WO2003082429A2

WO2003082429A2 - Liquid-liquid separation

Info

Publication number: WO2003082429A2
Application number: PCT/GB2003/001421
Authority: WO
Inventors: Kai Friedrich Hoettges; Derek Stevenson; Kevin Peter Homewood; Russell Mark Gwilliam
Original assignee: University Of Surrey
Priority date: 2002-04-02
Filing date: 2003-04-01
Publication date: 2003-10-09
Also published as: AU2003214475A1; AU2003214475A8; GB0208766D0; WO2003082429A3

Abstract

The invention provides a method of separating two liquid phases comprising flowing the liquid phases along a flow passage for example a junction or channel having wall regions having surfaces whose surface properties differ, whereby the phases are separated on the basis of their respective affinities to the respective surfaces. The invention also provides an apparatus for separating two liquid phases, an apparatus for extracting or partitioning between two liquid phases prior to liquid-liquid separation, and a method of separating, purifying, isolating or analysing one or more compounds by extracting or partitioning them between two liquid phases prior to separating the two phases.

Description

LIQUID-LIQUID SEPARATION

The present invention relates to the separation of two liquid phases from each other and is particularly useful for the separation, isolation, purification or analysis of compounds dissolved in either or both of the liquid phases.

Background to the Invention

The extent to which a particular compound dissolves in a particular liquid (i.e. the solubility of a potential solute in a potential solvent) depends on the polarity of each of the solute and solvent and the chemical structure of each of the solute and solvent. For example, ionic compounds usually dissolve well in aqueous liquids, polar organic compounds dissolve well in polar organic liquids and non-polar organic compounds dissolve well in non-polar organic liquids.

Mixtures of liquids or solutions may consist of separate liquid phases, each of which is a substantially homogeneous, physically distinct and mechanically separable portion of the mixture. Where two liquid phases are present, then given sufficient time and/or mixing, a compound or mixture of compounds (potential solutes) will partition between the two phases according to the solubility of each compound in each phase.

The differential partitioning of compounds between two liquid phases forms the basis of a wide range of useful chemical techniques. Preparative and analytical methods often require compounds to be separated from mixtures. Many types of mixtures (to name just a few: crude reaction products; biological fluids such as urine, saliva and plasma; and fluids which are important to analyse for environmental reasons such as waste water and river water) can be purified, separated or analysed on the basis of compounds' differing solubility in different phases. Depending on the nature of the mixture and the phases, the partitioning may result in a desired compound being effectively the only solute in a phase, or it may result in a partial but nevertheless useful separation of solutes. As an example of the former, where a mixture consists of a single organic compound and several inorganic compounds, the organic compound can often be isolated by partitioning the mixture between an aqueous phase and an organic phase, separating off the organic phase and removing the organic solvent. Further purification steps may be carried out if necessary.

In the field of analytical chemistry, liquid-liquid extraction or partitioning between phases is often used to partially clean up a sample or produce a solution in a suitable solvent prior to further analysis, for example by chromatography or mass spectrometry. Furthermore, the principle is not only useful for isolation or clean up purposes but also in its own right for characterization purposes since the extent to which a compound partitions between two phases provides useful information about the compound's properties; for example, octanol-water partition coefficients are important parameters to measure.

The separation, isolation, purification or analysis of compounds discussed above requires the two liquid phases to be separated. Separation does not necessarily need to be complete. In some cases, for example, the user is only concerned with obtaining some pure organic phase, and the aqueous phase and remaining organic phase is discarded.

Gravity-based separation is by far the most common method of separation and relies on the fact that two phases often differ in density. For example, the classical separating funnel allows each layer to be tapped off separately. Lipophilic and or hydrophilic inserts have also been used to enhance gravity- based separation. For example, Teflon fibre bundles and strips have been used to help direct the organic phase to the desired outlet. Membrane phase separators have also been used. An article by Kuban - "Liquid-liquid extraction flow injection analysis", Critical Reviews in Analytical Chemistry 1991, 22(6), 11-511 - discusses on pages 501-513 some phase separators that have been used. However, such arrangements have their disadvantages. Gravity-based separation may not work well if the layers do not separate easily, for example, if their densities are not very different. Furthermore, separators using lipophilic or hydrophilic inserts are complex to fabricate, and where more than one of these separators is used, the cost and complexity is multiplied. Teflon inserts need to be cleaned frequently, hydrophilic/lipophilic filters often require frequent replacement, and membrane phase separators often suffer in terms of stability and ease of operation.

In addition to the above, the miniaturisation of laboratory apparatus and processes is becoming increasingly important. Miniaturisation allows processes to be carried out on a small scale on reliable integrated devices using small quantities of materials. One miniaturised liquid flow system is described in Hibara et al. "Integrated Multilayer Flow System on a Microchip", Analytical Sciences 2001, 17, 89-93.

The present invention seeks to provide an improved apparatus and method for separating liquid phases which is adaptable for use on a miniaturised scale.

Summary of the invention

From a first aspect, the invention provides an apparatus for separating two liquid phases, comprising a junction, said junction branching into at least two channels, said channels having surfaces whose surface properties differ from each other whereby the phases are separated on the basis of their respective affinities to the respective surfaces.

From a second aspect, the invention provides a method of separation of two liquid phases comprising flowing the liquid phases into a junction branching into at least two channels, said channels having surfaces whose surface properties differ from each other whereby the phases are separated on the basis of their respective affinities to the respective surfaces. Thus the present invention is based on the differential affinity of liquids for solid surfaces. Liquids differ in the extent to which they have an affinity for solid surfaces. For example, polar liquids have a greater affinity to polar surfaces and non-polar liquids have a greater affinity to non-polar surfaces. One well-known consequence of this is capillary action whereby the pressure, p, driving a liquid into a capillary is described by p=2 σ(cosα)/r where σ is the surface tension of the liquid, α is the contact angle between the liquid and capillary surface and r is the radius of the capillary. Accordingly, a liquid is drawn more strongly into a capillary with a similar surface polarity (i.e. where there is a low contact angle).

Thus by flowing a liquid into a junction whose outlet channels have differing properties, the different phases of the liquid will wish to flow preferentially into one or other of the channels, leading to a degree of separation at the junction.

Alternatively, or additionally, separation may occur on the basis of phases' differing affinities for surfaces within the junction, so that regardless of what the surface properties are outside the junction, a degree of separation occurs inside the junction so that the respective phases are preferentially directed into one or other of the channels.

From a third aspect, therefore, the invention provides an apparatus for separating two liquid phases, comprising a junction, said junction branching into two channels and having two regions, said regions having surfaces whose surface properties differ from each other whereby the phases are separated on the basis of their respective affinities to the respective surfaces and accordingly are preferentially directed down one or the other channel.

Likewise, from a fourth aspect, the invention provides a method of separation of two liquid phases comprising flowing the liquid phases into a junction branching into two channels and having two regions, said regions having surfaces whose surface properties differ from each other whereby the phases are separated on the basis of their respective affinities to the respective surfaces and accordingly are preferentially directed down one or the other channel.

Preferably, therefore, the respective channel surfaces and/or surfaces of regions within junctions have differing polarities. Accordingly, one channel's surface is preferably more polar than the other, and/or one region's surface is preferably more polar than the other.

Preferably the more polar surface is an inorganic compound, for example glass, acid-activated glass or silicon, or a polar organic polymer, for example polyethylene glycol. Metals could also be considered, for example titanium.

Preferably the less polar surface is a non-polar organic compound for example an organic polymer or silicone. Suitable silicones are dimethylsiloxane, octadecylsilane or polymers thereof. Other suitable polymers for the less polar surface include trichloro octadecylsilane, polyethylene and polypropylene.

Each channel's entire surface may have the same surface properties, but that is not essential. What is important is that the channels should present different resistance to the flow of the respective liquid phases, or alternatively or additionally that the regions within the junctions, through their differing affinities for each phase, should differentially direct the flow of the respective liquid phases.

The dimensions of the channels and junctions are sufficiently small for the differences in affinity to allow the separation of the two liquid phases to occur. Preferably the channels are capillary channels. Preferably the width and depth of the channels are within the range 0.1-2000 μm, more preferably 1-1000 μm, more preferably 5-500 μm, most preferably 5-100 μm. Preferably the dimensions of the junctions are within the range 0.1-4000 μm, more preferably 1-2000 μm, more preferably 5-1000 μm, most preferably 5-200 μm. A junction may be T-shaped, or more preferably generally Y-shaped, as this gives reduced resistance to flow and improved separation.

The separation may be partial or complete: for many applications a partial separation is sufficient. While the invention in its broadest terms covers separation occurring at a single junction, this may not in some circumstances produce a sufficient degree of separation. Preferably, therefore, the apparatus comprises a plurality of interconnected junctions. Thus, for example, the output of a channel leading from a first junction may itself be input to a second junction, whereby further separation can take place at the second junction, and so on.

Preferably the junctions are arranged such that outlet channels from two preceding junctions flow into the junction. Most preferably, the channels flowing into the junction are those having different surface properties. In this arrangement, the junction may have a generally X-shaped configuration.

In a preferred arrangement, there are two sets of channels having different surface properties, so that one liquid phase is preferentially directed down one set of channels and the other liquid phase is preferentially directed down the other set of channels.

In a preferred arrangement, the sets of channels intersect each other thereby providing an interconnected array of channels, with junctions formed where the channels intersect. Preferably the channels in a set are arranged parallel to each other.

Most preferably, each of the two sets of channels is configured such that the separated flows in said channels are directed into respective single outlet channels. This considerably facilitates collection of the separated phases.

The channels and junctions may be formed in any suitable manner. Preferably, however, they are formed at the interface between two substrate surfaces. In this way, for example, a groove in one surface may cooperate with the opposed surface to form a channel. This provides a simple and effective way of fabricating a complex array of channels, such as the intersecting array of channels discussed above and avoids the need for micro fabrication.

The invention also extends to a method of manufacture of apparatus suitable for separating two liquid phases, comprising forming a set of grooves in a surface of a first substrate, and locating said surface against a surface of a second substrate so as to define a set of channels therebetween.

The grooves may be formed on just one substrate surface, but preferably they are formed on both substrate surfaces as this facilitates the provision of differing surface properties in the grooves.

In a preferred embodiment, therefore, the grooves formed in one substrate may have a first surface property and those in the second substrate a second surface property. With such an arrangement, one side of each channel will be of the 'wrong' surface chemistry. However, the majority of the channel surface will be of the "right" surface chemistry, so that at the entry junction to the channels, the two phase liquid will see one channel with predominantly one property and another with predominantly the other property and will separate accordingly.

The cross-sectional profile of the grooves may be of any form. They may, for example, be straight sided for example rectangular, trapezoid or N-shaped, or rounded, for example semi-circular or elliptical.

The grooves may be prepared by any suitable method including photolithography, etching (for example with hydrofluoric acid), reaction moulding, injection moulding, embossing, cutting, precision milling, laser ablation and fast prototyping.

In one embodiment, the surface chemistries of the sets of grooves differ by virtue of the fact that sets of grooves are formed on two substrates with different chemistries. For example, one substrate could be glass and the other an organic material. Metals and semi-conductors might also be possible materials.

In a different embodiment, however, one or both substrate surfaces can be provided with a suitable coating, either before or after groove formation. Suitable methods of coating include, for example, spin coating, evaporative coating, sputtering or chemical vapour deposition. The coating may be applied directly to the substrate or there may be an intermediate layer for improving adhesion of the coating to the substrate. For example the surface may be vinylised (e.g. by reaction with vinyl trichlorosilane) and then coated (e.g. spin-coated) with a further substance such as a mixture which will bind to the vinyl layer and result in the desired surface properties (e.g. a silicon monomer mix comprising vinyl-terminated dimethylsiloxane, methylhydrosiloxane-dimethylsiloxane copolymer and a polymerisation catalyst).

Preferably the two substrates are bonded together. Preferably, therefore, at least one of the substrates is coated with or formed from a substance which functions as a bonding medium, for example when the two substrates are pressed together and the substance is allowed to react (e.g. heat is applied so that the substance polymerises and simultaneously bonds the two substrates together). Most preferably the bonding coating is the groove surface coating, whereby the need for a separate bonding coating is avoided.

Other bonding methods are also suitable. Direct bonding of the two substrates may be achieved by activating them, bringing them together and annealing them: for example, glass or silicon substrates can be hydrophilically or hydrophobically activated (an example of the former is the formation at the surface of silanol groups which initially bond the two substrates together by hydrogen bonds and Van der Waals forces) and then annealed at elevated temperatures to form Si-O-Si bonds between the layers. Anodic bonding can be used, for example, to bond a glass substrate to a silicon substrate. Bonding with spin-on glass (a solution of silicon hydroxide) can be used, for example, to bond glass substrates together. An example of bonding with an intermediate polymer layer has been described above. For some materials simple mechanical clamping may suffice.

It will be appreciated that a large array of separation junctions can be formed on a substrate for example in the manner described above. To increase the scale, a plurality of arrays can be joined together in parallel. In a preferred arrangement, individual separators may be stacked one upon the other and supplied with liquid to be separated from a common source. For example, supply and/or outlet channels may be formed, e.g. drilled, to extend through the stack, communicating with the inlets and/or outlets of each individual separator. This will provide a particularly compact construction.

As has been mentioned above, the present invention is particularly suited for use in liquid-liquid extraction, particularly on a miniaturised scale.

Accordingly, the invention extends to cover liquid-liquid extraction apparatus incorporating separation apparatus in accordance with the invention and also a method of separating, purifying, isolating or analysing compounds by extraction between liquid phases followed by separation of the liquid phases by a method in accordance with the invention.

Typically in such arrangements, two liquid phases are first brought together, a large interface formed between the two phases and the phases then separated. Preferably therefore, the extraction apparatus further comprises means for bringing two liquid phases together and allowing extraction or partitioning between the two phases prior to the separator stage.

Preferably the two liquid phases are introduced into a channel where they proceed in the form of a slug flow, rather than a laminar flow. Slug flows are associated with internal circulation (see for example Burns et al., "The intensification of rapid reactions in multiphase systems using slug flow in capillaries", Lab on a Chip 2001, /, 10-15) and improve contacting between the liquid phases such that compounds are more easily partitioned between the two. The length of the channel is such that sufficient partitioning or extraction occurs. The contacting channel is preferably of generally the same dimensions as the separation channels discussed above. Preferably the two liquid phases are brought together at a T junction to form a slug flow.

The flow rate is such as to allow the desired extent of separation. Preferably the flow rate is less than 100 μl per minute, more preferably between 0.1 and 10 μl per minute. Fast flow rates may be associated with less efficient separation. Unlike laminar flow systems which are inherently unstable, the slug flow arrangement above is better able to cope with variations in flow rates and noisy pumping systems.

The flow rate of one liquid phase may be different to the flow rate of the other liquid phase; this allows one phase (e.g. an organic phase) to be more concentrated than a second phase (e.g. an aqueous phase).

The extraction apparatus may also comprise a detector (e.g. a spectrophotometer) or further separation device (e.g. a chromatographic device such as an HPLC or GC device) downstream from the separator to allow for more detailed analysis or separation.

As stated above, the invention also extends to an apparatus which comprises a plurality of the above-mentioned apparatus stacked together in parallel such that all the inlets are joined together and all the outlets are joined together. This allows separations to be carried out on larger scales.

The apparatus and methods described above can of course be used to separate two liquid phases even when these are free from solutes. Brief Description of Drawings

A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows, schematically, a liquid-liquid extraction apparatus incorporating a separator in accordance with the invention;

Figure 2 shows, schematically, a detail of the apparatus of Figure 1;

Figure 3 shows, schematically, a detail of Figure 2;

Figure 4 shows, schematically, a further detail of Figure 2; and

Figure 5 shows a sectional view along the line N-V of Figure 4.

Detailed Description of a Preferred Embodiment.

With reference to figure 1, a liquid-liquid extraction apparatus, device 2 comprises an inlet 4 for a first liquid and inlet 6 for a second liquid, a T junction 8 at which the liquids are mixed, an extraction channel 10 and a liquid-liquid separator or separation apparatus 12. Outlets 14, 16 are provided from the separator 12.

With reference to figure 2, the separator 12 comprises an inlet channel 18 and two outlet channels 20, 22. The inlet channel 18 enters a junction 24 from which leave outlet channels 26, 28. The inlet channel 18 and outlet channels 26, 28 are arranged in a generally Y shaped configuration. A first set of parallel passages 30 extend from the outlet passage 28 to the outlet channel 20 and a second set of parallel channels 32 extend from the channel 26 to the outlet channel 22. In this embodiment, the channel 26, outlet channel 20 and the set of parallel channels 30 have a predominantly non- polar surface while the channels 28, 22 and 32 have a predominately polar surface.

As can be seen from figure 2, the channels 30 and 32 intersect with each other forming generally X shaped junctions 34 at their points of intersection; in this particular embodiment there are 16 such junctions 34. The channels 30 and 26 also intersect with the channel 28, and the channels 32 and 28 with the channel 26, forming generally Y-shaped junctions 24 at their points of intersection; in this particular embodiment there are 9 such junctions 24.

A mixture of a polar liquid phase and a non-polar liquid phase which enters the inlet 18 will arrive at the junction 24 and due to the relative affinity of the liquids for the polar and non-polar surfaces of the channels 28, 26, they will be separated at least partially at that junction 24. As the partially separated liquid travels up the passages 26, 28, it will reach further Y junctions 24 and separate further and also X junctions 34 separating further still. As a result, the polar component of the mixture will flow towards to the outlet channel 22 and the non-polar towards the channel 20.

The respective liquids may be then removed from the apparatus and, if desired, subjected to further examination or analysis.

As mentioned above, the separator may be used in a liquid-liquid extraction device 2. In this device respective liquids which may contain solutes to be partitioned between the liquids are introduced into the inlets 4, 6 and mixed together, preferably in a slug formation at the T junction 8. The two phase liquid mixture then passes along the extraction channel 10 into the inlet junction 18 of the separation device, where the liquid components, with solute, are separated as described above for analysis.

With reference now to figures 4 and 5, construction of the separation device 2 will be described.

The separation device, and indeed the other components shown in figure 1 are all formed on a "chip" which typically may be 20mm wide by 40mm long. The chip is formed in two layer 42, 44. Each of these layers is typically 0.25 - 2mm thick.

The opposed surfaces of the layers 42, 44 are formed with respective grooves 46, 48. The grooves 48 in the lower layer 44 are arranged in the pattern of the channels 28, 22 and 32 while the grooves 46 in the upper layer 42 are arranged in the pattern of channels 24, 20 and 30. When the two layers are placed together, the respective channels and junctions are formed at the interface of the layers, as shown in figure 5. The grooves are about 25-35 μm deep and 100-500 μm wide. Figure 5 illustrates that each junction comprises a polar surface region 48 towards which a polar phase will be drawn and a non-polar surface region 46 towards which a non-polar phase will be drawn.

In the particular embodiment shown, the lower layer 44 is formed from a glass material in to which has been etched or otherwise suitably formed the grooves 48. The upper layer 42 comprises a glass substrate 50 whose surface is acid-activated and then vinylised by reaction with vinyltrichlorosilane to form a vinyl layer 52 bonded onto the glass surface. A non-polar layer 54 is then deposited on the vinyl layer 52 in order to provide the groove 46 with the appropriate surface chemistry. In this particular embodiment, the non- polar layer comprises a silicon monomer mix comprising vinyl-terminated dimethylsiloxane (about 95% by weight), methylhydrosiloxane- dimethylsiloxane copolymer (about 5% by weight) and a catalyst (platinum carbonyl cyclovinylmethylsiloxane complex, about 0.15% by weight). This coating is spin coated onto the vinyl layer.

After coating, the layers are pressed together face-to-face and polymerisation initiated by heating the composite to 100°C. This acts to bond the layers together and produce a silicone coating on the upper layer 42. Once bonded, the two layers together define the desired pattern of intersecting channels. The non-coated glass acts as a polar surface and the coated glass acts as a non-polar surface. The extraction channel 10, mixing junction 8 and the passages leading from inlets 4 and 6 may also be formed in this manner in the chip.

To exemplify the use of the apparatus described above, in one experiment, iso-octane was admitted in inlet 4 of the apparatus and water dyed with methylene blue to provide a visual illustration of the separation was admitted into the inlet 6. Each phase was pumped through the apparatus at 3.7 μl per minute using a modified syringe pump so that the total flow rate through the apparatus was 7.4 μl per minute, with the phases being present in a 1: 1 ratio.

The pump was operated in such a manner that the respective phases entered the extraction channel 10 in a slug flow. After passing through the separator 2, it was found that the outlet 14 contained 99.3% iso-octane and 0.7% water by volume while the other outlet contained 96% water and 3.9% iso-octane. This demonstrates the efficiency of the separation procedure and of course further separation could take place to completely separate the phases.

It will be appreciated that the above description is illustrative of the invention and not limiting. The skilled person will be able to apply the method and apparatus of the invention to a wide range of separations, purifications, isolations or analyses. Also, the method of manufacture of the device may be modified from the process specifically described.

By appropriate choice of types of channel surface, a wide variety of two-phase systems can be separated. Some examples of separable liquid phase systems are: (i) one phase being aqueous and the other organic; (ii) one phase being polar and the other non-polar; and (iii) one phase being water and the other octanol. The invention is applicable to the analysis, separation, purification or isolation of many compounds and mixtures, and these include, but are not limited to, crude reaction products, pesticides (e.g. organochlorines, phenylureas), drugs (e.g. morphine, clenbuterol, propanolol, tamoxifen), biological fluids (e.g. urine, saliva, plasma), fluids which are important to analyse for environmental reasons (e.g. waste water and river water) and other environmental pollutants (e.g. polyaromatic hydrocarbons). In very broad terms, the invention provides a method of separating two liquid phases comprising flowing the liquid phases along a flow passage for example a junction or channel having wall regions having surfaces whose surface properties differ, whereby the phases are separated on the basis of their respective affinities to the respective surfaces.

Claims

1. An apparatus for separating two liquid phases, comprising a junction, said junction branching into at least two channels, said channels having surfaces whose surface properties differ from each other whereby the phases are separated on the basis of their respective affinities to the respective surfaces.

2. A method of separation of two liquid phases comprising flowing the liquid phases into a junction branching into at least two channels, said channels having surfaces whose surface properties differ from each other whereby the phases are separated on the basis of their respective affinities to the respective surfaces.

3. An apparatus for separating two liquid phases, comprising a junction, said junction branching into two channels and having two regions, said regions having surfaces whose surface properties differ from each other whereby the phases are separated on the basis of their respective affinities to the respective surfaces and accordingly are preferentially directed down one or the other channel.

4. A method of separation of two liquid phases comprising flowing the liquid phases into a junction branching into two channels and having two regions, said regions having surfaces whose surface properties differ from each other whereby the phases are separated on the basis of their respective affinities to the respective surfaces and accordingly are preferentially directed down one or the other channel.

5. An apparatus or method as claimed in any preceding claim wherein the surface properties differ by virtue of the surfaces having differing polarities.

6. An apparatus or method as claimed in claim 5 wherein one surface is more polar than the other.

7. An apparatus or method as claimed in claim 6 wherein the more polar surface is an inorganic compound, a polar organic polymer, or a metal.

8. An apparatus or method as claimed in claim 6 or claim 7 wherein the less polar surface is a non-polar organic compound or a silicone.

9. An apparatus or method as claimed in any preceding claim wherein the channels are capillary channels.

10. An apparatus or method as claimed in any preceding claim wherein the width and depth of the channels are within the range 0.1-2000 μm and the dimensions of the junctions are within the range 0.1-4000 μm.

11. An apparatus or method as claimed in any preceding claim comprising a plurality of interconnected junctions.

12. An apparatus or method as claimed in claim 11 wherein outlet channels from two preceding junctions flow into a junction.

13. An apparatus or method as claimed in claim 11 or claim 12 wherein there are two sets of channels having different surface properties, and the sets of channels intersect each other thereby providing an interconnected array of channels and junctions where the channels intersect.

14. An apparatus or method as claimed in any of claims 11 to 13 wherein the channels and junctions are formed at the interface between two substrate surfaces, and wherein a groove in one surface co-operates with the opposed surface to form a channel.

15. A method of manufacture of apparatus suitable for separating two liquid phases, comprising forming a set of grooves in a surface of a first substrate, and locating said surface against a surface of a second substrate so as to define a set of channels therebetween.

16. A method as claimed in claim 15 wherein grooves are formed on both substrate surfaces, the grooves formed in one substrate have a first surface property and the grooves formed in the second substrate have a second, different, surface property.

17. A method as claimed in claim 16 wherein the different surface properties are a consequence of the different chemistries of the substrates.

18. A method as claimed in claim 16 wherein the different surface properties are a consequence of coating the grooves of at least one of the substrates with a suitable material.

19. A method as claimed any one of claims 16 to 18 wherein at least one of the substrates is coated with or formed from a substance which functions as a bonding medium.

20. A method claimed in claim 19 wherein the substance which functions as a bonding medium is the same as the coating which gives one set of grooves its surface properties.

21. An extraction apparatus comprising means for bringing two liquid phases together and allowing extraction or partitioning between the two phases prior to an apparatus for separating two liquid phases as defined in any of claims 1, 3 or 5-14.

22. A method of separating, purifying, isolating or analysing one or more compounds by extracting or partitioning them between two liquid phases prior to separating the two phases according to a method defined in any of claims 2, 4 or 5-14.

23. An extraction apparatus or method as defined in claim 21 or 22 wherein extraction or partitioning between the two phases occurs whilst the two phases proceed in a slug flow.

24. An apparatus or method as defined in any of claims 1-14 or 21-23 wherein the flow rate is within the range 0.1-100 μl per minute.

25. A method of separating two liquid phases comprising flowing the liquid phases along a flow passage having wall regions having surfaces whose surface properties differ, whereby the phases are separated on the basis of their respective affinities to the respective surfaces.