The Soret coefficient ($S_T$) of positively charged magnetic colloids was measured as a function of the nanoparticles’ diameter. The $Z$-scan technique and the generalization of the thermal lens model proved to be a reliable technique to measure $S_T$. We show that $S_T$ is negative and increases with the particle's diameter, being best described by a functional dependence of the type $S_T\propto d_0$. Potentiometric and conductometric experiments show that the particle's surface charge decreases as the temperature increases, changing the electrostatic interaction between the nanoparticles. The temperature gradient imposed in the ferrofluid by the Gaussian laser beam leads to the formation of the particle's concentration gradient. The origin of this phenomenon is discussed in terms of the decrease of the particle's surface charge in the hottest region of the sample and the thermoelectric field due to the inhomogeneous distribution of hydrogenous ions present in the colloidal suspension.

• Received 6 December 2013

DOI:https://doi.org/10.1103/PhysRevE.89.032308