Jovana Radulovic

On the effect of pH on spreading of surfactant solutions on hydrophobic surfaces

Surfactants are invaluable in a number of agricultural applications in products such as pesticides and herbicides. In these products, surfactants are very often used in conjunction with acidifiers in order to improve their half-life. In this paper, we investigate how the change in pH affects surfactant wetting and spreading. We compare the performance of a conventional surfactant, Triton X-100, with that of a trisiloxane superspreader, Silwet L-77, on a number of polymer coated surfaces exhibiting various degrees of hydrophobicity. Silwet L-77 in water based solutions showed very good wetting capability on all surfaces. However, its wetting ability was drastically reduced with the addition of acetic acid. On the other hand, Triton X-100 was not affected by the addition of acid and exhibited the same spreading behaviour as in water-based solutions.

Spreading and Wetting Behaviour of Trisiloxanes

Wetting and spreading processes which involve surfactant solutions are widely used in numerous industrial and practical applications nowadays. The performance of different non-ionic surfactants may vary significantly and so far superspreader solutions show the most promising spreading ability. The addition of trisiloxane surfactants to water was proven to enhance wetting, even on hydrophobic surfaces, on which conventional surfactants seem to have little or no effect. Although these extraordinary surfactants have been extensively studied over recent years, complete understanding of their underlying mechanisms and a suitable mathematical model are still lacking. Here we present a possible explanation for the impressive performance of trisiloxane, which is compared to wetting enhancement of a conventional surfactant. Additionally, we will explain why the hydrophobicity of the surface is a crucial factor for the spreading phenomenon. Light will be also shed on the effect of the pH of the solution to which surfactants are added. Finally, we will investigate long-term effects of the water environment on trisiloxane wetting ability and discuss if ageing may significantly affect their performance.

Spreading Exponents: Dynamics of Trisiloxane Wetting of Hydrophobic Surfaces

The excellent spreading and wetting behaviour of superspreader solutions has been known and extensively studied over recent years. However, explanations for spreading dynamics and accompanying mathematical models have not yet proved completely successful. Many attempts have been made to quantify the spreading exponents, but none of the models so far was able successfully to describe the whole wetting process of trisiloxane solutions, especially on hydrophobic surfaces. We have investigated the partial wetting of Silwet L-77® superspreader solutions of high concentrations (well above CMC) on polymer coated substrates of varying hydrophobicity. Results obtained can be explained in terms of the Marangoni effect as the major driving force for trisiloxane enhanced spreading. A simple theory, which involves surface tension gradients governing the spreading process, was developed in order to explain the specific evolution of the drop radius and consequent decrease in the contact angle. The proposed model was found to be in excellent agreement with the experimental results. Determined equation coefficients were shown to be dependent on both surfactant concentration and the hydrophobicity of the substrate.