258 JOURNAL OF COSMETIC SCIENCE
and agitation type (i.e., mixing, homogenization). Key composition factors were also
scrutinized: impact of gel viscosity by increasing the doses of a suitable rheology modifier
emulsifier level ratio between the internal aqueous phase and the external oily phase and
nature of the oil. This work also aimed to find an appropriate test to check quickly that the
formulation is optimized without waiting for the conventional stability tests. With this in
mind, flow and oscillatory rheology experiments were explored to test the structure and
resistance of gel-in-oils.
MATERIALS AND METHODS
INGREDIENTS AND COMPOSITION OF THE FORMULATION BASE
Formulation experiments were carried out by changing one parameter at a time. A formula
base with a reduced number of ingredients was chosen to facilitate analysis of the impact
of changes (Table I).
Ingredients were weighed using an ML 1602 precision balance (Mettler Toledo GmbH,
Greifensee, Switzerland) to prepare 300 g of each formulation.
CHARACTERIZATION AND STABILITY OF FORMULATIONS
Viscosity measurement. The viscosity of the samples was measured using a Brookfield
LVDVI+™ viscometer using an appropriate spindle and speed 6 (Brookfield Engineering
Laboratories Inc., Middleboro, MA, USA). Measurements were taken one day after
manufacturing (D1), after 7 days (D7), after 1 month (M1), and after 3 months (M3).
Microscopic observations. The stable formulations were observed using an optical microscope
Meiji® MT-9300 (Meiji Techno Co., Ltd., Saitama, Japan) with a magnification of ×400,
one week after manufacturing (D7), and images were captured by an attached charge-
coupled device camera GR500W (Shanghai Goldroom Im/Export Trade Co., Ltd.,
Shanghai, China).
Table I
Formulation Base With Fixed Ingredients and Variables Studied
Phase Ingredient %(w/w)
Gel Demineralized water Up to 100
phase Hydroxyethyl acrylate/sodium acryloyldimethyl taurate copolymera 0.6/0.7/0.8/1/1.2
Xylitylglucoside and anhydroxylitol and xylitolb 3.00
Phenoxyethanol and ethylhexylglycerinc 0.80
Oily Octyldodecanol and octyldodecyl xyloside and PEG-30
dipolyhydroxystearated
0.50/1/1.5/2/2.5/3
phase Caprylic/capric triglyceridee 3/5/8/13/18/23/28
a Polyelectrolyte rheology modifier (Seppic, La Garenne Colombes, France).
b Moisturizing active ingredient playing here the role of antifreeze agent (Seppic, La Garenne Colombes,
France): fixed ingredient in all the trials.
c Preservative (Thor Specialty Chemical Co., Ltd., Zhenjiang, China): fixed ingredient in all the trials.
d Liquid emulsifying system (Seppic, La Garenne Colombes, France).
e Oil (Croda International Plc, Goole, United Kingdom).

259 CRITICAL FACTORS TO OBTAIN STABLE HIP GEL-IN-OIL EMULSIONS
Conductivity measurement. Conductivity measurements were carried out at room temperature
at D1, D7, M1, and M3 using a SevenMulti™ dual pH/conductivity meter (Mettler Toledo
GmbH, Greifensee, Switzerland).
Stability monitoring. The appearance of the formulations was checked after storage at D1,
D7, M1, and M3 in different temperature conditions: room temperature at 45°C using a
BD 400 incubator (Binder GmbH, Tuttlingen, Germany) in −5°C to 40°C freeze–thaw
cycles using an MIR-154 Cooled Incubator (SANYO Electric Co., Ltd., Osaka, Japan), at
−18°C in a BCD-232ESN refrigerator/freezer (Electrolux, Senlis, France).
RHEOLOGY EXPERIMENTS
Experiments were conducted at around 20°C, D7 after manufacturing, using a rotational
controlled stress/strain Discovery Hybrid Rheometer DHR-2® (Waters—TA Instruments,
New Castle, DE, USA). In accordance with the thin particle size of the trials, an anodized
aluminum cone with a diameter of 40 mm, and forming an angle of 2° with the plate, was
selected for all tests.
Oscillatory frequency sweeps from 0.1 to 100 rad/s were carried out with an anti-evaporation
cap within the linear viscoelastic domain. Viscoelasticity was then analyzed to evaluate the
level of structuration of the gel-in-oil and its resistance or any change occurring in the
structure during the experiment (frequency expressed in Hz in the Figures according to
international units). Evolution of storage modulus (G’) and calculation of mean G’/G” (i.e.,
loss modulus) ratio were followed to easily compare the formulations. The higher the value
of mean G’ and relative G’/G” ratio, the greater the elasticity and the stronger the structure
of the formulation.
The samples were also subjected to a shear rate ramp ranging from 0 to 1,200 s−1 for 120
seconds (up and down ramp steady state flow protocol) to determine the global flow profile
and yield stress. Curve analysis was performed using the Herschel–Bulkley mathematical
model to extract yield stress and rate index. Rate index, which varies from values close
to zero for a strong shear-thinning profile to one for Newtonian behavior, was used as
an additional indicator to supplement the curves. The purpose of this experiment was
to subject the formulations to greater stress that can be representative of real stressing
situations such as pouring, stirring, mixing, pumping,22,23,24 etc.
IMPACT OF MANUFACTURING PROCEDURE: VARIATIONS AND RESULTS
VARIATIONS ON MANUFACTURING PROCEDURE
Some parts of the manufacturing process remained fixed for all the trials, according to
conclusions of previous work.17 The gel phase was prepared by dispersing the rheology
modifier (a) in water using a serrated disc stirrer between 500 and 1,000 rpm (IKA
Eurostar 60 digital stirrer, IKA® Works, Guangzhou, China). The antifreeze agent (b) and
preservative (c) were added, and stirring continued until a smooth gel texture was obtained.
The oily phase was prepared by addition of the emulsifier (d) to the oil and manual stirring
for a few seconds with a spatula. Then one phase was introduced into the other, added in
one shot.