(95c) Effect of Surfactant Kinetics on Spreading of Aqueous Trisiloxane Solutions

Effect of
surfactant kinetics on spreading of aqueous trisiloxane solutions

Nina M. Kovalchuk, Jacques Dunn, Jack Davies, Mark J.H. Simmons

School of Chemical
Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK

Key words:  spreading, trisiloxane surfactants, Marangoni
effect.

Surfactants are broadly used to enhance spreading
performance of aqueous formulations over hydrophobic substrates (1)
with applications in, for example, coating, painting, printing, agriculture and
medicine. The contact angle of pure water on many polymeric materials of
industrial importance as well as on biological surfaces is around or above 90
°. Surfactants decrease the interfacial tension on liquid/air and liquid/solid
interface and therefore improve the wetting. 

It is not straightforward to find the surfactants providing
complete wetting on highly hydrophobic surfaces, such, for example as
polyethylene or plant leaves. Even solutions of fluorosurfactants, which reduce
the surface tension of water to very low values of 17-20 mN·m^-1 do
not provide complete wetting of these surfaces due to low adsorption on
liquid/solid interface (2).
Often synergetic surfactant mixtures are used to facilitate spreading (3),
but the best performance so far is achieved with trisiloxane surfactants often
called superspreaders (4).

Superspreaders are able to spread over large surfaces
forming a film of micron thickness. They spread very fast, much faster than
pure liquids with similar properties. In particular, the spreading of pure
liquids follow the power law S ~
t^0.2 (5),
where S is the spread area and t is time, whereas for
superspreaders S ~
t (4).
Another peculiarity in the behaviour of superspreading solutions is a maximum on
the dependencies of spreading area on surfactant concentration and on substrate
hydrophobicity. Several mechanisms are proposed to explain superspreading and
its regularities, but none of them is generally accepted so far, because of lack
of stringent experimental corroboration. Understanding
of the superspreading mechanism is a necessary step in developing new effective
spreading agents with improved biocompatibility and biological degradability.

One of the proposed mechanisms explains superspreading by
Marangoni flow at the leading edge of spreading (6).
When surfactant solution spreads over the substrate surface, liquid/air and
liquid/solid interfacial areas expand. Surfactant from the bulk adsorbs on both
interfaces. If the surfactant transfer to the leading edge of spreading is not
fast enough to replenish its loss due to adsorption, surface tension there
increases due to surfactant depletion. This results in the Marangoni flow in
direction of spreading, with scaling S ~
t according to (6).

This mechanism explains why dependence of spread area on
concentration has a maximum: at large concentrations surfactant transfer
becomes fast enough to avoid depletion at the leading edge of spreading and
therefore Marangoni flow does not contribute noticeably to the spreading
kinetics. The diffusive flux of surfactant is proportional to the concentration
difference and diffusion coefficient. Therefore, if Marangoni flow provides a
crucial contribution to superspreading, a decrease in the diffusion coefficient
should result in the shift of the maximum spreading to the larger
concentrations.

Comparison of spreading performance of two surfactants with
different properties has shown that the surfactant which equilibrates slower
spreads faster (7)
as confirmation of importance of Marangoni flow in superspreading. However this
confirmation was not entirely conclusive, because other properties of these
surfactants can be more important that the equilibration rate. Therefore here
the effect of surfactant equilibration rate is studied for the same surfactant
by reducing its diffusion coefficient. The last was achieved by adding glycerol
to surfactant solutions. Addition of glycerol does not change noticeably
surface tension and density of solutions, but changes the viscosity and
therefore the surfactant diffusion coefficient. Viscosity, μ, itself has
rather weak effect on the spreading kinetics with S ~ μ^0.1, therefore by varying viscosity
in the narrow range effect of the surfactant kinetics can be seen clearly.  

Study of spreading characteristics of solutions of three
trisiloxane surfactants (Evonic), BREAK-THRU S 278 (BT-278), BREAK-THRU S 240 (BT-240)
and BREAK-THRU S 233 (BT-233) was performed on two substrates, polyethylene
(PE) and polyvinylidenefluoride (PVDF). Contact angle of water on these
substrates was 102 ° and 84 ° respectively.  Surfactants for this study were
provided as a gift by Dr J. Venzmer (Evonic). BT-278 and BT-240 are
superspreaders, whereas BT-233 is not. Solutions of these surfactants at
concentrations above critical micelle concentration were prepared in double-distillate
water and in 20, 30 and 40 % glycerol/water mixtures with viscosities 1.7, 2.4
and 3.5 mPa·s respectively .

Spreading of 5 μL drop released from micropipette close
to the substrate surface was recorded by high speed camera Photron SA3 at 60 fps
with an exposure time of 0.5 – 4 ms. Corresponding videos were processed by
ImageJ free software to find kinetics of spreading. For comparison, kinetics of
spreading of two silicone oils with viscosities 1 mPa·s (similar to water) and
4.6 mPa·s was studied. The results were average of 3-7 measurements. The
experimental error did not exceeded 10 % for silicone oils and 20 % for
surfactant solutions.  

Surfactant adsorption kinetics was followed by measurement
of dynamic surface tension in the time range 0.01 – 20 s relevant for the
spreading process using Maximum Bubble Pressure tensiometer (Sinterface).

Silicone oils demonstrated similar spreading kinetics with
spreading exponent α = 0.26 on PE and α = 0.29 on PVDF
for both silicone oils. The spreading exponent is slightly higher than
theoretical prediction α = 0.2 (5).
The difference is possibly related to the substrate roughness, because PVDF has
higher roughness than PE. Pre-exponential factor is ~ 5 % larger for less viscous silicone oil.

For all three studied surfactants there is a maximum in
spreading rate vs concentration, but the spreading on the whole is much slower than
for non-superspreading surfactant and the maximum is much less pronounced. Fig.
1 compares the spreading kinetics of less viscous silicone oil with surfactant
solutions in water (the same viscosity) at concentrations providing the best
spreading. It is seen from Fig. 1 that solution of non-superspreading
surfactant, BT-233 spreads much slower than superspreaders. On the time scale ~ 2 s spreading exponent for
this surfactant is close to that of silicone oils α = 0.3, but
later it increases to α = 0.5 demonstrating an additional mechanism
accelerating spreading. For solutions of BT-240 and BT-233 superspreading with α
= 1 was observed at t > 2 s. Before that, spreading followed the power law
with exponent α = 0.5.

An increase of liquid viscosity due to addition of glycerol
results in slower changes of dynamic interfacial tension as the result of
smaller diffusion coefficient. It also results in shifting to the larger values
the concentration at which the maximum spreading rate is observed. For superspreading
surfactants, BT-240 and BT-278, the concentration of maximum spreading
increases from 1.25 g/L for solutions in pure water to 10 g/l for solutions in
40 % GL/water. The changes for non-superspreading surfactant, BT-233, are not
that conclusive, but nevertheless increase from 2.5 g/L to 5 g/l was observed. These
results support the hypothesis about a crucial role of Marangoni flow in
superspreading mechanism and provide a tool to control the spreading rate.
Comparison the spreading results with data on dynamic surface tension reveal
that the fast spreading is observed at surfactant concentrations at which the
dynamic surface tension reaches an equilibrium value within 1 s and the most
sharp decrease of surface tension occurs within the time span of 0.01 – 1 s.

Fig. 1. Comparison of spreading kinetics on PVDF for
silicone oil 1 mPa·s and trisiloxane surfactant solutions in water.       

References

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