Dispersity

Not to be confused with Dispersal (disambiguation) or Dispersion (disambiguation).
IUPAC definition

ĐM = Mw/Mn
where Mw is the mass-average molar mass (or molecular weight) and
Mn is the number-average molar mass (or molecular weight).

Pure Appl. Chem., 2009, 81(2), 351-353

A uniform (monodisperse) collection
A non-uniform (polydisperse) collection

In chemistry, the dispersity is a measure of the heterogeneity of sizes of molecules or particles in a mixture. A collection of objects is called uniform if the objects have the same size, shape, or mass. A sample of objects that have an inconsistent size, shape and mass distribution is called non-uniform. The objects can be in any form of chemical dispersion, such as particles in a colloid, droplets in a cloud,[1] crystals in a rock,[2] or polymer macromolecules in a solution or a solid polymer mass.[3] Polymers can be described by molecular mass distribution; a population of particles can be described by size, surface area, and/or mass distribution; and thin films can be described by film thickness distribution.[citation needed]

IUPAC has deprecated the use of the term polydispersity index, having replaced it with the term dispersity, represented by the symbol Đ (pronounced D-stroke[4]) which can refer to either molecular mass or degree of polymerization. It can be calculated using the equation ĐM = Mw/Mn, where Mw is the weight-average molar mass and Mn is the number-average molar mass. It can also be calculated according to degree of polymerization, where ĐX = Xw/Xn, where Xw is the weight-average degree of polymerization and Xn is the number-average degree of polymerization. In certain limiting cases where ĐM = ĐX, it is simply referred to as Đ. IUPAC has also deprecated the terms monodisperse, which is considered to be self-contradictory, and polydisperse, which is considered redundant, preferring the terms uniform and non-uniform instead. The terms monodisperse and polydisperse are however still preferentially used to describe particles in an aerosol.

  1. ^ Martins, J. A.; Silva Dias, M. A. F. (2009). "The impact of smoke from forest fires on the spectral dispersion of cloud droplet size distributions in the Amazonian region" (PDF). Environmental Research Letters. 4 (1): 015002. Bibcode:2009ERL.....4a5002M. doi:10.1088/1748-9326/4/1/015002.
  2. ^ Higgins, Michael D. (2000). "Measurement of crystal size distributions" (PDF). American Mineralogist. 85 (9): 1105–1116. Bibcode:2000AmMin..85.1105H. doi:10.2138/am-2000-8-901. S2CID 101422067. Archived from the original (PDF) on 2017-08-08.
  3. ^ Okita, K.; Teramoto, A.; Kawahara, K.; Fujita, H. (1968). "Light scattering and refractometry of a monodisperse polymer in binary mixed solvents". The Journal of Physical Chemistry. 72: 278–285. doi:10.1021/j100847a053.
  4. ^ Stepto, R. F. T.; Gilbert, R. G.; Hess, M.; Jenkins, A. D.; Jones, R. G.; Kratochvíl P. (2009). "Dispersity in Polymer Science" Pure Appl. Chem. 81 (2): 351–353. DOI:10.1351/PAC-REC-08-05-02.