CLAY FACIAL MASKS 49
(�5.0 ± 0.5 °C ----22 ± 2 °C ____.,._45_0 ± 0.5 °C -----10.0 ± 0.5 °C/ 24 h 45.0 ± 0.5 °C/ 24 h
Z' 5000
·0 4000
•
U) 3000 5-
.....a..
C: � 2000
�
:
cu 1000
(o
1 3 7 14
Days of analyses wi.
5.0 ± 0.5 °e --a--:-22 ± 2 °C �45.0 :t 0.5 °C � .;.10.0 :t 0.5 °C/-24tt45.0 ± 0.5 °C/-24 h
� 5000 ·0
�
8
4000 -�
U)
� �3000
� 2000
a.
1000
(
0
1 3 7 14
Days of analyses
Figure 1. Apparent viscosity as a function of days of analysis at different temperatures in storage conditions
(a =Fl formulation b =F3 formulation).
stacked in sandwich fashion. The faces of these platelets carry a negative charge, while
their edges have a slightly positive charge. The net negative charge of the platelet is
mostly balanced by inorganic cations. These charge-balancing ions are associated with
platelet faces and are termed "exchangeable" since they can be readily substituted by
other cations (17 ,18). When the clay is dispersed in water, the latter penetrates into the
platelets, forcing them further apart. As this happens, the exchangeable ions begin to
diffuse away from the platelet faces. Further penetration of water between the platelets
then proceeds until complete separation (17,19). Generally, the speed with which plate-
let separation occurs is directly related to the amount of mechanical and thermal energies
introduced during hydration (7). Therefore, the technological conditions of clay masks'
preparation were concisely controlled in this study to achieve reproducible results (see
Formulations Studied, above).
Once the clay is hydrated, the weakly positive charges are attracted to negatively charged
platelet faces. The resuiting three-dimensionai structure builds rapidly ar firsr, giving a
quick increase in viscosity. As time passes, the remaining free platelets take a longer

50 JOURNAL OF COSMETIC SCIENCE
time to find an available site in the structure, and so viscosity increases at a progressively
slower rate (18). This was observed in the Fl and F3 formulations (Figure la and b,
respectively) which present a progressive increase of apparent viscosity in the days of
analysis at room temperature (22° ± 0 2C). Apparent viscosity as a function of the days
of analysis at different temperatures in storage conditions is illustrated in Figure 1. In
both formulations, at 5.0° ± 0.5°C and temperature cycles, apparent viscosity decreased
in relation to room temperature. Moreover, these formulations presented a slight visual
aspect modification with cluster formations. Lower temperatures possibly decrease the
clay hydration speed, promoting particle agglomerates. In contrast, the Fl and F3
formulations at high temperature (45.0° ± 0.5°C) presented a viscosity increase in re-
lation to room temperature (as shown in Figure 1), which was perceived visually. This
fact could be attributed to clay-accelerated hydration caused by high temperature, which
accelerates the fitting of free platelets in the structure. Jefferson and Rogers (14) ob-
served that thermally induced structural rearrangements, which are shown by slight
swelling, occur when clay suspensions are stored at higher temperatures. Formulations
Fl and F3 also presented notable drying due to water loss at high temperature, which
could also explain the increase in apparent viscosity in this storage condition. These
results indicate that the amount or kind of humectants in the formulations was not
enough to retain water.
The association between clays and gums is indicated to obtain stable clay dispersions at
different temperatures since most gums, in contrast to clays, have a chemical structure
without the tendency of association (11). Thus, studies have been developed with dif-
ferent concentrations and kinds of gums, as well as with other humectant concentrations,
in order to stabilize these formulations in light-and-temperature storage conditions.
In most clay mask formulations, clays are used in aqueous suspensions, and they present
gelification under certain conditions of pH values, electrolytes, clay concentration, and
temperature (14,18-20). The results obtained in this study indicate the behavior of these
formulations at different temperatures in storage conditions. These results are of a great
importance and can determine in which conditions the clay can be used, since formu-
lators are more concerned with the behavior of clay in the presence of the other ingre-
dients, rather than in water alone. Understanding the mechanism of gel structure
formation has important implications for its optimization in order to control the settling
of clay suspensions, although this is beyond the scope of this paper and needs further
study.
CONCLUSIONS
This study provides useful results to estimate physicochemical stability under the aging
of clay-based formulations. Magnesium aluminum silicate did not improve the physico-
chemical stability of formulation F2 in the preliminary stability test, while gums
stabilized formulations Fl and F3 in the same test. Formulations Fl and F3 were
unstable in the accelerated stability test, presenting variations in visual aspect and
apparent viscosity in storage conditions at different temperatures. Moreover, humectants
added to the formulations did not retain water under the same conditions, promoting
slight drying at a temperature of 45.0° ± 0.5°C.
ACKNOWLEDGMENTS
This work was supported by the National Council for Scientific and Technological