Determination of the effective charge of different nanocolloids at high ionic strength using conductivity and acoustophoresis

Abstract Conductivity and acoustophoresis experiments were performed on two charged colloidal systems as a function of the volume fraction, while the pH and the ionic strength were maintained constant. The two experimental methods can be used to determine the effective charge of nanocolloids. An exceptionally large colloidal solid volume fraction can be used because these two techniques are not based on light detection. We developed a theoretical approach to describe the resulting signal without neglecting the signal from the nanocolloid (conductivity) or from the salt (acoustophoresis). We applied this approach to experimental results of conductivity and ultrasonic vibration potential (UVP) for: (i) spherical nanomagnetic colloids ( R i  = 5 nm) and (ii) plate-like colloids (laponite). As the sizes are known from other techniques, we deduced in a coherent way the effective charges of the particles, which permit to reproduce conductivity and acoustophoresis experiments simultaneously. The variation of the charge of the magnetic nanocolloids shows an unusual behavior when decreasing the volume fraction.