Surface tension measurements of N-alkylimidazolium ionic liquids

We have measured the surface tensions, with use of a ring tensiometer, of a series of closely related ionic liquids [C(n)mim] [X] where [C(n)mim] represents the 1-CnH2n+1-3-methylimidazolium cation (n = 4 [bmim], 8 [omim], and 12 [C(12)mim] and X = PF6-, BF4-, Cl-, or Br-. The values of surface tension span an unusually wide range for compounds of such similar structure, from 45 mJ/m(2) ([bmim] [PF6]) to 24 mJ/m(2) ([C(12)mim][PF6]) at 336 K. They all show a linear variation of surface tension with temperature allowing a separation of the surface excess entropy and energy components. The surface excess quantities are alkyl chain length-dependent; both the surface excess entropy and energy decrease as the alkyl chain in the 1-position of the cation is lengthened for a particular anion. For a common cation, a reduction in the surface excess entropy occurs with decreasing anion size. A similar effect occurs in the surface excess energy when the anion decreases in size for the shorter alkyl chain compounds. The wide range of values of the surface properties for these materials probably reflects changes in surface orientation of the cation.