Fig. 1. Absorption spectra and calculated absorption spectra of monomer (- - -) and dimer (—) of rhodamine B (2.4 × 10⁻⁴ mol l⁻¹) in 5 °C intervals between 20 and 80 °C at pH 7.50: (a) in water; (b) 3 M NaCl.

Fig. 2. Absorption spectra and calculated absorption spectra of monomer (- - -) and dimer (—) of rhodamine 6G (5.89 × 10⁻⁵ mol l⁻¹) in 5 °C intervals between 20 and 80 °C at pH 7.50: (a) in water; (b) 2 M NaCl.

Fig. 3. The van’t Hoff equation plot at different concentrations of rhodamine B dye: (a) 5.89 × 10⁻⁶ mol l⁻¹; (b) 1.12 × 10⁻⁵ mol l⁻¹; (c) 1.77 × 10⁻⁵ mol l⁻¹; (d) 2.36 × 10⁻⁵ mol l⁻¹ and rhodamine 6G: (e) 3.24 × 10⁻⁵ mol l⁻¹; (f) 5.89 × 10⁻⁵ mol l⁻¹ at pH 7.50.

Fig. 4. Molar ratio of rhodamine B and rhodamine 6G dyes monomer () and dimer (○), compared to molar ratios predicted by the temperature dependence of the equilibrium constant (shown as line) at different concentrations of rhodamine B dye: (a) 5.89 × 10⁻⁶ mol l⁻¹; (b) 1.12 × 10⁻⁵ mol l⁻¹; (c) 1.77 × 10⁻⁵ mol l⁻¹; (d) 2.36 × 10⁻⁵ mol l⁻¹ and rhodamine 6G: (e) 3.24 × 10⁻⁵ mol l⁻¹; (f) 5.89 × 10⁻⁵ mol l⁻¹ in water (temperature unit is K scale).

Fig. 5. Plot of TΔS vs. ΔH° for dimerization of rhodamine B and rhodamine 6G of different concentrations in various ionic strengths medium.

Scheme 1. 

Dimeric constant (K_D) and thermodynamic parameters values of rhodamine B and rhodamine 6G dyes at different concentrations without adjusting ionic strength in water

Dimeric constant (K_D) and thermodynamic values of rhodamine B and rhodamine 6G dyes at different concentrations of salts