Polymer-Mediated Clustering of Charged Anisotropic Colloids

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.

Microstructure and Soft Glassy Dynamics of an Aqueous Laponite Dispersion

“…However, beyond a certain molar mass of PEO, Mongondry et al 155 propose bridging between Laponite particles, which leads to the formation of clusters. Such bridged Laponite-PEO structure has also been reported by Thuresson et al 154 , Loizou et al 159 and Baghdadi et al 160 The delay in formation of the arrested state in Laponite-PEO dispersion has been monitored using rheology, 153,160 DLS 60,161,162 and SANS studies. 155,157,158 Atmuri et al 163 studied the effect of molecular weight and concentration of PEO on aging behavior of Laponite dispersion.…”

Equilibrium clusters in suspensions of colloids interacting via potentials with a local minimum

“…The term "clusters" refers to a large variety of objects that range in size from small multimers to mesoscopic domains [1] and arise as a consequence of monomer self-association in a large variety of soft materials [2]. Most often, clusters are discussed in reference to colloidal suspensions [3], where they exist in equilibrium with monomers, but they were also reported for proteins [4], synthetic clays [5] and metal nanoparticles [6]. Equilibrium clusters become the dominant species in the solution at appropriate thermodynamic conditions.…”

Polyampholyte‐ and nanosilicate‐based soft bionanocomposites with tailorable mechanical and cell adhesion properties

“…With increasing concentrations of laponite in the system, the mechanical strength increases . These mechanical behaviors could be attributed to the presence of both physically and covalently crosslinked structures within the same nanocomposite …”

“…47 These mechanical behaviors could be attributed to the presence of both physically and covalently crosslinked structures within the same nanocomposite. 20,21,50,51 The PLL (0.65) compound used in this study is a polyampholyte; the charge on this polymer can be altered by varying succination and the pH of the solution. In this study, we focused only on PLL (0.65) at pH 7.4 with the goal of designing a system for biomedical application.…”