The role of solvency in the stabilization of a high internal phase emulsion explosive

Abstract

The stability of an industrial emulsion explosive comprising a high internal phase water-in-oil emulsion, stabilised by a polyisobutylene based surfactant (PIBSA) was investigated as a function of the oil phase and related parameters. The emulsion was characterised using a small angle and ultra small angle neutron scattering (SANS and USANS) in the size range 10 {u00C5} to 20 ?m using the technique of contrast matching. In addition to the characterisation, solution viscometry experiments were performed to enable the measurement of the surfactant-solvent interaction through the Flory-Huggins constant. In the industrial setting, diesel is used as the oil phase. To enable a systematic study of the effect of the solvent, in addition to diesel, toluene; cyclohexane; and hexadecane were used to approximate the naphthenic, aromatic and aliphatic character of diesel. The solution viscometry experiments showed that the nature of the oil phase has a marked effect on the interaction between the solvent and the PIBSA surfactant. This effect was measured as a change in the Huggins constant as a function of the solvency. Diesel was found to be a theta solvent for PIBSA at 25{u00B0}C. As diesel is the choice of solvent in the industrial setting, this result showed that there is a correlation between the extreme stability of the emulsion and favourable solvent-surfactant interaction. Measurements on toluene, hexadecane, the 50:50 v/v hexadecane:toluene mixture and cyclohexane presented a trend in solvency showing that aromatic character in the solvent diminishes solvent quality as do saturated cyclic compounds (cyclohexane). The structural characterization through neutron scattering revealed the presence of reverse micelles; micron sized aggregates of surfactant; and the behaviour of the PIBSA surfactant at the droplet-oil interface. All of the above were found to be highly sensitive to the nature of the oil phase. The structural and thermodynamic information fit with a depletion stabilization model of emulsion stability. The depletion stabilisation theory enabled the calculation of the depletion potential barrier to coalescence based on the knowledge of the droplet and reverse micelle sizes, and the volume fraction of the reverse micelles. The sensitivity of the emulsion to the nature of the solvent phase was explained in terms of the varying population of reverse micelles as a function of the oil phase and the Huggins constant. As a result of this theory, a clear connection was established between solvent quality and emulsions stability.