Surface electrical potential, definition

The definition of Gibbs elasticity given by Eq. (19) corresponds to an instantaneous (Aft t ) dilatation of the adsorption layer (that contributes to o ) without affecting the diffuse layer and o. The dependence of o on Ty for nonionic surfactants is the same as the dependence of o on Ty for ionic surfactants, cf Eqs (7) and (19). Equations (8) and (20) then show that the expressions for Eq in Table 2 are valid for both nonionic and ionic siufactants. The effect of the surface electric potential on the Gibbs elasticity Eq of an ionic adsorption monolayer is implicit, through the equilibrium siufactant adsorption T y which depends on the electric properties of the interface. To illustrate this let us consider the case of Langmuir adsorption isotherm for an ionic surfactant (17) ... 

In a similar way, electrochemistry may provide an atomic level control over the deposit, using electric potential (rather than temperature) to restrict deposition of elements. A surface electrochemical reaction limited in this manner is merely underpotential deposition (UPD see Sect. 4.3 for a detailed discussion). In ECALE, thin films of chemical compounds are formed, an atomic layer at a time, by using UPD, in a cycle thus, the formation of a binary compound involves the oxidative UPD of one element and the reductive UPD of another. The potential for the former should be negative of that used for the latter in order for the deposit to remain stable while the other component elements are being deposited. Practically, this sequential deposition is implemented by using a dual bath system or a flow cell, so as to alternately expose an electrode surface to different electrolytes. When conditions are well defined, the electrolytic layers are prone to grow two dimensionally rather than three dimensionally. ECALE requires the definition of precise experimental conditions, such as potentials, reactants, concentration, pH, charge-time, which are strictly dependent on the particular compound one wants to form, and the substrate as well. The problems with this technique are that the electrode is required to be rinsed after each UPD deposition, which may result in loss of potential control, deposit reproducibility problems, and waste of time and solution. Automated deposition systems have been developed as an attempt to overcome these problems. 

It should be recalled that the term surface potential is used quite often in membranology in rather a different sense, i.e. for the potential difference in a diffuse electric layer on the surface of a membrane, see page 443.) It holds that 0 = 0 + X (this equation is the definition of the inner electrical potential 0). Equation (3.1.2) can then be written in the form... 

The outer electrical potential of a phase is the electrostatic potential given by the excess charge of the phase. Thus, if a unit electric charge is brought infinitely slowly from infinity to the surface of the conductor to a distance that is negligible compared with the dimensions of the conductor considered (for a conductor with dimensions of the order of centimetres, this distance equals about 10 4cm), work is done that, by definition, equals the outer electric potential ip. 

The formation of 2D Meads phases on a foreign substrate, S, in the underpotential range can be well described considering the substrate-electrolyte interface as an ideally polarizable electrode as shown in Section 8.2. In this case, only sorption processes of electrolyte constituents, but no Faradaic redox reactions or Me-S alloy formation processes are allowed to occur, The electrochemical double layer at the interface can be thermodynamically considered as a separate interphase [3.54, 3.212, 3.213]. This interphase comprises regions of the substrate and of the electrolyte with gradients of intensive system parameters such as chemical potentials of ions and electrons, electric potentials, etc., and contains all adsorbates and all surface energy. Furthermore, it is assumed that the chemical potential //Meads is a definite function of the Meads surface concentration, F, and the electrode potential, E, at constant temperature and pressure Meads (7", ). Such a model system can only be... 

Alkali can be generated by the cathodic half of a corrosion reaction or the cathodic reaction may be driven by means of an electrical potential. When the cathodic reaction occurs between the rubber and metal surface the pH of the solution under the rubber may be as high as 14. Many factors (summarised by Leidheiser [3]) concerned with cathodic delamination are detailed. No definitive mechanism for this type of delamination has been determined although a number of suggestions have been put forward [3]. These include alkaline attack on the polymer, surface energy considerations and attack of the oxide at the interface. 

The rate of formation of the complex at the interface is described by the rate constant Kr. It is important to note that Kr depends on the surface charge of the membrane. This is simply a consequence of the definition of KrI the number of associations per square centimeter per second is set equal to CmKr, where Cm is the bulk concentration of the cation in the aqueous phase. Now, as a consequence of the negative charge of, for example, the phosphatidylinositol membrane, the cation concentration Cm at the membrane surface is larger than the bulk concentration by a Boltzmann factor exponent ( —4>), where cj) is the electric potential (expressed in units RT/F) at the membrane surface [330]. 

The electric potential of a particle varies gradually from the value of the surface potential (Vo) to zero. For the majority of particles without a complex surface structure, the variation is monotonic. Thus, electric potential at the shear plane is in between vj/o and zero (Figure 6.2). This potential is called the zeta potential ( potential). Zeta potential is different from surface potential /o but is a measurable amount. Since the shear plane is located at the frontier of the particle surface, any interfacial interaction of particle with other species (ions, neighboring particles, etc.) in medium will be encountered at the shear plane. Zeta potential actually has more direct influence compared with surface potential. Zeta potential is determined by many factors, mainly 1) surface potential, 2) potential curve, which is determined by the concentrations and valences of the co- and counter ions in the system and the dielectric constant of the continuous phase, and 3) location of the shear plane. There is no definite relationship between surface potential and zeta potential. For example, in different environments and particle surface conditions, the same zeta potential may correspond to different surface potentials and the same surface potential may result in different zeta potentials. Figure 6.2 describes the relation between... 

The position of the mercury meniscus in the capillary depends on the surface tension between the mercury and the sulphuric acid and this, in turn, depends on the electrical potential between the mercury and the acid. If this potential is altered, as, for example, by connecting the two mercury electrodes with a cell or with two points of a circuit between which there is a difference of potential, the meniscus will movej and for small differences of potential, the amount of movement is proportional to the difierence of potential. In order, however, that the meniscus shall take up a definite position, the two electrodes must be connected together except when making a measurement This is efiected by means of a triple contact Morse key, shown in Fig. 72. The electrical connections between the terminals a, b, c and the contacts d, V, d are indicated by means of the dotted lines. The electrodes of the electrometer are connected with the terminals b and c, so that they are connected together when the key is in its normal position. The terminals a and c are connected with the rest of the circuit, so... 

Clavilier eta/.196,794-796have studied CO adsorption on electrochemi-cally faceted Pt(lll) and Pt(110) electrodes and from the charge transients, with the provision that the CO dipole has a negligible contribution to the electrical double-layer potential these authors have provided a definite determination of ( =o- However, electrochemically faceted Pt(lll) electrodes have a polycrystalline surface structure, and thus the value of Eq-q for such electrodes lies between fiULO for terraces and forst s.197 786 787... 

---