Abstract
The spreading of solutions of three trisiloxane surfactants on two hydrophobic substrates, polyethylene and polyvinylidenefluoride, was studied with the addition of 0–40 mass % of glycerol. It was found that all the surfactant solutions spread faster than silicone oil of the same viscosity, confirming the existence of a mechanism which accelerates the spreading of the surfactant solutions. For the non-superspreading surfactant, BT-233, addition of glycerol improved the spreading performance on polyvinylidenefluoride and resulted in a transition from partial to complete wetting on polyethylene. The fastest spreading was observed for BT-233 at a concentration of 2.5 g/L, independent of glycerol content. For the superspreading surfactants, BT-240 and BT-278, the concentration at which the fastest spreading occurs systematically increased with concentration of glycerol on both substrates from 1.25 g/L for solutions in water to 10 g/L for solutions in 40% glycerol/water mixture. Thus, the surfactant equilibration rate (and therefore formation of surface tension gradients) and Marangoni flow are important components of a superspreading mechanism. De-wetting of the solutions containing glycerol, once spread on the substrates, resulted in the formation of circular drop patterns. This is in contrast to the solely aqueous solutions where the spread film shrank due to evaporation, without any visible traces being left behind.
Highlights
Surfactants are broadly used to enhance spreading performance of aqueous formulations over hydrophobic substrates [1,2,3,4] with example applications in coating, painting, printing, agriculture and medicine
The spreading kinetics of the two silicone oils used was similar with a spreading exponent α =
The spread area for the two studied silicone oils was proportional to t0.26−0.29, within the whole time of observation
Summary
Surfactants are broadly used to enhance spreading performance of aqueous formulations over hydrophobic substrates [1,2,3,4] with example applications in coating, painting, printing, agriculture and medicine. It is not straightforward to find surfactants which can provide complete wetting on highly hydrophobic surfaces such as polyethylene or plant leaves. Even solutions of fluorosurfactants, which reduce the surface tension of water to very low values (17–20 mN/m) do not provide complete wetting of these surfaces due to low adsorption at the liquid/solid interface [5]. Branched surfactants, such as dioctyl sodium sulfosuccinate (Aerosol OT) [6,7] or synergetic surfactant mixtures are often used to facilitate spreading [8,9,10]. The best performance is achieved with trisiloxane surfactants, often called superspreaders [11,12,13]
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