In a previous column1, we addressed the measurement of the contact angle of water on a solid surface and its utility as a gauge of solid surface contamination. In measuring contact angle, the properties of the water drop are taken as constant. However, residue of surfactant can lead to a false indication of cleanliness because the surfactant, when it dissolves in the water drop, modifies the properties of the drop by lowering the surface tension. This results in a shallower contact angle with the surface; and that is normally an indication of a clean surface.
The surface tension of the liquid per se is an important parameter with utility for many processes. In this series, we will describe the surface tension phenomenon, outline some applications, and explore techniques for measuring surface tension.
Why measure surface tension?
High surface tension inhibits surface wetting. That is why surfactants are added to a liquid in cases where wettability is important. Surfactants, or “surface active agents” are a class of materials that specifically align themselves at a surface to decrease surface tension. Surfactants are used in a number of industrial applications such as removal of contaminants (cleaning), defoaming, and formulations of inks and other coatings.
Since surfactants modify surface tension, measurements are performed to study the effects of surfactant concentration and action. But surfactants are not the only materials that alter surface tension, so measurements are also used to study or monitor the effects of the presence of other materials in liquids, either beneficial additives or contaminants, in general process control and monitoring. Other examples include studies of pharmaceutics absorption rates and of dipalmitoyl phosphatidylcholine (DPPC), a naturally occurring surfactant that is critical to air absorption in normal lung function.
What is surface tension
In a liquid, attractive forces between molecules are strong enough to bind to an adjacent molecule. Normally a molecule is surrounded by adjacent molecules which pull at it from all directions. At the surface, however, molecules on the vapor side are relatively sparse, so most of the attractive forces are pulling in one direction, away from the vapor. This causes the liquid to tend to form a spherical drop, being pulled towards the center from all directions. It takes other forces (e.g. gravity or a solid surface) or a change in the strength of these attractive forces (e.g. via heating or introduction of “contaminants”) to alter this tendency of a liquid to form a spherical drop.
Surface tension is usually expressed in the units of milli-Newtons/meter (mN/m). This is also the same as dyne/cm. Distilled water has a value ~70 mN/m. Most organic liquids (e.g. alcohols and other solvents) are considerably lower (~20 mN/m) due to their weaker non-polar attractive forces.
A pure chemical or a solution in equilibrium is characterized by a single static surface tension. There are a number of static surface tension measurement techniques. The classic technique, dating from the late 1800’s, is the Du Noüy Ring technique. Another related method is the Wilhelmy Plate technique. Both use a balance; and frequently both can be determined using a given commercial instrument. More recent techniques include the spinning drop and pendant drop.
However, when a surfactant is either added to a liquid, or when a fresh liquid/air surface forms (e.g. during bubble formation) in a liquid containing surfactants, it takes a finite time for the surfactant molecules to diffuse to the surface and actively align to lower the surface tension. Thus, the surface tension can vary during this time and measurement techniques that can follow this time changing behavior are called dynamic surface tension techniques. Two techniques for dynamic measurement are the bubble pressure and drop volume techniques. Eventually, the surfactant concentration at the surface reaches equilibrium and the surface tension reaches a new static value.
As this series continues, we will discuss both static and dynamic surface tension techniques along with some applications.
1 “Measuring Thin Film Surface Contamination”, B. Kanegsberg and M. Chawla, A2C2, September, 2001.