Less known facts about Excipients: Bentonite

This article is part of the “Less Known Facts About Excipients” series by Philippe Tschopp.

Bentonite: A natural adsorbent and stabilizer

Bentonite is one of the few pharmaceutical excipients that originated from volcanic ash. Formed over millions of years through geological processes, this naturally occurring clay has unique swelling and adsorption properties that continue to make it valuable in pharmaceutical formulations.

Unlike many mineral excipients that are used primarily as fillers, bentonite interacts strongly with water, forming highly viscous dispersions and stable suspensions. These properties have made it an important excipient for topical, oral and pharmaceutical processing applications.

Bentonite is a naturally occurring clay composed predominantly of the mineral montmorillonite, a layered aluminium silicate. It is formed when volcanic ash undergoes weathering and hydrothermal alteration over geological timescales.¹–³

How is pharmaceutical bentonite produced?

The production process consists of several carefully controlled steps:

1. Mining High-purity bentonite deposits are selectively mined, typically from ancient volcanic formations.
2. Crushing and Drying The raw clay is crushed and dried to reduce moisture while preserving its layered mineral structure.
3. Purification Undesirable minerals such as quartz, feldspar and other naturally occurring impurities are removed using physical separation techniques.
4. Milling and Classification The purified bentonite is milled and classified to achieve the particle size and quality required for pharmaceutical applications before complying with pharmacopeial specifications.¹–⁵

What makes bentonite unique?

Bentonite owes its remarkable functionality to the layered structure of montmorillonite.

When dispersed in water, the layers hydrate and separate, allowing the material to absorb large amounts of water and swell many times its original volume.

Its most valuable pharmaceutical characteristics include:

• Exceptional swelling capacity
• High adsorption capacity
• Excellent suspension stability
• Thixotropic behaviour
• High specific surface area
• Cation exchange capacity

One particularly important property is thixotropy: bentonite dispersions become less viscous when stirred or shaken but rapidly recover their viscosity once at rest. This behaviour helps maintain stable pharmaceutical suspensions.

Pharmaceutical applications

Pharmaceutical-grade bentonite is used in:

• Oral suspensions
• Topical creams
• Ointments
• Gels
• Dermatological preparations
• Pharmaceutical processing
• Clarification and purification processes¹,²

Its primary functions include:

• Suspending agent
• Viscosity modifier
• Stabilizer
• Adsorbent
• Thickening agent

Considerations for formulators

Because bentonite is a naturally occurring mineral, pharmaceutical manufacturers carefully select deposits with consistent mineral composition and low levels of impurities. Purification and quality control are essential to ensure compliance with pharmacopeial requirements, particularly regarding heavy metals and microbiological quality.

Formulators should also consider bentonite’s strong adsorption capacity, as it may interact with certain active pharmaceutical ingredients and influence drug availability if not properly evaluated during formulation development.¹–⁴

Major pharmaceutical manufacturers

Examples of established manufacturers and suppliers include:

• Clariant
• LKAB Minerals
• Ashapura Group
• Imerys
• Laviosa Minerals S.p.A.

Did you know?

The name “bentonite” originates from Fort Benton, Wyoming (USA), where one of the first commercially significant deposits was described in the late 19th century.

Some bentonite deposits used today were formed from volcanic eruptions that occurred tens of millions of years ago, making bentonite one of the oldest naturally occurring pharmaceutical excipients in geological terms.

By Philippe Tschopp
Philippe Tschopp writes about pharmaceutical excipients, formulation and oral drug delivery, with a focus on excipient functionality, practical formulation considerations and lesser-known material properties.

Source:

1. Rowe RC, Sheskey PJ, Quinn ME (Eds.). Handbook of Pharmaceutical Excipients. Pharmaceutical Press / American Pharmacists Association.
2. European Pharmacopoeia (Ph. Eur.). Monograph: Bentonite.
3. United States Pharmacopeia–National Formulary (USP–NF). Monograph: Bentonite.
4. Clariant. Pharmaceutical-grade bentonite technical documentation.
5. Imerys. Pharmaceutical Mineral Solutions and bentonite product information.
6. Murray HH. Applied Clay Mineralogy: Occurrences, Processing and Application of Kaolins, Bentonites, Palygorskite-Sepiolite, and Common Clays.
7. Bergaya F, Lagaly G (Eds.). Handbook of Clay Science.

Disclaimer

This article is based on publicly available information and industry sources at the time of writing and supported by AI. While every effort has been made to ensure accuracy, completeness, and fair representation, the author does not guarantee that all information is current, error-free, or reflective of the latest developments.

The content is provided for informational and educational purposes only and does not constitute technical, regulatory, or commercial advice. No liability is accepted for decisions made based on this material. Readers are encouraged to consult the original manufacturers and official sources to verify details before making professional or business decisions.

Read also the other article from the series “Less known facts about…” here:

• Calcium Carbonate – Less known facts about: Calcium Carbonate
• Tragacanth – Less known facts about: Calcium Carbonate
• Magnesium Stearate – Less known facts about Excipients: Magnesium Stearate
• Xanthan Gum – Less known facts about Excipients: Xanthan Gum