How fast and how far a chemical reaction goes in a liquid depends very much on what the reacting chemicals are dissolved in. The solvent that chemists commonly rely on is water, but it is not an ideal medium for carrying out reactions because many materials, particularly organic compounds, do not dissolve in water. For this reason, chemists have, for many years, exploited a wide range of non-aqueous solvents and mixtures of solvents, to dissolve these water-insoluble compounds.
Sometimes, however, reactions have to be carried out between molecules, some of which are soluble in water while others are not. Traditionally, this means combining two immiscible solvents which are then mixed by stirring. Reactions carried out in this way are much less efficient because of the relatively limited contact between the two phases.
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Recently, some researchers have started to use specially tailored solvents – so-called ‘functional fluids’ – to carry out their reactions. This is developing into an important and exciting area of chemistry and there is now available a range of novel media for reactions. One particular kind of functional fluid, known as a microemulsion, shows great potential. Microemulsions contain an organic solvent (for example, a hydrocarbon oil such as octane), water and a dispersing agent, which are organised in a structured way. Microemulsions provide a highly flexible and unique medium for carrying out chemical reactions.
Everyone knows that oil and water do not mix. They avoid being in contact as much as possible. If you shake a mixture of oil and water, one liquid disperses in the other as tiny droplets to form a ‘cloudy macroemulsion’.
Macroemulsions are not stable indefinitely. The droplets coalesce into ever larger globules thus reducing the area where the oil and water touch. Ultimately, the oil and water minimise their contact by separating into two phases. However, adding materials called surfactants can stabilise the emulsion. This is what happens when you make a salad dressing. If there is more oil than water present, droplets of water disperse in the oil to give a water-in-oil emulsion, as in mayonnaise; more water and less oil produces an oil-in-water emulsion, as in milk.
As Richard Templer described in his article on lipid membranes last week, surfactant molecules possess two distinct parts that make them behave in an interesting way: a hydrophilic headgroup that likes to associate with water molecules and a hydrophobic hydrocarbon tail that tries to avoid contact with water and prefers to be associated with oil (see Figure 1). As a result, the surfactant molecules tend to gather at the interface, with their headgroups sticking into the water and the hydrocarbon tails penetrating into the oil.FIG-mg17706501.jpg
Some surfactants actually form a fluid skin at the interface where the oil and water meet so as to eliminate the contact between the two phases. The surfactant molecules then line up in a closely packed array at the interface. This can result in ultra-small droplets of one phase being dispersed in the other. Such a system is completely stable and is called a ‘microemulsion’. It contains much smaller droplets than are found in ordinary emulsions.
Even in the absence of oil, surfactant molecules organise themselves in water to form ‘supermolecular’ assemblies called micelles. About 100 molecules combine in each micelle. The headgroups face outwards to the water while the hydrocarbon tails are tucked away inside. Micelles are not held together by any obvious chemical bonds; rather it is the desire of the hydrophobic tails to escape from the water that holds these structures together.
Although the overall micellar system is stable, individual micelles readily break up and reform in the solution, as surfactant molecules in the water rapidly change places with the surfactant molecules in the micelles. Surfactant molecules can also organise themselves in oil to form reverse micelles. Here, the tails are directed outwards into the oil. At very high concentrations of surfactant in both water and oil, the systems become highly structured to give a range of liquid-crystalline structures as described in Templer’s article.
Like microemulsions, solutions of micelles can dissolve both water-soluble and water-insoluble materials. Indeed, some molecules are more soluble in micellar solutions than in either water or a pure hydrocarbon solvent. This unusual property was recognised in the 1960s by Kozo Shinoda and others. They could explain why this was so in terms of the way molecules interact with the micelles. Some molecules like to associate with the hydrophilic micelle surface because they are chemically similar, while others prefer to bury themselves within the micelle close to the hydrophobic tails of the surfactant. When micellar solutions are prepared using an ionic surfactant such as sodium dodecyl sulphate, the micelle surface carries a net electrical charge. The surface can then attract or repel charged molecules in the solu-tion, depending on whether the charges are opposite or the same. The micelle surface can also catalyse reactions between molecules absorbed at the interface and hydrophobic molecules adsorbed in the micelle interior, but still close to the micelle surface.
This led some researchers to believe that micelles were mimicking enzymes, and so could be used as possible substitutes – as catalysts in speeding up chemical reactions. Enzymes are biological catalysts which are extremely selective as to which reac-tions they speed up. They have a characteristic three-dimensional structure, part of which is a pocket or ‘active site’ where molecules that have a complementary structure must fit, rather like a jigsaw piece, in order to react. Various research groups tried to modify the headgroup region of the surfactant so that it mimicked the active site of an enzyme. These attempts, however, were not very successful, simply because the structure of a micelle is far too loose in comparison with the region around the active site of an enzyme which is virtually rigid.
Nevertheless, micelles can speed up reactions dramatically: many proceed up to 10,000 times as fast as those carried out in simple water-based systems. These increases in rates of reaction compare favourably with those achieved using enzymes. In the 1970s, an American chemist Janos Fendler and his colleagues successfully used reverse micelles to catalyse a range of organic reactions including the molecular rearrangement of glucose in benzene using dodecylammonium propionate (DAP) as a surfactant. They also catalysed the binding of vitamin B12 to certain small molecules in benzene containing DAP. In the absence of a surfactant, these reactions happen only very slowly or not at all.
If you add a hydrocarbon oil to an aqueous solution of micelles, you can produce an oil-in-water microemulsion. Chemists have not studied these systems very much with a view to using them as catalysts, because the reacting organic compounds are often trapped inside the oily core of the micelle and cannot be reached by other chemicals. Adding water to a system of reverse micelles produces a water-in-oil microemulsion, which again has only a limited catalytic ability in its own right.
Nevertheless, several research groups have recently used reverse micelles to make catalyst particles of noble metals, for example platinum, and also semiconductors such as cadmium sulphide. Reactions involving these catalytic materials happen at their surfaces. They therefore work much more efficiently when divided into tiny particles, because this creates the largest possible surface area. Microemulsions provide an excellent way of preparing these highly efficient microparticles. The water cores of microemulsion droplets are extremely small, between 1 and 10 nanometres (a nanometre is a billionth of a metre). The size of the droplets depends on the ratio of water to surfactant in the system, and this allows droplets to be prepared with dimensions to order. The core of the water droplet then provides a microreactor tailored to produce a catalyst particle of the required size. Mixing droplets containing cadmium ions with droplets containing sulphide ions produces minute semiconductor particles of cadmium sulphide in a matter of seconds (see Figure 2).FIG-mg17706502.jpg
Another example where chemists have used water-in-oil microemulsions is in carrying out polymerisations. Researchers at the Charles Sadron Institute in Strasbourg introduced acrylamide into the water cores of the microemulsion drop-lets. They then polymerised the acrylamide with an initiator (a compound that starts off the polymerisation) dissolved in oil. The size that the polymers can grow to is limited by the size of the droplet cores. This gives a much better defined range of molecular size than can be achieved using more conventional methods of polymerisation.
Enzymes in action
In our research at the University of East Anglia, we are particularly interested in dispersing enzymes in the water cores of water-in-oil microemulsion droplets. The enzymes can then be used to prepare compounds that would be difficult or even impossible to make in the conditions in which most enzymes are designed to work. More chemists are now starting to use enzymes to catalyse the synthesis of organic compounds in the laboratory (see ‘The greening of chemistry’, 91av, 21 April, 1990). Enzymes are highly selective in catalysing one particular reaction rather than another. This can lead to the synthesis of much purer products necessary, for example, in the pharmaceutical industry. Enzymes also have the added advantage that they work in moderate conditions of temperature, pressure and acidity or alkalinity.
But there is a problem. Enzymes usually work in watery conditions, and many compounds that chemists would like to work with do not dissolve easily in water. This is where water-in-oil microemulsions are useful. They can act as solvents for virtually any molecule, irrespective of its chemical nature. Although water-in-oil microemulsions look like a single phase to the eye, the system really consists of three distinct domains: oil, water and a surfactant domain which lies at the interface between oil and water. Compounds that are insoluble in water – such as fats and steroids – may be dissolved in the oil region, while water-soluble materials such as enzymes, ethanol or even salts can be dissolved in the water core of the micelle or located in the interfacial region.
Using water-in-oil microemulsion droplets also solves another problem. Organic solvents can deactivate enzymes by disrupting their three-dimensional structure. But in these microemulsions, the enzyme is doubly protected from the potentially destructive effects of the organic solvent by a shell of water immediately surrounding the enzyme plus a coat of surfactant (see Figure 3).FIG-mg17706503.jpg
To carry out reactions between water-soluble and water-insoluble materials, the molecules must be brought close together. The interface where the surfactant molecules lie provides an ideal meeting place for the enzyme, the oil-soluble and oil-insoluble compounds. In ordinary emulsions, the rate at which reacting molecules travel to the interface and across it may control the rate of a chemical reaction, but this is rarely so in microemulsions. This is because there is such a large area of interface resulting from the small size of the microemulsion droplets. For example, in a typical microemulsion prepared in a hydrocarbon oil, such as octane, containing a small amount of water and 5 per cent of surfactant, the total amount of interface per litre is 33,000 square metres. This is equivalent to finding the surface area of 33 Olympic-size swimming pools in 55 pence worth of petrol.
The speed of the reaction depends in many cases on the subtle dynamic changes that the microemulsion droplets undergo. Each microemulsion droplet lasts for about one-thousandth of a second. During this short lifetime, the droplets tumble around; the surfactant molecules encapsulating the droplets may diffuse in and out of the oil; and most importantly, the droplets collide with each other. These collisions may cause two droplets to fuse. The contents of the two droplets can then mix. These events have a very important consequence. First, because the surfactant film is fluid, it readily allows reacting molecules and the products of the reaction to pass between the oily solvent and the water core of the droplet. Secondly, because fusion between droplets is rapid, the exchange processes do not limit speed of the reaction – at least for reactions taking longer than a second or so.
One intriguing feature of dispersing enzymes in organic solvents is that it allows some enzymes to work backwards. In other words, the direction of the reaction catalysed by the enzyme reverses. This can happen in reactions using a group of enzymes called hydrolases. They employ water to split large molecules into two or more smaller ones. Both a-chymotrypsin, a gastric enzyme which breaks down proteins, and lipase, which breaks down fats, fall into this category.
Take the classic lipase reaction, which is to break down triglycerides of the type commonly found in cooking oils and butters. The ultimate breakdown products are glycerol, a viscous water-soluble material, and various fatty acids which are usually water-insoluble. As Figure 4 shows, the reaction can go forwards or backwards. Under the conditions found in the body, there is always plenty of water present to push the reaction forwards. However, if we carry out the lipase reaction in an environment where there is not much water around, we can reverse the reaction to produce glycerides. In water-in-oil microemulsions, this is done by starting with a mixture of water and glycerol confined to the droplet core, and dissolving our fatty acids in the oil region of the microemulsion. By controlling the amount of water in the microemulsion, we can also control the ratios of the various products formed.FIG-mg17706504.jpg
Using a similar approach, lipases can also be used to make a wide variety of ‘natural’ food flavour esters from their parent alcohols and fatty acids. Food companies such as Unilever are extremely interested in such processes. Microemulsions operate as a single phase so they do not need stirring. This approach would make the large scale manufacture of chemicals using enzymes much more attractive.
There are drawbacks, however. Enzymes are quite expensive and because they are so finely dispersed in a continuous medium they cannot be simply filtered out and reused. It is, however, possible to recover most of the enzyme by making the microemulsion separate into two phases – an oil-rich phase containing products and a water-rich phase containing the enzyme. Unfortunately, some of the enzyme is invariably lost. A further problem is that you also have to separate the surfactant from the products of the reaction. This has been a significant obstacle in exploiting microemulsion enzyme technology for large scale processes.
One way of solving the problem is to use what is called a ‘hollow-fibre reactor’. This is a series of thin hollow tubes whose walls have a mesh-like structure. The size of the gaps in the mesh is carefully chosen so as to allow small molecules such as the reaction products to pass through the wall, but not to allow the passage of large molecules such as enzymes. By filling the inside of these hollow fibres with a microemulsion containing the enzyme, it is possible to collect the products of a reaction by dipping the fibres in a water-immiscible organic solvent (see Figure 5). Pier Luigi Luisi and his group at the ETH in Zurich employed a microemulsion containing chymotrypsin in a hollow-fibre reactor to make a peptide. Peptides are made by joining up amino acids – the fundamental units of proteins. Peptides are a class of molecules that are very important to the pharmaceuticals industry because many drugs used now, as well as new ones being developed, are based on peptides.FIG-mg17706505.jpg
Recently, we have developed another way of carrying out synthetic reactions with enzymes such as lipase in microemulsion-based systems. By adding a long polymer such as gelatin to an enzyme-containing microemulsion you can make the microemulsion ‘set’. These so-called microemulsion-based gels, or MBGs, have a stiffness similar to that of fruit pastilles. Interestingly, MBGs show no tendency to dissolve when placed in contact with the oil with which the microemulsion is made – the surfactant, gelatin, enzyme and water all remain locked within the gel. The structure of MBGs appears to consist of a network of interconnecting gelatin rods coated with surfactant, which co-exist with water-in-oil microemulsion droplets (see Figure 6).FIG-mg17706506.jpg
You can also make the MBGs into pellets which when placed in oil, remain quite separate and do not stick together. This is particularly useful because it allows an MBG containing an enzyme to be used in chemical reactors where the reactions are carried out in batches, as well as in those where there is a continuous flow of reacting materials. Instead of the reacting compounds being added to the original microemulsion, they can be added to the oil phase which is separate from the MBG pellets. Molecules diffuse into and through the MBG until they meet an enzyme molecule and react. The product of the reaction then diffuses out of the MBG and into the oil phase – which, in the case of column reactors is a continuously flowing phase.
We have found that lipases immobilised in MBGs are very effective in carrying out reversal reactions in which an alcohol and a carboxylic acid join to make esters in mild conditions. Indeed, in many cases we found that the reaction is reversed so decisively that the oil phase does not contain any alcohol or acid, only the ester.
A great advantage of lipases is that they can perform reactions with compounds that are structurally quite dissimilar to those they normally encounter. When doing so they retain their ability to discriminate between chemically reactive groups of atoms at different positions in the structure of the reacting molecules, or reactive groups with different spatial orientations. This means that chemists can use this method to prepare new compounds of real value.
Research in this exciting multidisciplinary area is developing rapidly. With the interest that industry is taking in applying microemulsion-based systems in both synthesis and separation science, this field promises much for the future.
Gareth Rees is a research fellow and Brian Robinson is professor of physical chemistry in the School of Chemical Sciences at the University of East Anglia.