ABSTRACT
Electrostatic forces between small groups of colloidal particles are measured using blinking optical tweezers. When the electrostatic screening length is longer than the interparticle separation, forces are found to be non-pairwise-additive. Both pair and multiparticle forces are well described by the linearized Poisson-Boltzmann equation with constant potential boundary conditions. These findings may play an important role in understanding the structure and stability of a wide variety of systems, from micron-sized particles in oil to aqueous nanocolloids.
ABSTRACT
Electrostatic interactions of colloidal particles are typically screened by mobile ions in the solvent. We measure the forces between isolated pairs of colloidal polymer microspheres as the density of bulk ions vanishes. The ionic strength is controlled by varying the concentration of surfactant (NaAOT) in a nonpolar solvent (hexadecane). While interactions are well-described by the familiar screened-Coulomb form at high surfactant concentrations, they are experimentally indistinguishable from bare Coulomb interactions at low surfactant concentration. Interactions are strongest just above the critical micelle concentration, where particles can obtain high surface potentials without significant screening, kappaa << 1. Exploiting the absence of significant charge renormalization, we are able to construct a simple thermodynamic model capturing the role of reverse micelles in charging the particle surface. These measurements provide novel access to electrostatic forces in the limit where the particle size is much less than the screening length, which is relevant not just to the nonpolar suspensions described here, but also to aqueous suspensions of nanoparticles.
Subject(s)
Alkanes/chemistry , Colloids/chemistry , Dioctyl Sulfosuccinic Acid/chemistry , Static Electricity , Surface-Active Agents/chemistry , Microspheres , Osmolar Concentration , Particle Size , Surface PropertiesABSTRACT
We study the electrostatic and hydrodynamic interactions of colloidal particles in nonpolar solvents. Using blinking optical tweezers, we can extract the screening length, kappa-1, the effective surface potential, |ezeta*|, and the hydrodynamic radius, ah, in a single measurement. We apply this technique to suspensions of polystyrene and poly(methyl methacrylate) particles in hexadecane with soluble charge control agents, aerosol sodium di-2-ethylhexylsulfosuccinate (AOT) and polyisobutylene succinimide (OLOA-1200). We find that the electrostatic interactions of these particles depend sensitively on surface composition as well as on the concentration and chemistry of the charge control agent.
Subject(s)
Alkanes/chemistry , Colloids/chemistry , Polymers/chemistry , Polymethyl Methacrylate/chemistry , Polystyrenes/chemistry , Succinates/chemistry , Succinimides/chemistry , Aerosols/chemistry , Optical Tweezers , Particle Size , Solvents/chemistry , Static Electricity , Surface Properties , ThermodynamicsABSTRACT
We describe and implement a technique for extracting forces from the relaxation of an overdamped thermal system with normal modes. At sufficiently short time intervals, the evolution of a normal mode is well described by a one-dimensional Smoluchowski equation with constant drift velocity v, and diffusion coefficent D. By virtue of fluctuation dissipation, these transport coefficients are simply related to conservative forces, F, acting on the normal mode: F=kBTv/D. This relationship implicitly accounts for hydrodynamic interactions, requires no mechanical calibration, makes no assumptions about the form of conservative forces, and requires no prior knowledge of material properties. We apply this method to measure the electrostatic interactions of polymer microspheres suspended in nonpolar microemulsions.
