BACKGROUND: Macroalgae has high potential as a bio-fuel feedstock due to its high carbohydrate content. Conventional aqueous systems require a huge amount of energy to catalyze depolymerization and hydrolysis of macroalgae. Recently various task-specific ionic liquids (ILs) successfully depolymerized different biomasses, but were thermally unstable. A variety of dicationic acidic ILs (DAILs) connected by oligo-ethylene glycol (EG) chains bearing hydrogen sulfate (HSO4-) anions were synthesized with better thermal stability and utilized in hydrolysis of macroalgal biomass, Gelidium amansii. RESULTS: The structures of DAILs were confirmed by H-1 and C-13 NMR as well as FT-IR. EG-linker chain lengths and dicationic structure increased the thermal stability of DAILs and influenced their catalytic performance for G. amansii hydrolysis. DAILs linked with short oligo-EG chains afforded higher sugar yields and exhibited faster hydrolysis rates compared with those linked by long oligo-EG chains (i.e. PEG400 and PEG600). A maximum sugar yield of 67.45wt% was recovered from G. amansii using [Tri-EG-(MIm)(2)]2HSO(4) DAIL, after 3 min at 120 degrees C. CONCLUSION: The synthesized DAILs showed dual properties as catalyst and reaction medium for hydrolysis of macroalgae in solvent-less conditions. Shorter oligo-EG chain linked DAILs exhibited faster reaction rates with higher sugar yields. (C) 2016 Society of Chemical Industry