Validation study of using the free volume approximation to confined thermotropic and lyotropic liquid-crystalline fluids

. We examined the accuracy of the free volume approximation (FVA) to calculate the isotropic-nematic (IN) transition properties of thermotropic and lyotropic rods between two parallel hard walls. This approximation has been proposed to ease the calculation of the confined systems. It approximates the free energy of the confined particles with a bulk free energy. It predicts a special point for these two types of liquid crystals where the first-order IN transition changes to the second one by decreasing either the temperature, the density or the pore width. This prediction is in contradiction (in spite of some qualitative agreement) with those of the other publications where the authors note that the discontinuous transition terminates at the critical point when the walls are completely impenetrable. Graphical abstract