Orientation and Optical Polarized Spectra (380–900？nm) of Methylene Blue Crystals on a Glass Surface

The crystallographic directions of the crystal toward the vector of polarized light can accurately be positioned, so the information that we gain from polarized spectra can be consistently interpreted according to known crystal structure. The orientation and optical properties of the methylene blue (MB) crystals were analyzed by XRD, XRPD, and polarized VIS-NIR spectroscopy. Cationic dye, MB, was polymerized into crystals on a glass slate. The blue color crystals showed pronounced dichroism, twin lamellar structure and bladed to fibrous habit. According to XRD data, [010] direction lies perpendicular to the crystal surface, so we recognized it as (0k0) face, while [100] and [001] directions coincide with crystal elongation and crystal thickness respectively. In this paper, the polarized spectra of MB crystal are presented, measured with the aim of acquisition of referent values, which could be helpful for the identification of MB molecular aggregation. 1. Introduction Organic dyes can change color and other properties in accordance with their environment. Changes that occur after interaction of dyes with clay minerals are easily determined and followed by spectrophotometry. The formation, structure, and aggregation of methylene blue (MB) molecules into dimers on the surface of clays have been investigated intensively; hence a large amount of data is available [1–8]. Different types of MB molecular packing upon clays and other materials have also been investigated which led us to new information about properties and applications of MB [9–14], and in due time we have encountered different theoretical studies concerning these questions [15]. In accordance with the literature, depending on the concentration, two absorption maximums in the methylene blue (MB) spectra can be observed. These maximums are the result of dipole electron transitions and their intensities. They are dependent upon monomer-dimer structure in which the degree of dimer growth is a dominant factor. MB solution is predominantly of a monomer structure with a pronounced absorption maximum at 664？nm and a coefficient of absorption ε = 95000？L？mol？1？cm？1. At MB concentrations higher than 7 × 10？6？mol/L in water solutions, formation of dimers occurs. They are identified [7] by an absorption maximum in the short wavelength region, between 570？nm and 600？nm. When the concentration of solution increases, the degree of molecular aggregation also increases leading to the formation of higher polymers. When the concentration is high enough, the final result of aggregation is the formation of the