Efficiency of the intermolecular interaction of salicylic acid neutral form and monoanion with Cd2 + ion studied by methods of absorption and fluorescence

Abstract

The methods of absorption and fluorescence were used to study the efficiency of the interaction between salicylic acid derivatives SAD (neutral SA form and SA monoanion) and Cd2+ ions (in CdBr2 salt) within the range рН=1.5÷8. The efficiency was determined from the change in both the absorption band contour and the fluorescence intensity of various SAD forms. It has been established that depending on the SAD form, the addition of CdBr2 to a starting solution leads to the formation of additional absorption for both the shorter wave lengths in the absorption spectrum of the neutral form (at рН<3) and the longer wave lengths in the absorption spectrum for the НSal− monoanion (at рН>4). In the fluorescence spectra, the intensity was observed to increase for the neutral SAD form (at рН<3) and to decrease for the НSal− monoanion (at рН>4) after addition of CdBr2.The spectral changes were interpreted in the framework of common notions about the effect of three physicochemical factors that determine the interaction between the SAD and the Cd2+ ion and affect the parameters of absorption and fluorescence spectra. These factors are: (1) the decrease in рН after addition of CdBr2 to the SAD solution, (2) the decrease in the efficiency of the Н-bonding of SAD molecules to the water ones, and (3) the existence of electrostatic ion-ion interaction between the НSal− monoanion and the Cd2+ ion.The bimolecular fluorescence quenching constants Kq of НSal− monoanion fluorescence quenching by the Cd2+ ion appeared to be substantially less than those of the quenching which would follow either the dynamic (diffusion) or the concentration (static) mechanisms.