Extraction of butyric acid by novel hydrophobic phosphonium carboxylates with a pour point below 273 K and enhanced biocompatibility

Abstract

Four new hydrophobic phosphonium carboxylate ionic liquids (ILs) were synthesized: tetradecyltrihexylphosphonium 2-butyloctanoate ([C₁₄C₆C₆C₆P][2-BOct]), tetradecyltrihexylphosphonium 2-hexyldecanoate ([C₁₄C₆C₆C₆P][2-HDec]), dodecyltrioctylphosphonium 2-butyloctanoate ([C₁₂C₈C₈C₈P][2-BOct]), and dodecyltrioctylphosphonium bis(2,4,4-trimethylpentyl)phosphinate ([C₁₂C₈C₈C₈P][BTMPP]). These ILs exhibit high extraction efficiency for carboxylic acids, comparable to commercially available phosphinate and decanoate ILs [C₁₄C₆C₆C₆P][BTMPP] and [C₁₄C₆C₆C₆P][Dec]. The pour point of the new dry ILs with branched carboxylate anions is below 255 K, providing technological advantages, while decanoate IL has a pour point above 308 K. In a water-saturated state, the viscosity of ILs [C₁₄C₆C₆C₆P][2-BOct] and [C₁₄C₆C₆C₆P][2-HDec] is similar or only slightly higher compared to decanoate IL and [C₁₄C₆C₆C₆P][neodecanoate], synthesized previously. However, except for decanoate IL, all studied carboxylate ILs with [C₁₄C₆C₆C₆P⁺] cation are toxic to Bacillus coagulans. ILs with a larger [C₁₂C₈C₈C₈P⁺] cation are biocompatible with Bacillus coagulans. Dry ILs with phosphinate anions [BTMPP] have higher viscosity compared to carboxylate ILs, complicating their use in separations. The viscosity of [C₁₂C₈C₈C₈P][2-BOct] is lower compared to phosphinate IL and slightly higher compared to decanoate IL. New carboxylate ILs with branched alkyl chains of anions and [C₁₂C₈C₈C₈P⁺] cation demonstrate favourable properties, making them promising candidates for various applications, especially in situations when low solubility in water, good biocompatibility, and advantageous pour points are essential considerations.