Microwave-assisted synthesis of trisiloxane superspreader and its superspreading behavior on plant leaves surfaces

This paper describes a facile microwave-assisted synthesis of a series of novel trisiloxane surfactants, their surface and aggregation properties, and superspreading behaviors on plant leaves surfaces. Kinetic study showed that hydrosilylation reaction followed a second-order rate law and that the activation energy was greatly reduced in case of microwave irradiation (84.395 kJ/mol) as compared to the conventional heating method (109.272 kJ/mol). The molecular structures of trisiloxane surfactants were characterized by Fourier transform infrared spectroscopy (FTIR) and H-1 nuclear magnetic resonance spectroscopy ((HNMR)-H-1). Their surface and aggregation properties were investigated by surface tensiometry, transmission electron microscopy (TEM), and dynamic laser particle size analysis (DLS). Dynamic spreading behavior of droplets of aqueous trisiloxane surfactant solutions on the surfaces of the rice and mango leaves were investigated by contact angle measurement, scanning electron microscopy (SEM) and atomic force microscopy (AFM). Effects of various factors on the advancing contact angle (CA), radius (r), wetted area (S), velocity of spreading (dr/dt), spreading exponent (n), and critical wetting concentration (CWC) were analyzed. These surfactants were found to have low critical aggregation concentration (CMC) and surface tension (gamma(CMC)). With increase in number of ethoxy units, the values of CMC, gamma(CMC), occupied area per surfactant molecule (Amin), and standard free energy of aggregation (Delta G(theta)mic) increased, whereas Gamma max and standard free energy of adsorption (Delta G theta ads) decreased. The surfactant, with an optimal HLB value of about 10 (10.37 for S-2), showed the best superspreading behavior. The superspreading behavior was related to molecular structure, the concentration of surfactant, and substrate wettability. A super spreading model was presented to illustrate the superspreading process. Synergistic effects of precursor water film and Marangoni effect were mainly responsible for the superspreading behavior. (C) 2016 Elsevier B.V. All rights reserved.