Difference between revisions of "Cohan Mechanism"

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The Cohan mechanism, also known as the Cohan theory of capillary condensation, describes the condensation of liquid in a cylindrical pore. The Cohan mechanism states that on adsorption, pores do not fill vertically, but instead fill radially. This it thought to explain the hysteretic behavior seen in the adsorption-desorption process for porous materials.  
 
The Cohan mechanism, also known as the Cohan theory of capillary condensation, describes the condensation of liquid in a cylindrical pore. The Cohan mechanism states that on adsorption, pores do not fill vertically, but instead fill radially. This it thought to explain the hysteretic behavior seen in the adsorption-desorption process for porous materials.  
  
Figure 1 shows a typical adsorption-desorption isotherm is shown for a porous solid. A hysteresis is evident, indicating that some adsorbate is retained during desorption and released at p/p0 value less than that required to cause adsorption.
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Typically for larger structures, the adsorption and desorption process is understood by the Kelvin equaiton:
  
 
<math>p/p0 = exp (-2\sigma * v_m / rRT) </math>
 
<math>p/p0 = exp (-2\sigma * v_m / rRT) </math>
 +
 +
 +
Figure 1 shows a typical adsorption-desorption isotherm is shown for a porous solid. A hysteresis is evident, indicating that some adsorbate is retained during desorption and released at p/p0 value less than that required to cause adsorption.
 +
 +
When the first layer of liquid is condensed, the radius of the pores is decreased, thus causing further condenstion at a fixed p/p0. Rebalancing the surface area to volume ratios of the new sized pores gives a modified version of the Kelvin equation p_{adsorption}.
 
   
 
   
 
[[Image:Porous.jpg|thumb|300px| Isotherm for a porous solid of uniform pore radius.]]
 
[[Image:Porous.jpg|thumb|300px| Isotherm for a porous solid of uniform pore radius.]]

Revision as of 19:10, 5 October 2009

The Cohan mechanism, also known as the Cohan theory of capillary condensation, describes the condensation of liquid in a cylindrical pore. The Cohan mechanism states that on adsorption, pores do not fill vertically, but instead fill radially. This it thought to explain the hysteretic behavior seen in the adsorption-desorption process for porous materials.

Typically for larger structures, the adsorption and desorption process is understood by the Kelvin equaiton:

<math>p/p0 = exp (-2\sigma * v_m / rRT) </math>


Figure 1 shows a typical adsorption-desorption isotherm is shown for a porous solid. A hysteresis is evident, indicating that some adsorbate is retained during desorption and released at p/p0 value less than that required to cause adsorption.

When the first layer of liquid is condensed, the radius of the pores is decreased, thus causing further condenstion at a fixed p/p0. Rebalancing the surface area to volume ratios of the new sized pores gives a modified version of the Kelvin equation p_{adsorption}.

Isotherm for a porous solid of uniform pore radius.


For a given pore radius r, adsorption with radial capillary condensation occurs at

<math>p_{adsorption} = p_0 exp (-\sigma * v_m / rRT) </math>

whilst for desportion,

<math>p_{desorption} = p_0 exp (-2\sigma * v_m / rRT) </math>

Radial filling of pores during adsorption (left) and desorption (right) as explained by the Cohan Mehanism