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Prinderre et al. (1998) applied factorial design methods to optimize the
stability and suggested the required hydrophilic lipophilic balance (HLB)
of o/w emulsions prepared with sodium lauryl sulfate as the surfactant.
The independent variables and their levels (low/high) were mixing rate
(rpm) (500/900), homogenization (no/yes), and mixing time (min)
(10/20). Dependent variables were the average size of the droplets, the
emulsion viscosity, and the conductivity. Experimental design determined
the required HLB with good approximation in fi ve runs for the average
diameter and viscosity studies, while the conductivity study needed at
least eight runs.
Similarly, Simovic et al. (1999) investigated the infl uence of the
processing variables on performance of o/w emulsion gels stabilized by a
polymeric emulsifi er (Pemulen ® TR-2NF). A two-factor three-level
experimental design at two sets was applied: using a laboratory mixer
and a disperser. Independent variables were mixing speed and time,
whereas dependent variables were millimeters of oil phase separated after
centrifugation at 3500 rpm in a laboratory centrifuge, and viscosity at
shear rate of 180 l/s. The responses were fi tted into a second-order model
by means of multiple regression analysis. The authors (Simovic et al.,
1999) could defi ne the most favorable conditions for preparing stable
o/w emulsions, using the laboratory mixer, with a mixing speed at
1500 rpm and mixing time of 20 minutes.
Rahali et al. (2009) found the optimal preservative combination and
concentration for preparing topical emulsions by using a D-optimal
experimental design (mixture design). In this study, three preservatives
were tested, benzoic acid, sorbic acid, and benzyl alcohol. The preservative
effects were evaluated using the antimicrobial preservative effi cacy test
(challenge test) of the European Pharmacopeia (EP). The results of this
study were analyzed with the help of Design Expert ® software. The
authors (Rahali et al., 2009) were able to formulate topical emulsions in
accordance with the requirements of the EP.
Simultaneous study of the infl uences of different factors for emulsion
systems is diffi cult, due to the problems of complicated nonlinear
correlations. The artifi cial neural networks (ANN) technique seems to
provide a useful tool for solving these problems. Gašperlin et al. (1998)
investigated the infl uence of different ratios of individual components on
the viscoelastic behavior of semisolid lipophilic emulsion systems using
this technique. The creams were prepared according to a preliminary
experimental design (mixture design). ANN involved 3 input, 12 hidden,
and 9 output neurons. The input neurons were the contents of the
particular emulsion components (silicone surfactant Abil ® WE 09,
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