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 "title": "Cloud-point extraction",
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 "excerpt": "Cloud-point extraction is a sample-preparation method in analytical chemistry that uses surfactant micelles to separate and preconcentrate analytes from aqueous samples, introduced by Watanabe and co-workers in 1976.",
 "snippet": "Cloud-point extraction is a sample-preparation method in analytical chemistry that uses surfactant micelles to separate and preconcentrate analytes from aqueous samples, introduced by Watanabe and co-workers in 1976.",
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 "markdown": "# Cloud-point extraction\n\nCloud-point extraction (CPE) is a sample-preparation method in analytical chemistry that uses surfactant micelles to separate and preconcentrate analytes from aqueous samples before instrumental analysis. A small amount of nonionic or zwitterionic surfactant is added to the sample, eliminating the organic solvents required in conventional liquid-liquid or solid-liquid extraction.<sup>[1](https://pubs.acs.org/doi/abs/10.1021/ie980389n)</sup> On heating, the solution splits into a small surfactant-rich phase that carries the extracted analytes and a larger aqueous phase, so trace metals and organic pollutants are delivered to the detector in a concentrated, small-volume extract.<sup>[2](https://www.jocpr.com/articles/cloud-point-extraction-as-a-sample-preparation-technique-for-trace-element-analysis-an-overview.pdf)</sup>\n\n| Key fact | Detail |\n|---|---|\n| Product | A small surfactant-rich phase (0.2–0.5 ml for metals) holding the extracted analytes, ready for instrumental determination<sup>[2](https://www.jocpr.com/articles/cloud-point-extraction-as-a-sample-preparation-technique-for-trace-element-analysis-an-overview.pdf)</sup> |\n| Driving mechanism | Dehydration of surfactant polar groups on heating reduces micelle repulsion and aggregates the micelles into a separate phase<sup>[3](https://www.mdpi.com/2227-9717/13/2/430)</sup> |\n| Typical thermal conditions | Equilibration 4–15 min at 60–70 °C<sup>[2](https://www.jocpr.com/articles/cloud-point-extraction-as-a-sample-preparation-technique-for-trace-element-analysis-an-overview.pdf)</sup>, usually 15–20 °C above the cloud point temperature<sup>[3](https://www.mdpi.com/2227-9717/13/2/430)</sup> |\n| Detection limits | With electrothermal atomic absorption spectrometry, quantitation limits reach the order of ng/L and recovery may reach 100%<sup>[3](https://www.mdpi.com/2227-9717/13/2/430)</sup> |\n| Example enrichment | Enrichment factor \\( 10^{2} \\) with recovery up to 99% for Sm³⁺ by ICP-OES; enhancement factor 110 and LOD 1.49 µg/L for Pb²⁺ by FAAS<sup>[3](https://www.mdpi.com/2227-9717/13/2/430)</sup> |\n| Main surfactants | Triton X-100, Triton X-114, Triton X-45, and Genapol X-080; Triton X-114 is preferred for biological fluids<sup>[3](https://www.mdpi.com/2227-9717/13/2/430)</sup> |\n| Green credential | Only a small amount of surfactant is needed, avoiding the organic solvents of conventional extraction<sup>[1](https://pubs.acs.org/doi/abs/10.1021/ie980389n)</sup> |\n\n## How it works\n\nAbove the critical micelle concentration (CMC), the concentration at which surfactant molecules self-assemble, surfactants form micelles with hydrophobic tails directed inward to form a core and hydrophilic heads facing the water. Hydrophobic analytes, or metal ions rendered hydrophobic by a chelating ligand, are encapsulated in this core.<sup>[3](https://www.mdpi.com/2227-9717/13/2/430)</sup>\n\nMost nonionic surfactants form micelles in water and become turbid when heated to a specific temperature, the cloud point temperature (CPT); above it, the micelle solution separates into two phases.<sup>[4](https://www.jstage.jst.go.jp/article/analsci/23/3/23_3_351/_pdf)</sup> The separation is driven mainly by dehydration of the polar groups in the surfactant molecules on heating, which reduces repulsion between micelles and facilitates their aggregation.<sup>[3](https://www.mdpi.com/2227-9717/13/2/430)</sup> Triton X-114 illustrates the process: warming from 0 to 20 °C increases its micelle weight and decreases its CMC, inducing intermicellar interactions that produce turbidity and phase separation at 20 °C.<sup>[5](https://link.springer.com/protocol/10.1385/0-89603-487-9:23)</sup> Above the cloud point the solution separates into a small surfactant-rich phase and a larger surfactant-poor phase in which the surfactant concentration remains near the CMC.<sup>[2](https://www.jocpr.com/articles/cloud-point-extraction-as-a-sample-preparation-technique-for-trace-element-analysis-an-overview.pdf)</sup> Phase separation is reversible by cooling the mixture below the cloud point.<sup>[6](https://www.degruyterbrill.com/document/doi/10.1515/revac-2015-0022/html?lang=en)</sup>\n\n## How it is done\n\nA standard workflow runs as follows. A complexing agent is added to convert metal ions into hydrophobic chelates, then surfactant is added above its CMC. The solution is heated above the CPT in a water bath, centrifuged to accelerate phase separation, and cooled in an ice bath to raise the viscosity of the surfactant-rich phase. The aqueous phase is removed by decantation, and the surfactant-rich phase is diluted, typically with methanol or ethanol plus mineral acid, before quantification.<sup>[7](https://www.mdpi.com/2306-5710/10/3/67)</sup>\n\nEfficiency depends on several controllable variables: sample pH, salt concentration, equilibration temperature and time, the Kraft point, centrifugation time, and surfactant selection and concentration.<sup>[2](https://www.jocpr.com/articles/cloud-point-extraction-as-a-sample-preparation-technique-for-trace-element-analysis-an-overview.pdf)</sup> Optimal thermal conditions usually exceed the CPT by 15–20 °C,<sup>[3](https://www.mdpi.com/2227-9717/13/2/430)</sup> and equilibration times of 4–15 min at 60–70 °C are often optimal.<sup>[2](https://www.jocpr.com/articles/cloud-point-extraction-as-a-sample-preparation-technique-for-trace-element-analysis-an-overview.pdf)</sup>\n\n## Origin\n\nCPE extracts zinc(II) ions with polyoxyethylene nonyl phenyl ether; one review dates the work to 1978,<sup>[3](https://www.mdpi.com/2227-9717/13/2/430)</sup> while another states that in 1976 Watanabe and co-workers introduced CPE as a separation and extraction technique alternative to organic solvents, initially for preconcentration of metals.<sup>[8](https://www.sciencedirect.com/science/article/abs/pii/S0165993605000609)</sup> Metal ions as metal chelate complexes were subsequently extracted and enriched from aqueous media with nonionic surfactants.\n\n## Variants\n\n**Chelate-mediated CPE** is the classical form for metals: ligands such as TAN, APDC, 8-hydroxyquinoline, [dithizone](https://www.edgepedia.org/dithizone), DDTC, 5-Br-PADAP, and PAN convert ions into hydrophobic chelates that partition into the micelles.<sup>[2](https://www.jocpr.com/articles/cloud-point-extraction-as-a-sample-preparation-technique-for-trace-element-analysis-an-overview.pdf)</sup>\n\n**Energy-assisted variants** replace water-bath heating: ultrasound-assisted (UA-CPE), microwave-assisted (MA-CPE), and vortex-assisted (VA-CPE) CPE are all faster than conventional heating, with UA-CPE used more frequently; ultrasound accelerates clouding by increasing the intensity and rate of interaction between the surfactant and the aqueous phase.<sup>[7](https://www.mdpi.com/2306-5710/10/3/67)</sup>\n\n**Salting-out at room temperature** exploits the fact that added salt lowers the CPT, allowing phase separation near room temperature without heating.<sup>[3](https://www.mdpi.com/2227-9717/13/2/430)</sup> Micro-cloud point extraction (M-CPE), a greener variant yielding final extracts of tens of microliters, reduces the solvent needed to dilute the surfactant-rich phase and can run at room temperature via salting-out with Na₂SO₄; it has been applied to Hg(II), Cu(II), and Zn(II) in tap water.<sup>[7](https://www.mdpi.com/2306-5710/10/3/67)</sup>\n\n**Ionic-liquid and deep eutectic solvent variants** use surfactants such as TEGII with TPPP, or mixed-micellar [C4MIM][PF6] with Triton X-114 and HECAT, for Hg(II) preconcentration before spectrofluorimetric or spectrophotometric quantification.<sup>[7](https://www.mdpi.com/2306-5710/10/3/67)</sup> A rapidly synergistic deep eutectic solvent CPE (RS-DES-CPE) method for aluminum ions, chelated with 2-hydroxy-5-p-tolylazobenzaldehyde with Triton X-114 and a DES, clouds at room temperature and required less time than conventional CPE.<sup>[7](https://www.mdpi.com/2306-5710/10/3/67)</sup> Deep eutectic solvents are cheaper, easier to prepare, and less environmentally impactful than ionic liquids, and can be tailored to be target-specific.<sup>[7](https://www.mdpi.com/2306-5710/10/3/67)</sup>\n\n**Recent variants** use air bubbles, ionic liquids, nanoparticles, auxiliary reagents, deep eutectic solvents, mixed micelles, and novel surfactants to improve efficiency, environmental friendliness or detection limits.<sup>[9](https://www.sciencedirect.com/science/article/abs/pii/S0165993623002005)</sup> A 2024 study extracted heavy metal ions using biodegradable nonionic surfactants combined with ionic surfactants instead of chelating agents, and demonstrated simultaneous removal of an organic pollutant and a toxic heavy metal. A 2026 paper reports rapid synergistic CPE of copper using a triazole-based [Schiff base](https://www.edgepedia.org/schiff-base), with greenness and toxicity evaluation of the method included.<sup>[10](https://www.nature.com/articles/s41598-026-35659-3)</sup>\n\nCPE coupled with back extraction adds a step that transfers the analyte from the surfactant-rich phase into an aqueous receiving phase, and has been suggested as a promising alternative to liquid-liquid extraction.<sup>[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC8112826/)</sup> CPE has also been implemented as a continuous process in a stirred extraction column operating countercurrently, based on measured Triton X-114/water/phenol phase equilibria.<sup>[12](https://onlinelibrary.wiley.com/doi/10.1002/cite.201100256)</sup>\n\n## Applications\n\n**Trace metals and speciation.** Metal chelate CPE followed by spectroanalytical determination of the micellar phase is the classical application, and micellar extraction is also used in metal speciation analysis and in on-line coupling of CPE with analytical techniques.<sup>[13](https://www.tandfonline.com/doi/abs/10.1080/05704920500230880)</sup> Reported performance includes an enrichment factor of \\( 10^{2} \\) with recovery up to 99% for Sm³⁺ by ICP-OES, and an enhancement factor of 110 with LOD 1.49 µg/L for Pb²⁺ by FAAS.<sup>[3](https://www.mdpi.com/2227-9717/13/2/430)</sup>\n\n**Organic pollutants.** CPE has been established for extracting organic pollutants from water, solid, and biological samples, including phthalates, polycyclic aromatic hydrocarbons, polychlorinated biphenyls, [polychlorinated dibenzodioxins](https://www.edgepedia.org/polychlorinated-dibenzodioxins) and dibenzofurans, chlorophenols, and organochlorine pesticides.<sup>[14](https://www.chromatographyonline.com/view/surfactant-mediated-extractions-part-1-cloud-point-extraction)</sup>\n\n**Biological and beverage samples.** Triton X-114 is preferred for biological fluids because of its low cloud point temperature and dense surfactant-rich phase that separates by simple centrifugation.<sup>[3](https://www.mdpi.com/2227-9717/13/2/430)</sup> CPE has been reviewed specifically for trace elements in beverages.<sup>[7](https://www.mdpi.com/2306-5710/10/3/67)</sup> The surfactant-rich phase couples to HPLC-UV, HPLC-MS, HPLC-FLD, ICP-OES, GC, FAAS, ETAAS, and spectrophotometry.<sup>[3](https://www.mdpi.com/2227-9717/13/2/430)</sup>\n\n## Limitations and alternatives\n\nCompared with liquid-liquid extraction, CPE replaces bulk organic solvents with a small amount of surfactant,<sup>[1](https://pubs.acs.org/doi/abs/10.1021/ie980389n)</sup> and coupling with back extraction has been proposed as a promising alternative to liquid-liquid extraction.<sup>[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC8112826/)</sup>\n\nKnown failure modes include spectral overlap between surfactant and analyte, possible analyte decomposition on heating, low recovery for polar analytes with nonionic surfactants, and automation challenges.<sup>[3](https://www.mdpi.com/2227-9717/13/2/430)</sup> Excessive temperature elevation can reduce extraction efficiency by degrading thermolabile compounds such as vitamins or metal complexes with chelating agents.<sup>[3](https://www.mdpi.com/2227-9717/13/2/430)</sup> For metals, the main limitation is the relatively low partition coefficients of several metal species with certain chelates, which can be circumvented with highly hydrophobic ligands.<sup>[2](https://www.jocpr.com/articles/cloud-point-extraction-as-a-sample-preparation-technique-for-trace-element-analysis-an-overview.pdf)</sup>\n\n## References\n\n1. [Surfactant-Mediated Cloud Point Extractions: An Environmentally Benign Alternative Separation Approach](https://pubs.acs.org/doi/abs/10.1021/ie980389n)\n2. [Cloud point extraction as a sample preparation technique for trace element analysis: An overview](https://www.jocpr.com/articles/cloud-point-extraction-as-a-sample-preparation-technique-for-trace-element-analysis-an-overview.pdf)\n3. [Cloud Point Extraction as an Environmentally Friendly Technique for Sample Preparation](https://www.mdpi.com/2227-9717/13/2/430)\n4. [Analytical Sciences article on cloud point temperature phase separation](https://www.jstage.jst.go.jp/article/analsci/23/3/23_3_351/_pdf)\n5. [Partitioning of Proteins in Triton X-114](https://link.springer.com/protocol/10.1385/0-89603-487-9:23)\n6. [Cloud point extraction in flow-based systems](https://www.degruyterbrill.com/document/doi/10.1515/revac-2015-0022/html?lang=en)\n7. [Cloud Point Extraction in Beverage Analysis: Innovations and Applications for Trace Elements](https://www.mdpi.com/2306-5710/10/3/67)\n8. [Micelle-mediated separation and cloud-point extraction](https://www.sciencedirect.com/science/article/abs/pii/S0165993605000609)\n9. [Recent innovations in cloud point extraction towards a more efficient and environmentally friendly procedure](https://www.sciencedirect.com/science/article/abs/pii/S0165993623002005)\n10. [Rapid synergistic cloud point extraction of copper in environmental samples with greenness and toxicity evaluation using a triazole based Schiff base](https://www.nature.com/articles/s41598-026-35659-3)\n11. [Cloud point extraction coupled with back extraction: a green methodology in analytical chemistry](https://pmc.ncbi.nlm.nih.gov/articles/PMC8112826/)\n12. [Aqueous Surfactant Two-Phase Systems for the Continuous Countercurrent Cloud Point Extraction](https://onlinelibrary.wiley.com/doi/10.1002/cite.201100256)\n13. [Cloud Point Extraction as a Procedure of Separation and Pre-Concentration for Metal Determination Using Spectroanalytical Techniques: A Review](https://www.tandfonline.com/doi/abs/10.1080/05704920500230880)\n14. [Surfactant-Mediated Extractions, Part 1: Cloud-Point Extraction](https://www.chromatographyonline.com/view/surfactant-mediated-extractions-part-1-cloud-point-extraction)\n\n---\n*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Chemical principles and methods › Analytical chemistry › Extraction and sample preparation*\n\n*Initially written Sep 29, 2026 · Edited: Oct 15, 2026*\n\n*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*\n\nLicense: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license\n",
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