ILOCCA
AMBA
OIS
*
**
Department of Chemistry, Princeton University, USA
Dipartimento di Scienze Chimiche, Fisiche e Matematiche, UniversitaÁ dell'Insubria at Como
DOI : 10.1388/SSC(2003)-CM-141
ABSTRACT ± La non comune stabilitaÁdel complesso indigo-palygorskite (noto come
Maya Blue) eÁstata studiata per mezzo di simulazioni di dinamica molecolare classica.
A temperatura ambiente, le molecole di indigo diffondono lungo i canali del clay finche raggiungono dei siti particolarmente stabili che ne impediscono l'ulteriore traslazione, anche ad alta temperatura. Questo fenomeno sembra spiegare l'elevata stabilitaÁ del
Maya Blue: in quest'ambito si eÁinoltre cercato di razionalizzare l'importanza delle interazioni elettrostatiche e steriche tra clay e pigmento organico, sia dirette che mediate da acqua strutturale.
The unusual stability of the indigo±palygorskite clay complex (Maya Blue paint) has been investigated by classical molecular dynamics simulations. At room temperature, the bulky indigo molecules migrate within the channels of the clay until they reach stable sites which prevent their further translation even at higher temperatures. This trapping is the most likely reason of the observed high stability of the Maya Blue paint. In order to sort out the mechanism locking the dye in these sites, we examined the role of electrostatic and van der Waals guest-host interactions, either direct or mediated by water molecules.
Maya Blue is an ancient turquoise-blue paint used in the Maya civilisation, which has recently attracted considerable attention from the scientific community due to its unusually high resistance to natural and chemical attacks 1, 2, 3 . Thanks to this property, ceramics and other ancient artefacts painted with Maya Blue have hardly faded until today. The reasons for this high stability are still unclear. Maya Blue is a mixture of the
4 N
2 natural clay palygorskite , and the organic pigment indigo (C
16
H
10
O
2
). Purely inorganic pigments produced in the chemical industry show similar stability, but they include large amounts of environmentally unfriendly metals. Therefore, a microscopic understanding of the basis of the stability of Maya Blue could open the way for a new class of metal-free and environmentally benign paints.
3
4
1
2
M. J. YacamaÂn, L. RendoÁn, J. Arenas, M. Carmen, and M. C. S. Puche, Science (1996), 273 , 223.
L. A. Polette, N. Ugarte, J. M. YacamaÂn, and R. R. Chianelli, Discovering Archaeol.
(2000), page 46.
L. A. Polette, G. Meitzner, J. M. YacamaÂn, and R. R. Chianelli, Microchem. J.
(2002), 71 , 167.
G. Artioli and E. Galli, Mat. Sci. Forum (1994), 166-169 , 647.

142
SCIENCE AND SUPERCOMPUTING AT CINECA - REPORT 2003
F IGURE 1. ± The starting simulation cell, with four indigo molecules included. Color codes: Si
(yellow); Mg (cyan); oxygen (red); carbon (green); hydrogen (white); nitrogen (blue).
Classical molecular dynamics simulations were performed with the DL POLY program 5 using a ionic forcefield to model the flexibility of the palygorskite lattice, coupled to a bonded force field for the indigo molecule, with partial charges obtained from DFT quantum chemical calculations of the indigo molecule using the Gaussian98 program 6 .
The starting configuration (3088 atoms) is shown in Fig. 1.
It includes 2 2 7 palygorskite unit cells. The composition of a unit cell is
Si
8
(Mg,Al)
4
[O
20
(OH)
2
(H
2
O)
4
] 4H
2
O (Z ˆ 2); silica tetrahedral layers are alternated to layers of octahedral Mg orAl, resulting in monodimensional channels directed along z , which are large enough to host an indigo molecule. Structural water molecules are bonded to Mg/Al, while zeolitic waters are found deeper in the channels.
First, we performed some calculations on the palygorskite lattice which highlighted a higher degree of stability for the structure with only Mg in the octahedral sites: therefore no alumimum was included in all the following simulations. Zeolitic water was removed from four channels where an indigo molecule was inserted, selecting
5
6
W. Smith and T. R. Forrester, DL POLY 2.13
. CCLRC, Daresbury Laboratory, Warrington, England, 2001.
J. A. Pople and et al., Gaussian 98, Revision A.7. Gaussian, Inc., Pittsburgh PA, 1998.

CHEMISTRY
143
F IGURE 2. ± z coordinate of the inversion center of each indigo molecule in a 180 ps run at high temperature.
different initial positions and orientations for each molecule, as shown in Fig. 1. With
2 2 7 unit cells included in the simulation box, the channels extend enough along z
( 37 A ) to minimize the interaction between the periodic images of indigo molecules in the same channel.
An initial energy minimization showed that indigo tends to be adsorbed in the xy plane, confirming experimental observations [, 2]. The temperature was then slowly increased to 300K and then to 423K in a 90 ps trajectory, during which each molecule migrated by 3-5 A along the channel axisand finally settled in a stable site. The next high temperature trajectories allowed to check the stability of indigo adsorbed on these sites by performing relatively short ( < 200 ps) MD runs. These trajectories showed that the adsorption sites effectively lock indigo inside the channel: during 90 ps at 423 K followed by 90 ps at 523 K no diffusion of indigo was observed, as shown by the flat evolution of the z coordinates in Fig. 2.
The main guest-host interactions are hydrogen bonds (Hbs) between structural water and the indigo carbonyls: the radial distribution function for the water hydrogenindigo C ˆ O is shown in Fig. 3.

144
SCIENCE AND SUPERCOMPUTING AT CINECA - REPORT 2003
F IGURE 3. ± H(water) - O (indigo) radial distribution functions: the red curve is calculated from the normal trajectory, while the blue curve comes from the run without coulombic interactions.
On average, 1.6 Hbs per indigo molecule are present, and in 44.3% of the MD trajectory both carbonyl groups of indigo are involved in Hbs. In order to understand if these electrostatic interactions alone are able to lock indigo inside the channels, we performed further MD runs turning off the coulombic terms in the force field.
Although the indigo-palygorskite hydrogen bonds are weaker in this case (1.3
average Hbs per molecule and 22.6% configurations with both carbonyl H-bonded, see also Fig. 3), the molecules do not migrate away from the stable sites during 90 ps at 423 K. This observation seems to point out that the stability of Maya Blu e arises from the almost perfect fit of indigo in the channels, controlled by steric (van der
Waals) interactions.
As some of the structural water molecules can desorb during the synthesis process 7 , some undercoordinated Mg ions can be exposed on the inner surface of channels. The undercoordination makes these kind of ``defect'' sites very reactive toward adsorption: we checked their tendency to adsorb indigo by performing MD simulations of a system where some structural water was removed from a channel.
Immediately after starting the dynamics the molecule residing in that channel moved to
7 G. Artioli, E. Galli, E. Burattini, G. Cappuccio, and S. Simeoni, N. Jb. Miner. Mh.
(1994), 5 , 217.

CHEMISTRY
145
F IGURE 4. ± Side view of a channel in a snapshot taken from the 573 K trajectory. Some structural water has been removed from thet op layer. The direct indigo-Mg ion interaction is visible in the top of the channel.
the pocket created in the water layer, with its carbonyl coordinated to the magnesium ion (Fig. 4). This adsorption mode is very stable and the molecule did not desorb in
90 ps at 573 K.
In conclusion, our simulations confirmed the hypothesis that the trapping of indigo inside the palygorskite channels determines the high stability of Maya Blue. Van der Waals interactions appear to play the major role in this trapping, while hydrogen bonds, albeit present, do not seem to be so important. At the same time, indigo can be locked in channel ``side pockets'' formed by desorption of structural water molecules.
Part of the calculations of this work were carried out thanks to a grant of computer time on the IBM-SP3 parallel machine at CINECA, Italy.
[1] E. F OIS , A. G AMBA , and A. T ILOCCA .
Microporous and Mesoporous Materials (2003), 57 , 263,.