Reversible Adhesion with Polyelectrolyte Brushes Tailored via the

Apr 24, 2015 - Phone: 773-834-2001. This article is part of the Steven J. Sibener Festschrift special issue. Cite this:J. Phys. Chem. C 119, 26, 14805...
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Reversible Adhesion with Polyelectrolyte Brushes Tailored via the Uptake and Release of Trivalent Lanthanum Ions Robert Farina,† Nicolas Laugel,† Jing Yu,‡,§ and Matthew Tirrell*,‡,§ †

Department of Chemical Engineering, University of California, Santa Barbara, California 93106, United States Institute for Molecular Engineering, University of Chicago, Chicago, Illinois 60637, United States § Institute for Molecular Engineering, Argonne National Laboratory, Argonne, Illinois 60439, United States ‡

S Supporting Information *

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, La3+. 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 La3+ 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 I0 = 0.003 M and I0 = 0.3 M, the sign of the interaction forces between overlapping brushes changes from repulsive to attractive when La3+ 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.

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INTRODUCTION Investigations 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 materials12 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 oppositely charged counterions with respect to the polyelectrolyte brush system. © 2015 American Chemical Society

The structure and behavior of polyelectrolyte brushes can be dramatically influenced by the surrounding environment. For example, chains that are extended in purely aqueous environments can transition to more compact structures when charges are electrostatically screened with monovalent salt2,3,20−22 or neutralized by adjusting pH.13−16 Furthermore, as we have reported in past work22 and will be expanded upon here, the collapse of polyelectrolyte chains can be exaggerated in the presence of multivalent counterions as a result of multivalent electrostatic bridging between chain segments. Additionally, in the presence of multivalent counterions adhesion has been observed between polyelectrolyte brushes tethered to opposed cross-cylindrical surfaces,22 as well as with spherical polyelectrolyte brushes in solution.2,3 This adhesion is a direct result of intermolecular electrostatic bridging between opposite brushes. Adhesion and exaggerated chain collapse exist only when multivalent counterions have replaced their monovalent counterparts inside polyelectrolyte brushes.22 As will be described, these multivalent electrostatic effects are also completely reversible (Figure 1). Special Issue: Steven J. Sibener Festschrift Received: March 4, 2015 Revised: April 22, 2015 Published: April 24, 2015 14805

DOI: 10.1021/acs.jpcc.5b02121 J. Phys. Chem. C 2015, 119, 14805−14814

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produced via Langmuir−Blodgett deposition of amphiphilic surfactant, N-octadecyltriethoxysilane (OTE). For electrochemical experiments, a 1-heptanethiol self-assembled monolayer was used in order to render the electrode surface hydrophobic. All details of brush formation, as well as SFA and electrochemical experimental measurements, are published in past work.22 Trivalent lanthanum was added to solution in the form of lanthanum(III) nitrate hexahydrate (La(NO3)3·6H2O). Surface Forces Apparatus (SFA). An SFA Mk II was utilized to measure intermolecular forces between two crosscylindrically oriented surfaces, each with an adsorbed polyelectrolyte brush. In all SFA experiments, ruby mica, grade 1 (V-1/V-2), was obtained and used from S and J Trading, Inc. This mica was cleaved into smaller step-free, molecularly smooth pieces. Silver (99.99% pure) was purchased from Cerac Incorporated and deposited onto the backside of this freshly cleaved mica by thermal evaporation, resulting in a metal thickness of approximately 55 nm. The silvered mica was then cut into small rectangular pieces of approximately 1 cm2 area, and glued silver side down onto curved cylindrical silica disks (ca. 2 cm radius of curvature). The glue was EPON Resin 1004F from Momentive Specialty Chemicals Inc. The ionic strength of the solution was justified by concentrated aliquots of 10−20 mL of salt solutions into the 375 mL SFA chamber to adjust the bulk concentrations as desired. More details of experiments can be found in our previous studies.22 Brushes were compressed and separated at average speeds of approximately 5 nm/s (±1 nm/s). The force distance profiles do not significantly change within a speed range of approximately 1−9 nm/s. Surface force data plotted in Figures 2−7 were taken in 1 s intervals, corresponding to one force measurement every second. All force measurements were repeated at least three times to ensure the reproducibility of the measurements. Brush height data is an average of at least six compressions of the two brush-coated surfaces; the error bars represent the range of those measurements. Brush heights plotted in Figures 8 and 9, as well as mentioned throughout the article, were systematically determined from the point at which repulsive forces upon compression reach 100 μN/m. Surface force data in Figures 2 and 3 were measured using a spring constant of k = 890 N/m. In Figures 4−6, k = 250 N/m. F/R represents the normalized force (energy per unit area) exerted between two polyelectrolyte brushes covering the opposite, cross-cylindrically aligned surfaces. The zero distance point was established during solid−solid contact measurements between hydrophobically modified mica surfaces before the self-assembly of PtBS20-NaPSS420 diblocks. R is the geometric mean of the radii of the surfaces. Electrochemistry/Cyclic Voltametry. All electrochemical experiments were performed using a three-electrode setup. A polycrystalline gold disk of 2 mm in diameter was used as the working electrode and a platinum coil as the auxiliary electrode. The reference potential was set with the use of a classical Ag/ AgCl electrode. Cyclic voltamograms were recorded between +200 and −500 mV vs Ag/AgCl and at a scan rate of 100 mV/ s. Polyelectrolyte brushes tested in electrochemical experiments presented in Figures 8 and 9 were exposed to solutions of identical trivalent concentrations and exposure times as brushes in the respective surface force experiments. The value of θ used in Figures 8 and 9 is defined below:

Figure 1. Reversible transition between collapsed/adhesive polyelectrolyte chains and extended/repulsive chains. Through adjusting the external salt environment surrounding end-tethered polyelectrolyte chains, a polyelectrolyte brush can transition from a collapsed, adhesive structure (left) to an extended, repulsive one (right). This reversible transition is driven by the uptake and release of multivalent counterions, respectively. As with our investigation, this cartoon depicts anionic polyelectrolyte chains and cationic mono- and trivalent counterions.

We have studied a system of strong polyelectrolyte chains in a brush by using a diblock copolymer of poly(t-butylstyrene)− poly(sodium styrenesulfonate), PtBS-NaPSS. The neutral, hydrophobic PtBS blocks are included in the design of these molecules for the sole purpose of anchoring the longer, negatively charged NaPSS chains to a solid substrate. The synthesis and characterization of these molecules,20−22 as well as self-assembly onto hydrophobically modified surfaces,22−26 have been extensively studied. In order to investigate the interactions of multivalent entities with these polyelectrolyte brushes, we employed a combination of two independent experimental techniques; the surface forces apparatus (SFA) and cyclic voltammetry.22 Measurements using these combined techniques in identical environmental conditions provide a direct comparison of polyelectrolyte structure and behavior (i.e., brush height and intermolecular forces of interactions between two polyelectrolyte brushes) to the amount of multivalent counterions inside a brush. Here, our earlier work22 using the complex ruthenium hexamine (Ru(NH3)63+) is expanded upon using the trivalent cation lanthanum (La3+), which interacts with the negatively charged polyelectrolyte segments as a counterion. Although trivalent La3+ does not have the ionization energy suitable for cyclic voltammetry experiments, similarities measured during independent polyelectrolyte brush surface force experiments using Ru(NH3)63+ and La3+ allow a comparison of brush behavior with these, structurally very different, trivalent ions.22 Additionally, polyelectrolyte brush behavior in the presence of La 3+ counterions has some unique components, such as measured adhesion strengths significantly greater than adhesion observed in experiments with Ru(NH3)63+, which in part is attributable to the small size and, therefore, high local charge density of La3+ cations.

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EXPERIMENTAL METHODS Polyelectrolyte Brush Formation. PtBS-NaPSS block copolymers used in this work were composed of approximately 20 units of PtBS and 420 units of NaPSS. Polyelectrolyte brushes composed of these PtBS20-NaPSS420 copolymers were self-assembled onto hydrophobic surfaces using a solution of 80 ppm PtBS20-NaPSS420 in aqueous I0 = 0.3 M NaNO3 solution. For SFA experiments, hydrophobically modified mica was

θ= 14806

Q Γ ;Γ= Γ sat nFA

(1) DOI: 10.1021/acs.jpcc.5b02121 J. Phys. Chem. C 2015, 119, 14805−14814

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The Journal of Physical Chemistry C

Figure 3. Compression and separation force vs distance profiles of NaPSS brushes in the presence of both Na+ and La3+. The bulk concentration of NaNO3 can be seen in the legend in the upper right. The concentration of La3+ is slightly varied as monovalent salt is increased; from 1.1 × 10−5 M to 8.9 × 10−6 M. (a) Shows compression and separation of the surfaces with normal scaling. (b) Compression forces are plotted on semilog scale.

Figure 2. Normalized force between two surfaces coated with selfassembled PtBS20−NaPSS420 brushes (F/R) is plotted as a function of distance. This experiment shows polyelectrolyte brushes in the presence of only La3+. The bulk concentration of La3+ can be seen in the legend in the upper right. (a) Compression and separation of the surfaces, where solid data points represent forces upon compression and open data points were measured upon separation (this is the same for all surface force figures). (b) Compression forces are plotted on a semilog scale.

behavior of polyelectrolyte brushes in the presence of La3+, as well as to quantify the magnitude of adhesion observed with La3+ counterions interacting with the brush. After polyelectrolyte brush collapse and adhesion between two surfaces was measured, additional testing of polyelectrolyte brush behavior was performed by adding Na+ ions to the bulk solution at a fixed concentration of La3+ (Figure 3). The results of this work, seen in Figures 2 and 3, show much stronger adhesion, with La3+ cations acting as counterions than with Ru(NH3)63+ complexes.22 These experiments also demonstrate the ability of polyelectrolyte brushes to reversibly change their physical properties from extended and repulsive to collapsed and adhesive structures. Collapse and Adhesion with La3+. The experiment presented in Figure 2, as with all experiments presented in this paper, was performed according to protocol outlined in previous work.22 As this protocol dictates, these brushes were originally tested in equilibrium environments of 0.3 M NaNO3, followed by a complete dilution of the SFA chamber to pure water and further surface force measurements. After ensuring proper behavior of polyelectrolyte brushes in these environments, a concentrated aliquot of La3+ was injected into the SFA chamber to create a bulk solution of 1.0 × 10−7 M La3+. At each respective bulk ionic environment inside the SFA chamber, two NaPSS brushes were brought in and out of contact, resulting in the corresponding force profiles. The compression and

where Q is a charge quantity associated with the amount of the cationic complex Ru(NH3)63+ inside a brush, n = 1 is the number of electrons involved in the redox reaction of Ru(NH3)63+, F is the Faraday constant, and A is the area of the electrode. Γ, the surface concentration of Ru(NH3)63+ inside a brush, was normalized by a saturation value Γsat to calculate θ. Γsat is the surface concentration where all electrostatic charges inside the brush are compensated by trivalent Ru(NH3)63+ cations; the Γsat solution has low overall ionic strength, I0 = 0.003 M, and a high bulk concentration of Ru(NH3)63+ of 3 × 10−5 M. The Γsat solution contains the same mono- and trivalent concentrations used in the high La3+ ionic environment (blue diamonds) presented in Figures 5−7. The reduction potential of La3+ is lower than the reduction potential of H+ in water, therefore, La3+ is not a suitable electrochemical probe and electrochemical measurements using La3+ were not performed.

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RESULTS AND DISCUSSION Initial measurements of PtBS20−NaPSS420 brushes in the presence of La3+ were performed without the presence of monovalent sodium (Na+) ions in bulk solution (Figure 2). These measurements were largely intended to understand the 14807

DOI: 10.1021/acs.jpcc.5b02121 J. Phys. Chem. C 2015, 119, 14805−14814

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work,22 equilibrium between multivalent counterions and polyelectrolyte brushes is mostly observed when the brush is collapsed/saturated with multivalent ions, or when sufficient monovalent ions are present in solution (i.e., high ionic strength) to control the exchange of multivalent ions into and out of the brush. Another observation to show that La3+ uptake is occurring can be observed in the abrupt change in slope of the force profiles as the brushes are compressed more heavily. This change in slope, which occurs at approximately 5000 μN/ m, is an indication of the onset of strong attractive forces within the polyelectrolyte brush. The steeper slope beyond 5000 μN/ m has a closer resemblance to the force profile of a collapsed brush, while the more gradual slope below 5000 μN/m is more similar to the typical force profile of an extended brush. These unique types of force profiles are seen before the measurement of adhesion and a more abrupt collapse of polyelectrolyte chains. In comparison with electrochemistry experiments from previously published work,22 this corresponds to a scenario where trivalent counterions are beginning to replace their monovalent counterparts. This type of brush can also be termed as an “intermediate” brush state, found between more extended and fully collapsed brushes, and explains the decreased brush height in comparison to the experiment at fixed ionic strength. As bulk La3+ concentration was increased by 1 order of magnitude, the polyelectrolyte brushes were observed to undergo a dramatic change in structure and behavior. While clearly the heights of the polyelectrolyte chains can be observed to have decreased, there is also the appearance of adhesion upon the separation of the two brushes (adhesion is represented by negative forces). This adhesion is observed at the two highest concentrations of La3+, 1.0 × 10−6 M (blue, circles) and 1.1 × 10−5 M (green, diamonds), and is followed by a rapid separation of the surfaces (“jump-out”); the jumpout occurs when the gradient of the force−distance curve equals the spring constant of the cantilever force measuring spring. The magnitude of adhesion is observed to increase as the bulk concentration of La3+ is again increased from 1.0 × 10−6 M to 1.1 × 10−5 M. Adhesion in the presence of La3+ ions was measured to be as high as approximately 30000 μN/m. The magnitude of this adhesion between two polyelectrolyte brushes, which is consistent with multivalent bridging between La3+ counterions and chains of opposite brushes, is unique to experiments with La3+. Another interesting difference in the force profiles at these two concentrations, which had not previously been observed with other multivalent counterions, is the existence of a “jumpin” on the compression of the brushes at 1.1 × 10−5 M. This subtle jump-in is believed to be a result of multivalent attractions between the trivalent counterions inside the polyelectrolyte brushes and the charged monomer segments of oppositely tethered chains. A typical compression force profile, which contains a jump-in, can be observed more clearly in Figure 7 and will be discussed in more detail. Reversal of Brush Properties with Na+. Beginning with the strongly adhesive, collapsed brushes from the final concentration of Figure 2, additional measurements in this experiment were performed by adding NaNO3 to the bulk solution inside the SFA chamber. The resulting data of these measurements are plotted in Figure 3. The bulk concentration of Na+ ions was increased, as seen in the legend, via injections of concentrated aliquots. Here, the lowest two concentrations of monovalent Na+ correspond to a salt environment consistent

separation data for each force profile are distinguished apart by closed and open data points, respectively. After force measurements at the 1.0 × 10−7 M La3+ concentration, the La3+ concentration inside the SFA was again changed to 1.0 × 10−6 M, and finally to a 1.1 × 10−5 M solution. The results plotted in Figure 2 demonstrate a transition from more extended, repulsive brushes to collapsed, adhesive ones as the concentration of La3+ is increased. One interesting aspect of the force profile at the 1.0 × 10−7 M La3+ concentration (red, triangles) is that the brushes are initially less extended than corresponding force profiles from 1.0 × 10−7 M La3+ concentrations in the fixed ionic strength experiment of I0 = 0.003 M, as shown in Figure 4. Here, the

Figure 4. Compression and separation force vs distance profiles performed in a fixed overall ionic strength solution of I0 = 0.003 M, using both Na+ and La3+. Bulk lanthanum concentrations were sequentially increased from the lowest concentration (0.1 × 10−6 M) to the highest (3.0 × 10−6 M). (a) Shows compression and separation of the surfaces with normal scaling. (b) Compression forces are plotted on semilog scale.

average brush height is approximately 75 nm, while in Figure 4 the average polyelectrolyte brush height is approximately 97.5 nm. This measured difference in brush height is most likely a result of trivalent La3+ uptake by the brush, which has been described in previous work as a dynamic event limited only by diffusion at low concentrations of Na+.22 Here, we want to further acknowledge that the brushes in this ionic environment have not yet reached equilibrium. As emphasized in previous 14808

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concentrations.22 In all cases, however, the transition to a collapsed brush is a direct result of trivalent counterion uptake. The small range of La3+ concentration (0.1 to 3.0 × 10−6 M) plotted in Figure 4 was purposely selected for this presentation to emphasize the sensitivity of the collapse of these polyelectrolyte chains. Note that pronounced shrinkage of the brush, eventually leading to attractive forces between brushes, sets in when the molar ratio of trivalent to monovalent ions is close to 0.001. In this small range of bulk La3+ concentration, the concentration of La3+ becomes sufficient enough that diffusion no longer limits the uptake of the counterions into the brushes. Subsequently, the trivalent La3+ cations replace monovalent Na+ as the counterions inside the brushes. The replacement of monovalent Na+ with trivalent La3+ is driven in part by the entropic gain of the system when trivalent cations are associated with the brush. When one La3+ counterion is taken up by the brush, three Na+ counterions can be released.22 The 0.1 and 0.5 × 10−6 M La3+ bulk concentration force profiles show that the repulsion upon approach of the two brushes begins at approximately 195 nm. The onset of this repulsion signifies that two brushes with brush heights of 97.5 nm (half the repulsion distance) have come into physical contact with one another. These brushes are considered “extended” brushes and contain essentially all Na+ cations acting as their counterions; as a reference, the fully stretched contour length of the chains is approximately 105 nm.20 At the La3+ concentration of 1.5 × 10−6 M, the range of repulsion can be observed to decrease; on doubling the concentration to 3.0 × 10−6 M, the polyelectrolyte chains undergo a dramatic transformation. Not only do the brush heights decrease to approximately 22.5 nm, but the presence of adhesion is measured upon separating the two brushes. As mentioned earlier, this adhesion is a result of multivalent bridging between the two opposite brushes made possible by trivalent La3+ counterions now directly interacting with the brushes. Lanthanum Release Experiment. Here, we show that the complete reversal of brush collapse and adhesion, measured previously in Figure 3, is directly related to the release of lanthanum counterions from the polyelectrolyte brushes. In our past work,22 we measured the release of Ru(NH3)63+ complexes acting as counterions from an electrostatically trivalent saturated brush via cyclic voltammetry experiments. Results showed that approximately 99% of Ru(NH3)63+ counterions were released from a saturated brush after only 30 s of submersion in a purely monovalent environment of high ionic strength (I0 = 0.3 M, SB). Conversely, we demonstrated that if a Ru(NH3)63+ saturated brush was placed in a low ionic strength environment (I0 = 0.003 M, OsB), the Ru(NH3)63+ counterions remained in the brush for time scales on the order of weeks. Figure 5 presents SFA measurements demonstrating the disappearance of intermolecular adhesion between brushes; this experiment was performed using similar methodology as the aforementioned electrochemical “release” experiment. The SFA experiment in Figure 5 was initiated by first electrostatically saturating polyelectrolyte brushes with La3+ counterions in a low ionic strength (I0 = 0.003 M) solution containing 3 × 10−5 M bulk La3+ cation concentration. Surface force measurements of brushes in this environment (blue diamonds) represent extremely adhesive and highly collapsed structures (approximately 6.8 nm brush height). The grafting density of PtBS-NaPSS in our study is close to 1 chain per 57 nm2 (0.0176 chains/nm2). This grafting density gives rise to a loosely packed NaPSS brush layer, which gives enough space

with an osmotic brush (OsB) regime, as dictated by monovalent salt theory.27,28 The highest concentration, 54.0 mM Na+ (green, diamonds), is considered to be a salted brush (SB) regime, also defined by the same theory. The incremental addition of Na+ ions results in a slight alteration of the bulk concentration of La3+ ions, ranging from 1.1 × 10−5 to 1.1 × 10−5 to 8.9 × 10−6 M, respectively. From Figure 3, the effect of these monovalent sodium cations in solution is quite clear; the brushes are observed to transition from collapsed and adhesive, at the lower concentrations of Na+, to once again extended and repulsive at the highest monovalent concentration of 54.0 mM. Although the polyelectrolyte brushes remain adhesive in both the 3.5 × 10−5 M (red, triangles) and 7.8 mM (blue, circles) Na+ concentrations, changes between the force curves were observed; most notably a decrease in adhesion strength from −18,000 to −7,500 μN/m. This observation also coincides with the disappearance of the previously described subtle jump-in, which was also observed in the 3.5 × 10 −5 M Na + concentration. Both the stronger adhesion and the disappearance of a jump-in at the higher monovalent concentration can be associated with an increased competition between mono and trivalent counterions. This can also be explained by an increase in electrostatic screening; dictated by a decreased Debye length from 30.6 nm at 3.5 × 10−5 M to 3.5 nm in the 7.8 mM Na+ concentration. As a result of a further increase in bulk Na+ concentration to 54.0 mM (which is in the SB regime of monovalent salt theory), the polyelectrolyte chains of the brushes were observed to re-extend themselves, as adhesion disappeared; that is, the compression and separation of the polyelectrolyte brushes once again becomes nonhysteretic. This reversal of physical properties is a direct result of the release of La3+ ions by the polyelectrolyte brushes. This phenomenon will be discussed in greater detail later in this paper with respect to surface force measurements performed using an identical protocol to electrochemistry experiments, which demonstrated the release of Ru(NH3)63+ from electrostatically saturated polyelectrolyte brushes. Brush Collapse in Fixed Ionic Strength Experiment (I0 = 0.003 M). Figure 4 demonstrates typical intermolecular force results from the compression and separation of two endtethered NaPSS brushes as they transition from extended to collapsed structures in the presence of trivalent La3+ at a fixed ionic strength in the OsB regime. The specific experiment represented in Figure 4 was performed in a solution of fixed overall ionic strength, I0 = 0.003 M, using NaNO3 aqueous media. In this experiment, the bulk trivalent concentration of La3+ was systematically increased with concentrated injections of La3+ solutions, employing identical concentrations and conditions used in previously run SFA and electrochemistry experiments using Ru(NH3)63+;22 bulk concentrations of La3+ can be seen in the legend of the graph. A systematic La3+ injection method was necessary to control the exposure time of the polyelectrolyte brushes to multivalent La3+ cations. Controlling the exposure time of solutions to these polyelectrolyte brushes was critical for a proper comparison to previous work due to the dynamic nature of these measurements. As previously described, in low ionic strength environments such as this, the uptake of trivalent ions into a polyelectrolyte brush is limited only by the diffusion of cations into the brush. Polyelectrolyte brush collapse in solutions of higher ionic strength occur at increased bulk trivalent 14809

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independent measurements were performed in identical ionic conditions; 3 × 10−5 M trivalent concentration and overall ionic strength of I0 = 0.003, OsB regime. The La3+ data in these figures are the same as the data plotted in Figure 5. In our past work,22 we showed that polyelectrolyte brushes in this environment are electrostatically saturated with Ru(NH3)63+ complexes acting as counterions. As will be described in more detail with Figure 8, results indicate that this is also true for brushes with La3+. However, there are also distinct differences between the force profiles measured in the La3+ experiment compared to the Ru(NH3)63+ one. From Figure 6, the most obvious difference between the surface force data measured in these separate experiments is the

Figure 5. Compression and separation force vs distance profiles demonstrating the release of trivalent La3+. This SFA experiment begins by first electrostatically saturating brushes with La3+ in an ionic environment of 3 × 10−5 M bulk La3+ concentration and ionic strength, I0 = 0.003 M (blue diamonds). Lanthanum cations in the bulk environment were then removed and the brushes were placed in a purely aqueous environment for 10 h (orange circles). Finally, the brushes were placed in a purely monovalent high ionic strength environment, I0 = 0.3 M, using NaNO3 for 30 min (brown squares).

for the NaPSS chains to reconfigure into a very compact layer, as reported by our previous study.22 The adhesive strength measured on separation of these collapsed brushes reaches almost 30000 μN/m. This collapse and adhesion of brushes is notably more exaggerated than the data presented in Figure 4 as a result of excess La3+ ions in the bulk environment. As mentioned previously, adhesion strength on this order of magnitude was never measured with surface force experiments in the presence of Ru(NH3)63+, which in part is a result of the increased size and hence decreased charge density of the Ru(NH3)63+ complexes in comparison to La3+ cations. After this measurement of strong adhesion in the high bulk La3+ concentration, the surrounding ionic environment was changed to pure water. The brushes remained in this aqueous solution for 10 h and subsequently remained collapsed and highly adhesive (orange circles); further supporting our previously described electrochemical data22 which showed that trivalent counterions remain associated with the polyelectrolyte chains if the bulk solution does not contain sufficient oppositely charged ions. The surrounding ionic environment was then changed to a purely monovalent salt system at high ionic strength (SB regime), I0 = 0.3 M (brown squares). Here, the brushes undergo a dramatic transformation to extended and purely repulsive after only 30 min of contact with the high monovalent ionic strength environment. Average brush heights in this high ionic strength environment were measured at 38.2 nm, which agrees with brush heights measured in monovalent solutions of I0 = 0.3 M from our previous work.20−22 This full recovery of brush height and disappearance of adhesion further supports the complete release of La3+ counterions from the brushes. We have found this reversible switching of adhesion to be consistently repeatable via surface force measurements. Comparison between La3+ and Ru(NH3)63+ Surface Force Data. Figures 6 and 7 are included in order to provide a direct comparison between surface force measurements of NaPSS brushes in the presence of La3+ and Ru(NH3)63+. These

Figure 6. Compression and separation force vs distance profiles from independent experiments demonstrating the difference between the effects of trivalent La3+ and Ru(NH3)63+ inside a brush. Here, the NaPSS brushes were immersed in identical solutions of trivalent concentration of 3 × 10−5 M, and an ionic strength of I0 = 0.003. This ionic environment of higher trivalent concentration is consistent with solutions which saturate the brushes with trivalent counterions. The La3+ data are identical to the force profile in Figure 5 at this same concentration.

adhesive energy upon separation of the brushes; which is over an order of magnitude greater for brushes containing La3+ counterions than adhesion between Ru(NH3)63+-compensated brushes. Another clear observation is the reduced brush height in the collapsed form of the brushes from the different experiments. These differences between polyelectrolyte brush behavior is a direct result of increased multivalent attractions between La3+ counterions and the charged monomer segments of the NaPSS chains. Figure 7 highlights the compression of these NaPSS brushes and demonstrates adhesion measured on the approach of the two La3+ brushes, which is clearly not observed in the experiment with Ru(NH3)63+ saturated brushes. This jump-in is also believed to be associated with electrostatic multivalent attraction between the trivalent La3+ counterions inside a brush and the polyelectrolyte chains of an opposite surface. Jump-ins are commonly observed upon the compression of polyelectrolyte brushes in the presence of La3+, and are dependent upon the ionic environment. However, jump-ins have not been observed in surface force measurements using other multivalent counterions with NaPSS polyelectrolyte brushes. These other counterions have consisted of ruthenium hexamine Ru(NH3)63+, divalent calcium Ca2+, trivalent aluminum Al3+, and 14810

DOI: 10.1021/acs.jpcc.5b02121 J. Phys. Chem. C 2015, 119, 14805−14814

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throughout surface force experiments with different positively charged ions, complexes, and aggregates. Comparison to Electrochemistry Experiments with Ru(NH3)63+ (I0 = 0.003 M). Figure 8 compares surface force results with trivalent La3+ cations to previously reported22 surface force measurements using the trivalent complex of Ru(NH3)63+. These experiments were performed at the fixed overall ionic strength of I0 = 0.003 M, which is within the osmotic brush regime where many of the ions remain within the brush to keep it globally electrically neutral. Additionally, the two sets of experiments were performed imposing identical trivalent concentrations, as well as exposure times between measurements, in order to produce comparable data sets. The red squares represent polyelectrolyte brush height data from La3+ SFA measurements and include the results presented in Figure 4. The blue triangles in Figure 8 represent polyelectrolyte brush heights from SFA experiments with Ru(NH3)63+.22 The open/unfilled data points designate measurements where adhesion (negative force) was observed upon separation of the brushes. The solid data points represent measurements without adhesion. Upon a comparison of the La3+ and Ru(NH3)63+ data, the most obvious observation is that the collapse of polyelectrolyte chains and the presence of adhesion occur at the exact same trivalent bulk concentration. This phenomenon occurs as a direct result of uptake of trivalent cations into the brushes, which was confirmed via electrochemistry experiments using Ru(NH3)63+. Ru(NH3)63+ counterion concentration inside a polyelectrolyte brush, represented by the orange circles in Figure 8, was measured via cyclic voltammetry experiments and was previously reported.22 For these experiments, Ru(NH3)63+ was used as both a trivalent counterion and a probe intended to measure the population of trivalent counterions inside a polyelectrolyte brush. The secondary y-axis in Figure 8 corresponds to θ, the normalized amount of Ru(NH3)63+ inside a polyelectrolyte brush for a given system. In the scenario where θ = 1, the polyelectrolyte chains are associated with only trivalent counterions (i.e., trivalent electrostatic saturation of the brushes has been reached). Conversely, θ = 0 coincides with polyelectrolyte brushes containing only monovalent Na+ counterions. Figure 8 shows a clear and dramatic increase of θ at the bulk Ru(NH3)63+ concentration of 3.0 × 10−6 M. This measurement coincides precisely with the independent observation of reduced chain length and adhesion measured via SFA experiments. While this has been previously established with past work in purely Ru(NH3)63+ trivalent environments,22 based on the aforementioned comparison between La3+ and Ru(NH3)63+ surface force results, we now conclude that the uptake of trivalent entities by polyelectrolyte brushes is a consistent and repeatable property of various trivalent ions. Comparison to Electrochemistry Experiments with Ru(NH3)63+ (I0 = 0.3 M). A final comparison is made between polyelectrolyte brush heights with La3+ and Ru(NH3)63+, respectively, in a I0 = 0.3 M (salted brush regime) surrounding ionic environment. The brush heights in this plot come from surface force measurements, with La3+ cations measured in a similar systematic way, as described with the data from Figure 8. The Ru(NH3)63+ data comes from previously presented work.22 From Figure 9, similar trends can be observed as mentioned in the comparison between these two counterions in the low ionic strength experiments. Again, physical property changes of polyelectrolyte brushes in the presence of La3+ and

Figure 7. This data highlights the compression force profiles plotted in Figure 6; separation data was removed and the y-axis is altered. Adhesion between NaPSS brushes is measured upon compression of the brushes in the presence of La3+, in the form of a jump-in. This is not observed during compression of brushes with Ru(NH3)63+.

also multivalent aggregates of cetyltrimethylammonium bromide (CTAB).29,30 There is no general understanding yet of what factors produce this strong adhesion on approach in La3+, but not in any other multivalent ion we have examined. Size, valence, and hydration likely all play important roles. Naturally, the valence state of the counterion plays a significant role in expected behavior; in past work29 adhesive forces measured upon separation with divalent calcium Ca2+ were observed to have similar adhesive strengths as Ru(NH3)63+ saturated brushes. However, there are other factors which come into play, such as the size and charge of the counterions, as well as their corresponding hydration radii. Many of these differences have been discussed in various studies, including those of the Hofmeister series, where differences between anions and cations were studied in regard to their ability to dissolve or precipitate proteins.31−33 Other studies on the effects of ions have consisted of surface forces measured with the SFA.34−36 In all cases, differences among ions of the same valence state have been observed. In this work, size effects can be used to explain certain aspects of interactions between counterions and polyelectrolyte brushes. In Figure 6, for example, the aforementioned larger collapsed brush heights with Ru(NH3)63+ are believed to be a result of the size of the complex ion, which is significantly larger than that of La3+. We hypothesize that the greater size of Ru(NH3)63+ counterion complexes creates more resistance to multivalent bridging on both compression (jump-ins) and separation (jump-outs) of the surfaces. In the case of surface force experiments with Al3+ cations from past work,29 hydration effects play an important role in comparison to experiments with La3+. As mentioned earlier, in Al3+ experiments, jump-in measurements on the compression of two NaPSS brushes were not observed. While aluminum ions are actually smaller than La3+ ones, Al3+ ions have a larger hydrated ionic radius (0.48 nm)36 as compared to that of lanthanum (0.31 nm).37 This increased radius is a direct result of a larger hydration layer surrounding the Al3+ cations, which, in turn, creates increased resistance to multivalent adhesion. These size and hydration effects clearly have an impact on some of the trends seen 14811

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counterions inside the brush is increased. On a comparison between brush height data and electrochemistry data, one can see that the physical property changes in brush height and adhesion are a direct result of steady increases in trivalent population within these brushes. Unlike brush collapse in the OsB regime, which occurs abruptly, here changes are observed over a wide range of trivalent concentrations. This can be explained by the increased competition between mono and trivalent counterions in this high ionic strength solution. Also noted from this comparison of trivalent counterions in the SB regime are shorter brush heights when the polyelectrolyte chains are collapsed, as well as stronger adhesion (again by an order of magnitude) observed with La3+ compensated brushes. This observation is consistent with results in the OsB regime and further supports the theory that the increased size of Ru(NH3)63+ counterions creates more resistance to multivalent bridging and attractions.

Figure 8. Comparison of independent polyelectrolyte brush surface force experiments using La3+ and Ru(NH3)63+ at the fixed ionic strength of I0 = 0.003 M; also plotted with trivalent counterion population measured via electrochemistry. This figure shows the combination of three independent experiments performed with identical protocol and plotted with respect to the bulk trivalent concentration of La3+ and Ru(NH3)63+ (x-axis). Polyelectrolyte brush heights (red squares, La3+ and blue triangles, Ru(NH3)63+) were measured using the SFA and are plotted on the y-axis. Open data points represent surface force measurements where adhesion was present on separation of two brushes. The normalized amount of Ru(NH3)63+ inside a polyelectrolyte brush (θ, orange circles) was measured via electrochemistry experiments and is plotted on the secondary y-axis.

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CONCLUSIONS Our data show that the chains of NaPSS polyelectrolyte brushes are able to reversibly transition from extended and repulsive to collapsed and extremely adhesive in the presence of trivalent lanthanum. These findings directly support our previous contention22 that trivalent counterions can be reversibly up-taken and released by polyelectrolyte brushes. This work also demonstrates that La3+ cations interact with polyelectrolyte chains in a unique fashion due to their small size and hydration radii. Adhesion measured upon the separation of brushes saturated with La3+ cations acting as counterions is over an order of magnitude greater than adhesion measured with Ru(NH3)63+ complexes in identical ionic environments. Additionally, adhesion can be produced upon the compression of La3+ saturated polyelectrolyte brushes in the form of a characteristic “jump-in,” which has not been observed with Ru(NH3)63+ or various other cations and complexes tested in past work. Our results on the collapse of NaPSS brushes in the presence of La3+ and Ru(NH3)63+ agree well with various theoretical work, showing that multivalent counterions can cause full collapse of polyelectrolyte brushes.38−41 Indeed, it has also been suggested that counterion size can be an important factor affecting the properties of polyelectrolyte brushes,39 but no computer simulation work has studied this topic carefully. Moreover, to our best knowledge, no theoretical work has predicted the full recovery of brush height and disappearance of adhesion via release of La3+ cations from negatively charged polyelectrolyte chains. We believe that this work can have a significant impact in a wide range of applications involving polyelectrolytes, ranging from responsive and tunable materials to drug delivery to energy storage. For example, this work could alter mechanisms in which therapeutics or drugs are delivered to the body, or could potentially lead to breakthroughs in batteries designed with higher energy densities by employing our described mechanism of multivalent cation uptake and release. Studies of polymer brushes have developed a great deal since the early years of end-tethered polymer theory described by Alexander42,43 and de Gennes44 and original discussions of polyelectrolyte brushes by Pincus.28 We believe there still remains a great deal to be learned from these highly controlled polyelectrolyte structures, especially in regard to polyelectrolyte interactions with multivalent ions and complexes.

Figure 9. Comparison of independent polyelectrolyte brush surface force experiments using La3+ and Ru(NH3)63+ at the fixed ionic strength of I0 = 0.3 M.

Ru(NH3)63+ can be observed to occur at similar trivalent bulk concentrations. From the graph, one can see a clear decrease in brush height for both sets of experiments as trivalent bulk concentration is increased, consisting of small changes at lower concentrations, followed by a more abrupt collapse at higher concentrations. While this final decrease in polyelectrolyte brush height occurs at a slightly higher La3+ concentration than Ru(NH3)63+, the onset of adhesion appears at identical trivalent concentrations for each of these independent experiments, again, when the molar ratio of trivalent to monovalent ions is close to 0.001. As with Figure 8, adhesion and brush height changes in Figure 9 are observed to occur as the amount of trivalent 14812

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

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S Supporting Information *

Four supporting figures. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b02121. Corresponding Author

*E-mail: [email protected]. Phone: 773-834-2001. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the contribution of Ms. Cui Fan to some of the experimental work reported here. This work was supported by the U.S. Department of Energy, Office of Science, Program in Basic Energy Sciences, Division of Materials Science and Engineering.

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DEDICATION This paper is dedicated to our colleague Steve Sibener on the occasion of his 60th birthday. REFERENCES

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