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Effect of different surfactant mixtures on the stabilisation mechanism of highly concentrated water-in-oil emulsions
Author(s)
Sanatkaran, Neda
Date Issued
2014
Type
Thesis
Publisher
Cape Peninsula University of Technology
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.
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.
Additional information
Thesis (DTech (Chemical Engineering))--Cape Peninsula University of Technology, 2014
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