Process Nano-Technology

Appeared earlier in TG magazine 6 of 2003.

At first glance, the title may seem a contradictio in términis because for nano-technology the relevant length scale is the nanometer – one billionth of a meter or the length of a row of 10 hydrogen atoms laid side by side – whereas for process technology the relevant length scale is meters or beyond. It is exactly this discrepancy of more than nine orders of magnitude in length scales that allows the new direction in chemical process technology that will be discussed below: in essence another way of Nature Inspired Chemical Engineering (NICE), or even better, Nature Inspired Colloidal Systems (NICS), as a new direction for our Colloid Science group.

Nano-physics

“It is a staggeringly small world that is below,” said the famous physicist Richard Feynman in his 1959 speech about nano-technology There’s Plenty of Room at the Bottom [1]. “In the year 2000, when they look back at this age, they will wonder why it was not until the year 1960 that anybody began seriously to move in this direction”. Indeed, 40 years after, nano-technologists have come quite far and it is not easy to cover all the contributions that have been made. Surely nano-tubes come to mind as well as molecular motors, but also improvements in sub-micron lithography fall within the realm of nano-technology. A specific definition of this new field is however not so easily given. A generic definition would involve a characteristic length scale of about one na­no­meter. Quite unlike their wont, physicists do not bother too much about a definition and name nano-technology whatever lets them (i) Get essentially every atom in the right place; (ii) Make almost any structure consistent with the laws of physics that one can specify in molecular detail; (iii) Have manufacturing costs not greatly exceeding the cost of the required raw material and energy. Two main concepts associated with nano-technology are positional assembly and self-replication. The first concept implies an interest in molecular robotics. i.e. robotic devices that are molecular both in their size and precision. Self-replication on the other hand is of course required to scale up the production of the materials.

Contemporary chemical processes are crude at the molecular level moving around atoms in bulk quantities. It is the Law of Large Numbers that governs them and our main control mechanisms involve bulk properties such as flow rates and temperature. Process technology indeed does look like a giant’s attempt to make an origami butterfly. But surely, this is not the whole story about chemical process technology. There are many processes that make use of colloidal systems such as emulsions and particulate dispersions. Examples are easily found in the food or paint industry. For the relevant characteristic length scale one usually takes the dimension of the particles that constitute colloidal systems. This yields length scales in the range from tenths of nanometers up to hundreds of micrometers.

Interfacial Engineering

Fig. 1:    Schematic representation of the principle of charge stabilization. The diffuse double layer around the colloidal particles consists of co-ions (closed) and counter-ions (open). Upon closer approach the bare charge of the particles becomes less screened.

Most, if not all, properties of the colloidal systems derive from the interface between the dispersed phase and the continuous phase. The essential task of the interface is to keep the two phases separated and is commonly known as colloidal stabilization; see e.g. [2,3]. There are basically two different methods to achieve colloidal stability. One method involves dissociated groups at the interface that render electrical charge to the interface and the particle as a whole. Since equally charged objects repel each other, this charge prevents particles from aggregating or coalescing. The counter ions and added salt ions will, however, accumulate close to the interface in what is called the diffuse double layer, see figure 1. They do so in such a way as to diminish the effect of the charge over larger distances; a phenomenon called screening. The associated DLVO-theory is due to Derjaguin, Landau, Verwey and Overbeek. The multitude of contributions to this subject by Dutch colloid scientists is reflected by the two last names mentioned in the acronym DLVO by which this theory is known. The Debye length measures the thickness of the electrical double layer that, depending on ionic strength, varies from several tenths of nanometers down to less than a nanometer. The other method is called steric stabilization and, despite its frequent use in industrial processes, much less is formally known on its mechanism of which the essence is depicted in figure 2. The adsorbed or chemically grafted polymer chains prevent close encounters of colloidal particles. A (too) simple argument for the repulsion lies in the gain of the free energy of mixing when the polymer chains of neighboring particles are overlapping. The thickness of an adsorbed or grafted polymer layer can be varied at will. Since its function is to shield the omnipresent Van der Waals attraction between the colloidal particles, a thickness of a few nanometers is sufficient.

Fig. 2 Schematic representation of the principle of steric stabilization. Upon closer approach the polymer chains would mix which is entropically unfavorable.

The characteristic size of the colloidal interface is in the nanometer regime. And there is quite some of it! In a system with the rather modest fill factor of 10% consisting of colloidal particles of a typical size of one micrometer there is about 300 m2 of interfacial area per liter. Interfacial Engineering, a word coined by Stokes and Evans [3] as an alternative to the rather old fashioned name of colloid and interface science, therefore certainly belongs to the nano-technologies!

A classical example where interfacial engineering plays a dominant role on an industrial scale is emulsion polymerization for the preparation of polymer particles (latex) by radical polymerization. The dispersed phase is initially formed by the monomers. A small amount of monomers, in equilibrium with the dispersed monomer, is dissolved in the continuous phase. The initiator, also in the continuous water phase, starts the polymerization by penetrating through the particle surfactant membranes. Some initiator may encounter monomer in the continuous phase and start polymerization there, but at some point the growing molecules become insoluble and form surfactant coated polymer particles themselves. Excess heat is easily removed through the continuous water phase. Depletion of monomer terminates the reactions. The process is fully controlled by the interface and by the surfactant molecules that constitute the interface. Moreover, even at quite significant volume fractions the viscosity if the dispersion is quite low which renders processing extremely simple. Modern developments include the use of spherical and bicontinuous micro emulsions for polymer and inorganic particle templating.

Controlled Release and Separation

A multitude of present day products, such as high quality paper or water borne paints, are made by sophisticated use of interfacial engineering. In all these products and processes the interface plays the simple role of separating two phases, albeit a given level of permeability to certain constituents is programmed. Recently, new functions have been introduced and one has made interfaces that are responsive to their environment in a predetermined manner. An example is the potion containing cod liver oil, which of old is fed to young children in winter to supplement their vitamin A and D intake. In wintertime there is less sunlight and hence the body production of these vitamins is low, for instance causing rachitis.  Few children are fond of this potion in its classical form. The present form is in fact an emulsion where the yellow oil is dispersed in an orange juice. Due to the interface, the badly tasting oil will not free in the mouth. Upon digestion, the interfacial membranes in the emulsion are destroyed and the cod liver oil can release the vitamin in the intestines.

Nowadays, many pharmaceuticals make use of this principle of controlled release. However, when controlled release is one way to govern a process, then controlled separation would be another. The exposure of an absorbent when exterior conditions are fitting is also a technique and it is already used as a means to catch poisonous molecules before these can permeate into the blood circulation.

Nano-structured Interfaces

Much more control would be attainable if it were possible to integrate more functions in the interfaces. Such nano-structured interfaces could respond to changes in their environment in a predetermined manner, much in the same way as the simple examples given above. More interestingly, these nano-devices could also be made to respond to external signals brought about by magnetic fields or light. Imagine for instance two solutions con­tai­ning strong reagents. Simply mixing them would yield incomplete reaction and uncontrolled heat production. These may become well mixed, without reacting, when one component is dispersed in impermeable colloidally sized capsules. At some stage in the process, where mixing is deemed sufficient, the permeability of the capsules is increased proportional to an external signal. The reaction will now run in a controlled manner under optimal conditions. This maybe sounds too fantastic, but one way to achieve this would be to make the capsules of a material that can be selectively modified by the action of microwaves or ultra­violet light.



Fig. 3 Synthesis of a nano-capsule by sequential adsorption of counter charged polyelectrolyte layers and subsequent removal of the core particle.

What is needed to turn this fantasy into reality is first of all methods to make nano-structured interfaces. Currently, there are quite some promising developments in that direction. One such development uses layer-by-layer deposition of alternately cationic and anionic polyelectrolyte layers on a core particle. The core particle can be a polymer particle itself, or a metal or inorganic particle, see fig. 3. The result is a multilayer core-shell particle with predetermined surface properties. In an extension of the technique, one removes the core particle after which the shell remains of which the permeability can be tuned by external means. Another development involves the adsorption of nano-sized particles onto the surface of a colloidal particle, see fig. 4. Under the right conditions, capillarity forces the nano-particles to fully cover the colloidal particle. Extensions of this technique involve the fusion of the nano-particles after which a core-shell particle with specific properties is created. Another extension is, just as with the polyelectrolyte coated particles, the removal of the core after which a so-called colloidosome is created. The size of the shell particles depends on the size of the initiating core particle. These above examples deal with so-called self-assembling systems but of course direct synthesis constitutes another possibility.


Fig. 4: Synthesis of nano-structured colloids by adsorption of anano-particle monolayer. Subsequent removal of the core particleleaves a capsule (top right), and fusion of the adsorbed monolayerresults in a coreshell particle (bottom right).

The way nano-structured interfaces are made already suggests methods to control them. The multilayer polyelectrolyte capsules can be controlled by pH or ionic strength whereas the colloidosomes are quite insensitive to these variables. Other methods to induce a particular response in nano-structured interfaces require the incorporation of, for instance, inorganic particles or more complicated structures. Some macromolecules such as present in biological cells could also introduce useful functions. In this sense, nano-structured interfaces are not new at all as Nature has designed its systems likewise.

Control of processes by means of nano-structured interfaces can be based on two complementary approaches: (i) extreme control, an internal stabilization by coupling rate phenomena to the environmental variables such as concentrations; and (ii) process control, by using the functions of the interfaces either as sensors, as actuators, or both. The dynamics of the system will be completely different from what is presently known. It will for instance not be possible to treat the system as a continuum and it will be necessary to find ways of including nano-devi­ces that are acting almost at the molecular level into a complete system description. It poses a serious challenge to process engineers to devise new models to perform process nano-control. Certainly, there is the potential of more powerful process control with the added degrees of freedom provided by the nano-structured interfaces.

Sustainability

Nano-structured interfaces also provide a new pathway to design and operate processes in a sustainable manner according to a philosophy of closed material cycles and maximum energy efficiency. At first glance, there is the disadvantage of introducing components into the product that were used to build the nano-structured interfaces. It might contaminate the product and it may even pose a hazard. In terms of mass, the amount of these materials is quite low and in many cases the end product often comes in dispersed form anyway. The cost involved in making the nano-structured interfaces should be recovered by the added value to the products.

However, the nano-structured interfaces can actually be used to advantage. Firstly, one may contain toxic or otherwise harmful reagents in impermeable capsules so as to transfer the material in a safe and efficient way towards the relevant process step. Secondly, one might conceive the possibility of capturing selectively some products in capsules so as to separate them efficiently from the environment. Essentially, once a process is properly designed, sustainability can be achieved at any level.

More fantasy

Once the way to make nano-structured is known, many more tasks may become feasible. Some of those derive back to controlled release and separation between continuous and dispersed phase. And there is no limit to the number of dispersed phases. It is easy to conceive multiphase systems where exchange between phases is controlled by internal or external means. But the interface can act as a scavenger itself. In order to achieve that, we can dress the interface with molecular entities, in essence nano-devices, that complex specific molecular species such as contaminations. Recently, new molecules have been designed that allow the control over complexation by means of light. In one conformation the molecule is able to complex a metal ion and in the other not. Transition between these two conformers is affected by light.

An interesting possibility is to design nano-structured interfaces in such a way that they could also be used to bring molecules in the right proximity to synthesize special molecules, so as to produce for instance straight alkanes from methane without producing syngas. This would require more accurate positioning of groups within the interface. With present day polymerisation methods that are used to make for instance block copolymers this might become reality as well. Extending this idea, nano-devices could act as catalysts themselves or allow the external control of the action of catalysts. And similarly nano-devices could be used to control the number of crystallization nucleation sites and maybe even crystal shape and maximum size comes to mind.

Epilogue

The ideas presented above, of which some are actively being researched, constitute a new challenge to chemical process technology. It is in fact a combination of old and new science because colloidal stability will remain an issue, in particular when interfaces are changing their function. The challenge to good old colloid science is to rejuvenate with the newly available nano-possibilities while treasuring its scientific standards with respect to colloidal stability.

Let’s now come back to what the physicists believed nano-technology would allow them to do and see whether the above discussed process nano-technology. The first issue was “Getting essentially every atom in the right place”. Surely this is an easier job with fluids than with the solid state, but nano-structured interfaces indeed can do just that. But chemists can go further, as it is within their capabilities to synthesize new molecules. The second issue was “Making almost any structure consistent with the laws of physics that one can specify in molecular detail”. The relevant physical laws are those of quantum mechanics. Even though in principle these laws are known, their predictive power in chemistry is rather limited. In actual fact, chemists often devise methods that seem to lure quantum chemistry. Finally, the third issue was “Have manufacturing costs not greatly exceeding the cost of the required raw material and energy”. As argued before, the cost involved in making and using nano-structured interfaces should at least balance with the added value of the resulting products. In addition, legislation may make the difference, as was the case for water borne paints. The necessity to produce a solvent-free product made manufacturers introduce dispersions of binder and pigment despite their intrinsically higher cost. In conclusion, there is no contradictio in términis.

References

  • Feynman RP, see http://www.zyvex.com/nanotech/feynman.html
  • Frens G, Cahiers Fysische Chemie, Delft University Press 2001.
  • Stokes RJ and Evans DF, Fundamentals of Interfacial Engineering, Jossey-Bass 1996.

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