Mixtures:Non-ideal Mixtures

Non-ideal Mixtures

The equilibrium condition for a mixture at given pressure p and temperature T is that the Gibbs free energy assumes a minimum. We denote the Gibbs free energies of the unmixed and the mixed states by Gunmixed, Gmixed, respectively. Both are related as

observed when the intermolecular forces between like particles are much larger than those between unlike particles.

Due to its dipole structure, water has strong polar bonds between its molecules, while oil is non-polar. Breaking up the water bonds to form water- oil pairs instead of water-water pairs requires energy, and hence the enthalpy of mixing, Hmix, is positive. From the fact that oil separates from water, we can conclude that the energetic effect exceeds the entropic effect, that is Hmix > T Smix.

Table salt (sodium chloride, NaCl) dissociates in water into charged ions, Na+ and Cl−, which have energetic bonds with water. Nevertheless, for water alt solutions, the enthalpy of mixing is positive, but smaller than the entropic term T Smix. In dilute solutions, the energetic interaction between salt ions and water molecules can be ignored, and the solution can be approximated as an ideal mixture. Dissociation requires shielding of salt ions by the polar water molecules, where several water molecules are shielding one salt ion. If all water molecules are used for the hydrogen shells, no additional salt ions can be dissolved, the solution becomes saturated.