Capacitance measurement

The electrical potential distribution has been extensively studied in the 1980s and 1990s by capacitance measurements, as excellently reviewed in 1998 by Samec [48], The first model of polarized ITIES is that of Verwey-Niessen [49], dating back to 1939, of two back-to-back diffuse layers, and then adapted by Gavach et al. [50,51] by considering the presence of an inner layer. The key problem with 

Which characteristics of the ionic density profiles determine the observed dependences of the capacitance on the nature of the ions  

Which parameters control the sign of the deviation of the capacitance from the Gouy-Chapman result and the asymmetry of the capacitance curves as a function of the potential  

What information on the free energy profile of the ions across the interface can be obtained from the capacitance data  

To answer these questions, these authors developed an analytical model based on a modified nonlinear Poisson-Boltzmann equation, taking into account the overlap of the two back-to-back diffuse layers and the resulting differential equation  

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Usually one varies the head of mercury or applied gas pressure so as to bring the meniscus to a fixed reference point [118], Grahame and co-workers [119], Hansen and co-workers [120] (see also Ref. 121), and Hills and Payne [122] have given more or less elaborate descriptions of the capillary electrometer apparatus. Nowadays, the capillary electrometer is customarily used in conjunction with capacitance measurements (see below). Vos and Vos [111] describe the use of sessile drop profiles (Section II-7B) for interfacial tension measurements, thus avoiding an assumption as to the solution-Hg-glass contact angle. 

The variation of the integral capacity with E is illustrated in Fig. V-12, as determined both by surface tension and by direct capacitance measurements the agreement confrrms the general correctness of the thermodynamic relationships. The differential capacity C shows a general decrease as E is made more negative but may include maxima and minima the case of nonelectrolytes is mentioned in the next subsection. 

As outlined above, electron transfer through the passive film can also be cmcial for passivation and thus for the corrosion behaviour of a metal. Therefore, interest has grown in studies of the electronic properties of passive films. Many passive films are of a semiconductive nature [92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102 and 1031 and therefore can be investigated with teclmiques borrowed from semiconductor electrochemistry—most typically photoelectrochemistry and capacitance measurements of the Mott-Schottky type [104]. Generally it is found that many passive films cannot be described as ideal but rather as amorjDhous or highly defective semiconductors which often exlribit doping levels close to degeneracy [105]. 

Impedance Some of the errors arising from the use of linear polarisation resistance led to interest and development in a.c. systems.An early development used a fixed a.c. frequency and a commercial instrument was produced in the UK. Inaccuracies still occurred, however, and were due to the electrode impedance which is fequency dependent. Electrode reactions have a capacitance component, in addition to resistance, resulting in a requirement to measure the impedance. However, the total impedance comprises values for the reaction, solution, diffusion and capacitance. Measurements at different frequency are more reliable, particularly where high solution resistances occur. Simplifications for industrial monitoring have been developed consisting of two measurements, i.e. at a high (10 kHz) and low frequency (0-1 Hz). The high-frequency measurement can identify the... 

The Hg/dimethyl sulfoxide (DMSO) interface has been studied by electrocapillary and capacitance measurements in a range of temperatures.291,304 Eamo was measured using the streaming electrode method. All potentials were recorded in a nonisothermal cell against a 0.1 M NaCl calomel electrode (CE) in water at 25°C. The potential difference of the cell CE/0.1 M NaC104 (aq.)/0.1 M NaC104 (DMSO)/CE was -0.096 V. This value was used to recalculate the data.312... 

The pzc of a pc-Cd renewed by cutting was determined in dilute fluoride and sulfate solutions by capacitance measurements.645,646 The C, E curves exhibited distinct minima whose depth increased with increasing dilution of the solution (Table 15). This value is ca. 30 mV more negative than that for polished electrodes and reflects the more disturbed surface stmcture of a renewed electrode. Adsorption of aliphatic alcohols and acids has also been studied on these electrodes.645,646... 

Met. = Method. EC = electrocapillary method, PC = electrode capacitance measured with a pulse method, T = ten-sammetry. See also list of symbols. 

Several methods exist that can be used to measure changes of ESE for solid electrodes as a function of potential or other factors, but the accuracy of such measurements is much lower than that for Uquid electrodes. A plot of ESE vs. potential is called the electrocapillaty cutye (ECC). Typical ECCs measured at a mercury electrode in NaF solutions of different concentration are shown in Fig. 10.6. Also shown in this figure is a plot of values vs. potential calculated via Eq. (10.27). This plot almost coincides with that obtained from capacitance measurements (Fig. lO.lfc). This is evidence for the mutual compatibility of results obtained by these two methods of measurement. 

Kakiuchi et al. [75] used the capacitance measurements to study the adsorption of dilauroylphosphatidylcholine at the ideally polarized water-nitrobenzene interface, as an alternative approach to the surface tension measurements for the same system [51]. In the potential range, where the aqueous phase had a negative potential with respect to the nitrobenzene phase, the interfacial capacity was found to decrease with the increasing phospholipid concentration in the organic solvent phase (Fig. 11). The saturated mono-layer in the liquid-expanded state was formed at the phospholipid concentration exceeding 20 /amol dm, with an area of 0.73 nm occupied by a single molecule. The adsorption was described by the Frumkin isotherm. 

Capacitance measurements of phospholipid monolayers at the ITIES have been proposed as a suitable tool for studying the enzyme activity under the precise control of the electrical state of the monolayer [81]. Kinetics of hydrolysis of phosphatidylcholine... 

Capacitance and surface tension measurements have provided a wealth of data about the adsorption of ions and molecules at electrified liquid-liquid interfaces. In order to reach an understanding on the molecular level, suitable microscopic models have had to be considered. Interpretation of the capacitance measurements has been often complicated by various instrumental artifacts. Nevertheless, the results of both experimental approaches represent the reference basis for the application of other techniques of surface analysis. 

The experimental data bearing on the question of the effect of different metals and different crystal orientations on the properties of the metal-electrolyte interface have been discussed by Hamelin et al.27 The results of capacitance measurements for seven sp metals (Ag, Au, Cu, Zn, Pb, Sn, and Bi) in aqueous electrolytes are reviewed. The potential of zero charge is derived from the maximum of the capacitance. Subtracting the diffuse-layer capacitance, one derives the inner-layer capacitance, which, when plotted against surface charge, shows a maximum close to qM = 0. This maximum, which is almost independent of crystal orientation, is explained in terms of the reorientation of water molecules adjacent to the metal surface. Interaction of different faces of metal with water, ions, and organic molecules inside the outer Helmholtz plane are discussed, as well as adsorption. 

It may be noted that the statement made above—that the surface potential in the electrolyte phase does not depend on the orientation of the crystal face—is necessarily an assumption, as is the neglect of S s1- It is another example of separation of metal and electrolyte contributions to a property of the interface, which can only be done theoretically. In fact, a recent article29 has discussed the influence of the atomic structure of the metal surface for solid metals on the water dipoles of the compact layer. Different crystal faces can allow different degrees of interpenetration of species of the electrolyte and the metal surface layer. Nonuniformities in the directions parallel to the surface may be reflected in the results of capacitance measurements, as well as optical measurements. 

F. Wei, B. Sun, Y. Guo, and X.S. Zhao, Monitoring DNA hybridization on alkyl modified silicon surface through capacitance measurement. Biosens. Bioelectron 18, 1157-1163 (2003). 

D. Jiang, J. Tang, B. Liu, P. Yang, and J. Kong, Ultrathin alumina sol-gel-derived films allowing direct detection of the liver fibrosis markers by capacitance measurement. Anal. Chem. 75, 4578 4584 (2003). 

C. Fernandez-Sanchez, C.J. McNeil, K. Rawso, and O. Nilsson, Disposable noncompetitive immunosensor for free and total prostate-specific antigen based on capacitance measurement. Anal. Chem. 76, 5649-5656 (2004). 

P. Bataillard, F. Gardies, N.J. Renault, C. Martelet, B. Cohn, and B. Mandrand, Direct detection of immunospecies by capacitance measurements. Anal. Chem. 60, 2374—2379 (1988). 

Capacitance measurements are quite simple. A typical drawback is the need of coaxial cables that introduce a thermal load which is not negligible in low-power refrigerators. On the other hand, capacitance bridges null the cable capacitance. Multiplexing is more difficult than for resistance thermometers. In principle, capacitors have low loss due to Joule heating. This is not always true losses can be important, especially at very low temperatures. Dielectric constant thermometers have a high sensitivity capacitance differences of the order of 10-19F can be measured. 

Various systems have been used to measure the mercury level change in the stem during intrusion. Indirect methods include resistance or capacitance measurements along the stem of the dilatometer [39], These readings are taken in conjunction with pressure readings, to correlate the number of pores at a specific pore size. 

Capacitive Measuring is based on detecting changes in the dielectric constant as a result of changing gas concentration. These sensors are mostly used for measuring humidity, but can also detect C02. One major drawback of these systems is... 

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