Identifying colloid-stabilizing forces in aqueous and non-aqueous media using atomic force microscopy and extended DLVO theory

The interfacial forces that dictate colloid stability in aqueous and non-aqueous systems are a function of the polar and apolar properties of the medium. This study quantifies these colloidal forces and energies between a glass microsphere and a glass surface in water and n-decane solutions using Atomic Force Microscopy (AFM). These solutions were amended systematically to identify the magnitudes of various contributing forces in aqueous and non-aqueous systems. For comparison, classical DLVO and Extended-DLVO (XDLVO) theory were used to calculate the free energy of interaction for each system. Long-range repulsive forces were measured in aqueous solutions and were generally predicted by DLVO theory. Small repulsive forces were measured in non-aqueous solutions at close separation distances (< 10 nm), and were not predicted by DLVO or XDVLO theory. Zeta-potentials of the colloids measured in n-decane (-10.1 mV) and in water (-45.1 mV) indicate the importance of electrostatic interactions in these systems. The measured increase in repulsive forces in the n-decane after the addition of a small aliquot of water compared to the pure n-decane, shows the importance of miniscule amounts of water on the stability of colloids in non-aqueous systems. Colloid stability was additionally influenced by the surfactant, sodium di-2-ethylhexylsulfosuccinate (AOT), in water; a relatively strong repulsive force was measured and indicated the presence of steric interactions. Force data collected in commercial motor oil recorded stronger repulsions than in the pure n-decane. These data suggest that chemical amendments to the motor oil may enhance long-range repulsive interactions that are not typically present in simple non-aqueous media. These forces may stabilize colloidal particles in motor oil suggesting that motor oil additives inhibit the aggregation of particles to a size that can be filter from the oil, thus impacting the oil longevity and filter efficiency.