Surfactant induced crystallisation of emulsion explosives via spinodal decomposition

• Hydrolysis of PIBSA surfactants reduces the stability of ammonium nitrate emulsions. • Hydrolysis of ester to acid weakens the interfacial hydrogen bonding network. • The mechanism of crystallisation occurs rapidly within the drops. • Crystallisation occurs through spinodal decomposition, initiated at the surface. Supersaturated ammonium nitrate emulsion explosives are prone to crystallization during manufacturing, long term ageing and when in contact with contaminants in the ground. With infrared spectroscopy, microscopy, and rheology, the nature of the emulsifier head group was found to affect the droplet stability depending on the extent of hydrogen bonding at the droplet interface. Emulsions were made with sorbitan monooleate (Span 80), and polyisobutylene succinic anhydride (PIBSA) surfactants with different head groups. Emulsions formed with the imide or acid head groups were less stable due to weaker hydrogen bonding and crystallised quickly. Ester groups formed more stable interfaces owing to stronger interfacial hydrogen bonding with the surfactant head group and greater disruption of OH water hydrogen bonds relative to bulk water. Hydrolysis of PIBSAs containing ester head groups resulted in a weakening of the hydrogen bonding network and resulted in an increased crystallisation rate. The mechanism for crystallisation was shown to occur through a very fast spinodal decomposition process. Understanding the factors that enhance the interfacial bonding is important for long term emulsion stability, transportation, and efficient blasting in emulsion explosive systems.