Viscosity of aqueous solutions of n-butylamine, sec-butylamine and tert-butylamine

Viscosities of the systems, water (W) + n-butylamine (NBA), W + sec-butylamine (SBA) and W + tert-butylamine(TBA) have been measured in the temperature range 298.15-323.15 K. The viscosities (eta) and excess viscosities (eta (E)) have been plotted against mole. fraction of amines (X-2). On addition of amines to water, viscosities first increase rapidly, then pass through maxima at 0.2 mole fraction of amines and then decline continuously as the addition of amines is continued. eta (E) show large positive values, with maxima also at 0.2 mole fraction of amines. The maxima of the curves of eta and eta (E) vs. mole fraction of butylamines follow the order, W + TBA > W + SEA > W + NBA. The ascending part of the eta vs. X-2 curves in the water-rich region is explained by the hydrophobic hydration caused by the hydrocarbon tails and the hydrophilic effect due to -NH2 group of amines. Following the maxima, amine - amine association is preferred, which accounts for the steady decrease of viscosity up to the pure state of amines.