OECD TG 115 is the standard titled “Surface Tension and Interfacial Tension of Aqueous Solutions, ” which is included in Section 1 (Physicochemical Properties) of the OECD Test Guidelines for the Testing of Chemicals.
Surface tension is defined as the force per unit length acting on the surface of a liquid in contact with air and is typically expressed in mN/m. Interfacial tension, on the other hand, is the tension that arises at the interface between two immiscible liquids. OECD TG 115 is specifically intended to determine the surface tension (at the air interface) of aqueous solutions, as well as the interfacial tension between water and a liquid that is immiscible with water.
The basic principle of the test involves measuring the surface or interfacial tension of an aqueous solution of the test substance at a specific temperature. For surfactants, the tension decreases as the concentration increases and stabilizes in the range of the critical micelle concentration (CMC). Therefore, the measurements are typically conducted as a concentration series.
OECD TG 115 does not prescribe a specific method but allows for the use of appropriate and validated methods. Common methods include:
Ring Method (du Noüy Ring):
- A platinum-iridium ring is pulled upward from the surface of the liquid, and the pull-off force is measured
- Frequently used in the measurement of surface and interfacial tension
- Correction factors are applied based on the ring geometry and wetting conditions
Plate method (Wilhelmy plate):
- A glass or platinum plate is immersed in the liquid, and the force acting on it is measured
- Provides high accuracy and repeatability in surface tension measurements
- The contact angle and the cleanliness of the plate are of critical importance
Drop Volume/Drop Weight Method:
- The volume or weight of a drop falling from a capillary is measured
- Surface and interfacial tension are calculated
- Requires careful use with viscous or surfactant-containing solutions
Pendent-drop and spinning-drop methods:
- The tension is determined by optical analysis of the droplet profile
- The spinning-drop method is particularly well-suited for very low interfacial tensions
Other methods:
- Other validated techniques that have been calibrated using appropriate reference substances
The basic parameters reported in the study are:
- Surface tension (mN/m) and/or interfacial tension (mN/m)
- Test concentration and concentration series
- Measured Temperature (°C)
- Information on the method and equipment used
- Non-miscible liquid used for interfacial tension (e.g., n-hexane, cyclohexane)
- If applicable, CMC value and pH value
- Data on Measurement Uncertainty and Repeatability
Why is the test performed?
OECD TG 115 is being conducted for the following scientific and regulatory purposes:
- Characterization of Surface Activity: Determination of the test substance's potential to reduce surface tension and its surface-active behavior
- CMC determination: Determination of the critical micelle concentration of surfactants; this value is important for formulation and environmental behavior
- Prediction of Environmental Behavior: Surface tension provides insights into the substance's film-forming, distribution, and bioavailability behavior on the water's surface
- Design of Ecotoxicity Tests: Provides information for selecting test concentrations and dosages for surfactants
- Formulation and Product Performance: Evaluation of Properties as a Detergent, Emulsifier, Wetting Agent, and Dispersant
- Classification and Labeling: Surfactant properties are used in some regulatory frameworks to classify the substance
- Quality Control: Monitoring the consistency of raw materials, intermediate products, and finished products in production processes
- Weighting Approach: A Contribution to the Holistic Assessment of the Substance in Conjunction with Other Physicochemical, Toxicological, and Environmental Data
OECD Test Guideline 115 is not a standalone toxicity or hazard assessment.
It provides a physicochemical characterization; the results should be interpreted in conjunction with other physicochemical properties, toxicological data, and an exposure assessment.
As part of our nano-testing services, we determine surface and interfacial tension in accordance with OECD TG 115, applying scientific rigor based on our accredited laboratory infrastructure and our quality management system. Our scope of services includes the following:
Method Selection and Validation
Selection of the appropriate method (ring, plate, drop volume, hanging/rotating drop) based on the properties of the test substance (surface activity, expected stress range, viscosity) and regulatory requirements
Validation of the selected method and verification using appropriate reference substances (e.g., pure water, reference solutions for surfactants); evaluation and reporting of measurement uncertainty; documentation of method-specific limitations and conditions of use
For information on method selection, the design of concentration series, sample acceptance criteria, the test plan, and reporting procedures for your surface and interfacial tension measurements in accordance with OECD TG 115, please feel free to contact Nano-Test.