Electrical term, chemical

Although polyacetylene has served as an excellent prototype for understanding the chemistry and physics of electrical conductivity in organic polymers, its instabiUty in both the neutral and doped forms precludes any useful appHcation. In contrast to poly acetylene, both polyaniline and polypyrrole are significantly more stable as electrical conductors. When addressing polymer stabiUty it is necessary to know the environmental conditions to which it will be exposed these conditions can vary quite widely. For example, many of the electrode appHcations require long-term chemical and electrochemical stabihty at room temperature while the polymer is immersed in electrolyte. Aerospace appHcations, on the other hand, can have quite severe stabiHty restrictions with testing carried out at elevated temperatures and humidities. 

Separation into chemical and electrical terms is possible with gradients but not with quantities, i.e., p and < >, themselves. The reason is simple. The electrochemical potential p was only conceptually separated into a chemical term p and an electrical term z F< >. The conceptual separation was based on thought experiments in practice, no experimental arrangement can be devised to correspond to the thought experiment described in Section 6.3.13.1, Thus, e.g., one cannot switch off the charges and dipole layer at the surface of a solution as one can switch off the externally applied field in a transport experiment Only the combined effect of lj and ZjFij) can be determined. 

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Secondly, selectivity is not always achievable. For example, permselectivity of ion-exchanging polymer films fails at high electrolyte concentration. We have shown that even if permselectivity is not thermodynamically found, measurements on appropriate time scales in transient experiments can lead to kinetic permselectivity. To rationalise this behaviour we recall that the thermodynamic restraint, electrochemical potential, can be split into two components the electrical and chemical terms. These conditions may be satisfied on different time scales. Dependent on the relative transfer rates of ions and net neutral species, transient responses may be under electroneutrality or activity control. 

The effects of DBS on the cortex-basal-ganglia-thalamus-cortex motor loop appear to be more complex than initially believed. The paradox of DBS is that electrical stimulation of brain tissue (which presumably induces brain activation), has a similar effect as that of a surgical lesion of that same structure (which effectively destroys brain tissue). These two realities are hard to reconcile. As indicated by [64] the ultimate elucidation of this paradox depends on the nature of the complex and interactive neural connections in the brain that communicate through electrical and chemical processes. There is an emerging view that DBS has both excitatory and inhibitory effects on how brain circuits communicate with one another depending on the distance from the electrode, the cell structures activated and the direction of the activation (ortho- versus anti-dromic). The effect appears to modulate the activity of a network as well as neural firing patterns. Long term effects on neurotransmitters and receptor systems cannot be excluded [64]. 

Dividing the electrochemical potentials formally into their chemical and electrical terms according to (4), and separating the rf into those present in the metallic and the aqueous phases, (14) becomes... 

Equation (6.119) indicates that the chemical work in electrolytes contains a chemical term fidNj and an electrical term ZjFipdNj and the sum is called the electrochemical potential jlI of the ionic species i... 

Figure 2-7. Contributions to the chemical po tential of species j, w (Eq. 2.4). The related fluxes are discussed throughout the text. Note that the Nemst-Planck equation (Eq. 3.8 Chapter 3, Section 3.2A) involves both the concentration term and the electrical term.

The influence of electrical potential on the chemical potential of an ion is expressed by the term ZjFE in Equation 2.4, where Zj is an integer representing the charge number of species /, F is a constant known as Faraday s constant (to be considered in Chapter 3, Section 3.1 A), and E is the electrical potential. Because water is uncharged (zw = 0), the electrical term does not contribute to its chemical potential. However, electrical potential is of central importance when discussing ions and the origin of membrane potentials both of these are examined in detail in Chapter 3 (e.g., Section 3.ID), where we explicitly consider the ZjFE term. 

Many solute properties are intertwined with those of the ubiquitous solvent, water. For example, the osmotic pressure term in the chemical potential of water is due mainly to the decrease of the water activity caused by solutes (RT In aw = —V ri Eq. 2.7). The movement of water through the soil to a root and then to its xylem can influence the entry of dissolved nutrients, and the subsequent distribution of these nutrients throughout the plant depends on water movement in the xylem (and the phloem in some cases). In contrast to water, however, solute molecules can carry a net positive or negative electrical charge. For such charged particles, the electrical term must be included in their chemical potential. This leads to a consideration of electrical phenomena in general and an interpretation of the electrical potential differences across membranes in particular. Whether an observed ionic flux of some species into or out of a cell can be accounted for by the passive process of diffusion depends on the differences in both the concentration of that species and the electrical potential between the inside and the outside of the cell. Ions can also be actively transported across membranes, in which case metabolic energy is involved. 

Such a unit, consisting of Avogadro s number of electronic charges (i.e., 1 mole of single, positive charges), is called Faraday s constant, F. This quantity, which appears in the electrical term of the chemical potential (Eq. 2.4), equals 9.65 x 104 C mol-1 or 9.65 x 104 J mol-1 V-1. 

To illustrate the rather small contribution that the pressure term, VjP, makes to differences in the chemical potential of a charged substance across a membrane, we will compare VjAP with the contribution of the electrical term, ZjFAE. We will use a typical electrical potential difference (AE) across a biological membrane of 100 mVand a hydrostatic pressure difference (AP) of 0.5 MPa (= 0.5 x 106 Pa = 0.5 x 106 N m 2 = 0.5 x 106 J m 3), and we... 

As indicated in Chapter 2 (Section 2.2B), the terms in the chemical potential can be justified or derived by various methods. The forms of some terms in i can be readily appreciated because they follow from familiar definitions of work, such as the electrical term and the gravitational term. Hie comparison with Fick s first law indicates that RT In a, is the appropriate form for the activity term. Another derivation of the / Tln a, term is in Appendix IV, together with a discussion of the pressure term for both liquids and gases. Some of these derivations incorporate conclusions from empirical observations. Moreover, the fact that the chemical potential can be expressed as a series of terms that can be added together agrees with experiment. Thus a thermodynamic expression for the chemical potential such as Equation 2.4 does the folio whig (1) summarizes the results of previous observations, (2) withstands the test of experiments, and (3) leads to new and useful predictions. 

The electrical term in the chemical potential of H+ can also power ATP formation. For instance, when an EM of 0.16 V is artificially created across lamellar membranes, ATP formation can be induced in the dark. This is consistent with our prediction that an electrical potential difference of at least 0.13 V is necessary (Fig. 6-6). In chloroplast thylakoids, EM in the light is fairly low, e.g., near 0.02 V in the steady state (see Fig. 6-5). However, the electrical term can be the main contributor to A/xh for the first 1 or 2 seconds after chloroplasts are exposed to a high photosynthetic photon flux (PPF). The electrical component of the H+ chemical potential difference can be large for the chromatophores of certain photosynthetic bacteria such as Rhodopseudomonas spheroides, for which Em can be 0.20 V in the light in the steady state. 

Thus the H+ chemical potential is higher in the lumen than in the matrix (Fig. 6-6). For some cases in which mitochondrial ATP formation occurs, EM is 0.14 V and pH° - pH1 is 0.5, in which case 16 kJ (mol H+) 1 is available for ATP formation from the chemical potential difference of H+ across the inner mitochondrial membrane (Fig. 6-6). We indicated previously that at least 13 kJ per mole H+ is required for ATP formation if four H+ s are used per ATP synthesized. We also note that for chloroplasts, most of the Ais due to the pH term, whereas for mitochondria the electrical term is usually more important for ATP formation. 

Synapses are electrical or chemical communicative contacts between neurons. Electrical synapses (neuronal gap junctions) function by the propagation of electrical impulses from one cell to another (and vice versa) via direct, physical contact. As a consequence, these synapses are characterized by a relatively simple organization of membrane structure and associated organelles (Zoidl et al. 2002). Electrical synapses are also less mutable, in terms of their function and molecular characteristics, and thus exhibit little of the plasticity that typifies the chemical synapse. 

Note that jli, Eq. 10, includes the species standard chemical potential, an activity term, and an electrical energy term. The electrical term is composed of the electrical work required to bring the molar charge ZiF on a given ionic species from infinity into the species phase, and (]) is the standard iimer potential or work function of the phase in question (e.g., that of the metal, ( )m, or of a particular ion in solution, 0s) (Ref. 21, p. 20). 

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