semi-ideal solution

A semi-ideal solution is a type of solution where the excess entropy of mixing is zero, but the excess enthalpy is not. This means that while the mixing process does not change the disorder of the components, there is an absorption or release of heat. This contrasts with an ideal solution, where both excess entropy and enthalpy are zero, meaning mixing occurs without any change in disorder or heat.1

In semi-ideal solutions, the intermolecular forces between the components are not identical to those within the pure components. This can lead to deviations from ideal behavior, such as changes in volume or heat absorption/release upon mixing. For example, mixing nonpolar molecules can be endothermic, meaning it absorbs heat from the surroundings, and the volume of the mixture may be larger than the sum of the volumes of the individual components.23