Measuring Surface Tension: Part 1
Barbara Kanegsberg and Ed Kanegsberg, July 2003
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.
Barbara Kanegsberg and Ed Kanegsberg are independent consultants in critical cleaning, precision cleaning, surface preparation, and contamination control. They are the editors of “Handbook for Critical Cleaning,” CRC Press. Contact them at BFK Solutions LLC., 310-459-3614; info@bfksolutions.com; www.bfksolutions.com.
