Lyotropic liquid crystals, definition

The traditional definition of emulsions (1) as consisting of two liquids, of which one is dispersed in the other in the form of macroscopic droplets, was modified by the lUPAC Commission for Nomenclature (2) to include lyotropic liquid crystals. This change was justified by the fact that a large number of commercial emulsions within the areas of foods, pharmaceutics, and personal care contain such structures. Commercial emulsions frequently also contain solid particles, but such systems are usually not called emulsions, but rather emulsions-suspensions to avoid having the term emulsions covering the majority of dispersed systems. 

The results lately obtained, although very interesting and promising, definitely do not cover all the aspects and do not answer all the questions addressed by the fascinating field of lyotropic liquid crystals and their applications as drug delivery systems. Clearly, many additional experiments need to be carried out in order to clarify the detailed structure, the exact properties, and specific potential of these systems. We hope that we at least opened a new window and provided new thoughts and interest into this rapidly growing field of research. 

A perhaps more significant example of a lyotropic liquid crystal is the cell membrane. The phospholipid bilayers of all cells behave as liquid crystals, maintaining a definite director and displaying only limited fluidity. Interestingly, most membranes are not pure phospholipids, but contain additives whose function is presumably to tune the structural and dynamic properties of the membrane. In animals, the most common additive is cholesterol, while plants use a similar steroid. As such, cell membranes can truly be thought of as cholesteric liquid crystalline phases. 

The expression single phase means that all observable properties are independent from the macroscopic locus of the sampling point. By using this definition, micellar solutions, microemulsions, lyotropic liquid crystals or colloids covered by surfactants are single-phase fluids, while emulsions, i.e. microphase... 

Relaxation measurements provide another way to study dynamical processes over a large dynamic range in both thermotropic and lyotropic liquid crystals (see Sec. 2.6 of Chap. Ill of Vol. 2A). The two basic relaxation times of a spin system are the spin-lattice or longitudinal relaxation time 7] and the spin-spin or transverse relaxation time T2. A detailed description, however, requires a more precise definition of the relaxation times. For spin 7=1, for instance, two types of spin-lattice relaxation must be distinguished, related to the relaxation of Zeeman and quadrupolar order with rates 7j"2 and Jfg. The relaxation rates depend on spectral density functions which describe the spectrum of fluctuating fields due to molecular motions. A detailed discussion of spin relaxation is beyond the scope of this... 

Gray has provided an excellent book on the liquid crystalline state as known in 196222. It includes many detailed definitions. Although he notes the wide occurrence of liquid crystals in biological tissue, he limited his discussion of such crystals to the introduction. Materials in the liquid crystalline state can be divided into two major groups, those that are thermotropic and lyotropic. 

Liquid crystals (LCs) are molecules that have the ability to self-assemble into organized mesophases with properties intermediate between those of crystalline solids and isotropic liquids [1,2]. In LC phases, the molecules are dynamic and collectively behave as a viscous liquid but retain on average a degree of organization reminiscent of an ordered, crystalline solid. Consequently, they can be considered ordered fluids, as a more accurate definition. LCs can be subdivided into two general classes—thermotropic LCs and lyotropic LCs—depending on the environmental and molecular factors that govern how they form ordered fluid phases. 

Liquid crystallinity can appear for more than one reason. Materials in which liquid crystalline properties are induced by the presence of a solvent are called lyotropic. If liquid crystallinity appears in definite temperature intervals, we have thermotropic liquid crystals. Hsiao, Shaw and Samulski" found that liquid crystalline properties can be also brought about by elevation of pressure I have called such LCs barot-ropic. Their existence is not surprising, since pressure and temperature changes produce similar (although not identical) effects in terms of affecting free volume. 

The existence of the liquid-crystalline state of matter is not only a function of temperature. A large number of organic materials show liquid crystallinity in properly chosen solvents. Systems which exist in the liquid-crystalline state in a definite range of temperature are called thermotropics, while the second group is known as lyotropics. Both groups show a rich polymorphism. While mesophases of rod-like thermotropic liquid crystals can essentially be subdivided into two... 

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