On 24 September 2023, New Zealand lost one of its senior physical chemists. Leon Phillips, born in 1935 in Thames, was a Professor at the Department of Chemistry at the University of Canterbury from 1966 until 2016. He worked mainly in the areas of chemical kinetics, photochemistry and gas-liquid interfaces. Leon was an incredibly animated scientist, a man on a mission in a permanent state of gusto. His long career involved multiple areas of physical chemistry and some interesting lines of research.

I first met Leon in 2006 as a third-year student in CHEM363, a core physical chemistry course at Canterbury. Leon taught two parts of that course. The first part was on molecular spectroscopy and the second was on photochemistry. Leon’s lectures were never boring. He talked quickly, with a tone similar to a horse racing commentator. He rejected the accepted lecturing practice of standing on stage, preferring to slouch cross-legged in a chair. His lecture notes usually consisted of an illegible scrawl on a single piece of paper, however he would rarely look at it. From his seat he would disseminate all things physical chemistry in a verbal hailstorm, pausing occasionally to ask whether we were familiar with Fourier transforms or Marcus theory. When visual explanations were required, he would launch out of his seat like an electron imbued by a photon, covering the whiteboard with glorious electronic structure diagrams and term symbols while slowly returning to his seat as if coupled to some kind of vibrational mode. Dexterous in mind and in supreme command of his topic. None of us could have guessed that he was over 70 years old at the time.

Leon’s laboratory was just as interesting. Many students would pass through here during his long career, beginning with Colin Freeman and Murray McEwan in the 1960s (who would themselves become prominent Canterbury faculty members) and ending with me in 2010. His lab, which consisted of an elaborate meshwork of steel, glass, lasers, and mirrors, all under a darkened ambience, was not for the fainthearted. It was here where Leon was usually to be found when he was not lecturing. Unlike the sedate managerial professors of today, Leon was fully active as an experimentalist throughout his entire career. He worked closely with his students and got to know them well. When an experiment was particularly important to him he would push the student hard and take over at any opportunity. By doing so he would set the pace. Students who could maintain control over their projects would usually end up becoming Leon’s good friend; those who could not were politely tolerated.

Leon was a self-identified experimentalist, holding ambivalent attitudes towards theoretical work. He would often remark that theory ‘was easy’ and ‘is the kind of thing that could be done in a doctor’s waiting room’. Yet he was thoroughly versed in the theory of his subject and utilised it extensively in his own research. His 1965 monograph on quantum chemistry was one of the earliest on the topic and remains well regarded. He was somewhat cold towards my interest in theoretical research, yet managed to give me the single best piece of feedback I have ever received. “I can’t see the physical picture,” he said, looking with befuddlement at one of my early attempts at theorising. “You need to make a model.” My project took off after that. That was Leon’s supervision at its finest – a curt, well-timed comment to put you on the right track.

The following provides a very brief overview of Leon’s research. As a scientific article it is highly unsatisfying. However, a scientist educated in the 2000s could never truly understand the motivations behind a 1960s photochemistry paper. The scientific record only records what was written, and Leon tended to be a terse writer. Rather than focusing on scientific completeness, I have instead tried to illustrate an important principle: that over the course of a long career, interesting research begets interesting research.

Flames, photochemistry and atmospheric chemistry

Leon began his research career in one of the hottest areas of chemistry at the time – flame chemistry. This work spanned his doctoral work (conducted at Cambridge in around 1960,under the supervision of Morris Sugden), his postdoctoral work (conducted at McGill University in the early 1960s, under the supervision of H. I. Schiff), as well as the first several years of his academic career at Canterbury. During the 1960s, Leon’s research expanded from flame chemistry to cover the areas of gas-phase chemical kinetics and photochemistry more broadly.

The area of flame chemistry involves adding reactants to a flaming gas and studying the subsequent chemical reaction. The basic experimental principle is explained in one of Leon’s first papers: “Little or no disturbance (to the flame) is produced…by small amounts of additives, and the main flame gasses can be regarded as a medium or ‘solvent’ in which the reactions of the additive take place” (Can. J. Chem. 38, 1960, 1804). The progress of the chemical reaction was followed by recording its light emission and everything took place within specially designed burners equipped with ‘spectroscopes’ and gas-lines for supplying the flame gases. Leon and his co-workers studied reactions involving atoms and small molecules such as lead and halogen gases. They elucidated their mechanisms by pen-and-paper chemical kinetics, just as is taught in physical chemistry courses. Thus, they would set up candidate reaction mechanisms, apply steady state approximations and test predicted relationships between rate constants, emission intensities and additive concentrations using their measurements. An interesting dimension of this work arose from the heterogeneity of the flame (being spatially non-uniform in temperature), which would cause the rate constants to show a strong spatial dependence. Leon remarked upon this point several times in his work, although it would have been difficult to investigate rigorously with the tools of the time.

Leon was perhaps best known for his work in photochemistry, a topic on which he published frequently during a period from the 1960s and well into the 1990s at Canterbury. This work broadly studied the kinetics of gas-phase chemical reactions involving excited species. Leon’s papers would usually focus on relatively simple compounds such as NH₃, CO₂ and ethane. These were excited directly by irradiation or by sensitisation by excited atoms such as triplet mercury. Reaction progress was usually monitored by recording light emission over time. On an analytical level, this work involved proposing reaction mechanisms, making linear or steady-state approximations, and determining rate constants through applications of Beer’s law. This makes for admittedly dry reading for a modern-day chemist, however the life of the work is not to be found in its formalities. Rather, one must look to its technical side, which involved the development of specialised reaction chambers equipped with monochromators, lamps, voltage coils and other items. Leon’s papers were quick to refer the reader to Figure 1, which would usually present a new contraption especially designed for particular measurements. Much innovation was achieved through the use of chopping wheels to modulate exciting light sources, using phase differences of the emitted light to infer something about the  of the excited species (e.g. Chem. Phys. Lett. 8, 1971, 226; Rev. Sci. Inst. 42, 1971, 1078). Leon’s photochemistry papers therefore record a long legacy of interesting experimental devices and contraptions, many of which remained on the back shelves of his lab in the late 2000s.

From about the mid-70s onwards, Leon broadened his photochemistry research in two ways. The first was by incorporating elements of atmospheric chemistry, starting with a highly readable review published with Murray McEwan in 1970 (Acc. Chem. Res. 3, 1970, 9), followed by a textbook (Chemistry of the Atmosphere, 1975, Wiley) and several papers. These works lay well within the square of photochemistry but included an increased emphasis on aerosols and aerosol interfaces as a reaction medium in later publications (e.g. J. Photochem. Photobiol. A: Chem. 74, 1993, 7; 93, 1996, 83). In fact, Leon continued his work on aerosol chemistry right through into the 2000s, with a small section of his darkened lab remaining fenced-off for that purpose. The second way in which Leon broadened his research was through theory. In terms of research technique, this was arguably the greater deviation from Leon’s previous works. The resulting paper trail includes reports of a new numerical scheme for solving differential equations related to excited atom fluorescence decay (J. Photochem. 5, 1986, 277), electronic structure calculations for various small molecules (which was no easy feat in those days, see e.g. Theoret. Chim. Acta 21, 1971, 205) and numerous applications of collision theories and trajectory simulations to calculate rate constants (e.g. J. Phys. Chem. 94, 1990, 7487; Chem. Phys. Lett. 165, 1990, 545l; J. Comput. Chem. 11, 1990, 88). Thus through an elaborate juggling act of experiment, application and theory, Leon had come to know the photochemistry field very well. By the mid-90s he was ready for something new.

Irreversible thermodynamics of the gas-liquid interface

Leon was now a master photochemist, a competent theoretician and highly knowledgeable of atmospheric chemistry. Now in his 50s and perhaps seeking a magnum opus, Leon would perform an interesting synthesis of these areas and embark in an entirely new direction. The rise of atmospheric CO₂ concentrations was gaining widespread recognition at that time, and questions were being raised about the solubility of CO₂ in the ocean. To this end, several groups attempted to measure CO₂ fluxes through the surface of water on the basis of Fick’s law of diffusion. Leon was not impressed. In a conversation with me, he vividly described these data as “looking like a shotgun pattern” when plotted. There was clearly something being overlooked in their measurements and Leon had a hunch as to what it was.

At some point during his foray into theoretical work, Leon had studied non-equilibrium thermodynamics. Non-equilibrium thermodynamics is somewhat tangential to mainstream physical chemistry, although quite important in theoretical physics and biology. The part of non-equilibrium thermodynamics relevant here is the theory of coupled transport, developed by Lars Onsager and others during the early 20th century. A key result from this theory is that mass transport does not occur in isolation. Indeed, if a flux of CO₂ molecules occurs under a temperature gradient, then the flux carries mass as well as heat. Noting that a temperature gradient would be present at the surface of the ocean, Leon conjectured that previous measurements of CO₂ fluxes had neglected the effect of a coupled heat flux.

Leon therefore aimed to quantify the extent to which heat fluxes coupled to mass fluxes through liquid surfaces. I am sure that he was the only person in the world who was trying to do this at the time. The quantity he sought to measure is known as the heat of transport (Q*),which acts as a kind of coupling coefficient in Onsager’s theory. Indeed, it can be shown that in a flux of gas molecules through a region with a temperature gradient (T, T + dT), the heat flux contribution is exactly Q*/RT times the mass flux contribution, where R is the gas constant. It is here where we can see all of the threads of Leon’s previous research coming together. Leon’s previous theoretical work would have made him aware of coupled heat fluxes. Through his extensive knowledge of atmospheric chemistry, Leon would have also been aware of the question of CO₂ solubilities in the ocean. How, then, did he put his extensive photochemistry background to work?

The answer is surprising: rotational spectroscopy. Measuring the heat of transport requires introducing small amounts of target gas (CO₂ or otherwise) into a chamber containing a reservoir of liquid. The partial pressure of the gas as a function of a temperature gradient across the liquid surface then needs to be measured (as can be seen from equation 5 of Leon’s review in Acc. Chem. Res. 37, 2004, 982). However, such measurements are difficult with conventional barometers, as the partial pressure of the target gas is a negligible fraction of the vapour pressure of the liquid. As a photo chemist, however, Leon knew that for certain gases, the intensities of certain rotational lines were highly sensitive to pressure and temperature. Over the course of several years and many experiments, Leon devised an elaborate setup consisting of a stainless steel chamber for holding the liquid reservoir and gas coupled to a diode laser for measuring rotational spectra. The resulting meshwork of glass tubes, stainless steel chambers, dewars, mirrors and oscilloscopes was unambiguously the product of an experimental photochemist.

It was with this setup that Leon and his students would measure the heat of transport measurements for various liquids (such as heptanol, sulfuric acid, glycerol and water) and gases (such as water, ammonia and nitrous oxide). For the full list of measurements as of 2010, the reader is referred to another review (Chem. Phys. Lett. 495, 2010, 1). These results are quite striking, showing that the quantity Q*/RT varies from about 3 (for the case of a N₂O flux through the surface of water) through to about 25 (for the case of heptanol vapor flux through the surface of heptanol). Leon completed this line of work with the important case of a CO₂ flux through the surface of water, the results of which he published in 2011 (J. Non-Equil. Thermo. 36, 2011, 273). The result of over 15 years of experimentation came down to two measurements: Q* = -6.9 kJ/mol and Q*/RT = -3.0. In his own words,“…the fractional temperature gradient is at least three times as important as the fractional partial-pressure gradient in controlling the magnitude and direction of the steady-state CO₂ flux through a water surface.”

Leon’s measurements ought to have been quite consequential for climate change research. After all, if CO₂ fluxes measured on the basis of Fick’s law of diffusion are out by a factor of three, then what do we really know about the solubility of CO₂ in the ocean? Yet Leon’s work in this area remains unacknowledged. While several of his papers on this topic are cited occasionally, the above paper, reporting Q*/RT for the case of CO₂, has only been cited once. Why? It cannot be that the atmospheric chemistry field has moved on; a paper published by another group in Nature Communications in 2020 describes the very problem of measuring CO₂ fluxes through ocean surfaces, without ever mentioning the heat of transport (Nat. Commun. 11, 2020, 4422). And who is to say that their measurements are wrong?

I cannot speculate on the potential impact of Leon’s work. However, I can suggest some reasons why Leon’s work has not been taken up by its target audience. The rough-and-ready types who ride on ocean barges and thrust steel instruments into the stormy seas are probably not so interested in learning irreversible thermodynamics. Leon’s insistence on terse writing also left his papers somewhat incomplete. For example, he never explained how field workers should apply his measurements to their work. Moreover, Leon’s work stopped short at measuring the heat of transport; he never tried to measure the CO₂ flux directly, at least not in his later work. My guess is that Leon considered these points trivial or a distraction.

I have hope that Leon’s work will be picked up in the future. Indeed, a quick Google Scholar investigation suggests that his efforts to measure the heat of transport have been appreciated by some theoretical physicists. This point is further supported by the fact that much of this work was published in the Journal of Non-Equilibrium Thermodynamics, a theoretician beehive. If Onsager’s abstract theory is ever coupled with molecular dynamics techniques, thereby allowing for Q* to be estimated by simulation, then Leon’s measurements may become important for computational benchmarking. Such work might also explain the molecular origin of the large values that he measured, potentially opening further avenues for research. In an odd twist, it may be theoreticians who cement Leon’s experimental legacy.

Final words

“That’s a very crude way of putting it,” Leon said in response to my explanation of the Fourier transform in his CHEM363 class. He would probably make the same remark about what I have written above. I have hardly done justice to his work on photochemistry and flame chemistry. Nor have I mentioned his wider research network, such as the Air UCI consortium at the University of California, Irvine, which was so important to him. Nor have I mentioned the warm kindness shown by Leon and his wife Pamela outside of the lab, where they would often take students out for dinner or on their yacht for a sailing lesson. The latter was an essential part of the Phillips lab experience.

It seems oddly fitting to finish this article with a combative tone. Leon was always happy to engage in a brawl, especially over administrative matters. Towards the end of his career, Leon became appalled at the creeping corporatisation of New Zealand’s universities and showed little respect for academics who acquiesced to their inflated bureaucracies. He was on the right side of an unwinnable battle and fought it using blunt weapons. His most effective attacks were found in the letters page of The Press, while his most ineffective ones were those directed at his department. For the latter he might be excused, for dividing houses is the very tactic by which academic corporatisation advances. Like coronavirus, the corporate university is now a fixture of New Zealand society. It is sad that the old physical chemistry professor, driven by a simple love of scholarship, is not.

Contributed by Associate Professor Daniel Packwood, Institute for Integrated Cell-Material Sciences, Kyoto University