Density, surface tension, interfacial tension with water, and contact angles on Teflon and glass Surfaces of 21 ionic liquids based on functionalized imidazolium, ammonium, phosphonium, and guanidinium cations combined with several anions, such as chloride, tetrafluoroborate, bis(trifluoromethanesulfonyl)imide, dicyanamide, p-toluenesulfonate, and ethylsulfate, were completely measured. The structure-property relationship of the selected ionic liquids was considered in order to predict their potential applications. Particularly relevant were the lower surface tension values (at 293.15 K) observed for tetrahexyldimethyl-guanidinium dicyanamide, [(di-h)(2)dmg][DCA] (21.35 +/- 0.03 mN.m(-1)), and trioctylmethylammonium bis(trifluoromethanesulfonyl)imide, [Aliquat][NTf(2)] (22.77 +/- 0.05 mN.m(-1)), and the remarkably low contact angles on Teflon surfaces of [(di-h)(2)dmg][DCA] (39.66 degrees) and trihexyltetradecylphosphonium p-toluenesulfonate, [P(6,6,6,14)][p-TsO] (39.77 degrees).