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Comparatively Speaking: Surface Tension in Water vs. in Formulation
By: Anthony J. O'Lenick, Jr., Siltech LLC
Posted: November 25, 2009
Many published studies of surfactants use pure surfactant in distilled water. Under these conditions, the performance of a pristine surfactant can be observed. An example is PEG-8 dimethicone in water. The surface tension as a function of concentration is shown in Figure 1.
However, a formulation is almost never a single surfactant in water. The difference is that interactions occur between the surfactant and other surfactants, oils and ingredients in the formulation. Since a surfactant is added to a formulation for its effect with other materials, the surfactant's properties must be studied in formulation, not in pure water.
Adding a silicone surfactant to a fatty surfactant rather than water alone will yield different results. A distortion of the curve shown in Figure 1 occurs, as seen in Figure 2. Since the starting surface tension is much lower in the formulation due to the additional surfactant, the curve in Figure 2 lacks the crispness of shape observed in Figure 1.
The efficacy of a silicone surfactant added to a formulation and its impact on feel, foam and other surfactant properties are dependant upon the interactions with the other components of the formulation. To the formulator, this means the selection of the most efficient additive will be determined by the actual formulation.