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. 2017 Jan 31;33(4):881-890.
doi: 10.1021/acs.langmuir.6b03863. Epub 2017 Jan 19.

Microelectrophoresis of Silica Rods Using Confocal Microscopy

Henriëtte E Bakker  1 , Thijs H Besseling  1 , Judith E G J Wijnhoven  1 , Peter H Helfferich  1 , Alfons van Blaaderen  1 , Arnout Imhof  1
Affiliations

Microelectrophoresis of Silica Rods Using Confocal Microscopy

Henriëtte E Bakker et al. Langmuir. .

Abstract

The electrophoretic mobility and the zeta potential (ζ) of fluorescently labeled colloidal silica rods, with an aspect ratio of 3.8 and 6.1, were determined with microelectrophoresis measurements using confocal microscopy. In the case where the colloidal particles all move at the same speed parallel to the direction of the electric field, we record a xyz-stack over the whole depth of the capillary. This method is faster and more robust compared to taking xyt-series at different depths inside the capillary to obtain the parabolic flow profile, as was done in previous work from our group. In some cases, rodlike particles do not move all at the same speed in the electric field, but exhibit a velocity that depends on the angle between the long axis of the rod and the electric field. We measured the orientation-dependent velocity of individual silica rods during electrophoresis as a function of κa, where κ-1 is the double layer thickness and a is the radius of the rod associated with the diameter. Thus, we determined the anisotropic electrophoretic mobility of the silica rods with different sized double layers. The size of the double layer was tuned by suspending silica rods in different solvents at different electrolyte concentrations. We compared these results with theoretical predictions. We show that even at already relatively high κa when the Smoluchowski limiting law is assumed to be valid (κa > 10), an orientation dependent velocity was measured. Furthermore, we observed that at decreasing values of κa the anisotropy in the electrophoretic mobility of the rods increases. However, in low polar solvents with κa < 1, this trend was reversed: the anisotropy in the electrophoretic mobility of the rods decreased. We argue that this decrease is due to end effects, which was already predicted theoretically. When end effects are not taken into account, this will lead to strong underestimation of the experimentally determined zeta potential.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
(a) Schematic showing the orientation dependent velocity of a negatively charged spherocylinder subjected to a DC-electric field adapted from Van den Heuvel et al. (b) Sample cell used during microelectrophoresis. (c) Schematic of flow inside the capillary during microelectrophoresis.
Figure 2
Figure 2
Electrophoretic mobility profiles of R2 rods measured in three different ways. Particles were dispersed in 0.55 mM LiCl 85 wt % glycerol–water, κa ≈ 24.5. (a) Measured perpendicular to gravity, ζ = −47.0 ± 0.5 mV. (b) Measured parallel to gravity at a field strength of 3 V mm–1, ζ = −47.0 ± 1.6 mV. The dashed vertical lines indicate the stationary planes. The lines indicate a parabolic-fit through the data. We set z = 0 μm at the maximum of the parabolic profile. Error bars, for xyt cross-correlation and xyt particle tracking, indicate the standard error of the measurement, which is typically smaller than the size of a symbol. The given standard deviation on ζ is the standard deviation on the three different estimates of ζ obtained from the parabolic fits through the data.
Figure 3
Figure 3
(a) Confocal images from an xyt-series of the electrophoresis of R2 silica rods in DMSO-–ater κa = 2.2. Silica rods show an orientation dependent velocity with respect to the applied electric field. (b) Overlay of frames from an xyt-series of the electrophoresis of a silica rod dispersed in CHC κa = 0.04, 30 frames, t = 11 s, Δt = 0.37 s, E = 2 V mm–1. At t = 0, the color of the rod is blue. The rods still exhibit rotational diffusion, due to Brownian motion. (c) Several trajectories of SR29 rods dispersed in CHC, κa = 0.04, during electrophoresis. Position of the center of mass is plotted. Minimum length of trajectory displayed here is 20 frames.
Figure 4
Figure 4
Electrophoresis measurement of R2 silica rods dispersed in DMSO–water, without salt added, κa = 2.2. (a) Orientation dependent mobility in x-direction (red symbols), parallel to electric field, and y-direction (green symbols), perpendicular to applied electric field. Raw data is binned in 4° wide bins. The error bars indicate the standard error on the binned data points. The solid lines are a fit through the raw data using eq 11 (μx) and eq 12 (μy). (b) Histogram of displacements of rods in x and y direction. An anisotropy of μ⊥/μ��� = 0.664 ± 0.006 was found and ζ = −75 mV. The direction of the electric field was in the positive x-direction, E = 1.14 V mm–1. Δt = 0.374 s. The error on μ⊥/μ∥ is the estimated standard error obtained from the covariance matrix of the fitted parameters.
Figure 5
Figure 5
Anisotropy in mobility of silica rods (μ⊥/μ∥) plotted as a function of κa. The black solid line depicts Ohshima’s analytical expression of Henry’s solution, for infinitely long rods and low zeta potential ζ < kBT/e. The dashed, dotted, and dashed-dotted lines depicts Ohshima’s approximate analytical expression for infinitely long rods and moderate zeta potentials, taking the relaxation effect into account, for |ζ| ≈ 25 mV, |ζ| ≈ 50 mV, and |ζ| ≈ 75 mV, respectively. The symbols are experimental data points of the measured anisotropy in mobility. Squares (green) are SR29 rods in CHC, from left to right: without TBAC added, with ∼0.026 μM, and with ∼0.26 μM TBAC. Circles (purple) are R2 rods in polar solvents (DMSO–water and glycerol–water), from left to right: 78 wt % DMSO–water, 78 wt % DMSO–water with LiCl, 85 wt % glycerol–water, and 85 wt % glycerol–water with LiCl. The error bars indicate the estimated standard deviations.
Figure 6
Figure 6
Anisotropy in electrophoretic mobility μ⊥/μ∥ plotted as a function of κa. Experimental data from Figure 5 are plotted together with a modified expression for “finite” rods and moderate zeta potentials. This was done by assuming that rods oriented parallel to the electric field are subjected to the same retardation function as rods oriented perpendicular to the electric field: μ⊥/μ∥ = f⊥(κa)/f⊥(κL/2). This assumption predicts that for finite rod sizes μ⊥/μ∥ increases as the size of the double layer increases (κa < 1).
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