Solvent effect on the solution, ionization, and structure of aminosulfonic acids

We have determined thermodynamic parameters for the solution and ionization in dimethylsulfoxide and water of four aminosulfonic acids: NH2(CH2)nSO3H with n = 0,1, and 2, and NH2C6H4SO3H. Potentiometric and calorimetric methods were used. The solubilities of the four acids in Me2SO are either nearly equal to or lower than in water, but the enthalpies of solution are 15–40 kJ mol−1 more exothermic in Me2SO. The three aliphatic acids are much weaker (2.1 ≤ ΔpKa ≤ 5.5) in Me2SO than in water and their heats of ionization are some 30 kJ mol−1 more endothermic in Me2SO. The discussion of our results is based on comparisons with corresponding data for related substituted amines and ammonium ions, and sulfonic acids and sulfonate ions. We conclude that the four aminosulfonic acids are mostly present as zwitterions in Me2SO just as in water.