Conductivity values for the first 30 days decreased for
the formulation that was kept at 40°C with 75% RH
relative to the conductivity values of the fresh sample.
This may be due to the formation of larger droplet size or
phase separation. But no phase separation was observed
at this condition. However, an increase in the droplet
size was observed. A similar justification has been offered
previously [26] where the reduction in the observed conductivities
has been attributed to phase separation and
larger droplet size. After 8 and 12 months of storage,
conductivity values (51.7 ± 3.06, 29.3 ± 0.30 respectively)
tend to decrease but yet no phase separation occurred even
after heating the samples at 80°C. Normal pH of skin is in
between 5 and 6, and 5.5 is considered to be the average
pH of skin [27]. Results of our pH analysis indicate that
there is no variation in the pH of multiple emulsions kept
at different conditions of storage and even after 8 and
12 months of storage (5.4 ± 0.01, 5.6 ± 0.01 respectively)
under varying temperature and humidity conditions.
Results of this study (Table 4) indicate that different
storage conditions did not have any influence on the
pH and conductivity.
Thermal stress test with repeated centrifugation
Thermal stress test with repeated centrifugation has been
applied on multiple emulsions to study the stability of these
emulsions under extensive stress conditions of temperature
change and centrifugal force. The obtained results of this
test have been shown in Table 5.
Obviously the generated multiple emulsions found to
be extremely stable against elevated temperatures and
the thermal stress could not produce any change in the
globule integrity, phase inversion or phase separation.
Even when the multiple emulsion was subjected to elevated
temperatures and repeated centrifugation at 5000 rpm for
20 min, no phase separation was observed and globules
have been found to be intact as observed through microscopic
examinations. Hence, when the emulsion was tested
after 12 months of storage at 25 ± 10°C with 40 ± 10% relative
humidity it was found to be very stable and expected
not to deteriorate soon. This resistance to phase separation
appears due to the addition of Cetomacrogol 1000®, which
acts as a film stabilizer and this has not been reported so
far. The possible mechanism behind Cetomacrogol 1000®
as film stabilizer is probably due to its self-bodying action
in which rheological properties of the emulsions are related
to gel networks formed in the continuous phases. This
phenomenon has been presented previously when mixtures
of emulsifiers of the surfactant-fatty alcohol type are used
to stabilize oil in water emulsions [28,29].
Rheological analysis
Consistency of cosmetic formulations is a very important
aspect and is determined by the rheological methods.