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Is there a consensus on scientific definition of porous materials? There are many definitions out there which do not seem accurate enough:

from wikipedia (accessed July 2017): A porous medium or a porous material is a material containing pores (voids).

-problem : Then almost all materials are porous, since there is some sort of void in everything.

from Merriam Webster (website accessed July 2017): Containing pores, permeable to fluids, capable of being penetrated.

-problem : A piece of tubing would be porous since fluid passes through it.

At what scale do we consider the pores to qualify a material as porous? How much does the permeability matter in a formal definition? Is there a convention on defining pores and porous materials that is agreed upon by scientific organizations like IUPAC or ASTM etc?

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    $\begingroup$ It all depends on your scale. A cellophane membrane is impermeable to starch but porous to water. Gold appears to be non-porous, but mercury poured on it soaks in, leaving a silvery stain until it disperses throughout the gold. $\endgroup$
    – DrMoishe Pippik
    Commented Jul 19, 2017 at 23:10
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    $\begingroup$ Mercury is dissolving gold. Permeable means can go through generally most of the liquid. $\endgroup$
    – user2617804
    Commented Jul 19, 2017 at 23:58
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    $\begingroup$ "some sort of void" in everything? Not in most substances unless you call the space between electrons and protons a void. Even then this is practically irrelevant as the space isn't accessible in any meaningful way as interactions between atoms are controlled by the electrons. Most solids are not porous in practice. And tubing isn't a material, it is an object and it would be pretty poor tubing if it were porous to the flowing substance across the walls of the tube. $\endgroup$
    – matt_black
    Commented Jul 20, 2017 at 16:27
  • $\begingroup$ @matt_black a lower bound for pore size / void size should be defined as well a definition of material vs object. Many synthesized and modified porous materials are made up of non-porous wall and could be perceived as object. $\endgroup$
    – Kinformationist
    Commented Jul 20, 2017 at 19:17

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I have used the following definition in my presentations:

"Porosity is where a material has accessible voids which are permeable to liquids or gases".

Another definition:

"a solid with pores, i.e. cavities, channels or interstices, which are deeper than they are wide."

IUPAC also defines different types of porosity based on the diameters of the pores:

  • Micropores: d < 2 nm
  • Mesopores: 2 < d < 50 nm
  • Macropores: d > 50 nm

M. Thommes, K. Kaneko, A. V. Neimark, J. P. Olivier, F. Rodriguez-Reinoso, J. Rouquerol, K. S. W. Sing, Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report), Pure Appl. Chem. 2015, 87, 1051.

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  • $\begingroup$ Although this answer is helpful it is directed towards defining porosity not porous materials. The question is rather to distinguish between porous and nonporous material. What level of porosity is agreed upon to qualify a material as porous? 1%, 30%, 80% ? Is there a consensus at all? Is there any informal definition anywhere? $\endgroup$
    – Kinformationist
    Commented Jul 20, 2017 at 0:09

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