Editing Hydrophobic effect (section)

== Cause ==

{{See also|Entropic force#Hydrophobic force}}
[[File:Liquid water hydrogen bond.png|right|thumb|200px|Dynamic hydrogen bonds between molecules of liquid water, the shape of the molecules is sometimes compared to that of boomerangs.]]
The origin of the hydrophobic effect is not fully understood.
Some argue that the hydrophobic interaction is mostly an [[entropy|entropic]] effect originating from the disruption of highly dynamic [[hydrogen bond]]s between molecules of liquid water by the nonpolar solute.<ref name=Silverstein_1998>{{cite journal|author=Silverstein TP|title=The Real Reason Why Oil and Water Don't Mix|journal=Journal of Chemical Education|date=January 1998|volume=75|issue=1|pages=116|doi=10.1021/ed075p116|bibcode=1998JChEd..75..116S}}</ref> A hydrocarbon chain or a similar nonpolar region of a large molecule is incapable of forming hydrogen bonds with water. Introduction of such a non-hydrogen bonding surface into water causes disruption of the hydrogen bonding network between water molecules. The hydrogen bonds are reoriented tangentially to such surface to minimize disruption of the hydrogen bonded 3D network of water molecules, and this leads to a structured water "cage" around the nonpolar surface. The water molecules that form the "cage" (or [[Clathrate hydrate|clathrate]]) have restricted mobility. In the solvation shell of small nonpolar particles, the restriction amounts to some 10%. For example, in the case of dissolved xenon at room temperature a mobility restriction of 30% has been found.<ref name =Xenon>{{cite journal |vauthors=Haselmeier R, Holz M, Marbach W, Weingaertner H | title = Water Dynamics near a Dissolved Noble Gas. First Direct Experimental Evidence for a Retardation Effect | journal = The Journal of Physical Chemistry | volume = 99 | issue = 8 | pages = 2243–2246 | year = 1995 | doi = 10.1021/j100008a001 }}</ref> In the case of larger nonpolar molecules, the reorientational and translational motion of the water molecules in the solvation shell may be restricted by a factor of two to four; thus, at 25&nbsp;°C the reorientational correlation time of water increases from 2 to 4-8 picoseconds. Generally, this leads to significant losses in translational and rotational [[entropy]] of water molecules and makes the process unfavorable in terms of the [[Gibbs free energy|free energy]] in the system.<ref>{{cite book | author = Tanford C | title = The hydrophobic effect: formation of micelles and biological membranes | date = 1973 | publisher = Wiley | location = New York | isbn = 978-0-471-84460-0 | url-access = registration | url = https://archive.org/details/isbn_0471844608 }}</ref> By aggregating together, nonpolar molecules reduce the [[Accessible surface area|surface area exposed to water]] and minimize their disruptive effect.

The hydrophobic effect can be quantified by measuring the [[partition coefficient]]s of non-polar molecules between water and non-polar solvents. The partition coefficients can be transformed to [[Gibbs free energy|free energy]] of transfer which includes [[enthalpy|enthalpic]] and entropic components, ''ΔG = ΔH - TΔS''. These components are experimentally determined by [[Differential scanning calorimetry|calorimetry]]. The hydrophobic effect was found to be entropy-driven at room temperature because of the reduced mobility of water molecules in the solvation shell of the non-polar solute; however, the enthalpic component of transfer energy was found to be favorable, meaning it strengthened water-water hydrogen bonds in the solvation shell due to the reduced mobility of water molecules. At the higher temperature, when water molecules become more mobile, this energy gain decreases along with the entropic component. The hydrophobic effect depends on the temperature, which leads to "cold [[Denaturation (biochemistry)|denaturation]]" of proteins.<ref name="pmid23396077">{{cite journal | vauthors = Jaremko M, Jaremko Ł, Kim HY, Cho MK, Schwieters CD, Giller K, Becker S, Zweckstetter M | title = Cold denaturation of a protein dimer monitored at atomic resolution | journal = Nat. Chem. Biol. | volume = 9 | issue = 4 | pages = 264–70 | year = 2013 | pmid = 23396077 | doi = 10.1038/nchembio.1181 | pmc=5521822}}</ref>

The hydrophobic effect can be calculated by comparing the free energy of solvation with bulk water. In this way, the hydrophobic effect not only can be localized but also decomposed into enthalpic and entropic contributions.<ref name='pmimd27442443' />