Ion specific hydration in nano-confined electrical double layers.

We probe counter-ion specific effects in nano-confined electrical-double-layers (EDLs) by means of direct force measurements across a model slit pore in the extended surface forces apparatus. By variation of solution composition, we compare four different counter-ions with dissimilar hydration properties, namely Na+, K+, Cs+ as well as H3O+, all confined between (001) mica surfaces. We discuss the results in terms of a recently proposed π-transition model, evoking hydrated ion layering as the recurrent structural element in highly confined EDLs. We demonstrate that the π-transition essentially proceeds in multiple steps in coherence with the event of single- or multiple- hydrated-ion layering.

Collective dehydration of ions in nano-pores.

Hydrated ions can enter nanometre pores that are smaller than the hydrated ion diameter - the associated dehydration mechanism is still poorly understood. Using an adjustable model slit pore between negatively charged mica surfaces, we have followed the dehydration of highly confined Na+ counter-ions as a function of salt concentration. We applied external load to the slit pore and resolved the induced sub-nanometre film-thickness transitions, in order to gain information about any structural elements present. At a given concentration, the pull-off force required to reopen the collapsed pore is a sensitive measure for the final hydration state of the confined ions at the interface. Remarkably, we observe a two-step evolution of pull-off force, suggesting two-stage collective ion dehydration. There is a notable coincidence between this process and the occurrence of hydrated-ion layering, as previously observed for K+ ions, suggesting that a similar mechanism is at work. The gained insights into equilibrium collective ion dehydration in nano pores add to our fundamental understanding of confined electrical double layers. This may be ultimately translated into design criteria for future nano-porous electrode materials and nanofiltration membranes used for water treatment, or electrical-double-layer capacitors.

Stepwise collapse of highly overlapping electrical double layers.

When two charged surfaces and their accompanying electrical double layers (EDLs) approach each other in an electrolyte solution, the EDLs first begin to overlap and finally collapse under confinement. During this collapse we can observe repulsive forces and film-thickness transitions, which contain valuable information about different structural elements present at the interface. Sensing and discriminating these transitions by size and frequency of occurrence is possible via direct force measurements. Changing salt concentration or pH provide additional means to shift chemical potentials and interfacial populations, and therefore also to shift the relative stability of these structural elements. We provide new evidence that the previously observed oscillatory surface force appearing at the final stages of collapse of the EDL is initially due to layering transitions between hydrated ions, which then develop into smaller transitions between highly confined adsorbed ion states.

Protein-mediated boundary lubrication in arthroplasty.

Wear of articulated surfaces can be a major lifetime-limiting factor in arthroplasty. In the natural joint, lubrication is effected by the body's natural synovial fluid. Following arthroplasty, and the subsequent reformation of the synovial membrane, a fluid of similar composition surrounds the artificial joint. Synovial fluid contains, among many other constituents, a substantial concentration of the readily adsorbing protein albumin. The ability of human serum albumin to act as a boundary lubricant in joint prostheses has been investigated using a pin-on-disc tribometer. Circular dichroism spectroscopy was employed to follow the temperature- and time-dependent conformational changes of human serum albumin in the model lubricant solution. Effects of protein conformation and polymer surface hydrophilicity on protein adsorption and the resulting friction in the boundary lubrication regime have been investigated. Unfolded proteins preferentially adsorb onto hydrophobic polymer surfaces, where they form a compact, passivating layer and increase sliding friction-an effect that can be largely suppressed by rendering the substrate more hydrophilic. A molecular model for protein-mediated boundary friction is proposed to consolidate the observations. The relevance of the results for in vivo performance and ex vivo hip-joint testing are discussed.