The physical properties of aqueous solutions of the ionic liquid [BMIM][BF4]

We report here the systematic study of the effect of concentration on the physical properties of aqueous solutions of the room-temperature ionic liquid [BMIM][BF4]. The measurements of density, rho, refractive index Delta n, viscosity eta, specific conductivity kappa and surface tension, gamma, were made over the whole concentration range. The equivalent conductance Delta(m) was calculated. The observed linear variations of density and refractive index with the molar concentration are established as those of an ideal solution. The surface tension varied most rapidly in the dilute region whereas the viscosity changed much more rapidly in the concentrated region. Two regions with different composition dependences were found after the analyses of the relationship between the conductivity and the concentration of [BMIM][BF4]. A proposed model for a structural change in the mixtures was described. The physical origin of the observed concentration dependence of these properties is discussed. The physical properties of the solutions vary with changes of association between anions and cations and the interaction between [BMIM][BF4] and water.