The Nature of Ionic Interactions

If we consider two point charges of equal but opposite unitary charge at a separation of 2.5 x 1CH° m, the attractive force is equal to 3.7 x 10 9N. 

This is, of course, an aspect of the question that Pauling addressed and that led to the definition of the electroneutrality principle. If we consider the interaction of six water molecules with a Cr3+ ion, we may write two limiting valence bond structures. One of these forms localises the positive charge on the metal centre and depicts a Cr3+ ion surrounded by six water molecules. The covalent representation places a single positive charge on each of the water oxygen atoms and a —3 charge on the chromium centre (Fig. 2-13). 

The truth lies somewhere between these two extremes, and the assessment of covalent and ionic contributions to bonding has attracted considerable attention. Pauling devised the electroneutrality principle and suggested that the relative importance of ionic and covalent components was such that the overall charge on any one atom did not exceed 1. In the context of the [Cr(H20)6]3+ ion, this leads to a description in which the metal centre is neutral and each oxygen bears a charge of + 2. The Cr-0 bonds may be described as 50 % covalent. 

Various semi-empirical relationships have been developed to describe the ionic potential actually observed by the valence electrons in an A-B bond. An example of such a relationship is shown in Eq. (2.3). 

The exponential term is a Thomas-Fermi screening factor which accounts for the screening by the core electrons. Direct measurement of the ionic character of a bond is a complex operation. In principle, a number of techniques such as X-ray or neutron diffraction, nmr, photoelectron or Mossbauer spectroscopy provide information about electron distribution and charge density in practice the results are usually far from unambiguous. 

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Due to the nature of ionic interaction between gelatin and ILs we also verify that hydrogen bonding played an important role on the mechanism of interaction between IL and gelatin. In fact Table 1 shows a selection of cation/anion combinations that provided the formation of either Ion Jelly or solid structures, l-ethyl-3-methyl-imidazolium [emim], l-butyl-3-methyl-imidazolium [bmim], l-butyl-3-methyl-imidazolium [bdmim], l-decyl-3-methyl-imidazolium [Ciomim] and tri-n-octyl-methylammonium (Aliquat336 ). 

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