The Liquid Crystalline State of Matter

Areas of particular interest to the vision area were in defining the rules which determined the absorption effectiveness of a chromophore, the elucidation of the "liquid crystalline" state of matter, the electronic characteristics of useful chromophore and the mechanisms of interfacing between chromophores and their associated "signal" receivers. 

Terms specific to the liquid crystalline state of matter... 

Brown, G. (1977) Structures and properties of the liquid crystalline state of matter. J. Colloid Interface Sci. Vol. 58, pg. 534... 

The liquid crystalline state of matter is found in several different situations, particularly related to large organic molecules but also occurring in the case of water. 

Before achieving its recent commercial importance, the liquid crystalline state of matter has been a laboratory curiosity since at least 1888101. 

In recent times, many if not most biological materials have been recognized as liquid crystals. In the consumer electronics area, nearly every item sold incorporates a liquid crystal display device. The liquid crystalline state of matter exhibits characteristics not found anywhere else in science and engineering. The important and relevant characteristics include ... 

Before proceeding, it is important to note the additional requirements that must be met by chemicals defined as chromophores on the basis of their structure. To be effective, it is also necessary that they be in the liquid crystalline state of matter. And for maximum effectiveness, they must be associated with a suitable structure that can provide a de-excitation path. This is particularly important in the case of the carboxyl-ion system. Oxygen exhibits a unique... 

Many applications have taken only a year or so to move from the laboratory curiosity to the commercial product We have only begun to take advantage of the liquid crystalline state of matter as we develop new products for the consumer market ... 

The book is subdivided into three parts. The first three introductory chapters include consideration of the nature of the liquid crystalline state of matter, the physical properties of mesophases related to their electroop-tical behavior, and the surface phenomena determining the quality of liquid crystal cells giving birth to many new effects. The second part (Chapters 5-7) is devoted to various electrooptical effects in nematic, cholesteric, and smectic mesophases including ferroelectric compounds. Here major emphasis is given to explaining the physical nature of the phenomena. The last part (Chapter 8) is a rather technical one. Here recent applications of liquid crystalline materials in electrooptical devices are discussed. 

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... 

Two basic requirements for superconductivity are the pairing of electrons and phase coherence. Both of these can be satisfied in highly anisotropic arrays of loosely associated double bonds which are not directly chemically bonded as further association due to extra electrons would then be impossible. These criteria seem to be fulfilled by proper liquid crystalline state of matter. 

From X-ray measurements in the liquid crystalline phase it is impossible to determine the conformation of the molecules in the condensed state. Computer simulations give us information about the molecules internal freedom in vacuum, but the conformations of the molecules in the condensed state can be different because of intermolecular repulsion or attraction. But it may be assumed that the molecular conformations in the solid state are among the most stable conformations of the molecules in the condensed matter and therefore also among the most probable conformations in the liquid crystalline state. Thus, as more crystallo-graphically independent molecules in the unit cell exist, the more we can learn about the internal molecular freedom of the molecules in the condensed state. 

The liquid crystalline state may be identified as a distinct and unique state of matter which is characterised by properties which resemble those of both solids and liquids. It was first recognised in the middle of the last century through the study of nerve myelin and derivatives of cholesterol. The research in the area really gathered momentum, however, when as a result of the pioneering work of Gray in the early 1970 s organic compounds exhibiting liquid crystalline properties were shown to be suitable to form the basis of display devices in the electronic products. 

In the case of vision, the chromophores of vision are usually explored in the laboratory while in solution. However, in-vivo, they are not in solution but in the liquid crystalline state. This state of matter causes the chromophores to exhibit an entirely separate spectral absorption characteristic. 

In addition, these chromophores are very delicate molecules from an energy state perspective and should not be subject to detergents and other reactants such as sodium based complex salts. Although, it is possible (and actually easy) to return the chromophores of vision to the liquid crystalline state after extraction and purification, it is not easy to prevent conversion of the chromophores into their chromogens or similar retinoids. These decomposition products do not exhibit the resonance phenomena (and resulting spectral characteristics) no matter what state of matter they are in. 

These states of matter are summarized in Figure 8-3. It is not meant to be exclusive, in that there are other molecular arrangements, such as those found in the liquid crystalline state, that are both interesting and of great technological importance. Eut that is beyond the scope of what we want to cover here. (However, you may want to read about the discovery of Kevlar which involves liq-... 

The liquid crystal state for low molecular weight compounds such as cholesterol esters has been known for more than 75 years. This is a mesomorphic state of matter, intermediate between the crystal state and the liquid state. In the liquid crystalline state the molecules retain preferential orientations relative to one another over large distances—a property normally associated with the crystalline state yet liquid crystals from low molecular weight substances are highly fluid in their thermodynamically stable condition. 

In 1888 the Austrian botanist and chemist Friedrich Reinitzer, interested in the chemical function of cholesterol in plants, noticed that the cholesterol derivative cholesteryl benzoate had two distinct melting points. At 145.5°C (293.9°F) the solid compound melted to form a turbid fluid, and this fluid stayed turbid until 178.5°C (353.3°F), at which temperature the turbidity disappeared and the liquid became clear. On cooling the liquid, he found that this sequence was reversed. He concluded that he had discovered a new state of matter occupying a niche between the crystalline solid and liquid states the liquid crystalline state. More than a century after Reinitzer s discovery, liquid crystals are an important class of advanced materials, being used for applications ranging from clock and calculator displays to temperature sensors. 

From these and many other studies there was by 1950 plenty of evidence pointing to the possible importance of liquid crystals in tissue. Since then the subject has been explored by direct and experimental methods. My purpose is to examine the chemical forms in which liquid crystals occur and to identify more closely their precise structure and integration into protoplasm and biological fluids. This is often a matter of great difficulty for obvious technical reasons. My own work in this respect has been concerned mainly with lipids and lipoproteins, so I shall refer mainly to these though several other important classes of substances, especially certain proteins and polynucleotides, can exist in the liquid crystalline state and may be present as such in living tissue. 

The distinction between macromolecular liquid crystals and condis crystals may be helped by experience, although the differences are usually only a matter of degree. Liquid crystals seem always to convert fully from the liquid state to the liquid crystalline state, and the conversion to the mesophase on cooling shows rdatively little supercooling (few kelvins). Condis crystals, in contrast, show more often than not partial conversion from the melt and a relatively high supercoolir (10 or more kelvins), i.e. they are much closer in their behavior to fully-ordered, macromolecular crystals. The crystallization kinetics of liquid crystals has foimd little attention in the past. It seems describable by Avrami expressions with exponents of 2 to 4 >. Condis crystals, in contrast, have in the few analyzed cases shown exponents of less than 2 2 W) tjie transition entropy of the liquid crystal to the isotropic melt is... 

Sometimes referred to as the fourth state of matter, the liquid crystalline state possesses the properties of both a liquid and a solid. The liquid crystalline state is usually associated with small molecules, but many polymeric systems exhibit similar types of order to those found in low molecular weight liquid erystals. It is appropriate to consider the factors that influence the formation of liquid crystalline phases in small molecules before considering polymer systems. 

Author of this article has made an attempt to extend further our notions of liquid [11,12]. It is considered now that sublimation is a direct transition from solid (crystalline) state of matter into gas. The author has propounded and substantiated the principle of least time for first-order phase transitions [11,13] it is shown by means of this principle that sublimation goes in two steps through a certain intermediate state in the form of surface film. It is concluded that this film consists of nonstructural liquidlike substance which is a certain antipode of liquid this liquidlike state of matter is named second licjuid [12]. 

Fig. 10.8. (a) A perfect crystalline state of matter. Molecules preserve both positional and orientation order, (b) An intermediate state of matter. Molecules preserve orientation order but no positional order, and the matters preserve fluidity. A state of anisotropic liquid. (c) A totally disordered state of matter, for example, an isotropic liquid. The molecules preserve neither orientation nor positional order... 

Liquid crystals represent a state of matter with physical properties normally associated with both soHds and Hquids. Liquid crystals are fluid in that the molecules are free to diffuse about, endowing the substance with the flow properties of a fluid. As the molecules diffuse, however, a small degree of long-range orientational and sometimes positional order is maintained, causing the substance to be anisotropic as is typical of soflds. Therefore, Hquid crystals are anisotropic fluids and thus a fourth phase of matter. There are many Hquid crystal phases, each exhibiting different forms of orientational and positional order, but in most cases these phases are thermodynamically stable for temperature ranges between the soHd and isotropic Hquid phases. Liquid crystallinity is also referred to as mesomorphism. 

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