Attractive forces classification

Chemisorption and Physisorption. One classification of adsorption phenomena is based on the adsorption energy the energy of the adsorbate-surface interaction. In this classification there are two basic types of adsorption chemisorption (an abbreviation of chemical adsorption) and physisorption (an abbreviation of physical adsorption). In chemisorption the chemical attractive forces of adsorption are acting between surface and adsorbate (usually covalent bonds). Thus, there is a chemical combination between the substrate and the adsorbate where electrons are shared and/or transferred. New electronic configurations are formed by this sharing of electrons. In physisorption the physical forces of adsorption, van der Waals or pure electrostatic forces, operate between the surface and the adsorbate there is no electron transfer and no electron sharing. 

Intermolecular forces are the attractive forces that hold molecules and ions together. These forces should not be confused with the intramolecular forces that hold the atoms together in a covalent molecule (see Lesson 11, Molecular Structure ). Intermolecular forces are grouped into four classifications, each supporting the existence of the condensed states of matter solids and liquids. In addition, these forces can also explain the nonideal behavior of certain gases. 

CLASSIFICATION OF ATTRACTIVE FORCES OF ADSORPTION CHROMATOGRAPHY BETWEEN SOLUTES AND STATIONARY PHASES... 

The concepts of physical adsorption (physisorption) and chemical adsorption (chemisorption) were introduced above. The nature of the two classifications is linked to the heat of adsorption, A//ads, which is defined as the binding energy of the adsorbed species. Physical adsorption is caused by secondary attractive forces (van der Waals) such as dipole-dipole interaction and induced dipoles and is similar in character to the condensation of vapor molecules onto a liquid of the same composition. 

Table 20 suggests that van der Waajs and repulsive forces contribute to the temperature coefficient of fiow (in cal./mol.) just as they do to the latent heat of sublimation in the latter instance the attractive forces, which are small, are predominant, but in the former the much larger repulsive forces are predominant. This provides a basis for the classification of activated diffusion processes into specific types (Hg-Pd) and non-specific types (He-SiOg), the basis being the nature of the... 

Their classification is nsually based on their cross-linking nature, which can be (i) of a physical type, that is, involving noncovalent attractive forces which are often hydrophobic forces, hydrogen bonding, and ionic interactions and (ii) of a chentical type, that is, using covalent bonds as leticn-lating nodes. These latter have thus a permanent stmctuie nnless chemically labile connections have been introdnced in the network. Another possible classification is related... 

Following a somewhat arbitrary - but convenient - classification of the attractive forces into physical and chemical ones, we will discuss them separately. In each case, through theoretical considerations and an Example, we will try to identify the relationship between these forces and macroscopic behavior. 

This sophisticated picture is reflected by the many test procedures dealing with degradation or biodegradation that are published by different national, international, or industry-driven organisations (e.g. ISO, ASTM). The aim of all these efforts is to obtain comparable data on the behaviour of the polymer under consideration, but a driving force is also the marketing need to present an attractive classification and labelling for the polymer product. 

Simple physical considerations enable us to understand, qualitatively, how weak forces of attraction may operate between saturated molecules, and lead to a system of classification as follows ... 

When secondary complications are surely eliminated, it is worth while to examine the possibilities of solvation in comparing reaction rates in various solvents and in the gas phase. An important classification of different types of solvation has been made recently by Moelwyn-Hughes and Sherman8 emphasizing the different kinds of electrostatic attractions. Quantum mechanical calculations are satisfactory in explaining strictly homopolar bonds but electrostatic forces are important not only in electrolytic solutions but also in many reactions which are not considered to be ionic. 

A preliminary classification of the weak forces which hold together well-separated units such as the individual molecules in ice or napthalene has already been made (p. 82). The strongest of these are often termed electrostatic, signifying that the forces can be attributed to an interaction of the unmodified, static charge distributions of the separate systems. Examples are the ion-dipole bonds which occur in hydrates and ammoniates. These are usually weaker than ordinary covalent bonds and relatively easily break on heating. The binding force arises (Fig. 62) from the attraction between the... 

Instead of following conventional classifications, it would be reasonable to classify the intermolecular forces into three categories on the basis of their origin [2]. The first is the forces caused by the electronic polarization, i.e. van der Waals attraction, such as London dispersion (electronic polarization-electronic polarization) and Debye interaction (dipole-dipole-induced electronic polarization). The second is the forces caused by the electrostatic charges and/or the dipoles of the molecule these forces are based on the molecular structure and are independent of electronic polarizability. And the last category is the forces caused by exchange of elemental particles, such as an electron (covalent bond) and a proton (hydrogen bond). 

Traditionally, the classification of crystals includes an important category the ionic crystals. These crystals are composed of ions, and the cohesion arises from the balance between the attractive coulombian forces and short-distance repulsive interactions, which prevents the collapse of the crystal This category includes the alkali halides, all crystals possessing a similar structure (MgO, CaO), the oxy-salts (carbonates, nitrates, sulfates, silicates). Sometimes even corundum is included in this categoryI... 

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