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| '''Physisorption''', also called '''physical adsorption''', is a process in which the electronic structure of the atom or molecule is barely perturbed upon [[adsorption]].<ref>{{citation|author=K. Oura et al.|title=Surface Science, An Introduction|location= Berlin|publisher= Springer|year= 2003| isbn =978-3-540-00545-2}}</ref><ref name=ConceptsinSurfacePhysics>{{citation |author=M. C. Desjonqueres et al |title=Concepts in surface physics |edition=2nd |url=http://books.google.com/?id=XW_Wvjwt5nIC&printsec=frontcover&dq=Concepts+in+surface+physics |place=New York |publisher=Springer-Verlag |date= 1996. Corrected printing 1998 |isbn=3-540-58622-9 |accessdate=29 August 2012}}</ref><ref>{{Citation|author=Hans Luth et al.|title=Surfaces and interfaces of solids|publisher= Springer-Verlag|year= 1993| isbn =978-3-540-56840-7}}</ref>
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| ==Introduction==
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| The fundamental interacting force of physisorption is caused by [[van der Waals force]]. Even though the interaction energy is very weak (~10–100 meV), physisorption plays an important role in nature. For instance, the van der Waals attraction between surfaces and foot-hairs of [[gecko]]s provides the remarkable ability to climb up vertical walls.<ref>{{Citation | author=K. Autumn ''et al.''|title= Adhesive force of a single gecko foot-hair| journal=Nature| volume=405 | pages= 681–5| year=2000 | doi=10.1038/35015073 | pmid=10864324 | issue=6787}}</ref> Van der Waals forces originate from the interactions between induced, permanent or transient electric dipoles.
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| In comparison with [[chemisorption]], in which the electronic structure of bonding atoms or molecules is changed and covalent or ionic bonds form, physisorption, generally speaking, can only be observed in the environment of low temperature (thermal energy at room temperature ~26 meV) and the absence of the relatively strong chemisorptions. In practice, the categorisation of a particular adsorption as physisorption or chemisorption depends principally on the [[binding energy]] of the adsorbate to the substrate.
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| ==Modeling by image charge==
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| [[Image:physisorption 1.jpg|thumbnail|200px|Fig. 1. Schematic illustration of an adsorbed hydrogen atom near a perfect [[electrical conductor|conductor]] interacting with its [[image charge]]s.]]
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| To give a simple illustration of physisorption, we can first consider an adsorbed hydrogen atom in front of a perfect conductor, as shown in Fig. 1. A nucleus with positive charge is located at '''R''' = (0, 0, ''Z''), and the position coordinate of its electron, '''r''' = (''x'', ''y'', ''z'') is given with respect to the nucleus. The adsorption process can be viewed as the interaction between this hydrogen atom and its image charges of both the nucleus and electron in the conductor. As a result, the total electrostatic energy is the sum of attraction and repulsion terms:
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| :<math>V = {e^2\over 4\pi\varepsilon_0}\left(\frac{-1}{|2\mathbf R|}+\frac{-1}{|2\mathbf R+\mathbf r-\mathbf r^'|}+\frac{1}{|2\mathbf R-\mathbf r^'|}+\frac{1}{|2\mathbf R+\mathbf r|}\right).</math>
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| The first term is the attractive interaction of nucleus and its image charge, and the second term is due to the interaction of the electron and its image charge. The repulsive interaction is shown in the third and fourth terms arising from the interaction of nucleus-image electron and electron-image nucleus, respectively.
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| By [[Taylor expansion]] in powers of |'''r'''| / |'''R'''|, this interaction energy can be further expressed as:
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| :<math>V = {-e^2\over 16\pi\varepsilon_0 Z^3}\left(\frac{x^2+y^2}{2}+z^2\right)+ {3e^2\over 32\pi\varepsilon_0 Z^4}\left(\frac{x^2+y^2}{2}{z}+z^3\right)+O\left(\frac{1}{Z^5}\right).</math>
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| One can find from the first non-vanishing term that the physisorption potential depends on the distance ''Z'' between adsorbed atom and surface as ''Z''<sup>−3</sup>, in contrast with the ''r''<sup>−6</sup> dependence of the molecular [[van der Waals]] potential, where ''r'' is the distance between two [[dipoles]].
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| ==Modeling by quantum-mechanical oscillator==
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| The [[van der Waals force|van der Waals]] binding energy can be analyzed by another simple physical picture: modeling the motion of an electron around its nucleus by a three-dimensional simple [[harmonic oscillator]] with a potential energy ''V<sub>a</sub>'':
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| :<math>V_a = \frac{m_e}{2}{\omega^2}(x^2+y^2+z^2),</math>
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| where ''m<sub>e</sub>'' and ''ω'' are the mass and vibrational frequency of the electron, respectively.
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| As this atom approaches the surface of a metal and forms adsorption, this potential energy ''V<sub>a</sub>'' will be modified due to the image charges by additional potential terms which are quadratic in the displacements:
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| :<math>V_a = \frac{m_e}{2}{\omega^2}(x^2+y^2+z^2)-{e^2\over 16\pi\varepsilon_0 Z^3}\left(\frac{x^2+y^2}{2}+z^2\right)+\ldots</math> (from the Taylor expansion above.)
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| Assuming
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| :<math> m_e \omega^2>>{e^2\over 16\pi\varepsilon_0 Z^3},</math>
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| the potential is well approximated as
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| :<math>V_a \sim \frac{m_e}{2}{\omega_1^2}(x^2+y^2)+\frac{m_e}{2}{\omega_2^2}z^2</math>,
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| where
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| :<math>
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| \begin{align}
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| \omega_1 &= \omega - {e^2\over 32\pi\varepsilon_0 m_e\omega Z^3},\\
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| \omega_2 &= \omega - {e^2\over 16\pi\varepsilon_0 m_e\omega Z^3}.
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| \end{align}
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| </math>
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| If one assumes that the electron is in the ground state, then the van der Waals binding energy is essentially the change of the zero-point energy:
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| :<math>V_v = \frac{\hbar}{2}(2\omega_1+\omega_2-3\omega)= - {\hbar e^2\over 16\pi\varepsilon_0 m_e\omega Z^3}.</math>
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| This expression also shows the nature of the ''Z''<sup>−3</sup> dependence of the van der Waals interaction. | |
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| Furthermore by introducing the atomic [[polarizability]],
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| :<math> \alpha= \frac {e^2} {m_e\omega^2},</math>
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| the van der Waals potential can be further simplified:
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| :<math>V_v = - {\hbar \alpha \omega\over 16\pi\varepsilon_0 Z^3}= -\frac{C_v}{Z^3},</math>
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| where
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| :<math>C_v = {\hbar \alpha \omega\over 16\pi\varepsilon_0},</math>
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| is the van der Waals constant which is related to the atomic polarizability.
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| Also, by expressing the fourth-order correction in the Taylor expansion above as (''aC<sub>v</sub>Z''<sub>0</sub>) / (Z<sup>4</sup>), where ''a'' is some constant, we can define ''Z''<sub>0</sub> as the position of the ''dynamical image plane'' and obtain
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| [[Image:physisorption table.jpg|thumbnail|400px|Table 1. The van der Waals constant ''C<sub>v</sub>'' and the position of the dynamical image plane ''Z''<sub>0</sub> for various rare gases atoms adsorbed on noble metal surfaces obtained by the jellium model. Note that ''C<sub>v</sub>'' is in eV/Å<sup>3</sup> and ''Z''<sub>0</sub> in Å.]]
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| :<math>V_v = - \frac{C_v}{(Z-Z_0)^3}+O\left(\frac{1}{Z^5}\right).</math>
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| The origin of ''Z''<sub>0</sub> comes from the spilling of the electron wavefunction out of the surface. As a result, the position of image plane representing the reference for the space coordinate is different from the substrate surface itself and modified by ''Z''<sub>0</sub>.
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| Table 1 shows the [[jellium]] model calculation for van der Waals constant ''C<sub>v</sub>'' and dynamical image plane ''Z''<sub>0</sub> of rare gas atoms on various metal surfaces. The increasing of ''C<sub>v</sub>'' from He to Xe for all metal substrates is caused by the larger atomic [[polarizability]] of the heavier rare gas atoms. For the position of the dynamical image plane, it decreases with increasing dielectric function and is typically on the order of 0.2 Å.
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| ==Physisorption potential==
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| [[Image:physisorption 2.jpg|thumbnail|400px|Fig. 2. Calculated physisorption potential energy for He adsorbed on various [[jellium]] metal surfaces. Note that the weak van der Waals attraction forms shallow wells with energy about few meV.<ref name="Kohn"/>]]
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| Even though the [[van der Waals interaction]] is attractive, as the adsorbed atom moves closer to the surface the wavefunction of electron starts to overlap with that of the surface atoms. Further the energy of the system will increase due to the orthogonality of wavefunctions of the approaching atom and surface atoms.
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| This [[Pauli exclusion]] and repulsion are particularly strong for atoms with closed valence shells that dominate the surface interaction. As a result, the minimum energy of physisorption must be found by the balance between the long-range van der Waals attraction and short-range [[Pauli repulsion]]. For instance, by separating the total interaction of physisorption into two contributions- a short-range term depicted by [[Hartree–Fock]] theory and a long-range van der Waals attraction, the equilibrium position of physisorption for rare gases adsorbed on jellium substrate can be determined.<ref name="Kohn">{{Citation | author=E. Zaremba and W. Kohn|title= Theory of helium adsorption on simple and noble-metal surfaces| journal= Phys. Rev. B| volume=15 | issue=4 | pages= 1769| year=1977 | doi=10.1103/PhysRevB.15.1769}}</ref> Fig. 2 shows the physisorption potential energy of He adsorbed on Ag, Cu, and Au substrates which are described by the jellium model with different densities of smear-out background positive charges. It can be found that the weak van der Waals interaction leads to shallow attractive energy wells (<10 MeV). One of the experimental methods for exploring physisorption potential energy is the scattering process, for instance, inert gas atoms scattered from metal surfaces. Certain specific features of the interaction potential between scattered atoms and surface can be extracted by analyzing the experimentally determined angular distribution and cross sections of the scattered particles.
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| ==Comparison with chemisorption==
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| * Physisorption is a general phenomenon and occurs in any solid/fluid or solid/gas system. [[Chemisorption]] is characterized by chemical specificity.
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| * In physisorption, perturbation of the electronic states of adsorbent and adsorbate is minimal. For chemisorption, changes in the electronic states may be detectable by suitable physical means.
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| * Typical binding energy of physisorption is about 10–100 meV. Chemisorption usually forms bonding with energy of 1–10 eV.
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| * The elementary step in physisorption from a gas phase does not involve an activation energy. Chemisorption often involves an activation energy.
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| * For physisorption, under appropriate conditions, gas phase molecules can form multilayer adsorption. In chemisorption, molecules are adsorbed on the surface by valence bonds and only form monolayer adsorption.
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| ==See also==
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| *[[Adsorption]]
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| *[[Chemisorption]]
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| *[[van der Waals force]]
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| ==References==
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| {{reflist}}
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| [[Category:Surface chemistry]]
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Alfred Halfnight is a London-based cleaning company employee and freelance journalist. " "If you already are now living in heaven, where would you go once you die. This cleanliness making you and the workers happier and productive. It is your responsibility to ensure that all your employees fully understand all areas of Health and Safety.
Tendency will be the cleaning is done in a very haste not giving much focus on corners and also other areas at risk of dust accumulation. Theyve quoted you at a price which fits your companys budget, may offer you flexible scheduling alternatives for cleaning your working environment, which is willing to complete it all having a smile. Under current legislation you happen to be legally obliged never to put the employees into situations where they are able to come to harm, whether that harm is immediate, temporary or potentially long term.
There are certain things you'll want to know before you engage a company. Here are a handful of other benefits to opening an advert cleaning company:. Whilst every care could possibly be taken to be sure no problems occur, there is obviously the outside possibility. If a Las Vegas window cleaning company likes you their image, they're planning to care about your image.
Partial refunds could be presented any moment purchaser expectations are certainly not thoroughly met. Clean your carpet, and make your home look better, effortlessly. When a house is built, drywall dust gets adhered on the duct walls as being a thin layer of sand. This article will provide you with some good tricks to follow. Company certifications will also be important to hunt for while browsing around the company websites.
Assuming needless to say that you implement some with the checks recommended below when hiring your best commercial cleaning company. Here are a couple of examples of issues your cleaning company and employees might come upon:. But do you think you're really receiving the most bang for the buck. Now, when you ask for your professional help of the cleaning service, make sure that there is a list from the areas in your house that you need to be cleaned up.
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