ramé-hart Newsletter

                   

August 2025
 

Owens-Wendt (OWRK) Method

The OWRK method posits that the total surface energy of a solid can be additively separated into two components: a dispersive component and a polar component

• Dispersive Component: Represents the contribution from London dispersion forces (van der Waals forces), which are universally present in all materials.
• Polar Component: Encompasses all other types of molecular interactions, including dipole-dipole interactions, hydrogen bonding, and inductive forces.

The OWRK method typically uses the geometric mean for combining rules to describe the interactions between the solid and probe liquid. While the geometric mean method is most common, the harmonic mean method (Wu method) is optimized for low-energy polymers.

Both the OWRK and Wu methods require contact angle measurements with at least two different probe liquids with known dispersive and polar components (e.g., a polar liquid like water and a non-polar liquid like diiodomethane). In both DROPimage Pro and Advanced, the Tool is the same but the user can choose "geometric" for the OWRK method or "harmonic" for the WU method.¹

Pros of Owens-Wendt:

• Simplicity and Wide Acceptance: It's conceptually straightforward and mathematically less complex than the Acid-Base method, making it widely adopted for routine analysis.
• Good for General Wettability: Provides a good general understanding of a surface's wettability by separating universal van der Waals interactions from more specific polar interactions.
• Sufficient for Many Engineering Applications: Often sufficient for assessing surface treatments, coating adhesion on many polymers, and general material characterization where a detailed understanding of specific acid-base interactions is not critical.

Cons of Owens-Wendt:

• Limited Detail of Polar Interactions: The primary drawback is that it lumps all non-dispersive interactions into a single "polar" component. It does not differentiate between electron-donating (basic) and electron-accepting (acidic) characteristics of a surface.
• Potential for Misleading Results: For surfaces where specific acid-base interactions are dominant (e.g., biological materials, certain minerals, or some highly functionalized polymers), the single polar component can be an oversimplification and may not accurately predict adhesion or other interfacial phenomena.
• Can Yield Negative Components: In some cases, particularly with certain liquid combinations or highly unusual surfaces, the calculation can theoretically result in negative polar components, which lack physical meaning.

ramé-hart Model 260 Contact Angle Goniometer / Tensiometer with DROPimage Advanced Software (p/n 260-U4)

Acid-Base (van Oss-Chaudhury-Good or vOCG) Method

The Acid-Base method provides a more detailed breakdown of the surface energy. It divides the total surface energy into three components:

• Lifshtiz-van der Waals Components: Equivalent to the dispersive component in OWRK, accounting for non-polar interactions (London dispersion, Keesom, Debye forces) but also requires more work as three liquids are necessary.
• Electron-Acceptor (Acidic) Component: Represents the ability of the surface to accept electrons (act as a Lewis acid).
• Electron-Donor (Basic) Component: Represents the ability of the surface to donate electrons (act as a Lewis base).

The Acid-Base method uses a different combining rule for the acid-base interactions and typically requires contact angle measurements with at least three probe liquids: one non-polar liquid (to determine the LW component) and two polar liquids, one predominantly acidic and one predominantly basic, with known components.

Pros of Acid-Base:

• Detailed Interaction Insight: Provides a more comprehensive understanding of surface chemistry by resolving specific acid-base interactions. This is important for systems where hydrogen bonding and electron transfer play key roles.
• Better for Biological and Complex Systems: Highly valuable in fields like biomaterials, pharmaceuticals, and colloid science, where specific acid-base interactions govern adhesion, protein adsorption, and cell-surface interactions.
• Improved Predictive Power: Often offers better predictive power for adhesion, biocompatibility, and wetting behavior in systems dominated by polar and acid-base interactions.
• Avoids Negative Components: Generally less prone to yielding non-physical negative components compared to OWRK, as it explicitly accounts for the electron-donor and electron-acceptor nature.

Cons of Acid-Base:

• Increased Complexity: The theoretical framework and calculations are more complex than OWRK, requiring more probe liquids and a deeper understanding of the method.
• Dependency on Liquid Selection: The accuracy of the results heavily relies on the precise characterization of the probe liquids' acid-base components, which can sometimes be debated or less precisely known.
• Interpretation Challenges: Interpreting the individualalues and their implications requires more specialized knowledge.

Conclusion:

The choice between Owens-Wendt and the Acid-Base method depends on the specific research question and the nature of the materials being investigated.

• Use Owens-Wendt for a general assessment of wettability, particularly for moderately polar or non-polar surfaces, and when a simpler, quicker analysis is preferred. It's often sufficient for many industrial quality control and process optimization tasks.
• Opt for the Acid-Base method when a detailed understanding of specific acid-base interactions, hydrogen bonding, and electron transfer capabilities of the surface is critical, especially in fields like biomaterials, adhesion science, and pharmaceuticals, where these interactions dictate material performance.

Notes
¹ Both the OWRK and Wu methods are integrated in the Two-Liquid Surface Energy Tool in ramé-hart DROPimage Pro and DROPimage Advanced software packages.

Interested in a Model 260? Feel free to contact us today for a no-obligation quotation for this tool or any other ramé-hart instrument model.

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