Salt concentration and particle density dependence of electrophoretic mobilities of spherical colloids in aqueous suspension

University of Malaga, Málaga, Andalusia, Spain
Journal of Colloid and Interface Science (Impact Factor: 3.37). 05/2007; 309(2):315-22. DOI: 10.1016/j.jcis.2007.01.006
Source: PubMed


Using laser Doppler velocimetry in the superheterodyne mode, we conducted a systematic study of the electrophoretic mobility of dispersions of small silica spheres (a=18 nm) suspended in water at different salinities and particle concentrations. The concentration of NaCl was varied from 40 microM up to 16 mM, while the particle concentrations were varied between 4.2x10(18) and 2.1x10(20) m-3. We find a decrease of mobility with increasing salt concentrations and an increase with increased particle number densities. The latter observation is not backed by the standard cell model of electrophoresis with Shilov-Zharkikh boundary conditions. Rather, if the experimental data are interpreted within that model, an unexpected change of the zeta potential at constant added salt concentration results. Interestingly, all experimental data collapse onto a single master curve, if plotted versus the ratio C* of particle counterions to added salt ions. We obtain a logarithmic increase of mobility for C*<1 and a plateau for C*>1. This may indicate a change of the Stern layer structure not yet included in the theoretical model.

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    • "These instruments are based on Laser Doppler Velocimetry (LDV) and Phase Analysis Light Scattering (PALS) techniques. Nevertheless these instruments give ensemble averaged results which have been reported to be dependent on the particle concentration [7] and are not adequate when the shape of particles differs significantly from that of a sphere as in the case of the bacterial spores investigated in this work. In the last three decades, optical tweezers [8] [9] (OT), have revealed as a formidable tool in many areas of science. "
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    ABSTRACT: In this work we report on the simultaneous measurement of the hydrodynamic coefficient and the electric charge of single Bacillus subtilis spores. The latter has great importance in protein binding to spores and in the adhesion of spores onto surfaces. The charge and the hydrodynamic coefficient were measured by an accurate procedure based on the analysis of the motion of single spores confined by an optical trap. The technique has been validated using charged spherical polystyrene beads. The excellent agreement of our results with the expected values demonstrates the quality of our procedure. We measured the charge of spores of B. subtilis purified from a wild type strain and from two isogenic mutants characterized by an altered spore surface. Our technique is able to discriminate the three spore types used, by their charge and by their hydrodynamic coefficient which is related to the hydrophobic properties of the spore surface.
    Full-text · Article · Feb 2014 · Colloids and surfaces B: Biointerfaces
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    • "electro-osmotic µ eof and electrophoretic µ e mobilities [10] [11]. To separate the two contributions the electrophoretic mobility has been studied in detail in cells where the contribution of electro-osmosis is either small or well known [1] [12]. It has been shown that µ e depends on colloid and salt concentration. "
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