kinetic of aggregation will be key issues to estimate the potential impact of those coloration alternatives. 4. CONCLUSIONS In recent years there have been significant advances in the design, synthesis and preliminary characterization in clay and dye based nanopigments. Our initial results confirm that a vast field of study on nanopigments and their industrial applications lies ahead for the coming years. It is quite likely that in some color formulas the use of nanopigments significantly reduces the incorporation of other additives needed with conventional dyes, providing better weather resistance and increasing mechanical properties to the substrate. Moreover, there are great expectations about a whole field of study on the direct use of natural dyes (based mainly in plants and microorganisms) in the design and
289
synthesis of nanopigments, combining the best of indigenous traditions (American, Indian, etc) with the best of recent advances in nanomaterials science and technology of color. 5. ACKNOWLEDGMENTS This research is supported by the Spanish Ministry of Science and Innovation by the grant number DPI2008-06455-C02-02. 6. REFERENCES [1] P. Knauth and J. Schoonman, Nanostructured Materials: Selected Synthesis Methods, Properties and Applications, New York: Kluwer Academic Publishers, 2004. [2] S. Raha, I. Ivanov, N.H. Quazi, and S.N. Bhattacharya, "Photostability of rhodamine-B/montmorillonite nanopigments in polypropylene matrix," Applied Clay Science , vol. 42, 2009, pp. 661-666. [3] A. Meunier, Clays, Berlin: Springer-Verlag Berlin Heidelberg, 2005. [4] F. López Arbeloa, V. Martínez, T. Arbeloa López, and I. López Arbeloa, "Photoresponse and anisotropy of rhodamine dye intercalated in ordered clay layered films," Journal of Photochemistry and Photobiology C , vol. 8, 2007, pp. 85-108. [5] A. Czímerová, J. Bujdák, and R. Dohrmann, "Traditional and novel methods for estimating the layer charge of smectites," Applied Clay Science, vol. 34, 2006, pp. 2 - 13. [6] J. Bujdák and N. Iyi, "Optical properties of molecular aggregates of oxazine dyes in dispersions of clay minerals," Applied Clay Science, 2009, pp. 157-165. [7] J. Bujdák and N. Iyi, "Molecular Orientation of Rhodamine Dyes on Surfaces of Layered Silicates," J. Phys. Chem. B, 2005, pp. 4608-4615. [8] J. Bujdák, "Effect of the layer charge of clay minerals on optical properties of organic dyes. A review," Applied Clay Science, vol. 34, 2006, pp. 58-73. [9] S. Yang and P. Sheng, Physics and Chemistry of Nanostructured Materials, Philadelphia: Taylor & Francis Inc, 2000. [10] J. Bujdák, A. Czímerová, and N. Iyi, "Structure of cationic dyes assemblies intercalated in the films of montmorillonite," Thin Solid Films, vol. 517, 2008, pp. 793-799. [11] F. López Arbeloa, J. Bañuelos Prieto, T. Arbeloa López, and I. López Arbeloa, "Characterization of Rhodamine 6G Aggregates Intercalated in Solid Thin Films of Laponite Clay. 1. Absorption Spectroscopy," J. Phys. Chem. B, 2004, pp. 2003020037. [12] N. Iyi, T. Fujita, C.V. Yelamaggad, and F. López Arbeloa, "Intercalation of cationic azobenzene derivatives in a synthetic mica and their photoresponse," Applied Clay Science, 2001, pp. 47-58. [13] F. López Arbeloa, S. Salleres, V. Martínez, C. Corcóstegui, and I. López Arbeloa, "Effect of surfactant C12TMA molecules on the
290
self-association of R6G dye in thin films of laponite clay,"
Materials Chemistry and Physics , vol. 116, 2009, pp. 550-556. [14] a. Gürses, C. Doğar, M. Yalçin, M. Açikyildiz, R. Bayrak, and S. Karaca, "The adsorption kinetics of the cationic dye, methylene blue, onto clay.," Journal of hazardous materials, vol. 131, 2006, pp. 217-28. [15] J. Bujdák, F. López Arbeloa, and N. Iyi, "Spectral Properties of Rhodamine 3B Adsorbed on the Surface of Montmorillonites with Variable Layer Charge," Langmuir, 2007, pp. 1851-1859. [16] S. Salleres and T. Arbeloa, "Adsorption of fluorescent R6G dye into organophilic C12TMA laponite lms," Journal of Colloid and Interface Science, vol. 321, 2008, pp. 212-219. [17] E. Baez, N. Quazi, I. Ivanov, and S.N. Bhattacharya, "Stability study of nanopigment dispersions," Advanced Powder Technology, vol. 20, 2009, pp. 267-272. [18] J. Bujdák, K. Lang, and et. al., "Clay Mineral Particles As Efficient Carriers of Methylene Blue Used for Antimicrobial Treatment," Environmental Science & Technology , vol. 43, 2009, pp. 6202-6207. [19] F. López Arbeloa and V. Martínez, "Orientation of Adsorbed Dyes in the Interlayer Space of Clays. 2 Fluorescence Polarization of Rhodamine 6G in Laponite Films," Clay Minerals, 2006, pp. 1407-1416. [20] M.E. Davila and E. Portillo, "INTERNATIONAL SYMPOSIUM / WORKSHOP ON NATURAL DYES," Reflections on the actual use of natural dyes , Hyderabad, India: UNESCO, 2006. [21] P.M. Ajayan, L.S. Schadler and P.V. Braun,Nanocomposite Science and Technology, Weinheim, WILEY-VCH, 2003. [22] Y.-W. Mai and Z.-Z. Yu, Polymer Nanocomposites, Woodhead Publishing Ltd. and CRC Press LLC, London, 2006. [23] Q. Zeng, Fundamental studies of organoclays and polymer nanocomposites, University of New South Wales, 2004. [24] H. Fischer, Polymer nanocomposites: from fundamental research to specific applications, Mater. Sci. Eng. C, 23, 763–72, 2003. [25] G. Buxbaum and G. Pfaff, Industrial Inorganic Pigments, Weinheim, Wiley-VCH, 2005. [26] K. Hunger, Industrial Dyes: Chemistry, Properties and Applications, Weinheim, Wiley-VCH, 2003. [27] W. Herbst and K. Hunger, Industrial Organic Pigments, 3rd ed., Weinheim, Wiley-VCH, 2004. [28] P. Bamfield, Chromic Phenomena: Technological Applications of Colour Chemistry, Cambridge, The Royal Society of Chemistry, London, 2001. [29] S-H. Kim, Functional Dyes, Elsevier, Amsterdam, 2006. [30] E.B. Faulkner, R.J. Schwartz, High Performance Pigments, 2nd ed. Weinheim, Wiley-VCH, 2009. [31] M.E. Davila and E. Portillo, "INTERNATIONAL SYMPOSIUM / WORKSHOP ON NATURAL DYES," Reflections on the actual use of natural dyes, Hyderabad, India: UNESCO, 2006. [32] World Dyes & Organic Pigments, The Freedonia Group Inc., May 2009.
