Abstract
The ability of rhodamine cations to form cation-anionic associates with tetraphenylborate-anion in an aqueous solution has been studied. The thermodynamic values of the equilibrium association constants Kasт were determined spectrophotometrically. The values of a number of properties of dye molecules (surface area, effective volume, hydrophobicity index log P, standard enthalpy of hydration ΔНohydr) were calculated using the QSAR model. The possibility of dependence of the value of Kasт on these factors has been reviewed. It was shown that changes in lg Kasт in the series of rhodamine associates are consistent with changes in the parameters log P and ΔНohydr.
Highlights
An increase in the content of TFB at a constant concentration of rhodamine cation leads to a significant decrease in the intensity of light absorption, which reaches 45 – 55 %, and to a bathochromic shift of the absorption band of the dye
Ct+∙An– with the equilibrium constant of the association: Kas = [Ct+∙An–] × (CAn – [Ct+∙An–])– 1× (CCt – [Ct+∙An–])–1, where concentration of TFB (CAn) is the initial molar concentration of the anion, [Ct+∙An–] is the equilibrium concentration of the associate, CCt is the initial concentration of the cation.The content of the associate was calculated from spectral data by the equation:
[Ct+∙An–] =×(εСt – εas)–1×l–1, where A is the optical density at the appropriate concentration of TFB (CAn) under λmax of the absorbtion band of cation; l is the thickness of the absorbing layer; εСt and εas are the molar absorption coefficients of the rhodamine and associate cations, respectively
Summary
The processes of association of molecular or ionic forms of dyes can significantly change the properties of the solution. The problems of the relationship between the structure of the dye and its ability to associate have become especially relevant. Of particular interest is the development of theoretical propositions on the basis of which it would be possible to predict the structure of associates, their spectral-luminescent properties, based only on the peculiarities of the structure of dyes. To this end, the driving forces of the association are analyzing now. Regulation of the completeness of the association is considered as an effective factor in changing the spectralluminescent characteristics of the medium [4, 9, 11]
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