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The adsorption and action of various polymers on talc has been investigated. Two families of polymers (starch-based polysaccharides and synthetic polyacrylamides) with specific molecular weights and functional group chemistry were studied. Adsorption isotherms, acoustophoresis, and contact angle studies enabled polymer adsorption behaviour, adsorbed layer thickness, and the hydrophilicity of the polymer-treated talc surface to be determined. Flotation testing on the treated talc revealed a hierarchy of effectiveness within the group of polymers studied. The observed trends in flotation performance are discussed with reference to the measured polymer layer properties. A 3D correlation plot of adsorbed layer thickness and contact angle against flotation recovery indicates that it may be feasible to predict an order of effectiveness for polymeric depressants based solely on measurements of the adsorbed layer properties, without recourse to flotation testing. This approach would be desirable when screening large numbers of potential depressants.

The oscillatory force profile, observed in liquids due to molecular ordering at interfaces, has been extensively investigated by means of atomic force microscopy, but it remains unclear whether molecular ordering is present at the tip apex. Using a displacement-sensitive, low-noise atomic force microscope (AFM) operated in dynamic mode, with a tip of radius1 nm, we have investigated the force profile between two approaching surfaces of the same or different hydrophilic and hydrophobic character. By directly comparing different surface chemistry interactions, we have been able to elucidate whether an oscillatory force profile is due to structured water layers adjacent to the surface, the tip, or a combination of the two. We have found that an oscillatory force profile is observed when the surface is hydrophilic in nature, irrespective of whether the tip is hydrophilic or hydrophobic. When the surface is hydrophobic, an oscillatory force profile is not measured, but rather a monotonic repulsive or a short-range attractive force is observed for interactions with a hydrophilic or a hydrophobic tip, respectively. Thus, we attribute the measurement of an oscillatory force profile, in the absence of lateral confinement effects, solely to water layers adjacent to a hydrophilic surface rather than the structuring of water at the tip apex. This is the first direct evidence that solvation forces occur solely as a result of water layers adjacent to the substrate.

The interactions of salts with lipid bilayers are known to alter the properties of membranes and therefore influence their structure and dynamics. Sodium and calcium cations penetrate deeply into the headgroup region and bind to the lipids, whereas potassium ions only loosely associate with lipid molecules and mostly remain outside of the headgroup region. We investigated a dipalmitoylphosphatidylcholine (DPPC) bilayer in the gel phase in the presence of all three cations with a concentration of Ca2+ ions an order of magnitude smaller than the Na+ and K+ ions. Our findings indicate that the area per unit cell does not significantly change in these three salt solutions. However the lipid molecules do re-order non-isotropically under the influence of the three different cations. We attribute this reordering to a change in the highly directional intermolecular interactions caused by a variation in the dipole-dipole bonding arising from a tilt of the headgroup out of the membrane plane. Measurements in different NaCl concentrations also show a non-isotropic re-ordering of the lipid molecules.

We present definitive interaction measurements of a simple confined liquid (octamethylcyclotetrasiloxane) using artifact-free frequency modulation atomic force microscopy. We use existing theory to decouple the conservative and dissipative components of the interaction, for a known phase offset from resonance (90° phase shift), that has been deliberately introduced into the experiment. Further we show the qualitative influence on the conservative and dissipative components of the interaction of a phase error deliberately introduced into the measurement, highlighting that artifacts, such as oscillatory dissipation, can be readily observed when the phase error is not compensated for in the force analysis.