Nematic-isotropic transition latent heat

A drawback of the theory becomes apparent when we calculate the latent heat of transition from the nematic to the isotropic phase. The heat of transition is easily shown to be given by ... 

The transition from water to ice at 1 atmosphere pressure is a first-order transition, and the latent heat is about 100 J/g. The isotropic-nematic transition is a weak first-order transition because the order parameter changes discontinuously across the transition but the latent heat is only about 10 J/g. De Gennes extended Landau s theory into isotropic-nematic transition... 

Since S is a scalar order parameter, tire prior arguments leading to Eqs. (3.3)-(3.11) are valid. The isotropic-nematic phase transition is characterized by a small latent heat (0.5 kj mol i), and small a ( 0.06 J cm K i), which is nearly two orders of magnitude smaller than that at melting of a solid either to liquid crystal or isotropic liquid, indicating the delicate nature of these transitions. Due to the small latent heat, tirere are pretransitional effects on either side of the transition. 

Different liquid crystal phase transitions will be more or less difficult to detect using DSC. If the transition is first order, meaning that the order parameter is discontinuous across the phase boundary, a significant latent heat will be measurable, and usually a clear peak can be observed. An example of a first-order phase transition in liquid crystals would be the crystalline-to-smectic or -nematic phase. The nematic-to-isotropic phase is also first order. Some liquid crystal phase transitions are much more... 

The latent heat of the transition from nematic to isotropic liquid can be calculated from Eq. [6] and the fact that < P2 > changes from 0.4289 to zero at T ... 

The intermolecular interaction energy responsible for the nematic ordering can be estimated from the latent heat of isotropic-nematic phase transition AE 300 cal/mol (= 0.01 eV/molecule (see, for example, G. H. Brown, J. W. Doane, and D. D, Neff, A Review of the Structure and Physical Properties of Liquid Crystals, p. 43, CRC Press, Cleveland, Ohio, 1971). 

Discussed in this section are the effects of fluctuations and of external fields on the phase transition. Experimentally, the NA transition is usually indistinguishable from second order [48] however, for materials with a small nematic range, calorimetric measurements have detected a small latent heat associated with the phase change, which is interpreted as a mean-field, second-order phase transition that is driven first order by the coupling to a second, strongly fluctuating order parameter. The second order parameter is associated with a nearby transition, between the isotropic and nematic phases (IN). 

---