Point of zero net charge

Effect of pH The pH of a solution affects the solubilization characteristics of a protein primarily in the way in which it modifies the charge distribution over the protein surface. At pH values below its isoelectric point (pi), or point of zero net charge, a protein acquires a net positive charge, while above its pi the protein will be negatively charged. Thus, if eleetro-static interactions are the dominant factor, solubilization should be possible only with anionic surfactants at pH values less than the pi of the protein because at values above pi, eleetrostatic repulsion would inhibit solubilization. The opposite effeet would be anticipated in the case of cationie surfactants. 

The electrophoretic mobility of a protein solution may also be measured as a function of pH. By this technique it may also be observed that the colloid passes through a point of zero net charge at which its mobility is zero. The point at which charge reversal is observed electrophoretically is called the isoelectric point. 

However, proteins are almost always charged (Dello Orco et al., 2005), for otherwise they would drop out of the solution, that is, phase separate macroscopically. Indeed, this is what usually happens near the isoelectric point of the proteins, i.e. their point of zero net charge. If two proteins bind to become part of an assembly, charged patches on them get on average closer together and hence repel each other when of the same sign. In other words, Coulomb repulsion between the proteins... 

The term pHznc in Equation 3.24 represents the point of zero net charge (PZNC). It is the pH value at which the cation exchange capacity equals the anion exchange capacity (Fig. 3.28). Equation 3.24 shows that pHznc varies with ionic strength (n), whereas pH0 or PZC is an intrinsic property of the mineralogically heterogeneous soil (Uehara and Gillman, 1980). 

This is often referred to as the point of zero net charge. 

Figure 3.19, Effect of ionic strength, /, on the point of zero net charge (PZNC) (a) and the point of zero net proton charge (PZNPC) (b) of a soil containing both permanent (P) and variable (V) charge minerals.

The point of zero net charge (PZNC) is the pH value of a soil solution at which the difference CEC - AEC equals zero. This difference is proportional to either an optimal or a nonoptimal value of the intrinsic surface... 

Figure 4 Hypothetical structure of an Immobiline gel and mechanism of the focusing process. The acrylamide acid and basic groups are shown grafted onto the polyacrylamide matrix. Two proteins are shown migrating in the gel at the times f = 0, at f = 1 and finally at the steady state, where they reach their respective pi values (pli and pla) as points of zero net charge.

Pressure of gas i Point of zero net charge Point of zero net proton charge Point of zero net intrinsic charge Point of zero salt effect Isoelectric point Dipolar moment of species i... 

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