269 CRITICAL FACTORS TO OBTAIN STABLE HIP GEL-IN-OIL EMULSIONS
Internal phase concentration was also decisive in obtaining an optimized resistance of the
structure of gel-in-oil emulsions when subjected to high-stress conditions (Figure 13A).
From gel phase concentration greater or equal to 90% (F16, F15), the structure of gel-
in-oil emulsions was not able to withstand high shear. Conversely, for 80% and 85%,
the structure was not affected by shear and exhibited a typical shear-thinning behavior.
Furthermore, as shown in Figure 13B, F17 and F18 were able to recover their initial state
when the shear was stopped: immediately for F18, and in a slightly delayed manner for F17,
exhibiting thixotropic behavior. This means that the decrease in relative elastic character
seen during the oscillatory experiments between 85% and 80% of the internal gel phase
had no relationship to the structure’s resistance at higher stress.
DISCUSSION
The first aim of this study was to determine the factors affecting the creation and
stability of HIP gel-in-oil emulsions. The results demonstrated the importance of both
the manufacturing process and composition factors. Gel-in-oil emulsions were easily
manufactured in one step, at room temperature, by one-shot addition of one phase onto
the other (both indirect/oil-in-gel and direct/gel-in-oil emulsification modes are suitable).
One-shot addition simplified the procedure compared to the stepwise addition of the major
phase recommended in previous publications.6,8,9,11 The mixing procedure for emulsification
was found to be critical for the creation of gel-in-oil emulsion: on the basis of the almost
optimal composition tested, agitation using a low-shear planetary device, such as an anchor
or a scraper, was the most effective. This finding also differed from previous publications
on W/O HIPEs describing high-shear homogenization procedures from 3,500 rpm to
12,000 rpm.3,8,10,11,13,27 In addition, HIPEs manufactured in a one-step process were reported
to contain relatively large droplets, with size decreasing with increasing homogenization
time and intensity,12 generally around 20 µm or more,8,14 to around 10 µm (10), and around
5 µm after a required homogenization in an example containing the same emulsifier
(d).13 On the contrary, the gel-in-oil emulsions studied contained small internal droplets,
around 1 to 2 µm, without the need for high-shear preparation. An optimized droplet
size was reached from above 2% to 2.5% of emulsifier (d) without the interaction of other
parameters, which was also in line with a dosage greater than 1.5% required to achieve
Figure 13. Flow profile of gel-in-oil emulsions (A, B) according to internal gel phase concentration.

270 JOURNAL OF COSMETIC SCIENCE
long-term stability. The study pointed out critical requirements of the internal gel phase
to create gel-in-oil emulsions: a viscosity greater than or equal to 18,000–20,000 mPa·s
achieved with a suitable rheology modifier and a maximum gel phase concentration of
90–93% depending on the rheology modifier dose. A restricted range starting from 80%
of internal gel phase led to stable formulations after 3 months of storage (Figure 14). As
expected from the principle of HIPEs, the viscosity and texture consistency of gel-in-oils
were found to be connected mostly with the internal gel phase concentration.12 In addition,
it could be noted that the final viscosity of the gel-in-oil emulsion did not help to predict
the stability of the formulation, as evidenced in Table V.
The effect of the nature of the oil on gel-in-oil emulsions was also investigated while
keeping a dose of 8% combined with a fixed dose of rheology modifier (a) 0.8% and
emulsifier (d) 2%. Eight oils were tested: mineral oil (Hangzhou Refinery China, Zhejiang,
China), dimethicone (Dow Corning, Midland, MI, USA), C15–19 alkane (Seppic, La
Garenne Colombes, France), squalane (Aprinnova, Emeryville, MA, USA), caprylic/capric
triglyceride (Croda, Cowick, United Kingdom), cetearyl ethylhexanoate (Seppic, La Garenne
Colombes, France), coco-caprylate/caprate (Seppic, La Garenne Colombes, France), and
sweet almond oil (Vantage Specialty Chemicals®, Deerfield, MI, USA). All assays resulted
in gel-in-oil emulsions after production (data not shown), demonstrating that the nature of
the oil was not a critical parameter. Most gel-in-oil emulsions were stable after 3 months,
except the ones with dimethicone, C15–19 alkane, and sweet almond oil, suggesting that
combinations of silicone and plant oil with ester could be tried to optimize the stability.
Additional rheology experiments will need to be carried out to evaluate the effect of nature
of the oil on gel-in-oil viscoelasticity and behavior when subjected to a higher shear ramp.
This data might help to optimize oil combinations.
The second objective of the study was to find a tool to check soon after manufacture that
the gel-in-oil structure was optimized and, ultimately, to be able to anticipate instability
issues that often occurred 3 months after manufacture. Rheology, in particular oscillatory
experiments coupled with frequency or temperature sweeps, is a well-known technique to
test the robustness of the internal structure of a formulation. Correlations with formulation
stability were demonstrated on regular O/W and W/O emulsions.27,28–31 The flow profile is
also useful to understand and predict the behavior of the product in use, such as pouring
Figure 14. Summary of influence of the different composition factors and contribution of rheology to
conventional stability monitoring.