Review on Kinetics of Spreading and Wetting by Aqueous Surfactant Solutions - Drops and Bubbles in Contact with Solid Surfaces - page 63

Figure 19. Spontaneous increase in the wetting angle of aged 0.1 wt% Silwet ® L-77 aqueous solution.
Day 1—53 ◦ ; day 6—72 ◦ ; day 8—91 ◦ ; and day 10—103 ◦ [126].
et al. [119] about direct adsorption surfactant aggregates on the liquid/air interface
supporting superspreading.
Other attempts to reveal the true nature of the superspreading phenomenon have
involved simulations. Shen et al. [123] studied the influence of surfactant structure
on spreading using molecular dynamics simulations. They compared the T-shaped
structure of trisiloxanes to the linear chain surfactant, treating surfactant molecules
as united atom structures, with all interactions given by Lennard-Jones potentials,
while modelling the solvent as a monatomic liquid and the substrate as a particle
lattice. The results from this study were somewhat coarse and authors admitted im-
provement of the model was necessary. Other studies included molecular dynamics
simulations using all-atom force fields by Halverston et al. [124]. Spherical wa-
ter nanodroplets with surfactant molecules adsorbed at the liquid-vapour interface
were placed in the vicinity of a graphite substrate and allowed to spread freely at
room temperature. The authors disclosed that their simulations of superspreading
produced results that did not match the experimental observations, probably be-
cause the drop was too small, or the time interval studied was too short. Another
possible reason could be the insufficient length of the hydrophilic tail since they
used E 4 trisiloxane surfactant in their model.
Possibly the biggest drawback of trisiloxanes is their hydrolytical stability. It was
shown that practically instantaneous hydrolysation of the trisiloxane head takes
place at low pH [106], leading to complete loss of their wetting ability. At more
moderate pH values, somewhat longer shelf-life is expected [125]. However, a grad-
ual loss of trisiloxanes' interfacial activity was noted after several days even in
aqueous environment (Fig. 19), as a consequence of spontaneous hydrolysation of
Si-O bond [126].
This has led to synthesis of the alternative, more hydrolytically stable, trisiloxane
structures. Zhang et al. tested the spreading behaviour of new glucosamide-based
trisiloxane surfactants on wheat and cabbage leaves [127].
Their latest study on spreading of the same surfactants on polystyrene surfaces
proved that these molecules exhibit superspreading, but not to the extent ethylene
oxide trisiloxanes do [128]. Wetting enhancement trisiloxanes exhibit is undoubt-
edly superior to any other type of surface-active agents. Impressive and unique
superspreading phenomenon is still one of the mysteries of the interfacial science
and further research is necessary until light is shed on the underlying activity of
superspreaders.