Forces between Silica Surfaces in Aqueous Solutions of a Weak Polyelectrolyte

Abstract

The forces between negatively charged silica surfaces in the presence of a weak polyelectrolyte, poly (2-vinylpyridine), were measured as a function of polymer concentration, salt concentration, solution pH, and surface collision rates. The solubility of the polymer is highly dependent on the solution pH; that is, when the molar concentration of solution protons is equivalent to the molar concentration of pyridine groups the polymer is >70% protonated and is highly soluble. As the pH increases, the degree of protonation decreases and the polymer becomes insoluble and precipitates from solution. At low polymer concentrations, low salt concentration, and a low pH, the polymer adsorbs strongly with an essentially flat conformation. The forces during compression are well described by DLVO (Derjaguin−Landau−Verwey−Overbeek) theory with no steric forces apparent. During decompression, the adhesive forces are much greater than those between the bare silica surfaces, indicating a strong bridging between the surfaces after contact and a sub-monolayer coverage. At higher polymer concentrations and/or salt levels, a steric interaction is seen during the compression runs and a significant decrease in the adhesion is observed. Both of these results imply a more expanded conformation of the polymer at the surface and a higher surface coverage. Increased collision rates between the surfaces give rise to an increase in the magnitude of the observed steric forces. Such an increase is attributed to an increased apparent stiffness of the chains as the compression rate increases. Measurements in a poor solvent resulted in the appearance of a shallow long-range intersegmental attractive force.

Normal Forces between Cellulose Surfaces Measured with Colloidal Probe Microscopy

“…At small separation speeds (about 0.1 µm/s) and small polymer concentrations, adhesive force minima were more noticeable. Similar trends for polyelectrolytes have been observed (29,30). Small speeds also imply long surface contact times in the constant compliance regime.…”

“…Normal forces between surfaces in aqueous solutions of uncharged polymers (19)(20)(21)(22) and polyelectrolytes (9,(23)(24)(25)(26)(27)(28)(29)(30) have been studied extensively with the SFA and the AFM, the latter using the colloidal probe configuration. The surfaces employed were typically mica, silica, or alumina.…”

Direct force measurement of the stability of poly(ethylene glycol)–polyethylenimine graft films

“…For this layer, the retract curves exhibit a scaled adhesive force of magnitude ∼40 µN/m and consistent jump-out distances of about 15 nm relative to the "constant-compliance region." This adhesion upon retraction persists over most of the force curve collection and is most likely due to bridging of polymer segments during compression to empty patches on the opposing surface [50]. This adhesion, as expected, is absent at higher PEI bulk concentrations in which both surfaces are more densely covered.…”

Doping in poly(o-ethoxyaniline) nanostructured films studied with atomic force spectroscopy (AFS)

“…This occurs because the tip is at least partially covered with the polymer after a series of measurements, giving rise to the so-called bridge forces between tip and sample (Leite et al, 2007a). The bridge forces depend on the frequency in the tapping mode, because at high frequencies the relaxation time of the polymer may be exceeded (Biggs and Proud, 1997).…”