Approach of the critical point of gas-liquid transitions in an electrostatically stabilized colloidal suspension

We report on the study of the critical point of gas-liquid transitions in a n electrostatically stabilized colloidal suspension. Suspensions are consti tuted of spherical magnetic nanoparticles dispersed in water and stabilized by electrostatic repulsions. The coexistence line is constructed on the Pi -V diagram (Pi: osmotic pressure, Phi: volume fraction) by the determinati on of transitions' threshold by optical microscopy on a large range of volu me fractions. This coexistence line presents a maximum that corresponds to a "critical area" where density fluctuations are seen at the microscopic sc ale. The measurement of these density fluctuations when reaching transition s by small angle neutron scattering (SANS) shows that the transition is of the second order in the critical area and of the first order out of the cri tical area. SANS measurements also allow to conclude that the interparticle potential is attractive at long range near the coexistence lines of the ph ase diagram. Long-range attractions are due to dipolar magnetic interaction s between particles. Such a potential authorizes gas-liquid transitions and thus the existence of a liquid colloidal phase. (C) 2001 American Institut e of Physics.