AcoustoSizer measurements

Figure 4.9 shows AcoustoSizer measurements of the dynamic mobility of a 100 nm diameter silica slurry. The ratio of particle radius to double layer thickness, Ka, is around ten in this case. From these measurements it can be seen that the mobility magnitude and arguments have a similar form to the systems with surface conductance in the previous section, that is, the mobility has a phase lead and the magnitudes increase with frequency. In this case there are no inertia forces to slow the particles down, for the ratio of inertia to viscous forces ... 

The original Acoustosizer used a single frequency whereas a later development has a range of 13 frequencies between 0.3 and 13 MHz. This allows the measurement of the dynamic mobility spectrum and the determination of the zeta potential and particle size. In order to invert the mobility spectrum into a size distribution a log-normal distribution of particle size is assumed. A comparison with photon correlation spectroscopy for determining particle size and laser Doppler anemometry for particle charge eonfirmed the results using ACS [266]. These and additional sedimentation measurements confirmed that changes in particle size and zeta potential due to dilution effects are likely to occur in aqueous and non-stabilized systems. 

In many publications, the preparation of samples for electrokinetic measurements and the measurement itself are described in detail. In a few other papers, the description is limited to the type of instrument. The potential is unequivocally defined by the composition of the solution in contact with solid particles. Therefore, conditions such as the solid-to-liquid ratio, aging, and ultrasonication should not affect the lEP. The use of optimum conditions for electrokinetic measurements may improve the reproducibility of results, but the lEP under different experimental conditions will be the same. In fact, in only a few studies was the experimental protocol indeed optimized, and in many other studies the experimental protocol was copied from other publications or arbitrarily selected. When the potential and lEP depend on the experimental conditions, information on these experimental conditions is essential. Eor example, the acidic branch of the electrokinetic curves (using the Acoustosizer) reported in [307] was rather... 

The kinetic potential is usually denoted as the zeta (0 potential and it is determined from the electrophoretic mobility of the extremely dilute particles in an electric field. More recently, the nse of electrokinetic sonic amplitude (ESA), acoustosizer (AZR), or colloid (or ultrasonic) vibration potential (CVP) has become available for the determination of the potential in rather concentrated particle suspensions. Again the potential may be measured as a function of either the metal concentration or the pH. In the latter case the point where the mobility ceases is denoted the isoelectric point (pH,Ep Fignre 8.27). It correlates particnlarly well with the stability of the sol. 

The Smoluchowski formula, which is used in the AcoustoSizer from the measured dynamic mobility, is valid for a disperse suspension of spherical particles according to Eq. 1. 

To measure the particle size and -potential of pigment TiC>2 by the ESA (Electrokinetic Sonic Amplitude) method, the Acoustosizer 2 device ( Colloidal Dynamics , Sydney) was used. 

More recendy, Matec has introduced their AcoustoSizer, which measures electrokinetic sonic amplitude as a function of frequency (dififusion wave spectroscopy). Rather large samples (400 ml) are required in the concentradon range 1-40 vol%. The particle size range is 0.1-10 pm. The Pen Kem AcoustoPhor 8000 is based on the measuimient of attenuation of sound as it passes through the dispersion. It is claimed to have a range of 0.1 -10 pm plus, requires a sample volume of 1(K) ml and is applicable to dispersions greater than 40 voI%. 

One immediate effect of increasing the particle concentration in the emulsion is that the acoustic impedance, Zg, can no longer be approximated as equal to that of the dispersion medium. Since Eq. (1) remains valid at all concentrations commonly encountered, it is important that the correct value of Zg is used, so that the correct value of the dynamic mobility is obtained from the measured ESA signal. In principle, the value of Zg for the emulsion could be a complex function of the frequency and the properties of the suspension, but the exact behavior is of little consequence for measurements with the AcoustoSizer, since it measures the value at each frequency before calculating from ESA signal. 

Acoustosizer IIs (Colloidal Dynamics, Warwick, USA) with a built-in conductivity meter and thermometer was used to measure the potential and conductance of solutions of phosphoric and oxalic acid solutions in mixed (water-ethanol and water-methanol) solvents and of Ti02 dispersions (1-10 % by mass). The apparatus was kept in a fume-hood. A flow-through system and stirrer prevented sedimentation of dispersions. An external thermostat was used to keep the solutions and dispersions at 25 1 °C. 4 mL of 1 M acid was added in 0.1 mL portions to 170 mL of initial solution or... 

This instrument factor A is determined by calibration. This involves the measurement of the signal from a material that has a known ESA. In the AcoustoSizer II an electrolyte is used for this calibration. The reason for using an electrolyte, rather than a colloid, is that colloids are notoriously variable in their zeta potential over time, whereas the ESA signal from the electrolyte is reproducible and can be calculated from its thermodynamic and transport properties [5j. 

Before moving on to the link between dynamic mobility and particle parameters, we note that Equation 4.7 only applies to the AcoustoSizer II, which measures the ESA from the nearest electrode. It does not apply to the Matec ESA 9800, which uses a resonance type measurement. 

Manglesdorf and White [12] have produced a computer program for calculating the dynamic mobility of a particle with an arbitrary double layer thickness, and this can be used for determining zeta from the measured mobility spectrum, provided Ka is known. Thus, an independent measure of particle size is required to obtain the zeta in nanoparticle systems. Once again, the attenuation sizing in the AcoustoSizer can be used for this determination. 

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