A study of adsorption of dodecylamine on quartz surface using quartz crystal microbalance with dissipation

In this study the adsorption characteristics of dodecylamine hydrochloride (DACL) on quartz surface have been investigated using a high sensitivity surface characterization technique referred to as quartz crystal microbalance with dissipation (QCM-D) technique in conjunction with zeta-potential and FTIR analyses. The experimental results have demonstrated the versatility and accuracy of the QCM-D for surface adsorption characterization and for the first time revealed the changes in structure and orientation of the amine adsorption film on quartz surface during the adsorption process by real-time measurements of frequency and dissipation shifts with quartz coated sensor. Five distinct adsorption behaviors were identified from the different slopes of ΔD − Δf plots at different concentrations of DACL at pH 6 and 9.5. The physisorption, coadsorption of dodecylammonium and dodecylamine, and surface precipitation of neutral amine molecules with variable conformation and orientation were revealed on the quartz surface by FTIR and QCM-D. Physisorption of ammonium ion and coadsorption of dodecylammonium and dodecylamine dominated at concentrations <0.11 mM at pH 6 and 9.5 forming a rigid and thin adsorption layer. A compaction stage was present at pH 9.5 at concentrations lower than 1.13 mM. Surface precipitation of neutral molecules dominated at higher concentrations at pH 6 and 9.5 to form a thick but dissipated adsorption layer. The adsorption density was calculated with Sauerbrey equation and Voigt model and the results indicated the existence of a critical concentration of 0.45 mM at pH 6 and 1.13 mM at pH 9.5 which led to a significant increase in adsorption density and a structural change in adsorption layer.