Liquid-crystal transition

The liquid-crystal transition between smectic-A and nematic for some systems is an AT transition. Depending on the value of the MacMillan ratio, the ratio of the temperature of the smectic-A-nematic transition to that of the nematic-isotropic transition (which is Ising), the behaviour of such systems varies continuously from a k-type transition to a tricritical one (see section A2.5.91. Garland and Nounesis [34] reviewed these systems in 1994. 

Seddon, J.M., Squires, A.M., Conn, C.E., Ces, O., Heron, A.J., Mulet, X., Shearman, G.C. and Templer, R.H. (2006) Pressure-jump X-ray studies of liquid crystal transitions in lipids. The Royal Society of London. Philosophical Transactions. Series A. Mathematical, Physical and Engineering Sciences, 364 (1847), 2635—2655. 

Some polymers undego other thermal transitions in addition to Ts and Tm. These include crystal-crystal transitions (i.e., transition from one crystalline form to another and crystalline-liquid crystal transitions. 

A liquid crystal compound in more cases Ilian not takes on more than one type of mesomorphic structure as the conditions of temperature or solvent are changed. In thermotropic liquid crystals, transitions between various... 

Information on the crystal to liquid crystal transitions is scarce and is to be treated with caution since partial crystallization is prominent and polymorphism of the smectic phase is frequent. Only the data on poly(acryloyloxybenzoic acid) (entry 2 of Table 5) have been extrapolated to 100% crystallinity. As with the low molecular weight liquid crystals, the total heat of transition is lower than expected for fully ordered crystals. Various combinations of two phase structures as suggested by Fig. 3 could be produced for the poly(acryloyloxybenzoic acid)21>. 

The crystal to liquid crystal transition has been measured frequently, but not studied in detail. From the generally poor crystallization of the polyester types which make up the liquid crystals3) one expects typical samples to have low crystal-... 

Solutions at concentrations above the cmc may contain significant concentrations of both monomeric and micellar surfactant. Previous researchers have used FT-IR to investigate monomer-to-micelle transitions (27) and gel-to-liquid crystal transitions of lipid bilayers (28,29). These studies have demonstrated that, in the case of such two-state transitions, linear combinations of the infrared spectra of the initial and final states characterize the spectra of the intermediate states, where both forms coexist. However, changes in band frequency or width are not necessarily linear with the extent of the transition. Linear combinations of two highly overlapped Lorenztion bands can give rise to non-linear shifts in the band frequency and width (27-29). 

The correlation between molecular structure, liquid crystal transition temperatures and physical properties of the nematic phase of these materials of relevance to individual types LCDs is dealt with extensively in Chapter 3. 

Significant variations in the magnitude of the physical properties, such as the liquid crystal transition temperatures, viscosity, birefringence and the elastic and dielectric constants, of alkenyl-substituted compounds with a carbon-... 

The o c experimental results for the water/pentanol/potas-slum oleate system (18,19) show that a pentanol/potasslum oleate molecular ratio of 5.5 and lower should give a premlcellar aggregate/lamellar liquid crystal transition instead of the pre-micellar aggregate/Inverse micelle transition at high alcohol/ soap ratios. 

Albon N, Sturtevant JM. Nature of Gel to Liquid-Crystal Transition of Synthetic Phosphatidylcholines. Proc. Natl. Acad. Sci. U. 

As shown in Fig. 11, the melting peak obtained by DSC is often structured and the clearing transition can be rather broad. The structuring of the melting peak may be an indication of polymorphism in the solid state but more often it is caused by successive meltings and recrystallizations of regions of imperfect crystallinity. The existence of more than one mesophase in the melt has also been observed, and this behavior leads to the presence of two or more liquid crystal transitions in the thermogram... 

The temperature of the hydration step should be above the gel-liquid crystal transition temperature of the lipid to allow the lipid to hydrate in its fluid phase with adequate agitation. Hydration time may differ slightly among lipid species and structures. We believe that good hydration prior to sonication makes the sizing process easier and improves the homogeneity of the preparation. 

Experimental results (12) showed a transition to a lamellar liquid crystal for 14 added water molecules. Our calculations (to be reported at a later occasion) showed no discontinuity or any other indication of instability of the soap/acid water complex for the subsequent water molecules added in excess of 14. It appears reasonable to assume that the isotropic liquid/liquid crystal transition does not depend on the energy levels of the polar group interactions. The phase transition probably depends on the hydrophobic/hydrophilic volume ratio and estimations according to Israelachvili/Ninham (15) approach may offer a better potential for an understanding. 

We note here that systematic studies of the melting transition of dry or nearly dry phospholipids bilayers (e.g., vesicles) have been scarce. While there is an abundant experimental and theoretical literature concerning the structure and properties of bilayers in water, less is known about their behavior when water is removed. We have therefore initiated a systematic experimental study of the gel-liquid crystal transition of pure DPPC and DPPC-cholesterol vesicles freeze-dried with and without disaccharides and oxyanion-disaccharide complexes. Some of our results to date are shown in Figure 9.3. 

Some applications in the biomedical field have been also envisaged for ionic liquid crystals. Transition metal complexes have potential antitumor activity, so metallomesogens could in principle be useful in the therapy for cancer [68],... 

Transition-temperature determinations for liquid crystals have been the subject of numerous investigations 112-20). Not only can the melting points be determined, but also the liquid-crystal transitions. One of the first such determinations was that by Barrall ei al. (12) on anisaldazine, which is illustrated in Figure 7.41. A stable, linear heating rate is necessary because a rJC sudden departure for linearity can appear as a glass transition point on the DTA curve. The two curves in Figure 7.41 illustrate the use of temperature-axis amplification in order to separate closely spaced peaks. In... 

Garland C W, Nounesis G and Stine J J 1989 XY behavior for the heat-capacity at nematic-smectic-A liquid-crystal transitions Phys.Rev. A 39 4919-22... 

Transitions between a solid and a mesophase, or between two mesophases, or between a mesophase and an isotropic liquid, are thermodynamic events and are classified as either first or second order. In liquid crystals, transitions between phases are usually thought of as being weakly first order, although second-order transitions are not uncommon the melting transition is, however, first order. 

---