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John A. Quinn: Selected Career Achievements Just a few decades ago the number of nationally prominent researchers in the field of membrane science and separations could be counted on one hand. John Quinn was one of these select few individuals who essentially created new fields of study, pioneered new processes for which membranes hold the advantage, and educated individuals who went on to pursue membrane research and development and have had a measurable impact on the field. Today the database for the journal Ind. Eng. Chem. Res. under the reviewer expertise “membrane separations” lists 500 reviewers, more than any other category for this broad-coverage research journal. John Quinn is a major reason for the tremendous growth of interest in the science and technology of membranes. To label John Quinn a “membranologist”, however, is to risk receiving a scolding from him, and indeed that term is much too confining as applied to him. John Quinn is first and foremost an engineering scientist
interested in biological and synthetic membrane problems rather than a membrane specialist. His refusal to define himself solely as a membrane scientist, and thus limit the scope of his research, helps explain his impressive track record of appreciating and pioneering new vistas for chemical engineering and then pulling the field in previously unexplored directions. In introducing this special issue of Ind. Eng. Chem. Res., our purpose is to highlight a few of the central themes characteristic of the body of John Quinn’s academic work and illustrative of his technical breadth and impact. We also expand on some of the obvious and not-so-obvious connections that exist at the interface between these themes because, in our view, it is John Quinn’s “interfacial activity” that explains, in large part, the scope and richness of his legacy. We focus primarily on John’s earlier work and the evolution of his career interests because those topics will be less familiar to some readers than his recent research. We also apolo-
10.1021/ie0109612 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/12/2002
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Figure 1. Research interests of Professor John A. Quinn.
gize for the incompleteness of this review and for the fact that many of John’s students must go unnamed below for reasons of space limitations. We have only John Quinn to blame for this, however, because his productivity is the ultimate cause of our problem. First, a few facts and figures are in order. John grew up in the Midwest and did not venture far afield for his baccalaureate chemical engineering education, obtaining his B.S. from the University of Illinois in 1954. He moved east to Princeton, however, to do his Ph.D. work on fluidization under Joe Elgin and Leon Lapidus. Upon graduation in 1958, he returned to the University of Illinois to join the faculty there, being promoted to the position of Associate Professor in 1964 and thence to Professor just 2 years later in 1966, the same year in which he received AIChE’s Colburn Award for excellence in publications. Upon returning from his first sabbatical at Imperial College London, he was pursued by Art Humphrey, then the department chair at the University of Pennsylvania, with the result that John accepted a faculty position there in 1971. Five years later John Quinn would receive AIChE’s Alpha Chi Sigma Award for outstanding chemical engineering research and be inducted into the National Academy of Engineering. John’s rise to prominence within chemical engineering circles was rapid and well deserved. The Role of the Interface in Mass Transfer John Quinn’s work on the role of interfacial phenomena in mass transfer is what initially brought him to the attention of his chemical engineering peers (see Figure 1). Interfacial effects were only beginning to capture the imagination of serious chemical engineers when he began his academic career at the University of Illinois, several years in advance of the 1961 publication of the Davies and Rideal book on Interfacial Phenomena. Included among the leading academic researchers exploring rate-limiting processes in interphase transport were the likes of Sherwood at MIT, Pigford at the University of Delaware and his Ph.D. student Scriven (then at Shell and now at the University of Minnesota), Danckwerts in the U.K., and Levich in the U.S.S.R. This was distinguished company indeed, but the young Quinn seemingly had few qualms about
his ability to play in the big leagues with these rather eminent investigators. Indeed, one suspects that part of what attracted John to this research area was the possibility of challenging the dogma of interfacial equilibrium so deeply embedded in the film and penetration theories of mass transfer. John Quinn’s first Ph.D. students would be urged to explore various aspects of interfacial mass transport. Increasing attention was being given to the possibility that events at the plane of the interface itself (as opposed to within adjacent boundary layers) could have a significant influence not only on the rates of permeation across it but also on convective transport in the bulk phases on either side. In principle, of course, interfacial resistance to mass transport had to be present even at very clean interfaces because diffusion implied the existence of a nonequilibrium state there. In practice, however, these intrinsic interfacial resistances were expected to be small. To further complicate matters, surface-active impurities invariably and quite rapidly accumulate at any fresh interface and, as John and his students would amply demonstrate, such monolayers were capable of either suppressing or promoting convection in the fluid phases on either side of the interface. Experiments to elucidate these and other socalled “barrier effects” would prove far from simple to execute or interpret. John began his quest for interfacial mass-transfer resistance with experiments conducted on gas absorption by T. S. Govindan, his first Ph.D. student at Illinois. They sought to test the assumption of chemical equilibrium by focusing on freshly formed gas/liquid interfaces because, if significant intrinsic interfacial resistances were to be discovered, they would be found only at extremely short contact times before diffusion boundary layers could grow and dominate and before surfaceactive contaminants could accumulate at the interface. Thus, the challenge that confronted young Quinn and his student Govindan was to design a gas/liquid contacting apparatus that would permit transient absorption rates to be measured at contact times as short as a millisecond or less, well below the range of contact times accessible to previous investigators using short wettedwall columns, rotating drums, and the like. Moreover, contact between gas and liquid had to be brought about in a well-defined geometry that would give rise to accurately known liquid flow fields. The solution to this problem was the so-called movingband absorber, which Govindan constructed with the help of Roger Schmitz, an undergraduate at Illinois at the time and later a Professor. This apparatus caused a soluble gas in the absorption chamber to be contacted with a thin liquid film applied to an endless 4-mil-thick Nichrome ribbon rotating over 6-in.-diameter pulleys at speeds up to 10 000 rpm, with the film being scraped from the moving band as it exited the absorption chamber. Clearances above the moving band were on the order of a few thousandths of an inch, and the slightest misalignment of the apparatus had dramatic consequences. Legend has it that this apparatus had to be seen and heard to be fully appreciated, but even from a safe distance, the moving-band absorber so impressed Danckwerts that he described it in his book on Gas-Liquid Reactions. At the same time, efforts were underway in John’s group to measure interfacial resistance at liquid/liquidphase boundaries, where again the hallmark of the work
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would be the design of apparatusshere the laminar liquid-liquid jetsthat would again “push the envelope” in terms of achieving short contact times in well-defined geometries amenable to rigorous analysis. Early experiments involved discharging the aqueous liquid comprising the laminar jet into a tank containing a second immiscible organic liquid. However, W. J. (Bill) Ward, III, John’s first Ph.D. student in this area, redesigned the apparatus to permit the deployment of the organic liquid in the form of a thin annular sheath surrounding an aqueous core. Ward’s tube-in-orifice nozzle could apply organic coatings atop aqueous cores in films so thin as to exhibit interference colors, while other parts of the apparatus were designed to skim and collect this thin organic phase at suitably short organic/aqueous contact times. In addition to establishing upper bounds on the magnitude of interfacial resistance in such systemss an accomplishment in and of itselfsthis work would ultimately lead to fruitful encounters with various other interfacial effects that would productively occupy the Quinn research group for years to come. Among these digressions was the discovery by John and his students of the steadying influence of stagnant films that retarded convection, on the one hand, and of several types of convective instabilities that could accelerate mass transfer, on the other hand. R. L. (Larry) Merson was the first of John’s students to confront the fact that surface-active constituents would inevitably accumulate at interfaces and influence mass-transfer rates, either directly by virtue of their permeation resistance or indirectly by virtue of their effects on the hydrodynamics. Merson first encountered these stagnant films in his so-called radial-flow liquid/ liquid contactor, an apparatus initially designed to explore interfacial mass transfer. Once discovered, however, the effects of these interfacial films on transport were so pronounced and intriguing that they demanded his full attention. In subsequent studies conducted with a laminar-flow horizontal trough, Merson would characterize these surface monolayers by their compressibility, rheology, and growth kinetics. A complementary branch of John’s research on insoluble monolayers at interfaces would focus on various convective instabilities brought about by gradients in density or surface tension or both. With R. E. (Bob) Plevan, John would investigate the effect of monomolecular films on the rates of gas absorption in an apparatus that permitted one to effect a rapid step change in the pressure of the absorbing gas in contact with an initially quiescent liquid subphase. With L. M. (Les) Blair, he would extend these studies of buoyancyinduced convection and publish an elegant paper in J. Fluid Mech. that featured both a time-dependent Rayleigh number characteristic of the onset time for convection and tools for differentiating between buoyancydriven and surface-tension-driven convective flows. Through Schlieren photography, it was possible to visualize these convection cells, the development of which could be retarded by surface monolayers. Subsequent students including W. T. (Terry) Mitchell and E. D. (Ed) Burger would extend this work to temperaturedriven interfacial instabilities and their relationship to insoluble monolayers supported on stagnant fluids. Thus John Quinn’s interests gradually shifted from the intrinsic properties of the fluid/fluid interface itself to the properties of the insoluble monolayers or “mem-
branes” invariably found there. From this point forward, John would view these interfacial films not as mere impediments to the study of interfacial mass transfer but as worthy subjects of study in their own right. It soon became apparent that, to study the permeability of these thin films more thoroughly, it would be advantageous to support them on something other than underlying fluid phases. Transport Fundamentals To measure the very small mass-transfer resistances associated with individual monolayers, John Quinn needed to identify highly permeable supports for them that would afford minimal permeation resistance themselves. This would lead him to explore high-permeability polymers, porous membranes, and immobilized liquid films for use as composite membrane supports. The first thesis along these lines was that of G. D. (Gene) Rose, who deposited Langmuir-Blodgett (LB) stearate monolayers on high-permeability substrates comprised of dense films of a silicone copolymer obtained from GE’s Research Labs. These supported LB membranes excited John, in part, by virtue of their promise as model systems for the study of biological membranes, as discussed by Rose and Quinn in one of three articles that John would ultimately publish in Science. Even the thinnest of these silicone-based films exhibited appreciable permeation resistance, however, forcing John to use even lower-resistance microporous membranes as supports. This search would ultimately lead to track-etched mica membranes characterized by extremely small and uniform pores. These could be formed in thin mica sheets by first irradiating them and then etching them with HF to dissolve material along the radiation damage tracks. John and his students J. L. (John) Anderson, W. S. (Winston) Ho, and postdoc W. J. (Wilfred) Petzny described these membranes in a 1972 paper in Biophys. J. that documented their utility as a tool for studying various steric, hydrodynamic, and electrodynamic phenomena associated with the transport of solutes and colloidal particles through fine pores. Meanwhile, John’s postdoc Petzny had not forgotten that the original interest in track-etched mica membranes stemmed from their potential utility as highly permeable supports for built-up LB multilayers. Rather surprisingly, however, he discovered that stearate monolayers deposited on the face of a mica membrane would undergo surface diffusion into the membrane’s pores, forming oriented layers on the pore walls with thicknesses identical with those of the layers initially deposited on the membrane’s exterior surface. By this means, the surface properties of the pores, and their effective diameter, could be manipulated at will, as documented in a second Science paper. About a dozen of John’s students would ultimately become proficient in fabricating track-etched mica membranes, using them both to improve our understanding of hindered diffusion, filtration, and adsorption in nanometer-sized pores and to gain important insights on the electrokinetics of charged pores. For example, in a seminal paper published in Biophys. J. in 1974, Anderson and Quinn developed fundamental hydrodynamic equations to describe hindered diffusion in submicron pores; their equations avoided the centerline assumption relied on by others and incorporated directly the effects of both steric restrictions and Brownian
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motion. Mass-transfer studies of solutes and small particles involving track-etched membranes and/or molecular films and coatings would continue in John’s laboratory throughout the 1990s. A related area that attracted John Quinn’s attention is diffusiophoresis, a process whereby particles diffuse in response to a gradient in the concentration of an interacting solute. John’s student P. O. (Peter) Staffeld developed the stopped-flow diffusion cell in order to measure diffusiophoretic velocities directly. In J. Colloid Interface Sci., Staffeld and Quinn remark that the diffusiophoretic motion of particles has features in common with certain active transport processes in biology like chemotaxis, wherein “living” particles (i.e., cells) are set in motion by solute concentration gradients. This provides but one example of John Quinn’s lifelong fascination with biological processes and their counterparts in engineering, a connection nowhere more evident than in his research on facilitated transport and membrane reactors. Transport in Reactive Media and Biological Systems A third theme running throughout John Quinn’s research career has centered on transport in complex media, chemically reactive membranes, and biological systems. John’s interests in membrane transport in both synthetic and biological membranes were already well established in the early 1970s when he and colleague David Graves were contacted by physicians at Penn’s Institute for Environmental Medicine for help in understanding a problem in skin permeation: human subjects placed in elevated-pressure dive chambers under certain conditions were known to form skin lesions and/or bubbles of unknown origin within or underneath their skin. Professors Quinn and Graves, analyzing the skin as just another composite membrane structure, ultimately traced the cause of these manifestations to “counterdiffusion supersaturation”, a phenomenon wherein the total pressure between two adjacent membranes of different permeabilities can be higher than that on either side of the composite. This work, performed in part by student J. M. (Jerry) Collins, would serve as the subject of John’s third Science publication and would lead to related studies of skin perfusion, blood oxygenation, and transcutaneous blood gas analysis. At about this same time, John became intrigued with work of his former student Bill Ward at GE on a new class of immobilized liquid membranes in which reversible chemical reactions were made to take place between permeants and carriers. Physiologists had long known that the transport of oxygen and other species in biological systems could be augmented by reversible complexation with heme compounds and other carriers, a phenomenon termed carrier-mediated or facilitated transport. The connection between synthetic membranes engineered to effect facilitated transport, on the one hand, and their biological counterparts, on the other hand, would prove irresistible to John Quinn. Facilitated transport was a “natural” as an area of study for John, who viewed these membranes, in effect, as back-to-back interfaces. John and students N. C. (Norm) Otto, T. L. (Terry) Donaldson, R. J. (Russel) Lander, and D. R. (Doug) Smith would ultimately develop an analytical model of facilitated transport membranes based on two nonequilibrium reaction bound-
ary layers surrounding a reaction-equilibrium core, and they would incorporate catalysts in these membranes in order to improve their efficiency or to measure rapid solution-phase kinetics. Given this backdrop, it was only a matter of time before John would propose that someone consider the merits of conducting net chemical conversion of reactants to products in catalytic membrane structures. That lucky someone turned out to be S. L. (Steve) Matson, an author of this Preface. In 1973, having landed a job at GE working on facilitated transport under Bill Ward, the author had the opportunity to meet with John during one of his consulting visits and discuss the potential of membrane reactors. The venue for these discussions soon shifted to the University of Pennsylvania, where the author would obtain his Ph.D. degree under John’s direction. In addition to serving as the thesis topic for J. L. (Jorge) Lopez and T. J. (Tom) Stanley, membrane reactors would in 1984 become the basis for Matson’s co-founding of Sepracor, Inc., a specialty pharmaceutical company focused in large part on optically pure drugs. In Sepracor’s hands, the multilayer membrane reactor constructs initially explored in John Quinn’s laboratory would be commercialized in a more practical, hollow-fiber-based embodiment of the technology targeted principally at the enzymatic resolution of chiral drug intermediates. Whereas most of John Quinn’s work has been scientific in nature, membrane reactors complete the circle by illustrating his contributions to engineering technology as well. Having said this, much of John’s fascination with membrane reactors undoubtedly arises from the fact that biological membrane reactors, so prominent in nature, are so much more capable and versatile than their engineered counterparts. This may explain, at least in part, John’s recent preoccupation with various natural phenomena centered at the cell membrane, e.g., his work on the motility of living cells. His work on bacterial chemotaxis with Peter Staffeld, the migration of mammalian cells with P. A. (Paul) DiMilla, and the attachment of cells to surfaces with Cynthia CozensRoberts all involve study of interactions between the cell and its environment that are mediated by the cell membrane. John Quinn’s keen interest in the history of science is well-known to his colleagues, and observers of his career may be struck by similarities in the sorts of problems John has picked and those pursued by some of the great minds in chemistry and physics from somewhat earlier timessmen like Lord Rayleigh, Irving Langmuir, and G. I. Taylor, to name a few. Those great scientists often designed deceptively simple and elegant experiments to explore natural phenomena that, one suspects, John Quinn wishes had been left unexplained for him to explore. At a time when many of us are quick to throw supercomputers and sophisticated instrumentation at problems, John has chosen to apply his intellect first and technology only second. Perhaps nowhere is this more evident than in a 1986 publication in AIChE J. with former students C. H. Lin and John Anderson, where they rigorously analyze and then apply G. I. Taylor’s method of hydrodynamic stability for measuring diffusion coefficients. Taylor had demonstrated the technique in a crude fashion but had given few details, and so it was left to John Quinn and his associates to do it up right. One can sense John’s delight
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in effectively working in collaboration from afar with the great G. I. Taylor. Although we have already noted in passing a few of the ways in which John Quinn’s research contributions have been formally recognized, it is appropriate here to document the record of his recognition and professional service more completely. John Quinn received the Colburn Award of AIChE in 1966 and the Institute’s Alpha Chi Sigma Award in 1978. Twenty years ago he was named the first recipient of the Robert D. Bent Professorship at the University of Pennsylvania, an endowed chair established through a grant from the Atlantic Richfield Foundation. He delivered the Mason Lectures at Stanford University in 1981, the Katz Lectureship at the University of Michigan in 1985, the Reilly Lectures at Notre Dame University in 1987, the Distinguished Research Lectureship in Chemical Engineering at Carnegie-Mellon University in 1997, and the 1995 and 2001 Alan S. Michaels Distinguished Lectureship at MIT. He was elected to membership in the National Academy of Engineering in 1978 and to the American Academy of Arts and Sciences in 1992. He has served as a member of several commissions and boards operating under the auspices of the National Research Council, including the Engineering Research Board, the Board of Chemical Sciences and Technology, the Committee on Separation Science and Technology, and the Amundson Committee on Chemical Engineering Frontiers. John Quinn has been on the editorial boards of several journals, including Chem. Eng. Commun., J. Membr. Sci., Rev. Chem. Eng., and Ind. Eng. Chem. Fundam. He has served on the Scientific Advisory Board of Sepracor, Inc., and the Scientific Advisory Committee of the Whitaker Foundation. He has been a member of advisory councils to Departments of Chemical Engineering at Princeton, Delaware, and Caltech and the Harvard/MIT Advisory Committee to the Division of Health Sciences and Technology.
Although formal honors and external recognition certainly have their place, it is telling that John has always taken particular pride in the success of those whom he has influenced, namely, the many students and professional colleagues he has mentored. Many of his students (which include some 40 Ph.D.’s) have risen to positions of leadership in industry or academia, and several have been elected to the National Academy of Engineering. There is a predictable pattern evident in the relationships that invariably develop between John Quinn and his charges. The relationship begins formally as that of student and teacher; it soon develops into easy friendship (with wife Frances as well as John!); finally it matures into one of personal and professional respect. That so many of John’s students continue to experience him in each one of these ways is no accident. Perhaps it is fitting that John Quinn himself be given the “last word” on this occasion. Recently, in a “Reminiscences and Recollections” column written for the North American Membrane Society’s Membr. Quart., John had the following to say: “My advice to a budding academic embarking on a career in membrane research is: surround yourself with first-rate graduate students and then get out of their way! And remember to present no barriers, facilitate progress, and catalyze net forward reaction/interaction!” Stephen L. Matson* Arete Technologies Inc., 15 Withington Lane, Harvard, Massachusetts 01451 John L. Anderson Department of Chemical Engineering, Carnegie-Mellon University, Pittsburgh, Pennsylvania 15213 IE0109612
