What are excess properties and how do they relate to activity coefficients?

Excess properties quantify deviation from ideal solution behavior: G_E = G_real - G_ideal mixing. For ideal solution, G_E = 0. Activity coefficient is related to partial molar excess Gibbs energy: RT*ln(gamma_i) = G_E_bar_i. Total excess Gibbs energy: G_E = RT*sum(x_i*ln(gamma_i)). Excess enthalpy (H_E) gives heat of mixing; excess volume (V_E) gives volume change on mixing. Activity coefficient models (NRTL, UNIQUAC, Wilson) are essentially correlations for G_E. Measuring H_E and G_E provides data for model fitting.