Wetting agents
Wetting agents are substances used in aqueous solutions and mixtures in order to improve wetting and/or penetration of solid materials. They are surfactants, i.e., substances that reduce the surface tension of water. Wetting agents occur in a wide variety of industrial products and processes and also naturally.
Why are wetting agents needed?
Wetting agents are used wherever a liquid is intended to penetrate, adhere, or spread rather than bead up. They are an important component of formulations, particularly when aqueous liquids come into contact with plastics and other materials that are difficult to wet due to their low surface free energy. Examples include inks and coatings, as well as pesticides.
How do wetting agents work?
Water has a high surface tension and therefore tends to form droplets rather than creating a common interface with other materials. The surfactants used as wetting agents reduce the surface tension of water and, as a result, also the interfacial tension when in contact with solid material. In this way, wetting agents ensure better spreading on the surface and more effective penetration of porous materials.
Where are wetting agents used?
In countless technical processes, it is necessary to reduce the surface tension of aqueous solutions to improve the wetting and penetration of solids. Often, the surfactants which serve as wetting agents fulfill more than one function.
Paints, varnishes & other coatings
In liquid formulations for printing, painting, and coating processes, wetting agents improve spreading and, consequently, final adhesion to the substrate. When color pigments and other powders are present, these same surfactants also act as dispersants to improve distribution within the liquid and prevent clumping.
Plant protection
Many crops are naturally difficult to wet in order to defend against mold and fungal infestation. Pesticide formulations, on the other hand, must spread across the leaves to ensure protection and to minimize the amount of active ingredient that enters the soil. Wetting agents therefore play an important role in agricultural chemistry.
Firefighting
So-called wet water or wetted water is often used for fire suppression. Even in low concentrations, the surfactants this water contains improve the wetting and penetration of combustible materials. In this way, wetting agents enhance the extinguishing and cooling effects and prevent the material from reigniting. The active ingredient reduces the required amount of water by up to 50%, thereby limiting secondary damage caused by the extinguishing water.
Electroplating
In electroplating, wetting agents prevent air pockets and the adhesion of gas bubbles that may form during the chemical process. At the same time, they clean the materials and mobilize adhering oils and fats by incorporating them into micelles.
Textile industry
Wetting agents are used in textile dyeing to ensure rapid and complete wetting of the fabric. These should be distinguished from leveling agents, which control and slow down the dyeing process. They either adsorb directly onto the fiber or bind to the dye, thereby ensuring an even result and preventing localized over-dyeing. As a rule, both auxiliaries are surfactants, and often the same substance is used for both purposes.
Cleaning and washing
Surfactants used as detergents combine a wetting effect with the ability to bind hydrophobic particles and liquids and mobilize them in the rinse solution or wash water.
How is the effectiveness of wetting agents measured?
The effectiveness of a wetting agent and the wetting behavior of liquid formulations can be examined using various methods of interfacial analysis.
Force tensiometry: measurement of surface tension
Since the function of a surfactant is to reduce surface tension, the SFT of a formulation is the most important criterion. Force tensiometers offer various measurement methods for this purpose, such as the Wilhelmy plate method.
The lower the minimum surface tension value of an aqueous solution of the active ingredient, the more effective the wetting agent is. How efficient a wetting agent is depends on the concentration at which the minimum surface tension value is reached. This surfactant concentration is referred to as the critical micelle concentration (CMC) and can be automatically determined using concentration series.
Contact angle measurement: Characterization of wetting
The contact angle describes the “roundness” of a dispensed droplet on a surface and is a direct measure of wettability by a liquid. The corresponding measurement method is called drop shape analysis. On the one hand, it enables direct measurements of liquid formulations on the solid. On the other hand, the material can be characterized using standard test liquids to create a wettability profile based on surface free energy. This value provides a guideline for the targeted production of optimally wetting liquid formulations.
Bubble pressure tensiometry: Determination of time behavior
After a new surface forms, surfactant molecules need time to accumulate at that surface and exert their effect – meaning that the surface tension depends on the surface age. In dynamic processes such as spray painting or the application of pesticides, it is important to take this time behavior into account during formulation.
Bubble pressure tensiometers are used to measure time-dependent, dynamic surface tension, as they can measure SFT across a wide range of surface ages. The resulting measurement curve indicates whether the SFT has decreased sufficiently during the droplets’ flight time until contact with the surface, allowing for good wetting to be expected.
