Potential contact

The succeeding material is broadly organized according to the types of experimental quantities measured because much of the literature is so grouped. In the next chapter spread monolayers are discussed, and in later chapters the topics of adsorption from solution and of gas adsorption are considered. Irrespective of the experimental compartmentation, the conclusions as to the nature of mobile adsorbed films, that is, their structure and equations of state, will tend to be of a general validity. Thus, only a limited discussion of Gibbs monolayers has been given here, and none of such related aspects as the contact potentials of solutions or of adsorption at liquid-liquid interfaces, as it is more efficient to treat these topics later. 

The difference in Volta potential AV, which has been called the surface (or contact) potential in this book, is then given by... 

If two metals with different work functions are placed m contact there will be a flow of electrons from the metal with the lower work function to that with the higher work fimction. This will continue until the electrochemical potentials of the electrons in the two phases are equal. This change gives rise to a measurable potential difference between the two metals, temied the contact potential or Volta potential difference. Clearly... 

Wlien an electrical coimection is made between two metal surfaces, a contact potential difference arises from the transfer of electrons from the metal of lower work function to the second metal until their Femii levels line up. The difference in contact potential between the two metals is just equal to the difference in their respective work fiinctions. In the absence of an applied emf, there is electric field between two parallel metal plates arranged as a capacitor. If a potential is applied, the field can be eliminated and at this point tire potential equals the contact potential difference of tlie two metal plates. If one plate of known work fiinction is used as a reference electrode, the work function of the second plate can be detennined by measuring tliis applied potential between the plates [ ]. One can detemiine the zero-electric-field condition between the two parallel plates by measuring directly the tendency for charge to flow through the external circuit. This is called the static capacitor method [59]. 

Whilman, L.J. and Colton, R.J., Design and calibration of a scanning force microscope for friction, adhesion, and contact potential studies. Rev. Sci. fnsirum., 66, I (1995). Ba.selt, D.R. and Baldeschwieler, J.D., Imaging spectro.scopy with the atomic-force microscope. J. AppL Pliys., 76(1), 33-38 (1994). 

Conditions of equilibrium, 92 Conduction of heat, 48, 84, 454 Configuration, 22, 107 Connodal curve, 243 Conservation of energy, 35 Contact potential differences, 470 Continuity of states, 174 Corresponding states, 228, 237 Creighton. See Southern. 

Measurements [113,368] of interfacial (contact) potentials or calculated values of the relative work functions of reactant and of solid decomposition product under conditions expected to apply during pyrolysis have been correlated with rates of reaction by Zakharov et al. [369]. There are reservations about this approach, however, since the magnitudes of work functions of substances have been shown to vary with structure and particle size especially high values have been reported for amorphous compounds [370,371]. Kabanov [351] estimates that the electrical field in the interfacial zone of contact between reactant and decomposition product may be of the order of 104 106 V cm 1. This is sufficient to bring about decomposition. 

If the two Cu cables are short circuited while the cell is broken into two parts by splitting the liquid phase, it can easily be proved that the same AE as for cell (12a) is measured as a contact potential difference (cpd) between the two solutions. In fact... 

For correlating relative Eamo values with values in the UHV scale (0 values), two quantities must be known 0 and A0. Contact potential measurements at metal/solution interfaces can be measured.4 In that case the interfacial structure is exactly that in the electrochemical situation (bulk liquid phase, room temperature). However, 0 to convert E into 0 must be independently known. It may happen that the metal surface state is not exactly the same during the measurements of 0 and A0. 

A third experimental configuration was proposed by Kolb and Hansen40 emersed electrodes. If an electrode is emersed from a solution while the control of the potential is maintained, the solvent layer dragged off with the metal (Fig. 3) would reproduce UHV conditions, but with potential control and at room temperature, as in the actual electrode situation. This appears to be the most convenient configuration for measuring 0. However, there are doubts that the solvent layer retains the properties of a bulk phase. It has in fact been demonstrated41 that a contact potential difference exists between an electrode in the emersed state and the same electrode regularly immersed in solution. 

The contact potential difference between Hg and water (actually a dilute aqueous solution of a surface-inactive electrolyte) has been measured42,43 to be -0.25 V. The negative sign means that the work function of Hg decreases upon contact with water. Since 4.50( 0.02) cV is the currently accepted5 value for 0 of Hg, the value of 0 for the uncharged metal (at the potential of zero charge) is 4.25 eV. 

The two perturbation terms are specific to the given interface and are experimentally inseparable. They measure the contact potential difference at the M/S contact. However, since no cpd is measured in this case <5/M + S%s are grouped into a single quantity denoted by X, called the interfacial... 

Conversion of Earno into an absolute (UHV) scale rests on the values of ff-0 and for Hg used as areference surface. While the accuracy of is indisputable, the experimental value of contact potential difference between Hg and H20, are a subject of continued dispute. Efforts have been made in this chapter to try to highlight the elements of the problem. However, a specialized experimental approach to the measurement of 0 (and A0 upon water adsorption) of Hg would definitely remove any further ambiguity as well as any reasons not to accept certain conclusions. 

Thus if the working and counter electrodes are made of different metals, then Eq. (5.35) gives the cpd (contact potential difference) of the two metals ... 

Why in a contact potential difference (CPD) experiment the potentials are nonzero (and the O are unaffected) while in a NEMCA setup P vanishes and O is affected by Uwr ... 

As we have shown, the polarization force depends not only on the topography [through the f(R z) term] and dielectric constant e, but also on the local contact potential 4). As we shall see now, ac bias modulation and lock-in detection allow these contributions to be separated. 

Experiments using ac bias modulation for the purpose of separating topography and contact potential were first carried out by Schonenberger et al. [43] and later by Yokoyama et al. [44]. When the cantilever is driven by a voltage of frequency co, the force detected by the lever can be expressed as ... 

In contrast to the case of the 8CB liquid crystal, no contact potential differences between first and second layers were observed with these lubricants. This indicates that there is no special orientation of the dipole active end groups, or perhaps that the end groups form hydrogen-bonded pairs with neighboring molecules so as to give no net dipole moment. 

FIG. 28 Changes in contact potential of mica relative to a hydrophobic tip as a function of relative humidity. The tip-sample distance during measurements was maintained at 400 A. At room temperature the potential first decreases by about 400 mV. At -30% RH it reaches a plateau and stays approximately constant until about 80% RH. At higher humidity the potential increases again, eventually becoming more positive than the initial dry mica surface. The changes in surface potential can be explained by the orientation of the water dipoles described in the previous two figures. 

FIG. 30 Top Square voltage waveform applied to the tip. Bottom. Corresponding changes in tip deflection (converted to force after multiplying by the lever spring constant). There is a net attractive force for both the positive and the negative cycles, but it takes time to reach the final force value. Note that the square-wave voltage is not symmetrical around zero. An offset is applied to compensate for the contact potential difference between the tip and the surface. This offset is dependent on humidity and is equal to the potential difference between the tip and the sample. (From Ref. 78.)... 

The contact potential image (Ico) shows a striking contrast difference below and above point A. At low humidity there is a strong enhancement of Ac]) at the step edges i.e.. 

FIG. 32 Top Semilog plot of the time constant t for ionic motion as a function of RH for KF. Bottom Simultaneously measured contact potential. At a critical humidity A, there is a break or a change in slope in these two surface properties. Below A, water solvates preferentially cations at the step edges. Above A, the rates of dissolution (solvation) of anions and cations are similar and water uni-... 

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