Oral Presentation International Solvent Extraction Conference 2025

One could observe that 100 years ago coordination chemistry revolutionized the way that we approach solvent extraction (SX), both in theory and practice. Fifty years ago, host-guest chemistry (HGC) revolutionized coordination chemistry. Has HGC proceeded to revolutionize SX? This paper attempts to answer this question, sketching the role that HGC has come to play in SX.

As a matter of defining terms, a “host” molecule may be considered a multi-dentate ligand, charged or neutral, that presents a concave pocket or cavity with affinity for guest species.¹ These include macrocycles, cages, and open-chain structures bearing multiple arms that can wrap around guest species. Guests can include cations, anions, and neutral molecules. Generally, extractants function in SX by replacing the hydration of species extracted from water. Hosts can fulfill this function well by supplying more of the donor groups for binding guests than traditional mono- and bidentate ligands so often used in SX. To the extent that the host molecules are preorganized to complement the specific size, shape, and electronic properties of a guest, the binding and resulting extraction can be highly selective.^2,3 Especially when the selectivity achieved cannot be matched by traditional SX reagents, host molecules can find important applications.^4,5

An avid research community publishes actively on host compounds for the selective extraction of metal cations and anions across the periodic table, revealing unprecedented new properties in extraction selectivity and behavior. Publication rates of HGC as part of SX research are shown in the figure. These are data on publication counts obtained in a search of the Web of Science database through 2024. SX publications were identified under the lumped general search terms solvent extraction, liquid-liquid extraction, liquid membranes, extraction chromatography, or solvent-impregnated resins. Since ca. 1991, the yearly publication rate in the field of SX has been increasing in an approximately linear fashion. Among the SX papers, those featuring HGC are shown as the orange line. These publications were identified under the search terms host-guest, molecular recognition, receptors, or chemical families such as crown ethers, cryptands, or calixarenes. Rising interest in HGC beginning in the 1960s reached approximately 10% of the SX publications in the decade 1991–2001 but thereafter plateaued.

The table hints at the role of HGC within the field of SX. Overall, the publication rate for HGC within SX (i.e., HG ⊂ SX) has averaged 5.5% of the total. The orientation toward multidisciplinary science of HGC is clear: 62% for HG ⊂ SX vs 37% for SX, while the latter is clearly more engineering oriented. Both SX and HG ⊂ SX attract research on analytical, nuclear, and biological & medical applications. While papers on analytical applications are similar in number, nuclear applications are more numerous for HG ⊂ SX. Biological and medical applications constitute a larger category for SX. Mining and metallurgy applications garner much fewer numbers of publications for HG ⊂ SX. As a caveat, these observations flow from the Web of Science database, which likely does not capture some publications in SX that flow from specialty areas like mining, where research results are often reported in conference proceedings. 

Notwithstanding the superficial nature of the literature analysis, it does qualitatively frame HGC as a niche area within the greater area of SX. Cost is likely the primary limitation, steering researchers to high-value specialties in which the economics are not overwhelmed by the high cost of the host molecule, which can easily exceed $200/g. Attrition of the host molecules can exceed the cost of the recovered metal, discouraging applications in mining and metallurgy. Nuclear, analytical, and bio & medical arenas requiring high selectivity will continue to drive research. HGC will likely remain an important niche area of SX chemistry.

Figure. Number of publications in the indicated years for the general field of solvent extraction (vertical bars) and for the subset of papers on the topic of HGC (orange line, right axis).