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. | ||
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| + | Typically for larger structures, the adsorption and desorption process is understood by the Kelvin equaiton: | ||
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| + | <math>p/p0 = exp (-2\sigma * v_m / rRT) </math> | ||
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| + | 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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| + | 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}. | ||
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| + | [[Image:Porous.jpg|thumb|300px|Figure 1 - Isotherm for a porous solid of uniform pore radius.]] | ||
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<math>p_{desorption} = p_0 exp (-2\sigma * v_m / rRT) </math> | <math>p_{desorption} = p_0 exp (-2\sigma * v_m / rRT) </math> | ||
| − | [[Image:Cohan_Mechanism.jpg|thumb|300px| Radial filling of pores during adsorption (left) and desorption (right) as explained by the Cohan Mehanism]] | + | [[Image:Cohan_Mechanism.jpg|thumb|300px|Figure 2 - Radial filling of pores during adsorption (left) and desorption (right) as explained by the Cohan Mehanism]] |
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| + | ==References== | ||
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| + | [1] Cohan, L.H., J. Am. Chem. Soc. 66, 98 (1944) | ||
| + | [2] "Principles of adsorption and adsorption processes" D. M. Ruthven | ||
Latest revision as of 21:31, 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}.
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>
References
[1] Cohan, L.H., J. Am. Chem. Soc. 66, 98 (1944) [2] "Principles of adsorption and adsorption processes" D. M. Ruthven
