Anion structures, electrode/solution interface

Adsorption of a condensed 1-hydroxy-adamantane layer at the Hg elec-trode/(Na2S04 or NaF) solution interface has been studied as a function of temperature by Stenina et al. [174]. Later, Stenina etal. [175] have determined adsorption parameters and their temperature dependence for a two-dimensional condensation of adamantanol-1 at a mercury electrode in Na2S04 solutions. They have also studied coadsorption of halide (F , Cl , Br ) anions and 1-adamantanol molecules on Hg electrode [176]. More recently, Stenina etal. [177] have described a new type of an adsorption layer comprising organic molecules of a cage structure condensed at the electrode/solution interface. This phenomenon was discovered for adsorption of cubane derivatives at mercury electrode. 

Electrolytes The above issue of double layer structure is important to the mechanism of nucleation and growth in ionic liquids, it may therefore be possible to control the structure at the electrode/solution interface by addition of an inert electrolyte. In this respect most Group 1 metals are soluble in most ionic liquids, although it is only generally lithium salts that exhibit high solubility. In ionic liquids with discrete anions the presence of Group 1 metal ions can be detrimental to the deposition of reactive metals such as A1 and Ta where they have been shown to be co-deposited despite their presence in trace concentrations. 

The only models that exist for double layer structure in ionic liquids suggest a Helmholz layer at the electrode/solution interface [103, 104], If the reduction potential is below the point of zero charge (pzc) then this would result in a layer of cations approximately 5 A thick across which most of the potential would be dropped, making it difficult to reduce an anionic metal complex. Hence, the double layer models must be incorrect. 

Figure 3 schematically depicts the structure of the electrode—solution interface. The inner Helmholtz plane (IHP) refers to the distance of closest approach of specifically adsorbed ions, generally anions to the electrode surface. In aqueous systems, water molecules adsorb onto the electrode surface. 

Investigation of electrode solution interfaces by in situ vibrational spectroscopy has two principal advantages firstly the species present and their structures are directly characterized by their spectra and, secondly, these spectra are sensitive to the environment and therefore can be used to probe complex interactions. Raman spectroscopy is particularly well suited to the investigation of aqueous systems and in certain cases the adsorption of neutral species, of anions in the double layer and of the solvent (as well as interactions between these species) can now be characterized[48]. Vibrational spectroscopy of systems of practical importance is illustrated by the Surface Enhanced Raman Spectra (SERS) of the corrosion inhibitor thiourea adsorbed at silver and copper electrodes[49] it should be noted that inhibitors such as thiourea are also used as plating additives. 

Anion(s), role in boundary layer at metal-electrolyte solution interface, 126-127 Anion adsorption and charge transfer on single-crystal electrodes advantages and disadvantages, 169 anion coverages, 161,162/ ball models for adlayer structures, 161,163/ charge correction, 167,169 defects, influence on adsorption, 165,166/... 

At the Hg electrode/electrolyte-solution interface, the electrical and structural situation is particularly complex (Fig. Id) and can be thought of as the result of combining the electrical anisotropy of the metal interface with that of a dipolar liquid, together with the distribution of dissolved cations and anions that arises, depending on the net charge, q, on the metal, plus any specific chemisorption affinity cations, or especially anions, may have for the metal surface. 

While the structure at the electrode/ionic liquid interface is uncertain it is clear that in the absence of neutral molecules the concentration of anions and cations at the interface will be potential dependent. The main difference between aqueous solutions and ionic liquids is the size of the ions. The ionic radii of most metal ions are in the range 1-2 A whereas for most ions of an ionic liquid they are more typically 3-5 A. This means that in an ionic liquid the electrode will be coated with a layer of ions at least 6-7 A thick. To dissolve in an ionic liquid most metal species are anionic and hence the concentration of metal ions close to the electrode surface will be potential dependent. The more negative the applied potential the smaller the concentration of anions. This means that reactive metals such as Al, Ta, Ti and W will be difficult to deposit as the effective concentration of metal might be too low to nucleate. It is proposed, as one explanation, that this is the reason that aluminum cannot be electrodeposited from Lewis basic chloroaluminate ionic liquids. More reactive metals such as lithium can however be deposited using ionic liquids because they are cationic and therefore... 

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