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.

271 CRITICAL FACTORS TO OBTAIN STABLE HIP GEL-IN-OIL EMULSIONS
out of packaging, spreadability, etc.32–35 but can also contribute to analyzing the stability
of the formula, especially based on evaluations of yield stress and thixotropy (i.e., time for a
sheared product to recover its initial configuration).36,37 Predominant elastic character was
much reported in previous publications as a characteristic of HIPEs,3,8,10,11,26 but a clear
relationship with the stability of the emulsion was not shown. Oscillatory experiments
carried out in the different parts of this work demonstrated that stable gel-in-oil emulsions
were characterized by an elastic structure, stable to frequency sweep. Unstable gel-in-oil
emulsions or gel-in-oil evolving to cream gels during aging due to insufficient dose of
emulsifiers (F7 and F8) could be detected by a very weak elastic structure at low frequencies.
However, F9 showing delayed exudation after 3 months of storage is a counterexample as
weaknesses in its structure were not detected by this sole measurement. Defective cream
gels, obtained when the gel phase viscosity was too low, were also differentiated by a
significantly weaker structure than gel-in-oil emulsions. However, because the relative
elastic character (G’/G”) was also shown to be closely connected to the internal gel phase
concentration, it seemed complicated to predict instability issues on the basis of this
parameter alone. Shear-thinning behavior was also reported as a characteristic of HIPEs,
without any relationship to the stability of the formulation.3,25 In the case of the gel-in-
oil emulsion base studied, tracking the risk of breakage on the flow curve at high shear
helped to set the conditions to obtain the most stable structures. The most suitable ratio
between external oily phase and internal gel phase, revealed as decisive, was determined
to be from 20:80 to 15:85 W/W in the formula studied. The optimum concentration of
emulsifiers was refined to greater than or equal to 2% (Figure 14). The essential role of
internal gel phase concentration could explain why weaknesses of the structures were
observed when studying the impact of emulsification mode, gel viscosity, and emulsifier
dosage. The gel internal phase concentration was arbitrarily chosen around 90% in these
trials. It would be interesting to supplement the trials with ratios 20:80 and 15:85 to
confirm the low viscosity limit.
Thus, a combination of the two types of rheology experiment (application of a low
oscillatory stress at different frequencies and shear rate ramp) detected early signs of
structural weakness, 7 days after manufacturing, for inappropriate compositions. This
approach helped to anticipate instability issues occurring after 1 or 3 months of storage
with conventional stability monitoring and can support the development of customized
HIP gel-in-oil emulsions.
CONCLUSION
The study, combining conventional characterization methodology and rheology
experiments, helped to better understand the influence of the different factors on the
structure and behavior of HIP gel-in-oil emulsions under stress. Rheology experiments
highlighted a strong and highly elastic structure reflecting the organization of the HIP
for optimized formulations. Relationships among conductivity, rheology profiles (behavior
under stress and elastic character), and formulation stability were clearly observed. In
addition, rheology measurements can detect early signs of weakness of the structure
under stress without the need to wait for long-term stability, and can support formula
development in a more general context than the formulation base studied. The knowledge
acquired can also remove the legitimate uncertainty of formulators in the face of an
unusual formulation concept.