260 JOURNAL OF COSMETIC SCIENCE
For practical considerations, due to the high volume of the gel phase, gel-in-oil emulsions
were made by an indirect mode (i.e., adding oily phase to gel phase). The direct
emulsification mode had not been attempted in previous studies. Previous work also
demonstrated that high shear was not required for emulsification, and mixing using a
planetary agitation device, such as an anchor or scraper, to gradually wrap the gel with
the oil was recommended.17 This work therefore aimed to investigate the influence of
the emulsification mode (i.e., direct or indirect) and to confirm the most suitable mixing
procedure on the selected formulation base.
Direct emulsification mode. Addition of gel phase to oily phase.
Indirect emulsification mode. Addition of oily phase to gel phase.
Emulsification mixing procedure. Three devices, involving low to high shear, were tested.
• Stirring with an anchor at 75 rpm for 1 minute, followed by 9 minutes at 300 rpm (IKA
Eurostar 60 digital stirrer, IKA® Works, Guangzhou, China).
• Mixing with a serrated disc stirrer between 500 and 1,000 rpm for 10 minutes (IKA
Eurostar 60 digital stirrer, IKA® Works, Guangzhou, China).
• Homogenization with a rotor/stator turbine using a Silverson® L4RT high-shear mixer
at 4,000 rpm for 4 minutes (Silverson, East Longmeadow, MA, USA).
RESULTS
All the formulations were stable under the various temperature conditions at 3 months.
However, the high conductivity of the formulations obtained with serrated disc mixing and
rotor/stator homogenization indicated that defective cream gels with continuous aqueous
phase were obtained in these conditions instead of gel-in-oil emulsions. The result was
the same with indirect and direct emulsification mode (Table II). As anticipated, gel-in-
oil emulsion was characterized with a conductivity value close to zero.13,14 This outcome
Table II
Effect of Variations of the Manufacturing Procedure on a Fixed Formula Containing: Water 85.40%,
Rheology Modifier 0.80%, Emulsifier 2.00%, Oil 8.00%, Internal Gel Phase 90% (W/W %)
Emulsification mode Indirect Indirect Indirect Direct Direct Direct
Agitation Anchor Serrated disc Rotor/stator Anchor Serrated disc Rotor/stator
Conductivity D1/M1
(μm/cm)
0 ≅453 ≅441 0 ≅440 ≅453
Formula type Gel-in-oil Cream gel Cream gel Gel-in-oil Cream gel Cream gel
Viscosity D1 (mPa·s) ≅104,500 ≅19,600 ≅20,300 ≅99,800 ≅20,500 ≅19,100
Stability Stable Stable Stable Stable Stable Stable
Rheology data NT(g) NT(g) NT(g) NT(g)
Mean G’ (Pa) 430 411
Mean G’/G” 6.7 6.4
Yield stress (Pa) 30.5 31
Rate index 0.46 0.44
g NT: Not tested.
h w/w: Weight/weight

261 CRITICAL FACTORS TO OBTAIN STABLE HIP GEL-IN-OIL EMULSIONS
confirmed that stirring with low energy, such as with an anchor, is the most effective
manufacturing method to obtain gel-in-oil emulsions, irrespective of the emulsification
mode. Gel-in-oil emulsions were also characterized by a higher viscosity than defective
cream gels with the same composition. This finding was consistent with HIP content and
compressed droplets.1 Despite low mixing energy, gel-in-oil emulsions contained thin
internal droplets. As shown in the microscopic appearance in Figure 2, internal gel droplet
size was around 1 µm, with a few bigger particles of 2–2.5 µm. The microscopic appearance
was identical for indirect and direct emulsification mode.
The rheological profiles of gel-in-oil emulsions obtained with indirect and direct
emulsification mode were similar (Table II). Frequency sweep curves (Figure 3) showed that
both trials have a strong stable elastic structure with very close G’/G” ratios. Shear behavior
was also identical for the two procedures (Figure 4). Analysis of the curves between 0 and
500 s−1 indicated a similar yield stress and a medium shear-thinning behavior as signaled
by a rate index close to 0.5 (Figure 4, Table II). A change in the slope of the curve was
observed, indicating that the acceptable deformation of the product had exceeded above
600 s−1. This observation was not linked to visible side effects on the edges of the geometry.
Deeper investigation was required in the subsequent experiments to understand if it was a
measurement artifact or if it could be related to a characteristic of the product itself.
Figure 2. Typical microscopic appearance of gel-in-oil emulsion prepared by indirect mode and stirring
using an anchor device.
Figure 3. Viscoelasticity of gel-in-oil emulsions at different frequencies according to indirect or direct
emulsification mode.