Reduced Capillary Length Scale in the Application of Ostwald Ripening Theory to the Coarsening of Charged Colloidal Crystals in Electrolyte Solutions

James Baird, Department Chemistry, University of Alabama in Huntsville

A colloidal crystal suspended in an electrolyte solution will ordinarily exchange ions with the surrounding solution and develop a net surface charge density.  The interfacial tension of the charged surface has contributions arising from: (1) background surface tension of the uncharged surface, (2) the chemical energy associated with the adsorption (or desorption) of ions from the surface, and (3) the polarizing effect of the electrostatic field within the double layer.  The chemical and polarization effects are negative and serve to reduce the interfacial tension below the value to be expected for the uncharged surface. The diminished interfacial tension leads to a reduced capillary length scale.  According to the Ostwald ripening theory of particle coarsening, the reduced capillary length will cause the solute supersaturation to decay more rapidly and the colloidal particles to be smaller in size and greater in number than in the absence of the double layer.  Although the length scale for coarsening should is little affected in the case of inorganic colloids, such as AgI, it should be greatly reduced in the case of suspensions of protein crystals, such as apoferritin, catalase, and thaumatin.  The small capillary length scale predicted for protein crystals explains why these crystals not only grow slowly but also achieve only a relatively modest size at equilibrium.  Adjustments can be made to the growth solution pH to increase the capillary length and increase the equilibrium size of protein crystals.  The increased size has the effect of reducing the intensity of the X-ray beams required for the crystallographic determination of protein molecular structure.   

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