Ion trap effect

The activation energy for oxide ion conduction in the various zirconia-, thoria- and ceria-based materials is usually at least 0.8 eV. A significant fraction of this is due to the association of oxide vacancies and aliovalent dopants (ion trapping effects). Calculations have shown that the association enthalpy can be reduced and hence the conductivity optimised, when the ionic radius of the aliovalent substituting ion matches that of the host ion. A good example of this effect is seen in Gd-doped ceria in which Gd is the optimum size to substitute for Ce these materials are amongst the best oxide ion conductors. Fig. 2.11. 

The response of the acids is surprising in that significant uptake is observed despite the fact that only a small proportion of the compound exists in the neutral form. This could be attributed to the fact that this is an open system and the compound is taken away by the circulation as soon as it crosses the membrane. An ion trap effect may be of more significance. For example, in the case of a weak acid (piifa = 3), when there is a pH differential across a membrane and only the neutral species moves across that membrane, under equilibrium conditions, it can be seen that ions are trapped in the compartment where the pH favors the production of the anion (i.e., where the pH > p fa)- Since the pH of the circulating... 

If the pH differs in two plant compartments separated by a membrane (e.g., vacuole, pH 5.5 cytoplasm, pH 7.5), then there will be different proportions of the nonionized and ionized forms of an acid in the two compartments. The nonionized form, for example, will cross the membrane more rapidly than does the anion, and so nonionized molecules moving from the low-pH compartment will be substantially ionized in the higher-pH compartment and the anions so produced can only escape slowly. This process is the basis of the ion trap effect, whereby weak acids can be substantially accumulated in plant cells of high pH. 

Figure 9.2. Accumulation of weak acids within cells by the ion-trap effect. (Reproduced from Ref. 12.)...

Weak acids. Uptake of acidic compounds by roots is very different from that of nonionized compounds in that it is strongly dependent on the pH of the bathing medium. For example, uptake of 2,4-D into barley roots over 24 h from nutrient solution was 36 times greater at pH 4.0 than at pH 7.0. This dependency can be explained by the ion-trap effect discussed in Section 9.2.3.3, whereby weak acids are accumulated in compartments of higher pH by virtue of the greater permeation rates across membranes of the undissociated form compared to the anion. 

The efficiency of transport in phloem is now regarded as being determined by two processes. The first of these is the extent to which compounds are accumulated in phloem cells relative to the mesophyll cells, for example, by the ion-trap effect in the case of weak acids (Section 9.2.3.3). The second factor is the extent to which compounds are retained in phloem cells during transport. 

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