Physico-chemical studies in nonaqueous solvents. Part 13. Thermochemical studies of some 1:1 electrolytes in tetramethylurea

Heats of solution of alkali metal, ammonium and substituted ammonium perchlorates, lithium and sodium bromides and sodium and potassium thiocyanates have been measured in the concentration range 1.75-67.7×10−3 mol dm−3 in tetramethylurea (TMU) at 26.9°C. Slopes of the linear plots of heat of solution against square root of concentration of the electrolytes (except for NaClO4 and KSCN) are 2 to 9 times higher than the theoretical limiting slope, suggesting thereby slight association of these electrolytes in TMU. However, the experimental slopes of the plots of NaClO4 and KSCN are comparable to the theoretical plot, showing complete dissociation of these electrolytes. Ion solvation enthalpies have been computed using a modified Buckingham's model. The results indicate that ion solvation enthalpies decrease with the increase in the size of the ions, which is consistent with findings in other solvents