Carbon-based liquid-liquid separation membrane and preparation method and application thereof

Abstract

The application relates to a carbon-based liquid-liquid separation membrane and a preparation method and application thereof. First, a controllable oxidation method is adopted, graphite or carbon nanotubes and other carbon materials are oxidized by potassium permanganate in a high-chloric acid or chlorosulfonic acid aqueous solution, then a vacuum filtration method is adopted to uniformly deposit the oxidized carbon materials on the surface of a filter membrane, thus an excellent liquid-liquid separation membrane is obtained. The separation membrane can be used for efficient separation of a mixed liquid or emulsion of a polar solvent and a non-polar solvent, has a series of remarkable advantages such as simple method, high separation efficiency, repeated use and the like, and has a good market application prospect.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of membrane materials, in particular to a carbon-based liquid-liquid separation membrane and a preparation method and application thereof. BACKGROUND

[0002] Liquid-liquid separation is very important for many chemical processes, such as industrial wastewater treatment, fine chemical synthesis, drug production and purification, petrochemical manufacturing, and separation of analytical components. At present, distillation is mainly used to separate organic liquid mixtures in industrial production, but this liquid-liquid separation process consumes a large amount of energy, so it is urgent to develop a sustainable, efficient and energy-saving liquid-liquid separation method.

[0003] Membrane technology is mainly used to separate different molecules according to the chemical properties or size differences of the molecules, and is considered as a potential economic way to separate organic liquid mixtures. Membrane technology includes pervaporation and organic solvent reverse osmosis. Pervaporation has the problems of small permeation flux, phase change of permeate, and the need to provide vaporization heat. Organic solvent reverse osmosis requires a high-pressure environment and a precise sub-nanometer pore size manufacturing process, in addition to which the flow rate of organic solvent reverse osmosis is also very small. Therefore, energy saving and high flow rate are two major difficulties in the application of membrane technology to the separation of organic liquid mixtures.

[0004] In recent years, super-wetting technology has been widely combined with membrane technology, and methods for liquid-liquid separation have been realized, including: (1) using the extreme wettability difference of membranes in air for different organic liquids to separate liquid mixtures. For example, the mesh membrane prepared in Chinese patent CN104959043A can extremely repel polar organic liquids with a surface tension higher than a threshold value, while non-polar organic liquids with a surface tension lower than the threshold value can completely spread on the surface of the mesh membrane. The use of the mesh membrane with completely opposite wettability for different organic liquids allows non-polar organic liquids to permeate through the mesh membrane while retaining polar organic liquids, thereby realizing the separation of organic liquid mixtures. The main problem of this method is that it is difficult to accurately control the extreme wettability of the filter membrane for different organic liquids. (2) Pre-wetting is used to make the membrane have special wettability in the medium, thereby separating various organic liquid mixtures. For example, Chinese patent CN108126650A uses a polar liquid to pre-wet a fiber membrane, so that the fiber membrane has super-sparse properties for non-polar organic liquids in polar organic liquids, thereby realizing the separation of various organic liquid mixtures. The separation efficiency of organic liquid mixtures is related to the pre-wetting degree, and it is a big problem to control the appropriate pre-wetting degree. (3) Using the special wettability of the membrane in the medium to separate organic liquid mixtures. For example, Chinese patent CN106868559A deposits copper on the surface of a stainless steel mesh to prepare a stainless steel mesh membrane with special wettability, which can completely repel non-polar organic liquids in polar organic liquids and can be used for the separation of layered organic liquid mixtures. However, the pore size of the stainless steel mesh membrane is large, and it is difficult to separate stable organic liquid emulsions.

[0005] In summary, there are still a series of problems such as difficult to control the filter membrane wettability, low separation efficiency of stable emulsion, etc. in separating organic liquid mixture by super-wetting filter membrane, so it is necessary to develop a super-wetting filter membrane with stable wettability and high efficiency in separating layered organic liquid mixture and stable emulsion. SUMMARY

[0006] One of the purposes of the present application is to provide a preparation method of carbon-based liquid-liquid separation membrane, mainly comprising the following steps: first, oxidizing carbon material in a strong acid solution by using a strong oxidant, and then depositing the oxidized carbon material on the surface of the filter membrane, thereby obtaining a high-quality and high-performance liquid-liquid separation membrane.

[0007] Further, the strong acid solution is selected from one of perchloric acid aqueous solution and chlorosulfonic acid aqueous solution.

[0008] Further, the mass percentage concentration of the strong acid solution is not less than 65%. Specifically, if the perchloric acid aqueous solution is selected, the concentration is 65%-80%; if the chlorosulfonic acid aqueous solution is selected, the concentration is above 95%.

[0009] Further, the strong oxidant is specifically potassium permanganate.

[0010] Further, the carbon material is selected from one of graphite and carbon nanotube, wherein the graphite is selected from at least one of nano-graphite, natural graphite and synthetic graphite.

[0011] Further, the diameter of the carbon nanotube is 1-20 nm.

[0012] Further, the mass ratio of the carbon material to the strong oxidant is 1:1

[0013] Further, the filter membrane is selected from any one of polyvinylidene fluoride membrane, polypropylene membrane, nylon membrane and alumina membrane.

[0014] Further, the oxidation process of the carbon material is specifically as follows: first, dispersing the carbon material in the strong acid solution, and then adding the strong oxidant for oxidation at room temperature.

[0015] Further, the deposition process of the membrane is specifically as follows: dispersing the oxidized carbon material in water to obtain an aqueous dispersion, and then performing suction filtration by using the filter membrane.

[0016] Further, the concentration of the oxidized carbon material in the dispersion is not more than 5 g / L.

[0017] Further, the filter membrane after suction filtration is fully dried in an environment of 60-80℃.

[0018] Further, the loading amount of the deposit in the prepared liquid-liquid separation membrane is 0.1-1 mg / cm2 .

[0019] The second object of the present application is to provide a liquid-liquid separation membrane comprising a filter membrane and an oxidized carbon material deposited on at least one surface of the filter membrane, wherein the thickness of the filter membrane is about 200 μm and the thickness of the deposited layer is 0.2-5 μm.

[0020] Further, the liquid-liquid separation membrane has the property of being hydrophilic to non-polar solvents and hydrophobic to polar solvents in a medium, and the contact angle of the membrane to non-polar solvents in a polar solvent is 150°-165°, and the contact angle of the membrane to polar solvents in a non-polar solvent is 3°-10°.

[0021] The third object of the present application is to provide the use of the above-mentioned liquid-liquid separation membrane in the separation of polar and non-polar mixed liquids or emulsions.

[0022] Further, the polar and non-polar mixed liquid is composed of a polar solvent and a non-polar solvent, and includes a stratified mixed liquid and a stable emulsion.

[0023] Further, the polar solvent is selected from at least one of water, dimethyl sulfoxide, formamide, N,N-dimethylformamide, N,N-dimethylacetamide, 1,2-propanediol, and ethylene glycol, and the non-polar solvent is selected from at least one of liquid alkanes, liquid halogenated alkanes, liquid benzenes, liquid ethers, and liquid esters.

[0024] Further, the volume ratio of the polar solvent to the non-polar solvent in the stratified mixed liquid is 1:1-5:1, and the volume ratio of the polar solvent to the non-polar solvent in the stable emulsion is 5:1-10:1.

[0025] The present application aims to solve the problems of difficult control of the wettability of the filter membrane and low separation efficiency of stable emulsions in the prior art, and provides a liquid-liquid separation membrane composed of a commercial filter membrane and a carbon material deposited on the surface of the commercial filter membrane, which has the property of being hydrophilic to polar solvents and hydrophobic to non-polar solvents in a medium, and can be used to realize the efficient separation of liquid-liquid mixtures or stable emulsions, with a separation efficiency as high as 98.5%. Compared with the prior art, the progressiveness and beneficial effects of the present application mainly lie in the following aspects:

[0026] (1) The present application slightly oxidizes the hydrophobic carbon material to prepare a carbon material with good water dispersibility, and the aqueous dispersion of the carbon material can be kept stable for a long time without adding any stabilizer.

[0027] (2) The stable carbon material aqueous dispersion liquid of the present application is beneficial to filtration, and the strong interaction force between the atom-thick carbon material and the surface of the membrane base material promotes the stable coating of the carbon material on the surface of the membrane base material, increases the roughness of the surface of the membrane base material, and forms a special micro / nano composite structure. These atom-thick nanomaterials can change the wetting properties of the surface of the membrane base material, so that it exhibits the characteristics of being polar-solventophilic and non-polar-solventophobic in the medium.

[0028] (3) The present application can deposit carbon material on the surface of a commercial filter membrane by a relatively simple filtration method to prepare a separation membrane. The separation membrane has a small pore size and has completely opposite forces for polar solvents and non-polar solvents, and can efficiently separate organic liquid mixtures and stable emulsions. More importantly, the separation membrane can be repeatedly used. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 The contact angle photos of the separation membranes prepared in Examples 1 and 4 in polar solvents and non-polar solvents;

[0030] Figure 2 The contact angle photos of the separation membranes prepared in Examples 1 and 4 in polar solvents and non-polar solvents;

[0031] Figure 3 The scanning electron microscope photos of the separation membrane prepared in Example 2;

[0032] Figure 4 The comparison diagrams of the separation membranes prepared in Examples 1-2 before and after separating emulsions;

[0033] Figure 5 The efficiency comparison diagrams of the separation membranes prepared in Examples 1-7 for separating emulsions;

[0034] Figure 6 The efficiency comparison diagram of the separation membrane prepared in Example 1 after repeatedly separating emulsions for multiple times. DETAILED DESCRIPTION

[0035] In order for those skilled in the art to fully understand the technical solutions and beneficial effects of the present application, the following will be further described in combination with specific embodiments and drawings.

[0036] In order to fully understand the structure and properties of the products prepared in each embodiment of the present application, the following tests are performed:

[0037] 1. Contact angle test

[0038] The contact angle of the sample is tested by using a JC2000C1 type contact angle measuring instrument.

[0039] 2. Scanning electron microscope test

[0040] The sample was photographed using a Regulus 8230 scanning electron microscope at an accelerating voltage of 5 kV.

[0041] 3. Separation efficiency test

[0042] The tests were performed using a GC590 gas chromatograph.

[0043] 4. Separation membrane durability test

[0044] A separation system was constructed using the prepared separation membrane to separate mixtures or emulsions formed by polar solvents (such as dimethyl sulfoxide) and non-polar solvents (such as n-hexane). After separation, the separation membrane was rinsed with anhydrous ethanol and then dried thoroughly in a drying oven at 60-80°C. The separation was then repeated 10 times according to the aforementioned procedure. The separation efficiency of the emulsions was tested using gas chromatography.

[0045] Example 1

[0046] 0.1 g of carbon nanotubes with a diameter of 20 nm were added to 20 mL of 95% chlorosulfonic acid aqueous solution. After stirring for 5 minutes, 0.1 g of potassium permanganate was added, and the reaction was continued for 6 hours. After the reaction was complete, the mixture was vacuum filtered and washed. The solid obtained by filtration was added to 100 mL of water and fully dispersed. The dispersed solution was then allowed to stand for 12 hours. 5 mL of the solution was diluted 20 times, and the diluted solution was filtered through a polypropylene membrane. The completely filtered membrane was transferred to a drying oven and dried at 60°C for 30 minutes to obtain the separation membrane of this embodiment.

[0047] The separation membrane was used to build a separation system to separate dimethyl sulfoxide and n-hexane (volume ratio 10:1) emulsions.

[0048] The test results for the contact angle are as follows: Figures 1-2 As shown in a and c, the contact angle of the separation membrane in a polar solvent (dimethyl sulfoxide) to a nonpolar solvent (n-hexane) is 158.6°, and the contact angle in a nonpolar solvent (n-hexane) to a polar solvent (dimethyl sulfoxide) is 5.6°. The above test results demonstrate that the liquid-liquid separation membrane prepared in Example 1 of this invention does indeed possess excellent properties of being repellent to nonpolar solvents and attracted to polar solvents in the medium.

[0049] The above-mentioned dimethyl sulfoxide and n-hexane emulsions before and after separation using this separation membrane are shown in the following photographs. Figure 4 As shown in figure a, the separation membrane is highly effective in separating dimethyl sulfoxide and n-hexane emulsions.

[0050] The separation efficiency of this separation membrane for separating dimethyl sulfoxide and n-hexane emulsions is as follows: Figure 5The separation efficiency of the separation membrane for separating dimethyl sulfoxide and n-hexane emulsion is up to 98.4% as shown in the figure.

[0051] The separation efficiency test results of repeatedly separating dimethyl sulfoxide and n-hexane emulsion by using the separation membrane are shown in the figure. Figure 6 Figure 6 The results show that the separation membrane can be used repeatedly for many times, and the separation efficiency is still maintained at a high level even after being used for many times.

[0052] Example 2

[0053] 0.2 g of carbon nanotubes with a diameter of 15 nm was added to 20 mL of 70% mass concentration of perchloric acid aqueous solution, and then 0.2 g of potassium permanganate was added after stirring for 5 minutes, and the stirring reaction was continued for 6 hours. After the reaction was completed, the mixed solution was vacuum filtered and washed, and the solid obtained by vacuum filtration was added to 100 mL of water and dispersed, and then the dispersed solution was left to stand for 12 hours. 5 mL of the standing solution was diluted 20 times, and the diluted solution was vacuum filtered by a nylon 6 membrane, and the completely filtered membrane was transferred to a drying oven and dried at 60°C for 30 minutes to obtain the separation membrane of the example.

[0054] The separation membrane was sampled for scanning electron microscope test, and the results are shown in the figure. Figure 3 As can be seen from the figure, the carbon material is fully and uniformly loaded on the commercial filter membrane.

[0055] The separation system was built by using the separation membrane to separate dimethyl sulfoxide and petroleum ether (volume ratio 10:1) emulsion.

[0056] The contact angle test was carried out according to the foregoing method, and the results show that the contact angle of the separation membrane to non-polar solvent (petroleum ether) in polar solvent (dimethyl sulfoxide) is 154.6°, and the contact angle of the separation membrane to polar solvent (dimethyl sulfoxide) in non-polar solvent (petroleum ether) is 5.6°. The above test results show that the liquid-liquid separation membrane prepared in Example 2 of the application indeed has excellent properties of being non-polar solvent-repellent and polar solvent-philic in the medium.

[0057] The actual photos of the above dimethyl sulfoxide and petroleum ether emulsion before and after separation by using the separation membrane are shown in the figure. Figure 4 b. As can be seen from the figure, the separation membrane also has very obvious effect on separating dimethyl sulfoxide and petroleum ether emulsion.

[0058] The separation efficiency of the separation membrane for separating dimethyl sulfoxide and petroleum ether emulsion is shown in the figure. Figure 5 As can be seen from the figure, the separation efficiency of the separation membrane for separating dimethyl sulfoxide and petroleum ether is up to 98.7%.

[0059] Example 3

[0060] ​To 20 mL of a 70% by mass aqueous solution of perchloric acid, 0.2 g of natural graphite powder was added, and after stirring for 5 minutes, 0.2 g of potassium permanganate was added, and stirring was continued for 6 hours. After the reaction was complete, the mixed solution was suction-filtered under vacuum, washed, and the solid obtained by suction filtration was dispersed in 100 mL of water, and the dispersed solution was left to stand for 12 hours. 4 mL of the solution after standing was diluted 20 times, and the diluted solution was suction-filtered using a nylon 66 membrane, and the membrane after suction filtration was transferred to a drying oven and dried at 60°C for 30 minutes, to obtain the separation membrane of the present example.

[0061] An emulsion of dimethyl sulfoxide and dodecane (volume ratio 10:1) was separated using the separation membrane.

[0062] A contact angle test was performed in accordance with the aforementioned method, and the results showed that the contact angle of the separation membrane with respect to the nonpolar solvent (dodecane) in the polar solvent (dimethyl sulfoxide) was 158.6°, and the contact angle of the separation membrane with respect to the polar solvent (dimethyl sulfoxide) in the nonpolar solvent (dodecane) was 5.1°. This shows that the separation membrane has excellent properties of being nonpolar solvent-repellent and polar solvent-philic in the medium.

[0063] The separation efficiency of the separation membrane with respect to the emulsion of dimethyl sulfoxide and dodecane is shown in Figure 5 From the graph, it can be seen that the separation efficiency of the separation membrane with respect to the emulsion of dimethyl sulfoxide and dodecane was as high as 98%.

[0064] Example 4

[0065] To 20 mL of a 95% by mass aqueous solution of chlorosulfonic acid, 0.05 g of natural graphite powder was added, and after stirring for 5 minutes, 0.05 g of potassium permanganate was added, and stirring was continued for 6 hours. After the reaction was complete, the mixed solution was suction-filtered under vacuum, washed, and the solid obtained by suction filtration was dispersed in 100 mL of water, and the dispersed solution was left to stand for 12 hours. 3 mL of the solution after standing was diluted 20 times, and the diluted solution was suction-filtered using a nylon 66 membrane, and the membrane after suction filtration was transferred to a drying oven and dried at 60°C for 30 minutes, to obtain the separation membrane of the present example.

[0066] An emulsion of formamide and n-hexane (volume ratio 10:1) was separated using the separation membrane.

[0067] A contact angle test was performed in accordance with the aforementioned method, and the results are shown in Figures 1-2 The contact angle of the separation membrane with respect to the nonpolar solvent (n-hexane) in the polar solvent (formamide) was 153.4° (as shown in Figure 1 a), and the contact angle of the separation membrane with respect to the polar solvent (formamide) in the nonpolar solvent (n-hexane) was 5.2° (as shown in Figure 2 b). This shows that the separation membrane has excellent properties of being nonpolar solvent-repellent and polar solvent-philic in the medium.

[0068] The separation efficiency of the separation membrane for separating the formamide and n-hexane emulsion is shown in FIG. 2. Figure 5 As shown in the figure, the separation efficiency of the separation membrane for separating the formamide and n-hexane emulsion is as high as 98.1%.

[0069] Example 5

[0070] 0.1 g of nano-graphite powder was added to 20 mL of 70% mass concentration perchloric acid aqueous solution, and 0.1 g of potassium permanganate was added after stirring for 5 minutes, and the stirring reaction was continued for 6 hours. After the reaction was completed, the mixed solution was vacuum filtered and washed, and the solid obtained by the vacuum filtration was added to 100 mL of water for dispersion, and then the dispersed solution was left to stand for 12 hours. 6 mL of the solution after standing was diluted by 20 times, and the diluted solution was vacuum filtered by a polytetrafluoroethylene membrane. The membrane after complete filtration was placed in a drying oven and dried at 60°C for 30 minutes to obtain the separation membrane of the present example.

[0071] The separation system was built by using the separation membrane to separate formamide and petroleum ether (volume ratio of 10:1) emulsion.

[0072] The contact angle test was performed according to the foregoing method, and the results showed that the contact angle of the separation membrane for non-polar solvent (petroleum ether) in polar solvent (formamide) was 162.3°, and the contact angle of the separation membrane for polar solvent (formamide) in non-polar solvent (petroleum ether) was 6.4°. This indicates that the separation membrane has excellent properties of being non-polar solvent-repellent and polar solvent-philic in the medium.

[0073] The separation efficiency of the separation membrane for separating the formamide and petroleum ether emulsion is shown in FIG. 2. Figure 5 As shown in the figure, the separation efficiency of the separation membrane for separating the formamide and petroleum ether emulsion is as high as 98.8%.

[0074] Example 6

[0075] 0.3 g of nano-graphite powder was added to 20 mL of 70% mass concentration perchloric acid aqueous solution, and 0.3 g of potassium permanganate was added after stirring for 5 minutes, and the stirring reaction was continued for 6 hours. After the reaction was completed, the mixed solution was vacuum filtered and washed, and the solid obtained by the vacuum filtration was added to 100 mL of water for dispersion, and then the dispersed solution was left to stand for 12 hours. 6 mL of the solution after standing was diluted by 20 times, and the diluted solution was vacuum filtered by a polytetrafluoroethylene membrane. The membrane after complete filtration was placed in a drying oven and dried at 60°C for 30 minutes to obtain the separation membrane of the present example.

[0076] The separation system was built by using the separation membrane to separate formamide and petroleum ether (volume ratio of 10:1) emulsion.

[0077] The contact angle test was performed according to the aforementioned method, and the results showed that the contact angle of the separation membrane to the nonpolar solvent (dodecane) in the polar solvent (formamide) was 159.8°, and the contact angle of the separation membrane to the polar solvent (formamide) in the nonpolar solvent (dodecane) was 8.2°. This indicates that the separation membrane has excellent properties of being nonpolar solvent-repellent and polar solvent-philic in the medium.

[0078] The separation efficiency of the separation membrane for separating the formamide and dodecane emulsion is shown in FIG. 2. Figure 5 As can be seen from the graph, the separation efficiency of the separation membrane for separating the formamide and dodecane emulsion is as high as 97.7%.

[0079] Example 7

[0080] 0.2 g of synthetic graphite powder was added to 20 mL of a 95% concentrated aqueous chlorosulfonic acid solution, and after stirring for 5 minutes, 0.2 g of potassium permanganate was added, and the reaction was continued with stirring for 6 hours. After the reaction was completed, the mixed solution was vacuum filtered and washed, and the solid obtained by the vacuum filtration was dispersed in 10 mL of water, and then the dispersed solution was left to stand for 12 hours. 6 mL of the solution after standing was diluted 20 times, and the diluted solution was vacuum filtered using a polytetrafluoroethylene membrane. The membrane after the vacuum filtration was dried in a drying oven at 60°C for 30 minutes, and thus the separation membrane of the present example was obtained.

[0081] The separation system was constructed using the separation membrane to separate the formamide and toluene (volume ratio of 10:1) emulsion.

[0082] The contact angle test was performed according to the aforementioned method, and the results showed that the contact angle of the separation membrane to the nonpolar solvent (dodecane) in the polar solvent (formamide) was 159.8°, and the contact angle of the separation membrane to the polar solvent (formamide) in the nonpolar solvent (dodecane) was 8.2°. This indicates that the separation membrane has excellent properties of being nonpolar solvent-repellent and polar solvent-philic in the medium.

[0083] The separation efficiency of the separation membrane for separating the formamide and toluene emulsion is shown in FIG. 2. Figure 5 As can be seen from the graph, the separation efficiency of the separation membrane for separating the formamide and toluene emulsion is as high as 96.1%.

Claims

1. A method for preparing a carbon-based liquid-liquid separation membrane, characterized in that, Includes the following steps: First, the carbon material is oxidized in a strongly acidic solution using a strong oxidizing agent. Then, the oxidized carbon material is deposited onto the surface of a filter membrane. The oxidation process of the carbon material is as follows: the carbon material is dispersed in a strongly acidic solution, and then a strong oxidizing agent is added for oxidation at room temperature. After the reaction is complete, the mixed solution is aspirated, vacuum filtered, and washed to obtain the oxidized carbon material. The precipitation and film formation process is as follows: the oxidized carbon material is dispersed in water to obtain an aqueous dispersion with a concentration not exceeding 5 g / L, and then filtered using a filter membrane. The filtered filter membrane is then thoroughly dried at 60-80℃. The carbon material is selected from either graphite or carbon nanotubes; the strong acid solution is selected from either perchloric acid aqueous solution or chlorosulfonic acid aqueous solution, wherein the mass concentration of the perchloric acid aqueous solution is 65%-80% and the mass concentration of the chlorosulfonic acid aqueous solution is above 95%.

2. The preparation method according to claim 1, characterized in that, The strong oxidant is potassium permanganate; the filter membrane is selected from any one of polyvinylidene fluoride membrane, polypropylene membrane, nylon membrane, and alumina membrane; the mass ratio of carbon material to strong oxidant is 1:

1.

3. The preparation method according to claim 1, characterized in that, The graphite is selected from at least one of nano-graphite, natural graphite, and synthetic graphite; the diameter of the carbon nanotubes is 1-20 nm.

4. The preparation method according to claim 1, characterized in that, The deposition loading in the prepared liquid-liquid separation membrane was 0.1-1 mg / cm³. 2 .

5. The liquid-liquid separation membrane prepared by any one of claims 1-4, characterized in that, The liquid-liquid separation membrane includes a filter membrane and an oxidized carbon material deposited on at least one side of the filter membrane, with the thickness of the deposited layer being 0.2-5 μm.

6. The liquid-liquid separation membrane as described in claim 5, characterized in that, The liquid-liquid separation membrane has the characteristics of being both resistant to non-polar solvents and resistant to polar solvents in the medium. Its contact angle with non-polar solvents in polar solvents is 150°-165°, and its contact angle with polar solvents in non-polar solvents is 3°-10°.

7. The application of the liquid-liquid separation membrane of claim 5 or 6 in the separation of polar and non-polar mixtures or emulsions.

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