Isoelectric solution, definition

The isoionic point, for a protein solution not too low in concentration, is experimentally defined as that pH which does not change when a small amount of pure protein is added to the solution (46). This definition also applies to a protein which has been focused isoelectrically (7,26). If more protein is added to the zone, the pH does not change. The buffering at the site of focusing is considered to be completely dominated by the... 

Amphoteric surfactants by definition are chimeric, exhibiting anionic character in alkaline solution, nonionic character near their isoelectric point, and cationic character in acidic solution [73]. As a result of their complex charge characteristics, their interactions with interfaces must be examined iudividually and as a function of pH. For example, the adsorption of alkyl betaines firom solution onto wool keratin is much greater at acidic than alkalide pH values. Although amphoteric surfactants are used extensively to improve the cosmetic attributes of many consmner products [74], their interactions with skin have received little attention. 

It is possible to create a pH gradient in the solution of concern in the separation system by external means. At the location where pH has the isoelectric value for the ith protein species, Z, will be zero, leading to a zero external electrical force on the ith species. At other locations, the force will be nonzero. The forces on molecules or ions in solution due to an externally imposed primary field (i.e. electrical field) can then be suitably altered by the imposition of additional property gradients in the solution by external means. If we have macroscopic particles instead of molecules or macromolecules, the driving force per particle may be obtained from definition (3.1.8) simply by replacing Z, by Qp, the net particle charge in coulomb. 

It should be noted that the equilibrium conditions, Eqs. (2) and (6b), have the same form whether there is adsorbed surface charge or not. The magnitudes of (0) and Vc(0), however, will differ in these two cases because of the difference in the charge neutrality conditions, Eqs. (4) and (6a). An important consequence of the above consideration is that in the presence of adsorbed surface charge and constant x potential the space charge potentials Vs(0) and Fc(0) will not be zero at Cf/O), and in fact will not be zero at the same value of silver ion concentration in solution. The definition of isoelectric point therefore becomes ambiguous and it is necessary to define isoelectric point separately for each side of the interface corresponding to values of silver ion concentration in solution where F (0) = 0 or Fc(0) = 0. If only K (0) is measured, as in the electrophoresis experiment, the isoelectric point determined for F (0) = 0 does not ensure that Kc(0) is zero at the same time. 

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