Editing Colloid (section)

==Preparation==
There are two principal ways to prepare colloids:<ref>Kopeliovich, Dmitri.  [http://www.substech.com/dokuwiki/doku.php?id=preparation_of_colloids Preparation of colloids]. substech.com</ref>
* [[Dispersion (chemistry)|Dispersion]] of large particles or droplets to the colloidal dimensions by [[Chemical milling|milling]], [[Aerosol spray|spraying]], or application of shear (e.g., shaking, mixing, or [[High-shear mixer|high shear mixing]]).
* Condensation of small dissolved molecules into larger colloidal particles by [[precipitation (chemistry)|precipitation]], [[condensation]], or [[redox]] reactions. Such processes are used in the preparation of colloidal [[Stöber process|silica]] or [[colloidal gold|gold]].

=== 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]].

=== Destabilization ===
Destabilization can be accomplished by different methods:
*Removal of the electrostatic barrier that prevents aggregation of the particles. This can be accomplished by the addition of salt to a suspension to reduce the [[Debye length|Debye screening length]] (the width of the electrical double layer) of the particles. It is also accomplished by changing the pH of a suspension to effectively neutralise the surface charge of the particles in suspension.<ref name="Israelachvili-2011" /> This removes the repulsive forces that keep colloidal particles separate and allows for aggregation due to van der Waals forces. Minor changes in pH can manifest in significant alteration to the [[zeta potential]]. When the magnitude of the zeta potential lies below a certain threshold, typically around ± 5mV, rapid coagulation or aggregation tends to occur.<ref>{{Cite journal|last1=Bean|first1=Elwood L.|last2=Campbell|first2=Sylvester J.|last3=Anspach|first3=Frederick R.|last4=Ockershausen|first4=Richard W.|last5=Peterman|first5=Charles J.|date=1964|title=Zeta Potential Measurements in the Control of Coagulation Chemical Doses [with Discussion]|url=https://www.jstor.org/stable/41264141|journal=Journal (American Water Works Association)|volume=56|issue=2|pages=214–227|doi=10.1002/j.1551-8833.1964.tb01202.x|jstor=41264141|url-access=subscription}}</ref>
*Addition of a charged polymer flocculant. Polymer flocculants can bridge individual colloidal particles by attractive electrostatic interactions. For example, negatively charged colloidal silica or clay particles can be flocculated by the addition of a positively charged polymer.
*Addition of non-adsorbed polymers called [[Depletion force|depletants]] that cause aggregation due to entropic effects.

Unstable colloidal suspensions of low-volume fraction form clustered liquid suspensions, wherein individual clusters of particles sediment if they are more dense than the suspension medium, or cream if they are less dense. However, colloidal suspensions of higher-volume fraction form colloidal gels with [[viscoelastic]] properties. Viscoelastic colloidal gels, such as [[bentonite]] and [[toothpaste]], flow like liquids under shear, but maintain their shape when shear is removed. It is for this reason that toothpaste can be squeezed from a toothpaste tube, but stays on the toothbrush after it is applied.

===Monitoring stability===
[[File:MLS scan.gif|thumb|Measurement principle of multiple light scattering coupled with vertical scanning]]

The most widely used technique to monitor the dispersion state of a product, and to identify and quantify destabilization phenomena, is multiple [[light scattering]] coupled with vertical scanning.<ref>{{cite journal|doi=10.1016/S0378-5173(03)00364-8|title=Systematic characterisation of oil-in-water emulsions for formulation design|year=2003|last1=Roland|first1=I|journal=International Journal of Pharmaceutics|volume=263|pages=85–94|pmid=12954183|last2=Piel|first2=G|last3=Delattre|first3=L|last4=Evrard|first4=B|issue=1–2}}</ref><ref>{{cite journal|doi=10.1023/A:1025017502379|year=2003|last1=Lemarchand|first1=Caroline|last2=Couvreur|first2=Patrick|last3=Besnard|first3=Madeleine|last4=Costantini|first4=Dominique|last5=Gref|first5=Ruxandra|s2cid=24157992|journal=Pharmaceutical Research|volume=20|pages=1284–92|pmid=12948027|title=Novel polyester-polysaccharide nanoparticles|issue=8}}</ref><ref>{{cite journal|doi=10.1016/S0927-7757(98)00680-3|title=Characterisation of instability of concentrated dispersions by a new optical analyser: the TURBISCAN MA 1000|year=1999|last1=Mengual|first1=O|journal=Colloids and Surfaces A: Physicochemical and Engineering Aspects|volume=152|issue=1–2|pages=111–123 }}</ref><ref>{{cite book|author=Bru, P. |title= Particle sizing and characterisation|editor1=T. Provder |editor2=J. Texter |year=2004|display-authors=etal}}</ref> This method, known as [[turbidimetry]], is based on measuring the fraction of light that, after being sent through the sample, it backscattered by the colloidal particles. The backscattering intensity is directly proportional to the average particle size and volume fraction of the dispersed phase. Therefore, local changes in concentration caused by sedimentation or creaming, and clumping together of particles caused by aggregation, are detected and monitored.<ref>{{Cite journal|last1=Matusiak|first1=Jakub|last2=Grządka|first2=Elżbieta|date=2017-12-08|title=Stability of colloidal systems - a review of the stability measurements methods|url=https://journals.umcs.pl/aa/article/view/4877|journal=Annales Universitatis Mariae Curie-Sklodowska, sectio AA – Chemia|volume=72|issue=1|pages=33|doi=10.17951/aa.2017.72.1.33|doi-access=free}}</ref> These phenomena are associated with unstable colloids.

[[Dynamic light scattering]] can be used to detect the size of a colloidal particle by measuring how fast they diffuse. This method involves directing laser light towards a colloid. The scattered light will form an interference pattern, and the fluctuation in light intensity in this pattern is caused by the Brownian motion of the particles. If the apparent size of the particles increases due to them clumping together via aggregation, it will result in slower Brownian motion. This technique can confirm that aggregation has occurred if the apparent particle size is determined to be beyond the typical size range for colloidal particles.<ref name="Everett-1988" />

===Accelerating methods for shelf life prediction===
The kinetic process of destabilisation can be rather long (up to several months or years for some products). Thus, it is often required for the formulator to use further accelerating methods to reach reasonable development time for new product design. Thermal methods are the most commonly used and consist of increasing temperature to accelerate destabilisation (below critical temperatures of phase inversion or chemical degradation). Temperature affects not only viscosity, but also interfacial tension in the case of non-ionic surfactants or more generally interactions forces inside the system. Storing a dispersion at high temperatures enables to simulate real life conditions for a product (e.g. tube of sunscreen cream in a car in the summer), but also to accelerate destabilisation processes up to 200 times.
Mechanical acceleration including vibration, [[centrifugation]] and agitation are sometimes used. They subject the product to different forces that pushes the particles / droplets against one another, hence helping in the film drainage. Some emulsions would never coalesce in normal gravity, while they do under artificial gravity.<ref>{{cite book|url=https://books.google.com/books?id=hDOS5OfL_pQC&pg=PA89|page=89|author= Salager, J-L |title=Pharmaceutical emulsions and suspensions|editor1=Françoise Nielloud |editor2=Gilberte Marti-Mestres |year=2000|isbn=978-0-8247-0304-2|publisher=CRC press}}</ref> Segregation of different populations of particles have been highlighted when using centrifugation and vibration.<ref>{{cite journal|doi=10.1021/la802459u|title=Size Segregation in a Fluid-like or Gel-like Suspension Settling under Gravity or in a Centrifuge|year=2008|last1=Snabre|first1=Patrick|last2=Pouligny|first2=Bernard|journal=Langmuir|volume=24|pages=13338–47|pmid=18986182|issue=23}}</ref>