Carbon black nanopowders are employed in a wide range of applications, such as asphalt sealants and decorative concrete colorants, textiles and conductive/resistive applications, and paint, ink and coating industries. Most of the applications require the dispersion of carbon black through non-aqueous media. Research has been extensively carried out, often with empirical formulation, to determine the most suitable media in which carbon black particles can be dispersed with reduced aggregate formation.
Apart from the molecular structural differences, one parameter that is relative to the charge transfer capability, and hence the surface charge of dispersed particles in dispersion media, is the dielectric constant. The standard method to determine the suitable dispersion medium for carbon black particles is the measurement of its zeta potential in addition to its particle size in media of different relative permittivities.
Non-Aqueous Zeta Potential Measurement Considerations
It is quite difficult to measure the zeta potential of non-aqueous suspensions. As the electrophoretic mobility of particles is a function of the relative permittivity of the dispersion medium, the mobilities, and therefore the frequency shifts, for non-aqueous dispersants tend to be very small. The extremely high sensitivity (10 m/V.s) of phase analysis light scattering (PALS) makes it suitable for the detection of very small particle velocities. Further, PALS was found to be an ideal choice for measuring electrophoretic motion in non-aqueous suspensions. Non-aqueous measurements can be successfully achieved using an appropriate measurement cell. Using a universal dip cell accessory, Zetasizer Nano can create high field strengths at low voltages with improved solvent compatibility.
Experimental Procedure
A carbon black standard test powder (JIS Z 8901) used in this experiment was obtained from the Association of Powder Process, Industry and Engineering, Japan, with a specified size distribution ranging from 0.03 to 0.2 µm. The powder was dried in an oven overnight before using, and was dispersed in organic solvents butan-2-one, tetrahydrofuran, trichloroethane, chloroform, decane and toluene. The dispersion in powders was carried out at 0.1% w/v concentration by bath ultra-sonication for 3 minutes, and kept aside overnight before starting the measurement. The solvents were pure and obtained from Wako and Nacalai Tesque Japan, and their relative permittivities of the pure solvents were in the range of 4.8 to 19.2, respectively.
Upon achieving sufficiently stable dispersions, a Malvern Panalytical Zetasizer Nano ZS combined with a universal dip cell accessory was used to perform measurements. Each sample was subjected to a minimum of three repeat zeta potential measurements. Using the dynamic light scattering (DLS) capability of the Zetasizer Nano ZS, particle size measurements can be carried out to verify the colloidal state of the particles. The measurements were performed at 25 °C in both cases.
Results and Discussion
The mechanisms that generate surface charge, the electrical double layer structure, and the interpretation of the hydrodynamic plane of shear have not been widely explored for non-aqueous solvents. The presence of impurities in the solvent, acid-base or Lewis acid-base interactions between the solvent and the particles, acid-base interactions between a charging agent and the particles, and the presence of trace water in the solvent are some of the possible charging mechanisms in non-aqueous media.
The zeta potential and particle size measurement results obtained from the dispersion of the carbon black powder in media with different relative permittivities are shown in Figure 1. Due to unstable dispersions and aggregation and sedimentation of the carbon particles overnight in non-polar solvents, it was impossible to obtain measurement results in these pure solvents with the lowest relative permittivities (toluene = 2.4 and decane = 2.0).
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Figure 1. The variation of the zeta potential and intensity-weighted mean diameter of carbon black nanopowder dispersed in a variety of non-aqueous solvents with different relative permittivities.
Data for other solvents was revealed with respect to relative permittivities as the ordinate. The conversion of measured electrophoretic mobilities into zeta potentials was performed using Huckel's approximation. The intensity-weighted mean diameter was reported as per ISO13321. Two media having the same relative permittivities, namely trichloroethane and tetrahydrofuran, were used. These media were found to produce zeta potentials with opposite charges. The positive zeta potential values resulting from the halogenated solvents indicate that a Lewis acid-base interaction between the particle surface and the solvent leads to the carbon particle charge.
From Figure 1, it is evident that the optimization of dispersion in non-aqueous media depends on the relative permittivity and the Lewis acid base character of the solvent. The optimum dispersants for the carbon black powder can be selected through the size and zeta potential information from Figure 1. Chloroform, tetrahydrofuran, butan-2-one and propan-2-ol were found to have stable suspensions with small particle sizes. The intensity-weighted mean diameters were observed in the range of 240 to 330 nm. These low size values were related to significant zeta potential means (both negative and positive in sign).
Conclusion
This article has demonstrated the successful measurement of zeta potential of carbon black powders dispersed in non-aqueous solvents. The combination of zeta potential and size measurements helps to determine the dispersion stability of carbon black powders.
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