A thermodynamic formalism is developed for lamellar liquid crystals, in which the free energy is written as the sum of two terms : one which treats the two kinds of lamellae (water and oil) as bulk media and a second which (i) accounts for the thinness of the lamellae and (ii) includes the interactions among them. By "thin" we mean thicknesses small compared to the range of interaction forces between the molecules of each of the lamellae. On the basis of this free energy expression, equations are derived which can provide information regarding the maximum dilution of the liquid crystal with water or hydrocarbons. Considering planar layers and only van der Waals type interactions between molecules, we find that there are conditions under which the lamellar liquid crystals display the unlimited swelling observed experimentally. The effect of the undulation disorder of the interfaces between the lamellae is also discussed.