Adsorption of nonionic surfactants (C_nE _m) at the silica-water and cellulose-water interface

The adsorption of nonionic surfactants to the silica-water and cellulose-water interfaces was studied using optical reflectometry (OR) and soft-contact atomic force microscopy imaging. The polyethylene oxide alkyl ethers C₁₄E₆ and C₁₆E₈ were shown to readily adsorb to both interfaces. The kinetics of the adsorption process as well as the equilibrium surface excess was determined using OR. In agreement with previous studies, the short headgroup surfactant C₁₄E ₆ adsorbed to a greater extent than the longer headgroup surfactant C₁₆E₈ on silica. This trend was also observed for the cellulose-water interface. The structure of the adsorbed surfactant layer above the critical surface aggregation concentration (csac) was visualized using the soft contact imaging technique for both interfaces. On the silica surface, the layer structure for both surfactants mostly showed spherical aggregates, however, with some elongation into rods being more prevalent for C ₁₄E₆. Similar structures were observed on the cellulose surface, but imaging was more difficult due to the soft gel-like nature of the cellulose thin film in water. This suggests that these surfactants adsorb in a cooperative fashion above the critical micelle concentration (cmc) with a similar interaction between surfactant headgroup and surface for both silica and cellulose. No evidence was seen for the penetration of surfactant molecules into the cellulose surface or any solubilization of the interface.