Ionic attraction

In principle, simulation teclmiques can be used, and Monte Carlo simulations of the primitive model of electrolyte solutions have appeared since the 1960s. Results for the osmotic coefficients are given for comparison in table A2.4.4 together with results from the MSA, PY and HNC approaches. The primitive model is clearly deficient for values of r. close to the closest distance of approach of the ions. Many years ago, Gurney [H] noted that when two ions are close enough together for their solvation sheaths to overlap, some solvent molecules become freed from ionic attraction and are effectively returned to the bulk [12]. 

Acid Leveling Dyes. These are molecular dispersions at low temperatures (tme solutions) and are simple molecules. They have low affinity at neutral pH and exhibit ionic attraction at acidic pH when the wool becomes charged. They exhibit good leveling and migration behavior. Their low affinity however also results in low fastness. 

These dyes are not very commercially important, and the dyeing mechanism has been described in detail elsewhere (15,25). The difficulty in applying fiber-reactive dyes to wool is the result of the same reactions already described. They are negatively charged and the wool is positively charged so ionic attraction exists. The fiber-reactive dyes are essentiaUy acid leveling or milling dyes and so this attraction can be controUed by pH. Once the dye is fixed no... 

Also important for stabilizing a protein s tertiary stmcture are the formation of disulfide bridges between cysteine residues, the formation of hydrogen bonds between nearby amino acid residues, and the presence of ionic attractions, called salt bridges, between positively and negatively charged sites on various amino acid side chains within the protein. 

Salt bridge (Section 26.9) The ionic attraction between two oppositely charged groups in a protein chain. 

A limited number of elements form ionic compounds. As we describe in the next two chapters, most substances contain neutral molecules rather than charged ions. The trends in ionization energies and electron affinities indicate which elements tend to form ions. Ionic compounds form when the stabilization gained through ionic attraction... 

The precise nature of the adhesion of the polyelectrolyte cements to untreated dental enamel and dentine has yet to be established. The earliest theory was due to Smith (1968) who speculated that the polyacrylate chains of the cement formed a chelate with calcium ions contained in the hydroxyapatite-like mineral in enamel and dentine. Beech (1973) considered this unhkely since it involved the formation of an eight-membered ring. Beech studied the interaction between PAA and hydroxyapatite, identified the formation of polyacrylate and so considered that adsorption was due to ionic attraction. 

Electrolytes are used to promote the exhaustion of direct or reactive dyes on cellulosic fibres they may also be similarly used with vat or sulphur dyes in their leuco forms. In the case of anionic dyes on wool or nylon, however, their role is different as they are used to facilitate levelling rather than exhaustion. In these cases, addition of electrolyte decreases dye uptake due to the competitive absorption of inorganic anions by the fibre and a decrease in ionic attraction between dye and fibre. In most discussions of the effect of electrolyte on dye sorption, attention is given only to the ionic aspects of interaction. In most cases, this does not create a problem and so most adsorption isotherms of water-soluble dyes are interpreted on the basis of Langmuir or Donnan ionic interactions only. There are, however, some observed cases of apparently anomalous behaviour of dyes with respect to electrolytes that cannot be explained by ionic interactions alone. 

Another method for generating an epoxidation catalyst on a solid support is to simply absorb or non-covalendy attach the catalyst to the solid support <06MI493>. Epoxidation of olefin 6 with mCPBA and catalyst 8 provides 7 in quantitative yields and with 89% ee. The immobilization of 8 on silica gel improves the enantioselectivity of the reaction providing 7 with 95% ee. Recycling experiments with silica-8 show a decrease in both yield and the enantiomeric excess for each cycle (45% ee after 4 cycles). This is attributed to a leaching of the catalyst from the silica gel. Two other solid supports, a Mg-Al-Cl-LDH resin (LDH) and a quaternary ammonium resin (Q-resin) were also examined. It was expected that ionic attraction between 8 and the LDH or Q-resin would allow the catalyst to remain immobilized through multiple cycles better than with silica gel. Both of these resins showed improved catalytic properties upon reuse of the catalyst (92-95% ee after 4 cycles). 

A number of different molecular mechanisms can underpin the loss of biological activity of any protein. These include both covalent and non-covalent modification of the protein molecule, as summarized in Table 6.5. Protein denaturation, for example, entails a partial or complete alteration of the protein s three-dimensional shape. This is underlined by the disruption of the intramolecular forces that stabilize a protein s native conformation, namely hydrogen bonding, ionic attractions and hydrophobic interactions (Chapter 2). Covalent modifications of protein structure that can adversely affect its biological activity are summarized below. 

Donor-acceptor interactions also lead to strangely bent geometries in heavier F— M—F alkaline earth fluorides. Such bending can occur when strong ionic attractions force a filled fluoride pz orbital into proximity with an orthogonal metal pj orbital, for in this case symmetry-forbidden (pz)p —(p l)m interactions can turn on only when the strict a/n symmetry of a linear F—M—F arrangement is broken. 

The binding energies in Table 4.44 show the expected strong preference for anionic over neutral ligands in complexes of the metal cation. However, the geometries and other properties of these complexes reflect strong covalency effects (albeit enhanced by net ionic attraction) that will principally be considered. 

Since cations and anions have opposite charges, is negative. The force between two anions will yield a positive value of . We see how a positive value of implies an inter-ionic repulsion and a negative value implies an inter-ionic attraction. 

Since the stationary phase is basically nonpolar in RPC, it is not expected to effect solute retention by ionic attraction, hydrogen bonding, formation of charge transfer complexes, or by any of the other strong non-covalent interactions familiar to the chemist. The only attractive force between the stationary phase and the eluite would seem to be van der Waals forces. However, these farces also act between the eluite and the mobile phase so that the net effect is generally not sufficien to account for the strong retention often observed with nonpolar compotinds in RPC. 

For water at 25 °C, e (which is dimensionless) is 78.5, and for the very nonpolar solvent benzene, e is 4.6. Thus, ionic interactions are much stronger in less polar environments. The dependence on r2 is such that ionic attractions or repulsions operate only over short distances—in the range of 10 to 40 run (depending on the electrolyte concentration) when the solvent is water. 

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