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OPTICAL AND VIBRATIONAL SPECTROSCOPIC STUDIES OF
SYNTHETIC MAYA PIGMENTS AS A FUNCTION OF
CONCENTRATION OF INDIGOID DYES
SWATI KUMAR
Department of Chemistry
APPROVED:
______Russell R. Chianelli, Ph.D., Chair
______
Felicia Manciu, Ph.D.
______Wen-Yee Lee, Ph.D.
______Lori A. Polette-Niewold, Ph.D.
______Patricia D. Witherspoon, Ph.D. Dean of the Graduate school
© The University of Texas at El Paso, El Paso, Tx, USA, 2008
All rights reserved.
Dedicated to My Loving Parents
OPTICAL AND VIBRATIONAL SPECTROSCOPIC STUDIES OF
SYNTHETIC MAYA PIGMENTS AS A FUNCTION OF
CONCENTRATION OF INDIGOID DYES
By
SWATI KUMAR
THESIS
Presented to the Faculty of the Graduate School of
The University of Texas at El Paso
in Partial Fulfillment
of the Requirements
for the Degree of
MASTER OF SCIENCE
Department of Chemistry
THE UNIVERSITY OF TEXAS AT EL PASO, EL PASO
December 2008
ACKNOWLEDGMENTS
First of all, I would graciously like to thank Prof. Russell R. Chianelli, who gave me an opportunity to learn new things and taught me to take up new challenges in the studies and also for providing the necessary facilities for carrying out present research work. I also want to thank Dr. Lori Polette for her valuable guidance, constant encouragement, keen interest and affectionate attitude during the course of this work.
I gratefully acknowledge Dr. Felicia Manciu, for her tireless efforts and help in solving problems regarding spectroscopic data. I am deeply obliged to Prof. Keith H.
Pannell, Professor, Department of Chemistry, for his well wishes and suggestions. I also would like to convey my special thanks to Prof. R. N. Kapoor and Dr. Hemant K.
Sharma for their valuable suggestions and timely help. I would like to express my thanks to MRTI specially Alejandra Ramirez for her assistance, support and valuable discussion during my research work .
Finally I wish to express my deepest gratitude to my parents, and friends for all they have done for me. The love, the inspiration, the care and the support that they have given me has been too overwhelming. I want to also say a special thank to my husband Mukesh Kumar, one of the most important and influential person in my life, who always inspired me to achieve my goal, I want to thank him for all his unconditional academic and emotional support.
v
ABSTRACT
Pigments developed by the Mayan civilization around 8th century, represent some of the most versatile pigments known to date. Several derivatives of these pigments are popular subjects of current research interest. This is due to the characteristic stability which is provided by a bonding mechanism between the dye and the clay. One such pigment “Maya Blue”, a mixture of Indigo and Palygorskite, provides a dramatic background for murals and ceramics throughout Mesoamerica. Several research groups have devoted time and interest in unlocking its particular features. 1-3
The work embodied in this thesis is focused on the synthesis and characterization of three pigments: Maya Blue, Maya Purple and Royal Blue with varying concentrations (1-25%) of the organic dyes. Samples were prepared by heating the corresponding dye with Palygorskite (Inorganic clay) at 170 °C for 9 hours. Various factors which account for the stability of these complexes are discussed by a critical analysis of the results obtained.
Ultra Violet-Visible (UV-Vis) spectra of Maya Blue, Maya Purple and Royal Blue samples provide an evidence for variations in the electronic structure of the dyes after they have incorporated into the Palygorskite matrix. This is suggested by a bathochromic shift of π→π* transition associated with dyes [ λmax (Indigo) = 584 nm,
λmax (Maya Blue 6%) = 656 nm; λmax (Thioindigo) = 507 nm, λmax (Maya Purple 6%) = 590 nm]. In contrast, upon increasing the concentrations of the dye in the pigment, the absorption maxima shift to a lower wavelength which is suggestive of partial contribution of the dye at higher concentrations.
vi
Analysis of Fourier Transform Infrared (FTIR) spectra provides a qualitative bonding description of the C=O, N-H, C=C, O-H and Si-O-X (where X = H, Al, Si) groups. The stretching band due to C=O group shifts to at lower wavenumber after the pigments formation [( νC=O, cm-1) = 1626 (Indigo), 1622 (Maya Blue), 1655 (Thioindigo),
1627 (Maya Purple)]. The νN-H band disappeared at lower concentrations of the dye in the Maya Blue samples. These data support the involvement of such groups in bonding during the pigment formation. On the contrary, the bands due to C=O, N-H groups become more sharp at higher concentrations of the dye. In a linear argument, the appearance of sharp bands of the C=O group suggests an excess of Indigo and
Thioindigo dyes.
Fourier Transform Raman (FT Raman) spectroscopic, Powder X-ray diffraction
(XRD), and Differential Scanning Calorimetric (DSC) studies further provide evidences to develop the binding mechanism of the dye and the clay.
Based on all the results, it is envisaged that, at lower concentrations of the dye
(<6%), the dye molecules may penetrate into the channels of clay while on increasing the concentrations (>6% - <16%), the dye molecules bind with the exposed surface involving Si-O-Mn+ (M = Al, Fe) sites. At much higher concentrations (>16%) of the dye, the surfacial activity predominates and the dye accumulates in the form of layers on the outer surface of the clay.
vii
TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS……………………………………………………………………...v
ABSTRACT……………………………………………………………………………………...vi
TABLE OF CONTENTS………………………………………………………………………viii
LIST OF TABLES…………………………………………………………………………….....x
LIST OF FIGURES…………………………………………………………………………...... xi
STATEMENT OF PROBLEM………………………………………………………………...xvi
RESEARCH OBJECTIVES…………………………………………………………………..xvi
JUSTIFICATION OF WORK…………………………………………………………………xvii
Chapter
1. INTRODUCTION……………………………………………………………………………..1
2. MATERIAL SAMPLING AND CHARCTERIZATION TECHNIQUES………………….16
2.1 Material Sampling………………………………………………………………………16
2.2 Sample Preparation……………………………………………………………………16
2.3 Characterization Techniques………………………………………………………….16
3. RESULTS AND DISCUSSION……………………………………………………………19
3.1. Color Change…………………………………………………………………………19
3.2 UV-Vis Spectroscopy……………………………………………………………...... 22
3.2.1 Indigo/ Palygorskite complex: Maya Blue………………………………………...22
3.2.2 Thioindigo/ Palygorskite complex: Maya Purple…………………………………28
viii
3.3 FTIR Spectroscopy……………………………………………………………………32
3.3.1 Palygorskite FTIR …………………………………………………………………...32
3.3.2 Indigo FTIR…………………………………………………………………………..37
3.3.3 Thioindigo FTIR……………………………………………………………………..38
3.3.4 FTIR Studies of Maya Blue………………………………………………………...40
3.3.4.1 Vibrational Modes Associated with Palygorskite………………………………41
3.3.4.2 Vibrational Modes Associated with Indigo……………………………………..46
3.3.4.3 Concentration Change in Maya Blue …………………………………………...50
3.3.5 FTIR Studies of Maya Purple………………………………………………………53
3.3.5.1 Vibrational Modes Associated with Palygorskite………………………………55
3.3.5.2 Vibrational Modes Associated with Thioindigo………………………………...58
3.3.5.3 Concentration Change in Maya Purple………………………………………...59
3.4 FT Raman Study of Maya Blue……………………………………………………...62
3.5 Chemical Analysis of Palygorskite…………………………………………………..64
3.6 X-Ray Diffraction Study of Maya Blue and Maya Purple………………………….66
3.7 Royal Blue……………………………………………………………………………..69
3.8 Differential Scanning Calorimetry Studies………………………………………….73
3.9 Discussion and Conclusions…………………………………………………………78
REFERENCES………………………………………………………………………………..84
APPENDIX………………………………………………………………………………….....90
CURRICULUM VITA………………………………………………………………………….93
ix
LIST OF TABLES
Table 1 . Absorption intensities of Indigo and Maya Blue heated at 170 oC/9h………….27
Table 2. Absorption intensities of Indigo and Maya Purple heated at 170 oC/9h……….31
Table 3 . Frequencies and IR modes of Palygorskite…………...... 36
Table 4 . Frequencies and IR modes of Indigo……………………………………………..38
Table 5. Frequencies and IR modes of Thioindigo………………………………………..39
Table 6 . IR frequencies for water modes in Maya Blue (6%) heated at 170 °C/9h….....46
Table 7 . IR frequencies for carbonyl modes and N-H modes……………………………49
Table 8 . IR frequencies for carbonyl modes and N-H modes in Maya
Blue (1-25%) heated at 170 °C/9h…………………………………………………………....53
Table 9 . IR frequencies for water modes in Maya Purple (6%) heated at 170 oC/9h...... 57
Table 10 . IR frequencies for carbonyl modes in Maya Purple……………………………59
Table 11 . IR frequencies for carbonyl modes in Maya Purple (1-25%) heated
at 170 °C/9h……………………………………………………………………………………..61
Table 12 . WD-XRF chemical analysis……………………………………………………....65
Table 13(a). ∆H (Enthalpy) values calculated for Maya Blue……………………………..75
Table 13(b). ∆H (Binding Energies) values calculated for Maya Purple…………………76
Table 13(c) . ∆H (Enthalpy) values calculated for Royal Blue…………………………….76
Table 14. Mol% calculation of different concentration of Indigo in Maya Blue………….90
Table 15. Mol% calculation of different concentration of Thioindigo in Maya Purple…..90
x
LIST OF FIGURES
Figure 1. Mural from Bonampak, Chiapas, Mexico (Classic Period 250 AD-900 AD)…..1
Figure 2. Palygorskite unit cell, visualization on 001 plane (top)……………………….....5
Figure 3. Octahedral ribbon for Palygorskite [Image courtesy of Güven et al .]………….7
Figure 4. Schematic representation of Bronsted acids (left) and Lewis acids…………...9
Figure 5. (a) The structure of an Indigo molecule, where: Blue = Nitrogen,
Red = Oxygen, Grey = Carbon and White = Hydrogen) (b) Crystal packing structure of Indigo (Modelled by ‘Cerius 2 ’ Acelrys software)………………………………………..11
Figure 6. Schematic of irreversible oxidation of Indigo to Dehydroindigo and reversible reduction to Leucoindigo…………………………………………………...... 12
Figure 7. (a) The structure of the Thioindigo molecule, where: Yellow = Sulfur,
Red = Oxygen, Gray = Carbon and White = Hydrogen (b) Thioindigo crystal structure [Modelled by ‘Cerius2’ Acelrys software]………………………………………..14
Figure 8 . Representation of trans- and cis- forms of Thioindigo………………………...15
Figure 9. (a) The color change observed when Indigo is heated with Palygorskite at 170 o C for 9 h. (b) Variations in color upon altering the concentrations of Indigo
(BH = Before Heating, AH = After Heating) [Image taken by Swati Kumar]……………20
Figure 10. (a) The color change observed when Thioindigo is heated with
Palygorskite at 170 o C for 9 h. (b) Variations in color upon altering the concentrations of Thioindigo (BH = Before Heating, AH = After Heating)
[Image taken by Swati Kumar]...... 21
Figure 11. UV-Vis spectra of Indigo, Maya Blue before (MBBH) and after
xi
heating (MBAH) 170 oC/9h……………………………………………………………………23
Figure 12 . UV-Vis spectra of various concentrations of Indigo (1-25%) in Maya
o Blue 170 C/9h………………………………………………………………………………….26
Figure 13 . UV-Vis spectra of pure Thioindigo, Maya Purple before and after
heating at 170ºC/9h…………………………………………………………………………...29
Figure 14 . UV-Vis spectra of different concentrations of Thioindigo (1-25%) in
Maya Purple heated at 170 °C/9h……………………………………………………………30
Figure 15. FTIR spectra of Palygorskite Clay (a) Complete range spectrum
(4000-400 cm -1), (b) OH stretching region, (c) Water bending vibration region………...35
Figure 16 . IR spectrum of Indigo (4000-400 cm -1)………………………………………...37
Figure 17 . IR spectra of Thioindigo (4000-400cm -1)………………………………………39
Figure 18. IR spectra observed for Indigo, Palygorskite, and Maya Blue before heating (BH) and after heating (AH) at 170 °C/9h…………………………………………..41
Figure 19 . OH stretching modes of Palygorskite…………………………………………..42
Figure 20 . IR spectra showing bending modes of water……………………………….....44
Figure 21 . IR spectra showing stretching modes of Si-O-Si and SiO 3……………….....44
Figure 22 . IR spectra showing bending modes of SiO3…………………………………..45
Figure 23 . IR spectrum for carbonyl region...... 47
Figure 24 . IR spectrum for stretching N-H modes………………………………………...48
Figure 25 . IR spectrum for bending N-H modes…………………………………………..49
Figure 26 . IR spectra for different concentrations (wt%) of Indigo in Maya Blue at 170 °C/9h…………………………………………………………………………………….50
xii
Figure 27 . IR spectra for carbonyl region of different concentrations of
Maya Blue……………………………………………………………………………………...51
Figure 28 . IR spectrum for N-H region on altering concentrations of Indigo in
Maya Blue at 170 °C/9h……………………………………………………………………….52
Figure 29. IR spectrum for Thioindigo, Palygorskite, Maya Purple (6%) before
and after heating at 170 °C/9h………………………………………………………………..54
Figure 30 . IR spectrum showing OH region for Maya Purple before and after
heating………………………………………………………………………………………….55
Figure 31 . IR spectrum for Si-O-Si and SiO 3 region in Maya Purple at 170 °C/9h…….56
Figure 32 . IR spectra for C=O region of Maya Purple at 170 oC/9h……………………...58
Figure 33 . IR spectra for different concentrations (wt%) of Thioindigo in Maya Purple
at 170 °C/9h (a); Change in C=O region on altering concentration (wt%) of Thioindigo
in Maya Purple (b)…………………………………………………………………………….60
Figure 34 . Raman spectra for Indigo, Maya Blue (20%) heated 9h, and 24h at 170 oC
(a) Finger print region, (b) Metal-Oxide region, (c) Carbonyl region…………………….63
Figure 35 . XRD pattern of Palygorskite from Mintech. comparison with two simulated polymorphs (Monoclinic, Orthorhombic unit cell) of Palygorskite, and
Quartz impurity…………………………………………………………………………………66
Figure 36 . XRD pattern of 6% Thioindigo mixture (unheated) with Palygorskite, comparison with Thioindigo, Palygorskite, and heated mixture at 170 oC/9h……………67
Figure 37. XRD pattern for Maya Blue (1-25%) heated at 170 °C/9h…………………....68
xiii
Figure 38 . XRD pattern for Maya Purple (1-25%) heated 170 °C/9h…………………...68
Figure 39. The color change observed when a mixture of Thioindigo and Indigo is heated with Palygorskite at 170 o C for 9 hours (BH = Before Heating, AH
= After Heating)………………………………………………………………………………..70
Figure 40 . UV-Vis spectra of Royal Blue before heating (RB BH) and after
heating (RB AH) at 170 oC/9h………………………………………………………………..71
Figure 41. FTIR spectra of Thioindigo, Indigo and Royal Blue before and after heating at 170 oC……………………………………………………………………………….72
Figure 42 . DSC profiles of different concentration Maya Blue between temperature range 25-400 oC………………………………………………………………...73
Figure 43 .DSC profiles of Maya Purple between Temperature range 25-400 oC………74
Figure 44 . DSC Profiles of Royal Blue, Maya Blue and Maya Purple between
temperature range 25-400 oC………………………………………………………………..75
Figure 45 . Schematic representation of Palygorskite unit cell, Tetrahedral, and
Octahedral sheets…………………………………………………………………………….79
Figure 46. A schematic representation of Si-O Tetrahedral sheet and active binding sites of clay……………………………………………………………………………80
Figure 47 . Possible interaction of Indigo and Thioindigo with Al 3+ sites present inTetrahedral sheets…………………………………………………………………………..82
Figure 48 : A model for binding of the dye with the clay as a function of concentrations of the dye…………………………………………………………………….83
Figure 49. Concentration Vs Absorbance for Indigo (wt%)………………………………91
xiv
Figure 50. Concentration Vs Absorbance for Indigo (mol%)……………………………..91
Figure 51 . Concentration Vs Absorbance for Thioindigo (wt%)…………………………92
Figure 52. Concentration Vs Absorbance for Thioindigo (mol%)……………………….92
xv
STATEMENT OF THE PROBLEM
The problem addressed in this thesis is the basic understanding of the bonding interactions between palygorskite and indigoid dyes. The chemistry of Maya pigments and its components has witnessed an unseeming proliferation during the past decades.
However, the nature of bonding between the clay and dye is still unknown till date. It was thus desired to explore the structural aspects of these remarkable pigments especially Maya Blue and Maya Purple. The primary impetus was driven from the concept that functional groups of the dye and active sites of the clay are involved in bonding.
RESEARCH OBJECTIVES
The aim of this work is to synthesize a series of samples of Maya Blue (1-25%) and Maya Purple (1-25%) with varying concentration of dyes in clay matrix to understand the nature of interactions between organic dye and inorganic host palygorskite. Various concentrations of indigoid dyes have been used in order to determine the amount of dye binding to the inorganic host. Optical spectroscopy,
Fourier Transformed Infrared spectroscopy (FTIR), FT Raman, Differential Scanning
Calorimetric, and X-ray diffraction studies were used to understand the nature of interactions and changes occurring during formation of complex on increasing concentration.
xvi
JUSTIFICATION OF THE WORK
Organic, inorganic hybrid material exhibits excellent thermal and chemical stability and which does not contain any heavy metals. This is important to paint and pigment industry, which consumes a large amount of environmental unfriendly metals.
Stability coupled with environmentally friendly behavior will lead to replacement of metal-containing pigments with new types of tunable hybrid materials that offer durability and wide range of color for future development opportunities.
xvii
CHAPTER 1
INTRODUCTION 1.1 Background
The Mayan homeland, once known as Mesoamerica, spans the five modern countries of Mexico, Guatemala, Belize, Hondruas and El Salvador. There is evidence that the Mayan population migrated south from North America to the highlands of
Guatemala, in Central America, as early as 2600 B.C. This greatest of all
Mesoamerican civilization, flourished during the classic period (250 to 900 AD), developed a technical skill of producing “environmentally friendly” colored pigments. 1-8
This was done by mixing natural dyes (extracted from plants) with clay. Maya Blue is one such pigment displayed remarkable stability over hundreds of years of exposure to atmospheric conditions. 9 A mural from the Bonampak, Chiapas region in Mexico shown in Figure 1 has survived over a thousand years of harsh exposure.10
Figure 1. Mural from Bonampak, Chiapas, Mexico (Classic Period 250 AD - 900 AD). 10
1
Its beautiful display of color and ease of application made it, the most spectacular
pigments of Mesoamerican cultures; being widely used to decorate pots, sculpture,
murals, and panels throughout Central America and Mexico. 11 The distinct blue color, was created by using a combination of two materials: Indigo, the natural dye extracted from the leaves of the plant ( Indigofera surffruticosa ) and Palygorskite (called saklu'um or 'white earth' from the Yucatec Maya language ).12
While the exact technique that Mayans employed to synthesize such a sophisticated pigment remains a mystery, the pigment has been successfully reproduced in a laboratory setting. 13 Synthetic Maya Blue can be prepared by the
combination of two ingredients, the inorganic component the Palygorskite and the
organic component the natural dye. The two components are mechanically combined
and heated at temperatures between 150 to 200 °C for a specific time. 14 Heating of
mixture is a necessary step, as it incorporates molecules of indigo into the white
Palygorskite clay. The process of embedding or intercalating Indigo into the clay
ensures the colors stability, even under exposure to harsh elements such as: alkali,
nitric acid and organic solvents. 15 For more than fifty years, this astonishing feature of
pigment has generated much interest and debate among research scientists.12, 13
Several theories have been proposed in elucidating the stability and nature of the
bonding interactions between the dye and the clay mixture. Van Olphan et al. 16
suggested that heating the mixture at 150 °C facilitates the elimination of zeolitic water
and allows the incorporation of Indigo into the channels of the clay. Chiari et al .17 and
18 Giusetetto et al . based on X-Ray Diffraction, Fourier Transform Infrared, Raman
2
spectroscopy and computational molecular modeling, concluded that the Indigo resides
within the channels of the clay and simultaneously form hydrogen bonds between C=O
and N-H groups. Hubbard et al .19 utilized thermal analysis, textural analysis and nuclear magnetic resonance spectroscopy techniques to build a model in which the Indigo molecules are anchored at the entrances of the channels of the Palygorskite via hydrogen bonding to the silanol groups at the edges of the fiber. Domenech, et al ., 20 reported that the different shades of color in the mixture results from varying amounts of dehydroindigo, the oxidized form of indigo. Similarly, Polette-Niewold, et al.21 contributed to solving the mystery by contributing the experimental observation of oxidation of indigo to dehydroindigo during the formation of the pigment. In conjunction with this operation it is theorized that the carbonyl oxygen of Indigo binds to surface Al3+ cation in SiO 4 tetrahedral sheet.
More fascinating properties have been found among several other derivatives of
Maya pigment. For instance, Maya Purple, an unusual derivative of Mayan pigment, synthesized from the organic dye Thioindigo and Palygorskite, exhibits a broad range of color from pink to purple after the specified heating treatment. Reinen et al .22 , using the tetrachloro derivative of Thioindigo, observed similar color changes and suggested that hydrogen bonding between the dye and the clay. Recently, Manciu et al .23 reported the detailed Fourier Transformed Infrared (FTIR), and FT Raman spectroscopic studies on
Maya Purple and proposed the formation of hydrogen bonds between the carbonyl and silanol (Si-OH) groups at lower concentration and metal oxygen bonding at higher concentrations.
3
1.2 Structural Description of Inorganic and Organic Components of the Pigments
1.2.1 Palygorskite
Palygorskite, a naturally occurring fibrous clay, is a key constituent of the pigment called Maya Blue. 24 The clay is an aluminum silicate with the general formula
2+ 3+ 3+ n+ (R ,R )5(Si,R )8O20 (OH 2)4M (H 2O) 4 with varying amounts of cation substitutions
2+ 2+ 2+ 2+ including Ca , Mg , and Fe (R ). Due to its large surface area and the presence of acidic sites; Palygorskite posses catalytic and sorptive properties, which are discussed later in this section. 24, 25
The structure of this fibrous clay was first reported by Bradley in 1940 26 and later
27-30 refined by several other scientific authors. Palygorskite is a mixture of two polymorphs: a monoclinic polymorph whose space group is C2/m with cell parameters a
= 13.24, b = 17.89, c = 5.21 Å, and an orthorhombic (space group: pbmn). 30 The structure is formed by discontinuous octahedral layers elongated in the c-direction alternated with continuous tetrahedral layers. For every two chains of tetrahedral layers, the apexes of the SiO 4 tetrahedral units point alternatively upward and downward. This causes the structure to cross and form micro-channels throughout the structure in the c- direction. 31 The dimension of channels is 6.4 x 3.7 Å, which is sufficiently large enough to store four zeolitic water molecules in the channels and four additional water molecules coordinated to the M2+ (generally Mg 2+ ) molecules per formula unit. 32 In general, the clay contains three types of water in its structure: (a) zeolitic water, free water molecules in the channels (b) coordinated water, coordinated to edge cations
(Mg 2+ ), and (c) structural water, where OH groups complete the coordination of metal
4
cations in each octahedral layer as presented in Figure 2. 30
Figure 2. Palygorskite unit cell, visualization on 001 plane (top). Block unit representation showing dimensions of the channels (bottom) [Image courtesy of
U.S. Geological Survey at http:// pubs.Usgs.gov.]
5
Thermal studies of Palygorskite revealed that the water molecules present in the
clay are susceptible to dehydration under relatively low temperature. 31 Recently, G.
Chiari performed thermal analysis which reported that dehydration is completely
reversible up to 350 °C. At 120 °C free pore water and surface water is lost while zeolitic water and partially coordinated water is released between ranges 120-300 °C. However, structural water remains unaffected up to 350 °C. At temperature higher than 350 oC the clay structure begins to fold due to loss of structural water. 33
Loss of water exposes the surface cations of the clay. These cations could play
an important role in the binding mechanism of the dye. Cation distribution and
arrangement in octahedral sheet define the nature of clay being either dioctahedral or
trioctahedral. 34 Chemical analysis of clay published by Bradley et al. 26 and later
reviewed by Dirtis et al.35 elucidated that only four of the five octahedral cation sites are occupied. Serna et al.36 and Augsburger et al.3 confirmed these observations via chemical analysis and FTIR studies. These observations illustrated in Figure 3, assisted in describing the distribution of metal cation bonded to the bound (coordinated water) and structural water (OH groups) inside the clay structure. According to this model, each Mg 2+ , located at the edges of the octahedral sheet, is bound to two coordinated water molecule (M3) positions, while two trivalent cations Al 3+ and/or Fe 3+ located in inner octahedral sites (M2), are bound with two hydroxyl groups. This arrangement is called dioctahedral, where two of three octahedral sites are occupied by trivalent cation
R3+ (eg. Al 3+ , Fe 3+ ). 38
6
Figure 3. Octahedral ribbon for Palygorskite [Image courtesy of Güven et al .]
1.2.1.1 Sorptive Sites and Surface Area
Palygorskite is considered a porous structure and structural studies have revealed consistently a 300 Å effective pore size with observable 40-80 Å mesopores also called intermediate pores, and additional 20 Å micropores. The specific surfaces of the inner channels and pores makeup ~50% of the total surface (224m 2/g by BET method). 39 Shariatmadari et al.40 illustrated that most of the binding sites are located at the external surface of the clays particles, therefore surface area is the major factor controlling the contribution of these sites to total sorption. The structure of the fibrous clay possesses three kinds of sorptive sites on its external surface.
(i) An oxygen ion on the tetrahedral sheet of ribbons. These oxygen ions are part of the
siloxane groups and behave as weak donors due to substitution of Al 3+ in the
7
tetrahedral sheet, interaction with these ions will also be very weak.
(ii) Water molecules coordinated to Magnesium ions at the edges of structural ribbons
display acidic properties. However, Infrared studies performed by Serratosa et al. 41
revealed that these water molecules coordinated to edge cations do not show acidic
properties, therefore, are not considered to be active sorptive site.
(iii) Si-OH groups along the fiber axis. Silanol groups are formed because broken Si–O-
Si bonds at external surfaces compensate their residual charge by accepting a
proton and becoming Si-OH groups. These groups can interact with molecules
adsorbed on the external surfaces. The abundance of these groups is related to the
dimensions of the fiber and crystal imperfections. 42 Silanol groups present on the
surface of palygorskite have a certain degree of acidity and can act as catalysts or
reactive sites. 43 The term surface acidity in clay minerals refers to the capability for
proton donation or electron pair acceptance by a functional groups located at the
edges of alumino–magnesium–silicate framework of the clays. 44 This characteristic
acidity results from the presence of surface groups such as siloxanes, silanols, and
aluminols. The presence of these groups considered a determinant factor to the
catalytic surface and sorptive activity .45 Based on the capability of proton donation
and electron pair acceptance these active acid sites can be differentiated as
Bronsted acids (proton donor) and Lewis acids (electron acceptor) as shown in
Figure 4.
8
+ OH H 2 _ O Al Si Al O O OO O O O O O
Bronsted acid Lewis acid
Figure 4. Schematic representation of Bronsted acids (left) and Lewis acids (right).
Spectroscopic techniques such as FTIR spectroscopy used to characterize these surface acid active sites. 44 U. Shauli et al.45 determined the strength of active sites from the adsorption of n-butylamine, pyridine and trimethylpyridine over the clay and revealed that at 150 °C most of the acid sites are of the Bronsted type. This interaction was mainly due to the hydrogen bonding between nitrogen (proton acceptor) and hydrogen (proton donor). The concentration and strength of acid sites increases with the thermal loss of zeolitic and bound water. Bound water molecule is a stronger acid than that of zeolitic water, but in general, Bronsted acidity inside the channel is weak.
However, C. Blancod et al. 46 determined high concentration of thermally stable Lewis acid sites at 150 °C on the surface of the palygorskite by FTIR modes of pyridine adsorbed on the clay. They also studied the adsorption of pyridine with heated (at
150 °C) and unheated mineral and found that after evolution of zeolitic water the clay structure folded and pyridine adsorbed into the unheated sample was not protonated, even after the organo-clay complex was heated at 150 °C, while pyridine adsorbed on heated sample was protonated, indicating that after evolution of zeolitic water and reversible folding of clays Bronsted acidity of bound water increase, when the organo- clay complex was heated at higher temperature.
9
1.2.2 Indigo
Indigo, a blue colored dye, is the organic component of the pigment Maya Blue. 47
This historical dye, insoluble in any organic solvent, is also well known for its extensive
use in textile industry. The structure of dye, as depicted in Figure 5(a), was established
by Adolf von Bayer in 1833. 48, 49
Indigo forms dark blue colored needles upon crystallization, which sublimes at
300 oC. 50 In crystalline phase, (Figure 5b), it is monoclinic polymorph with space group
P2 1. The lattice parameters are: a = 10.84 Å, b = 5.89 Å, c = 12.28 Å. It absorbs light in the yellow region of the electromagnetic spectrum (602 nm), which gives it a intense blue color. A large bathochromic shift is observed in the electromagnetic spectrum. It is likely due to its conjugated system of donor and acceptor groups linked by a C=C bond. 51, 52 Indigo is found to be stable in the trans configuration, which is due to intramolecular hydrogen bonding between the amine and carbonyl functional groups.53
Indigo can also undergo reversible reduction and irreversible oxidation process in
presence of two carbonyl groups linked via a conjugated system as shown in Figure 6.
Reduction of the carbonyl group to a hydroxyl group with a reducing agent such as
sodium dithionate (Na 2S2O4) under alkaline conditions results in the “Leuco” form, which is a water soluble yellow dye. This “Leuco” form is easily oxidized back to Indigo upon exposure to atmospheric oxygen. This process is popularly used in the textile industries for thousands of years. 48 The oxidation of indigo with an oxidizing agent such as lead
dioxide in benzene, produces dehydroindigo as referenced earlier.53
10
(a)
(b)
Figure 5. (a) The structure of an Indigo molecule, where: Blue = Nitrogen, Red =
Oxygen, Grey = Carbon and White = Hydrogen) (b) Crystal packing structure of Indigo
(Modelled by ‘Cerius 2’ Acelrys software)
11
O N DEHYDROINDIGO N O
Oxidation
H O N INDIGO
N
O H
Oxidation Reduction
H O N LEUCO-INDIGO N
O H
Figure 6. Schematic of irreversible oxidation of Indigo to Dehydroindigo and reversible reduction to Leucoindigo.
12
1.2.3 Thioindigo
Thioindigo is a well known derivative of Indigo, reported by Friedlander in 1906,
in which N-H is replaced by sulfur as shown in figure 7(a). 54 Thioindigo is an intense red
colored powder at room temperature and forms dark red colored crystals by sublimation
at 290 oC. The sublimation temperature is lower than that of Indigo because Thioindigo
does not contain the same hydrogen bonding as indigo. As depicted in Figure 7b, it
crystallizes with monoclinic unit cell and space group symmetry P2 1/n. The lattice
parameters are: a = 3.981 Å, b = 20.65 Å, c = 7.930 Å. 55
In its solid state, Thioindigo is found to be the most stable in its trans-
configuration. Electrostatic charges on the slightly negative and positive charges of
oxygen and sulfur atoms maintains the Thioindigo in trans- configuration, and prevents isomerization. 56 While in solution, these electrostatic forces are lost due to the presence of the solvent or substitution in the Thioindigo molecule. The trans- form changes into cis form as presented in Figure 8. For example, Thioindigo, in benzene, is observed to be change from red purple at λmax ~545 nm to a cis orange yellow form at λmax ~485 nm.57
13
(a)
(b)
Figure 7. ( a) The structure of the Thioindigo molecule, where: Yellow = Sulfur, Red =
Oxygen, Gray = Carbon and White = Hydrogen (b) Thioindigo crystal structure
[Modelled by ‘Cerius2’ Acelrys software]
14
O O O
CS C C
S S S
O (Trans) (Cis)
Figure 8 . Representation of trans- and cis- forms of Thioindigo
15
CHAPTER 2
MATERIAL SAMPLING AND CHARACTERIZATION TECHNIQUES
2.1 Material Sampling
All starting materials were used as received. No further purification was performed. Indigo and Thioindigo dyes were purchased from Shangai and TCl America
Inc., respectively. Palygorskite was purchased from Mintech Inc.
2.2 Sample Preparation
A series of samples with varying weight percentages (1%, 3%, 6%, 16%, 20%, and 25%) were prepared with Indigo and Thioindigo. The mixtures were then heated in an oven at 170 oC for 9 hours. The samples obtained were labeled as MB(1%), MB(3%),
MB(6%), MB(16%), MB(20%), MB(25% ) (MB-Maya Blue), MP(1%), MP(3%), MP(6%),
MP(16%), MP(20%), MP(25% ) (MP-Maya Purple) and RB 6% (RB-Royal Blue) (RB is 3
wt % of Indigo and 3 wt % of Thioindigo). Spectroscopic studies were performed on
heated and unheated samples. Unheated samples were labeled %MBBH (before
heating), %MPBH and %RBBH and heated samples were labeled as %MBAH,
%MPAH, and %RBAH.
2.3 Characterization Techniques
In this section, experimental details of spectroscopic techniques are described.
2.3.1 Ultraviolet-Visible Spectroscopy (UV-Vis)
UV-Vis spectra were recorded using Schimadzu 3101 spectrophotometer, with
BaSO 4 as a standard. Reflectance values were acquired in the range of 200 to 800nm,
which were then converted to absorption using the Kubelka–Munk Method. UV Probe
16
software was used for data acquisition, refinement and analysis.
2.3.2 Fourier Transformed Infrared Spectroscopy (FTIR)
Infrared transmission measurements were carried out between 400 and 4000 cm -1 with a Bruker IFS 66v spectrometer, equipped with a DTGS detector and a KBr beam splitter. The samples for the IR studies were prepared in the form of pellets by
grinding approximately 1 mg of sample with oven dried and desiccated KBr. Again, an
accumulation of 32 scans was performed for each spectrum. The data were normalized
at each frequency, under vacuum throughout spectrum.
2.3.3 Fourier Transformed Raman Spectroscopy (FT Raman)
The FT Raman measurements were performed in the backscattering geometry
with a Bruker FRA-106 operating at 1064 nm with a continuous wave diode pumped
with a Nd:YAG laser light. The small cavity of cup holder was filled with the powder
sample and then placed at the focus of the excitation laser beam. In all cases, the laser
power was 77 mW and the spectral resolution was 4 cm -1. Each spectrum was produced by an accumulation of 256 scans. Also, a fluorescence background subtraction was performed for all the spectra.
2.3.4 Wide Range X-ray Fluorescence Spectrometry (WD-XRF)
WD-XRF analysis was performed by an external company (SGS Mineral
Analytical) with an Axios Panalytical spectrophotometer . For each sample preparation,
0.2g to 0.5g of clay powder was diffused with 0.7 g of lithium tetraborate/lithium metaborate (50/50). This was done to form a homogeneous glass disk. The loss on ignition was determined separately by gravimetric method and used in all calculations.
17
2.4.5 X-Ray Powder Diffraction (XRD)
X-ray diffraction patterns were obtained with a Scintag XDS 2000 seal tube
diffractometer, using nickel filtered CuK α radiation source ( λ = 1.5418 Å) and quartz as the calibration standard. The parameters used for measurement involved diffraction angle between 5-25 degrees, a step size of 0.02 degree per second, and a 1.5 sec time scan. The analysis was performed by comparing the experimental data with reported data from the Inorganic Crystallographic Database.
2.4.6 Differential scanning calorimetry (DSC)
DSC scans were performed using a SEIKO EXSTAR 6000DSC unit at a heating rate of 10 oC min -1 and a cooling rate of 30 oC min -1. The scans were carried out between the temperature ranges of 50-400 oC
18
CHAPTER 3
3.1 Color Change
Color changes in all heated and unheated samples of Maya pigments
combinations (Indigo/Palygorskite and Thioindigo/Palygorskite) is detected. An
interesting aspect to mention is that one can visually see the color change when the
mixture is grinded and heated in oven. To monitor these color changes all samples of
Maya Blue and Maya Purple were freshly prepared by blending the dye with the clay in
different wt (%) at 170 °C for 9 hours. The Indigo molecules produce a brilliant blue colored pigment when reacted with clay. The various shades of Maya Blue and Maya
Purple were observed as a function of altering the concentration of Indigo and
Thioindigo. These are presented in Figures 9 and 10 respectively. The vertical row
(Figure 9a) represents the color change when Indigo reacted with the clay at 170 oC for
9h while the horizontal rows represent the color change as a function of concentration
(Figure 9b). Thioindigo a brilliant red colored dye produced a purple pigment when reacted with clay at 170 oC (Figure 10a) and the color changes observed as a function of concentration are shown in figure 10b.
The change in color for all the Maya Blue and Maya Purple complexes may possibly be due to alteration in the electron density of the conjugated organic dye molecules after interaction with clay. This indicates strong interactions between the dye and the clay. To understand this aspect, detailed UV-Vis, FTIR, FT Raman spectroscopic and X-ray diffraction studies were performed (vide infra ).
19
MBAH (1%-25%) 170 o C/9h 25 0C Palygorskite Indigo
MBBH
170 o C/9h
1% 3% 6% 16% 20% 25%
MBBH (1%-25%)
MBAH
(a) (b)
Figure 9. (a) The color change observed when Indigo is heated with Palygorskite at
170 oC for 9h. (b) Variations in color upon altering the concentration of Indigo (BH =
Before Heating, AH = After Heating) [Image taken by Swati Kumar].
20
MPAH (1%-25%) 170 o C/9h
25 0C Palygorskite Thioindigo
MPBH
170 0 C/9h
1% 3% 6% 16% 20% 25%
MPBH (1% -25%)
MPAH
(a) (b)
Figure 10. (a) The color change observed when Thioindigo is heated with palygorskite at 170 oC for 9h. (b) Variations in color upon altering the concentration of Thioindigo (BH
= Before Heating, AH = After Heating) [Image taken by Swati Kumar].
21
3.2 Ultra Violet-Visible (UV-Vis) Spectroscopy
The UV-Vis spectroscopy, as compared to the other characterization techniques,
uniquely exploits the property of the pigments by characterizing the electronic
transitions present in the dye-clay complex and the shifts in the absorbance in the
visible range. It reflects direct evidence for the electronic structural change of dye
molecules when incorporated with clay. The electronic structural change of the dye
molecules can be easily predicted by comparing the λmax values of π→π* and n → π*
transitions of pure dye and the dye associated with clay.
3.2.1 Indigo /Palygorskite Complex: Maya Blue
The UV-Vis absorption spectra of pure Indigo powder, Maya Blue before and after heating are presented in Figure 11; while the electronic transitions and their corresponding wavelength are listed in Table 1.
A bathochromic shift (shift to a longer wavelength) from 584 nm (pure Indigo) to
636 nm (MB-6%) was observed in the UV-Vis spectrum of the sample prepared at room temperature by crushing the dye with the clay, and a further shift to 656 nm was detected when heated at 170 °C for 9h. These bathochromic shifts occurred due to the change in the electron density associated with chromophoric groups (C=O, N-H and
C=C) of Indigo molecules when adsorbed on the Palygorskite surface and/or in the channels. 58 It is well established in the literature that the monomer of indigo can function as a monochelate ligand that shows a red shift upon complexation.59 Heating of the mixture allows loosely bound and Zeolitic water molecule to evaporate. This facilitates surface activity which might involve the coordination of dye molecules via C=O group
22
with the metal ions present in the tetrahedral sheets, Si-O-Mn+ (M n+ = Al 3+ , Fe 3+ ).
Indigo, 584 nm MBBH, 636 nm MBAH, 656 nm
a-Indigo 1.0 b-MBBH(6%) c-MBAH(6%)
0.8
0.6
0.4 Absorbance Normalized 0.2 a b c 0.0 400 500 600 700 800 Wavelength (nm)
Figure 11. UV-Vis spectra of Indigo, Maya Blue before (MBBH) and after heating
(MBAH) 170 o C/9h.
23
Leona et al .60 have described that the palygorskite lattice can be compared to a polar solvent due to presence of coordinated water and structural water. This property of the clay allows Indigo to form intermolecular H-bonds. It is likely that intramolecular
H-bonds are favored in non-polar solvents with a subsequent aggregation of indigo molecules, whereas intermolecular H-bonds with indigo are more probable in polar solvent.
Jacquemin et al .62 and others 61 performed TD-DFT (Time Dependent-Density
Functional Theory) studies on Indigo and Thioindigo molecules and have described that the HOMO of Indigo are located at the N and C=C, while the LUMO is located on C-C single bond and oxygen atom of the carbonyl group. Any perturbation in the nature of these bonds changes the energy gap between HOMO and LUMO and consequently a variation in the wavelength. It is noteworthy to mention that HOMO and LUMO are associated with π and π* orbitals. The observation of large red-shift in the present case may be correlated in the light of TD-DFT calculations reported by Jacquemin et al .62 and others. 61 Any involvement of N-H and C=C groups in bonding with the clay will results in the destabilization of HOMO and increase the energy and subsequently decrease
HOMO-LUMO gap which give rise a bathochromic shift.
The possibility of oxidation of dehydroindigo from indigo at the clay surface can not be ruled out. The oxidation of indigo molecules involves N-H and C=C groups, and in that process N-H groups looses its H + and C=C transforms into C-C bond. This process will be highly destabilizing for HOMO orbitals of indigo molecule and that will give rise to a large bathochromic shift of π to π* transition due to lowering the HOMO-
24
LUMO gap. The formation of dehydroindigo during the preparation of Maya Blue pigment is already reported by Polette et al. and group 21, 23, 63 and also by others. 20
Dehydroindigo is electronically different molecule than the indigo and more flexible to fit into the channels of clay. So, it is possible that certain percentage of dehydroindigo entered into the channels and binds within channels.23b
The UV-Vis spectra of the Maya Blue samples, with varying concentrations (1%,
3%, 6%, 16%, 20% and 25%) of Indigo used for the pigment are presented in Figure 12 and corresponding wavelengths for each concentration are listed in Table 1. A closer look at the spectra of MB samples with increased concentrations from 1% (presented by the solid red line) to 25% (presented by the solid magenta line) showed that the extent of red shift decreases with concentration and the absorption peaks moves towards absorption region of pure indigo. After 6% of the dye concentration the shift becomes stronger (25 nm) and is visible in UV-Vis spectra. It is assumed that at lower concentration the channel activity predominates while upon increasing the concentration surface binding is preferred and any excess of the dye accumulates on the edges and outer surface of clay. In order to further aid the identification of possible sites and structural changes of the dye and the clay all the samples were analyzed by using FTIR,
FT-Raman spectroscopy and X-ray diffraction studies.
25
6%MBAH 3%MBAH 16%MBAH, 20%MBAH, 1%MBAH 25%MBAH, 659 nm 656 nm 638 nm 633 nm 660 nm, 631 nm
a-MB(1%) 1.0 b-MB(3%) c-MB(6%) d-MB(16%) e-MB(20%) 0.8 f-MB(25%)
0.6
0.4 AbsorbanceNormalized 0.2 f e d c b a 0.0 400 500 600 700 800 Wavelength (nm)
Figure 12 . UV-Vis spectra of various concentrations of Indigo (1-25%) in Maya Blue
o 170 C/9h.
26
Table 1. Absorption intensities of Indigo and Maya Blue heated at 170 oC/9h.
Sample Transition Wavelength
(nm)
Indigo (powder) π → π* 584
n → π* 697
MB (6%-unheated) π → π* 636
MBAH (1%) π → π* 660
MBAH (3%) π → π* 659
MBAH (6%) π → π* 656
π → π MBAH (16%) * 638
MBAH (20%) π → π* 633
MBAH (25%) π →π* 631
27
3.2.2 Thioindigo / Palygorskite Complex: Maya Purple
The UV-Vis spectra of Thioindigo and Thioindigo-Palygorskite complexes are depicted in Figure 13 and the corresponding wavelengths are listed in Table 2. The maximum absorption band ( λmax ) of Thioindigo reported at 507 nm corresponds to the lowest energy π-π* transition, from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). A bathochromic shift was observed
(as found in Maya Blue) when Thioindigo reacted with the clay at room temperature
(λmax = 546 nm) and a bathochromic shift of 44 nm was observed when heated ( λmax =
590 nm) at 170 oC/9h. The UV-Vis spectrum of pure Thioindigo exhibits a strong π __ > π* absorption at 507 nm and a weak n __ > π* absorption at 557 nm. The color of Thioindigo
64 is mainly due to the donor (Sulfur) and acceptor (carbonyl) chromogen.
Jacquemin et al.62 have also done TD-DFT studies on Thioindigo and described that the HOMO of Thioindigo are located on S and C=C, while LUMO are located on C-
C single bonds and oxygen atom of the Thioindigo. With respect to these notions, we assume that due to changes in the energies of HOMO and LUMO, a bathochromic shift is observed in Maya Purple samples, similar to that of Maya Blue described in previous section. Possibly, metal-oxygen bonding involving C=O group of Thioindigo and Al 3+ present in the Si-O tetrahedral sheet results such bathochromic shift.
Figure 14 shows the UV-Vis spectra of Maya Purple samples with varying concentration of Thioindigo (1%, 3%, 6%, 16%, 20%, and 25%). All the transitions corresponding to Maya Purple samples are presented in Table 2. Upon increasing the concentration of dye, the extent of red-shifts decreases from 594 -559 nm. This
28
behavior is exactly similar to Maya Blue and can be accounted to the presence of excess of dye on surface.
Thioindigo, 507 nm MPBH, 546 nm MPAH, 590 nm
1.0 507 546 590 a-Thioindigo b-MPBH(6%) c-MPAH(6%) 0.8
0.6
0.4 Absorbance Normalized 0.2 a b c 0.0 400 500 600 700 800 Wavelength (nm)
Figure 13 . UV-Vis spectra of pure Thioindigo, Maya Purple before and after heating at
170ºC/9h.
29
1% MPAH, 3% MPAH, 20% MPAH, 25% MPAH, 594 nm 6% MPAH, 16% MPAH, 592 nm 561 nm 559 nm 590 nm 567 nm
a-MP(1%) 1.0 b-MP(3%) c-MP(6%) d-MP(16%) 0.8 e-MP(20%) f-MP(25%)
0.6
0.4 Absorbance Normalized 0.2 f ed cb a 0.0 400 500 600 700 800 Wavelength (nm)
Figure 14 . UV-Vis spectra of different concentrations of Thioindigo (1-25%) in Maya
Purple heated at 170 °C/9h
30
Table 2. Absorption intensities of Indigo and Maya Purple heated at 170 oC/9h.
Sample Transition Wavelength
(nm)
Thioindigo π → π* 507
n → π* 557
6% MP (unheated) π → π* 546
MPAH (1%) π→ π* 594
MPAH (3%) π → π* 592
MPAH (6%) π → π* 590
MPAH (16%) π → π* 567
MPAH (20%) π → π* 561
MPAH (25%) π → π* 559
31
3. 3. FTIR SPECTROSCOPY
In this section, Infrared spectral data of Palygorskite, Indigo, Thioindigo, Maya
Blue, and Maya Purple are described.
3.3.1 Palygorskite :
FTIR spectroscopy has been employed to study the nature of binding sites as well as the functional groups present in the clay. The position of vibrational modes as
65 depicted in Figures 15 (a, b, c) are in accordance with the work of M. Suarez et al . A list of corresponding bands observed in our study (based on peak fit) is presented in
Table 3. The O-H stretching vibrations associated with water, M-OH and coordinated water were detected in the higher wave number region between 3200 to 3700 cm -1.
The band at 3616 cm -1 is assigned to stretching mode of hydroxyl group bonded to the different cations in the lattice. Suarez et al. 65 and others 36, 41, 66-68 have assigned this band as the stretching mode due to the 2(Al-OH) moiety present in the inner octahedral sites. The shoulder band at 3589 cm -1 belongs to the structural water. The weak band at 3411 cm -1 and shoulder band at 3270 cm -1 corresponds to zeolitic water while the weak band at 3550 cm -1 is assigned to coordinated water. The bands at 1656,
-1 1621 and 1630 cm correspond to the bending mode of adsorbed and zeolitic water.
Characteristic absorptions associated with SiO 4 tetrahedral sites present in the tetrahedral sheets appear at 1192, 1083, 1033 and 981 cm -1. The band at 1192 cm -1 is
a characteristic of Si–O-Si units of tetrahedral sheets while the bands at 1083 , 981, and
1033 cm -1 belong to the M-O stretching vibration bands or the Si-O bonds. The appearance of additional splitting in this region at 981 and 1033 cm -1 is due to lowering
32
of symmetry in the lattice in relation to the point group of the tetrahedral silicate species. 37 The deformation band of 2Al-OH appeared at 912 cm -1 also encloses the
65 symmetric stretching mode of the terminal O-SiO 3 group. In addition, a small shoulder band at 863 cm -1 is assigned to the bending vibration mode of Al-OH-Fe bond. The assignment of bands in the lower wavenumber region is difficult due to complexity of spectrum. Moreover, in this region the spectrum strongly depends on the nature of the trivalent cation and not on the octahedral or tetrahedral sites. Four bands observed at
480, 578, 649, 778 cm -1 are related to the bonds corresponding to tetrahedral sheet.
-1 The band found at 512 cm is probably due to Si-O-Al moiety. This peak is directly related to the aluminum content associated in tetrahedral sheet. Structural Si-OH groups, generally present in the Palygorskite near 3710 cm -1 were not detected in our study. 32 The position of stretching as well as bending vibration of 2Al-OH group at the
3616 cm -1 and at 912 cm -1 respectively, confirms the dioctahedral character of
Palygorskite. 65
33
W ater bending region 1 .0 Palygorskite
0 .8
0 .6 OH stretching region
0 .4 Absorbance
0 .2
0 .0
4000 3500 3000 2500 2000 1500 1000 500 wavenum ber cm -1
(a)
P alygorskite 3589 3550 3411 3616 3270 Absorbance
3800 3700 3600 3500 3400 3300 3200 3100 3000 W avelength (cm - 1 )
(b)
34
1.0 Palygorskite 981 1036
0.5 480 1083 512 Absorbance 693 578 649 1697 1621 912 1192 1656 778 863 1377 0.0 1800 1600 1400 1200 1000 800 600 400 W avenumber cm -1
(c)
Figure 15. FTIR spectra of Palygorskite clay (a) Complete range spectrum (4000-400 cm -1), (b) OH stretching region, (c) Water bending vibration region
35
Table 3 . Frequencies and IR modes of Palygorskite (M. Suarez et al. 65 )
Mode Frequency
Zeolitic water 3270(w), 3411(m) 1650(s)
Coordinated water 3550(s)
Water (structural water) 3589 (sh),
Si-O-Si 1192(s)
Si-O-Al 512(s)
2Al-OH 3616(m), 912(s)
3+ Al-Fe - OH 3589,863
Al (th) 778 (sh)
Si (th) 1083(sh),1036(vs),
981(vs)
Fe, Al, Mg-OH 693 (sh)
Where, th = tetrahedral, vs = very strong, s = strong, m = medium, sh = shoulder, w = weak.
36
3.3.2 Indigo
Indigo contains two functional groups: Amine (NH) and Carbonyl (C=O), that are involved in interactions with palygorskite. The observed IR spectrum for indigo between range (4000-400 cm -1) is presented in Figure 15. Tatsch et al.69 and others, 4,48,70,71 have suggested assignments of IR vibration mode and are listed in Table 4. The stretching bands for the carbonyl and the amine groups are reported at lower wavenumbers than should be expected. These results indicate that there is N-H…. O intramolecular hydrogen bonding that take place within the structure of indigo.
Ind igo 1.0 1626
0.8 1462 1072 1612 0.6 1177 1482
Absorbance 0.4 1199 1317 1392 1585 3268
0.2 569 748 510 879 1011 638 0.0 4000 3500 3000 2500 2000 1500 1000 500 W avenum ber (cm -1 )
Figure 16 . IR spectrum of Indigo (4000-400 cm -1)
37
Table 4 . Frequencies and IR modes of Indigo [Ref. Tatsch. et al.69 ]
Mode Frequency
νN-H 3268 (s)
δN-H 1392 (s), 638 (w)
νC=O 1626 (vs),
νC-C 1612 (s) 1585 (s),1482 (s),1462 (s),
1317 (s), 879 (w)
δC-H 1299 (s),1177 (s),1127 (m),1011 (vw)
νC-N 1199 (m), 1072 (m)
δ5 ring 765 (w), 748 (w), 565 (w)
δC=C-C=O 510 (vw)
Where (vs) = very strong, s = strong, m = medium, w = weak
3.3.3 Thioindigo
Thioindigo contains carbonyl (C=O) group as a functional group that can participate participates in interaction with Palygorskite. The assignments of vibrational modes are given in Table 5 .The broad peak at 3430 cm -1 is due to water in the KBr.
38
Thioindigo 1 .0 1655 1292
0 .8 739 1585 1077
0 .6 896 1451 1224
Absorbance 0 .4 3430 0 .2 687 586
0 .0 4000 3500 3000 2500 2000 1500 1000 500 W avenumber (cm-1)
Figure 17 . IR spectra of Thioindigo (4000-400cm -1)
Table 5. Frequencies and IR modes of Thioindigo
Mode Frequency
C=O 1655 (s)
C=C 1587 (s)
C-C 1452 (s), 1288 (s)
C-S 739(s)
C-H 1077 (s),1120 (w)
1144 (vs), 1052 (vs)
Vs = very strong, s = strong, m = medium, sh = shoulder, w = weak.
39
3.3.4 FTIR studies of Maya Blue
For the comparative IR studies of the starting material with synthetic Maya Blue, the chemical modes of interest are water modes, hydroxyl groups, carbonyl and N-H groups. The IR spectra for Palygorskite, Indigo, and Maya Blue before heating and after heating at 170 °C/9h are presented in Figure 18(a). The vibrational modes for Maya Blue are assigned as they reported in literature, as well as being compared to the starting material. Some Indigo bands are visible in the Maya Blue spectrum as shown in Figure
18(b) in the region between 1460 and 500 cm -1, where interference due to the clay is minimal. The assignments of other parts of spectra are presented in order as they were studied.
1 .2 a-In dig o b-Palygorskite C =O region 1 .0 c-M B(6% )BH d-MB(6% )AH
0 .8
0 .6
Absorbance 0 .4 O H strtching region
c N -H region b 0 .2 d
a 0 .0 4000 3500 2000 1500 1000 500 W avenum ber (cm -1 )
(a)
40
981 a-Indigo 994 b-Palygorskite c-MP(6%)BH 1626 d-MP(6%)AH d1036 c 1072 1462
1612 b 1131 1482 1177 Absorbance 1199 705 1585 1317 1299 1392 a d 563 638 753
c 715 508 b 879 791 597
1000 W avenumber( cm -1 )
(b)
Figure 18. IR spectra for Indigo, Palygorskite, Maya Blue before heating (BH) and after heating (AH) at 170 °C/9h.
3.3.4.1 Vibrational Modes Associated with Palygorskite
(a) Water modes: The IR spectra presented in Figure 19-21 revealed the effect of the heating process on water, hydroxyl groups, silicate tetrahedral, and metal hydroxide modes. The corresponding frequencies are given in Table 6. The zeolitic water modes between 3270 cm -1 and 3400 cm -1 become extremely weak and almost disappear as the clay is heated with Indigo at 170 °C/9h. This is evident from these results that zeolitic water is physically adsorbed on the clay. The coordinated water band appearing at 3550
41
cm -1 becomes weak and shifts to 3556 cm -1 after heating at 170 °C. Lori Polette 63 reported that the shift in coordinated water’s νO-H band is due to rupture of hydrogen bonds between zeolitic water and coordinated water. The loss of water exposes the clay surface and changes the chemical environment which allows the interaction of dye and clay. The sharp band at 3616 cm -1, was assignment to νO-H of Al-OH moiety of
Palygorskite. For Maya Blue heated (170 ° C/9h) this band shifts from 3616 cm -1 to 3620 cm -1 indicating that hydrogen bonds are broken in the tetrahedral layer as coordinated and zeolitic water are eliminated, exposing clay surface. The strong bands at 1695,
-1 1650, and 1621 cm , are due to the bending vibration of water. These bands disappeared in Maya Blue sample (Figure 20).
b-Palygorskite c-M B(6% )BH d-MB(6%)AH 3550 b c 3411 3589
3616 3 5 5 6 d 3620 3270 Absorbance
3600 3400 W avenum ber (cm -1 )
Figure 19 . OH stretching modes of Palygorskite
42
(b) Si-O-Si Modes : The IR spectra for the vibrations of symmetric and antisymmetric modes of silicate tetrahedral are shown in Figure 21, and Table 6 summarizes the modes and frequency shift for Maya Blue as well as Palygorskite. The most significant change in this region is found for the Maya Blue sample when heated at 170 °C/9h. The band at 981 cm -1 shifts to 994 cm -1 while the 1036 cm -1 band remains unaffected. The change in frequency is the indication of the bond angle change of Si-O-Si or Si-O-M (Al,
Fe) linkage. 41,48 . This fact is supported by Weiss et al., 72 who used neutron scattering and theorized that, when organic molecules interact with the clay surface there is reorientation of the OH - groups which initiates the elastic deformation of the interlayer spacing, thus allow some penetration of the guest molecule. IR evidence presented here also indicates that the OH-groups on the palygorskite surface reorient. The reorientation directly affects the Si-O-Si bond angle that occurs after adsorption of indigo on external surface. In correlation with the bond angle change, the band at 512 cm -1 shifts to 518 cm -1 for the Maya Blue sample heated at 170 °C/9h (Figure 22). The tetrahedral ribbons of palygorskite consist of Si-O-Si and Si-OX (H, Al and Fe) linkages which generate Lewis acidic sites. The substitution of Si 4+ by Al 3+ changes the surface properties of the clay.
43
b-P alygorskite c-M B(6% )BH d-MB(6% )AH 1 6 5 0 1695
b Absorbance
c d
1700 1680 1660 1640 W avenum ber (cm -1 )
Figure 20 . IR spectra showing bending modes of water. 994 981 a-In digo
1036 b-Palygorskite d c-M B(6% )BH c d-MB(6% )AH b Aborbance
a
1080 990 900 W avenum ber (cm -1 )
Figure 21 . IR spectra showing stretching modes of Si-O-Si and SiO3.
44
a-Indigo b-Palygorskite 518 c-MB(6% )BH d d-MB(6%)AH c b 512 Absorbance
a
540 510 W avenumber (cm-1)
Figure 22 . IR spectra showing bending modes of SiO3
45
Table 6 . IR frequencies for water modes in Maya Blue (6%) heated at 170 °C/9h.
Mode Palygorskite Indigo MBBH MBAH
Band (cm -1) (25 °C) 170 °C/9h
Zeolitic water 3270,1656, - Weak -
1695, 3411
Coordinated 3550 - 3550 3556
water
Structural 3589 - 3589 3589
water
2Al–OH 3616 - 3616 3620
Si-O-Si asym 981 982 994
SiO 3 1036 - 1036 1036
Si-O-Al 512 512 518
3.3.4.2 Vibrational Modes Associated with Indigo
The IR spectra of Maya Blue samples were analyzed carefully to detect any changes in the vibrational modes associated with Indigo. The most obvious changes were detected in the carbonyl and N-H region.
(a) Carbonyl region : As shown in the Figure 23, the bands at 1626 and 1622 cm -1 are assigned to C=O stretch. The assignments of bands for all samples are listed in Table
7. It is clearly evident from the figure that the intensity of C=O band decreases after
46
Indigo is mixed with Palygorskite at room temperature. In case of Maya Blue samples, heated at 170 °C, the intensity of C=O decreases further and shifts to a lower frequency,
-1 -1 from 1626 cm to 1622 cm . The peak remains broad due to the presence of various carbonyl modes in this region, including, in phase and out of phase vibrations. The C=O force constant is weakened as it binds to a cation such as Al 3+ or Fe 3+ resulting in lowering of carbonyl frequency. Formation of metal carbonyl complexes to clay followed by color changes is well documented in literature. 70
a-Indigo 1626 b-Palygorskite c-MB(6% )BH d-MB(6%)AH Absorbance 1626 1622 d c a b
1650 1600 1550 W avenumber (cm -1 )
Figure 23 . IR spectrum for carbonyl region
47
(b) N-H Region : The IR spectrum for the N-H region is shown in Figures 24 and 25, and the corresponding frequencies are listed in Table 7. The sharp band at 3268 cm -1 corresponds to the stretching mode of N-H, while the weak band at 638 cm -1 is assigned as the deformation band of N-H group. The intensity of νN-H and δN-H is lost after mixing indigo with palygorskite at room temperature. For Maya Blue sample the intensity of bands decreases and become broader when heated at 170 °C.
a-Indigo b-Palygorskite c-MB(6% )BH d-MB(6%)AH 3268 Absorbance
b a c d
0.0 3400 3200 W avenumber (cm -1 )
Figure 24 . IR spectrum for stretching N-H modes
48
a-Indigo b-Palygorskite c-M B(6% )BH d-MB(6%)AH 638 Absorbance c d b a
690 660 630 600 W avenum ber (cm -1 )
Figure 25 . IR spectrum for bending N-H modes
Table 7 . IR frequencies for carbonyl modes and N-H modes
Mode Palygorskite Indigo MBBH MBAH
Band (cm -1) 25 °C 170 °C/9h
C=O - 1626(s) 1626(w) 1622(w)
νN-H - 3268(w) - -
δN-H 638(w) - -
(vw = very weak, w = weak)
49
3.3.4.3 Concentration Change in Maya Blue
IR spectra shown in Figure 26 were collected for all the samples (1-25%), prepared by altering the concentration of Indigo to address the question: “why upon increasing the concentration of dye, π→π* transition shifts to higher energy ?” The corresponding frequencies are presented in Table 9. 1626 1482 3411 3589 1180 561 3268 1736 3616 1317 1392 638 Indigo
M B 25%
Absorbance MB (20%)
MB (16%)
MB (6%)
MB (3%)
MB (1%)
4000 3800 3600 3400 3200 2000 1000 W avenum ber(cm -1 )
Figure 26 . IR spectra for different concentrations (wt%) of Indigo in Maya Blue at
170 °C/9h.
50
(a) C=O region : As shown in the Figure 27, the bands corresponding to C=O group
-1 appeared at 1626 cm . The intensity of band is decreased when clay is mixed with 1%
Indigo and a closer look at the IR data revealed that at the higher concentration of dye
(beyond 6%) this band became intense and sharp. A very weak intensity band at 1736
-1 cm at higher concentrations is assigned to νC=O of dehydroindigo. Similar results were reported by Manciu et al .23b and Klessinger and Luettke.49
1626
1612
1585
Indigo MB(25%) MB (20%) Absorbance MB(16%) MB(6%) MB(3%) MB(1%)
1800 1700 1600 1500 W avenumber (cm -1 )
Figure 27 . IR spectra for carbonyl region of different concentrations of Maya Blue.
51
(b) N-H Region : The IR spectra for the N-H region of pure Indigo and Maya Blue with various concentrations of Indigo (1% to 25%) are shown in Figure 28 and their corresponding frequencies are presented in Table 8. The N-H band at 3268 cm -1 completely disappears in lower concentration (1 - 6%), but appears as a bump in higher concentrations (6% to 25%). These results again suggest the possibility of involvement of N-H groups binding with clay. A small bump observed in the N-H region possibly may arise due to excess of dye on the surface. 3268
Indigo MB(25%)
MB(20%)
Absorbance MB(16%)
MB(6%) MB(3%) MB(1%)
3400 3300 3200 3100 W avenumber (cm -1 )
Figure 28 . IR spectrum for N-H region on altering concentrations of Indigo in Maya
Blue at 170 °C/9h.
52
Table 8 . IR frequencies for carbonyl modes and N-H modes in Maya Blue (1-25%)
heated at 170 °C/9h.
Mode Indigo MBAH MBAH MBAH MBAH MBAH MBAH
(1%) (3%) (6%) (16%) (20%) (25%)
C=O 1626 1622 1622 1622 1624(m) 1626 (s) 1626(s)
(br) (br) (br) (br) 1736 1736 1736
(vw) (vw) (vw)
νN-H 3268(s) - - - 3268(vw) 3268(vw) 3268(vw)
δN-H 638(w) ------
(vw = very weak, br = broad, m = medium, s = strong)
3.3.5 FTIR studies of Maya Purple
The IR spectra for Thioindigo, Palygorskite and Maya Purple (MP) are shown in
Figure 29 (a) (4000-400 cm -1) and the assignment of different bands is listed in Table 9.
The presence of vibrational modes at 1451, 1292, 896, 739 and 697 cm -1 (Figure 29b)
confirms the existence of Thioindigo in the Maya Purple complex. These assignments
were compared with the work on Maya Purple done by Manciu et al .23a
53
a-Thioindigo C=O region 1 .0 b-Palygorskite c-MP(6% )BH d-M P(6% )AH 0 .8
0 .6
Absorbance 0 .4 OH stretching region b d 0 .2 c
a 0 .0 4000 3500 2000 1500 1000 500 W avenum ber (cm -1 )
1 .2 a-Thioindigo b-Palygorskite 1656 c-M P(6% )BH
1292 d-MP(6% )AH
0 .8 1585 1077 739 1451 896 1224 1052 Absorbance 0 .4 687 531 b 586 d 788 a c 0 .0 1500 1000 500 W avenumber (cm-1)
Figure 29. IR spectrum for Thioindigo, Palygorskite, Maya Purple (6%) before and after heating at 170 °C/9h.
54
3.3.5.1 Vibrational Modes Associated with Palygorskite
(a) Water modes : Similar to Maya Blue, IR spectra presented in Figure 30 revealed the effect of the heating process on water, hydroxyl, silicate tetrahedral and metal hydroxide modes. The corresponding frequencies are given in Table 9. The zeolitic water (3400-
3200 cm -1) bands become extremely weak as the clay is heated with Thioindigo at
170 °C. In addition, the band for coordinated water at 3550 cm -1 becomes weak and shifts to 3556 cm -1 after heating at 170 °C/9h. The loss of water exposes the clay surface and changes the chemical environment which allows the interaction of the dye and clay. A similar shift was reported for 2Al-OH from 3616 cm -1 to 3620 cm -1. The
-1 strong bands at 1695, 1650, and 1621 cm , are due to bending vibrations of water which also disappeared in Maya Purple when heated at 170 °C.
0 .4
b-Palygorskite c-M P(6% )BH 3550 d-M P(6% )AH 3589 3420 3411 3616
b Absorbance d c
0 .0 3600 3400 W avenum ber (cm -1 )
Figure 30 . IR spectra showing OH region for Maya Purple before and after heating.
55
(b) Si-O-Si Modes : The IR spectra of the vibrations for symmetric and antisymmetric modes of silicate tetrahedral are shown in Figure 31, and Table 9 summarizes the modes and frequency shift for Maya Purple as well as palygorskite. Similar to Maya
Blue, a shift from 981 cm -1 to 994 cm -1 is the most significant change in this region while the 1036 cm -1 band is hardly affected.
1.2 a-Thioindigo b-Palygorskite 981 c-MP (6% ) BH 994 d-MP (6% ) AH d c 0.8 b
Absorbance 0.4
a
0.0 1120 1050 980 W avenum ber (cm -1 )
Figure 31 . IR spectrum for Si-O-Si and SiO 3 region in Maya Purple at 170 °C/9h.
56
Table 9 . IR frequencies for water modes in Maya Purple (6%) before and after heating
at 170 °C/9h.
Mode Band (cm -1) Thioindigo MPBH MPAH
Palygorskite 25 °C 170 °C/9h
Zeolitic 3270,1656, - Weak -
water 1695, 3411
Coordinated 3550 - 3550 3556
water
Structural 3589 - 3589 3589
water
2Al–OH 3616 - 3616 3620
Si-O-Si asym 981 982 994
SiO 3 1036 - 1036 1036
Si-O-Al 512 512 518
57
3.3.5.2 Vibrational Modes Associated with Thioindigo
(a) C=O Region : The band corresponding to C=O stretching in pure Thioindigo (Figure
-1 -1 32) is assigned at 1655 cm . This band decreases in intensity and shifts to 1627 cm after heating a mixture of the clay and Thioindigo (6%). This suggests that C=O group may be a potential interaction site with clay in Maya Purple. At this percentage the involvement of C=O group in coordination may at its highest points since beyond this percentage of dye a splitting of C=O band is observed.
Thioindigo 1 6 5 5 Palygorskite M P (6% ) BH M P (6% ) AH 0 .8 1585 1627
0 .4 Absorbance 1655 1695
0 .0 1800 1750 1700 1650 1600 1550 1500 w avenum ber (cm -1 )
Figure 32 . IR spectra for C=O region of Maya Purple at 170 oC/9h.
58
Table 10 . IR frequencies for carbonyl modes in Maya Purple.
Mode Palygorskite Thioindigo MPBH 25 °C MPAH
Band (cm -1) 170 °C/9h
C=O - 1655 (s) 1651 (br) 1627 (br)
3.3.5.3 Concentration Changes in Maya Purple
IR spectra were collected on a series of samples prepared by varying the concentration of Thioindigo (from 1% to 25% of the total pigment) and the most obvious changes are presented in Figure 33. A closer look of the C=O stretching region showed that the bands at 1655 and 1627 cm -1 becomes stronger at higher concentration. The latter band corresponds to C=O group coordinated to M n+ while the band at 1655 cm -1 represents excess of Thioindigo. This suggests that after 6% of dye concentration there is excess of Thioindigo on the surface of clay.
59
1655 3550 1291 3420 1585 739 1451 3620 896 1224 3434 687 Thioindigo
MP(25% )
Absorbance MP(20% ) MP(16% ) MP(6% )
MP(3% ) MP(1% )
4000 3500 2000 1500 1000 500 W avenumber (cm-1)
(a) 1655 1585 1627
Thioindigo
MP(25% ) MP(20% ) Absorbance MP(16% ) MP(6% ) MP(3% ) MP(1% )
1800 1750 1700 1650 1600 1550 1500 1450 1400 W avenumber (cm-1)
(b)
Figure 33 . IR spectra for different concentration (wt%) of Thioindigo in Maya Purple at
170 °C/9h (a); Change in C=O region on altering concentration (wt%) of Thioindigo in
Maya Purple (b). 60
Table 11 . IR frequencies for carbonyl modes in Maya Purple (1-25%) heated at
170 °C/9h
Mode Thio- MPAH MPAH MPAH MPAH MPAH MPAH
Indigo (1%) (3%) (6%) (16%) (20%) (25%)
C=O 1655 1647 1641 1627 1627 1627 1627
(s) (br) (br) (br) (s) (s) (s)
1655 (s) 1655 1655
(s) (s)
61
3.4 Raman Study of Maya Blue
The Raman spectra of the Maya Blue samples for higher concentration MBAH
(20%) heated for 9 and 24 hours at 170 oC are presented in Figure 34. It showed the disappearance of band at 1701 cm -1 (C=O) (marked by red solid arrow) after 24 hrs and the appearance of two new bands at 425 (marked by black arrow) and 606 cm -1
(marked by green dashed arrow). A clearer view can be seen in Figure 34 (b) and (c).
The bands at 606 cm -1 and 425 cm -1 may correspond to the formation new Al-O and Al-
N bonds, wherein the source of oxygen is carbonyl group and the source of N may be indigo or dehydroindigo. Similar metal oxide peaks were reported by Polette et al .21 and
Manciu et al .23 in Maya Blue and Maya Purple (8% and 16 wt %) samples respectively.
1.0 c a-Indigo b-MBAH(20%)9h c-MBAH(20%)24h 0.8
0.6
Al-O 0.4 b
Al-N 0.2 C=O Raman Intensity Normalized a
0.0 500 1000 1500 2000 Wavenumber (cm -1 )
(a)
62
a-Indigo Al-O b-MBAH(20%)9h c-MBAH(20%)24h c
0.2
Al-N
b RamanIntensity Normalized a
0.0 450 540 630 Wavenumber (cm -1 )
(b)
a- indigo b-MB(20%)9h/170 oC o C=O c-MB(20%)24h/170 C
0.04
0.02 Raman Intensity
0.00 1680 1710 1740 Wavenumber (cm -1 )
(c)
Figure 34 . Raman spectra for Indigo, Maya Blue (20%) heated 9h, and 24h at 170 oC
(a) Finger print region, (b) Metal-oxide region, (c) Carbonyl region.
63
3.5 Wide Range X-Ray Fluorescence spectroscopy: (WD-XRF)
The elemental weight percentages reported for Palygorskite are provided in
Table 12. The chemical analysis indicates the large amount of Si and O followed by other metals such as Mg, Al and Fe. Based on chemical analyses, chemical formula of
Palygorskite was calculated as (Si 0.918 , Al 0.162 ) (Fe 0.00669, Mg 0.266, K0.017 )O 2.48 H2O1.056 .
64
Table 12 . WD-XRF chemical analysis.
%Element % Wt
%SiO 2 55.2
%Al 2O3 8.24
%Fe 2O3 4.27
%MgO 10.7
%CaO 0.29
%Na 2O 0.1
%K 2O 0.82
%TiO 2 0.69
%P 2O5 0.07
%MnO 0.12
%Cr 2O3 0.01
%V 2O5 0.02
LIO 19
Sum 99.5
65
3.6 X-ray Powder Diffraction.
The X-ray diffraction (XRD) pattern confirms the crystalline nature of palygorskite with minor amounts of the impurity of quartz as depicted in Figure 35. By using synchrotron radiation, Polette-Niewold et al .21 was the first, to observe the disruption of the crystal structure of the Indigo when interacting with clay palygorskite. Similar behavior was noticed in our study, no crystalline peaks were detected for Indigo and
Thioindigo at lower concentrations of Maya Blue and Maya Purple samples as shown in
Figure 38. The loss of crystallinity might have played a very important role in the binding of the dye with the clay. The XRD patterns obtained in the present study were not of very good quality but clearly showed the absence of the crystalline peak of the organic dye molecule in the lattice structure of MB and MP complexes at lower concentrations, while peaks start appearing as the concentration is increased.
Figure 35 . XRD pattern of Palygorskite from Mintech. comparison with two simulated polymorphs (Monoclinic, Orthorhombic unit cell) of Palygorskite, and Quartz impurity.
66
0 2 Quartz alpha 0 Thioindigo Palygorskite MPBH(6%) 80 MPAH(6%)
0 2 0
Intensity 40 0 1 1
0 10 20 2 Theta
Figure 36 . XRD pattern of 6% Thioindigo mixture (unheated) with palygorskite. comparison with Thioindigo, Palygorskite, and heated mixture at 170 oC/9h.
67
1 1 1 0 0 0 5 0 0 1 0 P a ly 2 4 0 0 M B 2 5 %
M B 2 0 % 3 0 0 M B 1 6 %
Intensity M B 6 % 2 0 0 M B 3 %
1 0 0 M B(1% )
In d ig o
0 5 10 15 20 25 2 T h e ta
Figure 37. XRD pattern for Maya Blue (1-25%) heated at 170 °C/9h.
0 2 0
1 0 0
0 0 P a lyg o rskite 1 2 8 0 1 1 M P 2 5 % M P 2 0 % 6 0 M P 1 6 % M P 6 %
Intensity M P 3 % 4 0 M P 1 %
2 0 T h io in d ig o
Q uartz aplha 0
1 0 2 0 2 T h e ta
Figure 38 . XRD pattern for Maya Purple (1-25%) heated 170 °C/9h.
68
3.7 Royal Blue
We have been interested in Maya Blue and Maya Purple in the light of bonding features between the dye and the clay, Royal Blue which is a pigment formed by the mixture of indigo and thioindigo and clay is also of interest and has only been investigated here. The objective of this reaction is to understand the preferential reactivity of Indigo or Thioindigo when heated with the clay. A mixture of Indigo and
Thioindigo (3% each) and Palygorskite forms Royal Blue pigment. Change in color was detected, from purple to royal blue when a mixture of Indigo (3%) and Thioindigo (3%) was reacted with Palygorskite at room temperature. A color change also ensured after heating under similar conditions as the above mentioned pigments (170 oC/9h).These color changes are presented in Figure 39.
UV-Vis and FTIR spectroscopy have been used to interpret the impact on one dye to another in binding with clay. Figure 40 shows the UV-Vis spectra of Royal Blue unheated and heated. It has been observed that a bathochromic shift of 34 nm occurred when the pigment was prepared at 170 o C for 9 h. Maya Blue sample prepared using
3% of Indigo showed absorption at 659 nm while Maya Purple prepared using 3% of
Thioindigo showed absorption at 592 nm. A comparison of the λmax values of pure dyes
[λmax (thioindigo) = 507 nm, λmax (indigo) = 584 nm] and λmax of Royal Blue (604 nm) showed larger red-shift for Thioindigo (97 nm) and smaller for indigo (20 nm). This may suggests the preferential activity of Thioindigo to be greater than indigo.
69
Indigo Thioindigo 25 oC
RBBH
170 oC
RBAH
Figure 39. The color change observed when a mixture of Thioindigo and Indigo is heated with Palygorskite at 170 o C for 9 hours (BH = Before Heating, AH = After
Heating)
70
RBBH (6%)(570 nm ) RBAH(6%)(604 nm )
570 604 a-RBBH(6%) 1.0 b-RBAH(6%)
0.8
0.6
0.4 AbsorbanceNormalized
0.2 a b 0.0 400 600 800 Wavelength (nm)
Figure 40 . UV-Vis spectra of Royal Blue before heating (RB BH) and after heating (RB
AH) at 170 oC/9h.
71
FTIR spectra on the Royal Blue sample revealed that the bands corresponding to
C=O groups both in Indigo (1625 cm -1) and Thioindigo (1655 cm -1) decreased in intensity (Figure 41). This behavior is identical to the Maya Blue and Maya Purple infrared spectra wherein the decrease in the C=O intensity is observed. It seems that the reaction of the mixture of Indigo and Thioindigo with clay was competitive and each took part in the reactions.
------6% royal blue heated 1.8 ------6% royal blue unheated 1.6 ------ndigo ------Thioindigo 1626.58 1.4
1.2
1.0 1655.04
0.8 1587.40
0.6 1451.90 1462.10
0.4 1612.87
0.2 Thioindigo 0.0 Absorbance -0.2 1482.75 -0.4
-0.6 R B AH
-0.8 1625.74 1585.10 1392.14 1655.96
-1.0 1694.82 1461.85
-1.2 1371.59 R B B H 1482.89 -1.4
-1.6 Indig o -1.8 1800 1700 1600 1500 1400 Wavenumbers (cm-1)
Figure 41. FTIR spectra of Thioindigo, Indigo and Royal Blue before and after heating at 170 oC.
72
3.8 Differential scanning calorimetry studies
Differential scanning calorimetric studies on Maya Blue, Maya Purple and Royal
Blue sample were conducted to gather more information about the binding energies and preferential reactivity of the dyes. ∆H (Enthalpy) were calculated for all the samples from the peak area. The thermograms of Maya Blue, Maya Purple are presented in
Figures 42 and 43 respectively and corresponding ∆H (enthalpy) values are listed in
Table 13(a) and 13(b). DSC profile of Royal Blue is compared with the DSC traces of
Maya Blue and Maya Purple as shown in Figure 44.
80000
70000 a b 60000 c 50000 d 40000 e f 30000 Heat Flow(uw) Heat 4 a-MB 1% 20000 b-MB 3% 3 c-MB(6%) d-MB(16%) 10000 e-MB(20%) f-MB(25%) 2 0 1 -10000 100 200 300 400
Temperature ( oC)
Figure 42. DSC profiles of mixture of Indigo and Palygorskite between temperature range of 25-400oC.
73
80000
70000 a
60000 b c 50000 d e 40000 f 30000 a-MP1%
Heat Flow Heat (uw) b-MP3% 20000 c-MP6% d-MP16% 10000 e-MP20% 2 f-MP25% 3 0 1
100 200 300 400 Temperature ( oC)
Figure 43 : DSC profiles of mixture of Thioindigo and Palygorskite between temperature range of 25-400 oC.
74
a-M P b-R B c-M B 10000
a 0 b Heat Flow (uw) FlowHeat c
2 -10000 1 100 200 300 400 Temperatute ( OC )
Figure 44 : DSC profiles of Royal Blue, Maya Blue and Maya Purple between temperature range 25-400oC
Table 13(a). ∆H (Enthalpy) values calculated for Maya Blue
Sample ∆Hb1 (mJ/mg) ∆Hb2 (mJ/mg)
1% MB 15.727 17.183
3% MB 19.764 20.274
6% MB 27.578 38.008
16% MB 33.477 59.637
20% MB 37.054 87.129
25% MB 39.348 94.950
75
Table 13(b). ∆H (Binding Energies) values calculated for Maya Purple.
Sample ∆Hb1 (mJ/mg) ∆Hb2 (mJ/mg)
1% MP 14.422 16.494
3% MP 19.170 19.388
6% MP 21.084 25.519
16% MP 25.504 37.827
20% MP 32.094 38.91
25% MP 38.699 41.533
Table 13(c) . ∆H (Enthalpy) values calculated for Royal Blue.
Sample ∆Hb1 (mJ/mg) ∆Hb2 (mJ/mg)
6% MP 21.084 25.519
6% MB 27.578 38.008
6% RB 23.038 27.447
Differential Scanning Calorimetry is used to investigate the hygroscopic, zeolitic, coordinated, and hydroxyl water within the prepared mixtures of Thioindigo, Indigo and
Palygorskite clay. Each of these different water types is lost at a different temperature range, as described by Polette-Niewold et al .21 Two endothermic peaks were noticed, attributed to the removal of water molecules that were adsorbed with distinct energies.
This was achieved by plotting the heat flow from the DSC thermograms (µw) versus the
76
temperature. This investigation may offer insight to estimating the activation energy of
the compounds during their reaction paths, thus leading to the construction of a two- component phase diagram, or a study in reaction path kinetics.
At the 1 st endothermic peak an increase in energy was observed and this endothermic reaction corresponds to the removal of physically adsorbed water from clay surface. At the 2 nd endothermic peak also, an increase in enthalpy was observed,
(Figures 42 and 43) along with a steeper rise in the higher concentrations of pigment to clay. It is assumed that at this point (2 nd endothermic reaction) Indigo or Thioindigo molecules replaces zeolitic water in the channels. A 3 rd endothermic peak was observed but only at the high concentrations of dye in pigment and could be arise due to removal of structural water while the 4 th endothermic peak (only in Maya Blue) may be assigned to the sublimation of excess of Indigo from the clay surface. Upon increasing the concentration of dyes, the extent of endothermic reactions becomes higher i.e. it absorbs more heat. In thermodynamic term, the reaction between the clay and the dye is more favorable at lower concentration. This may be explained that in the presence of excess of dye the removal of water from clay surface and from channels becomes difficult as the dye molecules covers the entire exposed surface and blocks the channels. The endothermic nature of the clay-dye interaction is already reported. 74, 20
Frost et al. 74 studied DSC of palygorskite and found endothermic peaks at 54.7 oC and
99.0 oC for removal of physically adsorbed water and zeolitic water respectively.
A comparison of the ∆Hb2 values (Tables 13a, b) of the Indigo-clay and the
Thioindigo-clay mixtures revealed that Thioindigo reacts more rapidly than Indigo. A
77
comparison of three pigments (MB, MP, RB) at 6% by weight suggests that Thioindigo may more likely be first of the pigments to enter into the channels (Table 13(c)).
3.9 Discussion and Conclusions
The nature of bonding between the clay and dyes still remains a mystery even when numerous studies have been done by several research groups. However, previous reports certainly help to understand the binding sites of the clay as well as that of the dyes. In the present study we have carried out detailed Ultra Violet-Visible,
Fourier Transform Infrared, Fourier Transform Raman spectroscopic and X-ray diffraction studies on Maya Blue, Maya Purple and Royal Blue pigments to investigate possible interactions between the components of the pigments. The interactions we considered are mainly coordinative type.
We already have discussed the structure of Palygorskite in the results section, a detailed pattern of the tetrahedral and octahedral sheets and bonding is shown in Figure
45. This structural form shows the regular structure and does not show any functional groups or active binding sites on the edges and in the channels. On the basis of the
32,34,63 detailed structural formula of clay, [(Mg 2, Al 2), (Si 7.8 , Al 0.2 ) O20 (OH) 2 (OH 2)4], it is calculated that ~ 2.5% of Si 4+ cations from the tetrahedral sites are occupied by Al 3+ cations. A schematic representation of this formulation is shown in Figure 46. The
3+ presence of Al cations in the SiO4 tetrahedral sheets are considered as potential interacting sites of the clay.
78
Figure 45 . Schematic representation of Palygorskite unit cell, Tetrahedral, and
Octahedral sheets.
79
Tetrahedral sheet - (X = H O, OH ) Lewis acid sites 2
X Edge O O O O O O 3+ O Si Si Al Si O H O O O O O OH H2O 2+ 3+ 3+ 2+ 2 Mg Al Al Mg OH2 H2O O O O O H O
O Si Si Si Si O O O O O O O O
Figure 46. A schematic representation of SiO4 Tetrahedral sheet and active binding sites of the clay.
The UV-Vis spectral data of all the Maya Blue, Maya Purple, and Royal Blue samples provide the evidence of the dye-clay bonding. The large bathochromic shift of π to π* transition associated with pure Indigo and Thioindigo is suggestive of changes in the electronic density of chromophoric groups (N-H, C=O, C=C) which may be due to the involvement of these groups in bonding. However, upon increasing the concentrations of the dye, the extent of red shift decreases and the absorption maxima shifts to higher energy side. This is indicative of partial contribution of dye at higher concentration.
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IR studies of all sample (heated at 170 °C/9h) showed reduction in the intensity of
C=O group. In case of Maya Blue, the C=O band shifts from 1626 to 1622 cm -1 and remains broad. It may be attributed to the fact that, as the C=O interacts with Al 3+ the force constant of C=O becomes weak, and results in lowering of carbonyl frequency.
Raman spectrum of Maya Blue samples also suggests the formation of new Al-O bond, which may be ascribed to the involvement of C=O group with Al 3+ coordination.
Figures 47a and 47b represent the coordination of Indigo and Thioindigo molecules (via
C=O group) with Al 3+ sites. The bonding in Maya purple involves C=O group of the dye and Al 3+ present in tetrahedral sheets.
The enthalpy values ( H) calculated from Differential Scanning Calorimetric studies suggest that at lower concentrations of the dye, removal of water molecules is thermodynamically favorable as compare to at higher concentrations. This may be due to the fact that at higher concentrations of the dyes, the dye molecules covers the surface and block the channels. This may results in the lesser reactivity when high concentrations of the dye present around the clay.
On the basis of all the results presented above, it is concluded that at lower concentration of dyes (<6%), the dye molecules may penetrates into the channels.
However, upon increasing the dye concentrations (6-16%), dye molecules binds with the exposed surface of the palygorskite involving the C=O, N-H groups of Indigo, C=O
3+ of Thioindigo and M (M = Al, Fe) cations present in the tetrahedral SiO 4 sheets. At much higher concentrations (>16%), it is observed that dye forms layers on the outer surface of the clay. A binding model is presented in Figure 48.
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O H O N S
N S H O O
O O O O O O O O O O O O O O Si Si Al Si O Si Si Al Si O H O O O H O O O O O O O OH H2O 2+ 3+ 3+ 2+ 2 H O 2+ 3+ 2+ OH2 Al Mg 2 3+ Mg Al OH Mg Al Al Mg H O 2 H O OH2 2 O 2 O O O O O H O O O H O
O Si Si Si Si O O Si Si Si Si O O O O O O O O O O O O O O O .
Figure 47 . Possible interaction of Indigo and Thioindigo with Al 3+ sites present in
Tetrahedral sheets.
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dye < 6% dye > 6%- < 16% dye > 16% Outer surface
Tetrahedral Si -O s heet Octahedral sheet Tetrahedral Si -O sheet Channel
a- axis
Figure 48 : A model for binding of the dye with the clay as a function of concentrations of the dye.
83
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APPENDIX
Table 14. Mol% calculation of different concentration of Indigo in Maya Blue
Molecular weight of Indigo=262.26g/mol
Molecular wt of Palygorskite = 751.49g/mol (per unit formula)
(wt%) moles of molar ratio, mol% Indigo in Indigo Palygorskite moles of Palygorskit Indigo / Indigo in Maya Blue (g) (g) Indigo e Palygorskite Maya Blue 1 0.05 4.95 0.0001 0.0065 0.028 2.89 3 0.15 4.85 0.0005 0.0064 0.088 8.85 6 0.3 4.7 0.0011 0.0062 0.182 18.26 16 0.8 4.2 0.0030 0.0055 0.545 54.53 20 1 4 0.0038 0.0053 0.715 71.58 25 1.25 3.75 0.0047 0.0049 0.954 95.44
Table 15 . Mol% calculation of different concentration of Thioindigo in Maya Purple
Molecular weight of thioindigo=296.36g/mol
Molecular wt of palygorskite =751.49g/mol (per unit formula)
(wt%) moles of moles of mole ratio mol % Maya Thioindigo Palygorskite Thioindig Palygorskit Thioindigo/ Thioindigo in Purple (g) (g) o e Palygorskite Maya Purple 1 0.05 4.95 0.0001 0.0065 0.025 2.56 3 0.15 4.85 0.0005 0.0064 0.078 7.84 6 0.3 4.7 0.0010 0.0062 0.161 16.19 16 0.8 4.2 0.0027 0.0055 0.483 48.31 20 1 4 0.0033 0.0053 0.634 63.41 25 1.25 3.75 0.0042 0.0049 0.845 84.55
90
665 R2 = 0.9675 660 655 650 645 640 Absorbance 635 630 625 0 5 10 15 20 25 30 Concentration (wt%)
Figure 49 . Concentration Vs Absorbance for Indigo (wt%)
665 R2 = 0.949
660 655
650 645
640 635 Absorbance (nm) Absorbance 630 625 0 20 40 60 80 100 Concentration (mole%)
Figure 50 . Concentration Vs Absorbance for indigo (mol%)
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600 R2 = 0.969 595 590 585 580 575 570
absorbance 565 560 555 550 0 5 10 15 20 25 30 concentration (wt%)
Figure 51 . Concentration Vs Absorbance for Thioindigo (wt%).
600 R2 = 0.9521 595 590 585 580 575 570
Absorbance 565 560 555 550 0 20 40 60 80 100 Concentration (moel%)
Figure 52. Concentration Vs Absorbance for Thioindigo (mol %).
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CURRICULUM VITA
Swati Kumar was born in Roorkee, India, a small town close to great Himalayan
Mountains. She earned her Masters degree in Industrial Chemistry from Gurukul
Kangari University, Haridwar, India, in 2001 and worked at Central Building of Research
Institute from 2001 to 2006 as a research Intern.
She enrolled in the Masters Program in the department of Chemistry at the
University of Texas at El Paso, El Paso, TX in fall 2006 under the guidance of Dr.
Russell R. Chianelli to study organic/inorganic complex - ancient Maya Blue pigment.
The project involved the synthesis and characterization of Maya Blue and its analogs.
While pursuing her Master’s degree in Chemistry, she worked part time with the company Mayan Pigment Inc. from May 2007 – April 2008 located at the University of
Texas at El Paso, El Paso, Tx.
Permanent Address:
4214 Walnut Street, Apt # 2F
Philadelphia – 19104, PA (USA)
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