Surfactancy is the word that we use on the wiki to describe a detergent's (or other surfactant's) relative ability to reduce surface tension. Here in bubble-land we are less interested in the surface tension per se than in the physical thickness (not viscosity) of a soap film at a given concentration. The surface tension of water/detergent mixes in the range useful for making bubbles are all nearly the same. However, the thickness of a soap film created by a 20 to 1 dilution (water to detergent)  made by different detergents can vary quite considerably.

As explained elsewhere on the wiki, the less detergent there is in a solution the THICKER the bubble walls are. A pure water film is thicker than a soap film and (up to a point) the more detergent that is added, the thinner the created film. (See Dilution and Color and Film Thickness for more about this topic.)

For our sake, we are interested in how thin or thick bubble walls are with a given concentration. A detergent with higher surfactancy is more potent than one with lower surfactancy and requires less detergent to accomplish a similar thinning of the soap film.

For example, a detergent with a high surfactancy might achieve a thin film (and reach the critical dilution) at a water:detergent ratio as high as 25:1 (such as Charmy, Power of Suds, when it is pH adjusted) while the Dawn Simply Clean (a weak version of Dawn) has much lower surfactancy and needs much more detergent to achieve a similar film thickness.

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