Surface film potential

In heterogeneous systems, potential differences exist across the various phase boundaries. The surface film potential, AV, due to a monolayer is the change in the potential difference between the bulk substrate liquid and a probe placed above the liquid which results from the presence of the monolayer. Surface film potentials can be measured by air-electrode and vibrating-plate methods. 

Surface film potential measurements can yield useful, if not absolute, information about the orientation of the film molecules. Treating the film as a parallel plate condenser leads to the approximate expression... 

Surface film potential measurements are also used to investigate the homogeneity or otherwise of the surface. If there are two surface phases present, the surface film potential will fluctuate wildly as the probe is moved across the surface or as one blows gently over the surface. 

There are a number of instances in which (with the aid of sensitive measurements) well-defined transitions between gaseous and coherent states are observed as the film is compressed. The tt-A curves show a marked resemblance to Andrews p-V curves for the three-dimensional condensation of vapours to liquids. The tt-A curve for myristic acid, given as an example, has been drawn schematically to accentuate its main features (Figure 4.26). Above 8 nm2 molecule-1 the film is gaseous and a liquid-expanded film is obtained on compression to 0.5 nm2 molecule-1. Fluctuating surface film potentials verify the heterogeneous, transitional nature of the surface between 0.5 nm2 molecule-1 and 8 nm2 molecule-1. 

It is a general fact that at an interface or phase boundary between two dissimilar materials, there exists a surface electrical potential that reflects differences in the electronic makeup of the two phases. Because almost all surface-active materials (for aqueous systems, at least) have a polar head group, when the molecules adsorb at the surface, the dipole moments of those groups become at least partially oriented with respect to the interface. As a result of the orientation of the dipoles (or charges), the potential difference across the interface will be altered. The surface film potential due to the monolayer, AF, is the change in the interfacial potential due to the presence of the monolayer. 

If the monolayer is treated as a parallel plate condenser, the measured surface film potential can be used to deduce information about the orientation of the adsorbed molecules. While the calculation is only approximate, if n is the number of molecules in the adsorbed film (a known quantity), ju. is the dipole moment of the head group (also known or accessible), and e is the permittivity of the film (its dielectric constant x the permittivity of a vacuum), the approximate relationship... 

Many current designs use far more material than is necessary, or use potentially scarce materials where the more plentiful would serve. Often, for example, it is a surface property (e.g. low friction, or high corrosion resistance) which is wanted then a thin surface film of the rare material bonded to a cheap plentiful substrate can replace the bulk use of a scarcer material. Another way of coping with shortages is by... 

A comprehensive list of standard potentials is found in Ref. 7. Table 2-3 gives a few values for redox reactions. Since most metal ions react with OH ions to form solid corrosion products giving protective surface films, it is appropriate to represent the corrosion behavior of metals in aqueous solutions in terms of pH and Ufj. Figure 2-2 shows a Pourbaix diagram for the system Fe/HjO. The boundary lines correspond to the equilibria ... 

For Fe it follows from Eq. (2-530 that = -0.64 V. This value is 0.21 V more negative than the protection potential i/cu-cuso4 = -0.85 V which is adopted in practice (see Fig. 1-12) [22,23]. This more positive potential results because 5is actually greater and the cathodically generated surface films give additional protection [24-26]. 

This criterion is derived from the fact that the free corrosion potential in soil is generally I/cu Cuso4 -0-55 V. Ohmic voltage drop and protective surface films are not taken into consideration. According to the information in Chapter 4, a maximum corrosion rate for uniform corrosion in soil of 0.1 mm a can be assumed. This corresponds to a current density of 0.1 A m l In Fig. 2-9, the corrosion current density for steel without surface film changes by a factor of 10 with a reduction in potential of about 70 mV. To reduce it to 1 jum a (0.14 V would be necessary. The same level would be available for an ohmic voltage drop. With surfaces covered with films, corrosion at the rest potential and the potential dependence of corrosion in comparison with act contrary to each other so that qualitatively the situation remains the same. More relevant is... 

Tin when made anodic shows passive behaviour as surface films are built up but slow dissolution of tin may persist in some solutions and transpassive dissolution may occur in strongly alkaline solutions. Some details have been published for phosphoric acid with readily obtained passivity, and sulphuric acid " for which activity is more persistent, but most interest has been shown in the effects in alkaline solutions. For galvanostatic polarisation in sodium borate and in sodium carbonate solutions at 1 x 10" -50 X 10" A/cm, simultaneous dissolution of tin as stannite ions and formation of a layer of SnO occurs until a critical potential is reached, at which a different oxide or hydroxide (possibly SnOj) is formed and dissolution ceases. Finally oxygen is evolved from the passive metal. The nature of the surface films formed in KOH solutions up to 7 m and other alkaline solutions has also been examined. 

As indicated above, when a positive direct current is impressed upon a piece of titanium immersed in an electrolyte, the consequent rise in potential induces the formation of a protective surface film, which is resistant to passage of any further appreciable quantity of current into the electrolyte. The upper potential limit that can be attained without breakdown of the surface film will depend upon the nature of the electrolyte. Thus, in strong sulphuric acid the metal/oxide system will sustain voltages of between 80 and 100 V before a spark-type dielectric rupture ensues, while in sodium chloride solutions or in sea water film rupture takes place when the voltage across the oxide film reaches a value of about 12 to 14 V. Above the critical voltage, anodic dissolution takes place at weak spots in the surface film and appreciable current passes into the electrolyte, presumably by an initial mechanism involving the formation of soluble titanium ions. 

Thus titanium by itself cannot function as an efficient anode for the passage of positive direct current into an electrolyte. The surface film of oxide formed upon the titanium has, however, a most useful property while it will not pass positive direct current into an electrolyte (more correctly, while it will not accept electrons from negatively charged ions in solution), it will accept electrons from, or pass positive current to, another metal pressed on to it. Hence a piece of titanium which has pressed on to its surface a small piece of platinum will pass positive direct current into brine and into many electrolytes, at a high current density, via the platinum, without undue potential rise, and without breakdown of the supporting titanium . ... 

Little work has been done on bare lithium metal that is well defined and free of surface film [15-24], Odziemkowski and Irish [15] showed that for carefully purified LiAsF6 tetrahydrofuran (THF) and 2-methyltetrahydrofuran 2Me-THF electrolytes the exchange-current density and corrosion potential on the lithium surface immediately after cutting in situ, are primarily determined by two reactions anodic dissolution of lithium, and cathodic reduc-... 

Another way to detect the short-circuit is by sweeping the potential of one of the pipettes while the second pipette is disconnected from the potentiostat. When the surface film is present, the voltammograms obtained in this way are practically identical for both pipettes. This can be expected because both curves are produced by IT across the same interface (i.e., the interface between the aqueous film and DCE) and the filling solutions in... 

The indicated formal potential E° n of the corresponding monomer (-1.17V) in solution is very near that of the surface film (-1.13V vs. SSCE). That formal potentials of surface films on chemically modified electrodes are near those of their corresponding dissolved monomers (13,18) is actually a common, and quite useful, observation. In the present case, it demonstrates that the electronic structures of the porphyrin rings embedded in the polymer film are not seriously perturbed from that of the monomer. 

When the electrolyte solutions are not too reactive, as in the case of ethereal solutions, there is no massive formation of protective surface films at potentials above Li intercalation potential, and most of the solvent reduction processes may occur at potentials lower than 0.3 V vs. Li/Li+. Hence, the passivation of the electrodes is not sufficient to prevent cointercalation of solvent molecules. This leads to an exfoliation of the graphite particles into amorphous dust (expholiated graphene planes). This scenario is demonstrated in Figure 2a as the reduction of the 002 diffraction peak21 of the graphite electrode, polarized cathodically in an ethereal solution. 

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