Eleven Ways to Use Host-Guest Chemistry in Liquid-Liquid Separations: A Tool Box for Understanding and Applications

Host-guest chemistry has created a new paradigm in liquid-liquid extraction (LLE), and in turn, LLE has provided a vehicle for exploring as well as exploiting the high selectivity offered by the creative design of host molecules for ion recognition. In this review, intended for both the supramolecular and extractive-separations communities, use of host-guest chemistry for ion separation is systematized according to eleven ways that the principle of charge-neutrality in LLE may be satisfied. The eleven ways include five systems for ion-pair extraction, three for cation exchange, and three for anion exchange. To fully understand the host function in LLE, three of the eleven ways represent the null cases in which there is no host present. In the null cases, principles of ion partitioning based on solvation govern ion selectivity according to a bias type selectivity normally favoring large ions. To achieve peak selectivity, host molecules must be designed to overcome the underlying selectivity bias. Factors influencing selectivity toward functional LLE systems are discussed, and illustrative examples for each of the eleven ways are presented from the author’s research as well as prominent research of others. A problem has been that selectivity observed in homogeneous solution usually does not match the selectivity obtained in LLE with the same host. Thermodynamic terms for ion partitioning and ion pairing in LLE clarify the picture.