Density and surface tension in binary mixtures of CnMIM-BF4 ionic liquids with water and ethanol

We present new experimental data of the surface tension and density in binary mixtures of the ionic liquid (IL) family 1-alkyl-3-methyl imidazolium tetrafluoroborate with water and ethanol. Measurements were performed at 25.0 °C and atmospheric pressure, covering all range of possible concentrations. From these data we calculate the surface tension deviations and, from density data, the molar volumes. We observe that all sets of mixtures are ideal because the corresponding excess molar volumes are very small, similar to our experimental uncertainties. Density data for some of the aqueous mixtures studied here have been recently published, but discrepancies among authors are great. In contrast the number of published papers for ethanol mixtures is very scarce. It also happens for surface tension measurements in both studied systems. Data analysis indicates that the IL mixtures with water and with ethanol are similar in the bulk of the samples, but at surface level the behaviour is completely different. Thus, for water mixtures surface seems to be composed mainly by IL ions down to high diluted ones, and so the IL acts like a surfactant. Surface for ethanol mixtures is equally composed by IL ions and ethanol molecules. In addition for molar fractions of ethanol higher than equimolar, the alkyl chain length of the IL does not play any role in the surface tension of the mixture, being its value common for the three IL + ethanol mixtures studied.