Viscosity of aqueous solutions of formamide, N-methylformamide and N,N-dimethylformamide

Viscosities of aqueous solutions of formamide, N-methylformamide and N,N-di methylformamide have been measured at temperatures 303.15, 308.15, 313.15, 318.15 and 323.15 K. For formamide + water system the viscosity increases e xponentially with respect to the mole fraction of formamide. In contrast, N -methylformamide+water and N,N-dimethylformamide +water systems exhibit max ima in water-rich region, the maxima of the latter being higher and sharper than those of the former system. The excess viscosities of formamide+water system are positive at 323.15 K, which turn to negative values at 303.15 K , the magnitude of the values being very small irrespective of their sign. On the other hand, N-methylformamide +water and N,N-dimethylformamide+water systems show large positive excess viscosities for the whole range of comp osition. The viscosities and excess viscosities for the system formamide+wa ter have been explained by assuming some complex formation between the comp onents with the simultaneous disruption of water structures. For N-methylfo rmamide+water and N,N-dimethylformamide +water systems, it has been assumed that cluster-like structure of water is formed around methyl group(s) atta ched to N-atom of the amides, which influences the viscosity behaviour of t hese systems strongly.