Liquid Effects on Surface Tension

Surface tension lowering trends have been observed with ionic liquids when the alkyl chains are increased. This trend in surface tension behavior seemed to be present irrespective of the alkyl chain position on the cation or the anionic portion of the ionic liquid. A direct relationship between alkyl chain length and surface tension was shown with the series of [RMIMjlR-OSOj [5], where the alkyl group of the anion was varied during the first part of the experiment The second set of spectra was obtained for [RMIM][R-0S03 where the cation alkyl chains were changed and the anion was kept the same. 

Surface tension measurements via the volume drop method determined surface tensions of the cation-anion pairs [RMIM][Anion], where R stands for alkyl chains on the cation ranging from C4 to Cjj, combined with various anions. Surface 

Effects of anion on surface tension. Surface tension measurements by the du Nouy ring method kept the type of cation constant while the anions were varied to include [BMIM][Bp4], [BMlM][PFg], BMlM][Tf2 N], and [BMIMJpFjSO ] and four additional cation-anion pairs. The dry ionic liquids showed the expected decreasing trends in surface tension with increasing temperature for each ionic liquid cation-anion series [5]. Increasing anion size appeared to correlate with a decrease in surface tension of the ionic liquid - as noted in reference [34]. 

Incorrect assumptions, however, may result if only one particular set of data is used to make a straightforward correlation between surface tension lowering effects and the type of anion. It is more informative to compare the results of several data sets including more than one type of anion. This becomes obvious when comparing the surface tension lowering trends with a homologous series of an ionic liquid that contains a different type of anion. It was demonstrated in a different study where the alkyl chain lengths were varied and the anion remained the same. 

Surface tension data from the series of [BMIM][BF4] show 44mNm at 293 K whereas the surface tension for the ionic Hquid [OMIM][BF4] was reported as 33 mN m at the same temperature. The surface tension decreases, but in this case, the anion remains the same whereas the cation alkyl chain was modified. 

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