Dispersions of poly(methyl methacrylate) (PMMA) latexes were prepared in a low dielectric, nonpolar solvent (dodecane) both with and without the oil-soluble electrolyte, tetradodecylammonium-tetrakis(3,5-bis(trifluoromethyl)phenyl)borate. For dispersions with a high concentration of background electrolyte, the latexes become colloidally unstable and sediment in a short period of time (<1 h). This is completely reversible upon dilution. Instability of the dispersions is due to an apparent attraction between the colloids, directly observed using optical tweezers by bringing optically trapped particles into close proximity. Simple explanations generally used by colloid scientists to explain loss of stability (charge screening or stabilizer collapse) are insufficient to explain this observation. This unexpected interaction seems, therefore, to be a consequence of the materials that can be dispersed in low dielectric media and is expected to have ramifications for studying colloids in such solvents.

Highlights•We review the process of delayed collapse in weak colloidal gels.•We present new data on the microscopic processes which occur during delay.•We propose a new microscopic model of collapse.•We validate the model by analyzing new experimental data on the effect of an external force on gel collapse.Attractive colloidal particles can form a disordered elastic solid or gel when quenched into a two-phase region, if the volume fraction is sufficiently large. When the interactions are comparable to thermal energies the stress-bearing network within the gel restructures over time as individual particle bonds break and reform. Typically, under gravity such weak gels show a prolonged period of either no or very slow settling, followed by a sudden and rapid collapse – a phenomenon known as delayed collapse. The link between local bond breaking events and the macroscopic process of delayed collapse is not well understood. Here we summarize the main features of delayed collapse and discuss the microscopic processes which cause it. We present a plausible model which connects the kinetics of bond breaking to gel collapse and test the model by exploring the effect of an applied external force on the stability of a gel.Graphical abstract

We present a straightforward strategy for the synthesis of highly charged poly(ionic liquid)-functionalized particles in low-polarity solvents. A series of cationic liquid monomers consisting of a tetraalkyl ammonium cation and a fluorinated tetrakis[phenyl] borate anion linked, via a C3-alkyl chain, to a methacrylate unit were synthesized. The addition of this ionic monomer to a conventional dispersion polymerization of methyl methacrylate and methacrylic acid at 80 °C in a mixed dodecane/hexane solvent yielded spherical, highly monodisperse particles with mean diameters of between ∼50 and 2500 nm with high electrophoretic mobility and stability in nonpolar solvents such as dodecane. The surface potential in dodecane could be adjusted in the range from 0 to 180 ± 9 mV by altering the ratio of ionic monomer to methacrylate monomers. The particles open up new opportunities for the electrostatic assembly of nanoparticles and organized structures in nonpolar environments.

We report measurements on the ageing dynamics of a colloid–polymer mixture with a large polymer–colloid size ratio of 0.62. Quenched into a two-phase region the system gels and forms a network with a characteristic radius Rc. We find three distinct regimes in the time evolution of Rc(t), reminiscent of the linear, late and gravity-dominated regimes of coarsening seen in classical spinodal decomposition kinetics of binary fluids. In the early stages of gelation, we observe a peak in the time-dependent structure factor S(q, t) which is stationary in q and grows in intensity characteristic of a linear-Cahn regime. The domain size then coarsens continuously with the age t of the sample. In the late stages the domain size follows the approximate algebraic law, Rc ∼ tθ. The growth exponent θ is a strong function of the quench depth: for small polymer concentrations θ is significantly larger than for large polymer concentrations. The gel networks formed are transient, and in the final stages of phase separation, collapse under gravity when the correlation length of the gel becomes ∼ 2π times the capillary length.

A novel method for creating charged polymer particles in non-polar organic solvents is demonstrated. A small amount of an oleophilic tetraalkylammonium tetraphenylborate group is incorporated into a poly(methyl methacrylate) particle to provide ionic groups which dissociate in low permittivity solvents.

Electrostatic forces are typically produced in low polarity solvents by the addition of surfactants or charge-control additives. Although widely used, there is no consensus on the mechanism by which surfactants control the level of particle charge. We report an investigation using highly sensitive, single particle optical microelectrophoresis measurements combined with a small-angle neutron scattering study to establish the mechanism of charging by the surfactant AOT in the nonpolar solvent n-dodecane. We show that polymer-grafted particles with no chemically bound surface charges only charge above the critical micellar concentration of the surfactant. The surface potential increases gradually with increasing surfactant concentration c, before finally saturating at high c. The increase in the surface potential is correlated to the amount of surfactant adsorbed onto the surface of the particle. Using deuterated AOT and contrast variation techniques, we demonstrate that the surfactant is adsorbed within the polymer layer surrounding the particle core, probably as individual molecules rather than surfactant aggregates. A simple thermodynamic model accounts for the concentration dependence of the observed surface potential.

Colloids dispersed in a nonpolar solvent become charged when reverse micelles are added. We study the charge of individual sterically stabilized poly(methyl methacrylate) spheres dispersed in micellar solutions of the surfactants sodium bis(2-ethyl 1-hexyl) sulfosuccinate [AOT], zirconyl 2-ethyl hexanoate [Zr(Oct)2], and a copolymer of poly(12-hydroxystearic acid)−poly(methyl methacrylate) [PHSA-PMMA]. Although the sign of the particle charge is positive for Zr(Oct)2, negative for AOT, and essentially neutral for PHSA-PMMA, the different micellar systems display a number of common features. In particular, we demonstrate that over a wide range of concentrations the particle potential is a constant, independent of the number of micelles added and independent of the colloid size. A simple thermodynamic model, in which the particle charge is generated by the competitive adsorption of both positive and negative micelles, is in good agreement with the experimental data.