The dialogue between science and applications for emulsions
By Gilbert Schorsch
Extract from L'Actualité Chimique, February 2007, n° 305

The widespread yet "transitory" milieu of emulsions

Milk and rubber sap are two examples of "formulations" where nature has shown the way for enabling two incompatible liquids to cohabit. A wide range of extraction processes for natural raw materials (from petrol to chemical products, rare earths to isotopes of uranium, via essential oils), and an equally wide range of processes for manufacturing synthetic polymers (interfacial polycondensation, radicular polymerisation for the production of latex), make use of the two most interesting properties of emulsions: fluidity, making it easier to agitate the milieu, and the large contact area between the two phases, thus encouraging stability and reactivity. Many formulations, based on natural and synthetic products, are also sold directly as emulsions.
These products, either on their own or mixed with other raw materials, are destined for large capacity industrial applications (bitumen emulsions for road surfaces, aqueous paint formulae, and so on).
Thanks to their particular sensorial properties, we also use such products on a daily basis for food (dairy produce, vinaigrettes or mayonnaise), everyday personal hygiene (nourishing face creams, shampoos) and even to heal our ills or those of the plants that feed us (pharmaceutical and phytosanitary suspo-emulsions).
Emulsions are present in both industrial activities and products in everyday use, thus representing one of the "heavyweights" of industrial activity and world economics.
Emulsions nevertheless retain a certain degree of mystery about them. Sometimes they look like solutions, but they are not really solutions. They are heterogenous milieux, also metastable, that is, outside their thermodynamic equilibrium. They are used in a transitory manner, precisely because of this metastability. They require excellent mastery of their stability, or more precisely of this very same metastability. In short, the "science of emulsions" has still not succeeded in taking up the challenges laid down by their industrial development and applications in our daily life.

The World Congress for Emulsions - a platform for exchange

It was thus normal that bridges be built between emulsions specialists and manufacturers and users of emulsions. All dues to the company Colas - the world specialist in road surfaces - and its intuition. Since 1992, its aim has been to dynamise information exchange at two complementary levels: between scientists from the public sector and industrialists first of all, then also between industrialists from different sectors of activity concerned by a single problem. Colas succeeded in convincing other emulsions industrialists of the advantages of such dialogue, and the concept of the World Congress for Emulsions was born…
The first three editions of the Congress showed that it was possible for apparently incompatible scientific and industrial communities to find common ground. With each Congress, from Paris to Lyon, via Bordeaux, things started to gel a little more.
For the 4th edition, more than 1,000 participants - divided equally between industry, university laboratories and technical centres - met up in Lyon to exchange their experiences. Presided by Dominique Langevin, who has excellent credentials in the milieu, plenary conferences and workshops made it possible to review the current state of affairs. During the workshops, the most interesting presentations retained by the Presidents of each workshop were given orally and then put to the floor for discussion. 

The current state of affairs

In the course of the three days, both "black" and "white" emulsions were the guests of honour.
"Black" emulsions were represented by petrol-based products in every form: emulsions made from more or less heavy crude oil, for extraction and assisted petrol recovery, bitumen emulsions for road surfaces, more recently fuel emulsions, for reducing particle content in gas emissions… Of course, there was also mention of the undesirable water in oil emulsions!
On the other side, "white" emulsions are much more varied. They are the basis for natural products, traditional mineral and vegetable oils, animal oils or fats, as well as for synthetic chemical products such as silicone for example, or active hydrophobic materials. Their applications are also more varied. This type of emulsion can be found in the food industry, and in the pharmaceutics, cosmetics and phytosanitary industries, which is often more worthwile.
It is a shame that the coloured emulsions were not more present! Admittedly, a certain number of coloured road surface emulsions were on show, but what were missing from the three days were paints and aqueous inks - still formulated from emulsions - which would certainly have further enhanced the discussions!

End-use properties

The Scientific Committee had Jean-Eric Poirier, Scientific Manager at Colas, as its facilitator. The Committee's road map proposed starting the Congress with the applications for emulsions before moving on to how they are prepared and characterised.
The discussions on bitumen emulsions were particularly interesting. Using bitumen emulsions removes the need to heat bitumen on site. At present, this type of emulsion represents approximately 10% of all bitumen used and seems set for success. These emulsions are cationic, allowing them to break easily when they come into contact with aggregate surfaces that have been negatively charged. The problem now is how to recycle them. This is the perfect illustration of the transitory nature, if possible reversible, of using emulsions!
Of the many examples of uses for emulsions in the fields of treating and covering surfaces, thanks in particular to their fluidity: cosmetic foundation by Frédéric Auguste for L'Oréal and waterproofing facades with silicone emulsions by Martial Deruelle for Rhodia, we noted that applying an emulsion to a surface completely "destabilises" it. First, there is modification of the concentration and thus initially of the rheology - depending on the relative volatility of the continuous or dispersed phase, in the case of cosmetic foundation. Then there is also modification in the distribution of the surfactants agents and silicone droplets depending on the porosity of the stone treated, in the case of silicone emulsions. Yet often, the initial formulation has been guided only by its stability prior to application. The formulator must be aware of this evolution and anticipate it.
Finally, the conference given by Professor Moncef Stambouli from the Ecole Centrale in Paris showed that emulsions, beyond liquid/liquid extraction, are a milieu of choice for a wide range of depollution operations - from wastewater to land - and beyond. In the context of collaboration with the Châtenay-Malabry Faculty of Pharmacy, it has effectively been shown that emulsions also provide a solution for the fight against accidental intoxication or poisoning. Emulsions for this type of application also seem to be facing a bright future. The pedagogical nature of the presentation, and the vibrant hommage paid by the speaker to interdisciplinary collaboration (between process engineering and biology in the present case) deserve highlighting.
Each application effectively has its own problems. It is for this reason that we will not go any further with the other applications mentioned.

Product design, Conception, Preparation

Every time an emulsion is manufactured, there is a double problem. First, its formulation, that is, its physical and chemical properties (choice of ingredients, additives and emulsifiers in particular) and then the conditions under which the mixture is made, that is, a process engineering problem (choice of equipment and operating conditions, for example).
The various presentations made it possible to review the technology used, unfortunately more in the form of a catalogue than a proposition of methodology. The technology differs essentially in the power of the energy used to mix the liquids and additives.
Broadly speaking, mixers of the Rotor/Stator type and reactor-mixers with blades and more recently the HP (high pressure) homogeneisers provide energy to shear the two liquids in the time needed to obtain good fineness, keeping in mind that there is simultaneous rupture and coalescence of the droplets.
On the contrary, the techniques based on phase inversions, of thermodynamic origin - by changing temperature or concentration - are spontaneous and need very little high power equipment.
Between these two extremes, there are first the membrane emulsification techniques, described in the conference given by Professor Enrico Drioli from the Research Institute of Membrane Technology. These techniques are not new and consume 10 times less energy than conventional technology.
Miniaturisation of channels or mixture zones was then presented by Professor Erich Windhab from the ETH in Zürich. This is what specialists in process engineering refer to as "process intensification" and it is seen as another possible approach.
The formulation of emulsions, that is, the choice of emulsifiers - small molecules or macromolecules - is complementary to emulsification technology. In fact, at present it appears to be better mastered than the technology itself. The conference given by Nikki Denkov from the University of Sofia showed that a global, coherent description of emulsion stabilisation conditions is currently available… On the condition that the nature of the emulsifiers used and the operating parameters implemented be taken into account simultaneously.
Concentrated emulsions, mono-disperse emulsions protected from Ostwald ripening and nano-emulsions obtained under extreme shear conditions, multiple emulsions, functional droplet emulsions - magnetic or electrically charged to introduce an additional parameter - such are the emulsions that continue to rouse the curiosity of scientists, academics and industrialists alike.
The absence of a genuine dialogue between the physical-chemists and process engineering specialists was the biggest disappointment of these three days. It will one day be necessary to guarantee, for example, that characteristic process times are compatible with the diffusion rate of the emulsifying species!

Behaviour, Characterisation and Processing

The determining areas of progress in emulsion science are precise structural characterisation of emulsions, evaluation of physical and chemical characteristics - rheological in particular - and forecasting stability by means of accelerated ageing tests. By multiplying and confronting complementary analytic techniques we will succeed in better describing emulsions. It is in this field that the most noteworthy consolidations were presented. With microscopic techniques, optical techniques are without a doubt the techniques of choice for the characterisation of emulsions. In his presentation, Professor Peter Schurtenberger from the University of Fribourg in Switzerland showed in a highly pedagogical manner the complementarity between diffraction and diffusion techniques for electromagnetic, under static or dynamic conditions. With multiple diffusion and diffusive wave spectroscopy, it now seems possible to characterise opaque, and thus concentrated, emulsions.
This is a good point to mention an interesting follow-up study of structure by neutron diffusion presented by Bernard Cabane from the ESPCI. The phase inversion of the system studied works by using a lamellar phase that then fragments in a few dozen seconds.
Once again, the advantage of rheo-optical techniques was raised, as well as that of acoustic attenuation techniques. The development of NMR techniques, particularly at low fields, should also be mentioned. These techniques make it possible to monitor the distances between droplets, and thus a detailed study of emulsion destabilisation is possible.
Monitoring and mastering the stability of emulsions is still the key factor for most applications for emulsions. Stabilisation by particles (Pickering effect), which has been known for around ten years, has been studied in a highly fundamental manner by Véronique Schmitt from the CRPP in Bordeaux. Solid particles effectively make it possible to achieve very good stability for emulsions. To the academics focused on increasing the stability of their emulsions as much as possible, industrialists reply that in most of their applications, the emulsions only "exist" at a transitory level. Sooner or later the emulsion will have to be ruptured. Mastery of the duration of the stability of an emulsion is far from guaranteed!

New challenges, New ambitions

It was Jean-Marie Lehn, who, in his introductory scientific conference was the most prospective. He set a very high standard. Could the participants not take their inspiration from supramolecular chemistry as a means of trying to prepare adaptative "dynamic emulsions"? One example was indeed mentioned during the Congress, though unfortunately it is not inspired by the work of Jean-Marie Lehn. It was the photosensitive emulsions developed by Patrick Perrin from the ESPCI. Under the effect of light, it is possible to provoke, on demand, a phase inversion. This is highly innovative in scientific terms. But this scientific innovation is still looking for a real application.
Without a doubt, it was in the field of micro-fluids that the most interesting creations were presented. Mathieu Joanicot from Rhodia's Laboratory of the Future showed how it is possible, by controlling fluidics, to visualise the formation of micro-droplets in a confined space: the "birth" of very fine droplets as they come out of a micro-syringe! The devices used and the micro-fluidics they determine then make it possible to manipulate these micro-droplets to sort them or make them coalesce. They thus transform into genuine micro-mixers or even micro-reactors. Could not this return to the genesis of a drop and an emulsion be the means of finally reconciling physical and chemical properties, and process engineering?
Micro-fluidics applied to biology gave us a preview of interesting prospects, presented by Andrew Griffiths from the ISIS in Strasburg. He uses micro-fluidics for high throughput screening, and the micro-drops as chemical or biological reactors as he encloses either cells or micro-organisms.
The presentation by Volker Hessel, from the Mainz Institute of Micro-fluidics focused on illustrating the various approaches for process intensification. Although the presentation was impressive, the audience remained unconvinced because it was too much like a catalogue.

Now that the Congress is finished, it is time to summarise the situation, as seen by the various highly competent people consulted: we are in a phase that is more consolidation and conceptualisation of knowledge of emulsions than a phase of discovery and innovation!

Artistic creation and technological innovation: two aspects of a single approach!

Let us conclude with the highly efficient and highly original role played by Package Organisation, the company responsible for organising the World Congress on Emulsions in Lyon. The company has gained considerably in expertise, and the participants were impressed by the very professional level of organisation. This allowed the Congress to take place without a hitch. Package Organisation has understood that any exchange of information requires a minimum of conviviality… and that any Congress that hopes to be a success is judged not just on the event itself, but also on its complementary cultural events. This year, Package Organisation was right on target: participants were invited in succession to a fashion show put together by young fashion designers from Lyon, and then a modern dance evening at the Maison de la Danse given by the Ballet du Grand Théâtre in Geneva. Package Organisation thus succeeded in showing that artistic creation and technological innovation use the same approach: major fashion designers and stylists - who design and create clothes - and choreographers - who choose music and how it will be interpreted - use models and dancers respectively to embody and interpret their creations. Similarly, researchers need industrialists to produce and bring to life their innovations. It would hardly be possible to imagine a stronger image to highlight the complementarity of research and industry.