Kotsi, Kristo; (2025) Synergistic Effects of Nonionic and Anionic Surfactant Mixtures on Interfacial Phenomena: Implications for Enhanced Surface Activity. Doctoral thesis (Ph.D), UCL (University College London).

Text Synergistic Effects of Nonionic and Anionic Surfactant Mixtures on Interfacial Phenomena_Implications for Enhanced Surface Activity.pdf - Accepted Version Access restricted to UCL open access staff until 1 June 2026. Download (9MB)

Abstract

For the formulation of agrochemical products, surfactants are often mixed to achieve desirable properties during their delivery, like spontaneous emulsification, enhanced emulsion stability, and droplet adhesion on leaves, but also to control droplet evaporation for efficient penetration of the active ingredient to the plants. However, the single surfactants might be effective for one of these steps, and ineffective, or even counterproductive, in regulating the others, highlighting the need for optimised combinations. This thesis seeks to identify guiding principles that could be used to optimise surfactant combinations in agriculture and other industries. As part of the SusAgriChem project, which connects fundamental experiments with molecular simulations, the thesis will focus on unravelling how molecular interactions affect interfacial phenomena, including surfactant laden droplet evaporation and coalescence. For the studies, a nonionic (EOT) and an anionic (NaDDBS) surfactant -both commonly employed in agriculture- were investigated, along with their mixtures at mole ratios nEOT/nNaDDBS=0.01, 0.1, 1, 4 where n denotes the molar amount of each surfactant in the mixture. Tristyrylphenol ethoxylates were used as the nonionic surfactant and sodium benzene sulfonates with alkyl chain lengths of C10-C13 as the anionic surfactant. The critical micelle concentration, molecular weights, and ionic strengths of these surfactants differ significantly, making it difficult to predict their behaviour when mixed. Initially, the equilibrium surface tension of the single and mixed surfactant systems was measured followed by dynamic surface tension measurements, to better understand surfactant interactions. For the dynamic surface tension measurements of the mixtures two cases were analysed: the premixed and the add one by one. It was found that the surface tension for single surfactants stabilised fast (in a few minutes), while mixtures needed long time to reach equilibrium; up to 15 h for the premixed mixtures and 40 min when surfactants were added one by one. The critical micelle concentration values for the nEOT/nNaDDBS=0.01, 0.1 premixed surfactant mixtures were found to be in between the critical micelle concentration values of the single surfactants, but those for the nEOT/nNaDDBS=1 and 4 mixtures were lower than the critical micelle concentrations of both single surfactants. Calculations based on the regular solution theory suggested that there are attractive forces in the mixed micelles and at the surface layers, while the supramolecular assemblies in the bulk (i.e., micelles) and at surfaces (surfactant films) are preferentially enriched in EOT, exhibiting high EOT-to-NaDDBS mole ratios. Next, considering the same systems, the evaporation of surfactant laden sessile droplets was studied on hydrophobic silane coated glass slides, which have similar wettability properties to banana leaves, commonly used in agriculture research for testing. In all cases studied, surfactants decreased the evaporation time compared to the pure water droplets. As the initial surfactant concentration increased, the evaporation time decreased. Interestingly, EOT laden droplets exhibited longer evaporation times, despite EOT lowering surface tension more than NaDDBS. For the EOT/NaDDBS mixtures, evaporation times fell between those of the single surfactants, attributed to synergistic effects. The presence of surfactants tends to flatten the droplet and increase the surface area, which is likely to contribute to reducing the evaporation time; on the other hand, surfactant molecules at the droplet surface will alter the water evaporation dynamics. The results were interpreted based on two distinct modes of evaporation, i.e., the constant contact radius and constant contact angle; while both modes of evaporation were observed in all cases, the duration of the constant contact radius mode increased with surfactant concentration. As the concentration increased, deviations were observed between results and predicted trends. To validate the applicability of these findings to natural plant surfaces, evaporation studies were conducted on real banana leaves using the same surfactant systems. Results showed that EOT also increased droplet evaporation times on these surfaces, while the leaf substrate itself contributed to longer evaporation times compared to silane coated glass slides -a 64.6% increase was observed for the DI water. This increase was attributed to the reduced available surface area on banana leaves during the prolonged mixed mode. Finally, the coalescence of aqueous droplets with oil-aqueous interfaces was studied for single EOT, single NaDDBS, and nEOT/nNaDDBS=0.01 mixed laden droplets. Experiments focussed primarily on concentrations above the critical micelle concentration, as commonly used in practical applications. Implementing shadowgraphy, planar laser induced fluorescence, and particle image velocimetry techniques the droplet rest times, neck growth, film drainage, and internal flows, during coalescence were studied. The EOT laden droplets notably exhibited longer rest times compared to pure and NaDDBS laden ones, attributed to the formation of saturated layers along the droplet surface and the extended oil film trapped beneath the droplet. Mixed surfactant laden droplets showed intermediate rest times, between those of the two individual surfactants, attributed to synergistic effects. Lastly, EOT laden droplets had longer merging times compared to the single NaDDBS or mixed cases. Prolonged merging was mostly observed at higher surfactant concentrations, likely due to the reduced vortex intensity inside the droplets after film breakage, caused by the greater presence of surfactant molecules. Overall, this thesis enhances the understanding of interactions in mixtures of nonionic and anionic surfactants by studying their effects on surface tension, droplet evaporation, and coalescence; key properties for agrochemicals. The findings show the complexity of surfactant mixtures and provide insights for the development of efficient surfactant-based formulations in agriculture and beyond.

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