Editing Colloid (section)

=== Stabilization ===
The stability of a colloidal system is defined by particles remaining suspended in solution and depends on the interaction forces between the particles. These include electrostatic interactions and [[van der Waals forces]], because they both contribute to the overall [[Thermodynamic free energy|free energy]] of the system.<ref name="Everett-1988">{{Cite book|last=Everett|first=D. H.|url=https://www.worldcat.org/oclc/232632488|title=Basic principles of colloid science|date=1988|publisher=Royal Society of Chemistry|isbn=978-1-84755-020-0|location=London|oclc=232632488}}</ref>

A colloid is stable if the interaction energy due to attractive forces between the colloidal particles is less than [[KT (energy)|kT]], where k is the [[Boltzmann constant]] and T is the [[absolute temperature]]. If this is the case, then the colloidal particles will repel or only weakly attract each other, and the substance will remain a suspension.

If the interaction energy is greater than kT, the attractive forces will prevail, and the colloidal particles will begin to clump together. This process is referred to generally as [[Particle aggregation|aggregation]], but is also referred to as [[flocculation]], [[Coagulation (water treatment)|coagulation]] or [[Precipitation (chemistry)|precipitation]].<ref>{{Cite journal|last1=Slomkowski|first1=Stanislaw|last2=Alemán|first2=José V.|last3=Gilbert|first3=Robert G.|last4=Hess|first4=Michael|last5=Horie|first5=Kazuyuki|last6=Jones|first6=Richard G.|last7=Kubisa|first7=Przemyslaw|last8=Meisel|first8=Ingrid|last9=Mormann|first9=Werner|last10=Penczek|first10=Stanisław|last11=Stepto|first11=Robert F. T.|date=2011-09-10|title=Terminology of polymers and polymerization processes in dispersed systems (IUPAC Recommendations 2011)|journal=Pure and Applied Chemistry|language=de|volume=83|issue=12|pages=2229–2259|doi=10.1351/PAC-REC-10-06-03|s2cid=96812603|doi-access=free}}</ref> While these terms are often used interchangeably, for some definitions they have slightly different meanings. For example, coagulation can be used to describe irreversible, permanent aggregation where the forces holding the particles together are stronger than any external forces caused by stirring or mixing. Flocculation can be used to describe reversible aggregation involving weaker attractive forces, and the aggregate is usually called a ''floc''. The term precipitation is normally reserved for describing a phase change from a colloid dispersion to a solid (precipitate) when it is subjected to a perturbation.<ref name="cosgrove2010" /> Aggregation causes sedimentation or creaming, therefore the colloid is unstable: if either of these processes occur the colloid will no longer be a suspension.[[File:ColloidalStability.png|thumb|upright=1.4|Examples of a stable and of an unstable colloidal dispersion.]]

Electrostatic stabilization and steric stabilization are the two main mechanisms for stabilization against aggregation.
* Electrostatic stabilization is based on the mutual repulsion of like electrical charges. The charge of colloidal particles is structured in an [[electrical double layer]], where the particles are charged on the surface, but then attract counterions (ions of opposite charge) which surround the particle.  The electrostatic repulsion between suspended colloidal particles is most readily quantified in terms of the [[zeta potential]]. The combined effect of van der Waals attraction and electrostatic repulsion on aggregation is described quantitatively by the [[DLVO theory]].<ref>{{Cite journal|date=2011-01-01|title=Intermolecular Force|journal=Interface Science and Technology|volume=18|pages=1–57|doi=10.1016/B978-0-12-375049-5.00001-3|last1=Park|first1=Soo-Jin|last2=Seo|first2=Min-Kang|isbn=9780123750495}}</ref> A common method of stabilising a colloid (converting it from a precipitate) is [[peptization]], a process where it is shaken with an electrolyte.
* Steric stabilization consists absorbing a layer of a polymer or surfactant on the particles to prevent them from getting close in the range of attractive forces.<ref name="cosgrove2010" /> The polymer consists of chains that are attached to the particle surface, and the part of the chain that extends out is soluble in the suspension medium.<ref>{{Cite book|url=https://www.worldcat.org/oclc/701308697|title=Colloid stability : the role of surface forces. Part I|date=2007|publisher=Wiley-VCH|author=Tadros, Tharwat F. |isbn=978-3-527-63107-0|location=Weinheim|oclc=701308697}}</ref> This technique is used to stabilize colloidal particles in all types of solvents, including organic solvents.<ref>{{Cite journal|last1=Genz|first1=Ulrike|last2=D'Aguanno|first2=Bruno|last3=Mewis|first3=Jan|last4=Klein|first4=Rudolf|date=1994-07-01|title=Structure of Sterically Stabilized Colloids|journal=Langmuir|volume=10|issue=7|pages=2206–2212|doi=10.1021/la00019a029}}</ref>
A combination of the two mechanisms is also possible (electrosteric stabilization).

[[File:ComparisonStericStab-ShearThinningFluids2.png|thumb|Steric and gel network stabilization.|276x276px]]A method called gel network stabilization represents the principal way to produce colloids stable to both aggregation and sedimentation. The method consists in adding to the colloidal suspension a polymer able to form a gel network. Particle settling is hindered by the stiffness of the polymeric matrix where particles are trapped,<ref name="Comba 2009 3717–3726">{{cite journal|last=Comba|first=Silvia|author2=Sethi|title=Stabilization of highly concentrated suspensions of iron nanoparticles using shear-thinning gels of xanthan gum|journal=Water Research|date=August 2009|volume=43|issue=15|pages=3717–3726|doi=10.1016/j.watres.2009.05.046|pmid=19577785|bibcode=2009WatRe..43.3717C }}</ref> and the long polymeric chains can provide a steric or electrosteric stabilization to dispersed particles. Examples of such substances are [[xanthan]] and [[guar gum]].