Mercury-water interface

Assume that an aqueous solute adsorbs at the mercury-water interface according to the Langmuir equation x/xm = bc/( + be), where Xm is the maximum possible amount and x/x = 0.5 at C = 0.3Af. Neglecting activity coefficient effects, estimate the value of the mercury-solution interfacial tension when C is Q.IM. The limiting molecular area of the solute is 20 A per molecule. The temperature is 25°C. 

Incidentally, a quantity called the rational potential is defined as E for the mercury-water interface (no added electrolyte) so, in general, = E + 0.480 V if a normal calomel reference electrode is used. 

Equation V-64 is that of a parabola, and electrocapillary curves are indeed approximately parabolic in shape. Because E ax tmd 7 max very nearly the same for certain electrolytes, such as sodium sulfate and sodium carbonate, it is generally assumed that specific adsorption effects are absent, and Emax is taken as a constant (-0.480 V) characteristic of the mercury-water interface. For most other electrolytes there is a shift in the maximum voltage, and is then taken to be Emax 0.480. Some values for the quantities are given in Table V-5 [113]. Much information of this type is due to Gouy [125], although additional results are to be found in most of the other references cited in this section. 

Assume that a salt, MX (1 1 type), adsorbs at the mercury-water interface according to the Langmuir equation ... 

Many organic electrode processes require the adsorption of the electroactive species at the electrode surface before the electron transfer can occur. This adsorption may take the form of physical or reversible chemical adsorption, as has been commonly observed at a mercury/water interface, or it may take the form of irreversible, dissociative chemical adsorption where bond fracture occurs during the adsorption process and often leads to the complete destruction of the molecule. This latter t q)e of adsorption is particularly prevalent at metals in the platinum group and accounts for their activity as heterogeneous catalysts and as... 

Only this equation has been used in practice. For a mercury/water interface, the (Ajj Pjo is equal to -0.25 V2,23 or-0.22 V. Its components [Eq. (7)] have been estimated to be -0.30 V and -0.05 V, respectively They show that in the presence of water, the electrons cannot expand freely as in a gas, but are pushed into mercury, and that the interaction of water... 

Table II. Assignments of the Major Infrared Bands for Isoquinoline at the Mercury/water Interface...

Buffle, J. and Cominoli, A. (1981). Voltammetric study of humic and fulvic substances. Part IV. Behaviour of fulvic substances at the mercury-water interface, J. Electroanal. Chem., 121, 273-299. 

Researches carried out in electrochemistry on solid electrodes and especially on the mercury-water interface have made a significant contribution to an understanding of interfacial phenomena. Although the electrode-water interfaces are typically... 

Systematic variation of the reaction conditions has increased the enantiomeric excess to 47.4 % in the presence of yohimbine [138]. Lowering both the cathode working potential and the pH improved the degree of asymmetric induction, and no further improvement in induction is achieved by raising the concentration of alkaloid above 0.4 mM. Enantioselectivity is due to an adsorbed layer of alkaloid and adsorption phenomena at the mercury water interface are dependent on the surface... 

Some compounds fomi a surface excess concentration at the mercury - water interface and optically active compounds in this class can generate a chiral surface... 

Values of the parameter 0 may be experimentally evaluated for the mercury-water surface from electrocapillary studies. The displacement of the coordinates of the electrocapillary maxima in Figure 7.23 reflects differences in the intrinsic adsorbability of various ions. Electrocapillary studies reveal that the strength of specific adsorption at the mercury-water interface for some monovalent anions follows the order... 

In general, the repulsion between similar electric charges present at a surface lowers the surface tension in Chap. II, 21, we have already seen cases where the development of similar charges, by dissociation, on the end groups of a surface film, increases the surface pressure. In the well-known capillary electrometer, in which a potential difference can be applied across a mercury-water interface, simultaneously with measurement of the surface tension, any changes in the potential difference will alter the density of electrification at the interface, and consequently alter the surface tension. 

The adsorption of the anions at the mercury-water interface is not exactly in the same order as the adsorption at an air-water interface (cf. Chap. Ill, 10), for the mercury exerts specific attractions on certain ions, notably the sulphide and iodide the degree of hydration of the ions, which was all-important for the air-liquid adsorption, is here only one of the factors controlling the adsorption. Talmud1 shows marked lowering of tension and a similar shift of the maximum to the right, with soap solutions, in which of course the anions are strongly adsorbed. 

In many electrolytes, one or more of the constituent ions are specifically adsorbed at the interface. Specific adsorption implies that the local ionic concentration is determined not just by electrostatic forces but also by specific chemical forces. For example, the larger halide ions are chemisorbed on mercury due to the covalent nature of the interaction between a mercury atom and the anion. Specific adsorption can also result from the hydrophobic nature of an ion. Thus, tetra-alkylammonium ions, which are soluble in water, are specifically adsorbed at the mercury water interface because of the hydrophobic nature of the alkyl groups. Specific adsorption of molecular solutes, such as the alcohols, occurs for the same reason. 

Bockris Reddy (1970) describes the Butler-Volmer-equation as the "central equation of electrode kinetics . In equilibrium the adsorption and desorption fluxes of charges at the interface are equal. There are common principles for the kinetics of charge exchange at the polarisable mercury/water interface and the adsorption kinetics of charged surfactants at the liquid/fluid interface. Theoretical considerations about the electrostatic retardation for the adsorption kinetics of ions were first introduced by Dukhin et al. (1973). 

Interaction of Prothrombin with Phospholipid Monolayers at Air- and Mercury-Water Interfaces... 

Direct adsorption of Prothrombin at the Mercury-water Interface. The adsorption rate of prothrombin on a hanging mercury drop electrode (HMDE) was studied by measuring the decrease of the differential capacity with time of contact of the mercury drop with the solution, at a fixed potential, - 0.5 V, in parallel with the increase of the areas of the voltametric peaks corresponding to the reduction of some S-S bonds of the adsorbed molecules (14). The representation on... 

The simplest interpretation of the compact-layer capacitance is represented by the Helmholtz model of the slab filled with a dielectric continuum and located between a perfect conductor (metal surface) and the outer Helmholtz plane considered as the distance of the closest approach of surface-inactive ions. Experimental determination of its thickness, zh, may be based on Eq. (12). Moreover, its dielectric permittivity, h, is often considered as a constant across the whole compact layer. Then its value can be estimated from the values of the compact-layer capacitance, for example, it gives about 6 or 10 (depending on the choice of zh) for mercury-water interface, that is, a value that is much lower than the one in the bulk water, 80. This diminution was interpreted as a consequence of the dielectric saturation of the solvent in contact with the metal surface, its modified molecular structure or the effects of spatial inhomogeneity. The effective dielectric permittivity of the compact layer shows a complicated dependence on the electrode charge, which cannot be explained by the simple hypothesis of the saturation effects on one hand or by the unperturbed bulk-solvent nonlocal polarizability on the other hand. 

For many systems, the maximum of the electrocapillary curve is located in the region of ideal polarizability of the electrode, and the maximum potential corresponds to the potential of zero total (thermodynamic) and zero free charge. At the mercury-water interface, the region of ideal polarization is a few volts when soluble mercury salts are absent (2.2 V), but at the interface between two immiscible... 

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