Electrostatic Interactions of Colloidal Particles at Vanishing Ionic Strength

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Original Entry: Tom Dimiduk APPHY Fall 2010

Electrostatic Interactions of Colloidal Particles at Vanishing Ionic Strength

Keywords

DLVO, Micelles, Coulombic Repulsion, Surface Charging, Poisson Boltzmann

Summary

Figure 1: Electrostatic forces as a function of NaAOT concentration
Figure 2: "Ionic strength (a), surface potential (b), and charge (c) from fits to data in Figure 1"
Figure 3: "Conductivity of AOT/hexadecane solutions without particles. Symbols indicate measurements. Red dashed line indicates reverse micelle contribution to conductivity. Blue dashed line indicates surfactant monomer contribution to conductivity. Solid black line indicates sum of reverse micelle and monomer contributions. "

The authors have developed a system where they can observe electrostatic repulsion between charged colloids in a system of sufficiently low ionic strength that they standard coulomb interactions apply. They work in a system of hexadecane with sodium aerosol-OT (NaAOT) sulfactant and 600 nm poly(methyl methacrylate) (PMMA) spheres. At low NaAOT concentrations the PMMA is essentially uncharged experiences minimal electrostatic repulsion. Once NaAOT concentration exceeds its CMC inverse micelles form, likely with water trapped in their cores. These micelles can stabilize ions in their cores, facilitating charging of the PMMA spheres. This turns on an electrostatic repulsion between the spheres. Because the total ion concentration in the solution remains very low, screening is minimal and standard (unscreened) coulomb repulsion applies between the spheres. At higher (~ 1 mM) NaAOT concentrations, thermal charging of micelles becomes significant and rising solution ionic strength begins to screen the electrostatic repulsion. (Figure 1)

They are able to obtain surface charge and potential, and solution ionic strength from fits to Figure 1, (Figure 2). These results agree with theory and with a seperate measurement of ionic strength by conductivity (Figure 3).

Soft Matter Discussion

The authors here are able to observe the onset of double layer screening as ionic strength increases from near zero. They also show a method of observing charging of these inverse micelles. These are possible using a numerical solution of the nonlinear Poisson Boltzmann equation.

They are also able to explain the onset of charging of the micelles with an entropic argument, more states are available if some charges break off into micelles. This is of course in equilibrium with the electronic energy increase of increased ionization.