Formulation of novel topical drug delivery systems requires investigation of large number of
formulation and processing variables. Generally, the one factor at a time (OFAT) approach is
used for the method development and validation, which uses trial and error method and
provides information about the effect of individual factor but does not reveal the interactions
(if any) between the factors and requires enormous amount of experimentation due to
inclusion of only one factor at a time to investigate its effect on the overall performance.
Since the inception of ICH Q8 guidelines for the product development, Quality by Design
(QbD) optimisation approach using Design of Experiment (DoE) methodology has become
integral part of formulation development and reveals critical factors, their interactions and
suggests the best combinations of the factors for optimised outputs with lesser number of
experiments (Rowe, 1993; Rowe, 1997; Lewis et al. 1999; Anderson and Whitcomb, 2005).
Thus, OFAT approach surrenders in favour of QbD.
Several formulation aids and processing variables affect the physicochemical characteristics
of the semisolid drug delivery systems (Eccleston, 1986; Sheikh et al., 2011). Formulation
aids include emulsifiers and stabilisers while the processes are the modes of heating, mixing,
Active pharmaceutical ingredients (API) and excipients may show polymorphism.
Conversion from one polymorphic form to another during processing can affect the
physicochemical properties of the final formulation (Miller and York, 1985; Ertel and
Carstensen, 1988; Rajala and Laine, 1995). Stearic acid, magnesium stearate and oils are the
common ingredients for semisolid preparations. Stearic acid exists as polymorphs and may be
composed of either pure or mixed homologues. The commonly used stearic acid is not pure
but a “triple pressed” homologue mixture of ~60% palmitic acid and 40% stearic acid
(Eccleston, 1997). It has been reported that pure stearic acid in liquid paraffin produces
unstable semisolid systems (Bozic et al., 1980).
Commercial stearic acid is available as a mixture of monohydrate and dihydrate and its
moisture treatment produces different pseudopolymorphic forms affecting its
physicochemical properties (Swaminathan and Kildsig, 2001; Bracconi et al., 2005; Okoye
and Wu, 2007). Milling of magnesium stearate (MgSt), obtained from stearic acid has also
shown to affect its particle size and surface area causing changes in the lubrication properties
which may be related to the polymorphic modifications in MgSt (Leinonen et al., 1992).
It is reported that aqueous semisolid systems containing stearic acid are markedly affected by
processing variables such as mixing, which produces metastable polymorphic form (Garti et
al., 1982; Lin et al., 1994; Eccleston, 1997). Swelling of aqueous stearate creams has been
attributed to the existence of –crystalline lamellar structure (Eccleston, 1997; Mueller-
Goymann, 2004). However, swollen lamellar structures appeared to be metastable and under
pressure broken down to non-swollen plate like crystalline structures, producing mobile
Liquid paraffin is synthetic, petroleum–based oil, composed of long chain saturated
hydrocarbons which has been used as an oil phase for semisolid formulations for long time.
In contrast, palm–olein, natural vegetable–based oil contains saturated and polyunsaturated
fatty acids of varying chain lengths and has rarely been used in topical pharmaceutical drug
delivery systems (Baie and Sheikh, 2000; Baie et al., 2005).
Therefore, the aim of this work was to investigate the effect of various formulation and
processing variables such as type of cooling and mixing in addition to the type and
concentration of emulsifier and the polymorphism in stearic acid on the physicochemical
properties of liquid paraffin and palm–olein formulations using QbD.
2. MATERIALS AND METHODS
Palm-olein was a gift from Lam Soon Edible Sdn Bhd. Liquid paraffin BP was purchased
from JM Loveridge Plc, Southampton, England. Span 80, Tween 80 and stearic acid were
obtained from Sigma Chemical Co. (St. Louis, USA). Nitrogen and oxygen gases used for
DSC were purchased from MOX Gas, Sdn. Bhd. Malaysia. Aluminium crucibles for DSC
(40mL capacity) were purchased from Mettler Toledo, Switzerland. Double distilled
de-ionised water was used as continuous medium.
2.2.1 Experimental Design
The design of experiment (DoE) approach was used to investigate the potential influence of
six formulation– and processing–related factors and their interactions on the physicochemical
characteristics of the semisolid formulations as listed in Table 1. The selection of investigated
factors was based on literature and previous experience affecting similar formulations and
included type of oils (liquid paraffin and Palm–olein), emulsifiers (Span 80 and Tween 80)
and their optimum concentrations, stearic acid concentration. The processing variables
included mode of cooling and mixing upon cooling.
Table 1: Formulation and process-related factors investigated using quality by design
variables Designation Factors
Lower level (-) Higher Level (+)
Oil phase A. Palm–olein (A-) Liquid paraffin (A+)
Emulsifer type B. Span 80 (B-) Tween 80 (B+)
Emulsifier concentrtion C. 1% (C-) 10% (C+)
Stearic acid concentration D. 1% (D-) 5% (D+)
Cooling type E. Slow (E-) Fast (E+)
Mixing upon cooling F. “Yes” (F-) “no” (F+)
After selection of factors (each at two levels), a layout (matrix) for the experimental
conditions (Table 2) was generated for 6 factors by the DX® 7.1.5, a commercial software for
DoE. A factorial design with minimum experimental runs resolution 5 (Minimum Run Res 5)
without centre points was selected to investigate the effect of all factors and the interactions
between the factors. A matrix of 22 experiments was generated by the software (Table 2).
Based on the generated matrix, the real experiments were carried out using the experimental
methodology (c.f. Preparation of Formulations (Section 2.2.2)) to capture the effect of
various factors on the appearance and stability of the formulations.
The data were visualised by graphical display using Microsoft Excel® 2007. To determine the
best combination of the factors studied, the data obtained on physical appearance and stability
of formulations were entered in the matrix obtained from the DX® 7.1.5 and analysed using
appropriate DX®-suggested model. The data for all responses were randomised and evaluated
by Box Cox plot, which helps to analyse whether any transformation of data is required
before the final analysis.
Table 2: Matrix of experimental factors generated by DX® 7.1.5 for the formulation of o/w
probability (p) value <0.05. Based on the significance level of p value, the factors were
included in or excluded from the model. The non–significant factors were excluded from the
model using backward elimination with alpha to exit set at 0.1, implemented in DX® 7.1.5
and the model was re–fitted with only significant factors and selected based on the goodness–
of–fit statistical criteria (Khuri and Cornell, 1987). The optimisation criteria were the visual
appearance (semisolid) and the physical stability (no syneresis). The desired level of
syneresis was set to be equal or less than 1%.
The DX® 7.1.5, on the basis of the contribution of the individual factors in addition to the
interactions between the factors predicted experiments with various combinations of the
variables producing maximum (100%) desirability to minimum (30%) desirability. The
experiment with 100% desirability predicted semisolid creams with no syneresis and the
experiment with 30% desirability showed structured fluids or semisolids with syneresis.
The confirmatory experiments were conducted in triplicates to access the accuracy of the
prediction of DX® 7.1.5 using the maximum desirability conditions (100%). The agreement
of predicted and experimental responses was evaluated by observing response values within
prediction at 95% confidence interval. An outcome within this interval validated the model
2.2.2 Preparation of Formulations
The oil phase composed of the oil (liquid paraffin/palm–olein), emulsifier and stearic acid
and the continuous phase (water and Span 80 or Tween 80) according to the concentrations
given in Table 2 were heated separately in a 100mL beaker to 70oC on a water bath. The oil
phase was added to the water phase at the same temperature and cooled slowly (the beaker
left on the water bath to cool to room temperature (~0.01–0.02oC/min)) or fast (beaker taken
out of water bath and left on the bench to cool to room temperature (3–5oC/min)). Upon
cooling, the formulations were either continuously mixed using a homogeniser until cold or
mixed for a while after initial mixing of two phases at 70oC and then cooling discontinued.
2.2.3 Characterisation of Formulations
220.127.116.11 Visual Inspection
The formulations were inspected for the appearance (structured fluid or semisolid) in addition
to any syneresis of the systems. The extent of syneresis was investigated by calculating the
amount of oil separated after one week of preparation.
18.104.22.168 Polarised Light Microscopy
A Leica DMLS compound microscope (Histocentre, Malaysia) was used to study the
microstructure of the raw materials (liquid paraffin, palm–olein) and all formulations. Slides
containing sample were placed between crossed polars and studied at various magnifications
(x 5, x 10, x 20 and x 40). Digital photomicrographs were taken using the Image Pro Express
22.214.171.124 Hot Stage Microscopy
A TMS 91 hot stage (Linkam Scientific Instruments, UK) attached to the Polyvar microscope
was also used to determine the melting point of the various raw materials in addition to
investigate the phase changes of all formulations. A thin smear of sample was sandwiched
between two 16 mm circular glass cover–slips. The stage was heated slowly from 25oC to
70oC at a rate of 5oC/minute and changes in the microstructure were observed at appropriate
magnification (x 20). The melting point was recorded as the temperature between which the
sample started to flow and the temperature at which the structure completely disappeared.
Thermal changes at various temperatures were recorded and photographed using the Leutron
126.96.36.199 Differential Scanning Calorimetry
A DSC 822e (Mettler Toledo, Leicester, UK) with the sample robot, was used for the thermal
analysis of all samples. Approximately 1.5–10mg of each sample was carefully weighed by
difference in 40µL aluminium DSC pans, the pans were sealed and placed in the appropriate
position of the sample robot. A standard heating cycle of 30oC–80oC at a rate of 5oC/min was
chosen to observe thermal changes in the samples. The results were plotted together after
normalisation to eliminate weight bias due to the weight differences of the samples.
The thermal properties of the formulations were studied by investigating changes in heat
transfer (H) during heating cycle.
A cone and plate Physica MCR 301, air–bearing Pelletier rheometer (Anton Paar, Germany)
was used to investigate the flow curves of the formulations. The dimension of the measuring
plate was 50 mm and the zero gap was fixed to be 1mm. All experiments were conducted in
triplicates at ambient temperature (~28oC).
Flow curves were obtained using shear rate vs shear stress experiments to investigate the zero
shear and apparent viscosities of the lipogels. Flow curves were obtained using up and down
curves with minimum shear rate of 10-3 (s-1) and maximum of 102 (s-1).
The rheological properties were studied by determining apparent viscosity (Pa.s) of each
formulation, obtained from the apex of the shear stress vs shear rate curves.
188.8.131.52 X-ray diffraction
A powder X–ray diffractometer (Bruker 8 Advances, Germany) was used for the analysis of
the formulations. The method was adopted from Koivisto et al.23 The samples were irradiated
with X–rays from a copper target using the following conditions and parameters: Filter Ni,
Generator 40kV, voltage 40kV, current 20mA, l 0.15410nm using a Soller slit. The samples
were continuously scanned from 2.3o–40o at a rate of 2.5o/sec with a step of 0.025s-1. The
XRD spectra were analysed using Diffract Plus software. CuKa1 values were used for the
analysis of each peak.
3.1 Characterisation of formulations
Formulation and stability studies were carried out using DoE by conducting 22 experiments
(each in triplicate) provided by the matrix generated by DX® 7.1.5 as described in the Table
2. The data on visual appearance and stability (no syneresis), microscopy, DSC, rheology and
XRD for each formulation generated are listed in Table 3.
3.1.1 Visual appearance
Formulations ranged from structured fluids to semisolid creams with or without syneresis
depending on the variables selected (Table 3). Generally, both oils produced semisolid (non–
syneretic) creams when Span 80 was used as an emulsifier at 10% concentration, maximum
concentration of stearic acid (5%), using slow cooling with no mixing upon cooling. In
contrast, structured fluids were obtained with minimum amount of emulsifier (1%), and
stearic acid (1%) with fast cooling. Mixing upon cooling had no significant effect on the
appearence of structured fluids. However, systems containing maximum amount of stearic
acid (5%) with continuous mixing upon cooling produced pressure sensitive semisolid
creams. The systems were semisolid immediately after cooling but changed to structured
fluids with gentle stirring. However, these systems changed back to semisolid creams upon
storgage for a week.
Table 3: Influence of factors on the physical chemical proeprties of formulations studied by
Design of Experiment
Note: MC- Maltese Crosses; PC- Plate-like crystals; MC/PC- Maltese crosses and plate-like
crystals; AC- Anisotropic crystals; AC/PC- Anisotropic crystals and plate-like crystals;
AM/CR- Mixture of amorphous and crystalline materials
3.1.2 Polarised Microscopy
Figure 1 shows the photomicrographs of the selected formulations. The semisolid (non–
syneretic) creams showed presence of numerous “Maltese crosses” in addition to the
anisotropic crystals (Figure 1a). The number of Maltese crosses increased with an increase in
the amount of emulsifier and the stearic acid. Contrarily, structured fluids showed only
anisotropic crystals (not shown). No “Maltese crosses” were observed in these systems. The
pressure sensitive semisolid creams showed presence of few Maltese crosses in addition to
the plate–like crystals (Figure 1b).
Figure 1: Photomicrographs of the selected formulations (a) Stable (non-syneretic) semisolid
creams and (b) pressure sensitive syneretic semisolid creams
3.1.3 Hot Stage Microscopy
All formulations showed melting of anisotropic crystals between ~40 and ~50oC. In contrast,
Maltese crosses in the stable semisolid creams appeared to melt at ~55oC. The plate–like
crystals in the pressure sensitive semisolid creams melted at ~42oC.
3.1.4 Differential Scanning Calorimetry
Figure 2 shows DSC data for the selected samples. All formulations showed different DSC
data each showing one to two endotherms. The stable (non–syneretic) semisolid creams
showed two endotherms peaking between ~48°C and ~55°C. In contrast, the structured fluids
showed only one endotherm peaking at ~45oC. The pressure sensitive semisolids showed two
endotherms peaking between ~43 and ~50oC. The stable semisolid formulations showed
significantly higher integration (H) (P<0.05) compared to structured fluids and the pressure
sensitive semisolid formulations (Table 3).
rate of 5oC/min was used
All formulations showed non–Newtonian behaviour with flow curves in the form of
anti-clock-wise hysteresis loops. Rheograms of all samples were different from each other
showing varying hysteresis loops i.e. stable semisolid formulations showing the broadest loop
All formulations showed different apparent viscosities, which were calculated from the apex
of the loop (100s-1). The semisolid (non–syneretic) formulations showed highest viscosities
(Table 3). In contrast, the structured fluids showed lowest apparent viscosities.
3.1.6 X-ray Diffraction
Figure 3 shows the XRD data for the selected samples. The structured fluids demonstrated
sharp peaks at 2q between 25° and 30°. The pressure sensitive syneretic semisolid creams
showed a broad peak at 2q=20°. No sharp peaks were seen in these systems. In contrast, all
stable semisolid creams showed two sharp peaks at 2q between 25° and 30° in addition to the
broad peak at 2q=20° (Figure 3).
Figure 3: XRD spectra of selected formulations showing sharp to broad peaks.
The data from Table 3 was analysed on DX® 7.1.5 using factorial design model. The software
randomised and evaluated the need for data transformation for all responses by Box Cox plot
and no data transformation was required, as the lambda was equal to 1 (best lambda= 1.48).
The data was analysed by ANOVA implemented in DX® 7.1.5, which shows influence of the
factors, depending on the p values (Table 4).
Table 4: ANOVA showing contribution of various factors on the physicochemical properteis
3.2 Influence of Factors
The Pareto chart demonstrates influence of various experimental variables and interactions
between them on the overall outcome and categorises the factors into significant, critical and
non-significant. The factors showing t–value more than the lower limit (2.77645) were
considered as significant whereas the factors with t-value above Bonferroni limit (6.84714)
were the critical factors (Figure 4).
Figure 4: Pareto chart showing significance of the factors and their combinations on visual
appearance and syneresisas of studied formulations.
3.2.1 Oil phase
The Palm–olein (A-) and liquid paraffin (A+) were used as oil phases. The influence of type
of oil phase and its interactions with other factors is explained by the Pareto chart (Figure 4)
as well as ANOVA statistics (Table 4). Type of oil phase showed a significant effect on the
overall outcome of the experiment (P<0.0002) showing positive effect (represented by the
colourless bars) as the t-value was above the lower line (Figure 4) but it was below the
Bonferroni limit (a measure of critical effect). However, its interactions with stearic acid
concentration (AD) and the type of emulsifier (AB) appeared as critical factors (above
Bonferroni limit). The interaction of factors AD showed positive effect. In contrast,
interaction of factors AB showed negative influence.
3.2.2 Type of emulsifier
The emulsifiers used were Span 80 (B-) or Tween 80 (B+). Eulsifer type appeared to be
insignificant variable (P value 0.067) as shonw in Table 4 and the t-value was below the
lower line in the Pareto chart (Figure 4). However, it showed significant interactions with
other experimental factors to produce critical effect especially with the concentration of
emulsifier (BC) producing positive effect (Figure 4).
3.2.3 Emulsifier concentration
The two emulsifiers, each at two concentrations 1% (C-) and 10% (C+) were used in this
sudy. The emulsifier concentration alone (C) and its interactions with other factors such as
Span 80 (BC), stearch acid (CD) and mode of mixing (CF) showed critical positive effect on
overall results as all of these factors were above the Bonferroni limit (Figue 4).
3.2.4 Stearic acid concentration
Two stearic acid concentrations 1% (D-) and 5% (D+) were selected. Stearic acid
concentration alone had a critical (Bonferroni value 8.94) but negative influence (Blue
colour) on the overall results. However, it showed significant interactions with other factors
such as oil phase (AD) and concentration of emulsifier (CD) to produce positive effect
(colourless bar) (Figure 4).
3.2.5 Mode of cooling
The formulations were cooled either slowly (E-) or fast (E+). The type of cooling appeared to
be the most critical factor (Bonferroni value 11.92) showing positive effect when taken into
consideration in isolation (slow cooling) producing non–syneretic semisolid creams.
However, its interaction with type of oil phase produced critically negative effect (AEcoloured
bar). Other interactions such as emulsifier type and mode of cooling (BE) and
emulsifier concentration and mode of cooling (CE) also showed significantly negative
influence (Figure 4). Fast cooling produced syneretic semisolids.
3.2.6 Mode of mixing upon cooling
To find out the effect of mixing on properties under study, the formulations were either
mixed continuously until cold (F-) or mixing stopped after initial amalgamation of phases at
70°C (F+). It showed critical effect on the final appearance of the formulation (Bonferroni
value 10.5) (Figure 4). In addition, its interaction with stearic acid concentration (DF) also
showed a significant (negative) effect on the overall results.
The equation 1 quantifies the overall influence of all factors and the interactions between the
factors on the appearance and stability of the formulations.
Semisolid creams = 4.28 + 0.21A + 0.09B + 0.71C - 0.34 D + 0.89 E + 0.61F + 0.65 AD +
0.39BC - 0.17 BE (Equation 1)
Order of effect of factors and interactions between the factors was as follows:
E > C > AD > F > BC > D > A > BE > B
The interactions between various factors for each formulation provided significant amount of
data and it is not possible to show all data here therefore typical interaction plots showing
effect of various factors as a function of appearance and stability of the formulations are
shown in Figures 5 and 6.
Figures 5 and 6 show the influence of factor interactions on the appearance of formulations.
The investigation of the interactions of various factors showed that when palm–olein was
used as an oil phase (A-) with slow cooling (E+) and high stearic acid concentration (D+),
changing cooling mode (F- or F+) have insignificant effect on the appearance of formulations
(Figure 5) as it produced semisolids in each case. However, liquid paraffin (A+) with same
variables showed significant contribution of the change in the type of mixing upon cooling
from (F-) to (F+) producing non–syneretic stable semisolids and syneretic structured fluids
respectively (Figure 5).
Figure 5: Influence of mode of mixing upon cooling and its interaction with other factors on
the physicochemical properteis of formulations.
Changes in emulsifier type showed significant effect on the physicochemcial properties of the
palm–olein formulations. In contrast, the liquid paraffin formulations showed no change in
the appearaence. When Span 80 (B-) was used as an emulsifier both oils produced semisolid
creams. However, with Tween 80 (B+), the palm–olein produced structured fluids (Figure
6).DoE suggested following combintion of formulation–aids and processing variables to
achieve the desired output i.e. non–syneretic semisolid creams:
Oil: Palm–olein; Emulsifier: Tween 80; Emulsifier concentration: 10%; Stearic acid
concentration: 5%; Mode of cooling: slow; Mixing upon cooling: yes.
Confirmatory experiments using the above formulation–aids and the processing variables
produced non–syneretic semisolids of similar microscopical, thermal and rheological
Figure 6: Influence of emulsifier type and its ineraction with other factors on the
physicochemical properties of formulations.
Formulations containing either liquid paraffin or palm–olein as an oil phase were prepared by
mixing the oil and water phases at 70oC using Span 80 or Tween 80 as emulsifiers and stearic
acid as stabiliser. The systems were cooled either slowly or fast with or without continuos
mixing. The influence of the variables was investigated on the physicochemical properties of
the systems using optimisation technique Quality by Design.
The systems obtained were either stable (non–syneretic) semisolid creams, syneretic
semisolid creams or mobile structured fluids showing very different microscopic, rheological,
thermal and polymorphic properteis.
Rheological properties of topical drug delivery systems such as semisolid or mobile nature
are associated with the presence of three– dimensional a-crystalline lamellar gel network
phase (Eccleston et al., 2000; Sheikh et al., 2010). The literature is vague since the a-
crystalline lamellar phase and the lamellar liquid crystals could not be discriminated. Stability
of colloidal dispersions has been associated with the formation of mesomorphic lamellar
structures such as micelles, vesicles, liquid crystals, hexagonal or nanoparticle (Rao et al.,
1992; Kriwet and Mueller–Goymann, 1993; Mueller–Goymann, 2004). Various liquid
crystalline phases (hexagonal, cubic and lamellar) that are produced in concentrated
surfactant solutions are known. However, lamellar liquid crystals are fundamentally different
that they do not swell significantly and convert to micelles instead forming liquids. The
lamellar liquid crystals may simply extend in the aqueous phase producing only
two-dimensional systems showing little swelling and entrap the oil droplets forming liquid
emulsions (Tadros and Vincent, 1983; Muller-Goymann, 2004; Tadros et al., 2005).
The present work describes existence of a-crystalline lamellar structures in the stable
semisolid creams. This view was supported by the polarised microscopy where stable
semisolids showed existence of “Maltese crosses” between cross-polars (Figure 1a), which
are indicative of a-crystalline lamellar phases Eccleston, 1986; Eccleston et al., 2000). In
contrast, unstable semisolids and structured fluids did not show “Maltese crosses”, but rather
clusters of crystals, suggesting absence of a-crystalline lamellar structures. The unstable
semisolid contained both plate–like crystals (Figure 1b) and lamellar structures (not seen); the
latter only disappeared on stirring producing liquids. Various researchers have reported
significant effect of process variables such as stirring, temperature and solvent on the
appearance and stability of the aqueous formulations containing stearic acid (Garti et al.,
1980; Timmins et al., 1990; Lin et al., 1994; Eccleston, 1997). Eccleston (1997) showed that
aqueous stearate creams are markedly affected by the mode of mixing and described the
effect of stress on the swollen crystalline structures. It was shown that in some systems, the
swollen lamellar structures appeared to be metastable and changed to non–swollen structures
under pressure showing plate–like crystals and attributed these differences to the marked
polymorphism in the stearate creams. Therefore, pressure sensitivity of syneretic semisolid
systems can be attributed to the plate–like crystals. The presence of plate–like crystals is
associated with the amorphous state of the stearic acid, confirmed by the XRD data (Figure
This view was further supported by the present DSC data as all stable semisolid formulations
showed similar thermal properties suggesting similar microstructures (Table 3). The high
temperature endotherm (55°C) in stable semisolid systems was related to the melting of
“Maltese crosses”, which was confirmed by hot stage microscopy, confirming the existence
of a-crystalline lamellar structure. In contrast, unstable semisolids or liquids did not show
this endotherm (55oC), which established lack of a-crystalline lamellar structures in these
systems. In addition, pressure sensitive semisolids showed broad endotherm peaking at 42°C,
which was related to the melting of plate–like crystals as confirmed by hot stage microscopy,
confirming weakening of a-crystalline lamellar structure.
Viscosity values measure the ability of any system to resist the structural breakdown during a
shearing process (Eccleston, 1977; Realdon et al., 2001; Ribeiro et al., 2004; Tadros, 2004).
Furthermore, viscosity determinations provide information about the structural organisation
of the formulations (Mueller–Goymann, 2004). There is an increased organisation of the
a-crystalline lamellar structure with an increase in the viscosity (Tadros, 2004). It can be
postulated that an increase in viscosity causes structural organisation mainly due to formation
of a-crystalline lamellar structure. This postulation was confirmed by the rheology data as
the stable semisolid systems showed significantly higher apparent viscosities than the
syneretic semisolid or liquid systems (Table 3).
The XRD data showed that stearic acid is present either mainly in amorphous or crystalline
state. The broad peaks in the XRD spectra are generally related to the amorphous state and
sharp peaks to the crystalline state (Gunstone, 1967). The stable semisolid creams showed
existence of two sharp peaks at 2=24-30° in addition to a broad peak at 2=20°, suggesting
that stearic acid is present essentially in the mixture of crystalline and amorphous states
producing a-crystalline lamellar structures giving rheological strength to the systems. In
contrast, the syneretic semisolid nature of unstable systems can be attributed to the broad
peak at 2=20°, suggesting existence of stearic acid mainly in the amorphous state. The
structured fluids on the other hand showed three sharp peaks in the same region suggesting
the presence of essentially crystalline material. Amorphous materials are easy to disintegrate
and penetration of the vehicle is rather easier due to lack of any ordered structure compared
to the mixture of amorphous and crystalline materials (Garti et al., 1982; Leinonen et al.,
1992). In the present study the syneretic semisolids were brittle, fragile and crumbled to
touch. Due to lack of structural organisation, lamellar structure was not fully developed and
the systems showed syneresis as a consequence.
In the present study, the effect of six (6) factors (Table 1) was investigated on the properties
of formulations. In this context, using OFAT approach, 64 experiments are needed to be
conducted to obtain a complete set of data for the investigation of the effect of 6 factors
(2K6). However, with the use of DoE, the same information was obtained with just 22
experiments. This observation was in agreement to the finding of Nielloud et al., (2003) who
also reported significant reduction in number of experiments with the use of quality by design
approach in the formulation of submicron emulsions. In addition, a set of parameters and
their combination were predicted to optimise the formulation, the objective of the present
study. The DoE revealed that concentration of emulsifier, type of cooling and the type of
mixing upon cooling were the most critical factors for the overall appearance and stability of
the formulations. Although, other factors also contributed in obtaining the optimised
formulations but statistically they appeared to be non–critical.
The type of cooling had a significant effect on the formulation properties. Slow cooling
produced stable systems whereas fast cooling resulted in syneresis, a characteristics of
unstable systems. The continued mixing upon cooling adversely (negatively) effected the
stability of formulations of liquid paraffin when stearic acid concentration was at maximum-
(AD) and (DF) interactions (Figure 5) producing syneretic semisolids. This observation is
believed to be due to the polymorphic modification in stearic acid from crystalline to
amorphous state upon continued mixing during cooling as confirmed by the XRD data
(Figure 3) and is attributed to the pressure sensitivity of the stearic acid (Sheikh et al., 2010).
However, systems were stable semisolids when mixing was discontinued immediately after
initial mixing of two phases at 70oC. In contrast, palm–olein formulations were not affected
by the change in mixing type, suggesting that the a-crystalline lamellar structures in these
systems were essentially protected by the palm–olein resisting change in the crystal state of
the stearic acid upon stirring.
The emulsifier type also showed a significant effect on the appearance of systems. The palm–
olein produced stable semisolids only in the presence of Tween 80 as an emulsifier (AB
interaction). Structured fluids were obtained with the Span 80. In contrast, liquid paraffin
formulations had indifferent effect of the type of emulsifier as all formulations were stable
semisolids (Figure 4).
Therefore, DoE by investigating influence of various factors and their interactions provided
various combinations to obtain optimised stable semisolid formulations using both oils
separately and in combination, proving to be a cost effective methodology.
Quality by design “QbD” optimisation technique was used to formulate palm–olein based
semisolid systems. It was possible to investigate the critical factors and their interactions on
overall outcome i.e. physicochemical properties of the semisolid systems, in a significantly
fewer number of experiments (22) compared to OFAT approach (66).
The formation of stable semisolid systems with both oils depends on the type and
concentration of emulsifier used and preparation technique. Generally, slow cooling and
discontinued mixing upon cooling produced stable semisolids. Fast cooling and continued
mixing upon cooling containing stearic acid (5%) produced unstable pressure sensitive
semisolids showing syneresis of oil in liquid paraffin systems. In contrast, low amount of
emulsifier (1%) generally produced fluids.
The stable semisolid systems showed presence of “Maltese crosses”, suggesting existence of
a-crystalline lamellar structures. The unstable semisolids and fluids showed to lack
a-crystalline lamellar structures and the unstable semisolids showed presence of plate–like
crystals, implying pressure sensitivity.
Stearic acid was essentially in the mixture of crystalline and amorphous state in stable
semisolid systems producing a-crystalline lamellar phases. In contrast, it was mainly in the
amorphous and crystalline states in unstable semisolids and structured fluids respectively.
Authors thank International Medical University for the financial support under Grant no
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