Determination of the Size Distribution of Charged Nanoparticles via Capillary Electrophoresis under Variation of Counter-Ion Type and Concentration
In the current study, three different types of NPs were used with varied size, namely, Ludox silica nanoparticles (SNP), polystyrene sulfate latex nanoparticles (PSSL) and cobalt oxyhydroxide nanoparticles (CoOOH-NPs). SNPs with three different size distributions have the nominal particle diameters...
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|Summary:||In the current study, three different types of NPs were used with varied size, namely, Ludox silica nanoparticles (SNP), polystyrene sulfate latex nanoparticles (PSSL) and cobalt oxyhydroxide nanoparticles (CoOOH-NPs). SNPs with three different size distributions have the nominal particle diameters 7, 12 and 22 nm. PSSLs with four different size distributions have the mean particle diameters 21, 41, 63 and 80 nm. CoOOH-NPs have a mean domain size of 3.7-4.5 nm. While SNPs and PSSLs are negatively charged in alkaline dispersion medium. COOH-NPs were dispersed in acidic medium having a positively charged surface.
This study focuses on considering the influence of the variation of the counterion type and its concentration on the determination of the particle size distribution (PSD) of charged nanoparticles using capillary electrophoresis (CE). CE provides a suitable method to measure the size of NPs through converting electropherograms into a PSD. This approach is based on an exact determination of the electrokinetic potential ζ by measuring the electrophoretic mobility in an electrolyte of known composition, in combination with a second independent method that determines the mean particle radius such as TEM or Taylor dispersion analysis (TDA).
TDA measurements were used to determine the mean collective diffusion coefficient and the mean hydrodynamic radius via Stokes-Einstein equation. Later, these values of the mean hydrodynamic radius were used in the calculation of the calibration functions to obtain PSDs for the three types of NPs under this study. In addition, preliminary investigations and UV-Vis spectroscopy measurements were made for CoOOH-NPs in an aqueous solution of a monoprotic acid with varied type of anion as counterion. Results obtained show that the continuous decrease in the colour intensity and the absorbance at band maximum for CoOOH-NPs dispersions are independent of the type of anion.
For electrophoretic mobility measurements, two series of SNPs were used with varied sizes with different counterion types, namely: Li+, Na+, K+, and guanidinium (Gdm+) with varied ionic strength ( I = 20-120 mmol L-1) at 25 oC. For PSSL with varied sizes, electrophoretic mobility measurements were made with Na+ as counterion in the ionic strength range 10-50 mmol L-1 and with Li+, Na+ and Gdm+ as counterion in the ionic strength range 40-120 mmol L-1, at 20 oC. In the case of CoOOH-NPs, the electrophoretic mobility measurements were made in acidic solution of pH 2 at 25 oC using different methods for coating of the inner capillary wall, because these NPs have a positive charge on their surface. Also, the influence of parameters such as injection parameters, applied electric field strength and concentration of CoOOH-NPs in the sample were investigated. In all investigations, the electrophoretic mobilities for NPs are dependent on the type of counterion, which can be attributed to Hofmeister effects also called the specific ion effects.
The modification proposed by Pyell et al. based on an analytic approximation introduced by Ohshima, was used to estimate the electrokinetic potential ζ for all NP types. For the determination of ζ from the obtained electrophoretic mobility, the procedure takes the limiting equivalent conductance of the counterion or its ionic drag coefficient into account neglecting the limiting equivalent conductance or ionic drag coefficient of the co-ion. Results for |ζ | follow the order Li+ > Na+ > K+ > Gdm+ for SNPs and the order Li+ > Na+ > Gdm+ for PSSLs, whereas for CoOOH-NPs there is a decrease in ζ in the order NO3¯ > Cl¯ > CH3SO3¯. This dependence of |ζ| on the type of the counterion is also reflected by the determined values for the electrokinetic surface charge densities |ζ|.
Finally, size distributions of NPs were obtained from using the method developed by Pyell and Pyell et al. Electropherograms are converted directly into size distribution functions. The results of using the developed method for SNPs are reliable independent of the type of counterion. There is a good agreement with the results from using TEM analysis for the dispersion (width), which indirectly confirms the validity of the theoretical approach for the calculation of ζ from electrokinetic data and the mean particle size. There are advantages of using Li+ or Na+ compared to the use of K+ or Gdm+ as counterion with regard to preventing particle aggregation and peak distortion via the stabilizing effect due to higher |ζ | and higher |ζ|. In addition, there is a positive impact of the higher ionic drag coefficients on the size-selectivity of the method.
For PSSLs, results show acceptable values for the width produced from using the developed method within expected experimental errors. High electrophoretic mobility values and corresponding calibration functions result in very large errors with considerable uncertainty if ζ > 60. However, for CoOOH-NPs the value calculated from the moment analysis for the dispersion is excessively large, which might be due to adsorption effects that influence the estimation of ζ and the corresponding calibration functions. In addition, because of the small mean domain size of the CoOOH-NPs and the use of a low ionic strength electrolyte there is a very small value for ka (ka >> 1). Hence, for CoOOH-NPs the results obtained show the limitations of the investigated approach in the case of this very low reduced radius.|
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