Visualization of Aluminum Ions at the Mica Water Interface Links Hydrolysis State-to-Surface Potential and Particle Adhesion

Abstract

When hydrolyzable cations such as aluminum interact with solid–water interfaces, macroscopic interfacial properties (e.g., surface charge and potential) and interfacial phenomena (e.g., particle adhesion) become tightly linked with the microscopic details of ion adsorption and speciation. We use in situ atomic force microscopy to directly image individual aluminum ions at a mica–water interface and show how adsorbate populations change with pH and aluminum activity. Complementary streaming potential measurements then allow us to build a triple layer model (TLM) that links surface potentials to adsorbate populations, via equilibrium binding constants. Our model predicts that hydrolyzed species dominate the mica–water interface, even when unhydrolyzed species dominate the solution. Ab initio molecular dynamics (AIMD) simulations confirm that aluminum hydrolysis is strongly promoted at the interface. The TLM indicates that hydrolyzed adsorbates are responsible for surface-potential inversions, and we find strong correlations between hydrolyzed adsorbates and particle-adhesion forces, suggesting that these species mediate adhesion by chemical bridging.

Structure of the Silica/Divalent Electrolyte Interface: Molecular Insight into Charge Inversion with Increasing pH

“…[13][14] Similarly, electrokinetic measurements on both mineral colloids and surfaces in the presence of divalent or trivalent ions have yielded positive zeta potentials at a pH where the surfaces should be negative. 2,4,11,[15][16] The driving force for overcharging has been attributed to either "physical" interactions like ion-ion/ion-site correlations and dispersion forces (favoured by physicists) or "chemical" interactions that involve the formation of specific surface complexes (favoured by chemists). 8,12,[17][18] However, separating the effects of such physical or chemical interactions has proven particularly difficult for mineral oxides and multivalent ions based on the tendency of the latter to form different hydrolyzed species at higher pH.…”

“…They found that the surface potential of mica reverses from negative to positive with increasing pH due to specific adsorption of hydrolyzed species (Al(OH) 2+ and Al(OH)2 + ). 16 Another method that is intrinsically surface selective and sensitive to molecular speciation is the second-order non-linear optical technique vibrational sum frequency spectroscopy (VSFS). VSFS has been extensively used to investigate the amount of ordered water at the silica/water interface, [25][26][27][28][29][30][31] which can be used to report on the interfacial potential within the electric double layer.…”

Real-time tracking of ionic nano-domains under shear flow

“…Atomic force microscopy (AFM) can image single ions adsorbed at various solid-liquid interfaces 7,17,30,33,[48][49][50] . One of the main advantages of the technique is its ability to probe individual ions in-situ but with local contextual information about the interface over tens of nanometres at the point of measurement 7,30,48,50 .…”

“…Additionally, key effects, such as water molecules dissociation, pH and mobility 39,41 , are difficult to account for. Experimental approaches such as X-ray reflectometry [42][43][44][45] and neutron scattering 46,47 can probe the interface exchange dynamics, but they average over hundreds of thousands of ions and can be limited in their time resolution.Atomic force microscopy (AFM) can image single ions adsorbed at various solid-liquid interfaces 7,17,30,33,[48][49][50] . One of the main advantages of the technique is its ability to probe individual ions in-situ but with local contextual information about the interface over tens of nanometres at the point of measurement 7,30,48,50 .…”

“…Experimental approaches such as X-ray reflectometry [42][43][44][45] and neutron scattering 46,47 can probe the interface exchange dynamics, but they average over hundreds of thousands of ions and can be limited in their time resolution.Atomic force microscopy (AFM) can image single ions adsorbed at various solid-liquid interfaces 7,17,30,33,[48][49][50] . One of the main advantages of the technique is its ability to probe individual ions in-situ but with local contextual information about the interface over tens of nanometres at the point of measurement 7,30,48,50 . The relatively slow temporal resolution of standard AFM (typically 30-100 s per image) limits its use for obtaining dynamical information, but multiple improvements to the technique over the last decade have enabled the emergence of high-speed AFM (HS-AFM).…”