Dynamic viscosities of aqueous dissolutions of sodium sulfadiazine (NaSD), sodium sulfamerazine (NaSMR), and sodium sulfamethazine (NaSMT) were determined as a function of drug concentration and temperature in water and normal saline solution (NaCl 0.9 % w/w). Results obtained allowed to determine the relative viscosities and the Falkenhagen (A coefficient) and Jones–Dole (B coefficient) coefficients for the three sodium sulfonamides in both solvent systems at the different temperatures. The viscous flow activation parameters, \(\Delta^{\ddag } \mu_{2}^{{\text{o}}}\) , \(\Delta^{\ddag } H_{2}^{{\text{o}}}\) , and \(\Delta^{\ddag } S_{2}^{{\text{o}}}\) were determined in water and normal saline solution, and the transfer B coefficients were calculated for the three sodium sulfonamides as a function of temperature. Obtained results show that the viscosity of the three sodium sulfonamides in both solvent systems increases with drug concentration and with the molar mass of the solute (owing the presence of additional –CH3 groups in the structure), which suggests strong solute–solvent interactions and also indicates that the solutes studied behave as formers of the structure of the solvent. Likewise, viscosity decreases with an increase in temperature indicating an increase in intermolecular distances as a consequence of thermal movement. The viscosity A and B coefficients indicate that the solute–solute interactions in solution are weak owing the low value of A, whereas the positive value for the B coefficient indicates strong ionic solvation (strong solute–solvent interactions) and also suggests that sodium sulfonamides in solution act as solvent structure formers due to hydrophobic hydration and steric hindrance owing the presence of rings in the molecules studied.
