Kinetic and amplitude measurements for the process of association of acridine orange studied by temperature-jump relaxation spectroscopy

Abstract

The kinetics of aggregation of Acridine Orange and the amplitudes associated with this process have been studied as a function of concentration and temperature in water and in an aqueous organic medium, and the results have been quantitatively evaluated by means of a general theory which is developed for associating systems.

The relaxation curves are complex and non-exponential due to the presence of a multi-equilibrium system. However, an analysis based on mean relaxation times (τ*) is possible. This important kinetic quantity enables a relaxation spectrum of similar relaxation times to be analysed, (i.e. τ₁≃τ_n), and is most useful when non-exponential relaxation curves are observed. The theory when applied to self-associating systems leads generally to the prediction of wavelength-dependent τ* values. The amplitude and kinetic results confirm a previous thermodynamic analysis (based on visible spectrophotometry) that oligomers are formed in the solution, and that, at an ionic strength of 0.1 M^‡ in water, the equilibrium constants K_{n,n+ 1} characterising aggregation decrease as n is increased (where n refers to an oligomer of n Acridine Orange units). The rate constant for stepwise association is close to but less than the diffusion-controlled limiting value. The Debye–Smoluchowski equation is applied to the system and the results are explained in terms of solvation changes and reorientation on stacking. The thermodynamic parameters, ΔH^°₁₂ and ΔS^°₁₂, characterising dimerization are derived from the amplitude analysis and discussed in relation to the factors responsible for dye stability.