Article pubs.acs.org/Langmuir
Polymer-Mediated Clustering of Charged Anisotropic Colloids Anand K. Atmuri† and Surita R. Bhatia*,†,‡,§ †
Department of Chemical Engineering, University of Massachusetts Amherst, Amherst, Massachusetts 01003, United States Department of Chemistry, Stony Brook University, Stony Brook, New York 11794, United States § Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11793, United States ‡
ABSTRACT: Formation of stable, dense nanoparticle clusters is interesting due to both the underlying physics and use of nanoclusters in applications such as digital printing, imaging and biosensing, and energy storage. Here, we explore formation of nanoparticle clusters in dispersions of the model disk-shaped colloid Laponite. Under basic conditions, the model disk-shaped colloid Laponite forms a repulsive glass in water due to strong electrostatic interactions. Addition of a nonadsorbing polymer, the sodium salt of poly(acrylic acid) (PAA), induces a depletion attraction between particles. Through dynamic light scattering (DLS) and rheology, we see that the polymer initially causes a transition from the glassy phase to an ergodic fluid. Samples at higher particle concentration age to a weak nonergodic state, while samples at lower Laponite remain as fluids. As the strength of attraction between particles is increased, we find an increase in the fast relaxation time measured via dynamic light scattering (e.g., slowing of the short-time diffusion of a single particle). While this may in part be attributed to an increase in the ionic strength, the aging behavior and glass-fluid transition we observe appear to be unique to the presence of polymer, suggesting that depletion plays an important role. DLS data on the fluid samples were consistent with two widely spaced diffusive relaxation modes, corresponding to motion of single particles and motion of large clusters, although other slow dynamic processes may be present. On the basis of the estimated volume fraction and depletion attraction, we believe the Laponite-PAA suspensions to be either fluids of stable clusters or glasses of clusters, although it is possible that the nonergodic state we observe is instead a gel of clusters. Additionally, the cluster size was found to be stable for at least 120 days and was directly related to the polymer concentration. This may serve as an important means of tuning cluster size in products and processes based on dense nanoparticle assemblies.
■
structure15−17 and mechanical properties18 that can be attributed to formation of a transient network of clusters. A local “frozen” cage structure is observed, and large fluctuations emerge at small wave vectors due to heterogeneities or clusters. At higher volume fractions, two different types of glassy arrested states can be present.19−21 In a recent study, Zukoski and co-workers observed transitions from different arrested states (glass to fluid to gel) by changing the interaction potential from repulsive, to near hard, to attractive in spherical and anisotropic particle suspensions where particles interact only by van der Waals forces and a soft electrostatic repulsion.22,23 A different scenario is seen in particles with short-range attraction complimented by long-range repulsion. In this case, competition between aggregation from the attractive portion of the potential and the stabilizing role of repulsion has been observed. When the repulsion is short-range (σ), simulations3,27 show formation of both Wigner fluids of clusters would form and a glass transition driven by the repulsive interaction. In all the cases studied above, particles are spherical and the polymer is uncharged. Here we report clustering, dynamics, and rheology in a model disk-shaped colloid with a charged nonadsorbing polymer. The colloid we use is Laponite, a diskshaped particle with a diameter of 30 and 1 nm thick. The Laponite crystal is composed of 1500 unit cells28 with the empirical formula Na+0.7[(Si8Mg5.5Li0.3)O22(OH)4]−0.7. Many studies suggest that Laponite disks have a slight positive charge along the rim and slowly dissociate when pH is less than 9, while the pH > 9, the disk has a uniform negative charge.29 These conditions correspond to repulsive electrostatic interparticle interactions.30 In aqueous dispersions at pH > 9, Laponite forms a disordered gel-like solid. Evidence from scattering (small-angle X-ray scattering (SAXS), small-angle neutron scattering (SANS), and light) confirms that this phase is a repulsive colloidal glass.31,32 However, a recent analysis suggests there may be some positive charge on the edge even at higher pH, with roughly 10% of the edge groups being positively charged and slightly decreasing with increasing pH for pH < 11.33 As the surface charge is pH dependent, there has been much debate on whether the structural arrest arises due to the repulsions from overlapping of double layers or from aggregation due to attractive interactions between the edges and faces.31 However, several studies from the literature now generally agree that, with no added salt, Laponite at basic pH forms a repulsive glass.33 The interactions between Laponite particles can be tuned with the addition of salt or polymer. Ruzicka et al. have done a kinetic study on the aging dynamics on Laponite at different particle concentrations34 with added salt35 and have observed different routes for structural arrest. At low concentrations of colloids and high salt concentrations, suspensions form a gel driven by attractions, whereas at higher colloid concentration and low salt concentration, the structural arrest is driven by repulsive interactions and a glassy phase is observed. Mongodry et al.36 observed slowing down of the formation of Laponite gels with salt in the presence of pyrophosphate and poly(ethylene oxide) (PEO). In the former case the rim charge is screened by adsorption of four valence ions, which increases the activation energy for binding, whereas steric hindrance of chains adsorbed onto the Laponite particles slows down aggregation in the latter case. Similarly, Zulian et al.37 have done a kinetic study on the aging dynamics of the repulsive Laponite glass with addition of PEO and observed a slowing of the aging dynamics upon addition of polymer. Labanda and Llorens38 developed a phase diagram for a system of Laponitesodium polyacrylate at different ionic strengths, and in the presence of salt they also observed a variation of viscoelastic properties of the suspension with different molecular weights39 and concentrations40 because of the change in the electrostatic interaction between the particles.41 In our previous studies of Laponite with an adsorbing polymer, PEO, we have explored long-term aging dynamics by varying the length42 and concentration43 of polymer chains and have observed a type of re-entrant behavior, where elasticity is lost or decreased upon addition of polymer. However, the presence of bridging chains complicates analysis of these systems. In this report we present a dynamic light scattering
■
MATERIALS AND METHODS
Laponite RD was obtained from Southern Clay Products (Gonzales, TX). Single platelets of Laponite have a diameter of approximately 30 nm and are 1 nm thick. The sodium salt of PAA with a molecular weight of 5.1 kg/mol was purchased from Sigma-Aldrich. Samples were prepared by first adding the clay to nanopure water adjusted to a pH of 10 by the addition of NaOH. A T25 Basic UltraTurrax homogenizer was used for about 1 min to fully disperse the clay and break up any large aggregates. Suspensions were then stirred for 20 min using a magnetic stirrer and filtered using a 0.45 μm filter. PAA was then added to obtain the desired polymer concentration, cp, and samples were again stirred for 1−2 min to dissolve the polymer. Samples were prepared at two concentrations of Laponite (2 wt % and 3 wt %) and cp varying from 0 to 0.75 wt %. Addition of PAA results in an increase in the ionic strength of the solutions; the concentrations we have chosen lead to samples with an ionic strength of 0.1 mM−1.5 mM. We consider an aging time t = 0 to be the time after the polymer is completely dissolved and stirring has been stopped. Table 1 shows some parameters important for our data interpretation for these experimental conditions, including the effective
Table 1. Estimated Effective Volume Fraction, c/c*, and Strength of Depletion Attraction for Samples under Study Laponite
cp, wt %
Φeff
c/c*
−U/kT
2 wt %
0.25 0.50 0.75 0.25 0.50 0.75
0.26 0.26 0.26 0.39 0.39 0.39
0.06 0.11 0.17 0.06 0.11 0.17
0.15 0.30 0.46 0.21 0.42 0.63
3 wt %
volume fraction of particles, accounting for the electrical double layer; the polymer concentration scaled by c*; and an effective strength of depletion attraction arising from the polymer chains, calculated from the Asakura−Oosawa (AO) potential. Neat Laponite at a concentration of 2 wt % at pH = 10 forms a repulsive glass,31,32 with a high effective volume fraction due to electrostatic repulsions. With the change in ionic strength resulting from added PAA, the effective volume fraction is lower; nevertheless, at both 2 wt % and 3 wt % Laponite, the samples are fairly crowded. Addition of PAA is expected to induce a weak depletion attraction (
