Dichroism in Dye-Doped Colloidal Liquid Crystals

Abstract

Nematic liquid crystals were obtained in sterically stabilized suspensions of rodlike particles of sepiolite clay, with an average length up to 900 nm and aspect ratio up to 40. In agreement with computer simulations for hard spherocylinders, the isotropic-nematic transition shifted to lower volume fractions with increasing aspect ratio. However, the coexistence gap was broadened noticeably due to particle polydispersity. The sepiolite crystal structure includes channels filled with zeolitic water, which can be replaced by indigo dye molecules. The indigo molecules are constrained inside the zeolitic channels to be aligned along the long axes of the rods. As a result, the colloidal nematic phase showed a marked dichroism, with an order parameter up to 0.5 for magnetically aligned samples, similar to typical values for dye-doped thermotropic liquid crystals.

The Incorporation of Indigo Molecules in Sepiolite Tunnels

“…The colour depends on the concentration, temperature and time exposed to heat. Sepiolite is a magnesium silicate [Si 12 Mg 8 O 30 (OH) 4 …”

“…Sepiolite and indigo/sepiolite systems forming liquid crystals are able to self-align in toluene; the indigo orientation in this clay is then the first step to multi-scale organisation. [3,4] If the presence of a large amount of indigo is the first step towards more sophisticated materials, the control of the indigo/sepiolite complexes, the stability of these complexes and the indigo organisation in nanotunnels has to be determined to enlarge the range of applications. New and exciting organomineral nanocomposites can be developed with potential applications in, for example, paints, polymer reinforcement, control release, carriers and optics.…”

“…[1,2] In critical concentration, imogolite in water and sepiolite in toluene can form a mineral nematic liquid-crystalline phase. [3,4] These properties allow the adoption of a new approach for the design of materials through nanoscale hierarchical morphology. [5] Controlling the insertion, location and organisation of organic molecules in the cavities of microtubes and nanotubes associated with the release and/or stability of these molecules is a crucial challenge in the evolution towards unusual properties in nano-and microtechnologies.…”

On the interpretation of the Raman spectra of Maya Blue: a review on the literature data

“…[37] These authors consider the possibility of a head-to-tail interaction of indigo molecules in the channels of the clay. Finally, Sánchez del Río et al [38] have provided evidence of the inhibition of rehydration of MB when compared with palygorskite, suggesting a significant blocking of the tunnel entry by indigo molecules with possibly only partial penetration occurring inside the tunnels.…”

“…The vibrational spectrum of indigo was first analysed by Tatsch and Schrader [47] and was recently refined by Tomkinson et al [48] With regard to the variety of MB-type samples and experimental conditions, it should be noted that the preparation of synthetic analogues to MB follow the three main methods initially proposed by Van Olphen: [7] embedding palygorskite with a solution of indoxylacetate, vat dyeing and 'dry' crushing indigo with powdered clay, all followed by heating at temperatures between 130 and 200 • C. This last method has been widely used in recent literature for preparing MB pigments from indigo with palygorakite and sepiolite which is a similar aluminosilicate of composition Si 6 Mg 4 O 15 (OH) 2 · 6H 2 O but with larger channels of dimension 10.6×3.7 Å and frequently used for preparing MB-type specimens. [17,25,27,28,37,43,44] In this report, we present a review of the literature concerning the Raman spectra of indigo, synthetic MB specimens and genuine MB samples. Most of this literature is more or less explicitly influenced by three 'uniqueness hypotheses': (1) there is a unique organic component in MB; (2) there is a unique type of attachment between indigo and palygorskite: and (3) there was a unique preparation recipe used by the ancient Maya people.…”