Читать книгу Electroanalytical Chemistry - Gary A. Mabbott - Страница 23

There are numerous electrochemical sensors that selectively respond to a specific chemical species of interest. For example, fluoride is routinely monitored in municipal drinking water by fluoride selective electrodes. Lithium ion can be determined in the blood or urine of a patient being treated for depression by lithium‐containing medications using a lithium ion selective electrode. These devices are popular because of their simplicity of use and their reliability. The increasing interest in monitoring select chemical species in clinical, environmental, industrial settings and, more recently, in private homes and for personal health monitoring is likely to encourage the development and implementation of even more sensors of this type.

The heart of all electrochemical sensing devices is the boundary between the sensor and the test solution. It is there that a charge separation develops. Because of its importance, it is very useful to take a closer look at the structure of the boundary. Consider, for example, a metal wire dipping into a salt solution. Assume, for the sake of discussion, that an excess of negative charge (i.e. electrons) appears on the wire. Electromagnetic theory predicts that the excess charge will appear at the surface of the metal. The arrangement of charges on the solution side is a bit more complicated. The excess electrons will naturally attract cations from solution. In the mid‐nineteenth century, the German scientist Herman Helmholtz imagined that all of the cations necessary to balance the charge on the metal surface migrate into position at a small distance from the surface forming a plane of charge [5]. It is now known that the cations do not actually come into contact with the metal surface because a monolayer of water molecules cling directly to the surface and are not easily displaced. Furthermore, individual cations are surrounded by a sphere of water molecules, known as the hydration sphere, that are also tightly bound. As a consequence, the cations approach the electrode surface no closer than about a distance equal to the length of two water molecules (about 5–6 Å total). Figure 1.5 shows cations with their hydration spheres parked in a line outside a layer of water molecules attached to the electrode surface. The centers of these cations represent a layer now known as the OHP. In the Helmholtz model, the charge on the OHP is equivalent in magnitude to the charge on the metal. This model closely resembles a simple capacitor.

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Figure 1.5 The Helmholtz model of the electrified boundary between a metal surface (dark spheres) with a net negative charge and an aqueous salt solution. (a) Cations are attracted to the surface forming a net positive layer to balance the negative charge in the metal. Water molecules occupy the first layer on the metal surface. They also surround ions in solution aligning their dipoles according to the type of charge on the ion. (Arrows point toward the oxygen atoms.) The charges on the solution side define a layer called the outer Helmholtz plane (OHP) [5]. (b) The double layer of charge behaves like a capacitor producing an electrical potential energy difference between the two layers whose magnitude is proportional to the charge.

In cases where the solid surface has a net positive charge, it attracts an excess of anions to balance the charge and the OHP is occupied by an excess of anions. In some cases, individual anions are able to come into direct contact with the metal surface. This phenomenon is called contact adsorption. Whether or not contact adsorption occurs depends upon the net free energy for three separate steps in the overall adsorption process. Two of the steps are obviously endothermic. Removing water molecules from the electrode surface to make room for the anion and removing part of the hydration sphere around the ion both cost energy. Therefore, only interactions between the ion and the electrode surface that lead to strong bonds make the adsorption process favorable. The electrostatic attractions between oppositely charged ions and the electrode are not decisive by themselves. Contact adsorption relies on London dispersion forces, overlap of electron orbitals, and image forces. An image force is similar to the mechanism known as London dispersion forces where a momentary dipole resulting from the instantaneous arrangement of charge density around a molecule induces a rearrangement of electron density in a neighboring molecule creating a momentary dipole that results in dipole–dipole attraction. Unlike London dispersion forces, the image force is created by a permanent dipole or a charge on the ion inducing a dipole or local excess of charge in the electrode that leads to attraction at that location. The plane that includes the center of ions that are contact‐adsorbed to the electrode surface is often called the inner Helmholtz plane (IHP) (see Figure 1.6). An interesting consequence of these additional forces is that some anions can remain attached to the electrode surface even when the electrode is also negatively charged. To account for contact adsorption, the equation for the charge balance becomes

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Figure 1.6 The inner Helmholtz plane (IHP) is a plane parallel to the electrode drawn through the center of ions or neutral molecules adsorbed directly in contact with the electrode surface. London and image forces as well as electron overlap between the metal and ion are responsible for the net attraction. These forces can result in binding even when charge considerations oppose adsorption [5].

The model of Helmholtz suggests that all of the counter ions necessary to balance the charge on the electrode are held rigidly near the electrode surface. However, the experimental evidence suggests that thermal forces are great enough to dislocate counter ions to some degree. These observations inspired work by Louis Gouy [6] and by David Chapman [7] that led to a different model [5]. They proposed that the counter ions are distributed in a nonuniform manner with a high concentration of counter ions near the electrode that falls off exponentially with distance from the electrode surface (Figure 1.7).

Figure 1.7 (a) Gouy–Chapman model of electrical double layer showing a diffuse region of excess charge on the solution side. (Solvent molecules are not shown for clarity.) (b) The excess concentration drops off exponentially with distance from the electrode surface.

Unfortunately, this diffuse–charge model seemed to overcompensate. Experiments indicate that only a fraction of the charge appears to be disrupted by thermal agitation of the surrounding solution, whereas the diffuse charge model implied that it all was susceptible to thermal motion. In 1924, Otto Stern proposed a new model that was a synthesis of the earlier two [8]. Stern proposed that part of the compensating charge on the solution side is held tightly in the IHP plus the OHP and the remaining fraction of charge is contained in a diffuse zone of freely moving counter ions with a concentration that decreases with distance from the electrode surface. The IHP and OHP are sometimes collectively called the Stern layer. This layer is considered a stagnant zone of solution that clings to the surface. Figure 1.8 shows the arrangement of ions at a negatively charged electrode according to the Stern model. The charge balance equation for the double layer becomes

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Figure 1.8 Stern model of the electrical double layer. Charges and solvent at the OHP cling to the solid surface. Not all of the charge on the surface of the electrode is compensated by the excess of cations at the outer Helmholtz plane (OHP). A fraction of the counter charge is represented in a diffuse region just beyond the OHP. (Only the charges for the excess cations – the number of cations greater than the local number of anions – are pictured for clarity.)

Notice how the electrical potential changes as a function of distance from the electrode surface. The Stern model has several implications. One is that at higher electrolyte concentrations, the diffuse region of excess counter ions becomes more compact. The distance from the surface to the outer edge of the diffuse region is known as the Debye length, λ_D. The Debye length is inversely proportional to the square root of the electrolyte concentration. A good benchmark to keep in mind is that the Debye length is about 100 Å (10 nm) for a sodium chloride solution of 0.01 M [9]. Most electroanalytical measurements are made at electrolyte concentrations of 0.01 M or higher. This profile for the potential has an important significance for voltammetry experiments where molecules must be transported to the electrode surface in order to exchange electrons with the surface. In most cases, the molecules can approach no closer than the OHP. Consequently, the electric potential that they experience is that of the OHP rather than the true potential of the electrode surface. As will be discussed in Chapter 5, this has important implications for the rate of electron transfer and the magnitude of the corresponding current in voltammetry experiments.

It seems appropriate to pause here and note an application of electrochemical principles to phenomena outside of the field of electroanalysis. Naturally occurring phase boundaries involving electrified surfaces often occur in a variety of environments. Here is just one example in which the structure of the electrical double layer is especially relevant. River waters frequently carry tiny soil particles that remain suspended in the water because of the negative charges on the mineral surface (see Figure 1.9). These surface charges arise from the crystal structure in which some Al³⁺ and Si⁴⁺ ions are replaced by lower valence cations, such as Mg²⁺. Cations from the surrounding solution form an electrical double layer with the surface of each clay platelet. In the river, where the electrolyte concentration is low, these charged particles have electrostatic fields that extend far enough into solution to keep neighboring particles from approaching each other closely. However, whenever a river flows into the sea, the ionic strength of the mixture increases suddenly (the sea is about 0.5 M in NaCl) decreasing the distance that the electrostatic field reaches from the surface of a particle. Under these conditions, collisions bring the particles close enough for attractive interactions, such as van der Waals forces and hydrogen bonding, to overcome the repulsion of the charges. The particles cling to each other and can settle out faster as a result.

Figure 1.9 The electrical double layer plays an important role in the suspension or sedimentation of tiny particles, such as clay platelets. (a) Montmorillonite clay mineral structure with lysine bridging plates. (b) The surfaces of clay platelets are negatively charged as a result of lower valence cations replacing Al³⁺ and Si⁴⁺ ions in the crystal lattice. (c) At low ionic concentrations, the diffuse charge region of the sides extends several nanometers out into solution effectively pushing neighboring particles apart. (d) At high electrolyte levels, the field from the diffuse part of the electric double layer compacts allowing clay particles to approach more closely [12]. (e) Neutral polymers, such as naturally occurring polysaccharides, can adsorb at multiple points to neighboring particles leading to aggregation. Aggregation processes are known as coagulation and flocculation.

Source: Adapted with permission from Zhu et al. [17]. Copyright 2019, Elsevier.

Seawater and soil particles are complicated mixtures and multiple mechanisms for binding particles together have been described. In some cases, calcium and other di‐ or tri‐valent cations can bridge between adjacent particles [10]. Another mechanism has been exploited in water treatment and industrial applications of clay materials. Water soluble, neutral polymers, such as polysaccharide chains, are added to clay suspensions in industrial processes to bridge between particles by hydrogen bonding to oxygen atoms in the clay surface [11]. Polymers that attach at several points but still loop out into the solution appear to work best. Presumably, the loops in the chains extend far enough to reach across the electric double layer of neighboring particles. The compression of the electrical double layer is a key part of the mechanism in both industrial and natural processes. As these particles agglomerate at the mouth of rivers, they settle out of solution‐carrying nutrients, and sometimes pollutants, into the sediments. This process has very important implications for the ecology of estuaries and the biological productivity of marine environments [12].