Solvents, mixed aqueous vapor pressure

Assays are often performed in an aqueous medium with water as the solvent. Most drugs are organic molecules with poor water solubility. To improve solubility or at least evenly disperse the compound throughout the assay well, compounds are commonly dissolved in dimethyl sulfoxide (DMSO). DMSO has several attractive properties. DMSO dissolves organic compounds well, has a low vapor pressure to limit evaporation, and is fairly inert. Furthermore, DMSO freezes around 18 °C. A solution of a compound in DMSO can be easily frozen for long-term storage. Because high concentrations of DMSO may affect the reliability of an assay, compound solutions must be prepared such that, when mixed in the assay, the final DMSO concentration is 1% or less. 

Although Johnson and Furter (1,2), among others, observed a surprising insensitivity of k to mixed-solvent composition in many alcohol-water-inorganic salt systems, such does not appear to be the case with ammonium bromide-ethanol-water. A linear dependence of k with x was observed and is demonstrated in Figure 4. The slope of this dependence is 2.63 and the intercept with the y-axis occurs at approximately a value of unity. This extrapolated salt effect when x = 0, that is, with water as the single solvent, is consistent with Raoult s Law in that the vapor pressure of the aqueous salt solution depends directly on the salt concentration. However the same behavior has not been observed for the ammonium chloride-ethanol-water system (3) as seen in Table VIII its salt effect parameter shows essentially no dependence on the liquid composition. Therefore the two systems differ in this respect. 

In Fig. 7, the critical phase behavior of binary aqueous solutions of several selected hydrocarbons and additionally fluorobenzene is shown, most of them having been measured in our laboratory [3]. The dashed curve is the vapor pressure curve of pure water, and the solid lines are parts of the branches of the binary critical p T) curves that start from the critical endpoint llg (systems 9 and 10) or from the critical point of pure water CP(H20). Whereas naphthalene H- water (system 9) and biphenyl -f water (system 10) show class-II behavior, all other systems belong to class III according to the classification of van Konynenburg and Scott, and thus exhibit gas-gas equilibria of the second kind. The consequence is that naphthalene and biphenyl are completely miscible with water already at quite low pressures near the vapor pressure curve of pure water. This behavior is of interest for measurements in mixed solvents and for separations. 

In a thermospray interface, a jet of vapour and small droplets is formed by heating the column effluent of an LC column or any other continuous liquid stream in a heated vaporizer tube. Nebulization takes place as a result of the disruption of the liquid by the expanding vapour that is formed upon evaporation of part of the liquid in the tube. A considerable amount of heat is transferred to the solvent prior to the onset of the partial inside-tube evaporation. This assists in the desolvation of the droplets in the lower pressure region. By applying efficient pumping directly at the ion source, up to 2 mL min i of aqueous solvents can be introduced into the MS vacuum system. The ionization of the analytes takes place by mixed mechanisms based on gas-phase ion-molecule... 

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