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


DLVO, Micelles, Coulombic Repulsion, Surface Charging


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.