Specific Ion-Dependent Attraction and Phase Behavior of Polymer-Coated Colloids

Abstract

This paper presents results demonstrating the role of temperature and specific ions in mediating attraction between polymer-coated colloids and determining their equilibrium phase behavior. In particular, theoretical predictions of continuum van der Waals attraction between poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO)-coated polystyrene colloids are used to explain measured temperature and specific ion-dependent fluid-gel transitions in dispersions of these particles. Building on previous studies of PEO-PPO-PEO-coated polystyrene colloids dispersed in aqueous NaCl media, this work reports rheologically measured fluid-gel transitions as a function of temperature and NaCl/MgSO4 composition. Adhesive-sphere predictions of percolation thresholds are fit to measured fluid-gel data by allowing the adsorbed copolymer layer thickness as a single adjustable parameter. This allows the attraction between the PEO-PPO-PEO layers to be interpreted as a function of temperature and NaCl/MgSO4 composition. Quantitative predictions of a polymeric van der Waals attraction associated with the layer collapse in diminishing solvent conditions provides a simple mechanism for explaining the measured fluid-gel data as a dynamic percolation transition. Ultimately, this work identifies the importance of continuum polymeric van der Waals attraction for explaining specific ion-dependent phenomena.

Size dependent thermodynamics and kinetics in electric field mediated colloidal crystal assembly

“…2,3 Within the broad range of approaches to colloidal assembly, our narrower focus includes methods where interactions can be reversibly tuned on the kT-scale to manipulate near-equilibrium self-assembly processes. Examples of this type of approach include tunable van der Waals, 18,19 depletion, 20,21 DNA, 22,23 Casimir, 24,25 and electric eld [26][27][28][29][30][31] mediated interactions. In our previous studies based on tuning kT-scale interactions, we directly measured potentials using video microscopy methods, which we connected to equilibrium microstructures and particle scale dynamics using a suite of modeling tools.…”

Specific Ion Effects on Adsorbed Zwitterionic Copolymers

“…Practically, in each salt composition, the ionic strength of NaCl was kept constant at the same value, as shown in Figure 3A−C, and MgSO 4 was added to understand its role in addition to NaCl. Based on our prior work on adding MgSO 4 to adsorbed PEOtb layers, 9,19 we first added 0.3 M MgSO 4 and 0.05 M NaCl to adsorbed PEOtb copolymers (Figure 3D), which results in stronger van der Waals attraction (∼0.7kT) compared to that with only NaCl (∼0.5kT) (Figure 3A), and shorter-range repulsive interactions between PEO layers. A 5 nm reduction in the range of repulsion corresponds to a 2.5 nm dimensional collapse of each adsorbed layer.…”

“…8,15,16 Although PEO is uncharged and NaCl does not affect its solubility, it is well established that MgSO 4 causes PEO to become insoluble in water 17 and renders it ineffective for the stabilization of colloidal particles. 9,18,19 MgSO 4 is often discussed in the context of Hofmeister series 20,21 for saltinduced protein aggregation via specific ion effects; such effects indicate ion-mediated behaviors that are not easily captured alone by electrostatic interactions independent of ion type. Aqueous polyelectrolyte solution behavior is generally captured by considering screening of charge moieties, 22,23 where increasing salt screens intramolecular repulsion to decrease the chain dimensions.…”

“…For example, polyethylene oxide (PEO) is one of few alkylene oxide polymers that fits into water structure. Increasing or decreasing the temperature changes water structure and decreases the solvent quality and PEO solubility in water. ,, Although PEO is uncharged and NaCl does not affect its solubility, it is well established that MgSO 4 causes PEO to become insoluble in water and renders it ineffective for the stabilization of colloidal particles. ,, MgSO 4 is often discussed in the context of Hofmeister series , for salt-induced protein aggregation via specific ion effects; such effects indicate ion-mediated behaviors that are not easily captured alone by electrostatic interactions independent of ion type.…”

Simulating asymmetric colloidal mixture with adhesive hard sphere model

“…2 This model represents the extreme case of the hard sphere system with the square well in the limit of infinitely strong and infinitesimally short ranged attraction, leading to a finite interparticle attractive strength. 15 In the work of Prinsen and Odijk, 16 the SHS model has been applied in a semiempirical way as the authors investigated theoretically the fluid-crystal coexistence of solutions of globular charged nanoparticles such as proteins and inorganic colloids using the stickiness parameters evaluated via the experimental second virial coefficients. SHS model is therefore especially suitable for the modeling of colloidal systems as the range of intercolloidal interactions in dispersions is usually much shorter than the size of colloidal particles.…”