(520d) Molecular Simulation of Trisiloxane Surfactants At the Air/Water Interface

Trisiloxane surfactants are widely used in various products such as paints, inks, and herbicides, because of their capacity to enable "superspreading"—the greatly enhanced spreading of aqueous solutions on hydrophobic surfaces. Despite numerous studies on trisiloxane surfactants and superspreading, the mechanisms that explain superspreading have not yet been identified. Proposed explanations include the extreme reduction of the air/water surface tension in the presence of trisiloxane surfactants, the self-association of the surfactants, the transport of the surfactants within water droplets to the interfaces, and the interaction of the surfactants with the substrate.

Molecular dynamics simulations have the potential to provide deeper insight into the superspreading mechanism. However, before the superspreading problem can be addressed, improved algorithms and  molecular models for trisiloxane surfactants are required. For efficient and accurate calculation of long-range dispersion forces, which are known to be especially relevant in simulations with surfaces, we have developed and optimized a PPPM method for dispersion interactions. Furthermore, we optimized the interactions of an existing all-atom force field for the polyethylene oxide chain of the surfactant to the TIP4P/2005 model, the best non-polarizable water model currently available for describing the liquid-vapor interface, to obtain an improved behavior of the trisiloxane surfactants in aqueous solutions. We use this model to study the adsorption of the trisiloxane surfactants at the air/water interface in various concentrations. The reduction of the surface tension, as well as the structural behavior of the trisiloxane surfactants and water near the interface that explain the simulated surface tensions, are reported. We also compare our findings to experimental results from sum frequency generation.