Ionic liquids based aqueous biphasic systems: Effect of the alkyl chains in the cation versus in the anion

Highlights: • Alkyl-3-methylimidazolium alkylsulfonate ILs for implemention of aqueous biphasic systems. • Study of the effect of alkyl chain length and position on ILs hydrophobicity. • Evaluation of ILs extractive power on L-tryptophan aqueous solutions. • The alkyl chain in the anion contributes more to the hydrophobicity of the IL. • Less hydrophobic ILs have the better extraction coefficients for L-tryptophan. -- Abstract: The use of alkyl-3-methylimidazolium alkylsulfonate ionic liquids for implementing aqueous biphasic systems is studied in this work for the first time. The ability of high charge density inorganic salts, such as K3PO4, to promote phase segregation in aqueous solutions containing the ionic liquids 1,3-dimethylimidazolium methylsulfonate ([C1mim][C1SO3]), 1-ethyl-3-methylimidazolium hexylsulfonate ([C2mim][C6SO3]), 1-ethyl-3-methylimidazolium butylsulfonate ([C2mim][C4SO3]), 1-butyl-3-methylimidazolium methylsulfonate ([C4mim][C1SO3]), 1-butyl-3-methylimidazolium ethylsulfonate ([C4mim][C2SO3]), 1-pentyl-3-methylimidazolium methylsulfonate ([C5mim][C1SO3]), 1-hexyl-3-methylimidazolium methylsulfonate ([C6mim][C1SO3]) and 1-hexyl-3-methylimidazolium ethylsulfonate ([C6mim][C2SO3]) was experimentally determined at 298.15 K and atmospheric pressure. In general, the hydrophobicity of the ionic liquids studied is affected by the increase of the alkyl chain length. However, the position of the alkyl chain, whether in the cation or in the anion affects in a different way the lipophilic effect of the ionic liquid. Two ionic liquids with the same number of carbon atoms, the one with a longer chain in the anion is the more hydrophobic. Furthermore, four ionic liquids were chosen to extract the aminoacid L-tryptophan from aqueous solutions. The chain lengths of the anion or cation were fixed and the partition coefficients compared. The extractions, carried out at 298.15 K, showed the good extractive power of these ionic liquids and also that the less hydrophobic ionic liquids have the better extraction coefficients. Overall, the results from this work identify relations between the length and position of the alkyl chain with the hydrophobicity and also with the extractive power of the imidazolium-alkylsulfonate based ionic liquids