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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>
| | Relaxed large heels that are as trendy as they are useful can be identified. People who have no idea what sides to search for have a tendency to try out substantial versions of types and settle for significantly less than the best. Those who know what to request out will have a shorter, narrower search with bigger achievements.<br><br>Stilettos will endlessly be stylish. They are a typical item that is in design every solitary calendar year. Night and cocktail dresses will often need to have the extra top and femininity that only stilettos can carry. Business enterprise garments will also normally will need a dash of professionalism that heels present.<br><br>There are three main components to search out for to keep away from stiletto based mostly difficulties. The fat of the entire entire body is held on the ball of the foot when carrying a higher heel. The suffering this results in can be aided by two components. A sole and inset will cut down challenges significantly. The toe is a further location that suffers in the course of the putting on of stilettos. The arch is the remaining location that provides ache when there isn't really more than enough support.<br><br>Extra width and depth in the toe place will supply lots of more consolation. This calendar year it's modern to dress in a rounder toe but it needs to exist on a shoe that functions with this. The bulkier toe region demands to be balanced with the relaxation of the shoe, in any other case it seems to be far too useful. A a little bit thicker heel is all that's needed to stability out the glance.<br><br>A shock absorbing inset can prevent a big sum of suffering in the ball of the foot. Orthopedic insets are helpful to give the place excess softness. Large high quality insets that come with the shoe can be sufficient if they're well made. Improperly made insets flatten solely inside as small as a few weeks, rendering them ineffective.<br><br>The sole is the most significant space for shock absorption to avoid soreness at the ball of the foot. Platforms are a huge development this calendar year and will not need to have to be overly thick. As tiny as a centimeter or two's width, if built from perfectly fabricated plastic or gum rubber, can avoid pain in the ball of the foot fully. Synthetic rubber wears down in just months. Given that most persons have their body weight inconsistently on their toes, the sole wears down far more on one aspect. This triggers misalignment of the spine and hips, leading to very long phrase injuries, spasms and discomfort.<br><br>Those with problematic arches are generally ideal off with an orthopedic inset. An espadrille heal will give the arch added aid that helps prevent suffering for individuals with and without flat arches. Espadrilles are coming back again into style future summer time and have been present in a amount of 2013 collections.<br><br>A thick alternatively than stiletto heel can aid the ankles far better to lower imbalance and soreness to that spot. Thick heels are also improved at balancing out thicker calves. No make a difference how nicely picked out at ease superior heels are, they must be worn for brief durations. It really is very best to carry a flat pair of sneakers together, carrying them when ever heels are not a requirement. This will also avert sneakers from becoming damaged all through lengthy intervals of going for walks.<br><br>If you have any kind of concerns with regards to wherever as well as how you can use [http://tinyurl.com/k7shbtq http://tinyurl.com/k7shbtq], you possibly can e mail us on the web page. |
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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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Relaxed large heels that are as trendy as they are useful can be identified. People who have no idea what sides to search for have a tendency to try out substantial versions of types and settle for significantly less than the best. Those who know what to request out will have a shorter, narrower search with bigger achievements.
Stilettos will endlessly be stylish. They are a typical item that is in design every solitary calendar year. Night and cocktail dresses will often need to have the extra top and femininity that only stilettos can carry. Business enterprise garments will also normally will need a dash of professionalism that heels present.
There are three main components to search out for to keep away from stiletto based mostly difficulties. The fat of the entire entire body is held on the ball of the foot when carrying a higher heel. The suffering this results in can be aided by two components. A sole and inset will cut down challenges significantly. The toe is a further location that suffers in the course of the putting on of stilettos. The arch is the remaining location that provides ache when there isn't really more than enough support.
Extra width and depth in the toe place will supply lots of more consolation. This calendar year it's modern to dress in a rounder toe but it needs to exist on a shoe that functions with this. The bulkier toe region demands to be balanced with the relaxation of the shoe, in any other case it seems to be far too useful. A a little bit thicker heel is all that's needed to stability out the glance.
A shock absorbing inset can prevent a big sum of suffering in the ball of the foot. Orthopedic insets are helpful to give the place excess softness. Large high quality insets that come with the shoe can be sufficient if they're well made. Improperly made insets flatten solely inside as small as a few weeks, rendering them ineffective.
The sole is the most significant space for shock absorption to avoid soreness at the ball of the foot. Platforms are a huge development this calendar year and will not need to have to be overly thick. As tiny as a centimeter or two's width, if built from perfectly fabricated plastic or gum rubber, can avoid pain in the ball of the foot fully. Synthetic rubber wears down in just months. Given that most persons have their body weight inconsistently on their toes, the sole wears down far more on one aspect. This triggers misalignment of the spine and hips, leading to very long phrase injuries, spasms and discomfort.
Those with problematic arches are generally ideal off with an orthopedic inset. An espadrille heal will give the arch added aid that helps prevent suffering for individuals with and without flat arches. Espadrilles are coming back again into style future summer time and have been present in a amount of 2013 collections.
A thick alternatively than stiletto heel can aid the ankles far better to lower imbalance and soreness to that spot. Thick heels are also improved at balancing out thicker calves. No make a difference how nicely picked out at ease superior heels are, they must be worn for brief durations. It really is very best to carry a flat pair of sneakers together, carrying them when ever heels are not a requirement. This will also avert sneakers from becoming damaged all through lengthy intervals of going for walks.
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