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Sustainable Surfactants Renewable Feedstocks for the 21st Century - Design Criteria for Sustainable Surfactants |
Design Criteria for Sustainable Surfactants
Dr Paul
Reynolds, Bristol Colloid Centre
Introduction
Colloid, polymer and surfactant surface
sciences are all used to design an ideal material, having identified the
market for a sustainable surfactant. This ideal material is then used as a
model for producing the new surfactant. Currently available and new
sources of materials are investigated and may be improved through
traditional agronomy or genetic engineering. The resulting raw material
may be further improved through chemical modification and/or changes in
the extraction process. The objective is a product that is eco-friendly,
relatively cheap, renewable and as good as the synthetic alternative.
However, if it should be kept in mind that if it is the same as the
synthetic material then it may have the same problems of disposal and
environmental impact.
Surfactants (surface active agents) are molecules having two components: hydrophobic (water insoluble) chain (e.g. alkyl) and hydrophilic (water-soluble) head group (e.g. sulphate). At high concentration, they organise into small spherical collections of surfactant molecules, micelles, with the hydrophilic part on the outside and the hydrophobic part in the centre. At higher concentrations, they become non-spherical and form long tubes, or ‘worm-like’ arrangements. Ultimately, they become lamella sheets of organised molecules. Polymers often act as surfactants, having both hydrophilic and hydrophobic components. This type of molecule can also be used as a dispersant, thickener or rheology modifier. Surfactants stick or adhere (adsorb) at surfaces, changing the nature of the surface so that it becomes hydrophobic, hydrophilic, has controlled wetting with a particular liquid or has adhesive or cohesive properties.
A traditional market for surfactants is in detergents to remove ‘soil’ (oil, grease, dust and particles) from solid surfaces (metal, plastics, ceramics, fabrics, fibres, etc). The micelles adsorb onto the soil, surrounding it. There is a rapid transfer of the surfactant through the water phase and a slow transfer through the solid phase, so the water surface tension is rapidly lowered, removing the soil. However, surfactants can be used in a large range of industries, processes and products. There are many niche markets for polymeric surfactants, not necessarily at small volumes. The trend toward speciality chemicals, with lower volume, higher added value materials targeted at a specific problem, can offer opportunities for sustainable materials.
Market Opportunity: Cement Plasticisers and Superplasticisers
Even very traditional markets, such as the construction industry, are
changing. This industry traditionally used lower cost additives (such as
lignosulphonates and melamine and formaldehyde derivatives) as
plasticisers to modify the handling properties of cement and concrete.
These additives are still dominant in the market place, but Japanese,
German and French commercial research laboratories in particular are
developing new materials.
Plasticisers are used to improve the early handling properties of cement so it can be manipulated, particularly in speciality applications. A slump test is used to assess the performance of cement. A cone is filled with cement and then upended onto a flat surface. The cone is removed and the difference in height between the original cone and the resulting cement cone is measured. This is repeated at different times after preparation of the cement, with the material slumping less as the cement sets. The slump height decreases as the yield stress increases. Colloid micro-rheology (deformation and flow) can be used to predict the role of a stabiliser.
Plasticisers are added to maintain the degree of slump over time, and as water reducing agents. Properties such as strength and porosity can be adjusted by altering the water to cement ratio, with the added benefit of improved handling properties. Cement chemistry is dependent on the source of material and is extremely complex. For example: quick set cement is high in alumina, cement for oil wells involves high temperatures and other applications (such as use under water, e.g. bridge pylons) may require changes in iron content, addition of calcium sulphate (to prevent flash set), etc. Although not universally agreed, it is thought that cement obtains its final strength by hydration of calcium silicates. However, the early stage of the reaction is dominated by the reaction of calcium aluminates (and ferrite phases), giving flash set. Addition of calcium sulphate to prevent flash set produces ettringite, a mineral containing calcium, aluminate, silicate and sulphate species in poorly defined stoichiometry. In addition, there are side reactions producing ‘monosulphates’. Grains become coated with adsorption products such as ettringite. The viscosity and yield stress are proportional to degree and strength of aggregation and the coating of the grains. The plasticiser modifies these interactions by adsorbing reaction products, thus loosing its effectiveness.
A new ‘comb-like’ material was designed that modified the interaction between grains and adsorption products. This ‘designed’ steric stabiliser would have a backbone that adsorbs on the particle surface with the ‘teeth’ of the comb sticking out into the solvent, providing a barrier to close approach of particles and thus modifying the interparticle forces. This new material should be a good stabiliser, have a high pH tolerance and be mobile so it can ‘pick up its feet’. The process in developing this new product would consist of identifying the optimum polymer architecture to give a correct combination of mobility and layer thickness. Although this product would be more expensive, it would be much more effective than the traditional alternatives.
Market Opportunity: Polymeric surfactants in emulsions
An
emulsion consists of droplets of one liquid dispersed in an immiscible
liquid, with a large interfacial area. Sustainable surfactants have been
traditionally used to form emulsions. For example, various forms of
natural products are used in foods, where the immiscible liquid phases are
generally water and a triglyceride oil (lipid). These natural surfactants
include monoglycerides, fatty acids, phospholipids and proteins, as well
as fatty acid esters of sorbitol and polyethylene oxide derivatives.
However, there are synthetic surfactants that perform better and are thus
preferred for some applications.
Ideally, a surfactant used in an emulsion should: provide easy emulsification; produce the desired form of emulsion (water/oil or oil/water); produce a stable emulsion that does not coalesce or cream; be adaptable to give optimum performance for the system; give reproducible results; be clear, colourless and tolerant to, for example, salt, hardness, temperature, pH and degradation.
Energy is required to form the emulsion, which is only stable if there is a ‘barrier’ to coalescence. A surfactant can act as an emulsifier and/or a stabiliser. An emulsifier produces a low interfacial tension between the two liquids, diffuses to the newly made interface rapidly and forms the desired type of emulsion (i.e. w/o or o/w). A stabiliser minimises droplet coalescence giving low interfacial tension. If the interfacial tension is reduced sufficiently ‘stable’ micro-emulsions can be obtained. Stabilisers also form a robust interface, promoting interfacial viscosity and elasticity; damp down the fluctuations in the interfacial film, enhancing the Gibbs Marangoni effect; and/or reduce molecular diffusion across the interface, reducing Oswald ripening.
A good emulsifier will sit at the interface, producing a low interfacial tension. There needs to be an optimum balance of lipophilic and hydrophilic groups in the molecule. This will depend on the type of emulsion and system. Generally guidelines are:
A good stabiliser will have the following properties:
Some synthetic block copolymer (such as Arlacel) produce stable water-in-oil emulsions because of the thick barrier formed around the drop where the polymer associates to form a multi-layered liquid crystal-like structure. These can make highly stable emulsions that can produce high internal phase volume emulsions. They can also be used as primary emulsifiers in multiple emulsions (e.g. w/o/w), or to tailor molecules to particular applications (e.g. synthetic oils – silicones). The flexibility of block size, architecture and chemical composition of blocks enable molecules to be built with the desired solubility in each phase and produces a thick layer.
Conclusion
To achieve the properties described above using
a sustainable material is difficult, especially since it is desirable that
chemical modification is kept to a minimum. One solution may be to find
the optimum configuration by a synthetic route then adapt this to
sustainable materials, repeating the process to find the best sustainable
material. This requires a multidisciplinary team involving synthetic
chemistry, colloid chemistry and biotechnology, which cannot usually be
provided by an existing company infrastructure. Hence, there is an
opportunity for new small hi-tech businesses that are probably offshoots
of large companies, supported by venture capitalists.
Contacts
Contact
Participant
© Copyright 2006 Policy Statements
Updated
by CPL Press:
03/07/2007
- biomatnet@biomatnet.org
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