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Silica gel packet. Image Credit: Shutterstock.com/FrameAngel

Silica Gel vs. Molecular Sieve: What Are the Differences?

Xomety X
By Team Xometry
January 31, 2024
 8 min read
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Silica gel and molecular sieves are two popular desiccants for absorbing moisture in a variety of sectors. Although their main purpose is to dry materials, their compositions and uses are very different. As a result of its absorbent qualities, silica gel — which is made up of amorphous silica particles with uneven shapes — is a versatile and often used material. 

On the other hand, crystalline aluminosilicates with homogeneous pores in a molecular sieve demonstrate molecular, size-dependent selective adsorption. The choice between them is based on the particular need for moisture control. 

This article will examine the features, uses, and relative benefits of silica gel and molecular sieves. 

What Is Silica Gel?

Silica gel is a desiccant made from silicon dioxide, a compound found in sand. Comprising small particles, it efficiently absorbs large amounts of water. Employed in products like clothes, electronics, medications, and food packaging, silica gel prevents damage from humidity. Manufacturers caution against ingestion due to reported cases of accidental consumption, particularly involving young children.

What Is the Purpose of a Silica Gel?

Silica gel is essential for maintaining dry conditions and protecting against damage caused by humidity. It has effective moisture-absorbing qualities that help maintain the longevity and quality of various products. It is widely used in packaging, electronics, and other industries. 

How Does a Silica Gel Work?

Water molecules stick to the surface of silica gel by a process known as adsorption. Effective moisture absorption is made possible by the structure's high porosity, which has many voids that can hold water. As liquid water forms bridges within the particles' cavities, the molecules tightly grip both each other and the silica gel. This process fills the internal structure of each pellet, allowing silica gel to maintain a dry feel even when saturated.

What Does a Silica Gel Look Like?

Silica gel typically appears as small, granular, or pellet-like beads. It is available in three main colors: white, blue (indicating dryness), and orange (indicating moisture absorption). Figure 1 below shows a silica gel enclosed in white paper packaging:

Slide 1 of 1
silica gel
silica gel
silica gel

Silica gel in white paper packaging.

Image Credit: Shutterstock.com/jakkrit pimpru

What Are the Benefits of Silica Gel?

Some of the benefits of silica gel are:

  1. It effectively removes moisture, preventing damage to goods and hindering mold and odor development.
  2. Resistant to combustion, making it a safe choice for applications in which fire safety is a concern.
  3. Exhibits a high moisture absorption capacity, efficiently absorbing up to 40% of its weight in water.
  4. Can be reactivated by heating, extending its adsorption capabilities and ensuring cost-effective, long-term use.
  5. Is chemically inert, meaning it does not corrode or react with other materials, making it suitable for use with a variety of items.
  6. Is versatile, as it comes in different packaging sizes, allowing for convenient use in a wide range of applications.

What Are the Downsides of Silica Gel?

The downsides of silica gel include:

  1. Exhibits low water exchange due to the high relative pressure required for adsorption, limiting its efficiency.
  2. Silica gel's storage capacity is restricted, necessitating a large quantity for effective long-term water storage.
  3. Silica gel packets may be mistaken for food sachets, such as those that contain salt or sugar, leading to potential ingestion. While generally non-toxic, packets coated in cobalt chloride can pose health risks.
  4. Many silica-based materials face stability issues at temperatures exceeding 60 °C, impacting their overall performance.

What Is Molecular Sieve?

Molecular sieves are synthetic zeolite materials having homogeneous, well-engineered pores. Based on the size and polarity of the molecules, these holes preferentially adsorb certain gases and liquids. They are crystalline solids that are extremely porous and members of the aluminosilicate chemical class. The chemical formulas of the four primary types — 3A, 4A, 5A, and 13X — variate and dictate pore size. Molecular sieves adsorb smaller molecules to filter out larger molecules. Molecular sieves are an essential tool for sorting substances at the molecular level in a variety of industries, with a wide range of uses.

What Is the Purpose of a Molecular Sieve?

Firstly, molecular sieves effectively dry gases and liquids by selectively adsorbing moisture based on molecular size. Secondly, they serve as powerful tools for separating molecules, leveraging their precise and uniform pore structures to exclude or retain substances according to their sizes and shapes. This makes them crucial in various industries for processes requiring precise control over moisture levels and molecular separation.

How Does a Molecular Sieve Desiccant Work?

Molecular sieve desiccants work by taking advantage of the well-designed pores in them. When molecules are similarly tiny and fit through these holes, molecular polarity plays an important role in the separation process. Specific moisture removal is possible due to the more polar molecule's preferred adsorption.

What Does a Molecular Sieve Look Like?

Molecular sieves typically appear as small, granular beads or pellets. They may vary in color depending on the specific type and composition. Figure 2 below shows molecular sieve beads:

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molecular sieve beads
molecular sieve beads
molecular sieve beads

Molecular sieve beads.

Image Credit: Shutterstock.com/MayPali

What Are the Benefits of Molecular Sieve?

Some benefits of molecular sieves are:

  1. Widely used for removing water and molecules from gas streams, purifying solvents, and eliminating impurities from liquids.
  2. Exhibit rapid water adsorption, surpassing other desiccant air dryers and reducing humidity levels more effectively than standard silica gel.
  3. Swift moisture removal enables reduced energy costs, as drying equipment doesn't need to operate for extended periods.
  4. Are eco-friendly, produce no hazardous waste, and are recyclable or reusable, making them a sustainable choice for companies aiming to minimize environmental impact.
  5. Outperforming desiccants such as silica gel in applications exceeding normal room temperature, molecular sieves maintain their effectiveness in diverse environmental conditions.

What Are the Downsides of Molecular Sieve?

Some downsides of molecular sieve are:

  1. Small plants may experience high attrition rates, requiring periodic replacement of sieve material and leading to elevated production costs.
  2. Are selective, making their applicability inconsistent after removing various molecules.
  3. Ingestion, inhalation, or contact with eyes or mucous membranes may cause irritation or thermal burns. Exposure can aggravate asthma and chronic respiratory conditions.
  4. Can break, potentially releasing particles in the process, posing risks in certain applications.

What To Choose Between Silica Gel and Molecular Sieve?

The unique needs and application requirements determine which of the two options — silica gel or molecular sieve — should be used. A flexible and affordable material, silica gel can be used in a variety of industries. Molecular sieves, on the other hand, provide exact moisture removal and molecule size-based selective adsorption. Take into account elements like: the substances involved, the surrounding circumstances, and the ideal moisture levels. For general applications, silica gel might be a better option, but when specific moisture control and molecule separation are needed, molecular sieves work well.

What Are the Properties of Silica Gel and Molecular Sieve?

Table 1 below highlights a comparison of properties between silica gel and molecular sieve:

Table 1: Comparison of Properties Between Silica Gel and Molecular Sieve
PropertySilica GelMolecular Sieve
Property
Composition
Silica Gel
Silicon dioxide particles
Molecular Sieve
Synthetic zeolite materials or crystalline aluminosilicates
Property
Structure
Silica Gel
Irregularly shaped amorphous particles
Molecular Sieve
Precisely engineered pores with uniform size
Property
Application
Silica Gel
Versatile; used in various industries
Molecular Sieve
Selective adsorption, molecular separation
Property
Regenerable by heating
Silica Gel
Regenerable by heating
Molecular Sieve
Regenerable; can be reused or recycled
Property
Moisture Capture Rate
Silica Gel
Good
Molecular Sieve
Excellent
Property
Low Relative Humidity (RH) Capacity
Silica Gel
Poor
Molecular Sieve
Excellent

What Are the Uses of Silica Gel and Molecular Sieve?

As desiccants, silica gel and molecular sieves protect medications, nutraceuticals, and medical equipment from damage caused by moisture by controlling relative humidity. Because of their strong attraction to water molecules, these materials are excellent at absorbing and holding onto moisture to keep an environment under control. Moisture control is essential in businesses in which humidity sensitivity might jeopardize the integrity of stored commodities. It helps to maintain the stability and efficacy of sensitive products, assuring their quality, and extending shelf life.

To learn more, see our full guide on the Uses of Silica Gel.

Does Silica Gel Absorb Water?

Yes, silica gel does absorb water. It can adsorb approximately 40% of its weight in moisture, effectively lowering the relative humidity in a suitably sized, closed container to around 40%. This makes silica gel a highly efficient desiccant for maintaining optimal moisture levels in various applications.

How Long Do Silica Gel and Molecular Sieve Last?

Adsorbent desiccants, such as silica gel and molecular sieve, have a three- to five-year shelf life in dryers exposed to air free of contaminants. It becomes important to ensure the inlet filtration is done correctly because impurities can undermine their effectiveness. These desiccants are protected by a strong filtering system, which prolongs their life and maintains their maximum capacity to absorb moisture. This is important for procedures in the pharmaceutical and industrial industries in which moisture control is critical.

Can Bacteria Grow in Silica Gel and Molecular Sieve?

No, bacteria generally do not thrive in silica gel and molecular sieves. Tests on Escherichia coli in silica gel revealed decreased metabolic activity and viability after one month, making these desiccants unfavorable for bacterial growth. Research on gram-negative bacteria, like Salmonella, highlighted the cell wall acting as a molecular sieve, restricting the diffusion of saccharides. These characteristics demonstrate the inhospitable environment these desiccants provide against bacterial proliferation.

Summary

This article presented silica gel and molecular sieve, explained each of them, and discussed their key differences. To learn more about both silica gel and molecular sieve, contact a Xometry representative.

Xometry provides a wide range of manufacturing capabilities and other value-added services for all of your prototyping and production needs. Visit our website to learn more or to request a free, no-obligation quote.

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Xomety X
Team Xometry
This article was written by various Xometry contributors. Xometry is a leading resource on manufacturing with CNC machining, sheet metal fabrication, 3D printing, injection molding, urethane casting, and more.