Chemical Unsaturation

The surface of a crystal is thus chemically unsaturated and the existence of a definite surface energy is due to the fact that the surface is unsaturated. Since the surface of a crystal consists of a regular lattice of orderly distributed atoms or ions the surface adhesional forces may be regarded as being distributed over the centres of these atoms (Haber, J.G.S.I. xxxiii. 50 Z it.f. Elektro-chem. XX. 521, 1914) although in all probability these points represent maxima in a continuous field. 

NorrishJ has sought to connect the catalytic activity of surfaces with their polar nature. By polar nature is meant a certain chemical unsaturation, usually associated with the presence of oxygen in the molecule. His work on the combination of ethylene and bromine has already been mentioned. The combination hardly takes place at all in vessels coated with paraffin wax, but is promoted by surfaces of such oxygen-containing substances as cetyl alcohol and stearic acid. To be quite conclusive such observations would have to be combined with others showing that the actual adsorption on paraffin wax is not very... 

Double bond - A double union of two carbon atoms instead of one union. Materials containing double bonds are chemically unsaturated. 

E. Paramagnetic Properties. All free radicals are chemically unsaturated and possess an odd number of electrons. As a consequence they are paramagnetic. Any technique capable of detecting paramagnetism is therefore a potential tool for the detection of free radicals, if there are present no stable molecules such as O2, NO, or NO2, which are also paramagnetic. 

Covalent Solids. These are substances such as graphite, diamond, and quartz in which the atoms are bonded to nearest neighbors by covalent linkages forming a macromolecular, two- or three-dimensional network. Atoms at the surfaces and edges of such crystals may be chemically unsaturated and can thus act as centers for initiating free radical or redox reactions. 

In considering the possible activity of a given catalyst it is important to note that the mere existence of centers of chemical unsaturation at the surface of the solid is not enough to guarantee catalytic activity. If these centers of activity arc sufficiently high in free energy, they will tend to stabilize themselves by forming permanent chemical bonds either with... 

As the surface state orbitals are directed outward from the surface, the center of their electron density does not correspond with that of the Si nucleus. Their occupation with electrons gives rise to a surface dipole moment. The dangling bonds formed on the unreconstructed Si(lll) surface behave as electron radicals as long as they are half filled. In the presence of a gas this will lead to rapid reaction. In a vacuum the chemically unsaturated nature of the dangling bonds leads to reorganization of the surfaces, resulting in a decrease of surface reactivity. Because of this surface stabilization, the surface energy decreases. 

Chemical Unsaturation Presence of double or triple bonds in a chain organic molecule such as that of some polymers, usually between carbon atoms. Unsaturation makes the molecule more reactive, espeeially in free-radical addition reactions such as addition polymerization, and polymers more susceptible to degradation, eross-linking, and chemical modification. Also called Polymer Chain Unsaturation. 

The surface of a fully dehydroxylated ionic oxide can be considered as an extended array of coordinately unsaturated oxygen ions and metal cations. From the acid-base point of view, they act as Lewis base and Lewis acid centers, respectively (structure I in Scheme 1). The net charge on each surface center depends on the stoichiometry, ionicity, and local surface structure, which in turn vary from one crystal plane to another. A few positions that represent geometrical defects on the surface (corners, edges, dislocations) may be characterized by enhanced acid-base character, as their energy is further increased due to additional chemical unsaturation,... 

In SFC, argentation columns separate TG according to degree of chemical unsaturation, chain length [e.g. POL (P = palmitate, O = oleate, L = linoleate) is separated from SOL (S = stearate) (Fig. 2.2)], position of double bonds in a fatty acid moiety [e.g. TG containing an a-linolenic moiety is separated from a TG where this moiety is exchanged for a y-linolenic moiety... 

Ozone is a particularly destructive gas. It is particularly likely to attack sites of chemical unsaturation in polymer molecules according to the following scheme. 

It is not, however, the case that all monomers are capable of forming macromolecules. (Chemically unsaturated) hydrocarbons are what primarily have the ability to create a polymer chain. In them, the carbon atoms have multiple bonds and the number of hydrogen atoms is reduced accordingly ... 

The high chemical reactivity of radicals is due to their open shells. The similarity between the chemical properties of carbon, nitrogen, oxygen, fluorine hydrides, and those of atoms with the same number of electrons is characteristic. For instance, the CH radical is chemically similar to the N atom CH2 and NH radicals are similar to the 0 atom CH3, NH2, OH radicals resemble the F atom, and finally CH4, NH3, H2O and HF molecules resemble, in a certain sense (in their inertness), the Ne atom. As radicals are chemically unsaturated, the activation energy for processes they are involved in is of the same order as that for atomic reactions. For this reason, the rates of these reactions are, as a rule, approximately the same as those of atomic processes. 

The experimental results of chain reaction mechanisms reveal that chemically unsaturated molecular fragments -- free atoms and radicals — represent the basic active centers of chain reactions. Chains carried on by free atoms and radicals are referred to as radical chains. 

Chemical Unsaturation n presence of double or triple bonds in a chain organic molecule such as that of some polymers, usually between carbon atoms. Unsaturation makes the molecule more reactive. 

Formulation. Unlike the natural rubber pressure sensitive adhesives, acrylic adhesives are often supplied in a ready-to-use form. Natural rubber adhesives require time-consuming and costly mastication of the rubber followed by dissolving in solvent. In addition, they must be formulated with a tackifier resin and antioxidants, neither of which is required with an acrylic adhesive. The acrylic adhesives are known for their good UV and oxidative stability while the rubber-based adhesives, because of their chemical unsaturation, are poor in these properties. In general, tackifiers are not required with acrylic adhesives as tack can be designed into the polymer by the proper choice of... 

The primary application for halobutyl rubber is in tires. The combination of low gas and moisture permeability, high heat and flex resistance, and ability to covulcanize with highly chemically unsaturated rubber has secured the use of these rubbers in the innerliners of tubeless tires. Passenger tires use chlorobutyl alone or in a blend with 20 to 40% natural rubber. High-service steel-belted truck tires use 100% bromobutyl innerliner compoimds. Chlorobutyl is also used for truck inner tubes for its superior heat resistance compared to butyl rubber. Halobutyl rubbers are added to sidewall compounds for improved ozone and flex resistance, and to certain tread compounds for improved wet skid resistance and traction. 

Generally, atoms that make up the surface layer of a solid have fewer neighbors than atoms within the bulk and so they are chemically unsaturated. Therefore, surface atoms can form new bonds with molecules impinging on the surface from the gas or liquid phases (chemisorption), or modify and even break the existing bonds of the impinging molecules (dissociative chemisorption). 

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