Effect of solvent isotopic substitution and temperature on some standard thermodynamic properties of tetramethylthiourea in H2O and D2O: A comparison with aqueous oxygen-containing analogue

Abstract Density measurements on solutions of 1,1,3,3-tetramethyl-2-thiourea (TMTU) in ordinary (H 2 O) and heavy (D 2 O) water were carried out with a precision of 0.02 kg·m −3 using the Anton Paar DMA 5000 M vibrating tube densimeter. All experiments were performed within the temperature range between (278.15 and 318.15) K, with a step of 10 K, at the pressure to be (99.6 ± 0.8) kPa. The solute aquamolality, m aq , was ranged from 0.01 to 0.06 and 0.04 mol·(55.50843 mol of solvent) −1 respectively in the protiated and deuterated solutions. The standard (apparent at infinite dilution) molar volumes, V x o , and expansibilities, E p , x o , of the solute as well as volume-related second virial coefficients, v xx , were derived from the data on density for each of H/D isotopically distinguishable systems. It was established that, unlike the solutions of 1,1,3,3-tetramethyl-2-urea (TMU) in H 2 O and D 2 O where the standard volume-isotope effect (IE) is negative over the whole temperature range studied, the δV x o (H 2 O → D 2 O) value for TMTU undergoes a negative-to-positive sign inversion near of 308 K. The same goes for the previously evaluated results on the enthalpy-isotope effect of TMTU hydration. The v xx quantities and corresponding IEs are negative, which indicate that the solute studied should have a prevailingly structure-making effect on the aqueous surroundings. However, based on the Hepler's criterion, neither structure-making nor structure-breaking effects do not have any predominating influence on the solvation process of TMTU in H 2 O or D 2 O.