Miscibility

See also: Mixture and Solubility

Miscibility (/ˌmɪsɪˈbɪlɪti/) is the property of two substances to mix in all proportions (that is, to fully dissolve in each other at any concentration), forming a homogeneous mixture (a solution). Such substances are said to be miscible (etymologically equivalent to the common term "mixable"). The term is most often applied to liquids but also applies to solids and gases. An example in liquids is the miscibility of water and ethanol as they mix in all proportions.^[1]

Diesel fuel is immiscible in water. The bright rainbow pattern is the result of thin-film interference.

By contrast, substances are said to be immiscible if the mixture does not form a solution for certain proportions. For one example, oil is not soluble in water, so these two solvents are immiscible. As another example, butanone (methyl ethyl ketone) is immiscible in water: it is soluble in water up to about 275 grams per liter, but will separate into two phases beyond that.^[2]

Organic compounds

In organic compounds, the weight percent of hydrocarbon chain often determines the compound's miscibility with water. For example, among the alcohols, ethanol has two carbon atoms and is miscible with water, whereas 1-butanol with four carbons is not.^[3] 1-Octanol, with eight carbons, is practically insoluble in water, and its immiscibility leads it to be used as a standard for partition equilibria.^[4] The straight-chain carboxylic acids up to butanoic acid (with four carbon atoms) are miscible with water, pentanoic acid (with five carbons) is partly soluble, and hexanoic acid (with six) is practically insoluble,^[5] as are longer fatty acids and other lipids; the very long carbon chains of lipids cause them almost always to be immiscible with water. Analogous situations occur for other functional groups such as aldehydes and ketones.^{[citation needed]}

Metals

Immiscible metals are unable to form alloys with each other. Typically, a mixture will be possible in the molten state, but upon freezing, the metals separate into layers. This property allows solid precipitates to be formed by rapidly freezing a molten mixture of immiscible metals. One example of immiscibility in metals is copper and cobalt, where rapid freezing to form solid precipitates has been used to create granular GMR materials.^[6]

Some metals are immiscible in the liquid state. One with industrial importance is that liquid zinc and liquid silver are immiscible in liquid lead, while silver is miscible in zinc. This leads to the Parkes process, an example of liquid-liquid extraction, whereby lead containing any amount of silver is melted with zinc. The silver migrates to the zinc, which is skimmed off the top of the two-phase liquid, and the zinc is then boiled away, leaving nearly pure silver.^[7]

Effect of entropy

Main article: Hydrophobic effect

If a mixture of polymers has lower configurational entropy than the components, they are likely to be immiscible in one another even in the liquid state.^[8][9]

Determination

Miscibility of two materials is often determined optically. When the two miscible liquids are combined, the resulting liquid is clear. If the mixture is cloudy the two materials are immiscible. Care must be taken with this determination. If the indices of refraction of the two materials are similar, an immiscible mixture may be clear and give an incorrect determination that the two liquids are miscible.^[10]

See also

• Emulsion
• Heteroazeotrope
• ITIES
• Miscibility gap
• Multiphasic liquid

References

1. Wade, Leroy G. (2003). Organic Chemistry. Pearson Education. p. 412. ISBN 0-13-033832-X.
2. Stephen, H.; Stephen, T. (2013-10-22). Binary Systems: Solubilities of Inorganic and Organic Compounds, Volume 1P1. Elsevier. ISBN 9781483147123.
3. Barber, Jill; Rostron, Chris (2013-07-25). Pharmaceutical Chemistry. OUP Oxford. ISBN 9780199655304.
4. Sangster, J. (1997-05-28). Octanol-Water Partition Coefficients: Fundamentals and Physical Chemistry. John Wiley & Sons. ISBN 9780471973973.
5. Gilbert, John C.; Martin, Stephen F. (2010-01-19). Experimental Organic Chemistry: A Miniscale and Microscale Approach. Cengage Learning. p. 841. ISBN 978-1439049143.
6. Mallinson, John C. (2001-09-27). Magneto-Resistive and Spin Valve Heads: Fundamentals and Applications. Academic Press. p. 47. ISBN 9780080510637.
7. Rich, Vincent (2014-03-14). The International Lead Trade. Woodhead Publishing. pp. 51–52. ISBN 9780857099945.
8. Webb, G. A. (2007). Nuclear Magnetic Resonance. Royal Society of Chemistry. p. 328. ISBN 9780854043620.
9. Knoll, Wolfgang; Advincula, Rigoberto C. (2013-02-12). Functional Polymer Films, 2 Volume Set. John Wiley & Sons. p. 690. ISBN 9783527638499.
10. Olabisi, Olagoke; Adewale, Kolapo (1997-03-19). Handbook of Thermoplastics. CRC Press. p. 170. ISBN 9780824797973.

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