a b s t r a c tDensities (q) and viscosities (g) of binary mixtures containing the Protic Ionic Liquid (PIL), pyrrolidinium octanoate with five molecular solvents: water, methanol, ethanol, n-butanol, and acetonitrile are determined at the atmospheric pressure as a function of the temperature and within the whole composition range. The refractive index of all mixtures (n D ) is measured at 298.15 K. The excess molar volumes V E and deviation from additivity rules of viscosities g E and refractive index D / n, of pyrrolidinium octanoate solutions were then deduced from the experimental results as well as apparent molar volumes V/ i , partial molar volumes V m;i and thermal expansion coefficients a p . The excess molar volumes V E are negative over the entire mole fraction range for mixture with water, acetonitrile, and methanol indicating strong hydrogen-bonding interaction for the entire mole fraction. In the case of longest carbon chain alcohols (such as ethanol and n-butanol) + pyrrolidinium octanoate solutions, the V E variation as a function of the composition describes an S shape. The deviation from additivity rules of viscosities is negative over the entire composition range for the acetonitrile, methanol, ethanol, and butanol, and becomes less negative with increasing temperature. Whereas, g E of the {[Pyrr][C 7 CO 2 ] + water} binary mixtures is positive in the whole mole fraction range and decreases with increasing temperature. the excess Gibbs free energies of activation of viscous flow (DG* E ) for these systems were calculated. The deviation from additivity rules of refractive index D / n are positive over the whole composition range and approach a maximum of 0.25 in PIL mole fraction for all systems. The magnitude of deviation for D / n describes the following order: water > methanol > acetonitrile > ethanol. Results have been discussed in terms of molecular interactions and molecular structures in these binary mixtures.

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