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{{Short description|Mixture of an insoluble substance microscopically dispersed throughout another substance}}
{{Use dmy dates|date=March 2021}}
[[File:SEM Image of Colloidal Particles.jpg|thumb|upright=1.2|[[Scanning electron microscope|SEM]] image of a colloid.]]
{{Condensed matter physics}}
A '''colloid''' is a [[mixture]] in which one substance consisting of microscopically [[Dispersion (chemistry)|dispersed]] [[insoluble]] [[particle]]s is [[suspension (chemistry)|suspended]] throughout another substance. Some definitions specify that the particles must be dispersed in a [[liquid]],<ref name="Israelachvili-2011">{{Cite book |last=Israelachvili |first=Jacob N. |title=Intermolecular and surface forces |date=2011 |publisher=Academic Press |isbn=978-0-08-092363-5|edition=4rd |location=Burlington, MA |oclc=706803091}}</ref> while others extend the definition to include substances like [[aerosol]]s and [[gel]]s. The term '''colloidal suspension''' refers unambiguously to the overall mixture (although a narrower sense of the word ''[[suspension (chemistry)|suspension]]'' is distinguished from colloids by larger particle size). A colloid has a dispersed phase (the suspended particles) and a continuous phase (the medium of suspension).

Since the definition of a colloid is so ambiguous, the [[International Union of Pure and Applied Chemistry]] (IUPAC) formalized a modern definition of colloids: {{blockquote|The term colloidal refers to a state of subdivision, implying that the molecules or polymolecular particles dispersed in a medium have at least in one direction a dimension roughly between 1 [[nanometre]] and 1 [[micrometre]], or that in a system discontinuities are found at distances of that order. It is not necessary for all three dimensions to be in the colloidal range…Nor is it necessary for the units of a colloidal system to be discrete…The size limits given above are not rigid since they will depend to some extent on the properties under consideration.<ref>{{Cite book|author1=International Union of Pure and Applied Chemistry. Subcommittee on Polymer Terminology|title=Compendium of polymer terminology and nomenclature: IUPAC recommendations, 2008|date=2009|publisher=Royal Society of Chemistry|author2=Jones, Richard G. |isbn=978-1-84755-942-5|location=Cambridge|oclc=406528399}}</ref>}} This IUPAC definition is particularly important because it highlights the flexibility inherent in colloidal systems. However, much of the confusion surrounding colloids arises from oversimplifications. IUPAC makes it clear that exceptions exist, and the definition should not be viewed as a rigid rule. D.H. Everett—the scientist who wrote the IUPAC definition—emphasized that colloids are often better understood through examples rather than strict definitions.<ref>{{Cite book |last=Everett |first=Dogulas H. |title=Basic Principles of Colloid Science |date=1988 |publisher=The Royal Society of Chemistry |isbn=978-0-85186-443-3|location=London}}</ref>

Some colloids are [[translucent]] because of the [[Tyndall effect]], which is the [[scattering]] of light by particles in the colloid. Other colloids may be [[Opacity (optics)|opaque]] or have a slight color.

Colloidal suspensions are the subject of [[interface and colloid science]]. This field of study began in 1845 by [[Francesco Selmi]],<ref>Selmi, Francesco "Studi sulla dimulsione di cloruro d'argento". ''Nuovi Annali delle Scienze Naturali di Bologna, 1845''.</ref><ref>Selmi, Francesco, Studio intorno alle pseudo-soluzioni degli azzurri di Prussia ed alla influenza dei sali nel guastarle, Bologna: Tipi Sassi, 1847</ref><ref>Hatschek, Emil, The Foundations of Colloid Chemistry, A selection of early papers bearing on the subject, The British Association Committee on Colloid Chemistry, London, 1925</ref><ref>Selmi, Francesco - Sur le soufre pseudosoluble, sa pseudosolution e le soufre mou, Journal de Pharmacie et de Chimie, tome 21, 1852, Paris</ref> who called them pseudosolutions, and expanded by [[Michael Faraday]]<ref>{{cite journal|doi=10.1162/posc.2006.14.1.97|title=Discovering Discovery: How Faraday Found the First Metallic Colloid |year=2006 |last1=Tweney |first1=Ryan D. |journal=Perspectives on Science |volume=14 |pages=97–121 |s2cid=55882753 }}</ref> and [[Thomas Graham (chemist)|Thomas Graham]], who coined the term ''colloid'' in 1861.<ref>{{cite journal|doi=10.1098/rstl.1861.0011|title=X. Liquid diffusion applied to analysis |journal=Philosophical Transactions of the Royal Society of London |year=1861 |volume=151 |pages=183–224 |s2cid=186208563 }}.  Page 183:  "As gelatine appears to be its type, it is proposed to designate substances of the class as ''colloids'', and to speak of their peculiar form of aggregation as the ''colloidal condition of matter''."</ref>{{Quote box
| title = [[IUPAC]] definition
| width = 35%
| quote = '''Colloid''': Short synonym for ''colloidal'' system.<ref name=quote1>{{cite book|title=Compendium of Polymer Terminology and Nomenclature (IUPAC Recommendations 2008)|year=2009|publisher=RSC Publ.|isbn=978-0-85404-491-7|page=464|edition= 2nd|editor1=Richard G. Jones |editor2=Edward S. Wilks |editor3=W. Val Metanomski |editor4=Jaroslav Kahovec |editor5=Michael Hess |editor6=Robert Stepto |editor7=Tatsuki Kitayama }}</ref><ref name=quote2>{{cite journal|title=Dispersity in polymer science (IUPAC Recommendations 2009)|journal=[[Pure and Applied Chemistry]]|year=2009|volume=81|issue=2|pages=351–353|doi=10.1351/PAC-REC-08-05-02|url=http://pac.iupac.org/publications/pac/pdf/2009/pdf/8102x0351.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://pac.iupac.org/publications/pac/pdf/2009/pdf/8102x0351.pdf |archive-date=2022-10-09 |url-status=live|last1=Stepto|first1=Robert F. T.|s2cid=95122531}}</ref>

'''Colloidal''': State of subdivision such that the molecules or polymolecular particles dispersed in a medium have at least one dimension between approximately 1 nm and 1 μm, or that in a system discontinuities are found at distances of that order.<ref name=quote1 /><ref name=quote2 /><ref>{{cite journal|title=Terminology of polymers<br/>and polymerization processes in dispersed systems (IUPAC Recommendations 2011)|journal=[[Pure and Applied Chemistry]]|year=2011|volume=83|issue=12|pages=2229–2259|doi=10.1351/PAC-REC-10-06-03|url=http://pac.iupac.org/publications/pac/pdf/2011/pdf/8312x2229.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://pac.iupac.org/publications/pac/pdf/2011/pdf/8312x2229.pdf |archive-date=2022-10-09 |url-status=live|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.|s2cid=96812603}}</ref>
}}

==Classification==
Colloids can be classified as follows:
{| class="wikitable" style="text-align:center; max-width:900px"
|-
! colspan="2" rowspan="2" | Medium/phase
! colspan="3" | Dispersed phase
|-
! style=width:30%; | Gas 
! style=width:30%; | Liquid 
! style=width:30%; | Solid
|-
! rowspan="3" | Dispersion <br />medium
! Gas
| style="vertical-align: top;" | '''No such colloids are known'''
----
Helium and xenon are known to be [[immiscible]] under certain conditions.<ref name="de Swaan AronsDiepen2010">{{cite journal|last1=de Swaan Arons|first1=J.|last2=Diepen|first2=G. A. M.|title=Immiscibility of gases. The system He-Xe: (Short communication)|journal=Recueil des Travaux Chimiques des Pays-Bas|volume=82|issue=8|year=2010|page=806|doi=10.1002/recl.19630820810}}</ref><ref name="de Swaan AronsDiepen1996">{{Cite journal|last1=de Swaan Arons|first1=J.|last2=Diepen|first2=G. A. M.|year=1966|title=Gas—Gas Equilibria|journal=J. Chem. Phys.|volume=44|issue=6|page=2322|doi=10.1063/1.1727043|bibcode=1966JChPh..44.2322D}}</ref>
| style="vertical-align: top;"|'''Liquid [[aerosol]]'''
----
Examples: [[fog]], '''[[cloud]]s''', [[condensation]], [[mist]], [[steam]], [[hair spray]]s[[File:2013-05-19 15-05-07-nuages.jpg|none|120px]]
| style="vertical-align: top;"|'''Solid aerosol'''
----
Examples: '''[[smoke]]''', [[ice cloud]], [[atmospheric particulate matter]] [[File:Smoke Incense AB.jpg|none|120px]]
|-
! Liquid
|  style="vertical-align: top;"|'''[[Foam]]'''
----
Examples: '''[[whipped cream]]''', [[shaving cream]][[File:Crème Chantilly.jpg|none|120px]]
|  style="vertical-align: top;"|'''[[Emulsion]] or [[Liquid crystal]]'''
----
Examples: '''[[milk]]''', [[mayonnaise]], [[hand cream]], [[latex]], {{nobr|[[biological membranes]]}}, liquid [[biomolecular condensate]][[File:Glass of Milk (33657535532).jpg|none|120px]]
| style="vertical-align: top;"|'''[[Sol (colloid)|Sol]]'''
----
Examples: [[ink|pigmented ink]], [[sedimentation|sediment]], '''[[mud]]''', [[precipitates]], solid [[biomolecular condensate]][[File:Bubbling Mud Volcano (3860838997).jpg|none|120px]]
|-
! Solid
| style="vertical-align: top;"|'''Solid foam'''
----
Examples: '''[[aerogel]]''', [[Ivory (soap)|floating soap]], [[Expanded polystyrene|styrofoam]], [[pumice]][[File:Aerogel hand.jpg|none|120px]]
| style="vertical-align: top;"|'''[[Gel]]'''
----
Examples: [[agar]], '''[[gelatin]]''', [[Fruit preserves|jelly]], {{nowrap|gel-like [[biomolecular condensate]]}}[[File:Jello Cubes.jpg|none|120px]]
| style="vertical-align: top;"|'''Solid sol'''
----
Example: '''[[cranberry glass]]'''[[File:Vintage cranberry glass.jpg|none|120px]]
|}

Homogeneous mixtures with a dispersed phase in this size range may be called ''colloidal aerosols'', ''colloidal emulsions'', ''colloidal suspensions'', ''colloidal foams'', ''colloidal dispersions'', or ''hydrosols''.

<gallery mode="packed">
File:Opaleszens Kolloid SiO2.jpg|Colloidal [[silica gel]] with light [[opalescence]]
File:Dollop of hair gel.jpg|A dollop of hair gel
File:Cream in round container.jpg|[[Cream (pharmacy)|Creams]] are semi-solid emulsions of oil and water. Oil-in-water creams are used for cosmetic purpose while water-in-oil creams for medicinal purpose
File:Why is the sky blue.jpg|[[Tyndall effect]] in an [[opalite]]:<br />it scatters blue light making it appear blue from the side, but orange light shines through.<br />[[Opal]] is a gel in which water is dispersed in silica [[Colloidal crystal|crystals]].
File:Mist - Ensay region3.jpg|Mist
</gallery>

==Hydrocolloids==

'''Hydrocolloids''' describe certain [[chemicals]] (mostly [[polysaccharides]] and [[proteins]]) that are colloidally dispersible in [[water]]. Thus becoming effectively "soluble", they change the [[rheology]] of water by raising the viscosity and/or inducing gelation. They may provide other interactive effects with other chemicals, in some cases synergistic, in others antagonistic. Using these attributes, hydrocolloids are very useful chemicals since in many areas of technology&nbsp;– from [[foods]] through [[pharmaceuticals]] and personal-care products, to industrial applications&nbsp;– they can provide stabilization, destabilization and separation, gelation, flow control, crystallization control, and numerous other effects. Apart from uses of the soluble forms, some of hydrocolloids have additional functionality in a dry form if, after solubilization, they have the water removed&nbsp;– as in the formation breath-strip films, artificial sausage casings, and wound-dressing fibers (some being more compatible with [[skin]] than others).  There are many different types of hydrocolloids, each with differences in structure, function, and utility; which is best suited to a particular application area may depend on the control of rheology and the physical modification of form and texture. Some hydrocolloids like [[corn starch]] and [[casein]] are useful foods, as well as rheology modifiers; others have some limited nutritional value, usually providing a source of [[dietary fiber]].<ref>{{Cite journal |last1=Saha |first1=Dipjyoti |last2=Bhattacharya |first2=Suvendu |date=6 November 2010 |title=Hydrocolloids as thickening and gelling agents in food: a critical review |journal=[[Journal of Food Science and Technology]] |volume=47 |issue=6 |pages=587–597 |doi=10.1007/s13197-010-0162-6 |pmc=3551143 |pmid=23572691 }}</ref>

The term ''hydrocolloid'' may also refer to [[Hydrocolloid dressing|a type of wound dressing]], designed to lock moisture in the skin and help the natural healing process of skin, to reduce scarring, itching, and soreness.

===Components===
Hydrocolloids contain some type of gel-forming agent, such as sodium carboxymethylcellulose (NaCMC) or gelatin. They are normally combined with some type of sealant, e.g. polyurethane to stick to skin.

== Compared with solution ==
A colloid has a [[Dispersion (chemistry)|dispersed phase]] and a continuous phase, whereas in a [[Solution (chemistry)|solution]], the [[solute]] and [[solvent]] constitute only one phase. A solute in a solution are individual [[molecule]]s or [[ion]]s, whereas colloidal particles are bigger. For example, in a solution of salt in water, the [[sodium chloride]] (NaCl) [[crystal]] dissolves, and the Na<sup>+</sup> and Cl<sup>−</sup> ions are surrounded by water molecules.&nbsp; However, in a colloid such as milk, the colloidal particles are globules of fat, rather than individual fat molecules. Because colloid is multiple phases, it has very different properties compared to fully mixed, continuous solution.<ref>{{cite journal | doi=10.1021/acs.langmuir.0c01139 | title=Differences between Colloidal and Crystalline Evaporative Deposits | year=2020 | last1=McBride | first1=Samantha A. | last2=Skye | first2=Rachael | last3=Varanasi | first3=Kripa K. | journal=Langmuir | volume=36 | issue=40 | pages=11732–11741 | pmid=32937070 | s2cid=221770585 }}</ref>

== Interaction between particles ==
The following forces play an important role in the interaction of colloid particles:<ref name="Lekkerkerker">{{cite book| last1=Lekkerkerker| first1=Henk N.W.| last2=Tuinier| first2=Remco| title=Colloids and the Depletion Interaction| publisher=Springer| location=Heidelberg| date=2011| doi=10.1007/978-94-007-1223-2| isbn=978-94-007-1222-5| url=https://cds.cern.ch/record/1399210| access-date=5 September 2018| archive-url=https://web.archive.org/web/20190414163235/https://cds.cern.ch/record/1399210| archive-date=14 April 2019}}</ref><ref name="vanAndersPNAS2014">{{cite journal|last1=van Anders| first1=Greg| last2=Klotsa| first2=Daphne| last3=Ahmed| first3=N. Khalid| last4=Engel| first4=Michael| last5=Glotzer| first5=Sharon C.| date=2014| title=Understanding shape entropy through local dense packing|journal=Proc Natl Acad Sci USA|volume=111| issue=45|pages=E4812–E4821|doi=10.1073/pnas.1418159111|arxiv=1309.1187| pmid=25344532| pmc=4234574|bibcode=2014PNAS..111E4812V| doi-access=free}}</ref>
*[[Excluded volume|Excluded volume repulsion]]: This refers to the impossibility of any overlap between hard particles.
*[[Coulomb's law|Electrostatic interaction]]: Colloidal particles often carry an electrical charge and therefore attract or repel each other. The charge of both the continuous and the dispersed phase, as well as the mobility of the phases are factors affecting this interaction.
*[[van der Waals force]]s: This is due to interaction between two dipoles that are either permanent or induced. Even if the particles do not have a permanent dipole, fluctuations of the electron density gives rise to a temporary dipole in a particle. This temporary dipole induces a dipole in particles nearby. The temporary dipole and the induced dipoles are then attracted to each other. This is known as van der Waals force, and is always present (unless the refractive indexes of the dispersed and continuous phases are matched), is short-range, and is attractive.
*[[Steric effects|Steric forces]]: A repulsive steric force typically occurring due to adsorbed polymers coating a colloid's surface. 
*[[Depletion force]]s: An attractive entropic force arising from an osmotic pressure imbalance when colloids are suspended in a medium of much smaller particles or polymers called depletants.

== Sedimentation velocity ==
[[File:Brownian Motion.gif|thumb|Brownian motion of 350 nm diameter polymer colloidal particles.|268x268px]]
The Earth's [[gravitational field]] acts upon colloidal particles. Therefore, if the colloidal particles are denser than the medium of suspension, they will [[Sedimentation|sediment]] (fall to the bottom), or if they are less dense, they will [[Creaming (chemistry)|cream]] (float to the top). Larger particles also have a greater tendency to sediment because they have smaller [[Brownian motion]] to counteract this movement.

The sedimentation or creaming velocity is found by equating the [[Stokes' law|Stokes drag force]] with the [[gravitational force]]:

:<math>m_Ag=6\pi \eta rv</math>

where

:<math>m_Ag</math> is the [[Archimedes' principle|Archimedean weight]] of the colloidal particles,

:<math>\eta</math> is the [[viscosity]] of the suspension medium,

:<math>r</math> is the [[radius]] of the colloidal particle,

and <math>v</math> is the sedimentation or creaming velocity.

The mass of the colloidal particle is found using:

:<math>m_A =V(\rho_1 - \rho_2)</math>

where

:<math>V</math> is the volume of the colloidal particle, calculated using the volume of a sphere <math>V = \frac{4}{3}\pi r^3</math>,

and <math>\rho_1-\rho_2</math> is the difference in mass density between the colloidal particle and the suspension medium.

By rearranging, the sedimentation or creaming velocity is:

:<math>v = \frac{m_Ag}{6\pi\eta r}</math>

There is an upper size-limit for the diameter of colloidal particles because particles larger than 1 μm tend to sediment, and thus the substance would no longer be considered a colloidal suspension.<ref name="cosgrove2010">{{Cite book|last=Cosgrove|first=Terence|title=Colloid Science: Principles, Methods and Applications|publisher=[[John Wiley & Sons]]|year=2010|isbn=978-1-4443-2018-3}}</ref>

The colloidal particles are said to be in [[sedimentation equilibrium]] if the rate of sedimentation is equal to the rate of movement from Brownian motion.

==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.|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]].<ref>{{cite book |last1=Dukhin |first1=Andrei S. |last2=Xu |first2=Renliang |title=Zeta potential: fundamentals, methods, and applications |date=2025 |publisher=Academic Press |location=London Cambridge, MA |isbn=978-0-443-33443-6 |url=https://shop.elsevier.com/books/zeta-potential/dukhin/978-0-443-33443-6}}</ref> 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|bibcode=2011IntST..18....1P |isbn=978-0-12-375049-5}}</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|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]|journal=Journal (American Water Works Association)|volume=56|issue=2|pages=214–227|doi=10.1002/j.1551-8833.1964.tb01202.x|jstor=41264141}}</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-Skłodowska, Sectio AA – Chemia|volume=72|issue=1|page=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>

==As a model system for atoms==
In [[physics]], colloids are an interesting model system for [[atom]]s.<ref>{{cite journal|last=Manoharan| first=Vinothan N. |title=Colloidal matter: Packing, geometry, and entropy| journal=Science| volume=349| issue=6251 | article-number=1253751| date=2015| doi=10.1126/science.1253751| pmid=26315444| s2cid=5727282 | url=https://dash.harvard.edu/bitstream/handle/1/30410808/Manoharan-Science-2015-postprint.pdf?sequence=1| doi-access=free}}</ref> Micrometre-scale colloidal particles are large enough to be observed by optical techniques such as [[confocal microscopy]]. Many of the forces that govern the structure and behavior of matter, such as excluded volume interactions or electrostatic forces, govern the structure and behavior of colloidal suspensions. For example, the same techniques used to model ideal gases can be applied to [[Scientific modelling|model]] the behavior of a hard sphere colloidal suspension. [[Phase transition]]s in colloidal suspensions can be studied in real time using optical techniques,<ref name=greenfield2013shockwave>{{cite journal|last=Greenfield|first=Elad |author2=Nemirovsky, Jonathan |author3=El-Ganainy, Ramy |author4=Christodoulides, Demetri N |author5=Segev, Mordechai |title=Shockwave based nonlinear optical manipulation in densely scattering opaque suspensions|journal=Optics Express|year=2013|volume=21|issue=20|pages=23785–23802|doi=10.1364/OE.21.023785 | pmid = 24104290 |bibcode = 2013OExpr..2123785G |url=https://stars.library.ucf.edu/cgi/viewcontent.cgi?article=5052&context=facultybib2010 |doi-access=free }}</ref> and are analogous to phase transitions in liquids. In many interesting cases optical fluidity is used to control colloid suspensions.<ref name=greenfield2013shockwave /><ref name=greenfield2011light>{{cite journal|last=Greenfield|first=Elad |author2=Rotschild, Carmel |author3=Szameit, Alexander |author4=Nemirovsky, Jonathan |author5=El-Ganainy, Ramy |author6=Christodoulides, Demetrios N |author7=Saraf, Meirav |author8=Lifshitz, Efrat |author9=Segev, Mordechai |title=Light-induced self-synchronizing flow patterns|journal=New Journal of Physics|year=2011|volume=13|issue=5|article-number=053021|doi=10.1088/1367-2630/13/5/053021|bibcode = 2011NJPh...13e3021G |doi-access=free }}</ref>

==Crystals==

{{Main|Colloidal crystal}}

A colloidal crystal is a highly [[Order (crystal lattice)|ordered]] array of particles that can be formed over a very long range (typically on the order of a few millimeters to one centimeter) and that appear [[analogous]] to their atomic or molecular counterparts.<ref>{{cite journal|author =Pieranski, P.|year =1983| title = Colloidal Crystals| journal= Contemporary Physics| volume= 24| pages =25–73|doi =10.1080/00107518308227471|bibcode = 1983ConPh..24...25P }}</ref> One of the finest [[natural]] examples of this ordering phenomenon can be found in precious [[opal]], in which brilliant regions of pure [[wikt:spectrum|spectral]] [[color]] result from [[close-packed]] domains of [[amorphous]] colloidal spheres of [[silicon dioxide]] (or [[silica]], SiO<sub>2</sub>).<ref>{{cite journal|author = Sanders, J.V.|year =1964|title = Structure of Opal|journal = Nature |volume=204|page =1151|doi=10.1038/204990a0|last2 = Sanders|first2 = J. V.|last3 = Segnit|first3 = E. R.|s2cid =4191566|bibcode = 1964Natur.204..990J|issue=4962}}</ref><ref>{{cite journal|author = Darragh, P.J.|year =1976|journal = Scientific American|volume=234|issue =4|pages=84–95|display-authors=etal|doi=10.1038/scientificamerican0476-84|title=Opals|bibcode=1976SciAm.234d..84D}}</ref> These spherical particles [[precipitate]] in highly [[siliceous]] pools in [[Australia]] and elsewhere, and form these highly ordered arrays after years of [[sedimentation]] and [[compression (physical)|compression]] under [[hydrostatic]] and gravitational forces. The periodic arrays of submicrometre spherical particles provide similar arrays of [[interstitial defect|interstitial]] [[wikt:void|voids]], which act as a natural [[diffraction grating]] for [[visible spectrum|visible]] [[light]] [[wave]]s, particularly when the interstitial spacing is of the same [[order of magnitude]] as the [[Optical physics|incident]] lightwave.<ref>{{cite journal |last1=Luck |first1=Werner |last2=Klier |first2=Manfred |last3=Wesslau |first3=Hermann |title=Über Bragg-Reflexe mit sichtbarem Licht an monodispersen Kunststofflatices. II |journal=Berichte der Bunsengesellschaft für Physikalische Chemie |date= 1963 |volume=67 |issue=1 |pages=84–85 |doi=10.1002/bbpc.19630670114}}</ref><ref>{{cite journal|author1=Hiltner, P.A.  |author2=Krieger, I.M.|year =1969|title = Diffraction of light by ordered suspensions|journal=J. Phys. Chem.|volume=73|page=2306|doi = 10.1021/j100727a049|issue = 7}}</ref>

Thus, it has been known for many years that, due to [[Coulomb's Law|repulsive]] [[Coulombic]] interactions, [[electrically charged]] [[macromolecule]]s in an [[aqueous]] environment can exhibit long-range [[crystal]]-like correlations with interparticle separation distances, often being considerably greater than the individual particle diameter. In all of these cases in nature, the same brilliant [[iridescence]] (or play of colors) can be attributed to the diffraction and [[constructive interference]] of visible lightwaves that satisfy [[Bragg's law]], in a matter analogous to the [[scattering]] of [[X-ray]]s in crystalline solids.

The large number of experiments exploring the [[physics]] and [[chemistry]] of these so-called "colloidal crystals" has emerged as a result of the relatively simple methods that have evolved in the last 20 years for preparing synthetic monodisperse colloids (both polymer and mineral) and, through various mechanisms, implementing and preserving their long-range order formation.<ref>{{Cite journal|last1=Liu|first1=Xuesong|last2=Li|first2=Zejing|last3=Tang|first3=Jianguo|last4=Yu|first4=Bing|last5=Cong|first5=Hailin|date=2013-09-09|title=Current status and future developments in preparation and application of colloidal crystals|journal=Chemical Society Reviews|volume=42|issue=19|pages=7774–7800|doi=10.1039/C3CS60078E|pmid=23836297}}</ref>

==In biology==
Colloidal [[phase separation]] is an important organising principle for compartmentalisation of both the [[cytoplasm]] and [[Cell nucleus|nucleus]] of cells into '''[[biomolecular condensate]]s'''—similar in importance to compartmentalisation via lipid bilayer [[biological membranes|membranes]], a type of [[liquid crystal]]. The term [[biomolecular condensate]] has been used to refer to clusters of [[macromolecules]] that arise via liquid-liquid or liquid-solid [[phase separation]] within cells. [[Macromolecular crowding]] strongly enhances colloidal phase separation and formation of [[biomolecular condensate]]s.

==In the environment==
Colloidal particles can also serve as transport vectors <ref>{{Cite book
|last = Frimmel|first = Fritz H. |author2=Frank von der Kammer |author3=Hans-Curt Flemming
|year = 2007
|title = Colloidal transport in porous media
|edition = 1
|publisher = Springer
|page=292
|isbn = 978-3-540-71338-8|url = https://www.springer.com/earth+sciences/book/978-3-540-71338-8?detailsPage=toc}}</ref>
of diverse contaminants in the surface water (sea water, lakes, rivers, freshwater bodies) and in underground water circulating in fissured rocks<ref>{{Cite journal
| last = Alonso| first = U.
|author2=T. Missana |author3=A. Patelli |author4=V. Rigato
 | year = 2007
| title = Bentonite colloid diffusion through the host rock of a deep geological repository
| journal = Physics and Chemistry of the Earth, Parts A/B/C
| volume = 32
| issue = 1–7
| pages = 469–476
| doi = 10.1016/j.pce.2006.04.021|bibcode = 2007PCE....32..469A }}</ref>
(e.g. [[limestone]], [[sandstone]], [[granite]]). Radionuclides and heavy metals easily [[sorption|sorb]] onto colloids suspended in water. Various types of colloids are recognised: inorganic colloids (e.g. [[clay]] particles, silicates, [[ferrihydrite|iron oxy-hydroxide]]s), organic colloids ([[humic]] and [[fulvic]] substances). When heavy metals or radionuclides form pure colloids, the term "''[[eigencolloid]]''" is used to designate pure phases, i.e., pure Tc(OH)<sub>4</sub>, U(OH)<sub>4</sub>, or Am(OH)<sub>3</sub>. Colloids have been suspected for the long-range transport of plutonium on the [[Nevada Nuclear Test Site]]. They have been the subject of detailed studies for many years. However, the mobility of inorganic colloids is very low in compacted [[bentonite]]s and in deep clay formations<ref>{{Cite journal
|last1 = Voegelin
|first1 = A.
|last2 = Kretzschmar
|title = Stability and mobility of colloids in Opalinus Clay
|publisher = Institute of Terrestrial Ecology, ETH Zürich
|volume = Nagra Technical Report 02-14.
|date = December 2002
|issn = 1015-2636
|url = http://www.nagra.ch/documents/database/dokumente/%24default/Default%20Folder/Publikationen/e%5Fntb02%2D14.pdf
|page = 47
|first2 = R.
|journal = Technischer Bericht / NTB
|access-date = 22 February 2009
|archive-url = https://web.archive.org/web/20090309172632/http://www.nagra.ch/documents/database/dokumente/%24default/Default%20Folder/Publikationen/e%5Fntb02%2D14.pdf
|archive-date = 9 March 2009
}}</ref>
because of the process of [[ultrafiltration]] occurring in dense clay membrane.<ref>{{Cite web
 |title       = Diffusion of colloids in compacted bentonite
 |url         = http://www.kth.se/che/divisions/nuchem/research/1.19965?l=en_UK
 |access-date  = 12 February 2009
 |archive-url  = https://web.archive.org/web/20090304210603/http://www.kth.se/che/divisions/nuchem/research/1.19965?l=en_UK
 |archive-date = 4 March 2009
}}</ref>
The question is less clear for small organic colloids often mixed in porewater with truly dissolved organic molecules.<ref>{{Cite journal
| last = Wold | first = Susanna
|author2=Trygve Eriksen
| year = 2007
| title = Diffusion of humic colloids in compacted bentonite
| journal = Physics and Chemistry of the Earth, Parts A/B/C
| doi = 10.1016/j.pce.2006.05.002
| volume = 32
| issue = 1–7
| pages = 477–484|bibcode = 2007PCE....32..477W }}</ref>

In [[soil science]], the colloidal fraction in [[soil]]s consists of tiny [[clay]] and [[humus]] [[particle]]s that are less than 1μm in [[diameter]] and carry either positive and/or negative [[Electric charge|electrostatic charges]] that vary depending on the chemical conditions of the soil sample, i.e. [[soil pH]].<ref>{{Cite book|title=Elements of the nature and properties of soils|author1=Weil, Ray|author2=Brady, Nyle C.|isbn=978-0-13-325459-4|edition= Fourth|location=New York, NY|oclc=1035317420|date = 11 October 2018}}</ref>

==Intravenous therapy==
Colloid solutions used in [[intravenous therapy]] belong to a major group of [[volume expander]]s, and can be used for intravenous [[fluid replacement]]. Colloids preserve a high [[colloid osmotic pressure]] in the blood,<ref name="gregory">{{Cite web|url=http://www.medscape.org/viewarticle/503138|title=An Update on Intravenous Fluids|last=Martin|first=Gregory S.|date=19 April 2005|website=[[Medscape]]|access-date=6 July 2016}}</ref> and therefore, they should theoretically preferentially increase the [[intravascular volume]], whereas other types of volume expanders called [[crystalloid solution|crystalloid]]s also increase the [[interstitial volume]] and [[intracellular volume]]. However, there is still controversy to the actual difference in [[efficacy]] by this difference,<ref name=gregory/> and much of the research related to this use of colloids is based on fraudulent research by [[Joachim Boldt]].<ref>{{Cite news|url=https://www.telegraph.co.uk/health/8360667/Millions-of-surgery-patients-at-risk-in-drug-research-fraud-scandal.html|title=Millions of surgery patients at risk in drug research fraud scandal|last=Blake|first=Heidi|date=3 March 2011|newspaper=The Telegraph|location=UK|archive-url=https://web.archive.org/web/20111104083124/http://www.telegraph.co.uk/health/8360667/Millions-of-surgery-patients-at-risk-in-drug-research-fraud-scandal.html|archive-date=4 November 2011|access-date=4 November 2011}}</ref> Another difference is that crystalloids generally are much cheaper than colloids.<ref name=gregory/>

==References==
{{reflist}}

{{Phase of matter}}
{{Chemical solutions}}

{{Authority control}}

[[Category:Colloids| ]]
[[Category:Chemical mixtures]]
[[Category:Colloidal chemistry]]
[[Category:Condensed matter physics]]
[[Category:Soft matter]]
[[Category:Dosage forms]]