Recovery of amino acids by imidazolium based ionic liquids from aqueous media

Abstract

In this paper, imidazolium based ionic liquids [C₄mim][PF₆], [C₆mim][PF₆], [C₆mim][BF₄] and [C₈mim][BF₄] were used as alternative solvents for the recovery of some amino acids (L-tryptophan, L-phenylalanine, L-tyrosine, L-leucine and D-valine) from aqueous media. The experimental results shown that logarithm of partition coefficients, log (P_IL/W), of the amino acids correlate well with the hydrophobicity of the amino acids, indicating the importance of the hydrophobic effect of amino acids as a driving force for their partition into a particular ionic liquid. The partition coefficients are found to be strongly affected by pH of aqueous phases and the water solubility in ionic liquids. They decrease steeply with increasing pH in the range of pH < pK₁, but increase with increasing solubility of water in ionic liquid phases. The higher extraction degree at low pHs has been interpreted by the strong electrostatic interactions between cationic form of the amino acids and anion of the ionic liquids. The importance of anionic nature and the alkyl chain length on the cation of ionic liquids have also been examined. It is noted that ionic liquid with BF₄⁻ anion has much higher extraction efficiency for the amino acids than those with PF₆⁻ anion because of the stronger effective charge in BF₄⁻. It seems likely that there is no correlation between the partition coefficients of the amino acids and the polarity of the ionic liquids. Preliminary results indicated that [C₆mim][BF₄] and [C₈mim][BF₄] ionic liquids may be adapted to liquid-liquid recovery process of some amino acids from aqueous media, and L-tryptophan can be effectively separated from fermentation broth in a multiple step process using [C₆mim][BF₄] as an extractant.