Abstract
Preface
The theme of supramolecular chemistry (SC), entailing the organization of multiple species through noncovalent interactions, has permeated virtually all aspects of chemical endeavor over the past several decades. Given that the observed behavior of discrete molecular species depends upon their weak interactions with one another and with matrix components, one would have to conclude that SC must indeed form part of the fabric of chemistry itself. A vast literature now serves to categorize SC phenomena within a body of consistent terminology. The word supramolecular itself appears in the titles of dozens of books, several journals, and a dedicated encyclopedia. Not surprisingly, the theme of SC also permeates the field of solvent extraction (SX), inspiring the framework for this volume of Ion Exchange and Solvent Extraction. It is attempted in the six chapters of this volume to identify both how supramolecular behavior occurs and is studied in the context of SX and how SC is influencing the current direction of SX.
Researchers and practitioners have long dealt with supramolecular interactions in SX. Indeed, the use of polar extractant molecules in nonpolar media virtually assures that aggregative interactions will dominate the solution behavior of SX. Analytical chemists working in the 1930s to the 1950s with simple mono- and bidentate chelating ligands as extractants noted that extraction of metal ions obeyed complicated mass-action equilibria involving complex stoichiometries. As chemists and engineers developed processes for nuclear and hydrometallurgical applications in the 1950s and 1960s, the preference for aliphatic diluents only enhanced the complexity and supramolecular nature of extraction chemistry. Use of physical techniques such as light scattering and vapor-pressure measurements together with various spectroscopic methods revealed organic-phase aggregates from well-defined dimers to small aggregates containing a few extractant molecules to large inverse micelles swollen with water molecules. Extraction systems involving long-chain cations such as alkylammonium species or long-chain anions such as sulfonates or carboxylates proved especially prone to extensive aggregate formation. The related phenomenon of third-phase formation in SX systems, long misunderstood, is now yielding to spectroscopic and scattering techniques showing extensive long-range organization. Over the last 50 years, tools for studying the structure and thermodynamics of aggregation have grown increasingly sophisticated, leading to a rich and detailed understanding of what we can now recognize as SC phenomena in SX.
In the 1970s and 1980s, the rapid growth of SC elicited a paradigm shift in SX. The influence of SC principles had two major effects on the course of SX research. First, it provided a framework for understanding the supramolecular behavior that was already well appreciated in the field of SX, though earlier without the SC terminology. Second, it provided the conceptual tools to control supramolecular behavior in SX, direct it for intended functionality, and to simplify it. Extraction by designed reagents has been steadily progressing ever since, with commercial applications emerging to successfully validate this approach. With the discovery of crown ethers in the late 1960s, the advancement of extractant design has fruitfully employed the concept of inclusion. While considerable initial progress occurred with such molecules, especially because of their affinity and selectivity for alkali and alkaline earth metals, other molecular platforms such as calixarenes have proven more versatile. Multidentate receptors for partial to full inclusion of cations, anions, ion pairs, as well as neutral species, have now become commonplace for selective extraction.
This volume of Ion Exchange and Solvent Extraction examines how the principles of SC are being employed both in advancing the design of new highly selective SX systems and in understanding aggregation phenomena in SX systems. Chapter 1 discusses the nature and definition of SC and how it is used generally in design of novel SX reagents. Major approaches using SC principles are outlined and illustrated. Chapter 2 expands upon the theme of ion-pair recognition and introduces outer-sphere recognition of metal complexes, a novel idea with the potential for structural control of solvation, casting a new light on solvent modifiers. Chapter 3 reviews the large literature of calixarenes as extraction reagents for metal ions, where the synthetic versatility of this family of compounds has produced vast possibilities for inclusion and selective separations. Chapter 4 extends such chemistry to extraction of biomolecules, where the potential for selective separations is only beginning to be explored through site recognition in macromolecules. In Chapter 5, a detailed examination of the liquid-liquid interface as an expression of supramolecular phenomena in SX is presented. While most SX chemical research has focused on the complex interactions occurring in the bulk organic solvent, all transport in SX must occur through the liquid-liquid interface, where the sharp gradient of properties between the two immiscible phases serves as a remarkable force for the organization of amphiphilic species, aka extractant molecules. Finally, Chapter 6 returns to the perennial problem of aggregation in SX and the progress that has been made recently within sight of the historical struggles to understand it.
Beyond this volume into the future, SC will play a critical role in the future directions of SX. Molecular design will become more sophisticated to the point that the selective binding of target ions and molecules by new extractants designed computationally will be assured, but a major question in fact concerns the quantitative prediction of equilibrium constants and distribution ratios. To approach this question will entail, yes, application of the principles of SC. These will have to be embodied in computational tools that take into account aggregative interactions, solvation, and hydration of organic-phase species, which are only now beginning to be examined in present-day modeling. New extractants will behave more simply, which in effect means that supramolecular behavior will be understood to the point where unwanted aggregation can be avoided or predicted. Not only will the extraction complexes be well-behaved, but free extractant molecules will be designed to either not aggregate or else to aggregate in a controlled fashion so that aggregated structures will be compatible with the solvent matrix. Solvent modifiers, presently used in the form of branched alkanols, alkylamides, or other monofunctional polar molecules, will also yield to computational design so that they will effectively function as selective solvating agents. That is, only the target metal complexes will be solvated and therefore receive selectively enhanced distribution. Self-assembly will become a refined concept for extractant design from small molecular units. Whereas extractants such as cryptands that fully encapsulate target ions have proven to involve difficult organic synthesis, the practicality of extractants that function by encapsulating guest species can be expected to improve if they can be designed to self-assemble from simple units. Self-assembly can be viewed as controlled aggregation, a logical outgrowth of the current body of research aimed at understanding the structural basis of aggregation phenomena in SX. Taking the above points dealing with control of complexation, solvation, aggregation, and phase behavior together, it would appear that the future progress and success in the field of SX in large part grows out of the principles of supramolecular chemistry.