View source for Adsorption

{{short description|Adhesion of atoms, ions, or molecules from a substance to a surface It can also be defined as the increase of adsorbate on the surface of adsorbent }}
{{distinguish|text=[[Absorption (chemistry)|Absorption]]}}
{{See also|Physisorption|Chemisorption|Segregation (materials science)}}
[[Image:BET Multilayer Adsorption.svg|thumb|Brunauer, Emmett and Teller's [[Adsorption#BET|model]] of multilayer adsorption is a random distribution of molecules on the material surface.]]

'''Adsorption''' is the [[adhesion]] of [[atoms]], [[ion]]s or [[molecule]]s from a gas, liquid or dissolved solid to a [[Surface science|surface]].<ref>{{cite web |url=http://www.brownfieldstsc.org/glossary.cfm?q=1 |title=Glossary |publisher=The Brownfields and Land Revitalization Technology Support Center |accessdate=2009-12-21 }}{{Dead link|date=September 2019 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from [[absorption (chemistry)|absorption]], in which a [[fluid]] (the ''absorbate'') is [[Dissolution (chemistry)|dissolved]] by or [[permeation|permeates]] a liquid or solid (the ''absorbent''), respectively.<ref>{{cite web |url=http://www.memidex.com/absorption+chemistry |title=absorption (chemistry) |work=Memidex (WordNet) Dictionary/Thesaurus |accessdate=2010-11-02}}</ref> Adsorption is a ''[[surface phenomenon]]'', while absorption involves the whole volume of the material. The term ''[[sorption]]'' encompasses both processes, while ''[[desorption]]'' is the reverse of it.

{{Quote box
 |title =  [[International Union of Pure and Applied Chemistry|IUPAC]] definition
 |width=50%
 |align=right
 |quote = Increase in the concentration of a substance at the interface of a condensed and a liquid or gaseous layer owing to the operation of surface forces.

Note 1: Adsorption of proteins is of great importance when a material is in contact with blood or body fluids. In the case of blood, [[albumin]], which is largely predominant, is generally adsorbed first, and then rearrangements occur in favor of other minor proteins according to surface affinity against mass law selection ([[Vroman effect]]).

Note 2: Adsorbed molecules are those that are resistant to washing with the same solvent medium in the case of adsorption from solutions. The washing conditions can thus modify the measurement results, particularly when the interaction energy is low.<ref name=goldbook1990>
{{cite book |title=Glossary of atmospheric chemistry terms (Recommendations 1990) |journal=[[Pure and Applied Chemistry]] |year=1990 |volume=62 |page=2167 |doi=10.1351/goldbook.A00155 |url=http://goldbook.iupac.org/A00155.html |isbn=978-0-9678550-9-7}}</ref> 
}}

Similar to [[surface tension]], adsorption is a consequence of [[surface energy]]. In a bulk material, all the bonding requirements (be they [[ionic bond|ionic]], [[covalent bond|covalent]] or [[metallic bond|metallic]]) of the constituent [[atoms]] of the material are filled by other atoms in the material. However, atoms on the surface of the adsorbent are not wholly surrounded by other adsorbent atoms and therefore can attract adsorbates. The exact nature of the bonding depends on the details of the species involved, but the adsorption process is generally classified as [[physisorption]] (characteristic of weak [[van der Waals force]]s) or [[chemisorption]] (characteristic of covalent bonding). It may also occur due to electrostatic attraction.<ref>{{cite journal |journal=[[Journal of Colloid and Interface Science|J. Colloid Interface Sci.]] |year=2010 |volume=347 |issue=1 |pages=15–24 |title=Interaction of cement model systems with superplasticizers investigated by atomic force microscopy, zeta potential, and adsorption measurements |last=Ferrari |first=L. |last2=Kaufmann |first2=J. |last3=Winnefeld |first3=F. |last4=Plank |first4=J. |doi=10.1016/j.jcis.2010.03.005 |pmid=20356605|bibcode=2010JCIS..347...15F }}</ref>

Adsorption is present in many natural, physical, biological and chemical systems and is widely used in industrial applications such as [[heterogeneous catalysts]],<ref>{{cite journal |last=Czelej |first=K. |last2=Cwieka |first2=K. |last3=Kurzydlowski |first3=K.J. |title=CO<sub>2</sub> stability on the Ni low-index surfaces: Van der Waals corrected DFT analysis|journal=Catalysis Communications |date=May 2016 |volume=80 |issue=5 |pages=33–38 |doi=10.1016/j.catcom.2016.03.017 |url= }}</ref><ref>{{cite journal |last=Czelej |first=K. |last2=Cwieka |first2=K. |last3=Colmenares |first3=J.C. |last4=Kurzydlowski |first4=K.J. |title=Insight on the Interaction of Methanol-Selective Oxidation Intermediates with Au- or/and Pd-Containing Monometallic and Bimetallic Core@Shell Catalysts |journal=Langmuir |year=2016 |volume=32 |issue=30 |pages=7493–7502 |doi=10.1021/acs.langmuir.6b01906 |pmid=27373791 }}</ref> [[activated charcoal]], capturing and using [[waste heat]] to provide cold water for air conditioning and other process requirements ([[Adsorption#Adsorption chillers|adsorption chillers]]), [[synthetic resins]], increasing storage capacity of [[Tunable nanoporous carbon|carbide-derived carbons]] and [[water purification]]. Adsorption, [[ion exchange]] and [[chromatography]] are sorption processes in which certain adsorbates are selectively transferred from the fluid phase to the surface of insoluble, rigid particles suspended in a vessel or packed in a column. Pharmaceutical industry applications, which use adsorption as a means to prolong neurological exposure to specific drugs or parts thereof,{{Citation needed|date=November 2013}} are lesser known.

The word "adsorption" was coined in 1881 by German physicist [[Heinrich Kayser]] (1853–1940).<ref>
{{cite journal
 |last1=Kayser |first1=Heinrich
 |year=1881
 |title=Über die Verdichtung von Gasen an Oberflächen in ihrer Abhängigkeit von Druck und Temperatur
 |url=https://books.google.com/books?id=ZxVbAAAAYAAJ&pg=PA526#v=onepage&q&f=false
 |journal=Annalen der Physik und Chemie
 |volume=248 |issue=4 |pages=526–537
 |bibcode=1881AnP...248..526K
 |doi=10.1002/andp.18812480404
}}.  In this study of the adsorption of gases by charcoal, the first use of the word "adsorption" appears on page 527: {{lang|de|"Schon Saussure kannte die beiden für die Grösse der Adsorption massgebenden Factoren, den Druck und die Temperatur, da er Erniedrigung des Druckes oder Erhöhung der Temperatur zur Befreiung der porösen Körper von Gasen benutzte."}}  ("Saussaure already knew the two factors that determine the quantity of adsorption – [namely,] the pressure and temperature – since he used the lowering of the pressure or the raising of the temperature to free the porous substances of gases.")</ref>

==Isotherms==
The adsorption of gases and solutes is usually described through isotherms, that is, the amount of adsorbate on the adsorbent as a function of its pressure (if gas) or concentration (for liquid phase solutes) at constant temperature. The quantity adsorbed is nearly always normalized by the mass of the adsorbent to allow comparison of different materials. To date, 15 different isotherm models have been developed.<ref name="FooHameed2010">{{cite journal |last1=Foo |first1=K. Y. |last2=Hameed |first2=B. H. |title=Insights into the modeling of adsorption isotherm systems |journal=Chemical Engineering Journal |volume=156 |issue=1 |year=2010 |pages=2–10 |issn=1385-8947 |doi=10.1016/j.cej.2009.09.013}}</ref>

=== Linear ===
{{Main|Henry adsorption constant}}

=== Freundlich ===
{{Main|Freundlich equation}}
The first mathematical fit to an isotherm was published by Freundlich and Kuster (1906) and is a purely empirical formula for gaseous adsorbates:

:<math>\frac{x}{m} = kP^{1/n},</math>

where <math>x</math> is the mass of adsorbate adsorbed, <math>m</math> is the mass of the adsorbent, <math>P</math> is the pressure of adsorbate (this can be changed to concentration if investigating solution rather than gas), and <math>k</math> and <math>n</math> are empirical constants for each adsorbent–adsorbate pair at a given temperature. The function is not adequate at very high pressure because in reality <math>x/m</math> has an asymptotic maximum as pressure increases without bound. As the temperature increases, the constants <math>k</math> and <math>n</math> change to reflect the empirical observation that the quantity adsorbed rises more slowly and higher pressures are required to saturate the surface.

===Langmuir===
{{See also|Langmuir equation}}
[[Irving Langmuir]] was the first to derive a scientifically based adsorption isotherm in 1918.<ref name=Langmuir/> The model applies to gases adsorbed on solid surfaces. It is a semi-empirical isotherm with a kinetic basis and was derived based on statistical thermodynamics. It is the most common isotherm equation to use due to its simplicity and its ability to fit a variety of adsorption data. It is based on four assumptions:
# All of the adsorption sites are equivalent, and each site can only accommodate one molecule.
# The surface is energetically homogeneous, and adsorbed molecules do not interact.
# There are no phase transitions.
# At the maximum adsorption, only a monolayer is formed. Adsorption only occurs on localized sites on the surface, not with other adsorbates.

These four assumptions are seldom all true: there are always imperfections on the surface, adsorbed molecules are not necessarily inert, and the mechanism is clearly not the same for the very first molecules to adsorb to a surface as for the last. The fourth condition is the most troublesome, as frequently more molecules will adsorb to the monolayer; this problem is addressed by the [[#BET isotherm|BET isotherm]] for relatively flat (non-[[microporous material|microporous]]) surfaces. The Langmuir isotherm is nonetheless the first choice for most models of adsorption and has many applications in surface kinetics (usually called [[Langmuir–Hinshelwood kinetics]]) and [[thermodynamics]].

Langmuir suggested that adsorption takes place through this mechanism: <math>A_\text{g} + S \rightleftharpoons AS</math>, where ''A'' is a gas molecule, and ''S'' is an adsorption site. The direct and inverse rate constants are ''k'' and ''k''<sub>−1</sub>. If we define surface coverage, <math>\theta</math>, as the fraction of the adsorption sites occupied, in the equilibrium we have:

:<math>K = \frac{k}{k_{-1}} = \frac{\theta}{(1 - \theta)P},</math>

or

:<math>\theta = \frac{KP}{1 + KP},</math>

where <math>P</math> is the partial pressure of the gas or the molar concentration of the solution.
For very low pressures <math>\theta \approx KP</math>, and for high pressures <math>\theta \approx 1</math>.

The value of <math>\theta</math> is difficult to measure experimentally; usually, the adsorbate is a gas and the quantity adsorbed is given in moles, grams, or gas volumes at [[standard temperature and pressure]] (STP) per gram of adsorbent. If we call ''v''<sub>mon</sub> the STP volume of adsorbate required to form a monolayer on the adsorbent (per gram of adsorbent), then <math>\theta = \frac{v}{v_\text{mon}}</math>, and we obtain an expression for a straight line:

:<math>\frac{1}{v} = \frac{1}{Kv_\text{mon}}\frac{1}{P} + \frac{1}{v_\text{mon}}.</math>

Through its slope and ''y'' intercept we can obtain ''v''<sub>mon</sub> and ''K'', which are constants for each adsorbent–adsorbate pair at a given temperature. ''v''<sub>mon</sub> is related to the number of adsorption sites through the [[ideal gas law]]. If we assume that the number of sites is just the whole area of the solid divided into the cross section of the adsorbate molecules, we can easily calculate the surface area of the adsorbent.
The surface area of an adsorbent depends on its structure: the more pores it has, the greater the area, which has a big influence on [[reactions on surfaces]].

If more than one gas adsorbs on the surface, we define <math>\theta_E</math> as the fraction of empty sites, and we have:

:<math>\theta_E = \dfrac{1}{1 + \sum_{i=1}^n K_i P_i}.</math>

Also, we can define <math>\theta_j</math> as the fraction of the sites occupied by the ''j''-th gas:

:<math>\theta_j = \dfrac{K_j P_j}{1 + \sum_{i=1}^n K_i P_i},</math>

where ''i'' is each one of the gases that adsorb.

===BET===<!-- linked in the caption of the image in the lead -->
{{Main|BET theory}}
Often molecules do form multilayers, that is, some are adsorbed on already adsorbed molecules, and the Langmuir isotherm is not valid. In 1938 [[Stephen Brunauer]], [[Paul H. Emmett|Paul Emmett]], and [[Edward Teller]] developed a model isotherm that takes that possibility into account. Their theory is called [[BET theory]], after the initials in their last names. They modified Langmuir's mechanism as follows:

:A<sub>(g)</sub> + S ⇌ AS,

:A<sub>(g)</sub> + AS ⇌ A<sub>2</sub>S,

:A<sub>(g)</sub> + A<sub>2</sub>S ⇌ A<sub>3</sub>S and so on.

[[File:Isothermes.svg|thumb|Langmuir (blue) and BET (red) isotherms]]

The derivation of the formula is more complicated than Langmuir's (see links for complete derivation). We obtain:

:<math>\frac{x}{v(1 - x)} = \frac{1}{v_\text{mon}c} + \frac{x(c - 1)}{v_\text{mon}c},</math>

where ''x'' is the pressure divided by the [[vapor pressure]] for the adsorbate at that temperature (usually denoted <math>P/P_0</math>), ''v'' is the STP volume of adsorbed adsorbate, ''v<sub>mon</sub>'' is the STP volume of the amount of adsorbate required to form a monolayer, and ''c'' is the equilibrium constant ''K'' we used in Langmuir isotherm multiplied by the vapor pressure of the adsorbate. The key assumption used in deriving the BET equation that the successive heats of adsorption for all layers except the first are equal to the heat of condensation of the adsorbate.

The Langmuir isotherm is usually better for chemisorption, and the BET isotherm works better for physisorption for non-microporous surfaces.

===Kisliuk===
[[File:Wiki kisliuk n2-tungsten.JPG|thumb|Two adsorbate nitrogen molecules adsorbing onto a tungsten adsorbent from the precursor state around an island of previously adsorbed adsorbate (left) and via random adsorption (right)]]

In other instances, molecular interactions between gas molecules previously adsorbed on a solid surface form significant interactions with gas molecules in the gaseous phases. Hence, adsorption of gas molecules to the surface is more likely to occur around gas molecules that are already present on the solid surface, rendering the Langmuir adsorption isotherm ineffective for the purposes of modelling. This effect was studied in a system where nitrogen was the adsorbate and tungsten was the adsorbent by Paul Kisliuk (1922–2008) in 1957.<ref name=Kisliuk/> To compensate for the increased probability of adsorption occurring around molecules present on the substrate surface, Kisliuk developed the precursor state theory, whereby molecules would enter a precursor state at the interface between the solid adsorbent and adsorbate in the gaseous phase. From here, adsorbate molecules would either adsorb to the adsorbent or desorb into the gaseous phase. The probability of adsorption occurring from the precursor state is dependent on the adsorbate’s proximity to other adsorbate molecules that have already been adsorbed. If the adsorbate molecule in the precursor state is in close proximity to an adsorbate molecule that has already formed on the surface, it has a sticking probability reflected by the size of the S<sub>E</sub> constant and will either be adsorbed from the precursor state at a rate of ''k''<sub>EC</sub> or will desorb into the gaseous phase at a rate of ''k''<sub>ES</sub>. If an adsorbate molecule enters the precursor state at a location that is remote from any other previously adsorbed adsorbate molecules, the sticking probability is reflected by the size of the S<sub>D</sub> constant.

These factors were included as part of a single constant termed a "sticking coefficient", ''k''<sub>E</sub>, described below:

:<math>k_\text{E} = \frac{S_\text{E}}{k_\text{ES} S_\text{D}}.</math>

As S<sub>D</sub> is dictated by factors that are taken into account by the Langmuir model, S<sub>D</sub> can be assumed to be the adsorption rate constant. However, the rate constant for the Kisliuk model (''R''’) is different from that of the Langmuir model, as ''R''’ is used to represent the impact of diffusion on monolayer formation and is proportional to the square root of the system’s diffusion coefficient. The Kisliuk adsorption isotherm is written as follows, where Θ<sub>(''t'')</sub> is fractional coverage of the adsorbent with adsorbate, and ''t'' is immersion time:

:<math>\frac{d\theta_{(t)}}{dt} = R'(1 - \theta)(1 + k_\text{E}\theta).</math>

Solving for Θ<sub>(''t'')</sub> yields:

:<math>\theta_{(t)} = \frac{1 - e^{-R'(1 + k_\text{E})t}}{1 + k_\text{E} e^{-R'(1 + k_\text{E})t}}.</math>

===Adsorption enthalpy===
Adsorption constants are [[equilibrium constant]]s, therefore they obey the [[van 't Hoff equation]]:

:<math>\left( \frac{\partial \ln K}{\partial \frac{1}{T}} \right)_\theta = -\frac{\Delta H}{R}.</math>

As can be seen in the formula, the variation of ''K'' must be isosteric, that is, at constant coverage.
If we start from the BET isotherm and assume that the entropy change is the same for liquefaction and adsorption, we obtain
:<math>\Delta H_\text{ads} = \Delta H_\text{liq} - RT\ln c,</math>
that is to say, adsorption is more exothermic than liquefaction.

===Single-molecule explanation===
The adsorption of ensemble molecules on a surface or interface can be devived into two processes: adsorption and desorption. If the adsorption rate wins the desorption rate, the molecules will accumulate over time giving the adsorption curve over time. If the desorption rate is larger, the number of molecules on the surface will decrease over time. The adsorption rate is dependent on the temperature, the diffusion rate of the solute, and the energy barrier between the molecule and the surface. The diffusion and key elements of the adsorption rate can be calculated use [[Fick's laws of diffusion]] and [[Einstein relation (kinetic theory)]]. The desorption of a molecule from the surface depends on the binding energy of the molecule to the surface and the temperature.

==Adsorbents==

===Characteristics and general requirements===
[[Image:Activated Carbon.jpg|thumb|Activated carbon is used as an adsorbent]]

Adsorbents are used usually in the form of spherical pellets, rods, moldings, or monoliths with a [[hydrodynamic radius]] between 0.25 and 5&nbsp;mm. They must have high [[Abrasion (mechanical)|abrasion]] resistance, high [[thermal stability]] and small pore diameters, which results in higher exposed surface area and hence high capacity for adsorption. The adsorbents must also have a distinct pore structure that enables fast transport of the gaseous vapors.

Most industrial adsorbents fall into one of three classes:
* Oxygen-containing compounds – Are typically hydrophilic and polar, including materials such as [[silica gel]] and [[zeolites]].
* Carbon-based compounds – Are typically hydrophobic and non-polar, including materials such as [[activated carbon]] and [[graphite]].
* Polymer-based compounds – Are polar or non-polar functional groups in a porous polymer matrix.

===Silica gel===
[[File:THC 2003.902.070 Silica Gel Adsorber for NO2.tif|thumb|right | Silica gel adsorber for NO<sub>2</sub>, Fixed Nitrogen Research Laboratory, ca.1930s]]

[[Silica gel]] is a chemically inert, nontoxic, polar and dimensionally stable (< {{convert|400|°C|F|disp=or|sigfig=2}}) amorphous form of SiO<sub>2</sub>. It is prepared by the reaction between sodium silicate and acetic acid, which is followed by a series of after-treatment processes such as aging, pickling, etc. These after-treatment methods results in various pore size distributions.

Silica is used for drying of process air (e.g. oxygen, natural gas) and adsorption of heavy (polar) hydrocarbons from natural gas.

===Zeolites===

[[Zeolite]]s are natural or synthetic crystalline aluminosilicates, which have a repeating pore network and release water at high temperature. Zeolites are polar in nature.

They are manufactured by hydrothermal synthesis of sodium aluminosilicate or another silica source in an autoclave followed by ion exchange with certain cations (Na<sup>+</sup>, Li<sup>+</sup>, Ca<sup>2+</sup>, K<sup>+</sup>, NH<sub>4</sub><sup>+</sup>). The channel diameter of zeolite cages usually ranges from 2 to 9 [[angstrom|Å]]. The ion exchange process is followed by drying of the crystals, which can be pelletized with a binder to form macroporous pellets.

Zeolites are applied in drying of process air, CO<sub>2</sub> removal from natural gas, CO removal from reforming gas, air separation, [[catalytic cracking]], and catalytic synthesis and reforming.

Non-polar (siliceous) zeolites are synthesized from aluminum-free silica sources or by dealumination of aluminum-containing zeolites. The dealumination process is done by treating the zeolite with steam at elevated temperatures, typically greater than {{convert|500|C|F|sigfig=2}}. This high temperature heat treatment breaks the aluminum-oxygen bonds and the aluminum atom is expelled from the zeolite framework.

===Activated carbon===

[[Activated carbon]] is a highly porous, amorphous solid consisting of microcrystallites with a graphite lattice, usually prepared in small pellets or a powder. It is non-polar and cheap. One of its main drawbacks is that it reacts with oxygen at moderate temperatures (over 300&nbsp;°C).[[Image:Demac isoth.jpg|right|thumb|230px|Activated carbon nitrogen isotherm showing a marked microporous type I behavior]]

Activated carbon can be manufactured from carbonaceous material, including coal (bituminous, subbituminous, and lignite), peat, wood, or nutshells (e.g., coconut). The manufacturing process consists of two phases, carbonization and activation. The carbonization process includes drying and then heating to separate by-products, including tars and other hydrocarbons from the raw material, as well as to drive off any gases generated. The process is completed by heating the material over {{convert|400|°C|F|sigfig=2}} in an oxygen-free atmosphere that cannot support combustion. The carbonized particles are then "activated" by exposing them to an oxidizing agent, usually steam or carbon dioxide at high temperature. This agent burns off the pore blocking structures created during the carbonization phase and so, they develop a porous, three-dimensional graphite lattice structure. The size of the pores developed during activation is a function of the time that they spend in this stage. Longer exposure times result in larger pore sizes. The most popular aqueous phase carbons are bituminous based because of their hardness, abrasion resistance, pore size distribution, and low cost, but their effectiveness needs to be tested in each application to determine the optimal product.

Activated carbon is used for adsorption of organic substances and non-polar adsorbates and it is also usually used for waste gas (and waste water) treatment. It is the most widely used adsorbent since most of its chemical (e.g. surface groups) and physical properties (e.g. pore size distribution and surface area) can be tuned according to what is needed. Its usefulness also derives from its large micropore (and sometimes mesopore) volume and the resulting high surface area.

== Water adsorption==
The adsorption of water at surfaces is of broad importance in [[chemical engineering]], [[materials science]] and [[catalysis]]. Also termed surface hydration, the presence of physically or chemically adsorbed water at the surfaces of solids plays an important role in governing interface properties, chemical reaction pathways and catalytic performance in a wide range of systems. In the case of physically adsorbed water, surface hydration can be eliminated simply through drying at conditions of temperature and pressure allowing full vaporization of water. For chemically adsorbed water, hydration may be in the form of either dissociative adsorption, where H<sub>2</sub>O molecules are dissociated into surface adsorbed -H and -OH,  or molecular adsorption (associative adsorption) where individual water molecules remain intact <ref>[https://www.researchgate.net/publication/304714130_The_effects_of_copper_doping_on_photocatalytic_activity_at_101_planes_of_anatase_TiO2_A_theoretical_study Adsorption of water at TiO<sub>2</sub> planes], dissociative vs molecular water adsorption. </ref>

==Carbon capture and storage ==
Typical adsorbents proposed for [[carbon capture and storage]] are [[zeolite]]s and [[Metal-organic framework|MOFs]].<ref>{{Cite book|title=Introduction to carbon capture and sequestration|last=Berend|first=Smit|last2=Reimer|first2=Jeffery A|last3=Oldenburg|first3=Curtis M|last4=Bourg|first4=Ian C|publisher=Imperial College Press|year=2014|isbn=9781306496834 |pages=}}</ref> The customization of adsorbents makes them a potentially attractive alternative to absorption. Because adsorbents can be regenerated by temperature or pressure swing, this step can be less energy intensive than [[Absorption (chemistry)|absorption]] regeneration methods.<ref>{{Cite journal|last=D'Alessandro|first=Deanna M.|last2=Smit|first2=Berend|last3=Long|first3=Jeffrey R.|date=2010-08-16|title=Carbon Dioxide Capture: Prospects for New Materials|journal=Angewandte Chemie International Edition|language=en|volume=49|issue=35|pages=6058–82|doi=10.1002/anie.201000431|pmid=20652916|issn=1521-3773|url=http://infoscience.epfl.ch/record/200571}}</ref> Major problems that are present with adsorption cost in carbon capture are: regenerating the adsorbent, mass ratio, solvent/MOF, cost of adsorbent, production of the adsorbent, lifetime of adsorbent.<ref>{{Cite journal|last=Sathre|first=Roger|last2=Masanet|first2=Eric|date=2013-03-18|title=Prospective life-cycle modeling of a carbon capture and storage system using metal–organic frameworks for CO2 capture|url=http://pubs.rsc.org/en/content/articlepdf/2013/ra/c3ra40265g|journal=RSC Advances |volume=3|issue=15|pages=4964|doi=10.1039/C3RA40265G|issn=2046-2069}}</ref>

==Protein  and surfactant adsorption==
[[Protein adsorption]] is a process that has a fundamental role in the field of [[biomaterials]]. Indeed, biomaterial surfaces in contact with biological media, such as blood or serum, are immediately coated by proteins. Therefore, living [[cell (biology)|cell]]s do not interact directly with the biomaterial surface, but with the adsorbed proteins layer. This protein layer mediates the interaction between biomaterials and cells, translating biomaterial physical and chemical properties into a "biological language".<ref>{{Cite journal| title=Mediation of Biomaterial-Cell Interactions by Adsorbed Proteins: A Review |journal=Tissue Engineering |volume=11 |issue=1 |pages=1–18| year=2005 |pmid=15738657| doi=10.1089/ten.2005.11.1| last1=Wilson| first1=CJ| last2=Clegg| first2=RE| last3=Leavesley| first3=DI| last4=Pearcy| first4=MJ}}</ref> In fact, [[cell membrane]] [[receptor (biochemistry)|receptor]]s bind to protein layer bioactive sites and these receptor-protein binding events are transduced, through the cell membrane, in a manner that stimulates specific intracellular processes that then determine cell adhesion, shape, growth and differentiation. Protein adsorption is influenced by many surface properties such as surface [[wetting|wettability]], surface chemical composition <ref>{{Cite journal|author1=Sivaraman B. |author2=Fears K.P. |author3=Latour R.A. |title=Investigation of the effects of surface chemistry and solution concentration on the conformation of adsorbed proteins using an improved circular dichroism method |journal=Langmuir |volume=25 |issue=5 |pages=3050–6| year=2009 |pmid=19437712 |pmc=2891683 |doi= 10.1021/la8036814}}</ref> and surface nanometre-scale morphology.<ref>{{Cite journal| title=The effect of surface nanometre-scale morphology on protein adsorption |journal=[[PLoS ONE]] |volume=5 |issue=7 |pages=e11862| year=2010 |doi= 10.1371/journal.pone.0011862| editor1-last=Zhang| editor1-first=Shuguang| last1=Scopelliti| first1=Pasquale Emanuele| last2=Borgonovo| first2=Antonio| last3=Indrieri| first3=Marco| last4=Giorgetti| first4=Luca| last5=Bongiorno| first5=Gero| last6=Carbone| first6=Roberta| last7=Podestà| first7=Alessandro| last8=Milani| first8=Paolo|bibcode = 2010PLoSO...511862S | pmid=20686681 | pmc=2912332}}</ref>
Surfactant adsorption is a similar phenomenon, but utilising surfactant molecules in the place of proteins.<ref>{{cite journal|last1=Cheraghian|first1=Goshtasp|title=Evaluation of Clay and Fumed Silica Nanoparticles on Adsorption of Surfactant Polymer during Enhanced Oil Recovery|journal=Journal of the Japan Petroleum Institute|date=2017|volume=60|issue=2|pages=85–94|doi=10.1627/jpi.60.85}}</ref>

==Adsorption chillers==
[[File:Adsorption chiller schematic.svg|thumb|A schematic diagram of an adsorption chiller: (1) heat is lost through evaporation of refrigerant, (2) refrigerant vapour is adsorbed onto the solid medium, (3) refrigerant is desorbed from the solid medium section not in use, (4) refrigerant is condensed and returned to the start, (5) & (6) solid medium is cycled between adsorption and desorption to regenerate it.]]
Combining an adsorbent with a refrigerant, [[Adsorption refrigeration|adsorption chillers]] use heat to provide a cooling effect. This heat, in the form of hot water, may come from any number of industrial sources including waste heat from industrial processes, prime heat from solar thermal installations or from the exhaust or water jacket heat of a piston engine or turbine.

Although there are similarities between adsorption chillers and [[absorption refrigerator|absorption refrigeration]], the former is based on the interaction between gases and solids. The adsorption chamber of the chiller is filled with a solid material (for example zeolite, silica gel, alumina, active carbon or certain types of metal salts), which in its neutral state has adsorbed the refrigerant. When heated, the solid desorbs (releases) refrigerant vapour, which subsequently is cooled and liquefied. This liquid refrigerant then provides a cooling effect at the evaporator from its [[enthalpy of vaporization]]. In the final stage the refrigerant vapour is (re)adsorbed into the solid.<ref>{{cite book | last = Pilatowsky | first = I. |author2=Romero, R.J.|author3= Isaza, C.A.|author4= Gamboa, S.A.|author5= Sebastian, P.J.|author6= Rivera, W. | title = Cogeneration Fuel Cell-Sorption Air Conditioning Systems | chapter-url = http://www.springerlink.com/content/lp0v4lt01h5040p2/ | accessdate = 10 May 2011 | series = Green Energy and Technology | year = 2011 | publisher = Springer | isbn = 978-1-84996-027-4 | pages = 99,100 | chapter = Chapter 5: Sorption Refrigeration Systems }}</ref> As an adsorption chiller requires no compressor, it is relatively quiet.

==Portal site mediated adsorption==
Portal site mediated adsorption is a model for site-selective activated gas adsorption in metallic catalytic systems that contain a variety of different adsorption sites. In such systems, low-coordination "edge and corner" defect-like sites can exhibit significantly lower adsorption enthalpies than high-coordination ([[basal plane]]) sites. As a result, these sites can serve as "portals" for very rapid adsorption to the rest of the surface. The phenomenon relies on the common "spillover" effect (described below), where certain adsorbed species exhibit high mobility on some surfaces. The model explains seemingly inconsistent observations of gas adsorption thermodynamics and kinetics in catalytic systems where surfaces can exist in a range of coordination structures, and it has been successfully applied to bimetallic catalytic systems where synergistic activity is observed.

In contrast to pure spillover, portal site adsorption refers to surface diffusion to adjacent adsorption sites, not to non-adsorptive support surfaces.

The model appears to have been first proposed for carbon monoxide on silica-supported platinum by Brandt ''et al.'' (1993).<ref name=Brandt/> A similar, but independent model was developed by King and co-workers<ref name=Uner1997/><ref name=Narayan/><ref name=Wiel/> to describe hydrogen adsorption on silica-supported alkali promoted ruthenium, silver-ruthenium and copper-ruthenium bimetallic catalysts. The same group applied the model to CO hydrogenation (Fischer–Tropsch synthesis).<ref name=Uner1998/> Zupanc ''et al.'' (2002) subsequently confirmed the same model for hydrogen adsorption on magnesia-supported caesium-ruthenium bimetallic catalysts.<ref name= Zupanc/> Trens ''et al.'' (2009) have similarly described CO surface diffusion on carbon-supported Pt particles of varying morphology.<ref name=Trens/>

==Adsorption spillover==

In the case catalytic or adsorbent systems where a metal species is dispersed upon a support (or carrier) material (often quasi-inert oxides, such as alumina or silica), it is possible for an adsorptive species to indirectly adsorb to the support surface under conditions where such adsorption is thermodynamically unfavorable. The presence of the metal serves as a lower-energy pathway for gaseous species to first adsorb to the metal and then diffuse on the support surface. This is possible because the adsorbed species attains a lower energy state once it has adsorbed to the metal, thus lowering the activation barrier between the gas phase species and the support-adsorbed species.

[[Hydrogen spillover]] is the most common example of an adsorptive spillover. In the case of hydrogen, adsorption is most often accompanied with dissociation of molecular hydrogen (H<sub>2</sub>) to atomic hydrogen (H), followed by spillover of the hydrogen atoms present.

The spillover effect has been used to explain many observations in [[heterogeneous catalysis]] and adsorption.<ref name=Rozanov />

==Polymer adsorption==
{{main|polymer adsorption}}
Adsorption of molecules onto polymer surfaces is central to a number of applications, including development of non-stick coatings and in various biomedical devices. Polymers may also be adsorbed to surfaces through [[polyelectrolyte adsorption]].

==Adsorption in viruses==
[[Viral entry|Adsorption]] is the first step in the [[viral life cycle]]. The next steps are penetration, uncoating, synthesis (transcription if needed, and translation), and release. The virus replication cycle, in this respect, is similar for all types of viruses. Factors such as transcription may or may not be needed if the virus is able to integrate its genomic information in the cell's nucleus, or if the virus can replicate itself directly within the cell's cytoplasm.

==In popular culture==
The game of [[Tetris]] is a puzzle game in which blocks of 4 are adsorbed onto a surface during game play. Scientists have used Tetris blocks "as a proxy for molecules with a complex shape" and their "adsorption on a flat surface" for studying the [[thermodynamics]] of [[nanoparticles]].<ref>[https://arstechnica.com/science/news/2009/05/the-thermodynamics-of-tetris.ars The Thermodynamics of Tetris], Ars Technica, 2009.</ref><ref>{{cite journal|doi=10.1021/la900196b |title=Structure, Thermodynamics, and Solubility in Tetromino Fluids|year=2009|last1=Barnes|first1=Brian C.|last2=Siderius|first2=Daniel W.|last3=Gelb|first3=Lev D.|journal=Langmuir|volume=25|issue=12|pages=6702–16|pmid=19397254}}</ref>

==See also==
* [[Cryo-adsorption]]
* [[Molecular sieve]]
* [[Pressure swing adsorption]]
* [[Micromeritics]]
* [[Kelvin probe force microscope]]
* [[Fluidized bed concentrator]]
* [[Dual-polarization interferometry]]
* [[Potential theory of Polanyi|Polanyi adsorption]]
* [[Random sequential adsorption]]

==References==
{{Reflist|35em|refs=

<ref name=Brandt>{{cite journal |last=Brandt |first=R. K. |first2=M. R. |last2=Hughes |first3=L. P. |last3=Bourget |first4=K. |last4=Truszkowska |first5=R. G. |last5=Greenler |year=1993 |title=The interpretation of CO adsorbed on Pt/SiO<sub>2</sub> of two different particle-size distributions |journal=Surface Science |volume=286 |issue=1–2 |pages=15–25 |doi=10.1016/0039-6028(93)90552-U |bibcode = 1993SurSc.286...15B }}</ref>

<ref name=Kisliuk>{{cite journal |last=Kisliuk |first=P. |year=1957 |title=The sticking probabilities of gases chemisorbed on the surfaces of solids |journal=Journal of Physics and Chemistry of Solids |volume=3 |issue=1–2 |pages=95–101 |doi=10.1016/0022-3697(57)90054-9 |bibcode = 1957JPCS....3...95K }}</ref>

<ref name=Narayan>{{cite journal |last=Narayan |first=R. L. |first2=T. S. |last2=King |year=1998 |title=Hydrogen adsorption states on silica-supported Ru-Ag and Ru-Cu bimetallic catalysts investigated via microcalorimetry |journal=Thermochimica Acta |volume=312 |issue=1–2 |pages=105–114 |doi=10.1016/S0040-6031(97)00444-9 }}</ref>

<ref name=Rozanov >{{cite journal |last=Rozanov |first=V. V. |first2=O. V. |last2=Krylov |year=1997 |title=Hydrogen spillover in heterogeneous catalysis |journal=Russian Chemical Reviews |volume=66 |issue=2 |pages=107–119 |doi=10.1070/RC1997v066n02ABEH000308 |bibcode = 1997RuCRv..66..107R }}</ref>

<ref name=Langmuir>{{cite journal |last=Czepirski |first=L. |first2=M. R. |last2=Balys |first3=E. |last3=Komorowska-Czepirska |year=2000 |title=Some generalization of Langmuir adsorption isotherm |journal=Internet Journal of Chemistry |volume=3 |issue=14|issn=1099-8292|url=http://www.yumpu.com/en/document/view/5463272/article-14-some-generalization-of-langmuir-adsorption-isotherm-agh}}</ref>

<ref name=Trens>{{cite journal |last=Trens |first=P. |first2=R. |last2=Durand |first3=B. |last3=Coq |first4=C. |last4=Coutanceau |first5=S. |last5=Rousseau |first6=C. |last6=Lamy |year=2009 |title=Poisoning of Pt/C catalysts by CO and its consequences over the kinetics of hydrogen chemisorption |journal=Applied Catalysis B: Environmental |volume=92 |issue=3–4 |pages=280–4 |doi=10.1016/j.apcatb.2009.08.004 }}</ref>

<ref name=Uner1997>{{cite book |last=Uner |first=D. O. |first2=N. |last2=Savargoankar |first3=M. |last3=Pruski |first4=T. S. |last4=King |year=1997 |title=The effects of alkali promoters on the dynamics of hydrogen chemisorption and syngas reaction kinetics on Ru/SiO<sub>2</sub> catalysts |volume=109 |issue= |pages=315–324 |doi=10.1016/S0167-2991(97)80418-1 |series=Studies in Surface Science and Catalysis |isbn=9780444826091 }}</ref>

<ref name=Uner1998>{{cite journal |last=Uner |first=D. O. |year=1998 |title=A sensible mechanism of alkali promotion in Fischer Tropsch synthesis:Adsorbate mobilities |journal=Industrial and Engineering Chemistry Research |volume=37 |issue= 6|pages=2239–2245 |doi=10.1021/ie970696d }}</ref>

<ref name=Wiel>{{cite journal |last=VanderWiel |first=D. P. |first2=M. |last2=Pruski |first3=T. S. |last3=King |year=1999 |title=A Kinetic Study of the Adsorption and Reaction of Hydrogen on Silica-Supported Ruthenium and Silver-Ruthenium Bimetallic Catalysts during the Hydrogenation of Carbon Monoxide |journal=Journal of Catalysis |volume=188 |issue=1 |pages=186–202 |doi=10.1006/jcat.1999.2646 |url=https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=13531&context=rtd }}</ref>

<ref name=Zupanc>{{cite journal |last=Zupanc |first=C. |first2=A. |last2=Hornung |first3=O. |last3=Hinrichsen |first4=M. |last4=Muhler |year=2002 |title=The Interaction of Hydrogen with Ru/MgO Catalysts |journal=Journal of Catalysis |volume=209 |issue=2 |pages=501–514 |doi=10.1006/jcat.2002.3647 }}</ref>

}}

==Further reading==
{{Refbegin}}
* {{cite book |last=Cussler |first=E. L. |year=1997 |title=Diffusion: Mass Transfer in Fluid Systems |edition=2nd |location=New York |publisher=Cambridge University Press |pages=308–330 |isbn=978-0-521-45078-2 }}
{{Refend}}

==External links==
{{Wiktionary}}
{{Refbegin}}
* [http://www.jhu.edu/%7Echem/fairbr/OLDS/derive.html Derivation of Langmuir and BET isotherms], at JHU.edu
* [https://web.archive.org/web/20090105233739/http://www.megtec.com/solvent-recovery-carbon-adsorption-p-685-l-en.html Carbon Adsorption], at MEGTEC.com
{{Refend}}

{{Authority control}}

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