Surface free energy with three probe liquids
Susanna Laurén Jun 25, ’24 > 3 min

Surface free energy with Acid-Base method

Surface free energy is a solid property equivalent to a liquid's surface tension. Surface free energy cannot be directly measured but is typically calculated through contact angle measurements. To calculate surface free energy some decisions, need to be made. The most important is the selection of the surface free energy theory applied. Several surface free energy theories are developed that consider solid-liquid interactions in different ways. One of these theories is called the Acid-Base theory or Van Oss-Chaudhury-Good method, according to its inventors.

What is surface free energy?

Figure 1Surface free energy is the excess energy at the surface of a material compared to its bulk.This energy originates from the imbalance of molecular forces experienced by molecules at the surface, as similar molecules on all sides do not surround them. This phenomenon is analogous to surface tension in liquids but extends to solids and interfaces between different phases.

Acid-base theory of surface free energy

Surface free energy is typically divided into different interactions; polar and dispersive. In Acid-Base theory, the polar interaction is further divided into acid and base interactions.  

After the introduction of the definition of the polar component into electron-donor (base) and electron-acceptor (acid), it soon became clear that many organic polymers, especially many polar biopolymers are mainly or only electron-donors, or to a lesser extent only electron-acceptors. The surfaces with only either electron-donor or electron-acceptor properties are called monopolar. Such monopolar surfaces have many unexpected properties that may give explanations to some well-known but less completely understood colloid and surface phenomena

Measurement in practice

Surface free energy cannot be directly measured but it is calculated through contact angle measurements. When acid and base theory is applied, there are three unknown components; dispersive, acid, and base components of the surface free energy. To solve these, contact angle measurements with three known probe liquids need to be made. The selection of the probe liquids is important. One of them should be non-polar and two bipolar. Most typically di-iodomethane is used as a non-polar liquid. Polar liquid can be water, ethylene glycol, or glycerol.

As three liquids need to be used, using three disposable tip dispensers together with an automated sample stage makes the measurements fast and fully automated.

If you would like to learn more about surface free energy measurement methods, please download the white paper through the link below.

White paper

Learn more about surface free energy


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