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Comparatively Speaking: Wetting vs. Superwetting
By: Anthony J. O'Lenick Jr., Siltech LLC
Posted: February 2, 2011
In this edition of "Comparatively Speaking," industry expert Tony O'Lenick discusses the terms wetting and superwetting in relation to the ability of a surfactant to wet a substrate.
The terms wetting and superwetting both relate to the ability of a surfactant to wet a substrate but the substrates being wetted in each test are fundamentally different and often confusing to the cosmetic formulator. Superwetting is a term generally applied to the ability of a drop of surfactant to spontaneously spread on a paraffin surface.1 This type of spreading is important in agricultural applications when fertilizer or herbicide is sprayed on a waxy leaf plant. The compounds demonstrating this phenomenon are trisoloxanes (see Figure 1). This type of wetting is generally not of interest to the cosmetic formulator, but the commonality in the terms can be confusing.
Wetting is a term that, when used by the cosmetic formulator, refers to hair and skin rather than paraffin, and is measured by a dynamic method such as the Draves wetting test.2 There is often a significant difference in the behavior of an aqueous solution of a particular surfactant as measured by these two types of tests, even when both substrates are hydrophobic. The superior behavior of the trisiloxane derivatives on hydrophobic substrates has been called superwetting or superspreading by a number of investigators of the phenomenon.3 Table 1 shows the comparison of a trisiloxane to a dimethicone copolyol. While the structure of the trisiloxane is given above, the structure of dimethicone copoylol is shown in Figure 2.
As Table 1 shows, the trisoloxane drop superwetter at 0.1% by weight spontaneously spreads to 56 times its original diameter on paraffin. None of the dimethicone copolyols can do this. Trisiloxane is unique in this regard. When evaluated on cotton using the Draves method, the wetting times of the low molecular weight dimethicone copolyol is similar to the trisiloxane. Formulators looking for wetting agents for hair care and skin care can use the low molecular weight dimethicone copolyols for their formulations.
M. Rosen,4 perhaps the best known expert in surfactant technology, along with his associate, Song, have studied the difference between wetting and super wetting and have concluded: