The molar surface Gibbs energy and its application to the binary mixtures of N-butylpyridinium dicyanamide [C4py][DCA] with alcohols

Highlights: • The density and surface tension for binary mixtures of [C₄py][DCA]+alcohols were measured over the whole composition. • We improved Li's model of surface tension so that a new function– molar surface Gibbs energy, g_s, was obtained. • According to new function g_s, a new Etvs equation was obtained and applied to binary mixtures and discussed. • A new method for predicting the surface tension, γ, of the mixture is proposed. -- Abstract: The density and surface tension for the binary mixtures of ionic liquid (IL) [C₄py][DCA] with ethanol, 1-propanol and 2-propanol were measured across the entire range of mole fraction (x₁ = 0.0000–1.0000) at T = (288.15–318.15) K. The average molar volume and the thermal expansion coefficient of the binary mixtures were calculated from the experimental density. The excess molar volumes, V^E, of the binary mixtures were also calculated and well fitted by Redlich−Kister equation. The obtained V^E are negative values over the whole composition range. The values of V^E are related to the alcohol chain length and temperature. In order to predict the surface tension of the mixtures, the new function – the molar surface Gibbs energy of mixtures was obtained from Li's model improved by us in this work. So, the molar surface Gibbs energy, g_s, and its excess function, g_s^E, of mixtures for [C₄py][DCA] with ethanol, 1-propanol and 2-propanol were calculated. The excess molar surface Gibbs energy, g_s^E, was well fitted by Redlich−Kister equation. In terms of the Redlich−Kister's parameters, a new method for predicting the surface tension, γ, of the mixtures is proposed and the predicted values, γ (pre.), are in good agreement with the corresponding experimental ones, γ(exp.). On a basis of molar surface Gibbs energy of mixtures, a new Etvs equation was obtained and applied to binary mixtures of [C₄py][DCA] with alcohols. The absolute value of the slope of the new Eötvös equation is the mole surface entropy, s, and the intercept is the molar surface enthalpy, h, which is temperature-independent.