SensaDyne: Why Surface Tension Is Important

Why Surface Tension Is Important

What is Surface Tension?

Fluid surface tension is the tangential force that keeps a fluid together at the air/fluid interface. It is the intermolecular force of attraction between adjacent molecules, expressed in force per unit width, as dynes/centimeter (dynes/cm.) or milliNewtons/meter (mN/m). Water, at ambient temperature, has a high surface tension in the range of 72 dynes/cm. while alcohols are in a low range of 20 to 22 dynes/cm. Solvents, typically, are in the 20 to 30 dynes/cm. range.

Why is Surface Tension important?

Quite simply, if any formulation changes at the molecular level then the surface tension will change. If the formulation changes due to the addition of another chemical, the addition of a surfactant, or if anything contaminates the fluid in question, then the surface tension will change. Measuring surface tension is a direct indicator of the quality of any chemical and any formulation.

What is Dynamic Surface Tension?

Many fluids and formulations contain surfactants, or other impurities that are surface active or time dependent. A surfactant is a chemical whose molecules have a hydrophobic segment and a hydrophilic segment. This partial solubility allows this molecule to migrate to and occupy the space at the gas/fluid interface.

Surfactants greatly reduce the surface tension of solvents, water and water-based solutions, inks, fountain solutions, adhesives and other coating formulations. To reduce the surface tension, however, the surfactant molecules have to migrate to the interface, and this takes some finite amount of time. Given enough time, the formulation will eventually reach equilibrium (static) surface tension. This takes several seconds or even several minutes depending on the type of surfactant and the concentration. If you do not allow enough time for the solution to reach equilibrium you are then operating in the dynamic zone, and the critical measurement parameter is dynamic surface tension.

How do surfactants work?

Classified by the ionic charge of the surface acting part of the molecule, anionic surfactants have a negative molecular charge, cationics positive, and nonionics no charge. In amthoterics there are both positive and negative charges. Anionics and nonionic surfactants provide most of industrial surfactant requirements and are the most common. Selection of surfactants is based on specific needs and often mixed surfactants are used.

In general, surfactants with a smaller (lighter) molecule mass (short hydrophobic tail) diffuse more rapidly to the interface, and are vertically adsorbed at the interface, causing a compressive force to act on the surface, thereby reducing surface tension. Most surfactants at higher concentrations exert strong molecular attractions between adjacent molecules causing strong surface films, the strength of which determines the surface properties of the surfactant solutions. Nonionic surfactants with ethylene oxide groups usually diffuse very rapidly to the surface while fluorinated surfactants are slower and more effective at equilibrium

Can you give me an example of how dynamic surface tension changes?

On a printing press, ink is picked up from the ink tray by a roller that rotates at a certain speed, through a number of degrees until the ink is deposited on the substrate to be coated (paper, plastic, etc.) The time it takes for the ink to get from the ink tray to the substrate is the process time for these press conditions, and during this time surfactant molecules in the ink migrate to the newly created air/ink interface.

A positive spreading coefficient results when the dynamic surface tension of the ink is lower than the surface energy of the substrate. If the speed of the press is increased, so that the migration time of the surfactant is reduced, then the press will operate higher on the dynamic curve and the dynamic surface tension of the ink may now be higher than the surface energy of the substrate, resulting in poor printing quality.

Is anyone using the instrument for my application?

Please refer to the Application Notes and Technical Papers for references. If you cannot find an application of interest, please contact us at our Mesa, Arizona office for assistance. We know of many usual and unusual applications that are not listed. We can also run one or two feasibility tests, at no charge, to determine if the instrument is applicable to your specific needs.

Why should I buy a SensaDyne tensiometer?

Surface tension relates directly to a wide range of practical relationships and applications, so that even when a specific surface tension value itself is not of direct interest, it relates directly to another important property of the chemical or formulation. Examples of this are surface tension versus additive or surfactant concentration, surface tension versus contamination, and Critical Micelle Concentration (CMC). All SensaDyne tensiometers measure both dynamic and equilibrium surface tension.

How can I use this tensiometer?

Most users of SensaDyne Tensiometers begin by characterizing their fluids and formulations in terms of dynamic surface tension curves. By generating curves on various (good and bad) samples, these can be compared with one another. This gives users information on why certain formulations work better than others. Users then develop criteria that defines quality levels and quality bandwidths for their various formulations. Once they know what the criteria is, they can use the tensiometer for routine QC testing.

Smart users extend the use of the tensiometer to check incoming chemicals for quality, so that what comes through the door is what is expected in terms of quality. In cases where the manufacturing is a batch or continuous process, instruments are often adapted to batch or on-line monitoring and control.

The net results of using SensaDyne Tensiometers is tighter control of incoming chemicals, tighter formulation control, defect reduction, yield enhancement, and increased revenues.