Abstract
Direct force measurements between negatively charged silica particles in the presence of a like-charged strong polyelectrolyte were carried out with an atomic force microscope. The force profiles can be quantitatively interpreted as a superposition of depletion and double-layer forces. The depletion forces are modeled with a damped oscillatory profile, while the double-layer forces with the mean-field Poisson-Boltzmann theory for a strongly asymmetric electrolyte, whereby an effective valence must be assigned to the polyelectrolyte. This effective valence is substantially smaller than the bare valence due to ion condensation effects. The unusual aspect of the electrical double layer in these systems is the exclusion of the like-charged polyelectrolyte from the vicinity of the surface, leading to a strongly nonexponential diffuse ionic layer that is dominated by counterions and has a well-defined thickness. As the oscillatory depletion force sets in right after this layer, this condition can be used to predict the phase of the oscillatory depletion force.
Highlights
Since the pioneering work of Asakura and Oosawa [1], depletion forces remained in the focus of the soft condensed matter community [2,3,4]
For a wide range of systems, these forces could be rationalized with a simple damped oscillatory profile, which follows from the large-distance asymptotics of the hard-sphere depletion potential [3,9]
The reason could possibly be that the double-layer force does not decay exponentially, as one would naively expect from the simple theory of the electrical double layer
Summary
Since the pioneering work of Asakura and Oosawa [1], depletion forces remained in the focus of the soft condensed matter community [2,3,4]. The larger particles will interact by repulsive double-layer forces [2] and shorter-ranged depletion interactions [16]. We investigate the interplay between the depletion and double-layer forces acting between charged colloidal silica particles in solutions of strong like-charged polyelectrolytes.
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