Association Equilibrium of Methylene Blue by Spectral Titration and Chemometrics Analysis: A Thermodynamic Study

Abstract

The monomer-dimer equilibrium of methylene blue (MB, Scheme I) has been investigated by means of UV-Visible spectroscopy in aqueous solutions. The self aggregation of MB in water has been investigated by recording absorption spectra in the wavelength range of 450-750 nm, and in different ionic strengths using concentrated KCl solutions in the temperature range of 20-90°C. Chemometrics analysis of the spectral data gave a dimerization constant, individual spectra of the monomer and dimer forms of the dye molecule. The quantitative analysis of the data of the undefined mixture was carried out by simultaneous resolution of the overlapping spectral bands in the whole set of absorption spectra. The dimerization constants of MB determined by mathematical deconvolution of the thermometric spectral titration data show dependency on temperature variations. The concentration range of MB was 6.00´10-5 -3.001 0 -4 M. Utilizing the van't Hoff relation, which describes the dependence of the equilibrium constant on temperature, the thermodynamics parameters DH°and DS°of the aggregation process were determined. The compensation effect was verified by the thermodynamics results of the dimerization process of the dye.

Adsorptive removal of methylene blue by agar: effects of NaCl and ethanol

“…In these sections, MB adsorbs on agar as monolayer and in spite of increase in dye concentration, due to alternately repeating 3,6-anhydro-L-galactopyranose units, MB molecules can not form surface aggregates on agar. Dyes aggregate as a function of concentration and in the commonly used concentration range (10 -6 -10 -3 M) the main equilibrium is a monomer-dimer reaction [ 20 ]. In the used concentration range of MB, its dimer aggregates form [ 15 ], Figure 1(d) and in sections 2A and 2B, MB molecules adsorb on agar as monomer and dimer, respectively.…”

“…Dyes aggregate as a function of concentration and in the commonly used concentration range (10 -6 -10 -3 M) the main equilibrium is a monomer-dimer reaction [ 20 ]. In the used concentration range of MB, its dimer aggregates form [ 15 ], Figure 1(d) and in sections 2A and 2B, MB molecules adsorb on agar as monomer and dimer, respectively. On the other hand, we studied adsorption of MB on pure agarose at 306 K. Binding constant (obtained from the Temkin isotherm), maximum experimental adsorption capacity ( q m , exp ) and mean adsorption energy ( E ) values (obtained from the Dubinin-Radushkevich equation) for adsorption of MB on agarose are 496946 M -1 , 9.78 mg g -1 and 10.1 kJ mol -1 , respectively that are similar to those of section 2A for adsorption of MB monomers on agar at 308 K, Table 1 .…”

Immobilization of methylene blue onto bentonite and its application in the extraction of mercury (II)

“…one quarter of the CEC capacity of the clay. This may be attributed to the large size of the MB molecules which may even exist as a dimmer or as aggregates at the surface, as well as a single protonated form depending on the concentration and the surface properties [24,25,33].…”

“…The supernatant solution was discarded, and the residual MB was estimated by UV spectrometry at 620.3 nm (isobestic point) by using calibration curve of MB solution. Isobestic point was chosen for measurement to avoid fluctuations due to different MB species which is known to form aggregates depending upon concentration and ionic strength [9,24,25]. About 500 mL DDW was then added to the clay slurry and shaken overnight.…”

Study Interionic Attraction of anionic dyes by Electrolytic Conductivity Measurement

“…Often one of the most common instances of stacking behaviors is the dye association. Many dyes display significant association with 10 -6 -10 -4 M concentrations and qualitative evidence of several studies indicates a number of multimers are produced [2,3]. There is great interest in dye aggregation and has been studied by many researchers [4][5][6][7].…”