Interaction of Methylene Blue with Reduced Charge Montmorillonite Juraj Bujda ´ k* and Peter Komadel Institute of Inorganic Chemistry, SloVak Academy of Sciences, SK-842 36 BratislaVa, SloVakia ReceiVed: June 5, 1997 X Interaction of methylene blue (MB) with reduced charge Li-montmorillonites (RCM) in the aqueous suspensions was investigated using visible absorption spectroscopy. Dye cation agglomeration and protonation at the clay surface depended very sensitively on layer charge density. With increasing layer charge reduction, the content of agglomerates of MB cations decreased in favor of monomers and the protonated form of MB. Lower negative charge density on the clay basal surface induces a greater distance between neighboring MB cations sorbed at the clay surfaces, which suppresses dye agglomeration. Since each form of MB absorbs visible light at a different wavelength, different layer charge densities induce different colors of the resulting clay-dye suspension. Therefore, visible spectra of MB-smectite suspensions may be a simple but extremely sensitive method for the detection of layer charge density of smectites. Introduction Methylene blue (MB) is a heteroaromatic, cationic dye (Figure 1) readily soluble in water. Visible spectra of dilute solutions contain a band associated with monomeric MB at 665 nm with a shoulder attributed to the 0-1 vibronic transition of monomers at 605 nm. In aqueous solutions of concentrations greater than 2.5 × 10 -6 mol dm -3 , MB cations partially associate as cationic dimers in a face-to-face arrangement to minimize their hydro- phobic interactions with water. The dimers absorb near 605 nm. 1 The surfaces of clay minerals exhibit a strong affinity for MB cations. 2 The cations are adsorbed via ion exchange, frequently accompanied by agglomeration, where micelles consisting of three or more cations at the clay surface orient via π-π interaction on both sides of the molecular plane with two neighboring cations. A partial protonation of MB cations may also take place at clay surfaces in a manner similar to that in very acidic solutions. 1,3-7 Visible spectra of MB-clay suspensions contain several bands assigned to the absorption of monomers, dimers, agglomerates (near 575 nm), and pro- tonated MBH 2+ cations (at about 765 nm). 3 MB agglomeration and protonation were found to be sensitive to many factors, such as the dye load rate, surface properties of the clay, exchangeable cations, pH, and the age of MB-clay suspension. 3-4,6 The greatest extent of agglomeration occurs at high dye loadings, while monomers and protonated cations predominate at low loadings. The type of exchangeable cations affects the swelling properties of smectite and hence the accessibility of basal surfaces for large MB cations. MB is a sensitive fingerprint molecule for the detection of surface properties of clays in suspensions. 3 However, some discrep- ancies still remain in previously reported interpretations of the site of dye agglomeration. 3-4,6 Methylene Blue Agglomeration on Clay Surface. The sites of MB agglomeration (edges, basal planes) have not been definitely identified yet. According to Gessner et al., 6 only monomers and protonated MB are present in the interlayer spaces of smectites. Agglomeration has been claimed to proceed at the external surfaces of clay layers. 3,6 Dimerization may proceed also at basal surfaces. 3 Consideration of the following discussion will, however, indicate that MB dimerization and higher agglomeration may occur also at the basal surfaces. (1) More than 75% of the dye may agglomerate at greater (MB amount/CEC; CEC ) cation exchange capacity) loadings. Large amounts of agglomerates were found also in fresh suspensions. 3-5 In dilute suspensions of Na-smectites, indi- vidual clay layers are present and both the edges and the basal surface sites are accessible for MB. 3 An absence of agglomer- ates of MB cations at the basal surfaces would mean that adsorbed MB cations are predominantly located at the clay particle edges, which is not probable. (2) The main argument for considering an absence of MB agglomerates at the basal planes is X-ray diffraction data of the basal spacings of MB-clay intercalates. 8-10 Assuming an orientation of the MB molecular plane parallel to the basal surface of the clay, it is possible for two layers of MB cations to fill the interlayer space, yielding a basal spacing d 001 = 1.77 or 1.72 nm observed for the MB-smectite complexes. 8,9 However, considering a nearly perpendicular orientation of the MB molecular plane with basal surfaces and the height of the MB cation (0.74 nm), 11 the existence of larger MB agglomerates on clay basal surface is possible. The orientation of MB cations nearly normal to the basal plane of clay surfaces would explain why a d 001 spacing of 1.72 nm was observed for MB- montmorillonite compounds at high (100% CEC) loadings. 9 At high load rates of MB, agglomerates are expected to be predominant, while monomers and dimers are practically absent in MB-clay complexes. Recently, a detailed study used two independent methods, i.e., X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy, to determine the orientation of MB at the surface of mica. 11 Both methods concluded that MB cations are tilted at an angle of 65-70° with respect to the clay basal surface rather than arranged parallel with the layer plane, as assumed earlier. 3,6 Indeed, a nearly perpendicular orientation of MB cations in the interlayer would enable a better alignment of adjacent aromatic rings of neighboring cations via hydrophobic interactions than orientation parallel to the clay surface. This would allow * To whom correspondence should be addressed. E-mail: uachjuro@ savba.sk. X Abstract published in AdVance ACS Abstracts, October 1, 1997. Figure 1. Molecular structure of methylene blue cation. 9065 J. Phys. Chem. B 1997, 101, 9065-9068 S1089-5647(97)01851-8 CCC: $14.00 © 1997 American Chemical Society