Critical Micelle Concentration (CMC) of sodium dodecyl sulfate (SIDS), an anionic surfactant, has been investigated in 30 mM aqueous solutions of 1-benzyl-3-methylimidazolium chloride (1) and 1-methyl-3-(pentafluorophenyl)imidazolium chloride (2). The CIVIC of SDS is significantly lower in the presence of 2 as compared to that of 1. The decreased CMC of SDS in the presence of 2 is in accordance with its increased hydrophobicity due to the fluorinated side chain. The thermogravimetric analysis of compounds 1 and 2 shows that they are thermally stable up to about 290degreesC. (C) 2004 Elsevier B.V. All rights reserved.