Electrostatic ionisation

Many anionic dyes (section 1.6) depend on their sulphonic acid groups for their solubility in water. Dye sulphonic acids have pK values within the range of pH 1-2 and are fully ionised under dyeing conditions as either the free acid or the sodium salt. The mutual electrostatic repulsion between dye sulphonate anions ensures their uniform separation and distribution in dilute aqueous solution. At higher concentrations, however, this repulsion is counterbalanced by mutually attractive forces of various kinds operating at shorter range [3] ... 

Figure 16.6—Linear time of flight (TOF) and principle of the reflectron. 1) Sample and sample holder 2) MALDI ionisation device 3 and 3 ) extraction and acceleration grid (5 000 V potential drop) 4) control grid 5) multichannel collector plate 6) electron multiplier 7) signal output. The bottom figure shows a reflectron, which is essentially an electrostatic mirror that is used to time-focus ions of the same mass, but which have different initial energies. This device increases resolution, which can attain several thousand.

The chief merit of the ionic theory is the ease with which quantitative calculations can be performed within a simple electrostatic approach. These will be discussed in Chapter 5. Even if it could be conclusively proven that crystals such as NaCl do not contain ions, chemists would be reluctant to discard the ionic description. The model even has successes-and this should arouse suspicion - in situations where few chemists would seriously entertain the real existence of ions. For example, the 4d and 5d elements tend to exhibit high oxidation states more readily than their counterparts in the 3d series. This can be explained if we note (see Section 4.3) that the energies required to attain highly ionised states are much smaller for the heavier atoms, provided that we are prepared to believe that compounds like WF6, Re03 and 0s04 are ionic ... 

The hydrophilic part of the most effective soluble surfactants (e.g. soaps, synthetic detergents and dyestuffs) is often an ionic group. Ions have a strong affinity for water owing to their electrostatic attraction to the water dipoles and are capable of pulling fairly long hydrocarbon chains into solution with them for example, palmitic acid, which is virtually un-ionised, is insoluble in water, whereas sodium palmitate, which is almost completely ionised, is soluble (especially above its Krafft temperature - see page 93). 

Ionic bonding occurs between molecules which have opposite charges and it involves an electrostatic interaction between the two opposite charges, the functional groups that most easily ionise are amines and carboxylic acids ... 

The origin of the electrostatic force is the surface charge that solid particles acquire when they are immersed in a liquid that contains a sufficient amount of ions. Possible charging mechanisms are ionisation, ion adsorption and ion dissolution, which are now discussed. 

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