How to measure the materials porosity

The properties of a material can be strongly influenced by the presence of porosity in its internal structure or on its surface, ultimately conditioning its application. One of the most significant examples can be given in drug delivery systems, where the porosity of the vector has a decisive influence on its loading capacity, almost completely defining its final application. Another field where porosity plays a decisive role is in construction, since the porosity of the material will define its resistance under adverse conditions. In today’s blog we tell you all the secrets of porosity and how to analyze it. Do not miss it!

What is the porosity of a material?

Porosity is the volumetric fraction of pores in the material. These pores can be located on its surface or in its internal structure. Porosity is associated with the density of the material, and with the nature of its compounds and the existence of empty spaces between them.

Types of porosity

The pores have different properties between them. The most important are its shape and size, its location, its connectivity, and its surface-related chemical properties.

• Size: The main property that defines a pore is its size, that is, its spatial dimension. Therefore, due to its easy analysis, pore size is often the main tool for characterizing a porous material. The pore size has a great influence on the properties of a porous material and, as a consequence, on its final applicability. The larger the pore size, the larger particles can pass through them, increasing the reactivity of the material. The pore size of a material is often referred to as its pore distribution, and according to the IUPAC (International Union of Pure and Applied Chemistry) it can be classified as follows:

Micropores: they are less than 2 nanometers in size.

Mesopores: they have a pore size between 2 and 50 nanometers.

Macropores: they have a size greater than 50 nanometers.

• Location: pores can be found on the surface of the material or in its internal structure. This property is strongly linked to the property of connectivity, since there are porous materials whose pore distribution is isolated, while there are other materials that have pores connected to form a more or less tortuous framework. The connectivity can be partial, since it only occurs between the pores of the internal structure of the material, or complete, where the pores of the internal structure are connected to the pores of the surface.

• Chemical properties: they imply the reactivity that the material can offer in different environmental conditions. If the pore distribution is interconnected, the material can show strong degradation under adverse conditions. However, if the pores are isolated, the materials may experience point degradation or even not degrade.

Influence of the material porosity

The porosity of a material is decisive when evaluating its durability and resistance to adverse conditions. The distribution of pores and their characteristics defines the permeability of the material, that is, its ability to store fluids, thus conditioning its physical and chemical properties.

To understand how porosity can influence a material, we are going to study two specific cases, where we will see that porosity can provide advantages and disadvantages to our material.

One of the most representative cases where porosity contributes positive factors to the applicability of a material is the case of controlled drug release systems. In this case, the goal of developing controlled drug release systems is to be able to control the release of a drug over time. When the delivery vector is porous, the loading capacity is greater than that of a non-porous vector, since the porous internal structure presents accessible deposits for the drug. Therefore, in this case, porosity offers the advantage of increasing the loading capacity of the vector, thus allowing a more sustained release of the drug over time.

A totally opposite case to the previous one, where porosity appears on the scene as a material defect, is in the case of metals. When a metal has pores on its surface and is in corrosive conditions, there can be superficial and internal corrosion of the metal, leading to pitting or the release of ions from the metal. On many occasions, metals susceptible to corrosion in working conditions are protected with layers of noble metals that serve as a barrier against adverse environmental conditions. However, many cases have been found where the barrier that the noble metal provides is ineffective. This is due to the fact that the porosity of this protective barrier has allowed the entry of corrosive agents and their contact with the metal that we want to protect, resulting in its corrosion in the form of surface defects.

Techniques for analyzing porosity

As we have seen, the parameters that characterize the porosity of a material are related to its structural properties. Therefore, the analysis techniques that allow us to evaluate the porosity of a material are those that allow us to study its surface, the layer distribution of its structure or the permeability against different elements. Next, we will see the most important techniques:

Microscopy techniques: they offer us information on the surface of materials with high resolution, being able to distinguish surface details of the order of nanometers. The most used are:

• SEM and TEM: both techniques allow us to observe the surface of a sample through the interaction of an electron beam. In addition, they allow us to perform the analysis of the composition of the surfaces.
• FIB-SEM: this technique is very useful, since the ion beam generates a precise opening on the surface of the sample, allowing us to observe the distribution of layers inside it using SEM.
• Confocal microscopy, profilometer and AFM: these techniques are considered one of the most useful for porosity analysis since they allow obtaining information about the topography and surface roughness, allowing us to obtain profiles of nanometric areas of the material’s surface.

Techniques of physical adsorption of a gas: A gas is injected into the sample (generally nitrogen or carbon dioxide) at a constant temperature, determining by gravimetric or volumetric methods, the adsorption isotherm, that is, the amount of gas that has been capable of adsorbing material. By analyzing the adsorption isotherm of a material, its surface area and pore volume and size distribution can be determined. Depending on the gas used, information on pores of different sizes can be obtained. When using nitrogen, pores from 3.5 to 400 nanometers can be determined, while the use of carbon dioxide gives us information on microporosity.

Mercury porosimetry: this technique is based on the intrusion of mercury into the porous structure of a material through the application of isostatic pressure. The technique is based on the Washburn equation. This equation relates the pressure applied to the diameter of the pore into which the mercury is introduced. Mercury porosimetry provides information on pores with a size of 900 microns up to 4 nanometers in diameter.

Helium pycnometry. Helium pycnometry is a technique for determining the specific gravity or bulk density of a material through volume displacement. In this technique, the helium gas is expanded to a cell of known volume, occupied by the sample to be analyzed within that cell, determining the apparent density of the material. By this measure we can know the porosity of the material.

Porosity analysis parameters

In general, the characterization of porosity usually occurs in relation to the following parameters, all of them in accordance with its structural character:

• Statistical distribution of pore size. It is usually given in average value and is called effective pore.
• Surface density of the pores, that is, the number of pores per unit area
• Porosity by volume, which refers to the fraction of the total volume of the material occupied by pores or voids
• Morphology of surface pores and roughness profile.
• Tortuosity, in the case of materials with interconnected pores, where the morphological differences between them are evaluated.

Examples of porous materials

Next, we show you some materials with a high degree of porosity to which this property is usually measured, which does not imply that any material can be measured if it is a property that is necessary to know:

• Minerals, rocks and other porous geological materials. These materials are an example of the consequence of a porous structure in a material in the presence of adverse conditions over time.

• Biological materials. One of the most representative examples is bones, in which the internal porosity is very high. As a consequence, bones are lightweight materials. Another very common case is the skin, whose porosity is related to its permeability.

• Porous polymers. New porous polymers are currently being developed because pores offer the possibility of storing different types of molecules depending on the type of cavity. Such an application may have future potential in the fields of chemical catalysis or molecule exchange.
• Food. The porosity of a food is directly related to its storage and maintenance conditions.

What do you think of our blog on porosity? Do you need to analyze your material because you think it is porous and may be affecting its quality? Do you need a porous material for your application? Contact us!