Comparatively Speaking: Wetting vs. Superwetting

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:

1. There is no necessary relationship between superspreading on the hydrophobic surface Parafilm and the Draves skin wetting test.

2. The low surface tension of the wetting solution at a short time (< 0.2 sec) is one of the critical conditions for good Draves skin wetting, whereas a low equilibrium surface tension (< 21 mN/m) is a necessary but not sufficient condition for superspreading on Parafilm.

3. Synergism in superspreading is possible with additives having the proper hydrophobicity.

4. Synergism and antisynergism in superspreading are not caused by a further change in the surface tension of the solution.

5. Synergism in superspreading is probably related to the molecular attractive interaction between trisiloxane surfactants and additives.

1. KP Ananthapadmanabhan, ED Goddard and P Chandar, Colloids Surf 44, 281 (1990) 
2. CZ Draves, and RG Clarkson, Am Dyest Rep 20, 20 (1931)
3. Z Lin, M He, HT Davis, LE Scriven and SA Snow, J Phys Chem 97, 3571 (1993)
4. M Rosen and L Song, Langmuir 12, 4945–4949 (1996)

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