Study of adsorption of trisiloxane surfactants on air-water and hydrophobic solid-water interface: An attempt to explain the 'super-spreading' behavior

The exceptional ability of the trisiloxane surfactants to enhance spreading of water on highly hydrophobic surfaces has been the focus of many recent studies. The reason behind their “super-spreading” ability is not yet clear. In this work, we explain the reason of their “super-spreading” ability by comparing their adsorption characteristics on air-water and hydrophobic solid-water interfaces with that of alkyl poly-ethoxylate surfactants (C ₁₂E_j).; In the first part, we have developed model hydrophobic surfaces using self-assembled silane monolayers. These surfaces have been used for surfactants' adsorption experiments. Using Atomic Force Microscope as a tool to study the morphology of the partial silane monolayers, we have developed a protocol to prepare mixed silane monolayers by sequential adsorption of different silanes. In these mixed monolayers, domains of one-chemical functionality are surrounded by another chemical functionality. The size and morphology of the domains can be controlled from nanometer to micrometer scale by controlling the deposition conditions.; The adsorption characteristics of trisiloxane surfactants on air-water interface have been investigated by pendant bubble technique and compared with that of C₁₂E_j surfactants. Due to the peculiar “T” structure of the hydrophobic groups, the trisiloxane surfactants have larger driving force to adsorb on the air-water interface. Consequently, they pack more densely than C₁₂E_j, and reduce the surface tension faster and to values lower than that of C₁₂E_j surfactants.; The adsorption of the trisiloxane and C₁₂E_j surfactants on hydrophobic silane monolayers has been investigated using FTIR spectroscopy in attenuated total internal reflection mode. We have observed the changes induced in the hydrogen bonding structure of water near hydrophobic surfaces, as a result of surfactants' adsorption, and the effect of surfactant structure on the hydrogen bonding of the interfacial water. The trisiloxane surfactants reduces the hydrophobic-solid water surface energy to lower values because of their ability to remove free OH present near the hydrophobic surface. This is again attributed to their “T” structure. In conclusion, the super-spreading ability of the trisiloxane surfactants is due to their ability to reduce hydrophobic solid- and air-water surface tensions, which in turn is attributed to the structure of their hydrophobic group.