Abstract
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 (84.395kJ/mol) as compared to the conventional heating method (109.272kJ/mol). The molecular structures of trisiloxane surfactants were characterized by Fourier transform infrared spectroscopy (FTIR) and 1H nuclear magnetic resonance spectroscopy (1HNMR). 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 (γCMC). With increase in number of ethoxy units, the values of CMC, γCMC, occupied area per surfactant molecule (Amin), and standard free energy of aggregation (△Gθmic) increased, whereas Гmax and standard free energy of adsorption (△Gθads) 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 superspreading model was presented to illustrate the superspreading process. Synergistic effects of precursor water film and Marangoni effect were mainly responsible for the superspreading behavior.
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