“Water-in-salt” super concentrated electrolyte clusters are “micelles”

Abstract

Water-in-salt electrolytes are super concentrated mixtures of salt and water with molal concentration exceeding 20 m. Previous microscopic measurements revealed the presence of clusters. We hypothesize that these clusters are “micelles” because salts contain large “amphiphilic” anions. For verifying this hypothesis, we study bis(trifluoromethanesulfonyl)imide (LiTFSI) within a wide concentration range from 0.1 molal up to 21 molal. We measure conductivity and sound speed. Both methods yield the same value of CMC: 5 m (≈2.9 Molar). Sound speed measurement indicates a decline of the solvating number from 14.2 to below 2 at highest concentration. Assuming micelle diameter as 2 nm, we calculated number of TFSI- anions in a single micelle as 17. Some of them could be neutralized by ion-pairing with Li ions. Nevertheless, the high electric charge causes internal stress that competes with “hydrophobic effect” and shifts the CMC value. Strong DLVO electrostatic repulsion between separate micelles prevents formation of super-micelles and gels at high concentrations, keeping the solution in liquid state.