Wetting Dynamics of Aqueous Surfactant Solutions on Polymer Surfaces - Drops and Bubbles in Contact with Solid Surfaces - page 89

Figure 19. Exponent values of the power function for SDS and C 12 E 5 on PVF.

factant molecules adsorbed at the solid-liquid interface occurs inside of the drop,
probably caused by the bi-layer formation due to low monomer solubility which,
in turn, may be affected by low surrounding humidity leading to water evaporation
of the drop. Second, there is other fairly realistic possibility should be considered
when analyzing the reasons for observed low spreading rates—the transfer of sur-
factant molecules onto the bare hydrophobic surfaces which is a relatively slow but
a spontaneous process. The characteristic time scale of the surfactant molecules
transfer onto the hydrophobic surface decreases with increasing surfactant concen-
tration for all surfactant/polymer systems studied as predicted in [40]. Experimental
data in [41, 52, 53] indicate that it is valid for both highly and moderately hy-
drophobic surfaces. Moreover, it was found that the characteristic time scale for
transfer of individual ethoxylated alcohol surfactants E m E 5 estimated for hydropho-
bic polypropylene surface increases with increasing the hydrocarbon chain length,
probably indicating steric limitations with increasing molecular size. As concluded
in [52], based on experimental data the characteristic scale time transfer which can
be act as a spreading characteristic sensibly responds to changes both the surfactant
nature (ionic, non-ionic) and surface free energy.
Dynamic contact angle measurements of aqueous solutions of technical sur-
factants were carried out on Parafilm surface. The equilibrium contact angles are
good reproducible within 1 to 2% (Fig. 20). Considering the spreading behaviour,
they spread faster on Parafilm surface than similar model surfactants. Especially at
relatively low concentrations 0.0625 · cmc and 0.125 · cmc, their aqueous solutions

already spread on highly hydrophobic Parafilm.
A methodology to analyze and compare aqueous solutions of different surfac-
tants with respect to their wetting and spreading on polymer surfaces is provided.
In order to better distinguish between very similar surfactants, highly hydrophobic
polymer samples with very smooth and homogeneous surfaces are recommended to
use as model surfaces. In many respects (costs, repeated use, reproducibility), the
best model sample seems to be the surface of Parafilm, a product of Pechiney Plas-
tic Packaging, USA. The developed method was verified by two selected technical
surfactants, both products of Sasol Germany (Marl).