A regular solution can also be described by
Raoult's law modified with a
Margules function with only one parameter \alpha: :\ P_1= x_1P^*_1f_{1,M}\, :\ P_2= x_2P^*_2f_{2,M}\, where the Margules function is :\ f_{1,M}= {\rm exp}(\alpha x_2^2)\, :\ f_{2,M}= {\rm exp}(\alpha x_1^2)\, Notice that the Margules function for each component contains the mole fraction of the other component. It can also be shown using the
Gibbs-Duhem relation that if the first Margules expression holds, then the other one must have the same shape. A regular solutions internal energy will vary during mixing or during process. The value of \alpha can be interpreted as
W/RT, where
W = 2
U12 -
U11 -
U22 represents the difference in interaction energy between like and unlike neighbors. In contrast to ideal solutions, regular solutions do possess a non-zero enthalpy of mixing, due to the
W term. If the unlike interactions are more unfavorable than the like ones, we get competition between an entropy of mixing term that produces a minimum in the
Gibbs free energy at
x1 = 0.5 and the enthalpy term that has a maximum there. At high temperatures, the entropic term in the free energy of mixing dominates and the system is fully miscible, but at lower temperatures the
G(
x1) curve will have two minima and a maximum in between. This results in phase separation. In general there will be a temperature where the three extremes coalesce and the system becomes fully miscible. This point is known as the
upper critical solution temperature or the upper consolute temperature. In contrast to ideal solutions, the volumes in the case of regular solutions are no longer strictly additive but must be calculated from
partial molar volumes that are a function of
x1. The term was introduced in 1927 by the American physical chemist
Joel Henry Hildebrand. ==See also==