Positively Charged Surfaces

Fig. 5. Variation of ion concentrations, C (-), and effective surface potential, / (-), with distance from a positively charged surface. The layer of...

Another class of DNA-binding proteins are the polymerases. These have a nonspecific interaction with DNA because the same protein acts on all DNA sequences. DNA polymerase performs the dual function of DNA repHcation, in which nucleotides are added to a growing strand of DNA, and acts as a nuclease to remove mismatched nucleotides. The domain that performs the nuclease activity has an a/P-stmcture, a deep cleft that can accommodate double-stranded DNA, and a positively charged surface complementary to the phosphate groups of DNA. The smaller domain contains the exonuclease active site at a smaller cleft on the surface which can accommodate a single nucleotide. 

Microbial cells transported with the stream of fluid above the surface interact with conditioning films. Immediately after attachment, microorganisms initiate production of slimy adhesive substances, predominantly exopolysaccharides (EPS) that assist the formation of microcolonies and microbial films. EPS create bridges for microbial cells to the substratum and permit negatively charged bacteria to adhere to both negatively and positively charged surfaces. EPS may also control interfacial chemistry at the mineral/biofilm interface. 

The end points of precipitation titrations can be variously detected. An indicator exhibiting a pronounced colour change with the first excess of the titrant may be used. The Mohr method, involving the formation of red silver chromate with the appearance of an excess of silver ions, is an important example of this procedure, whilst the Volhard method, which uses the ferric thiocyanate colour as an indication of the presence of excess thiocyanate ions, is another. A series of indicators known as adsorption indicators have also been utilized. These consist of organic dyes such as fluorescein which are used in silver nitrate titrations. When the equivalence point is passed the excess silver ions are adsorbed on the precipitate to give a positively charged surface which attracts and adsorbs fluoresceinate ions. This adsorption is accompanied by the appearance of a red colour on the precipitate surface. Finally, the electroanalytical methods described in Chapter 6 may be used to scan the solution for metal ions. Table 5.12 includes some examples of substances determined by silver titrations and Table 5.13 some miscellaneous precipitation methods. Other examples have already been mentioned under complexometric titrations. 

At low pH a positively charged surface prevails at high pH, a negatively charged surface is formed. At some intermediate pH the net surface charge will be zero. 

The slope of unity in region I indicates that the anionic surfactant adsorbs as individual ion through electrostatic interaction with the positively charged surface. 

Having chosen a particular model for the electrical properties of the interface, e.g., the TIM, it is necessary to incorporate the same model into the kinetic analysis. Just as electrical double layer (EDL) properties influence equilibrium partitioning between solid and liquid phases, they can also be expected to affect the rates of elementary reaction steps. An illustration of the effect of the EDL on adsorption/desorption reaction steps is shown schematically in Figure 7. In the case of lead ion adsorption onto a positively charged surface, the rate of adsorption is diminished and the rate of desorption enhanced relative to the case where there are no EDL effects. 

Effects of Pentavalent Sb on the Adsorption of Divalent Co-57. The emission Mossbauer spectra of divalent Co-57 adsorbed on hematite with pentavalent Sb ions (Figure 8) are complex and we have not yet succeeded in their analysis. It is certain, however, from the spectra that trivalent Fe-57 ions produced by the EC decay of Co-57 are interacting magnetically with the ferric ions of the substrate. This means that the divalent Co-57 are not adsorbed on the pentavalent Sb ions, but on hematite directly. The [Sb(OH)g]- anions are considered to facilitate direct adsorption of divalent Co-57 ions on the positively charged surfaces of hematite in the acidic region. 

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