4 research outputs found
Probing the Surface Properties of Gold at Low Electrolyte Concentration
Using
the surface force balance (SFB), we studied the surface properties
of gold in aqueous solution with low electrolyte concentration (∼10<sup>–5</sup> M and pH = 5.8), i.e., water with no added salt,
by directly measuring the interaction between an ultrasmooth gold
surface (ca. 0.2 nm rms roughness) and a mica surface. Under these
conditions, specific adsorption of ions is minimized and its influence
on the surface charge and surface potential of gold is markedly reduced.
At open circuit potential, the electrostatic interaction between gold
and mica was purely attractive and gold was found to be positively
charged. This was further confirmed by force measurements against
a positively charged surface, poly-l-lysine coated mica.
Successive force measurements unambiguously showed that once gold
and mica reach contact all counterions are expelled from the gap,
confirming that at contact the surface charge of gold is equal and
opposite in charge to that of mica. Further analysis of adhesion energy
between the surfaces indicated that adhesion is mostly governed by
vdW dispersion force and to a lesser extent by electrostatic interaction.
Force measurements under external applied potentials showed that the
gold–mica interaction can be regulated as a function of applied
potential even at low electrolyte concentration. The gold–mica
interaction was described very precisely by the nonlinearized Poisson–Boltzmann
(PB) equation, where one of the surfaces is at constant charge, i.e.,
mica, and the other, i.e., gold, is at constant potential. Consequently,
the gold surface potential could be determined accurately both at
open circuit potential (OCP) and under different applied potentials.
Using the obtained surface potentials, we were able to derive fundamental
characteristics of the gold surface, e.g., its surface charge density
and potential of zero charge (PZC), at very low electrolyte concentration
Normal and Frictional Interactions between Liposome-Bearing Biomacromolecular Bilayers
Highly
efficient lubricating boundary layers at biosurfaces such as cartilage
have been proposed to comprise phospholipids complexed with biomacromolecules
exposed at the surfaces. To gain insight into this, a systematic study
on the normal and frictional forces between surfaces bearing a sequentially
deposited model alginate-on-chitosan bilayer, bearing different adsorbed
phosphatidylcholine (PC) liposomes, was carried out using a surface
force balance. Structures of the resulting surface complexes were
determined using atomic force microscopy (AFM) and cryo-scanning electron
microscopy (cryo-SEM). The liposome/lipid–polymer complexes
could maintain their integrity up to high pressures in terms of both
normal and shear interactions between the surfaces, which were repeatable,
reproducible, and revealed very low friction (coefficient of friction
μ down to 10<sup>–3</sup>–10<sup>–4</sup>, depending on the PC used) up to pressures of hundreds of atm. We
attribute this remarkable lubrication capability ultimately to hydration
lubrication acting at the hydrated phosphocholine headgroups of the
PC lipids, either exposed at the liposome surfaces or through complexation
with the polyelectrolyte bilayer. Values of μ, while low, were
roughly an order of magnitude higher than for the same PC vesicles
adsorbed on bare mica, a difference attributed to their lower density
on the bilayer; the bilayer, however, stabilized the PC-vesicles far
better than bare mica against rupture and shear at high compressions
and sliding
Interactions of Hyaluronan Layers with Similarly Charged Surfaces: The Effect of Divalent Cations
We
used colloidal probe atomic force microscopy to measure the
normal forces between the surface of a silica colloidal particle and
a sparse layer of hyaluronan (hyaluronic acid, HA, MW ≈ 10<sup>6</sup> Da) covalently attached to a planar silica surface, both
across pure water and following the addition of 1 mM MgCl<sub>2</sub>. It was found that in the absence of salt the HA layer repelled
the colloidal silica surface during both approach and retraction.
The addition of the MgCl<sub>2</sub>, however, changes the net force
between the negatively charged HA layer and the opposing negatively
charged silica surface from repulsion to adhesion. This interaction
reversal is attributed to the bridging effect of the added Mg<sup>2+</sup> ions. Our results provide first direct force data to support
earlier simulation and predictions that such divalent cations could
bridge between negative charges on opposing surfaces, leading to an
overall reversal of force from repulsion to attraction
Normal and Shear Interactions between Hyaluronan–Aggrecan Complexes Mimicking Possible Boundary Lubricants in Articular Cartilage in Synovial Joints
Using a surface force balance, normal and shear interactions
have
been measured between two atomically smooth surfaces coated with hyaluronan
(HA), and with HA/aggrecan (Agg) complexes stabilized by cartilage
link protein (LP). Such HA/Agg/LP complexes are the most abundant
mobile macromolecular species permeating articular cartilage in synovial
joints and have been conjectured to be present as boundary lubricants
at its surface. The aim of the present study is to gain insight into
the extremely efficient lubrication when two cartilage surfaces slide
past each other in healthy joints, and in particular to elucidate
the possible role in this of the HA/Agg/LP complexes. Within the range
of our parameters, our results reveal that the HA/Agg/LP macromolecular
surface complexes are much better boundary lubricants than HA alone,
likely because of the higher level of hydration, due to the higher
charge density, of the HA/Agg/LP layers with respect to the HA alone.
However, the friction coefficients (μ) associated with the mutual
interactions and sliding of opposing HA/Agg/LP layers (μ ≈
0.01 up to pressure <i>P</i> of ca. 12 atm, increasing sharply
at higher <i>P</i>) suggest that such complexes by themselves
cannot account for the remarkable boundary lubrication observed in
mammalian joints (up to <i>P</i> > 50 atm)