Nonadsorbed electrolyte

An interesting correlation exists between the work function of a metal and its pzc in a particular solvent. Consider a metal M at the pzc in contact with a solution of an inert, nonadsorbing electrolyte containing a standard platinum/hydrogen reference electrode. We connect a platinum wire (label I) to the metal, and label the platinum reference electrode with II. This setup is very similar to that considered in Section 2.4, but this time the metal-solution interface is not in electronic equilibrium. The derivation is simplified if we assume that the two platinum wires have the same work function, so that their surface potentials are equal. The electrode potential is then ... 

In a very extensive and thorough study of silver electrodes in a nonadsorbing KPFg electrolyte, Valette determined the inner-layer capacities of Ag(lll), Ag(lOO), and Ag(llO) to be equal to 77,92, and 112//F... 

Amokrane and Badiali proposed a semiempirical approach to the determination of the solvent contribution C, to the capacitance of the double layer in aqueous and nonaqueous " solutions. They used the relation C = Cf - C m, where Q is the experimentally determined capacity of the inner layer and Cm is the contribution of the metal. The plots ofC, vs. (Tm were presented for various solvents and correlated with their properties.However, the problem of the supporting electrolyte was entirely neglected in the quoted papers. It was shown recently that the height and position of the maximum on the C, vs. Gm plots depend on the type of the supporting electrolyte. Experimental differential capacity data obtained on the Hg electrode in methanol and ethanol containing various electrolytes with nonadsorbing anions (F , PFg, ClOi) indicate that the type as well as concentration of the electrolyte influences the position and the height of the maximum on the C, vs. plots (Fig. 13). 

Such anion adsorption can be prevented by chemisorbing a mono-layer of a strongly adherent thiol molecule to the Au surfaces [97,98]. 1-Propanethiol (PT) was used here because the gold nanotubules can still be wetted with water after chemisorption of the PT monolayer [97].t The Em versus applied potential curves for an untreated and PT-treated gold nanotubule membrane, with KBr solutions present on either side of the membrane, are shown in Fig. 13. The untreated membrane shows only cation permselectivity, but the permselectivity of the PT-treated membrane can be switched, exactly as was the case with the nonadsorbing electrolyte (Fig. 12). 

If a nonadsorbed electrolyte (such as NaCl) is present in a large excess, then any increase in concentration of Na R will produce a negligible increase in Na" " ion concentration, and therefore becomes negligible. Moreover, d/tQ is also negligible, so that the Gibbs adsorption equation reduces to... 

Eor a nonadsorbed electrolyte such as NaCl, any increase in Na" " R" concentration produces a negligible increase in Na" " concentration (d/tf, is negligible and d//(- " is also negUgible. 

By applying equation 3.47 and the principle that electrolyte activities can be expressed as the product of activities of the component ions, equation 3.48 can be reexpressed in a form independent of the nonadsorbing Cl anion ... 

The concentration of a nonadsorbing indifferent electrolyte just sufficient to rapidly flocculate a lyophobic colloid is known to be strongly dependent on the charge number of the counterions (here, the ions of charge opposite to that of the colloid). However, the colloid stability is largely independent of the charge number of the coions and of the concentration of the colloid. These observations are embodied in what is known as the Schulze-Hardy rule, which states that the valence of the counterions has the principal effect on the stability of a lyophobic colloid. 

When a conductive electrode (e.g., metallic or glassy carlxMi) is in contact with an electrolytic solution, the excess electronic charge is accumulated at the electrode surface and charge distribution occurs in the solution only. This is related to the fact that as the number of charged species increases, the space in which the redistribution of charges occurs shrinks. At a metallic electrode-solution interface, the charge redistribution in solution depends on the applied potential and is described by the Guy-Chapman-Stem theory. The characteristic thickness of the diffuse layer in nonadsorbing electrolytes varies from 0.3 nm in 1 M to 3 nm in 0.01 M aqueous electrolyte, while the thickness of the Helmholtz layer is much smaller [17]. 

Water and water-methylcellulose solution have been replaced by polyethylene glycol (PEG) electrolyte solution or low Hounsfield value barium suspension, 0.1% ultra-low-dose barium with sorbitol, a nonadsorbable sugar alcohol that promotes luminal distention and limits resorption of water across the length of the small bowel (Megibow et al. 2006). 

Portions of the appropriate hydrolysis mixtures, prepared as described below, were loaded onto the tops of the columns and the excess electrolyte, as well as all nonadsorbed complex species, removed by washing first with distilled water and then with 0.05 M sodium perchlorate. Adsorbed species were eluted individually from the column by gradually increasing the concentration of sodium perchlorate in the eluting solution. The effluent from the columns were collected in small fractions, usually from 15 to 25 ml, which were stored in the dark at 0° until used in subsequent experiments. Typical elution curves from the anion and the cation-exchange resin columns are shown in Figs. 1 and 2, respectively. Detailed procedures for the separation of individual species are given below. 

We consider a nonadsorbing suporting electrolyte of high concentration, so that the charges in the interfacial region are well screened. Then, we define the electrosorption valency as... 

To obtain a detailed microscopic understanding of structure and processes at interfaces, one needs a microscope with sufficient resolution. Cyclic voltammetry is the metaphorical microscope of surface electrochemistry (Gileadi, 2011). The voltage scan rate, viz. the linear rate of potential change (in V s ), is its resolution and the considered potential window is the focus. A cyclic voltammogram (CV) of a Pt( 111) surface, in contact with a nonadsorbing aqueous electrolyte, is shown in Figure 3.16. 

FIGURE 3.18 A comparison of the proposed model of surface oxide formation and reduction at Pt(lll) in nonadsorbing acidic electrolyte with CV data of Gomez-Marin et al. (2013). The scan rate varies by four orders of magnitude from (a) to (f), as indicated in the plots. (Reprinted from Electrocatalysis, Mechanistic principles of platinum oxide formation and reduction, 2014, 1-11, Rinaldo et al. Copyright (2014) Springer. With permission.)... 

Figure 2 Illustration of a negatively charged biomolecular surface with charge density a in the presence of a mixed electrolyte. The surface may represent that of a colloidal or biophysical particle such as a membrane (plane), polynucleic acid (cylinder), or micelle (sphere) where the distance of closest approach of ions is designated x — a. n the solution of the Gouy-Chapman equation, and of the Poisson-Boltzmann equation in general, the charged surface is usually displaced from its actual position (relative to the solvent) to the plane of closest approach of nonadsorbed ions, also called the outer Helmholtz plane.

---