Ions, absolute properties conductivity

AQPs are highly insulating to the electrochemical potential across the cell membrane by sharp selectivity that absolutely excludes conduction of all charged molecules, all ions including hydroxide and hydro-nium ions (Finkelstein, 1987), and protons. An important fundamental question is how this is accomplished, and structural data on GlpF and AQP-1 have given rise to suggested mechanisms for this (Mitsuoka et al., 1999 Fu et al., 2000). Molecular mechanics has been harnessed to identify, and then evaluate the contributions of each factor to this property (Tajkhorshid et al., 2002). 

In a perfect crystal, all atoms would be on their correct lattice positions in the structure. This situation can only exist at the absolute zero of temperature, 0 K. Above 0 K, defects occur in the structure. These defects may be extended defects such as dislocations. The strength of a material depends very much on the presence (or absence) of extended defects, such as dislocations and grain boundaries, but the discussion of this type of phenomenon lies very much in the realm of materials science and will not be discussed in this book. Defects can also occur at isolated atomic positions these are known as point defects, and can be due to the presence of a foreign atom at a particular site or to a vacancy where normally one would expect an atom. Point defects can have significant effects on the chemical and physical properties of the solid. The beautiful colours of many gemstones are due to impurity atoms in the crystal structure. Ionic solids are able to conduct electricity by a mechanism which is due to the movement of fo/ 5 through vacant ion sites within the lattice. (This is in contrast to the electronic conductivity that we explored in the previous chapter, which depends on the movement of electrons.)... 

In many cases, heteropolar crystals conduct electric current through the motion of ions, and they can be electrolyzed by means of a sufficiently high voltage. Even when, in certain ranges of component activities, electronic partial conductivity predominates in an ionic crystal, its absolute value is always small in comparison with that of normal semiconductors or metals which will be discussed later. One final characteristic property should be mentioned Ionic crystals absorb strongly in the infrared by virtue of vibrations of the totality of the cations and anions in their sublattices. 

The changes in the film material with the current density, field, and temperature at which the films are made (in dilute, aqueous solution) and the changes which occur on subsequent annealing at temperatures of up to 200 C or so, represent large proportional changes in the concentration of mobile ions, that is, according to the usual theory, in the concentration of defects (interstitial metal ions and anion and cation vacancies). However, the absolute variations in terms of numbers of defects are probably small. Thus the variations in properties, which are not directly functions of defect concentrations, such as refractive index and density, are of the order of 1 %— not large for ordinary formation conditions—whereas the variations in those properties, such as ionic conductivity, electronic conductivity, and rate of dissolution in which are directly dependent on the concentra-... 

A great difficulty when dealing with electrolytes is to ascribe individual properties to individual ions. Individual thermodynamic properties cannot be determined, only mean ion quantities being measurable. Interionic and ion-solvent interactions are so numerous and important in solution that, except in the most dilute cases, no ion may be regarded as behaving independently of others. On the other hand, there is no doubt that certain dynamic properties such as ion conductances, mobilities and transport numbers may be determined, although values for such properties are not absolute but vary with ion environment. 

This approach will not be practical for some time to come. The fundamental properties of surfactants (micelle formation, enrichment at interfaces) mean that the activity of a surfactant will usually differ from its absolute concentration (1). Just as serious is the technical problem that current surfactant-selective electrodes suffer from response which varies with their past and recent history they are also sensitive to the concentration of nonsurfactant ions. The result is that quantitative applications use electrodes not in direct measurements relating potential to concentration, but as indicators of the end point of a titration. In this latter application, it is not important that the electrode potential be exactly reproducible, but only that the potential change sharply as the surfactant concentration changes. For the titration of an anionic surfactant with a cationic surfactant, the electrode used for end point detection can be chosen to respond to either surfactant. Because of the drift in electrode potential, titrations must be conducted to an inflection in the titration curve rather than to a specific millivolt value. Details of the potentiometric titration methods can be found earlier in this chapter. The electrodes have also been demonstrated as detectors for flow injection analysis. 

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