Salt Fractionation Effect for Spherical Macroions

• Published in: Langmuir Vol. 12; no. 10; pp. 2340 - 2347
• Main Authors: Smalley, M. V, Schärtl, W, Hashimoto, T
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 15-05-1996
• Subjects: Applied sciences; Chemistry; Colloidal state and disperse state; Exact sciences and technology; General and physical chemistry; Organic polymers; Physical and chemical studies. Granulometry. Electrokinetic phenomena; Physicochemistry of polymers; Properties and characterization; Solution and gel properties; Particle size; Fractionation; Electrolyte solution; Crystals; Theoretical study; Particle suspension; Latex; Numerical computation; Debye length; Charged particle; Monodispersed particle; Models; Spherical particle; Macroion; Colloid particle; Surface potential
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la950400d

Ordered structures and void formation in systems of highly charged spherical macroions are investigated using the Sogami potential. The crucial feature of the treatment is the salt fractionation effect; Sogami theory combined with the Dirichlet boundary condition (constant surface potential) yields a definite prediction for the distribution of simple electrolyte between the macroion-rich and macroion-poor regions. The treatment is restricted to the extreme case of an equilibrium between an ordered structure and voids. The voids are found to be stable in the range between aκ = 0.816 and aκ = 3.05, where a is the radius of the particles and κ is the inverse Debye screening length. In this range, added simple electrolyte fractionates between the macroionic crystals and the voids, accumulating strongly in the voids. Added electrolyte does not fractionate outside this range, leading to an extraordinary regime below aκ = 0.816 and to a homogeneous liquid-like structure above aκ = 3.05. Within the range, the hypothesis that the relevant interaction κ in the crystals is determined solely by the fractionated simple salt leads to a definite prediction for the variation in the lattice parameter with added salt. This prediction is in quantitative agreement with recent USAXS measurements and with a physically realistic value for the surface potential of the charged spheres. It is noted that the standard DLVO theory of colloid stability has nothing whatsoever to say about these interesting phenomena.