Abstract
The interaction between single- or double-charged anions of bromocresol purple (BP) and cyanine cations (quinaldine blue, QB, or quinaldine red, QR) at concentrations of dyes 5.0·10−7–4.0·10−5 mol/L has been investigated by vis-spectroscopy. The thermodynamic constants of dissimilar associations (Kas) have been studied. Comparison of the values of lg Kas shows that QB− associates of BP− are more stable (6.61 ± 0.07) than QR associates (4.84 ± 0.06); a similar phenomenon is observed for associates of the BP2− anion. Semi-empirical calculations (PM3 method) are in agreement with the vis-spectroscopy data and indicate that the association of dye into an associate is possible. The standard enthalpies of formation of associates (ΔfHo) and energy diagrams have been determined. The ΔfHo data indicate that the formation of an associate between dye ions is an energetically favourable process. The gain in energy significantly exceeds the systematic error of semi-empirical calculations and increases from 157 kJ/mol (associate ”BP− + QB+”) to 729 kJ/mol (associate “BP2− + QR+”). The most probable structures of dissimilar associates are presented. The study of the dissimilar association develops the concept of intermolecular interactions in solutions.
Highlights
Sulfonephthalein dyes have a wide range of applications, from acid–base or metallochromic indicators to analytical reagents for spectral determination of a number of organic substances [1,2,3]
The interaction between single- or double-charged anions of bromocresol purple (BP) and cyanine cations at concentrations of dyes 5.0·10−7–4.0·10−5 mol/L has been investigated by vis-spectroscopy
Comparison of the values of lg Kas shows that quinadine blue (QB)− associates of BP− are more stable (6.61 ± 0.07) than quinadine red (QR) associates (4.84 ± 0.06); a similar phenomenon is observed for associates of the BP2− anion
Summary
Sulfonephthalein dyes have a wide range of applications, from acid–base or metallochromic indicators to analytical reagents for spectral determination of a number of organic substances [1,2,3]. The bromine derivatives attract attention due to favorable specific features, such as the stability of protolytic forms, weak dimerization, good diversity of absorption bands of single- and double-charged anions, and high contrast of color reactions [3,4]. They are used in technologies for determining the acidity of pure and natural waters [5,6,7] and are the basis of sensitive elements of optical pH sensors, fiber optic biosensors, and chips [8,9,10,11]. Associates are able to be extracted into the organic phase (chloroform, dichloromethane [12,13,15,16,17,18,19,21,24,26])
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