A composition for separating a C 1 to C 4 carboxylic acid from water, the composition comprising: (A) a solvent comprising (a) a quaternary phosphonium phosphinate salt; and (b) a co-solvent selected from the group consisting of ethers, esters selected from ethyl acetate, n-propyl acetate, n-propyl formate, i-propyl acetate, i-propyl formate, n-butyl acetate, n-butyl formate, i-butyl acetate, i-butyl formate, n-propyl propionate, and i-propyl propionate, ketones, and aromatic hydrocarbons, wherein the phosphinate salt has the general formula 1: wherein R 1 , R 2 , R 3 and R 4 are each independently a C 1 to C 26 hydrocarbyl group, provided that R 1 , R 2 , R 3 , and R 4 collectively have a total of at least 24 carbon atoms; and R 5 and R 6 are each independently an alkyl or aryl group having 3 to 24 carbon atoms and may be connected together with the phosphorus atom to form a heterocyclic ring; (B) a C 1 to C 4 carboxylic acid; and (C) water; wherein the alkyl groups represented by R 5 and R 6 may be branched or straight-chained, and may contain functional groups such as alkoxy, olefinic, and halogen functionalities, and wherein the aryl groups represented by R 5 and R 6 may be homo- or polycyclic and may be substituted with a halogen, alkyl group, aryl group, halogen-substituted alkyl group, halogen-substituted aryl group, secondary alkyl or aryl amino group, tertiary alkyl or aryl amino group, halogen-substituted secondary alkyl or aryl amino group, halogen-substituted tertiary alkyl or aryl amino group, nitro group, alkyl or aryl ether group, halogen-substituted alkyl or aryl ether group, or combinations thereof. (A) a solvent comprising (a) a quaternary phosphonium phosphinate salt; and (b) a co-solvent selected from the group consisting of ethers, esters selected from ethyl acetate, n-propyl acetate, n-propyl formate, i-propyl acetate, i-propyl formate, n-butyl acetate, n-butyl formate, i-butyl acetate, i-butyl formate, n-propyl propionate, and i-propyl propionate, ketones, and aromatic hydrocarbons, wherein the phosphinate salt has the general formula 1: wherein R 1 , R 2 , R 3 and R 4 are each independently a C 1 to C 26 hydrocarbyl group, provided that R 1 , R 2 , R 3 , and R 4 collectively have a total of at least 24 carbon atoms; and R 5 and R 6 are each independently an alkyl or aryl group having 3 to 24 carbon atoms and may be connected together with the phosphorus atom to form a heterocyclic ring; (a) a quaternary phosphonium phosphinate salt; and (b) a co-solvent selected from the group consisting of ethers, esters selected from ethyl acetate, n-propyl acetate, n-propyl formate, i-propyl acetate, i-propyl formate, n-butyl acetate, n-butyl formate, i-butyl acetate, i-butyl formate, n-propyl propionate, and i-propyl propionate, ketones, and aromatic hydrocarbons, R 1 , R 2 , R 3 and R 4 are each independently a C 1 to C 26 hydrocarbyl group, provided that R 1 , R 2 , R 3 , and R 4 collectively have a total of at least 24 carbon atoms; and R 5 and R 6 are each independently an alkyl or aryl group having 3 to 24 carbon atoms and may be connected together with the phosphorus atom to form a heterocyclic ring; (B) a C 1 to C 4 carboxylic acid; and (C) water; <EOS>