Reversible Adhesion with Polyelectrolyte Brushes Tailored via the Uptake and Release of Trivalent Lanthanum Ions

Abstract

Applications of end-tethered polyelectrolyte "brushes" to modify solid surfaces have been developed and studied for their colloidal stabilization and high lubrication properties. Current efforts have expanded into biological realms and stimuli-responsive materials. Our work explores responsive and reversible aspects of polyelectrolyte brush behavior when polyelectrolyte chains interact with oppositely charged multivalent ions and complexes, which act as counterions. There is a significant void in the polyelectrolyte literature regarding interactions with multivalent species. This paper demonstrates that interactions between solid surfaces bearing negatively charged polyelectrolyte brushes are highly sensitive to the presence of trivalent lanthanum, La 3+ . Lanthanum cations have unique interactions with polyelectrolyte chains, in part due to their small size and hydration radius which results in a high local charge density. Using La 3+ in conjunction with the surface forces apparatus (SFA), adhesion has been observed to reversibly appear and disappear upon the uptake and release, respectively, of these multivalent cations acting as counterions. In media of fixed ionic strength set by monovalent sodium salt, at I 0 = 0.003 M and I 0 = 0.3 M, the sign of the interaction forces between overlapping brushes changes from repulsive to attractive when La 3+ concentrations reach 0.1 mol % of the total ion concentration. These results are also shown to be generally consistent with, but subtlety different from, previous polyelectrolyte brush experiments using trivalent ruthenium hexamine in the role of the multivalent counterion. ■ INTRODUCTIONInvestigations of polyelectrolyte brushes are directly relevant to a wide range of fields including colloidal behavior, 1−5 lubrication, 6−8 drug and therapeutic delivery, biological interactions, 9−11 coatings and adhesives, 12−17 and energy storage. Polyelectrolyte "brushes" consist of charged polymer chains extended outward into solution with one end tethered to an interface. These structures, which are found in nature and in industrial applications, and on hard and soft surfaces, exist when polymer chains are anchored to surfaces at high tethering densities. 5,18,19 The proximity of chains in polyelectrolyte brushes creates electrostatic and steric repulsion among monomer segments, which drive chains to stretch away from the surface into solution, resulting in a brush structure. Adding complexity to this system are charged counterions, which directly interact with the charged monomer segments and play a crucial role in polyelectrolyte chain behavior. Most studies of responsive polyelectrolyte materials 12 have centered on temperature, 16,17 pH, 13−16 or monovalent salt effects. 20,21 Here, however, we further investigate the interactions of polyelectrolyte brushes with multivalent cations, namely, using trivalent lanthanum. In previous work, 20−22 we have looked at polyelectrolyte interactions with a number of cations, complexes, and charged-aggregates, all of which act as opposit...

The effect of multivalent counterions to the structure of highly dense polystyrene sulfonate brushes

“…), even at very low concentration, can cause abrupt collapse of sparsely tethered PSS brushes [26,27]. Our NR results demonstrate that this conclusion also holds for densely tethered PSS brushes.…”

“…The scattering length density of PSS, ρ p = 2.248×10 -6 /Å 2 , is obtained from the fitting of the X-ray reflectivity data from dry PSSNa films. Both theoretical and experimental work has shown that adding multi-valent counterions can cause an abrupt change in the structure of polyelectrolyte brushes [17,25,27]. We have previously shown that the existence of tri-valent cations (e.g., La 3+ and Ru(NH 3 ) 6…”

“…Polyelectrolyte brushes experience an abrupt decrease in brush thickness in the presence of multi-valent counterions, analogous to the precipitation of polyelectrolytes in multi-valent ionic solutions [24,25]. SFA experiments have shown that adding multivalent counterions induces collapse of the PSS brushes, and adhesion force was measured when separating two PSS brushes as a result of multi-valent electrostatic bridging between chain segments [26,27]. This collapse of polyelectrolyte brushes is in part due to a decrease in the osmotic pressure of counterions through the replacement of multiple mono-valent counterions by multi-valent counterions, and in part due to the greater propensity of multi-valent ions for counterion condensation, leading to the possibility of effective electrostatic bridges.…”

Structure and Functionality of Polyelectrolyte Brushes: A Surface Force Perspective

“…Multivalent counterions can cause dramatic structural changes in polyelectrolyte brushes, because of an entropy‐driven ion‐exchange process and electrostatic crosslinks. Tirrell and co‐workers performed SFA measurements with PSS brushes in solutions that contained different divalent and trivalent counterions at fixed ionic strengths (Figure ) ,,. Compared to monovalent counterions, divalent and trivalent counterions caused rapid collapse of the polyelectrolyte brushes in both the osmotic brush and salted brush regimes,, as indicated by a rapid narrowing in the range of repulsive forces between two PSS brushes that were synthesized using SI‐ATRP .…”

“…Tirrell and co‐workers performed SFA measurements with PSS brushes in solutions that contained different divalent and trivalent counterions at fixed ionic strengths (Figure ) ,,. Compared to monovalent counterions, divalent and trivalent counterions caused rapid collapse of the polyelectrolyte brushes in both the osmotic brush and salted brush regimes,, as indicated by a rapid narrowing in the range of repulsive forces between two PSS brushes that were synthesized using SI‐ATRP . Additionally, attractive forces were measured when separating two physisorbed PSS brushes in the presence of trivalent [Ru(NH 3 ) 6 ] 3+ and La 3+ species, but the attractions induced by La 3+ ions were stronger than those caused by [Ru(NH 3 ) 6 ] 3+ complexes, presumably owing to the smaller hydration radius of La 3+ ions.…”