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

Viscosities of aqueous solutions of formamide, N-methylformamide and N,N-dimethylformamide have been measured at temperatures 303.15, 308.15, 313.15, 318.15 and 323.15 K. For formamide + water system the viscosity increases exponentially with respect to the mole fraction of formamide. In contrast, N-methylformamide+water and N,N-dimethylformamide +water systems exhibit maxima 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 composition. The viscosities and excess viscosities for the system formamide+water have been explained by assuming some complex formation between the components with the simultaneous disruption of water structures. For N-methylformamide+water and N,N-dimethylformamide +water systems, it has been assumed that cluster-like structure of water is formed around methyl group(s) attached to N-atom of the amides, which influences the viscosity behaviour of these systems strongly.