Acetonitrile solvent properties

The recent introduction of non-aqueous media extends the applicability of CE. Different selectivity, enhanced efficiency, reduced analysis time, lower Joule heating, and better solubility or stability of some compounds in organic solvent than in water are the main reasons for the success of non-aqueous capillary electrophoresis (NACE). Several solvent properties must be considered in selecting the appropriate separation medium (see Chapter 2) dielectric constant, viscosity, dissociation constant, polarity, autoprotolysis constant, electrical conductivity, volatility, and solvation ability. Commonly used solvents in NACE separations include acetonitrile (ACN) short-chain alcohols such as methanol (MeOH), ethanol (EtOH), isopropanol (i-PrOH) amides [formamide (FA), N-methylformamide (NMF), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA)] and dimethylsulfoxide (DMSO). Since NACE—UV may present a lack of sensitivity due to the strong UV absorbance of some solvents at low wavelengths (e.g., formamides), the on-line coupling of NACE... 

The properties of both organic matter and clay minerals may affect the release of contaminants from adsorbed surfaces. Zhang et al. (1990) report that desorption (in aqueous solution) of acetonitrille solvent from homoionic montmorillonite clays is reversible, and hysteresis appears to exist except for K+-montmorillonite. This behavior suggests that desorption may be affected by the fundamental difference in the swelling of the various homoionic montmorillonites, when acetonitrile is present in the water solution. During adsorption, it was observed that the presence of acetonitrile affects the swelling of different homoionic clays. At a concentration of 0.5 M acetonitrile in solution, the layers of K+-montmorillonite do not expand as they would in pure water, while the layers of Ca +- and Mg +-montmorillonite expand beyond a partially collapsed state. The behaviors of K+-, Ca +-, and Mg +-montmorillonite are different from the behavior of the these clays in pure water. Na+-montmorillonite is not affected by acetonitrile presence in an aqueous solution. 

Pic. 17. Dependence of solvent properties pertinent to RPC on composition of water-acetonitrile mixtures at 2S C. Surface tension y data were obtained from Timmermans U34)i the viscosity and dielectric constant < data were taken from Timmermans (134) and Doubdret and Morenas (137), respectively. Reprinted from Horvdth and Melander (129), J. Chromatogr. Sci., with permisskw from Preston Publications.

The effect of the solvent properties on the polarographic behavior of Cd(II) complexes with glycine. At-acetyl, and N-benzoylglycine was studied in DM SO, acetonitrile (AN), and DMF solutions [90]. The stability constants were found to depend linearly on the acceptor numbers of the solvents. 

Both in acetonitrile and in other non-aqueous solvents, a major problem arises in terms of the manner in which the potential values are reported by various investigators. Koepp, Wendt, and Strehlow [6] noted that hydrogen ion is the poorest reference material on which to base nonaqueous potentials because of the extreme differences in its solvation in various solvents. On the basis of an investigation of the solvent dependence of 18 redox couples, these investigators concluded that ferrocene/ferrocenium ion (i.e. bis(cyclopentadienyl)iron(III/II), abbreviated as Fc+ /Fc°) and/or cobal-tocene/cobalticenium ion represented optimal potential reference materials for nonaqueous studies. On the basis of their minimal charge (+1, 0) and their symmetry (treated as though they were roughly spherical), the potentials of these two redox couples are presumed to be relatively independent of solvent properties. 

Acetonitrile (mp, -45°C bp, 81°C) is a colorless liquid with a mild odor. Because of its good solvent properties for many organic and inorganic compounds and its relatively low boiling point,... 

The effect of solvent properties on double-layer capacitance of TiC-CDC was explored using triethylmethylammonium tetrafluoroborate in PC, DMK (dimethyl ketone), y-butyrolactone, and acetonitrile [97,98], The capacitance was shown to decrease in the order acetonitrile > y-butyrolactone > DMK > PC and was dependent on polarization. The same trend was found in relating solvent to cycling efficiency. Interestingly, CDC was shown to have a lower time constant than an advanced... 

Organic solvents influence the ionization constants of weak acids or bases in several ways (note that they influence the analytes and the buffer as well). Concerning ionization equilibria, an important solvent property is the basicity (in comparison to water), which reflects the interaction with the proton. From the most common solvents, the lower alcohols and acetonitrile are less basic than water. Dimethyl sulfoxide is clearly more basic. However, stabilization of all particles involved in the acido-basic equilibrium is decisive for the pKa shift as well. For neutral acids of type HA, the particles are the free, molecular acid, and the anion, A . In the equilibrium of bases, B, stabilization of B and its conjugated acid, HB, takes place. As most solvents have a lower stabilization ability toward anions (compared to water), they shift the pK values of adds of type HA to higher values in general. No such clear direction of the change is found for the pK values of bases however, they undergo less pronounced shifts. 

The solvent properties of alcohols with short carbon chains are similar to those of water and such alcohols could be used as the nonaqueous catalyst phase when the products are apolar in nature. The first commercial biphasic process, the Shell Higher Olefin Process (SHOP) developed by Keim et al. [4], is nonaqueous and uses butanediol as the catalyst phase and a nickel catalyst modified with a diol-soluble phosphine, R2PCH2COOH. While ethylene is highly soluble in butanediol, the higher olefins phase-separate from the catalyst phase (cf. Section 2.3.1.3). The dimerization of butadiene to 1,3,7-octatriene was studied using triphenylphosphine-modified palladium catalyst in acetonitrile/hexafluoro-2-phe-nyl-2-propanol solvent mixtures [5]. The reaction of butadiene with phthalic acid to give octyl phthalate can be catalyzed by a nonaqueous catalyst formed in-situ from Pd(acac)2 (acac, acetylacetonate) and P(0CeH40CH3)3 in dimethyl sulfoxide (DMSO). In both systems the products are extracted from the catalyst phase by isooctane, which is separated from the final products by distillation [5]. 

Using ethylammonium nitrate (EAN) as PIL ( . , =0.95, ji = 1.12, a=1.10, P = 0.46), following types of binary mixture models were selected for the analysis and quantification of the microscopic solvent properties (a) [molecular aprotic solvent with HBA ability + PIL cosolvent], (b) [molecular aprotic solvent with both HBD and HBA ability + PIL cosolvent] and (c) [molecular protic solvent + PIL cosolvent] [31]. The molecular solvents included in this analysis were dimethylsul-phoxide (DMSO) ( ., =0.44, 7r = 1.00, a=0.02 and p=0.76) as a polar aprotic HBA solvent, acetonitrile (AN) ( .,. =0.46, 7t =0.75, a=0.19, p=0.40) as polar aprotic HBA/HBD solvent and methanol ( .,. =0.76, 7t =0.60, a=0.98, p=0.66) as a protic solvent. EAN is a N-H-bond donor. In all cases, the pure component part of the mixtures was capable of forming associated species through hydrogen-bonding interactions. For the explored solvent mixtures, empirical parameters . n, a and P were calculated from the wave numbers of the absorbance maxima of the corresponding chemical probes at 25°C. 

The most pertinent feature in Table 3.7 is the vast range of mechanical properties that have been reported for PPy. It is apparent that the composition of the polymer (e.g., counterion type) and the polymerization conditions have a significant effect on the polymer properties. However, the relationships are not straightforward. For example, Wynne and Street113 have shown that acetonitrile solvent yields PPy films with very good mechanical properties, whereas Sun and coworkers11 and others have reported the opposite. It is clear that systematic analyses are required to elucidate the determinants of the mechanical properties of PPy s. 

Because of their crucial role in the ionization step, solvents have a profound effect on the rates of El reactions. These rates for a number of tertiary halides have been determined in a variety of solvents. For r-butyl chloride there are huge differences in the rates in water (log k = -. 54), ethanol (log k = -7.07), and diethyl ether (log k = — 2.1A)P Similarly, the rates of the El reaction of 1-methylcyclopentyl bromide range from 1 x 10 s in methanol to 2 x 10 s in hexane. Polar aprotic solvents such as DMSO (k = 2x lO s ) and acetonitrile (k = 9x 10 s ) are also conducive for ionization. The solvent properties that are most important are polarity and the ability to assist leaving group ionization. These, of course, are the same features that favor reactions, as we discussed in Section 3.8. 

The properties of solvents has been studied extensively by Snyder (5), who created a dassification of the solvent properties of common solvents. It has been found (7) that (excluding proton donors such as alcohols) the maximum difference in mobile-phase selectivity is obtained if the polar solvents have a large difference in basidty. Thus, for maximum selectivity differences, one solvent should have a low basidty. Solvents of this type are acetonitrile, ethyl acetate or other esters, and acetone or other ketones. The other solvent should have a high basidty examples are ethers such as tert-butyl methyl ether, diethyl ether or tetrahydrofuran, or amines such as triethylamine. Between these groups and alcohols, large differences in chromatographic selectivity can be obtained in normal-phase chromatography (10). 

Molecular properties of some common anal3 es. Log k was measured on an ODS silica gel in 80% aqueous acetonitrile at 40 °C. ac represents molecular properties calculated using a model acetonitrile solvent phase. Reproduced by permission of Oxford University Press, ref. 48. 

Molecular properties of standard compounds in a model acetonitrile solvent phase. Log k values were measured on an ODS-bonded silica gel in aqueous 70% acetonitrile containing 0.01% phosphoric acid." HOMO and LUMO energy values (eV) were calculated using the PM5 program. 

In water-organic solvent mixtures the decrease in the water concentration is generally accompanied by degradation of the water associates and hence by the formation of smaller associates, which finally leads to the liberation of monomeric water molecules. It must be noted, however, that this process does not result in enhancement of the donor properties of the water in every system. For example, Moreau and Douheret [Mo 74] reported that, in a water-acetonitrile solvent mixture, the breakdown of the water structure as a consequence of the addition of acetonitrile is not associated with an increase in the solvation of the proton. 

There are many other solvents which fall between the extreme cases which we have so far considered, for example, solvents of moderate basicity like the amides or acetonitrile, solvents with basic but no acidic properties such as the ethers, and solvents of moderate acidity such as the... 

The rates of reaction of copper, zinc and uranium with dinitrogen tetroxide are greatly increased in the presence of acetonitrile Reactions of this type lead to the formation of acetonitrile complexes of the corresponding nitrate, which may loose coordinated acetonitrile by pumping in vacvo or heating. Reactions in such mixed acceptor solvent-donor solvent systems may provide unusual solvent properties and it is desirable that such systems should be investigated in considerable detail. 

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