Binary mixtures, acetonitrile-water

Retention is reduced by increasing the solvent concentration, conversely, retention is increased by increasing the proportion of water. Binary mixtures of water and acetonitrile or water and methanol are, unfortunately, not simple binary mixtures because, as is well known, they associate strongly with one another. Thus, a nominally binary mixture of methanol and water is, in fact, a ternary mixture of water, methanol and water associated with methanol. It follows, that some discussion on aqueous solvent mixtures would be pertinent. 

FIGURE 13.2 Plots of viscosity versus eluent composition for binary mixtures of water and methanol (top curve) and water and acetonitrile (bottom curve). 

Variations in retention and selectivity have been studied in cyano, phenyl, and octyl reversed bonded phase HPLC columns. The retention of toluene, phenol, aniline, and nitrobenzene in these columns has been measured using binary mixtures of water and methanol, acetonitrile, or tetrahydrofuran mobile phases in order to determine the relative contributions of proton donor-proton acceptor and dipole-dipole interactions in the retention process. Retention and selectivity in these columns were correlated with polar group selectivities of mobile-phase organic modifiers and the polarity of the bonded stationary phases. In spite of the prominent role of bonded phase volume and residual silanols in the retention process, each column exhibited some unique selectivities when used with different organic modifiers [84],... 

Figure 1.10—Chromatograms of a separation. The mobile phase in each trace is a binary mixture of water/ acetonitrile a) 50/50 b) 55/45 c) 60/40 d) 65/35. The arrow indicates the dead time tM in minutes (from J.W. Dolan, LC-CC Int. 1994, 7(6), 333).

The separation of two compounds A and B is studied by HPLC on a column of type RP-18. The mobile phase is a binary mixture of water and acetonitrile. A linear relationship exists between the logarithm of the retention factor and the % of acetonitrile within the binary mixture (H20/CH3CN). 

Cell (D) has been used several times to determine the solvent transference number A of the sparingly soluble salt Ag2S04 in the binary solvent mixtures acetonitrile-water l, dimethylsulphoxide-water and dimethylsulphoxide-methanol In Fig. 3 the solvent transference number of Ag2S04 is plotted versus Xdmso =... 

When in a mixture of two solvents, both ions of a binary salt are solvated preferably by the same solvent, the term applied is homoselective solvation (Fig. 2-lOa). Similarly, the preferred solvation of the cation by one, and the anion by the other solvent, is termed heteroselective solvation (Fig. 2-lOb) [119], Thus, in a solution of silver nitrate in the binary solvent mixture acetonitrile/water, a preferential solvation of Ag by acetonitrile and of NO by water was observed (heteroselective solvation) [121, 369] . In contrast, in solutions of calcium chloride in water/methanol mixtures, both Ca and Cl are solvated largely by water (homoselective solvation) [122], Zn (from ZnCl2) in... 

Figure 4.23. Variation of viscosity with composition for binary mixtures of water and tetrahydrofuran (A), methanol (B) and acetonitrile (C).

In RPLC, by considering eqn [5] and the values from Table 1, to a first approximation, it may be expected that mixtures of the same solubility parameter will yield the same solute retention factor. In other words, if acetonitrile (ACN) or tetrahydrofu-ran (THF) is used instead of methanol (MeOH) in a binary mixture with water, a composition of 9 ACN = 0 78 MeOH < THF = 0-fi2 MeOH is expected to yield roughly the same retention factor as the composition... 

Katz, Lochmuller and Scott also examined acetonitrile/water, and tetrahydrofuran (THF)/water mixtures in the same way and showed that there was significant association between the water and both solvents but not nearly to the same extent as methanol/water. At the point of maximum association for methanol, the solvent mixture contained nearly 60% of the methanol/water associate. In contrast the maximum amount of THF associate that was formed amounted to only about 17%, and for acetonitrile the maximum amount of associate that was formed was as little as 8%. It follows that acetonitrile/water mixtures would be expected to behave more nearly as binary mixtures than methanol/water or THF/water mixtures. 

Bonded phases are the most useful types of stationary phase in LC and have a very broad range of application. Of the bonded phases, the reverse phase is by far the most widely used and has been applied successfully to an extensive range of solute types. The reverse phases are commonly used with mobile phases consisting of acetonitrile and water, methanol and water, mixtures of both acetonitrile and methanol and water, and finally under very special circumstances tetrahydrofuran may also be added. Nevertheless, the majority of separations can be accomplished using simple binary mixtures. 

In the binary water/acetonitrile mixture, however, water coordinates much stronger to Li+ than acetonitrile, such that addition of water immediately leads to the formation of [Li(H20)4]+ (93). It was observed that the position of the NMR signal moved significantly (from 4.5 to 6.4 ppm) even if the amount of added water was very small (only 10% H20). Then the chemical shift increased slightly and stayed almost constant while the amount of water was increased from 10 to 100%. This observation confirms that water coordinates much stronger to the Li+ ion than acetonitrile (Fig. 5). 

Figure 13.3 shows inlet pressure versus flow rate profiles calculated using relationships explained elsewhere.28 A 4.6 mm x 250 mm column packed with particles ranging in size from 1 to 5 /.un was used for each case. The solid lines in the figure represent an eluent of 100% water. The dotted lines above each solid line represent a binary mixture of 60 40 methanokwater, and the dashed lines represent binary blends of 50 50 acetonitrile water. As an example, a column packed with 5 fjm... 

The polarity values of binary acetonitrile/water and methanol/water mobile phases used in RPLC were measured and compared with methylene selectivity (acH2) for both traditional siliceous bonded phases and for a polystyrene-divinylbenzene resin reversed-phase material [82], The variation in methylene selectivity for both was found to correlate best with percent organic solvent in methanol/water mixtures, whereas the polarity value provided the best correlation in acetonitrile/water mixtures. The polymeric resin column was found to provide higher methylene selectivity than the siliceous-bonded phase at all concentrations of organic solvent. 

Reversed-phase chromatography employs a nonpolar stationary phase and a polar aqueous-organic mobile phase. The stationary phase may be a nonpolar ligand, such as an alkyl hydrocarbon, bonded to a support matrix such as microparticulate silica, or it may be a microparticulate polymeric resin such as cross-linked polystyrene-divinylbenzene. The mobile phase is typically a binary mixture of a weak solvent, such as water or an aqueous buffer, and a strong solvent such as acetonitrile or a short-chain alcohol. Retention is modulated by changing the relative proportion of the weak and strong solvents. Additives may be incorporated into the mobile phase to modulate chromatographic selectivity, to suppress undesirable interactions of the analyte with the matrix, or to promote analyte solubility or stability. 

Solvatochromism and piezochromism of a range of pentacyanoferrates(II) have been examined in binary aq ueous solvent mixtures, " and their solvatochromism in micelles and reversed micelles. The solvatochromism of [Fe(CN)5(nicotinamide)] has been established in several ranges of water-rich binary solvent mixtures, " of [Fe (CN)5(2,6-dimethylpyrazine)] in acetonitrile-water mixtures.The solvatochromism of [Fe(CN)5(4Phpy)] and [Fe(CN)5(4Bu py)] has been proposed as an indicator of selective solvation in binary aqueous solvent mixtures. ... 

VI. ACETONITRILE-WATER VERSUS DMSO-WATER BINARY MIXTURES... 

Usually, when an HPLC method is developed, an acceptable degree of separation for all the components of interest in our sample is required in a reasonable time. The mobile phases more frequently used are the classical mixtures of methanol-water and acetonitrile-water in different proportions. If a satisfactory separation cannot be achieved using a binary solvent mixture as mobile phase, a ternary composition may be used. 

P. Suppan, Time-resolved luminescence spectra of dipolar excited molecules in liquid and solid mixtures - dynamics of dielectric enrichment and microscopic motions, Faraday Discuss., (1988) 173-84 L. R. Martins, A. Tamashiro, D. Laria and M. S. Skaf, Solvation dynamics of coumarin 153 in dimethylsulfoxide-water mixtures Molecular dynamics simulations, J. Chem. Phys., 118 (2003) 5955-63 B. M. Luther, J. R. Kimmel and N. E. Levinger, Dynamics of polar solvation in acetonitrile-benzene binary mixtures Role of dipolar and quadrupolar contributions to solvation, J. Chem. Phys., 116 (2002) 3370-77. 

A simple example of modifier-solute selectivity effects is shown in figure 3. The retention of five different functional groups is compared for three different binary solvent mixtures with water. The acetonitrile and tetrahydrofuran concentrations have been choosen so as to provide the same retention for benzene, a k of 4.7. Different solvent modifiers are strongest for different solutes. The shifting of peaks relative to each other is quite pronounced among the different solvents. 

Polar group selectivity also occurs in ternary solvent systems (5.10). For example, the addition of 5% to 25% of a third solvent to a water-acetonitrile mixture can alter the relative retention of peaks, and often resolve overlapping peaks. Dolan et al (11) have employed ternary mobile phases of water, methanol and tetrahydrofuran to analyze vitamin tablets where interfering peaks could not be resolved with binary mixtures. See Figure 4. 

The proposed modeling approach has been validated for distillation of non-reactive mixtures. For this purpose, the use is made of the total reflux distillation data for the binary mixture chlorobenzene/ethylbenzene (CB/EB) and ternary mixture methanol/acetonitrile/water (MEOH/ACN/WATER) obtained by Pelkonen (1997) as well as for the ternary mixture methanol/ethanol/water (MeOH/EtOH/WATER) measured by Mori et al. (2006). The experiments of Pelkonen (1997) were carried out in a column of 100 mm diameter, equipped with Montz-Pak A3-500 structured packing. The measured concentrations, temperature and flow rates at the condenser outlet are used as input values for simulations. 

These very simple relationships can be verified experimentally as is shown in figure 3.16. The iso-eluotropic compositions of binary mixtures of THF and acetonitrile with water have been plotted against the binary methanol-water composition. The thin straight lines indicate the theoretical relationships from solubility parameter theory (eqns. 3.50 and 3.51). The thick lines correspond to average experimental data over large numbers of solutes [335]. An (average) experimental data point can be found as follows. 

Figure 3.16 Iso-eluotropic compositions for binary mixtures of THF and acetonitrile in water, relative to methanol/water mixtures. The solid lines represent the average experimental compositions for a large number of solutes. The thin lines represent calculated compositions from solubility parameter theory (eqns.3.50 and 3.51). Figure taken from ref. [311]. Reprinted with permission.

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