Nature and Structure

Thermotropic liquid crystals come in two types calamitic and discotic. Calamitic phases (from the Greek for tube ) are all those that are caused by rod-like mesogens. The more recently characterised discotic phases are caused by disc-like species. Calamitic phases may be either nematic (from the Greek for thread ), smectic (from the Greek for soap ) or cholesteric (named after the cholesterol derivatives such as 13.4, which exhibit this behaviour). 

There are three main techniques used in characterisation of mesophases polarised optical hot-stage microscopy, differential scanning calorimetry (cf. Box 9.1) and small-angle X-ray scattering. 

Blue phases have been known since the early days of liquid crystalline substances. Reinitzer, in a letter to his physicist collaborator, Lehmann, wrote of them in 1888  

On cooling [the liquid phase of cholesteryl benzoate] a violet and blue phenomenon appears, which then quickly disappears leaving the substance cloudy but still liquid. 

Solid Cholesteric. . Blue phase I Blue phase U 

---

Binuclear aromatic hydrocarbons are found in heavier fractions than naphtha. Trinuclear and polynuclear aromatic hydrocarbons, in combination with heterocyclic compounds, are major constituents of heavy crudes and crude residues. Asphaltenes are a complex mixture of aromatic and heterocyclic compounds. The nature and structure of some of these compounds have been investigated. The following are representative examples of some aromatic compounds found in crude oils ... 

The 2nd law is true only statistically and does not apply to individual particles nor to a small number of particles, i.e. thermodynamics is concerned with bulk properties of systems. Thermodynamics thus has many limitations, but is particularly valuable in defining the nature and structure of phases when equilibrium (a state that does not vary with time) has been attained thermodynamics provides no information on the rate at which the reaction proceeds to equilibrium, which belongs to the realm of chemical kinetics. 

There is a whole spectrum of heterogeneous catalysts, but the most common types consist of an inorganic or polymeric support, which may be inert or have acid or basic functionality, together with a bound metal, often Pd, Pt, Ni or Co. Even if the support is inert its structure is of vital importance to the efficiency of the catal ic reaction. Since the reactants are in a different phase to the catalyst both diffusion and adsorption influence the overall rate, these factors to some extent depending on the nature and structure of the support. 

This last equation contains the two essential activation terms met in electrocatalysis an exponential function of the electrode potential E and an exponential function of the chemical activation energy AGj (defined as the activation energy at the standard equilibrium potential). By modifying the nature and structure of the electrode material (the catalyst), one may decrease AGq, thus increasing jo, as a result of the catalytic properties of the electrode. This leads to an increase in the reaction rate j. 

The vast capacity of SCS-based methods are related to simplicity of manufacturing and application of sensitive elements as well as with their unique response to adsorption of chemically active particles of different nature and structure. 

The results of work [ 135] are of specific interest. The work surveyed the influence of the nature and structure of adsorbed layers upon the mechanism of deactivation of He(2 S) atoms. It has been shown that on a surface of pure Ni(lll) coated with absorbed bridge-positioned molecules of CO or NO, the deactivation of metastable atoms proceeds by the mechanism of resonance ionization with subsequent Auger-neutralization. With large adsorbent coverages, when the adsorbed molecules are in a position normal to the surface, deactivation proceeds by the one-electron Auger-mechanism. The adsorbed layers of C2H4 and H2O on Ni(lll) de-excite atoms of He(2 S) by the two-electron mechanism solely. In case of NH3 adsorption, both mechanisms of deactivation are simultaneously realized. Based on the given data, the authors infer that the nature of metastable atoms deactivation on an adsorbate coated metal surface is determined by the distance the electron density of adsorbate valance electrons is removed from the metal lattice. 

The two types of wood differ, however, in their nature and structure. The main structural characteristic of the hardwoods (which are botanically known as angiosperms, plants that flower to pollinate for seed reproduction) is that in their trunks or branches, the volume of wood taken up by dead cells, varies greatly, although it makes up an average of about 50% of the total volume. In softwoods (from the botanical group gymnosperms, which do not have flowers but use cones for seed reproduction) the dead cells are much more elongated and fibrous than in hardwoods, and the volume taken up by dead cells may represent over 90% of the total volume of the wood. 

Faraday Society (1953), Nature and Structure of Collagen, Butterworth, London. 

The electrochemical behavior of ferrocene-based polymers [15] and of complexes containing a large, well defined number of ferrocene-type units [16] had already been reported when several groups became interested in dendrimer research. In the past few years several dendrimers of different chemical nature and structure carrying ferrocene-type units in the periphery have been synthesized... 

One of the most promising tools in the study of the nature and structure of adsorbed molecules is photoelectron spectroscopy (40), and results from such experiments can be compared with EHT calculations. As an example, the experimental and calculated spectra of ethylene on Ni(l 11) are compared in Fig. 40. In the calculations, the model surface consisted of a Ni atom surrounded by six nearest neighbours in the surface plane and three in the plane below. The molecular plane of ethylene was taken to be parallel to the surface. 

The functionality of immobilized biomolecules is mainly governed by their nature, preparation method as well as the nature and structure of the immobilization matrix. 

Deffieux A, Cramail H, Radharkrishnen K, Pedeutour JN (2001) Reactivity of metallocene catalysts for olefin polymerization influence of activator nature and structure. Macromol Rapid Commun 22 1095-1123... 

Among the main goals of electrochemical research are the design, characterization and understanding of electrocatalytic systems, (1-2) both in solution and on electrode surfaces. (3.) Of particular importance are the nature and structure of reactive intermediates involved in the electrocatalytic reactions.(A) The nature of an electrocatalytic system can be quite varied and can include activation of the electrode surface by specific pretreatments (5-9) to generate active sites, deposition or adsorption of metallic adlayers (10-111 or transition metal complexes. (12-161 In addition the electrode can act as a simple electron shuttle to an active species in solution such as a metallo-porphyrin or phthalocyanine. 

Concerning the nature and structure of such amyloid peptide or protein channels, oligomers with annular morphologies have in fact been observed by EM for a-synuclein (Lashuel et al., 2002) and equine lysozyme (Malisauskas et al., 2003) even in the absence of any lipids or membranes. Channel-like structures have also been reconstituted in liposomes and observed by SFM for A/ i 4o, A/ j 42, human amylin, a-synuclein, ABri, ADan, and serum amyloid A (Fig. 5A Lin et al., 2001 Quist et al., 2005). Doughnut-shaped structures with a diameter of 10-12 nm and a central hole size of 1-2 nm (Fig. 5B) were imaged on top of lipid membranes (Quist et al., 2005). However, the radius of curvature of the SFM tips meant that it is not possible to say whether the pores were really traversing the lipid bilayer. 

The photoreactivity of the involved catalyst depends on many experimental factors such as the intensity of the absorbed light, electron-hole pair formation and recombination rates, charge transfer rate to chemical species, diffusion rate, adsorption and desorption rates of reagents and products, pH of the solution, photocatalyst and reactant concentrations, and partial pressure of oxygen [19,302,307], Most of these factors are strongly affected by the nature and structure of the catalyst, which is dependent on the preparation method. The presence of the impurities may also affect the photoreactivity. The presence of chloride was found to reduce the rate of oxidation by scavenging of oxidizing radicals [151,308] ... 

We shall consider the system isobutene - BF3 - HzO in a non-polar solvent, involving the anion BF3OH (discussed in detail by Skinner [7]), since we are still too ignorant of the nature and structure of the anions derived from the aluminium or Group IV halides. The reaction in question can be represented thus ... 

Whatever the technique used, it is important to note that (i) only an equivalent viscosity can be determined, (ii) the response of a probe may be different in solvents of the same viscosity but of different chemical nature and structure, (iii) the measured equivalent viscosity often depends on the probe and on the fluorescence technique. Nevertheless, the relative variations of the diffusion coefficient resulting from an external perturbation are generally much less dependent on the technique and on the nature of the probe. Therefore, the fluorescence techniques are very valuable in monitoring changes in fluidity upon an external perturbation such as temperature, pressure and addition of compounds (e.g. cholesterol added to lipid vesicles alcohols and oil added to micellar systems). 

Influence of Structure and Chain Length of Surfactant on the Nature and Structure of Microemulsions... 

Before describing how microemulsion nature and structure are determined by the structure and chain length of surfactant and cosurfactant, it is necessary first to briefly review the theories of microemulsion formation and stability. These theories will highlight the important factors required for microemulsion formation. This constitutes the first part of this review. The second part describes the factors that determine whether a w/o or o/w microemulsion is formed. This is then... 

As discussed before (4) it is perhaps convenient to classify these theories into three main categories interfacial or mixed film theories, solubilisation theories and thermodynamic theories. Below a brief description of each of these classes will be given with particular emphasis on the role of surfactant nature and structure. 

---