Applications of spectroscopic techniques to the study of monomer-dimer equilibria for methylene blue in aqueous solutions containing ionic liquid: Probing the structural interactions involving water and ionic liquids.

An understanding of the nature of interaction and bonding in dye aggregation process is important for such diverse problems and applications such as stacking interactions in biomolecules, staining properties, photodynamic therapy for cancer, energy transfer in lasing technology and energy transfer and electron transfer processes. In present communication we report visible absorption spectrums and their analysis for the dye methylene blue (MB) in concentration range of 1∙10-6 - 1∙10-4 M in aqueous solutions containing ionic liquids namely, 1-ethyl-3-methyl-imidazolium bromide, 1-butyl-3-methyl-imidazolium bromide and 1-hexyl-3-methyl-imidazolium bromide at 298 ± 1. In this concentration range of MB only Monomer ⇌ Dimer equilibria for the dye exist and hence the spectral characteristics in the visible range of 550-700 nm have been examined. The spectrums obtained are compared with those obtained in aqueous solutions of NaCl. Using suitable developed method of estimating molar absorption extinction coefficient values, the equilibrium constant values are obtained at various ionic strengths of imidazolium ions. It has been found that monomer and dimer absorption maximum occur at 665 and ≈605/610 nm, respectively, do not get much altered on addition of imidazolium ions. The extent of interaction between the MB and imidazolium cations varies with the concentration of imidazolium cations as well as the nature of imidazolium cations, that is, the chain length of the substituents. The dimer dissociation constant values extrapolated to zero ionic strength of ionic liquids exhibit systematic alteration with respect to alteration of chain length in imidazolium cations. The different interactional phenomena such as the formation of ion-pairs, ion-pair complexes, dimer dissociation, solubilization of monomers and binding of MB with imidazolium cations have been examined. The transfer standard free energy changes have been calculated for the transfer of dimers from aqueous solutions to aqueous solutions containing ionic liquids. The changes in dimer geometry from sandwich type and end-on-end types also have been studied. It is observed that the contribution to the interaction energy is mainly from van der Waals type and dispersion forces, in addition to short range forces involving multipoles. The binding of monomers with imidazolium cations is also examined in terms of formation of micellar type aggregates in solution phase. It is proposed that the water structural interaction and hydrophobic interactions are the major factors in the formation and dissociation of aggregates.

Thermodynamic studies of aqueous solutions of 2,2,2-cryptand at 298.15 K: enthalpy-entropy compensation, partial entropies, and complexation with K+ ions.

The osmotic coefficient measurements for binary aqueous solutions of 2,2,2-cryptand (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8] hexacosane) in the concentration range of ~0.009 to ~0.24 mol·kg(-1) and in ternary aqueous solutions containing a fixed concentration of 2,2,2-cryptand of ~0.1 mol·kg(-1) with varying concentration of KBr (~0.06 to ~0.16 mol·kg(-1)) have been reported at 298.15 K. The diamine gets hydrolyzed in aqueous solutions and needs proper approach to obtain meaningful thermodynamic properties. The measured osmotic coefficient values are corrected for hydrolysis and are used to determine the solvent activity and mean ionic activity coefficients of solute as a function of concentration. Strong ion-pair formation is observed, and the ion-pair dissociation constant for the species [CrptH](+)[OH(-)] is reported. The excess and mixing thermodynamic properties (Gibbs free energy, enthalpy, and entropy changes) have been obtained using the activity data from this study and the heat data reported in the literature. Further, the data are utilized to compute the partial molal entropies of solvent and solute at finite as well as infinite dilution of 2,2,2-cryptand in water. The concentration dependent non-linear enthalpy-entropy compensation effect has been observed for the studied system, and the compensation temperature along with entropic parameter are reported. Using solute activity coefficient data in ternary solutions, the transfer Gibbs free energies for transfer of the cryptand from water to aqueous KBr as well as transfer of KBr from water to aqueous cryptand were obtained and utilized to obtain the salting constant (ks) and thermodynamic equilibrium constant (log K) values for the complex (2,2,2-cryptand:K(+)) at 298.15 K. The value of log K = 5.8 ± 0.1 obtained in this work is found to be in good agreement with that reported by Lehn and Sauvage. The standard molar entropy for complexation is also estimated for the 2,2,2-cryptand-KBr complex in aqueous medium.