Effect of different surfactant mixtures on the stabilisation mechanism of highly concentrated water-in-oil emulsions

Abstract

The subject of this investigation was a highly concentrated water-in-oil emulsion (HCE), explosive grade, with volume fraction of approximately 88 vol%, wherein the dispersed phase was comprised of a super-cooled solution of inorganic salts. Explosive emulsions are thermodynamically unstable compounds and this instability is related to crystallisation in the dispersed phase, which is a supersaturated solution (>75 wt%) of an oxidiser (e.g. ammonium nitrate salt (AN) in water). Slow crystallisation of droplets can occur during shelf life storage, transportation and application, thereby suppressing the sensitivity of the emulsion to detonation. The structure of these emulsions with respect to their stability has been studied and their rheological properties have been well described. Explosive emulsions are commonly stabilised by poly (isobutylene) succinic anhydride (PIBSA)-based surfactants that provide optimal shelf life stability, but become unstable during high shear conditions. This adversely affects the quality of these emulsions during their transportation through long hosepipes, as occurs in the relevant industry. Other issues associated with the use of PIBSA surfactants include long refinement times required, which increase the energy costs to form stable explosive emulsion. The trend of using surfactant mixtures to provide overall stability, both during shelf life and high shear, has grown in recent years. Among other advantages of this approach are associated economic benefits, and improved safety and technological properties of emulsions. The choice of co-surfactants depends on the nature of the components of the emulsion and is mainly empirically-based. The key concept is using synergetic binary surfactant systems, which may impact on the stability and properties of the emulsions. This study presents results from such an investigation, bearing in mind that the emulsion performance depends on the fundamental physicochemical properties of the mixed surfactants. Initially, two groups of surfactants (block copolymers named Pluronics and water soluble surfactants named Tweens), as well as their combination with a PIBSA-based surfactant (PIBSA-Mea) and sorbitan monooleate (SMO) were selected to stabilise HCEs. Pluronics, when combined with PIBSA-Mea and SMO, were unsuccessful in forming stable emulsions, while the emulsions consisting of PIBSA-Mea/water soluble surfactants showed acceptable stability. Attempts at dissolving water-soluble surfactants in the aqueous phase were unsuccessful. This was attributed to the salting-out effect of Tweens in the presence of large quantities of AN in the water phase. In the current study, the water soluble surfactants were successfully dissolved in the oil phase containing industrial grade oil (Ash-H).

The stability and interfacial behaviour of one the most stable novel emulsions, stabilised by PIBSA-Mea/water soluble surfactants (Tween 80), and developed during this study, was then compared to the current standard industrial explosive formulation (PIBSA/SMO). Results showed an acceptable stability of the new emulsion formulation in both shelf life and under high shear. More interestingly, it was observed that there were markedly different interfacial behaviours of PIBSA-Mea/water soluble Tween 80 and PIBSA-Mea/oil soluble SMO at the water-oil interface over a wide range of surfactant/co-surfactant ratios. Based on the results obtained from the aforementioned comparative studies, a series of nonionic oil-soluble (Spans) and water-soluble (Tweens) compounds with systematically varying structure (length, presence of double bonds and number) of hydrophobic tails were identified and subsequently mixed with PIBSA-Mea. This was done in order to elucidate the effect of compatibility and synergism between PIBSA and co-surfactant, with particular reference to the interface to stability under shear and on-shelf of final explosive emulsions. An investigation of the effect/s of co-surfactant structure on interfacial properties at the water-oil interface was performed. The Rosen method was used to characterise synergism between the two surfactants. This was correlated with the stability on shelf and under shear as well as with the rheological properties/pumpability of the novel manufactured emulsions. The degree of synergism (interaction parameter) for PIBSA-Mea/Spans decreased, with a corresponding decrease in the length of alkyl tails, as well as the presence of a double bond in tail. There was a major antagonism noted for PIBSA-Mea/multi tails Span mixtures. In all the PIBSAMea/ Tweens mixtures the opposite effect of tail length on interaction parameter was observed. However, the effect of tail structure on synergism was less pronounced for the Tweens group than it was for Spans. Emulsification was markedly more rapid for the PIBSA-Mea/water soluble Tweens mixtures, and an improved stability on shelf and under high shear was recorded for this group when compared to PIBSA-Mea/Span mixtures. In the current study, depending on the structure of the surfactant, it was shown that synergism between the surfactant and co-surfactant is one of the major factors in determining stability of the emulsions. In addition, the influence of the chemical structure of co-surfactants on the rheological properties of the emulsions was studied. Higher pumpability of the explosive emulsions stabilised with water soluble Tween is attributed to a lower yield stress of the PIBSA-Mea/Tweens emulsions, compared to the PIBSA-Mea/Spans emulsions.

Finally, the partial replacement of PIBSA by certain suitable water-soluble Tweens offers a cost-effective, easily available and environmentally friendly alternate. Additionally, such a system could provide acceptable stability for different technological applications associated with emulsions, including droplet refinement during emulsion production, adequate long-term storage and acceptable pumping characteristics of these mixtures. Overall, this would reduce the cost of the final product on an industrial scale.