Review on Kinetics of Spreading and Wetting by Aqueous Surfactant Solutions - Drops and Bubbles in Contact with Solid Surfaces - page 62

Figure 17. Molecular structures of the alkyl polyethoxylates (left) and trisiloxane surfactants (right).
Grey atoms are oxygen; black atoms are carbon; small white atoms are hydrogen; and large white
atoms are silicon [105].

2 mmol/m − 3 Figure 18. BAM images for trisiloxanes E 6 ,E 7 ,E 8 at C = [121].

studies which confirmed slightly higher contact angles and surface tension values
for E 4 and E 5 [110, 121]. Optimal wetting abilities were found for chain lengths
in range of 6-9 ethoxy groups. Silwet ® L-77 in particular, and a number of simi-
lar commercial products, with an average of n = 7 . 5 were found to have the most

advanced wetting properties [105].
Recently, Ritacco et al. [121, 122] have measured dynamic surface tensions for
trisiloxane surfactants for the short time ( < 1 s). They discovered that in the case of
trisiloxanes (with relatively long EO n chains, n

6-9) possessing superspreading
character, two inflection points were detectable on dynamic surface tension curves
[121]. Using Brewster Angle Microscopy, the authors directly observed aggregates
appearing on the solution interface for those trisiloxanes in a certain range of con-
centrations, Fig. 18. This allows us to suggest that the surfactant molecules are
present at the liquid/air interface in two states: as monomers and as surface aggre-
gates [121, 122]. The aggregates could act as reservoirs of surfactant monomers
in the course of spreading, completely confirming the suggestion made by Kumar

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