Charge neutralisation

X-ray Photoelectron Spectroscopy analysis of the samples was performed with a Surface Science Instruments spectrometer (SSI 100) with a resolution (FWHM Au 4f7/2) of 1.0 eV. The X-ray beam was a monochromatised AlKa radiation (1486.6 eV). A charge neutraliser (flood gun) was adjusted at an energy of 6 eV. As the Cls spectra of these compounds were very complex, the binding energies were referenced to the binding energy of Ols, considered experimentally to be at 531.8 eV [8). 

Firstly charge density despite the fact that polyelectrolytes of comparable molecular weight are adsorbed in a flatter conformation and thus to a lesser extent as their charge density increases (see above), charge neutralisation is still achieved at a lower addition level for high than for low charge density polyelectrolytes (Figure 6.13). 

This type of mechanism is likely to be partly operative in systems containing inorganic electrolytes as, for example, in the case of aluminium species. Some polyelectrolytes may also induce flocculation by charge neutralisation but the adsorbed polymer may also be able to bridge from one particle surface to another ( polymer bridging ). 

Charge neutralisation is characterised by the isoelectric point (point of zero charge) being coincident with the point of optimum flocculation (minimum turbidity) — (Figure 7.4). A special case of... 

Figure 7.4 Typical flocculation and mobility behaviour for flocculation by charge neutralisation.

Figure 7.5 Typical flocculation, mobility and adsorption data for flocculation by the patch charge neutralisation mechanism.

High shear forces are prevelant in the approach flow system to the paper machine (i.e. as the fibre suspension approaches the point of deposition on the wire), and these have a large impact upon the efficiency of retention aids (Figure 7.8). A study of the effect of shear can often be helpful in establishing the mechanism of retention. Bridging flocculation is irreversibly sensitive to shear (i.e. when the shear forces are removed the suspension does not reflocculate) whereas charge neutralisation is reversibly sensitive to shear. 

The activation energies calculated for the two steps of the above reaction are + 160 kJ/mol for the ki step and -l- 78 kJ/mol for the k2 step [15]. The overall enthalpy of reaction is — 78 kJ/mol. It has been found that the half-life for the ki reaction is sensitive to the counterion concentration in case of SDS micelles. The effect of added counterion may be due to the charge neutralisation of the sulphate anion heads in the SDS micellar Stern layer, to facilitate approach and penetration of the CN- ions at the micelle-water interface. Hemin encapsulated in CTAB micelles reacts much faster with cyanide compared to that in SDS presumably because of the cationic Stern layer in CTAB. The... 

On the assumption that at a glass-electrolyte interface hydroxyl ions are adsorbed, the fraction of the original charge neutralised by adsorption can be calculated from the above equations as follows. From (a) and (b)... 

Early theories of the radiolysis of liquid water usefully illustrate two extreme possibilities. Samuel and Magee11 estimated that a 10 eV electron would travel approximately 20 A in 10 13 sec before being thermalised, after which, as it would still be effectively within the electrostatic field of the positive ion, charge neutralisation would take place to give an excited molecule. 

The suppression of polymerisation by 02 and benzoquinone was due to the fact that these species captured a significant fraction of the secondary electrons produced and not to their ability to scavenge free radicals. The negative ions thus produced underwent charge neutralisation with (CH3)3C+ thus reducing the yield of polymerisation initiators. In confirmation of the fact that ionic rather than free radical processes were important was the observation that the production of free radicals within the solution by the photolysis of diacetyl failed to initiate polymerisation. Spectroscopic evidence for the existence of (CH3)3C+ in the pulse radiolysis of isobutene was obtained subsequently31. 

It is probable that all scavengers may capture electrons, which in the absence of scavenger would not escape the parent positive ions (G = 2-3), in addition to those electrons which have escaped the positive ion (G a 0.1). Whereas the products of electron capture by most solutes still undergo charge neutralisation with neighbouring positive ions, it is assumed that N20 decomposes before charge neutralisation can take place. 

It is clear that in the gaseous phase the contribution from excited molecule reactions does not exceed that from ion-molecule reactions. This is probably to be attributed to the fact that the electron may more readily escape its parent ion in the gaseous phase and consequently the production of excited states by charge neutralisation is diminished. It would be interesting to examine the effect of let on the relative yields of these processes. 

If charge neutralisation is the predominant destabilisation mechanism, then there should be a stoichiometric relationship between the particle concentration... 

The colloid, as usually prepared, is electro-positive in character, and may be precipitated from solution by electrolysis, by the addition of small quantities of electrolytes, or by the action of an oppositely charged colloid, such, for example, as (negative) arsemous sulphide, whereby the two electrical charges neutralise each other.7 The smallest quantities of a few electrolytes required to precipitate colloidal ferric hydroxide from solution are given in the following table —8... 

It is now agreed, that for the most part, the radiolysis of polar liquids such as alcohol, water, etc. leads to the formation of ions and subsequently radicals rather than excited states. However, the picture is changed as the polarity of the liquid decreases, and in the extreme case of arenes such as benzene only excited states both singlet and triplet are observed. In other liquids such as alkanes, dioxane, etc. both ions and excited states are observed, charge neutralisation of the ions also giving excited states. 

Scavenging Studies. The solvent excited states produced by radiolysis of aromatic liquids could be produced directly, or formed, via charge neutralisation of solvent ions. The low oscillator strengths of the first and second excited states of benzene, toluene and p-xylene preclude direct excitation into these states. However, the third excited state could be excited with a yield as high as unity. 

By comparison with proteins, nucleic acids are much less sensitive to irreversible denaturation. Like proteins, nucleic acids are polar molecules which can be precipitated by charge neutralisation (by varying the pH) or by reducing the water activity by addition of organic solvents, etc. (Maniatis et al. 1989 Ausubel et al. 1989 Harwood 1996). 

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