Previously, a surfactant was defined as a surface active agent with two or more groups that are not soluble in one another in their pure form. In addition it was stated that a surfactant must be soluble and lower surface tension. Surface tension determines the properties of formulations. The question is: How does one measure surface tension?
The Department of Energy, Argonne Laboratories states, “There are many methods for measuring the surface tension of liquids. Each has its advantages and limitations. There is a reason why there are different methods: surface tension is a very complicated property of a liquid. It depends upon many variables, some of which are: temperature, composition of the liquid (it may be a solution or contain small amounts of substances that affect the surface tension), measurement time, the material of construction of the apparatus, and the viscosity of the liquid.”
Since realistically, there is more than one surface tension in a liquid—it should be referred to as the plural tensions—measurements of this characteristic can vary depending upon the method used.
One category of methods is based on the angle of contact a liquid makes to a solid surface. The solid may be a flat horizontal plate, a tilted plate, a vertical plate or the walls of a thin tube (capillary). Assuming the variables noted above are all under control, these methods have a common limitation in that the angle of contact is difficult to measure accurately. Everybody "sees" the angle a bit differently, and this difference results in a different surface tension value. In each case, the liquid/solid contact may be stationary or it may be moving.
A second category of methods is based on the shape of a drop of the liquid. The drop may be hanging stationary, it may be dripping, or it may be resting on a flat plate. The "problem" with this class of methods is that the mathematical analysis of the shape of a drop and the surface tension of the liquid is complicated. In fact, the relation between the shape and surface tension must be solved numerically since the formulas do not have a direct solution.
Each of these methods also comes in "flavors." One "flavor" is a static measurement in which the liquid is not moving. The other "flavor" is a dynamic measurement in which the liquid is moving. Often the static and dynamic surface tension have different values that are real and not just experimental error. The difference between the static and dynamic value can be very useful in understanding the surface properties of the liquid.
In relation to formulas, the static surface tension would be the surface tension in a bottle; an interesting determination, but perhaps not the most important one. As the product is poured from the bottle and used, dynamic surface tension becomes more important since the formulation is spread onto the hair or skin, creating a new surface area. This increase in surface area occurring during application results in a dynamic situation, where surfactant present in micelles transfers to the newly created surface to lower surface tension. Figure 1 illustrates the different forces in play during the complex situation of applying the formulation.
