A gemini-type superspreader: Synthesis, spreading behavior and superspreading mechanism

This paper describes the facile microwave-assisted synthesis of a series of trisiloxane gemini superspreaders, as well as their surface and aggregation properties and superspreading behavior on plant leaf surfaces. The molecular structures of the trisiloxane gemini surfactants were characterized by Fourier transform infrared spectroscopy (FTIR) and ¹H nuclear magnetic resonance spectroscopy (¹H NMR). The obtained thermodynamic parameters showed that an increase in the spacer group (CH₂) resulted in decreases in the critical aggregation concentration (CAC), corresponding surface tension (γ_CAC), and surface excess concentration (Γ_max) but increases in the occupied area per surfactant molecule (A_CAC) and absolute values of the standard free energies of aggregation (ΔG^θ _mic) and adsorption (ΔG^θ _ads). An increase in ethoxy units (CH₂-CH₂-O-) resulted in increases in the CAC, γ_CAC, and A_CAC but decreases in Г_max and the absolute values of ΔG^θ _mic and ΔG^θ _ads. The transmission electron microscopy and dynamic light scattering results showed that the average sizes of the aggregates of superspreader solutions increased with an increasing number of spacer units (CH₂) but decreased with an increasing number of ethoxy units (CH₂-CH₂-O-). The dynamic spreading behavior results demonstrated that the average spreading velocity increased with increasing spacer chain length, and the dependence of the maximum spreading velocity on the ethoxy chain length was nonmonotonous with a maximum at n(EO) = 8.68. The optimal HLB value was essential to obtaining good superspreading behavior, and the substrate wettability (hydrophobic rice plant and hydrophilic mango plant surfaces) greatly influenced the superspreading. The synergistic effects from the precursor film and Marangoni effect existed in the proposed superspreading model.