In situ kinetics and conformation studies of dodecylamine adsorption onto zinc sulfide using a quartz crystal microbalance with dissipation (QCM-D)

Abstract

Abstract The adsorption behavior and kinetics of dodecylamine onto zinc sulfide (ZnS) coated quartz crystal sensor (referred to as ZnS sensor hereafter) were investigated using quartz crystal microbalance with dissipation (QCM-D). The QCM-D data showed that the adsorption interactions, conformations and kinetics were critically dependent on the species distribution of dodecylamine in the solution. When the dodecylamine was primarily ion form or molecular form in the solution, the dodecylamine was adsorbed to a lesser extent, forming a thick and rigid adsorption layer without any change in conformation. Meanwhile, the pseudo-second order provided the best fit to the adsorption process. With an increase in pH and concentration, the micelle, or precipitation, of dodecylamine became the dominant species in the solution. The micelle of dodecylamine was adsorbed faster and in larger amounts onto the ZnS sensor, with two distinguishable adsorption stages: the first stage, characterized by a dissipative adsorption layer; and the second stage, which had a more compact adsorption layer. The initial stage was best fitted by the pseudo-first order model and the second stage was best fitted by the Elovich model. The pseudo-second order and Elovich model fittings demonstrated that the interactions between the adsorbed layer and the ZnS sensor involved chemisorption. The morphology of the adsorbed layer was also examined using AFM. AFM imaging of the adsorbed layer illustrated that the adsorption of dodecylamine onto the ZnS sensor was heterogeneous, and that the dodecylamine aggregated and scattered on the ZnS sensor, resulting in the formation of discrete surface agglomerations.