Two-Step Adsorption of a Switchable Tertiary Amine Surfactant Measured Using a Quartz Crystal Microbalance with Dissipation

The adsorption of a switchable cationic surfactant, <i>N</i>,<i>N</i>,<i>N</i>′-trimethyl-<i>N</i>′-tallow-1,3-diaminopropane (DTTM, Duomeen TTM), at the silica/aqueous solution interface is characterized using a quartz crystal microbalance with dissipation (QCM-D). The adsorption isotherms reveal that changes in the solution pH or salinity affect surfactant adsorption in competing ways. In particular, the combination of the degree of protonation of the surfactant and electrostatic interactions is responsible for surfactant adsorption. The kinetics of adsorption is carefully measured using the real-time measurement of a QCM-D, allowing us to fit the experimental data with analytical models. At pH values of 3 and 5, where the DTTM is protonated, DTTM exhibits two-step adsorption. This is representative of a fast step in which the surfactant molecules are adsorbed with head-groups orientated toward the surface, followed by a slower second step corresponding to formation of interfacial surfactant aggregates on the silica surface.