Non-aqueous systems

Corrosion in non-aqueous hquids such as fuels, lubricants, and edible oils is usually caused by the small amounts of water often present. Water is shghtly soluble in petroleum products, and its solubility increases with temperature. If a non-aqueous solvent is saturated with water and the temperature is lowered, then some of the water will separate out to attack any steel that it contacts. 

Small amounts of water inhibit corrosion in some non-aqueous solvents. Halogenated (containing chlorides, fluorides, bromides, or iodides), non-aqueous solvents can be particularly troublesome. Organic amines are effective inhibitors for steel degreasing vessels that contain hot chlorinated solvents. 

It has only recently been discovered that SER spectra can be obtained from systems involving non-aqueous electrolyte solutions [35, 36], The first report was for pyridine in N, iV-dimethylformamide solution at a silver electrode [35]. Variations in the relative intensities of the pyridine bands in the 1000 cm-1 wavenumber region as a function of electrode potential are shown in Fig. 19. These potential-dependent changes are quite different from those recorded for the corresponding aqueous system and have been interpreted in terms of the solvent effect on the surface morphology of the electrode. 

The wider electrochemical potential windows associated with non-aqueous solvents than with water opens the way to a far richer field of reaction studies. A report of SERS of the tris(2,2-bipyridyl)-ruthenium(II) complex ion, [Ru(bpy)3]2+, adsorbed from acetonitrile solution on to a silver electrode [36] has been followed, independently, by a report [37] on an in situ SERS study of the electroreduction reaction to [Ru(bpy)3]+. It had been 

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The term magic acid coined in G. A. Olah s laboratory for the non-aqueous system HSOaF/SbFs. 

A second hold-up correlation reported by T. Otake and K, Okada [55] represents a survey of considerable literature, and is applicable to aqueous and non-aqueous systems for Reynolds numbers from 10, - 20,000 [40]. 

Synthesis of fluoride compounds is performed in various media, such as aqueous solutions, non-aqueous systems and heterogeneous interactions. 

Dispersible silicone emulsions are generally preferred for aqueous systems, whereas silicone fluids and compounds are preferred for non-aqueous systems. Silicones are widely employed in cooling water treatment programs, less so in boiler plants because of higher operating costs than available alternatives, but also because of sometimes questionable emulsion stability at higher temperatures. 

Mann, C. K., and Barnes, K. K. (1970). Electrochemical Reactions in Non-Aqueous Systems , Marcel Dekker. 

To overcome some of the problems associated with aqueous media, non-aqueous systems with cadmium salt and elemental sulfur dissolved in solvents such as DMSO, DMF, and ethylene glycol have been used, following the method of Baranski and Fawcett [48-50], The study of CdS electrodeposition on Hg and Pt electrodes in DMSO solutions using cyclic voltammetry (at stationary electrodes) and pulse polarography (at dropping Hg electrodes) provided evidence that during deposition sulfur is chemisorbed at these electrodes and that formation of at least a monolayer of metal sulfide is probable. Formation of the initial layer of CdS involved reaction of Cd(II) ions with the chemisorbed sulfur or with a pre-existing layer of metal sulfide. 

H. Nae, W. Reichert, and A. C. Eng. Organoclay compositions containing two or more cations and one or more organic anions, their preparation and use in non-aqueous systems. Patent EP 681990,1999. 

Mann CK, Barnes KK (1967) Electrochemical reactions in non-aqueous systems, Marcel Decker New York, 1970. Chapter 9. J Electroanal Chem 13 1474... 

In studying the stability of colloidal dispersions it is of considerable advantage if the particles concerned are monodisperse and spherical. For aqueous, charge-stabilised systems polymer latices have proved invaluable in this regard. With non-aqueous systems, steric stabilisation is usually required. In this case it... 

In such systems the requirement of the electrostatic contribution to colloidal stability is quite different than when no steric barrier is present. In the latter case an energy barrier of about 30 kT is desirable, with a Debye length 1/k of not more than 1000 X. This is attainable in non-aqueous systems (5), but not by most dispersants. However when the steric barrier is present, the only requirement for the electrostatic repulsion is to eliminate the secondary minimum and this is easily achieved with zeta-potentials far below those required to operate entirely by the electrostatic mechanism. 

Any fundamental study of the rheology of concentrated suspensions necessitates the use of simple systems of well-defined geometry and where the surface characteristics of the particles are well established. For that purpose well-characterized polymer particles of narrow size distribution are used in aqueous or non-aqueous systems. For interpretation of the rheological results, the inter-particle pair-potential must be well-defined and theories must be available for its calculation. The simplest system to consider is that where the pair potential may be represented by a hard sphere model. This, for example, is the case for polystyrene latex dispersions in organic solvents such as benzyl alcohol or cresol, whereby electrostatic interactions are well screened (1). Concentrated dispersions in non-polar media in which the particles are stabilized by a "built-in" stabilizer layer, may also be used, since the pair-potential can be represented by a hard-sphere interaction, where the hard sphere radius is given by the particles radius plus the adsorbed layer thickness. Systems of this type have been recently studied by Croucher and coworkers. (10,11) and Strivens (12). 

The thallous derivative so formed was found to undergo metathesis with methyl iodide and to yield thallous iodide and methyl cellulose. Analyses of the latter product for methoxyl content after suitable purification yielded a measure of the accessible hydroxyl groups. The reactions were all conducted in non-aqueous systems. 

The DLVO theory, a quantitative theory of colloid fastness based on electrostatic forces, was developed simultaneously by Deryaguin and Landau [75] and Verwey and Overbeek [76], These authors view the adsorptive layer as a charge carrier, caused by adsorption of ions, which establishes the same charge on all particles. The resulting Coulombic repulsion between these equally charged particles thus stabilizes the dispersion. This theory lends itself somewhat less to non-aqueous systems. 

The use of non-aqueous media offered new attractive possibilities for the analysis of hydrophobic compounds, which are often difficult to be analyzed due to their low solubility in aqueous media. Selectivities that are difficult to be obtained in aqueous buffers can be easily obtained using non-aqueous systems, due to larger differences in the ionized—unionized equilibrium for two closely related substances in non-aqueous solvents compared to aqueous solvents. Organic solvents such as methanol, ACN, ethanol, formamide, dimethyl formamide. 

The non-aqueous system of spherical micelles of poly(styrene)(PS)-poly-(isoprene)(PI) in decane has been investigated by Farago et al. and Kanaya et al. [298,299]. The data were interpreted in terms of corona brush fluctuations that are described by a differential equation formulated by de Gennes for the breathing mode of tethered polymer chains on a surface [300]. A fair description of S(Q,t) with a minimum number of parameters could be achieved. Kanaya et al. [299] extended the investigation to a concentrated (30%, PI volume fraction) PS-PI micelle system and found a significant slowing down of the relaxation. The latter is explained by a reduction of osmotic compressibihty in the corona due to chain overlap. 

The survey summaries show that zeoHte adsorbents are most often employed for non-aqueous systems. This is because the material generally used as a binder to fabricate an agglomerated zeoHte, is a clay comprising silicon dioxide and aluminum oxide which tends to dissolve in water. This dissolution results in negative changes in physical characteristics of the adsorbent as well as silicon contamination of the solution which manifests itself as turbidity in the product. 

It is very instructive to compare the kinetics and plausible mechanisms of reactions catalyzed by the same or related catalyst(s) in aqueous and non-aqueous systems. A catalyst which is sufficiently soluble both in aqueous and in organic solvents (a rather rare situation) can be used in both environments without chemical modifications which could alter its catalytic properties. Even then there may be important differences in the rate and selectivity of a catalytic reaction on going from an organic to an aqueous phase. TTie most important characteristics of water in this context are the following polarity, capability of hydrogen bonding, and self-ionization (amphoteric acid-base nature). 

It is well documented that amines oxidise differently in non-aqueous environments to those pathways seen in aqueous systems. In the former systems, hydrogen abstraction of the a-carbon predominates. The reactivity is in the decreasing order tertiary > secondary > primary amines. Oxidation in non-aqueous systems results in amides, aldehydes and carbon-nitrogen cleavage products [67]. 

Most chemists are familiar with chemistry in aqueous solutions. However, the common sense in aqueous solutions is not always valid in non-aqueous solutions. This is also true for electrochemical measurements. Thus, in this book, special emphasis is placed on showing which aspects of chemistry in non-aqueous solutions are different from chemistry in aqueous solutions. Emphasis is also placed on showing the differences between electrochemical measurements in non-aqueous systems and those in aqueous systems. The importance of electrochemistry in non-aqueous solutions is now widely recognized by non-electrochemical scientists - for example, organic and inorganic chemists often use cyclic voltammetry in aprotic solvents in order to determine redox properties, electronic states, and reactivities of electroactive species, including unstable intermediates. This book will therefore also be of use to such non-electrochemical scientists. 

Conversely, stripping voltammetry and electrochemical biosensors are outlined in 13) and 14), though their applications in non-aqueous systems are still rare. 

The reference electrodes used in non-aqueous systems can be classified into two types. One type uses, in constructing a reference electrode, the same solvent as that of the solution under study. The other type is an aqueous reference electrode, usually an aqueous Ag/AgCl electrode or SCE. Some reference electrodes are listed in Table 6.2 and are briefly discussed below. For other types of reference electrodes used in non-aqueous solutions, see Ref. [4],... 

Aqueous reference electrodes, such as SCE and Ag/AgCl electrodes, are often used in noil-aqueous systems by dipping their tips into lion-aqueous solutions of the salt bridge. The tip should not be dipped directly into the solution under study, because the solution is contaminated with water and the electrolyte (usually KC1). When we use such aqueous reference electrodes, we must take the liquid junction potential (LJP) between aqueous and non-aqueous solutions (Table 6.2) into account. If we carefully reproduce the composition of the solutions at the junction, the LJP is usually reproducible within 10 mV. This is the reason why aqueous reference electrodes are often used in non-aqueous systems. However, the LJP sometimes exceeds 200 mV and it is easily influenced by the electrolytes and the solvents at the junction (Section 6.4). The use of aqueous reference electrodes should be avoided, if possible. 

Because there is no truly reliable reference electrode for use in non-aqueous solutions, various reference electrodes have so far been used. Thus, accurate mutual comparison of the potential data in non-aqueous systems is often difficult. In order to improve the situation, the IUPAC Commission on Electrochemistry proposed a method to be used for reporting electrode potentials [8]. It can be summarized as follows ... 

In electrochemical measurements in non-aqueous systems, we sometimes use a cell with a junction between electrolyte solutions in different solvents. The problem of the LJP between different solvents is rather complicated but is discussed in Section 6.4. 

The pH scale in non-aqueous solutions was briefly discussed in Chapter 3. In this section, practical methods of pH measurements in non-aqueous systems are considered, with emphasis on the differences from those in aqueous solutions. 

Recently, Ta2Os- and Si3N4-type pH-ISFETs have been used in non-aqueous systems, by preparing them to be solvent-resistant [17]. In various polar non-aqueous solvents, they responded with Nernstian or near-Nernstian slopes and much faster than the glass electrode. The titration curves in Fig. 6.5 demonstrate the fast (almost instantaneous) response of the Si3N4-ISFET and the slow response of the glass electrode. Some applications of pH-ISFETs are discussed in Section 6.3.1. 

Potentiometry and potentiometric titrations are widely used in studying various types of reactions and equilibria in non-aqueous systems (Sections 6.3.1-6.3.4). They also provide a convenient method of solvent characterization (Section 6.3.5). Moreover, if the electrode potentials in different solvents can accurately be compared, potentiometry is a powerful method of studying ion solvation (Section 6.3.6). 

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