Magnetic colloidal particles as probes for the determination of the structure of laponite suspensions

A new way to probe the microscopic structure of suspensions of discotic particles of Laponite is reported. Magnetic spherical colloidal particles of maghemite are incorporated into the Laponite suspensions to study their spatial repartition and their microrheological behavior. Chemicals conditions are found for the synthesis of stable mixtures. The determination of the equation of state of the obtained mixtures by osmotic measurements shows that the inclusion of the probes does not modify the structure of Laponite suspensions which exhibit a fluid-solid transition at a given volume fraction. The transition is related to the appearance of a pseudoplateau in the equation of state of the system. SANS and microrheological measurements show that the probes are homogeneously dispersed in the mixture out ofthe pseudoplateau and that there is a microscopic phase separation between Laponite particles and maghemite probes in the region of the pseudoplateau ofthe equation of state. This microscopic phase separation is consistent with the description of Laponite suspensions as microscopically segregated in the region of the pseudoplateau.