Driving forces electrical potential differences

The fourth fully developed membrane process is electrodialysis, in which charged membranes are used to separate ions from aqueous solutions under the driving force of an electrical potential difference. The process utilizes an electrodialysis stack, built on the plate-and-frame principle, containing several hundred individual cells formed by a pair of anion- and cation-exchange membranes. The principal current appHcation of electrodialysis is the desalting of brackish groundwater. However, industrial use of the process in the food industry, for example to deionize cheese whey, is growing, as is its use in poUution-control appHcations. 

Galvanic Corrosion. Galvanic corrosion occurs when two dissimilar metals are in contact in a solution. The contact must be good enough to conduct electricity, and both metals must be exposed to the solution. The driving force for galvanic corrosion is the electric potential difference that develops between two metals. This difference increases as the distance between the metals in the galvanic series increases. 

The net electrochemical driving force is determined by two factors, the electrical potential difference across the cell membrane and the concentration gradient of the permeant ion across the membrane. Changing either one can change the net driving force. The membrane potential of a cell is defined as the inside potential minus the outside, i.e. the potential difference across the cell membrane. It results from the separation of charge across the cell membrane. 

Allelopathic inhibition of mineral uptake results from alteration of cellular membrane functions in plant roots. Evidence that allelochemicals alter mineral absorption comes from studies showing changes in mineral concentration in plants that were grown in association with other plants, with debris from other plants, with leachates from other plants, or with specific allelochemicals. More conclusive experiments have shown that specific allelochemicals (phenolic acids and flavonoids) inhibit mineral absorption by excised plant roots. The physiological mechanism of action of these allelochemicals involves the disruption of normal membrane functions in plant cells. These allelochemicals can depolarize the electrical potential difference across membranes, a primary driving force for active absorption of mineral ions. Allelochemicals can also decrease the ATP content of cells by inhibiting electron transport and oxidative phosphorylation, which are two functions of mitochondrial membranes. In addition, allelochemicals can alter the permeability of membranes to mineral ions. Thus, lipophilic allelochemicals can alter mineral absorption by several mechanisms as the chemicals partition into or move through cellular membranes. Which mechanism predominates may depend upon the particular allelochemical, its concentration, and environmental conditions (especially pH). 

The portion AQ = AH - AG = TAS of AH is transformed into heat. Ideal theoretical efficiencies % determined by the types and amounts of reactants and by the operating temperature. Fuel cells have an efficiency advantage over combustion engines because the latter are subdued to the Carnot limitation. High thermodynamic efficiencies are possible for typical fuel cell reactions (e.g., e,h = 0.83 (at 25°C) for H2 + I/2O2 -> H20(i)). The electrical potential difference between anode and cathode, = -AG/W(f, which is also called the electromotive force or open-circuit voltage, drives electrons through the external... 

The driving force for diffusion of C+ from the membrane to the aqueous solution is the favorable solvation of the ion by water. As C+ diffuses from the membrane into the water, there is a buildup of positive charge in the water immediately adjacent to the membrane. The charge separation creates an electric potential difference ( ou,cr) across the membrane. The free-energy difference for C+ in the two phases is AG = —nFE(Mcr, where F is the Faraday constant and n is the charge of the ion. At equilibrium, the net change in free energy for diffusion of C+ across the membrane boundary must be 0 ... 

In order to describe the presence of electrolytes in the system, the driving force due to electrical potential difference needs additionally to be taken into account [16]. Therefore, the gradient of the electrical potential is introduced into the generalized driving force dp. 

As electrolyte species are available in the system considered, the driving forces caused by the electrical potential differences must be taken into account [16]. The migration is described through the Nernst-Planck equation (Eq. (24)). This implies that the electroneutrality condition, Eq. (25), is satisfied. 

In 1808, Rous, a colloid chemist, observed that imposing an electric potential difference across a porous wet clay led not only to the expected flow of electricity, but also to a flow of water. He later applied hydrostatic pressure to the clay and observed a flow of electricity. This experiment displayed the electrokinetic effect and demonstrated the existence of coupled phenomena where a flow may be induced by forces other than its own driving force. Therefore, the electric current is evidently caused by the electromotive force, but it may also be induced by the hydrostatic pressure. When two... 

In this chapter we turn our attention to the properties of solutes. We will compare chemical potentials in the aqueous phases on the two sides of a membrane or across some other region to predict the direction of passive solute fluxes as well as the driving forces leading to such motion. We will also show how the fluxes of charged species can account for the electrical potential differences across biological membranes. 

The electrical potential is lower inside the cell than outside (E1 < E°)y as is also indicated in Figure 3-3. Hence, the electrical driving force on the positively charged K+ tends to cause its entry into the cell. At equilibrium these two tendencies for movement in opposite directions are balanced, and no net K+ flux occurs. As indicated previously, the electrical potential difference existing across a membrane when K+ is in equilibrium is the Nernst potential for K+>ENk. Generally, and axK for both plant and animal cells are... 

Fluxes of many different solutes occur across biological membranes. Inward fluxes move mineral nutrients into cells, while certain products of metabolism flow out of cells. The primary concern in this section is the passive fluxes of ions toward lower chemical potentials. First, we indicate that the passive flux density of a solute is directly proportional to the driving force causing the movement. Next, the driving force is expressed in terms of the relevant components of the chemical potential. We then examine the consequences of electroneutrality when there are simultaneous passive fluxes of more than one type of ion. This leads to an expression describing the electrical potential difference across a membrane in terms of the properties of the ions penetrating it. 

Membranes may be hastily classified according to the driving force at the origin of the transport process (1) a pressure differential leads to micro-, ultra-, nanofiltration, and reverse osmosis (2) a difference of concentration across the membrane leads to diffusion of a species between two solutions (dialysis) and (3) an electric potential difference applied to an ion-exchange membrane (lEM) leads to migration of ions through the membrane (electrodialysis, membrane electrolysis, and... 

The diffusion of charged ions is more complicated because of the law of electroneutrality, which states that the sum of the positive charges on each side of the membrane must equal the sum of the negative charges. In addition to the concentration gradient, the electrical potential difference determines the Bnal equilibrium of a substance across the membrane. Therefore at equilibrium, the concentration of an ionic species may be unequal across the membrane and this gradient will balance the electrical difference across the membrane. The driving force for transport in this situation is defined as the electrochemical potential. The Nemst equation describes the equilibrium situation for ions... 

The heat flow is directly proportional to the difference in temperature between the two surfaces. The driving force of temperature difference is analogous to the potential difference (voltage) in an electric circuit and so XmAm/5 is the thermal conductance and its inverse... 

The mass transfer in dense membranes takes place by diffusion in the free volume between the polymer chains of the membrane material. The external driving force for this process is a difference of the chemical potential A/r, of the permeating species i on either side of the membrane. This difference can be expressed as a concentration difference Ac, a partial pressure difference Ap or an electrical potential difference A . The transport mechanism involves three distinctive steps, (a) selective adsorption of the feed components to the membrane, the feed components are dissolved in the membrane material (b) diffusion of the dissolved species through the membrane, and (c) desorption of the permeating species at the permeate-side of the membrane assisted by an applied sweep gas or vacuum. 

These equations imply that metal ions tend to transfer from the solid across the interface to the solution due to a decrease in the GFE (i.e., Gm+ < Gmo ). They tend to transfer in the opposite direction as a consequence of the difference in potential between the two phases (i.e., (( ) + >( ) o ) These concepts are summarized in Fig. 2.2. This result leads to the brief generalization At equilibrium, the GFE driving force to transfer ions from the metal to the solution is exactly balanced by the electrical potential difference attracting the ions back to the metal. 

For membrane separation processes, only driving forces which induce a significant flux of matter are of practical importance. These driving forces are hydrostatic pressure, concentration, and electrical potential differences. These driving forces can also lead to the separation of chemical species. 

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