The molar surface Gibbs energy and its application 3: The aqueous solution of ionic liquids [C_nmim][OAc](n = 3, 5)

Highlights: • ILs [C_nmim][OAc](n = 3, 5) were prepared and characterized. • A new function – the molar surface Gibbs free energy, g_s, was derived. • At constant pressure, c_p,s, is approximately close to zero. • Using the empirical equation, the surface tension of the solution was predicted. • Using the molar surface Gibbs free energy, a new Eötvös equation was obtained. The values of density and surface tension for aqueous solution of ionic liquids (ILs) 1-alkyl-3-methylimidazolium acetate [C_nmim][OAc](n = 3, 5) with various molarities were measured at 288.15–318.15 K with interval 5 K. Li's model was improved and based on this, a new function – the molar surface Gibbs energy, g_s, was derived. The g_s values of the aqueous solution for ILs [C_nmim][OAc](n = 3, 5) were calculated so that a linear empirical equation between the molar surface Gibbs energy and the molality of the solution was established. Using the empirical equation, the surface tension of the solution was predicted, and the predicted values and the corresponding experimental one are highly correlated and extremely similar. Thus, the molar surface Gibbs energy may become a semi-empirical method to predict the properties of ILs and theirs solution like parachor. In terms of the molar Gibbs energy, a new Eötvös equation is obtained in which each parameter has the clearer physical meaning. According to the new Eötvös equation, the molar surface entropy is the main factor that determines the polarity of the liquid and the larger the value, the smaller the polarity of the liquid.