Intercalation (chemistry)

In chemistry, intercalation is the reversible inclusion or insertion of a molecule (or ion) into compounds with layered structures. Examples are found in graphite intercalation compounds.

Model of intercalation of potassium into graphite

Many layered solids intercalate guest molecules. A famous example is the intercalation of potassium into graphite.[1] Intercalation expands the van der Waals gap between sheets, which requires energy. Usually this energy is supplied by charge transfer between the guest and the host solid, i.e., redox. Two potassium graphite compounds are KC8 and KC24. Carbon fluorides (e.g., (CF)x and (C4F)) are prepared by reaction of fluorine with graphitic carbon. The color is greyish, white, or yellow. The bond between the carbon and fluorine atoms is covalent, thus fluorine is not intercalated. Such materials have been considered as a cathodes in various lithium batteries.

Cartoon for intercalation of Li into a titanium disulfide cathode. The process involves swelling of one crystal axis and charge transfer from Li to Ti.

Treating graphite with strong acids in the presence of oxidizing agents, causes the graphite to oxidise. Graphite bisulfate, [C24]+[HSO4]−, is prepared by this approach using sulfuric acid and a little nitric acid or chromic acid. The analogous graphite perchlorate can be made similarly by reaction with perchloric acid.

Aside from graphite, well-known intercalation hosts are the layered metal dichalcogenides such as tantalum disulfide and iron oxychloride.[2] In characteristic manner, intercalation is analyzed by X-ray diffraction, since the spacing between sheets increases, and by electrical conductivity, since charge transfer alters the number of charge carriers.

Exfoliation

An extreme case of intercalation is the complete separation of the layers of the material. This process is called exfoliation. Typically aggressive conditions are required involving highly polar solvents and aggressive reagents.[3]

Related materials

In biochemistry, intercalation is the insertion of molecules between the bases of DNA. This process is used as an method for analyzing DNA and it is also the basis of certain kinds of poisoning.

Clathrates are chemical substances consisting of a lattice that traps or contains molecules. Usually, clathrate compounds are polymeric and completely envelop the guest molecule. Inclusion compounds are often molecules, whereas clathrates are typically polymeric. Intercalation compounds, which are not 3-dimensional, unlike clathrate compounds.[4] According to IUPAC, clathrates are "Inclusion compounds in which the guest molecule is in a cage formed by the host molecule or by a lattice of host molecules."[5]

Notes

  1. ↑ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. p. 794 ISBN 0-12-352651-5.
  2. ↑ S. Kikkawa, F. Kanamaru, M. Koizumi "Layered Intercalation Compounds" Inorganic Syntheses, 1983, Volume 22, 86. doi:10.1002/9780470132531.ch17
  3. ↑ V. Nicolosi, M. Chhowalla, M. G. Kanatzidis, M. S. Strano , J. N. Coleman, "Liquid Exfoliation of Layered Materials" Science 2013, Vol. 340 no. 6139. http://www.sciencemag.org/content/340/6139/1226419.abstract
  4. ↑ J. L. Atwood "Inclusion Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, 2012, Wiley-VCH, Weinheim. doi: 10.1002/14356007.a14_119
  5. ↑ http://goldbook.iupac.org/C01097.html

See also

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