Pore distribution and pore size

Porous materials are of paramount importance in various fields of science and industry. From soils and rocks in construction and geology to membranes, catalysts, filters, and even biological tissues in medicine, the properties of these materials play a crucial role in their performance. Among the most important properties of porous materials are porosity and pore size distribution .

Simply put, porosity is the ratio of a material’s pore size to its total volume. However, not all pores are the same. Understanding pore distribution, size, and relationships is therefore essential for the development, selection, and application of these materials.

Determination of porosity

Porosity is usually expressed as a percentage and corresponds to:

$frac{V_v}{V_t}\times 100$

Where:

$V_v$ Volume of the cavity
$_{=}$ $Total volume$ material

Generally, porosity is divided into two main categories:

Open porosity: Open pores on the surface of the material allow liquids to pass through.
Closed-cell property: The pores are enclosed within the material and do not connect to the outer surface.

Pores are classified according to their size.

According to the IUPAC standard, slot sizes are divided into three main categories:

Micropores: diameter less than 2 nanometers.
Medium-sized pores: Their diameter ranges between 2 and 50 nanometers.
Large pores: Their diameter exceeds 50 nanometers.

This classification is very important in the development of porous materials . For example:

Micropores play an important role in the absorption and storage of gases such as hydrogen or methane.
Medium porosity materials play a crucial role in catalysts and membranes.
Large pores are necessary for the permeability and transport of liquids.

Where are the holes located?

The pore size distribution (PSD) characterizes the distribution of pore sizes in a material. In other words, it determines the ratio of pores of different sizes to the total pore size.

This property, along with the overall porosity, plays a crucial role in the physical and mechanical properties of porous materials. For example:

In a filtration membrane, the pore size directly determines which molecules can pass through or be filtered out.
In agricultural soils, the particle size distribution determines the amount of water and oxygen that is retained.
In building materials such as concrete, the particle size distribution can impair mechanical strength and durability .

Methods for measuring hole distribution

There are various methods for determining the pore size distribution (PSD), each of which is applicable to a specific pore size range:

1. Gas absorption/gas removal

It is based on the principle of the absorption of nitrogen or other gases at low temperatures.
The data from the adsorption and absorption equations provide detailed information about microspores and intermediate spores.
The BET and BJH methods are among the best known.

2. Mercury porosity measurement (MIP)

This is done by applying pressure to the mercury, which forces it into the pores of the material .
Very useful for measuring large pores.
Limitations: High pressure can damage the material’s structure.

3. Electron microscope (SEM/TEM)

It provides direct images of the surface and cross-section of the material.
The shape, size, and distribution of the pores can be observed.

4. Computed tomography (CT) X-ray

This is a non-destructive 3D scanning method.
You can perform a network analysis of the pores and their connection methods.

5. Nuclear magnetic resonance and small-angle X-ray scattering methods (SAXS)

In specific cases, it can be used to analyze pores in the subnanometer range.

Factors that influence hole distribution

The composition and distribution of pores in a material depends on various factors, including:

Processes for the manufacture or shaping of materials
- Water-cement ratio in concrete
- Ceramic firing method and heating temperature
- Membrane manufacturing processes (phase reversal process, electrospinning process, etc.)
Chemical composition and crystal structure
- The presence of additives can lead to changes in pore size and homogeneity.
Environmental conditions
- Temperature, pressure, and humidity during manufacturing or use can affect the particle size distribution.

The importance of aperture distribution in various applications

1. Civil Engineering and Geology

In soil, the particle size distribution determines permeability , water storage , and mechanical stability .
In the case of concrete and building materials, there is a direct correlation between the density of the material and its compressive strength as well as its frost resistance.

2. Production of membranes and filters

The choice of membrane used in a separation process (e.g. in desalination plants or medical filters) depends on the accuracy of the particle size distribution.

3. Energy and Environment

In batteries and supercapacitors, micropores can form a large specific surface area, thereby improving energy storage capacity.
For activated carbon and zeolites, the particle size distribution plays a crucial role in the absorption of pollutants .

4. Medicine and Life Sciences

For tissue engineering, structures must have a suitable particle size distribution to enable the exchange of nutrients and oxygen with the cells.
In controlled-release formulations, the pore size determines the release rate of the active ingredient.

Problems and limitations

Despite advances in analytical methods, some limitations still exist when investigating particle size distribution in detail:

Many methods are only accurate and effective within a specific opening range.
Some methods are destructive and the samples cannot be reused.
With heterogeneous materials, it is difficult to determine the actual particle size distribution.

The future of PSD research

The following are some of the new trends:

PSD is predicted using computer-aided methods and molecular modeling .
Development of advanced 3D visualization methods for real-time monitoring of porous networks.
A more accurate analysis can be achieved by combining experimental data and modeling using artificial intelligence and machine learning.

Calgon Carbon Sorbitol

In conclusion

The pore size distribution is a fundamental criterion in materials science and engineering, as it determines how a material interacts with its environment. From permeability and mechanical strength to absorption capacity and mass transport – everything depends on the shape and distribution of the pores. Understanding and precisely controlling this property improves material properties in various industries, increases durability, and opens up new areas of application.