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ABSTRACT: Layer charge is one of the key parameters used for the characterisation of expandable clay minerals, smectites. It determines
most significant properties of the material which are important from the industrial application point of view. This work is
related to a novel method introduced to characterize the layer charge of smectites, based on using cationic organic dyes as
molecular sensors. One xanthene and four phenothiazine cationic dyes were tested using reduced charge montmorillonites (RCMs)
and compared with methylene blue, which has been used most frequently. The characterization of the charge was based on the
formation of molecular assemblies (H- and J-aggregates) composed by dye cations, which were easily detectable using absorption spectroscopy in the UV/VIS spectrum. More
detailed characterization of the spectra required calculations of second-derivative curves. For all of the reaction systems
tested in this work, the molecular aggregation increased with the layer charge of RCMs. Slight to moderate differences in
the formation of dye assemblies related to the differences in the molecular structures of the individual dye cations. For
example, the molecular asymmetry of azure A brought about the formation of coexistent species of similar structures. The structure
of the heteroaromatic skeleton affected the extent of the aggregation and spectral changes with time. The presence of reactive,
non-substituted amino groups in thionine cations probably partially decomposed in the clay mineral colloids based on high-charge
RCMs. Any of the tested dyes could be used as molecular sensors for empirical characterization of the layer charge of clays
taking into account the differences mentioned above.
Central European Journal of Chemistry 04/2012; 7(3):343-353. · 1.07 Impact Factor
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ABSTRACT: Single- and two-step fluorescence resonance energy transfer (FRET) was investigated between laser dyes rhodamine 123 (R123), rhodamine 610 (R610), and oxazine 4 (Ox4). The dye molecules played the role of molecular antennas and energy donors (ED, R123), energy acceptors (EA, Ox4), or both (R610). The dye cations were embedded in the films based on layered silicate laponite (Lap) with the thickness of several μm. Optically homogeneous films were prepared directly from dye/Lap colloids. Dye concentration in the films was high enough for FRET to occur but sufficiently low to prevent the formation of large amounts of molecular aggregates. The films were characterized by absorption and fluorescence spectroscopies, and their optical properties were compared with colloid precursors and dye aqueous solutions. The phenomenon of FRET was confirmed by means of steady-state and time-resolved fluorescence spectroscopies. Significant quenching of ED emission in favor of the luminescence from EA molecules was observed. FRET led to the decrease in the lifetimes of excited states of ED molecules. Molecular orientation of dye molecules was determined by polarized absorption and fluorescence spectroscopies. Almost parallel orientation with respect to silicate surface (∼30°) was determined for all fluorescent species of the dyes. Theoretical model on relationship between anisotropy and molecular orientation of the fluorophores fits well with measured data. The analysis of anisotropy measurements confirmed the significant role of FRET in the phenomenon of light depolarization.
Journal of Colloid and Interface Science 08/2011; 364(2):497-504. · 3.07 Impact Factor
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ABSTRACT: Montmorillonite monoionic forms with alkali metal and NH(4)(+)-cations were prepared by ion exchange. The hydration properties and binding of the ions to montmorillonite surface and the swelling properties of the mineral specimens were analyzed. Whereas Na(+)- and Li(+)-ions were fully hydrated over a large range of conditions, large size K(+), NH(4)(+), and mainly Rb(+) and Cs(+) ions were apt to bind directly to the oxygen atoms on the mineral surface. The forms with large ions exhibited reduced hydration and swelling and the absence of macroscopic swelling of the respective aqueous colloids. The interaction of laser dye rhodamine 6G (R6G) in montmorillonite colloids was investigated by absorption and steady-state fluorescence spectroscopies. Significant effects of the properties of both the inorganic ions and swelling properties of colloidal dispersions on R6G molecular aggregation were observed. Large amounts of the molecular aggregates were formed in the colloids of Na(+)- and Li(+)-montmorillonites. The aggregates absorbed light at significantly lower wavelengths (~460 nm) with respect to the light absorption by monomers (535 nm). Fluorescence spectroscopy provided a key evidence for the assignment of the type of the aggregates: The emission of the aggregates at relatively low energies proved these assemblies are rather a mixed H-/J-type than ideal H-aggregates. The presence of parent inorganic cations of larger size led to a significant lowering of the amount of the R6G aggregates in favor of the monomers. Investigations of the evolution of the dye aggregation with time indicated basic features of dye aggregation reaction: The size of parent inorganic ions did not affect the reaction mechanism, but rather limited the extent of the reaction. Probably the forms with large inorganic ions, such as Rb(+) and Cs(+), did not provide sufficient surface for the formation of the large size assemblies of the dye. This property can be explained in terms of strong association of the large alkali metal ions to clay mineral surface, as well as to reduced swelling in the colloidal systems of respective forms.
Journal of Colloid and Interface Science 01/2011; 357(2):322-30. · 3.07 Impact Factor
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ABSTRACT: Tetracationic porphyrin dyes TMPyP and ZnPyP were intercalated into hydrophobized layered silicate films of three smectites.
The smectites represented the layered silicate specimens of high (Fluorohectorite, Corning; FHT), medium (Kunipia F montmorillonite;
KF) and low layer charge (Laponite, Laporte; LAP). The molecular orientations of the dye cations were studied by means of
linearly-polarized ultraviolet-visible (UV-VIS) spectroscopy. The spectral analysis and consequent calculations of tilting
angles of the transition moments at the wavelengths of Soret band transitions were in the range of 25°-35°. The determined
angles indicated molecular orientation of the dye cations being almost parallel to the surface of the silicates. Slightly
higher values (above 35°), determined for a FHT film, indicated either a slightly tilted orientation of the dye cations or
the change of molecular comformation after the intercalation of the dye.
Central European Journal of Physics 05/2007; 5(2):236-243. · 0.91 Impact Factor
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ABSTRACT: Adsorption of a cationic dye, methylene blue (MB), on the surface of montmorillonite leads to the molecular aggregation of dye cations, reflected by significant changes of dye optical properties. Montmorillonite samples, saturated with various inorganic cations (mono-, bi-, and trivalent, including those of transition metals), were used. Influence of the exchangeable cations on the MB aggregation was tested. Various properties of cations were considered (charge, diameter, acidity, hydration energies). Both direct and potential indirect effects of the cations were taken into account, such as salting-out effect, influence of the ions on solvent polarity, influence on swelling, colloid properties of montmorillonite dispersions, cation hydration properties, hydrolysis, and interaction of the cations with the clay surface. The spectra of MB in dispersions of montmorillonite saturated with NH4+, K+, Rb+, and Cs+ were significantly different from those of other reaction systems. Direct association between large monovalent cations and basal oxygen atoms of silicate probably leads to a partial fixation of the cations, which affects the ion exchange reaction and dye aggregation. Thus, the presence of large monovalent cations leads to the formation of fewer ordered H-aggregates in favor of monomers and aggregates of lower size. In these cases, dye species absorbing light of low energies also appeared in significant amounts and were assigned to J-aggregates, characterized by a head-to-tail intermolecular association.
Journal of Colloid and Interface Science 07/2004; 274(1):126-32. · 3.07 Impact Factor
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ABSTRACT: Hybrid materials based on cationic organic dyes intercalated in oriented clay mineral films were prepared via dye adsorption from aqueous solutions on the films having been prepared by a spin-coating method. The films with four thiazine dyes thionine, azure A, azure B and methylene blue and one representative xanthene dye, pyronin Y (Py), were investigated. Four reduced charge montmorillonites (RCM) were used as clay mineral templates, representing a semi-synthetic series of materials of almost the same composition, but different in the layer charge and related properties. The interlayer structure was investigated using X-ray diffraction and linearly-polarized absorption spectroscopy in ultraviolet–visible region. The effects of the molecular structure of the dyes and the different layer charge densities of the clays mineral on the molecular orientation were studied. The structure of the assemblies was very similar for thiazine dyes, but different for the case of Py. In the latter case, the formation of bimolecular layers of Py cations led to a significantly higher interlayer expansion. Unusual structural properties of Py/RCM films were interpreted in terms of the presence of polar/non-polar zones in heteroaromatic skeleton of Py cation to influence dye cation aggregation to specific molecular assemblies.
Thin Solid Films.
