Agglomeration of magnetic particles and breakup of magnetic chains in surfactant solutions

This study investigates the transition from reversible secondary to irreversible primary-minimum aggregation of superparamagnetic particles in surfactant solutions. The magnetic induction at which this transition occurs is experimentally determined by visualization of chain formation under a magnetic field and chain breakup after the field is removed. The value of the theoretical transitional magnetic induction is calculated from the extended Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, which includes van der Waals, electrostatic, and magnetic-dipole forces, as well as non-DLVO steric repulsion. Experimental results show that the transitional magnetic induction increases with higher concentrations of sodium dodecyl sulfate (SDS). When the surfactant concentration is high, the theoretical value of transitional magnetic induction agrees well with the experimental value. Only when the surface of the particles is completely covered by surfactant molecules can the secondary-minimum chains break up quickly to form a uniformly dispersed particle suspension after the magnetic force is removed. Moreover, such investigations reveal that the primary-minimum chains are shorter when they are formed in solutions of higher concentrations of SDS. This phenomenon occurs because the nonequilibrium steric repulsion between adsorbed SDS layers on the surface of the particles allows the transition from secondary- to primary-minimum aggregation for some of the particles in a chain. When the SDS segments are not adequately compressed, long chains formed in SDS solutions break up at points of secondary-minimum aggregation after removal of the magnetic force. At these points, the adsorbed SDS layers keep particles away from the primary-minimum, leading to the breakup of long chains and formation of short primary-minimum magnetic chains.