Solvents, mixed aqueous surface tension

The bulk properties of mixed solvents, especially of binary solvent mixtures of water and organic solvents, are often needed. Many dielectric constant measurements have been made on such binary mixtures. The surface tension of aqueous binary mixtures can be quantitatively related to composition. ... 

More than 70 years elapsed between the first report of aqueous two-phase systems (11) and their subsequent applications to biochemical systems (12). In the last ten years in particular, there have been several innovative applications of aqueous two-phase systems (13). Aqueous two-phase systems consist of two immiscible fluids in a bulk water solvent. In such systems, the percentage of water in both phases is high, i.e. between 75 and 95%. As a consequence, the surface tension between the two immiscible phases may be as low as 0.1 dyne/cm so that a gentle mixing is sufficient to produce and maintain an emulsion (14). 

Water ean usually be used as a unique solvent for the study of biomedical materials. In other applications such as reverse-phase liquid chromatography, however, different solvents such as water-methanol mixed solvents are used. As the MeOH fraction increases, the surface tension of the solution decreases. In mixed solvents, the trend of contact angles as a function of concentration was generally similar to that of the aqueous case. As the proportion of methanol increases, the contact angle deereases dramatically as in the case of decreased concentration. 

The fluorinated surfactant-hydrocarbon surfactant mixtures have unique properties. In two-phase systems of water and a hydrocarbon solvent, the fluorinated surfactant reduces surface tension and the hydrocarbon surfactant decreases the interfacial tension. For example, an aqueous foam of mixed surfactants spreads on a hydrocarbon solvent because the fluorinated surfactant adsorbs preferentially at the air-water interface, whereas the hydrocarbon surfactant adsorbs at the water-oil interface (see Chapter 8, Fire-fighting Foams). 

We present in this work all pubhshed data on density, refractive index, viscosity, electrical conductivity and surface tension for all systems IL + water and + ethanol covering a broad range of concentrations. For density, refractive index and viscosity the data for mixtures with water or ethanol are very similar, and also their behaviour with concentration is not really dependent of the IL mixed with any solvent (except for its value). Density and refractive index can be deduced one from another using Newton s model, which demonstrates the close relationship between both magnitudes. For electrical conductivity and surface tension, the solvent nature determines the data behaviour obtained. Thus, the electrical conductivity value of the pntre IL for aqueous systems increases up to 10 times, while that increase is halved for ethanol systems. In the case of surface tension the behaviour is completely different depending on the solvent and IL studied. For alkyl-methyl-imidazolium tetrafluoroborate the IL acts like a surfactant in water, and the surface tension value decreases sharply from that of water to that of the pure IL for small concentrations of this last, effect that does not appear for halogenated imidazohum ILs. If we change the water for ethanol, that surfactant like effect disappear, and the surface tension value of the only four ILs measured decreases linearly with the ethanol content down to a common value at about equimolar mixture, and then all data has the same value. 

Back in 1983, the concept of mixed solvent layer [16] resulted from the determination of water surface excess concentrations at different interfaces by interfacial tension measurements that showed that, in the case of the H2O-DCE interface, and unlike the liquid water-vapor or the water-heptane interfaces, the water excess concentration was less than a monolayer as expected for aqueous 1 1 electrolyte. The molecular dynamics results of Wick and Dang seem therefore to corroborate this early concept of interfacial structure in the presence of electrolytes in the aqueous phase. 

---