(381f) Superspreading: Branched Ionic Trimethylsilyl Surfactant Vs Trisiloxanes

Surfactants are broadly used to enhance the spreading performance of aqueous formulations over hydrophobic substrates (1) with myriad applications including in coating, painting, printing, agriculture, medicine (2). The hydrophobic substrate of interest can be not only a solid, but a hydrocarbon liquid. For example, a fast spreading aqueous formulation is a key characteristic of Aqueous Film-Forming Foams (AFFFs) which are used to extinguish fires on flammable liquids, such as hydrocarbon fuels and solvents (3).

The best surface coverage is achieved in the case of complete wetting, but 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 the liquid/air and liquid/substrate interface and therefore improve the wetting. However, to achieve complete wetting the sum of liquid/air and liquid/substrate surface tension should be smaller than the surface tension of pure substrate in contact with air. That is why even solutions of fluorosurfactants, which reduce the surface tension of water to very low values of 17-20 mN/m, do not provide complete wetting on many hydrophobic substrates due to low adsorption on the aqueous/hydrocarbon interface (4). Often synergetic surfactant mixtures are used to facilitate spreading (5), in particular, mixtures of fluoro- and hydrocarbon surfactants, but the best performance so far is achieved with trisiloxane surfactants often called superspreaders (6).

Superspreaders are able to spread over large surfaces forming a film of micron thickness and they spread very fast, much faster than pure liquids with similar properties. The increase of spreading area of pure liquids follows a the power law with exponent of 0.2 (7), whereas for superspreaders, the spread area increases linearly with time (6). Fast spreading is important for aqueous formulations, because for relatively volatile liquids like water, evaporation is competitive with spreading and can considerably reduce the spread area for slow spreading formulation.

It should be noted, that fluorosurfactants with fluorocarbon chain lengths of 8 and above contain or degrade to perfluorooctane sulfonates which are very toxic and prone to bio-accumulation. Trisiloxane surfactants are also toxic to the environment. Additionally, trisiloxane solutions are prone to hydrolysis, reducing their superspreading performance with time. That is why development of new surfactants with superspreading properties is of great importance.

Here, a new surfactant, the magnesium salt of bis (3-(trimethylsilyl)-propyl) 2-sulfosuccinate, Mg(AOTSiC)₂, is presented and its spreading properties are compared with a known trisiloxane surfactant, BT-240 (Evonic). It was suggested earlier that surfactant equilibration rate (8) can play an important role in superspreading mechanism. Therefore, special attention was paid to comparison of the spreading performance with surfactant equilibration rate at the liquid/air interface assessed through the dynamic surface tension. The equilibrium surface tension of Mg(AOTSiC)2 at concentrations above critical aggregation concentration, 21.8 mN/m (9), is close to the surface tension of trisiloxane superspreaders (10). Thus, it is expected that Mg(AOTSiC)2 should wet completely substrates wetted by trisiloxanes.

The spreading of solutions of trisiloxane surfactant BT-240 and Mg(AOTSiC)2 was compared on Polyvinylidenefluoride (PVDF) film (GoodFellow) with a thickness of 0.05 mm. The contact angle of water on this substrate was 81 ± 3° (9). Glass microscope slides (Corning Incorporated) were used as a support for the films cut in pieces of 40 × 40 mm. A drop of surfactant solution with a volume of 5mm³ was deposited immediately on the substrate using an Eppendorf pipette. Spreading was recorded using a Photron SA3 camera equipped with an AF NIKON 24–85mm lens at 60 fps with an exposure time of 0.5 ms and a spatial resolution of 40 μm/pixel. Image processing was carried out by ImageJ.

The structure of two studied surfactants is shown in the left panel of Figure 1, which shows that they have very different architecture, but the spreading kinetics is similar, as shown in the right panel of Figure 1. Therefore, Mg(AOTSiC)₂ is a superspreader. It enables a very fast spreading kinetics for aqueous formulations with spread area being proportional to time. The spreading performance of Mg(AOTSiC)₂ is similar to the spreading performance of trisiloxane superspreader on the studied substrate. Notably, within 14 s of spreading, the area covered by both surfactant solutions increases more than 100 times and reaches 400 mm² for a drop of 5mm³ volume. This highlights that the specific hammer-like molecular architecture of trisiloxanes is not a necessary condition for superspreading (9).

The spreading performance of Mg(AOTSiC)₂ solution remains the same for at least 45 days. This is a considerable advantage in comparison with solutions of trisiloxane superspreaders, which are prone to hydrolysis and show a marked deterioration of spreading performance after 1−2 days (9).

Comparison of dynamic surface tension corresponding to the optimum surfactant concentration shows that the best spreading performance of trisiloxane superspreaders was observed for solutions reaching an equilibrium surface tension on the time scale of 1 s, whereas for Mg(AOTSiC)₂ the best spreading performance was observed at larger concentration, 30 CAC, and a much shorter equilibration time at air/liquid interface of 0.1 s, see the central part of Figure 1. Thus, the essential changes in the type of surfactant, namely in its architecture, activity and CAC value, result in considerable changes in the required surfactant equilibration rate at liquid/air interface, displaying the importance of surfactants adsorption on solid/liquid interface for spreading performance of surfactant solutions (9).

Acknowledgment: This work was funded by the EPSRC Programme Grant PREMIERE EP/T000414/1. We would like to express special thanks to Dr Joachim Venzmer (Evonic) for donating surfactant BT-240.

References

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