48 JOURNAL OF COSMETIC SCIENCE
stability of formulations (7). Magnesium aluminum silicate, which was added to the F2
formulation, is widely used as a thickening agent in many cosmetic preparations, due to
its thixotropic and pseudoplastic properties. The colloidal structure of this clay provides
excellent suspension of fine particles in aqueous systems (16) however, it did not offer
the suspension efficiency of other clay particles (kaolin and montmorillonite) in the
formulations studied.
Clays and organic thickeners, such as gums, are often used synergistically in cosmetic
products to provide a balance of suspension stability. The gums' protective colloidal
action improves clay stability in the presence of electrolytes, surfactants, water-solubles,
and even against phase separation (11,17). This fact was also observed in this study, once
hydroxypropyl starch phosphate (Fl) and xantham (F3) gums promoted stability in both
formulations after centrifuge testing, preventing phase separation. Therefore, only the
Fl and F3 formulations were classified as normal after PST, and they were submitted to
AST.
ACCELERATED ST ABILITY TEST (AST)
Formulations Fl and F3 maintained normal color and odor in all storage conditions.
These formulations did not present variation in the initial pH value (7 .3), superior to
± 1.0%, probably due to the buffer effect of clay that occurs when a suspension is at a pH
very near its natural pH (range of pH close to 6.0 or 7.5-8.0) (18). However, they
presented variations in visual aspect (Table I) and apparent viscosity (Figure 1) in the
same evaluated conditions. These alterations could be associated with temperature and
time influences on the clay hydration process.
The structural framework of clay is composed of thousands of submicroscopic platelets
Table I
Visual Aspects Observed in Fl and F3 Formulations at Different Temperatures in Storage Conditions in
the Accelerated Stability Test
Temperatures of
storage conditions
5.0° ± 0.5°C
22° ± 2.0°C
45.0° ± 0.5°C
-10.0° ± 0.5°C/24 h
45.0° ± 0.5°C/24 h
Days of
analysis
t
l 3
7
14
3
7
14
3
7
14
7
14
t
1 =24 h after preparation of clay masks.
SM =slightly modified.
N =normal.
Visual aspect
Fl F3
Homogeneous Homogeneous
SM SM
SM SM
SM SM
N SM
N SM
N SM
SM SM
SM SM
SM N
SM N
SM N

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