The surface tension of ionic liquid (IL) C5MIBF4 (1-methyl-3-pentylimidazolium tetrafluoroborate) with various amount of water was measured by the forced bubble method at 278.15 to 338.15 +/-0.05 K. In terms of standard addition method, the surface tension of pure IL C5MIBF4 was obtained. The properties of surface for pure C5MIBF4 were discussed in terms of the principle of independent surface action and Glasser's theory of lattice energy. Using the surface tension data, the interstice model was examined. (c) 2007 Elsevier Inc. All rights reserved.