Atomic force microscopy for sorption studies
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The hypothesis behind this research is that Atomic Force Microscopy (AFM) can be used to capture changes in the surface interaction force caused by sorption of ions, and thereby can be employed as a tool to study sorption at the nano-scale. The Derjaguin??dau??wey??rbeek (DLVO) theory was used to explain surface interactions and probe DLVO and other interparticle forces. In the first part of the work, sorption of copper ions onto a silica particle resulted in charge reversal detected by AFM. Transient measurements of the interaction force between the silica particle and a flat glass surface can be related to the kinetics of copper ion sorption. In the second part, the force-volume AFM mode was used to detect heterogeneously charged regions on quartz silica surfaces resulting from copper ion sorption. Conditions of copper ion concentration and pH were varied to confirm the sorption mechanism. The surprising result of this study was the observation of growing islands during sorption indicating a heterogeneous behavior of the surface. In the third part, AFM was used to measure interaction forces between a gold sample and the silicon nitride tip of the AFM in aqueous electrolyte solutions under various values of applied electrostatic potential. Along with the applied potential, pH conditions were also varied. Experimental results were compared to theoretical calculations using the non-linear Poisson-Boltzmann equation. This work showed a relationship between the surface potential and the externally applied potential. Finally, the total interaction force between a standard silicon nitride AFM tip and a gold-coated plate in the presence of cationic surfactant, cetyltrimethylammonium bromide (CTAB), was measured under various conditions of applied potential. The interaction force-distance profile between the surface and the tip can be related to the structure of surfactant molecules sorbed onto the surface, which is influenced by the magnitude of the applied potential. The results presented in this work are of importance in natural and engineered systems involving colloidal particles and charged species with implications in separations as well as naturally occurring facilitated transport.