Publi Th. Zemb

10 main publications selected and put into perspective:

• 4 in Separation Chemistry
• 4 in Colloidal Physical Chemistry
• 2 in Instrumentation

1. Attractive Interactions Between Reverse Aggregates and Phase Separation in Concentrated Malonamide Extractant Solutions

C. Erlinger, L. Belloni, Th. Zemb, C. Madic. Langmuir (1999 15, 2290 – 2300. DOI: 10.1021/la980313w

Three newly expressed general conditions required for successful extraction are introduced: 

• The existence of inverse aggregates composed of complexing agents, water molecules, and adducts,
• The stability of the extracting phase is driven by colloidal interactions between aggregates, under solvent-wetting conditions,
• The existence of a critical point close to the optimal formulation which is necessary for good extraction.

Before this seminal article, all modelling of liquid-liquid extraction experiments (>500 mainly technical articles per year) focused on extraction in all its forms, based on the differences in complexation in aqueous and solvent phases containing surfactants with “complexing properties”. This article lays the foundations for ienaics, the science of controlled electrolyte transfer between a complex fluid and a concentrated electrolyte solution.  Molecular forces and entropies of phases in dynamic equilibrium play as much a role as complexation with first neighbors. Moreover, to understand and be able to predict the stability limits of dispersions, it is necessary to determine the short-range attractive potential between aggregates (close to inverse micelles), which depends on pH, cation charge, and solvent wetting of inverse aggregates. The crucial role of the critical point was highlighted for the first time in this article.

2. How to Explain Microemulsions Formed by Solvent Mixtures Without Conventional Surfactants

Th. Zemb, M. Klossek, T. Lopian, J. Marcus, S. Schoettl, D. Horinek, S. F. Prevost, D. Touraud, O. Diat, S. Marcelja, W. Kunz. PNAS (2016) 113, 4260 – 4265. DOI: 10.1073/pnas.1515708113

Replacing a conventional solvent with a hydrotrope increases extraction efficiency, contrary to what was previously accepted. We explained this enhanced extraction by the presence of low-dynamic 2 nm aggregates resulting from a competition between mixing entropy and hydration forces due to the hydrotrope-enriched surface of emerging aggregates. 

The intensive use in proposed formulations of hydrotropic molecules in a “greener” recycling of metals but also biomass and decontamination results was recognized by the 2024 “Kash Mittal Surfactant in Solution” Prize awarded in Goa in June 2024.

3. Entropy drives Synergistic Solvent Extraction

M. Spadina, K. Bohinc, Th. Zemb, J.-F. Dufrêche. ACS Nano (2019) 13 (12), 13745 – 13758. DOI: 10.1021/acsnano.9b07605

The strong synergy between certain extractants in certain molar ratios is a phenomenon observed and sometimes used in hydro-metallurgy since the 50's, without quantitative explanation beyond an effective parameterization. We have given a model capable of predicting synergies when they exist by the configurational internal entropy of the various mixed aggregates that form around the cations to be extracted.

Synergy is sometimes used in metal recycling formulations but not yet used in the nuclear fuel cycle, as the mechanism was unknown before this article was published. Synergy and hydrotrope efficiencies will certainly be exploited in the Generation IV nuclear fuel cycle, as well as in greener processes in the nuclear fuel cycle of the future.

4. Molecular Forces in Liquid-Liquid Extraction

M. Spadina, J.-F. Dufrêche, S. Pellet-Rostaing, S. Marcelja, Th. Zemb. Langmuir (2021) 37 (36), 10637 – 10656. DOI: 10.1021/acs.langmuir.1c00673

We identify and quantify in kJ/mol the molecular forces acting in addition to the organo-metallic complexation between the cation and its first coordination sphere. The forces (derived from the free energy) related to the self-assembly of extractants in the form of inverse molecular aggregates are entropic in nature and oppose extraction initiated by complexation and depend on the thermodynamic distance to the critical point.

The general decomposition of the free energy of transfer into five well-defined terms now makes it possible to model the distribution coefficients via much simpler and more robust software for safety analysis than those based on more than a hundred competing complexation reactions currently used in the hydro-metallurgical industry.

1. Micellar Structure From Comparison of X-Ray and Neutron Small Angle Scattering

Th. Zemb, P. Charpin. J. Phys. (1985) 46, 249 – 256. DOI: 10.1051/jphys:01985004602024900

Before 1980, micelles and vesicles were difficult to study by X-ray scattering, as interactions between colloids distorted the signal. We have shown that a separation of the structure factor, combined with consideration of the “high-resolution” wide-angle asymptote by X-ray and neutron scattering, makes it possible to measure the mass, hydration, and charge of any micelle unambiguously.

This general result was recognized by the Prize of the Physical Chemistry Division of the French Chemical Society, which is also a section of the French Physical Society, in 1985. The method initiated in this work is now integrated into all standard software used on large neutron or X-ray instruments for the initial characterization of micelles.

2. Self-Assembly of Flat Nanodiscs in Salt-Free Catanionic Surfactant Solutions

Th. Zemb, M. Dubois, B. Demé, Th. Gulik-Kryzwicki. Science (1999) 283, 816 – 820. DOI: 10.1126/science.283.5403.816

Seeking to prepare solids from non-stoichiometric co-crystallization of acidic and cationic anionic surfactants in hydroxide form, we have observed and modeled the reasons for the formation and the stability of size-controlled crystallized nanodisks analogous to micelles made from lipid mixtures. 

These truly-salt-free “catanionic” that are crystalline and off-stoichiometry mixtures published in “Science” were the subject of dozens of publications annually after 2000, then left the realm of laboratory curiosity to be used twenty years later in the design of lipid nanoparticles that are the carriers of RNA vaccines.

3. Self-Assembly of Regular Hollow Icosahedra in Salt-Free Catanionic Solutions

M. Dubois, B. Demé, Th. Gulik-Krzywicki, J.-C. Dedieu, C. Vautrin, S. Désert, E. Perez, Th. Zemb. Nature (2001) 411, 672 – 675. DOI: 10.1038/35079541

In an attempt to elucidate the origin of the very intense neutron scattering of ultra-diluted samples of catanionic mixtures initially intended to serve as a “solvent reference”, we observed in a particular zone of the ternary phase diagram the systematic presence of giant (mass > 1 GDa), dilutable and manipulable colloids with a regular icosahedral shape analogous to that of certain viruses.

This experimental work showing that facetted giant colloids of 1 GDa molar mass was the innovation, at the origin of Solvay Prize of the European Colloid and Interface Society (ECIS), awarded in Almeria in 2003.

4. Shape Control Through Molecular Segregation in Giant Surfactant Aggregates

M. Dubois, V. Lizunov, A. Meister, Th. Gulik-Krzywicki, J.-M. Verbavatz, E. Perez, J. Zimmerberg, Th. Zemb. PNAS (2004) 101, 15082 – 15087. DOI: 10.1073/pnas.0400837101

Nanometer-thick disks, lamellar phases with holes, and giant faceted vesicles are observed in non-stoichiometric “catanionic” dispersions. Partial segregation of species between faces, edges, and vertices within a single colloidal cohesive object of controllable electric charge is the general underlying principle behind all observed morphologies.

This PNAS article, the result of an intense collaboration between four laboratories, was specially edited by Donald Caspar, the author of the then most successful theory of icosahedral viral capsid formation, which proved possible outside the world of proteins, by quenching a dispersion of vesicles.