AAPS PharmSciTech, Vol. 10, No. 2, June 2009 (# 2009)
`DOI: 10.1208/s12249-009-9218-1
`
`Research Article
`
`Characterization and Stability of Emulsion Gels Based on Acrylamide/Sodium
`Acryloyldimethyl Taurate Copolymer
`
`Giulia Bonacucina,1 Marco Cespi,1 and Giovanni F. Palmieri1,2
`
`Received 10 July 2008; accepted 1 March 2009; published online 2 April 2009
`Abstract. Sepineo P 600, a concentrated dispersion of acrylamide/sodium acryloyldimethyl taurate
`copolymer in isohexadecane, has self-gelling and thickening properties and the ability to emulsify oily
`phases, which make it easy to use in the formulation of gels and o/w emulsion gels. In this paper, gels
`were prepared using a Sepineo P 600 concentration between the 0.5% and 5% (w/w), and then emulsion
`gel was also prepared from the 3% Sepineo gel by adding a specific amount of almond oil. All the
`prepared systems were analyzed and characterized by oscillation rheology and acoustic spectroscopy. The
`particle size of the oil droplets and the microrheological extensional moduli (G′ and G″) of the systems
`were determined from acoustic parameters and used together with the classical oscillatory rheological
`tests to assess the stability of the systems. Classical oscillatory analysis revealed that the dynamic moduli
`were very dependent on polymer concentration; as this parameter increased, there was progressive
`improvement in the sample elasticity. In fact, the mechanical spectra of the 0.5% and 1% (w/w) Sepineo
`samples were characterized by strong frequency dependence and multiple crossover points, typical of
`dilute polymer solution with no organized structure. On the other hand, the 3–5% (w/w) concentration
`systems showed typical gel-like spectra, marked by the absence of crossover points between the dynamic
`moduli and by weak dependence on frequency. Nevertheless, the elastic properties of the gel-like
`structure even at elevated polymer concentrations were not strongly long-lasting, as demonstrated by the
`increase of the viscous contribution in the low frequency range during acoustic spectroscopy analysis.
`This fact could indicate that the gel structure is characterized by weak polymer–polymer interactions, an
`advantageous characteristic for topical administration, as the sample is thus easier to rub into the skin.
`Finally, both rheology and acoustic spectroscopy indicated that addition of the oily phase caused minimal
`changes to the elastic character of the gel. Thus, Sepineo P 600 gel and emulsion gel are very effective
`systems for use in topical and other types of applications.
`
`KEY WORDS: acoustic spectroscopy; emulsion gel; Sepineo; viscoelasticity.
`
`INTRODUCTION
`
`In recent years, there has been great interest in the use of
`novel polymers with complex functions as emulsifiers and
`thickeners because the gelling capacity of these compounds
`allows the formulation of stable emulsions and creams by
`decreasing surface and interfacial tension and at the same
`time increasing the viscosity of the aqueous phase. In fact, the
`presence of a gelling agent in the water phase converts a
`classical emulsion into an emulsion gel.
`Recently, new research with emulsion gels has focused
`on the characterization of milk protein (used as surfactant
`and gellifying agents) o/w emulsions (1–6). Other investiga-
`tions have examined the thickening and emulsification
`properties of water-soluble amphiphilic polymers, such as
`modified hydroxyethylcelluloses (7–8). Gelled emulsions have
`
`1 Department of Chemical Sciences, University of Camerino, via S.
`Agostino 1, 62032, Camerino, Italy.
`2 To whom correspondence should be addressed. (e-mail: gianfilippo.
`palmieri@unicam.it)
`
`been described as composite materials in which oil droplets
`behave as filler particles, and free proteins (referring to
`protein gel systems) form the gel matrix (9) and also act as
`emulsifiers. Thus, differences in emulsion gel rheology are
`due to the differences in interactions across the interfaces
`between the filler particles and the gel matrix (10). Previous
`studies demonstrated that the presence of oil droplets could
`modify the mechanical characteristics of the protein gel by
`modifying its viscoelastic properties (11), substantially in-
`creasing gel strength (12). In fact, oil droplets increase the
`bulk protein concentration so that the latter can strengthen
`the gel in the bulk phase of the emulsion (12). Furthermore,
`the oil droplets create an interface able to interact with the
`gel matrix (11), and the presence of the proteins adsorbed at
`the interface assures interactions within the matrix and filler
`(13).
`Investigations of viscoelastic properties have provided
`fundamental characterization of these systems and made
`rheology the most widely used technique for such study
`(1,2,7,10,12).
`In this work, the self-gelling properties of the acrylamide/
`sodium acryloyldimethyl taurate copolymer (Sepineo P 600),
`
`1530-9932/09/0200-0368/0 # 2009 American Association of Pharmaceutical Scientists
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`Characterization and Stability of Emulsion Gels
`
`369
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`both alone and as dispersing phase for the preparation of
`o/w emulsion gels, have been investigated by oscillatory
`rheological measurements and acoustic spectroscopy.
`Sepineo P 600, based on the concept of droplet hydro-
`swelling, is a concentrated droplet dispersion of acrylamide/
`sodium acryloyldimethyl taurate (a viscous liquid at room
`temperature) in isohexadecane as the oily dispersing phase.
`The presence in this dispersion of polysorbate 80 is important
`for keeping the resultant dispersion stable. When water is
`added, these polymer droplets disappear because the polymer
`molecules interact with it strongly, instantly forming a stable
`semisolid system (14).
`The possibility of obtaining stiff and stable gelled phases
`with this polymer makes it a good candidate for the
`formulation of emulsion gels. Obviously, the presence of the
`oily phase can influence the viscoelastic behavior of Sepineo
`P 600 gels, with consequent changes in the characteristics of
`the final emulsion.
`Thus, in this study, rheological oscillatory measurements,
`in particular, frequency sweep analysis, were used for the
`mechanical characterization of gels and emulsions, while
`acoustic spectroscopy provided a better understanding of
`sample microstructure. This non-destructive technique offers
`a unique possibility for characterizing concentrated colloidal
`dispersions while avoiding dilution, which is a limiting step,
`particularly when the tested sample is highly structured. The
`parameters defined by acoustic spectroscopy, such as attenu-
`ation frequency spectra and sound speed, allow the calcula-
`tion of particle size from 5 nm to 1,000 μm (15).
`The operating principle of acoustic spectroscopy is based
`on the generation of sound pulses that pass through a sample
`and are measured by a receiver. During the passage through
`the sample, sound is attenuated by the presence of the liquid
`medium. The energy changes in intensity and phase are
`measured.
`There are six mechanisms of sound interaction with a
`dispersed system: viscous (related to the shear waves
`generated by the particles oscillating in the acoustic pressure
`field due to the difference in the densities between particle
`and medium), thermal (related to the temperature gradients
`generated near the particles’ surface), scattering (the same
`principle as light scattering),
`intrinsic (losses of acoustic
`energy occur when the sound wave interacts with the particles
`and the medium as a homogeneous phase), structural (caused
`by the oscillation of a network of particles; this mechanism is
`specific to structured systems), and electrokinetic (ultrasound/
`double layer interactions). The electrokinetic losses are
`negligible in terms of the total attenuation, making it possible
`to separate electroacoustic spectroscopy from acoustic spec-
`troscopy (15).
`Through complex processing and modeling of these
`kinds of energetic contribution to the total acoustic attenua-
`tion, it is possible to calculate the particle size and the zeta
`potential of the dispersed particles (15).
`The acoustic spectrometer can also act as a micro-
`rheometer, taking into account the fact that, in this case,
`“longitudinal” viscoelastic properties are measured because
`the stress is not tangential, as it is in an oscillation experiment
`in a rotational rheometer, but normal.
`It is possible to demonstrate that ultrasonic absorption
`and velocity are related to the real and imaginary part of the
`
`complex modulus (16) and that G′ and G″ moduli are related
`to sound speed (V), sound attenuation (α), and frequency
`(ω), using the following equations (16):
`0 ¼ V2
`
`ð1Þ
`
`G
`
`0 ¼ 2V3=!
`
`G
`
`ð2Þ
`
`Based on these facts, a number of Sepineo P 600 systems
`were prepared using a Sepineo concentration ranging be-
`tween 0.5% and 4%, and after accurate characterization of
`Sepineo P 600’s gelling ability, almond oil was added to the
`3% Sepineo gel. This oil was chosen because it is widely used
`in pharmaceutical and cosmetic applications for its practically
`inexistent toxicity and its high tolerability. The emulsion
`stability over time was monitored by observing the variation
`of the rheological parameters and mean diameter of the
`droplets.
`
`MATERIALS AND METHODS
`
`Sepineo P 600 and Sepicide HB, an anti-microbial agent
`composed of a mixture of phenoxyethanol, methylparaben,
`ethylparaben, propylparaben, and butylparaben, were a kind
`gift of Seppic (Paris). Almond oil USP was obtained from
`ACEF s.p.a. (Fiorenzuola D’Arda, Italy), and deionized
`water was obtained from an MF3 ion-exchange system (San
`Salvatore di Cogorno, Genova, Italy).
`
`Gel Preparation
`
`Gels were prepared for simple dispersion by mechanical
`stirring (30 min at 300 rpm; Eurostar Digital, IKA Labor-
`technik) of the Sepineo in the required amount of deionised
`water. The Sepineo concentration ranged between the 0.5%
`and 5% (w/w).
`Systems were then left to rest for 24 h before being
`analyzed.
`
`Emulsion Preparation
`
`Emulsions were obtained by dispersing the 3% w/w Sepineo
`P 600 in the required amount (20% w/w) of almond oil and then
`adding the aqueous phase, containing 0.5% (w/w) of Sepicide
`HB, and stirring (Eurostar Digital, IKA Labortechnik,
`1,200 rpm).
`Samples were left at room temperature for 24 h before
`being analyzed and then stored for 3 months at 40°C to check
`sample stability over time.
`
`Rheological Characterization
`
`Non-destructive oscillatory measurements made it possi-
`ble to obtain the principal rheological parameters, such as the
`storage or elastic modulus (G′), the loss or viscous modulus
`(G″), and the loss tangent (tan δ). Rheological analyses were
`performed in triplicate using a stress control rheometer
`(Stress-Tech, Reologica) equipped with a cone-plate geome-
`try (4/40) operating in the oscillation mode. The gap was
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`370
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`Bonacucina, Cespi and Palmieri
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`150 μm. Several tests were carried out on gels and emulsion
`systems:
`◆ Oscillation stress sweep. The sample was exposed to
`increasing stress (0.05–10 and 0.05–50 Pa) at constant
`frequency (1 Hz) and temperature (20°C), and the G′
`values were plotted in logarithmic scale. This test
`makes it possible to determine the linear viscoelastic
`regime of the sample and therefore to choose the
`stress value for the other oscillation tests.
`◆ Temperature sweep. This test was performed to charac-
`terize sample behavior at constant frequency and stress in
`a range of temperatures. The experimental parameters
`were 1 Hz frequency, 1 Pa stress, a temperature range of
`10–60°C and a heating rate of 1°C/min.
`◆ Frequency sweep. The sample was exposed to stepwise
`increases of frequency (0.01–50 Hz) at constant stress
`(1 Pa) in the field of linear viscoelasticity, and the
`average values of G′, G″ and δ were calculated at 10,
`1, and 0.02 Hz. The frequency range, the G′, and the
`G″ values were plotted in logarithmic scale.
`◆ Creep/recovery. This test was carried out at 20°C at
`different values of stress, depending on the system
`studied. The selected stress was kept constant for
`100 s, then instantly removed, followed by a 200-s
`recovery. The creep compliance JC (defined as the
`ratio between the measured strain and the applied
`stress) was monitored against time. The test was also
`used to calculate the viscosity of the sample from the
`linear stress/strain region of the retardation curve.
`
`The rheological behavior of emulsions was also studied
`by performing the following test:
`◆ Shear test + time sweep. This test was performed by
`applying a constant stress (200 Pa) for 5 min (shear
`test) and then observing the sample recovery for
`90 min in the oscillation mode at 1 Pa of stress and
`1 Hz of frequency at the temperature of 20°C (time
`sweep test).
`
`Emulsion stability was checked 1 and 3 months after
`preparation, using stress sweep and creep-recovery tests and
`shear analysis followed by time sweep.
`
`Acoustic Spectroscopy Measurements
`
`The cell of the DT-1200 acoustic and electroacoustic
`spectrometer (Dispersion Technology, USA) was filled with
`
`15 ml of Sepineo P 600 gel or the emulsion, and the sound
`attenuation and speed were monitored. Analyses were
`the gap interval of 0.325–20 mm,
`performed at
`in the
`frequency range of 3–100 MHz at 20°C. Particle size and
`rheological G′ and G″ moduli were calculated (15). The
`possible variation of droplet size and modification of the
`microrheological parameters over time were adopted as
`stability criteria of the semisolid systems.
`
`RESULTS AND DISCUSSION
`
`Rheological Analysis of the Gels
`
`The rheological behavior of the gels was strictly related
`to polymer concentration. Stress sweep results and viscosity
`values obtained from the creep recovery test showed
`increasing elasticity in the samples, as the Sepineo concen-
`tration was increased from 0.5% to 3%, as demonstrated by
`the values of elastic modulus G′ and viscosity (Table I). This
`behavior was also confirmed by the temperature sweep test,
`which showed higher and very similar values of G′ modulus
`for the 3% and 5% samples. This test also showed that there
`was no significant variation in G′ or G″ moduli at increasing
`temperatures (Fig. 1).
`From the frequency sweep analyses, it was possible to
`observe that the 3–5% (w/w) concentration samples showed
`typical gel-like spectra, characterized by the absence of
`crossover points between the dynamic moduli and by weak
`dependence on frequency (Fig. 2, Table II). In contrast, as
`expected, the 0.5% and 1% (w/w) systems presented different
`viscoelastic behavior with mechanical spectra characterized
`by stronger frequency-dependence and multiple crossover
`points. These considerations were confirmed by the slope
`values of the G′–G″ curves versus frequency (Table III). It is
`known that
`the rheological behavior of a polymer in
`dispersion can show a modification in the slope values of
`the G′–G″ versus frequency curves at increasing concentra-
`tions or molecular weights. In this case, given the strong
`frequency dependence, the Zimm or Rouse models seemed
`the best-fitting (17). These models concern the viscoelastic
`behavior of an isolated chain that relaxes independently of
`the presence of the others and, for this reason, are usually
`employed for dilute polymer solutions. However,
`in this
`study, they failed to provide correctly fitting results, even for
`the 0.5% concentration, perhaps because of the co-presence
`of an entanglement network at this concentration. Certainly,
`at low polymer concentrations, it is possible to assume the
`
`Table I. G′ and G″ Obtained from the Stress Sweep Tests and Calculated at the Stress of 1 Pa and Viscosity Values Obtained from the Creep-
`Recovery Tests of the 0.5–5% Sepineo Gels and of the Sepineo 3%/Almond Oil Emulsion at Different Storage Times
`
`Gels
`
`Emulsion
`
`0.5%
`1%
`3%
`5%
`1 day
`1 month
`3 months
`
`G′ (Pa)
`
`0.630
`6.670
`615.7
`665.7
`619.6
`611.4
`608.6
`
`SD
`
`0.03
`3.17
`16.1
`44.4
`7.70
`4.00
`6.50
`
`G″ (Pa)
`
`1.100
`4.620
`90.67
`94.73
`68.20
`67.80
`60.60
`
`SD
`
`0.03
`0.84
`1.98
`11.2
`12.5
`8.31
`5.76
`
`Viscosity (Pa s)
`
`SD
`
`0.4265
`47.180
`84,414
`112,647
`92,346
`92,123
`92,011
`
`0.045
`2.540
`2,740
`6,215
`3,421
`3,836
`4,971
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`Fig. 1. Temperature sweep test of Sepineo P 600 systems (0.5–5% w/w)
`
`existence of topological constraints and thus to imagine that
`there are entanglement sites, rather than specific interactions
`between polymeric chains. More in-depth analysis of the
`frequency sweep plot of the 0.5% sample revealed that it
`could easily be fitted with the Doi–Edwards model (17,18)
`(Fig. 3), which assumes that the motion of the chain is
`confined to a tube-like region formed by the surrounding
`polymer molecules. Running along the center of the tube is a
`primitive chain that represents the shortest path down the
`tube, and every deviation from this path is considered a
`defect. The motion of these defects allows the chain to move
`along the tube with a reptilian motion (reptation). The
`plateau modulus, which refers to the plateau observed in
`the plot of G′ versus frequency at intermediate frequencies
`and is related to the relaxation modulus, can be calculated
`using this type of modeling. In addition, it was possible to
`obtain the characteristic system relaxation time (0.5185 s.),
`the zero shear viscosity (2.303 Pa s), and the related steady-
`state compliance from the values of the plateau modulus and
`
`of the system relaxation time. The low value of zero shear
`viscosity and plateau modulus (5.338 Pa) obtained from the
`fitting, and the fact that this model
`is usually applied to
`semidilute and concentrated solutions demonstrated that the
`0.5% systems did not possess an organized structure. In fact,
`the Doi–Edwards theory adequately describes the dynamic
`behavior of ideally flexible polymers in concentrated non-
`entangled solution, where the polymer chains do not possess
`permanent cross-links, and the chain is free to move along the
`“tube.”
`Slightly different considerations can be made for the 1%
`concentration sample. In fact, the modeling of its frequency
`spectra showed better correlation with the “tube linear
`dilution theory” (17,19,20).
`This theory considers the shortcomings of reptation
`theory applied to linear chains, treating them as two-armed
`stars (21–23). In this model, relaxation phenomena are
`related to arm retraction. This means that the retraction
`motion of the arm (for each star) is very similar to the
`
`Fig. 2. Frequency sweep test of Sepineo P 600 systems (0.5–5% w/w)
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`Table II. G′ and G″ from the Frequency Sweep of Sepineo P 600
`Gels (0.5%, 3% and 5% w/w) Calculated at Frequencies of 10, 1, and
`0.02 Hz
`
`% (w/w)
`
`Frequency (Hz)
`
`0.5
`
`1
`
`3
`
`5
`
`10
`1
`0.02
`10
`1
`0.02
`10
`1
`0.02
`10
`1
`0.02
`
`G′ (Pa)
`
`1.830
`1.257
`0.001
`11.26
`9.130
`2.680
`760.0
`650.7
`530.0
`811.3
`686.6
`545.0
`
`SD
`
`0.93
`0.07
`0.00
`2.84
`1.46
`1.40
`7.21
`8.62
`4.00
`16.8
`13.0
`13.1
`
`G″ (Pa)
`
`2.320
`1.103
`0.061
`11.06
`4.240
`2.000
`123.7
`78.87
`62.73
`147.0
`90.40
`73.30
`
`SD
`
`0.26
`0.05
`0.00
`0.92
`0.21
`0.13
`5.51
`5.80
`6.40
`1.73
`0.95
`1.97
`
`fluctuation motion of the contour along the linear chain
`(fluctuation-driven stretching and contraction of the chain
`along the tube). It should be taken into account that, in linear
`chains, the tube segments, which do not have retracting ends,
`will be relaxed by reptation. For these linear chains, the fast
`retractions will involve motion near the chain end, and thus,
`this model must consider the center of the chains as fixed.
`Application of this model and calculation of the relative
`spectra revealed the increasing elasticity of
`this system
`compared to the 0.5% concentration, as confirmed by the
`higher values of
`the plateau modulus (491.6 Pa), shear
`viscosity (870.6 Pa s), and characteristic relaxation time
`(170.6 s.). This means that at 1% concentration, Sepineo is
`present in dispersion in a more “organized” structure, which
`implies more complex mechanisms of relaxation and higher
`polymer–polymer or polymer–solvent interactions despite the
`absence of a three-dimensional network (Fig. 4).
`Concentrations from 3% to 5% showed very different
`mechanical spectra. As confirmed by the slope values very
`close to 0, these systems can be considered as gels from a
`rheological point of view, and in this case, the spectra were
`typical of a solid-like sample with a three-dimensional
`network structure.
`Once again, these results confirmed that the rheological
`behavior depended strongly on polymer concentration. In-
`creased concentrations from 0.5% to 5% gave rise to a
`progressive change in system mechanical spectra, which could
`lead one to think that increasing chain interactions and
`formation of entanglements foster the development of
`connectivity across the entire system until a network is
`formed.
`
`Rheological Analysis of the Emulsions
`
`added in order to assess the influence of an oily phase on the
`rheology of the dispersing phase and the stability of the
`emulsions themselves.
`The stress sweep test was used to follow emulsion
`stability because the elastic modulus slowly decreases when
`the dispersed droplets become bigger but less numerous
`(24–27).
`The stress sweep analyses performed 1 day after the
`preparation of these emulsions, and then at 1 and 3 months,
`showed good stability (Table I). It is important to note that
`the dynamic moduli values were just slightly higher than
`those found for the corresponding Sepineo gel, proving that
`the addition of
`the oily phase did not affect Sepineo
`rheological behavior too much (Table I).
`The time sweep performed to check sample recovery
`after shear showed that emulsions were unable to recover
`their structure during the test, as confirmed by the smaller
`moduli values (data not shown). This indicated a degree of
`sample deformability after shear, which can improve system
`spreading and absorption, making it easy to rub the sample
`into the skin after topical administration (28,29).
`The frequency sweep tests (Fig. 5), performed 1 day after
`preparation, confirmed the behavior previously observed
`from the stress sweep: The almond oily phase did not
`substantially affect the mechanical spectrum of the Sepineo
`aqueous phase, showing a very similar dependence on
`frequency for both moduli as revealed by the slope values
`in Table III.
`Furthermore, the emulsion behavior with temperature
`(data not shown) confirmed that
`the systems were not
`affected by temperature, as already observed for the Sepineo
`gel.
`
`Acoustic Spectroscopy Analysis of the Gels
`
`Acoustic spectroscopy afforded a microrheological char-
`acterization of the gel samples. It is important to note that the
`dynamic moduli obtained from acoustic spectroscopy meas-
`urements are not comparable with those derived from
`rotational rheometers working in the oscillation mode, since
`the applied stress is not tangential, as in an oscillatory
`experiment, but “normal,” and the tested frequencies are
`much higher.
`In any case, analysis of the rheological parameters and,
`in particular, of the G″ modulus (Fig. 6), which is the
`frequency-dependent modulus, showed some differences
`within the various systems, in agreement with the oscillatory
`results. Basically, the spectra of the 0.5% and 1% Sepineo
`
`Table III. Slope Values Calculated by the Linear Fitting of the G′
`and G″ Average Curves Obtained from the Frequency Sweep of the
`0.5–5% Sepineo Gels and of the Sepineo 3%/Almond Oil Emulsion
`
`Based on the rheological characterization of the different
`gelled systems, the 3% (w/w) concentration was selected as
`the external dispersing phase for the preparation of the
`emulsions. In fact, the 0.5% and 1% samples were not stiff
`enough, while the considerable elasticity of the 5% system
`could compromise correct emulsion formation. As mentioned
`in the “MATERIALS AND METHODS”, almond oil was
`
`0.5% gel
`1% gel
`3% gel
`5% gel
`Emulsion
`
`Slopes G′
`
`Slopes G″
`
`1.568
`0.243
`0.056
`0.060
`0.057
`
`0.696
`0.272
`0.115
`0.102
`0.120
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`Characterization and Stability of Emulsion Gels
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`373
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`progressive loss of the network structure made the viscous
`contributions more important, giving rise to the remarkable
`increase of the G″ modulus.
`On the other hand, the G′ modulus did not show
`significant differences among the various systems, even
`though the 3% (w/w) and particularly the 5% concentrations
`showed slightly higher values for this modulus (Fig. 6). In this
`case, the increase in the density and sound speed of the
`systems at greater polymer concentration could explain the
`results obtained (see Eq. 1).
`
`Acoustic Spectroscopy Analysis of the Emulsions
`
`Ultrasound technique was used to monitor emulsion
`microrheological behavior and stability by considering the
`variation of both their rheological parameters and particle
`size (such as mean diameter).
`The frequency spectra used to analyze system stability
`over time (Fig. 7) confirmed the stability of the emulsion. In
`fact, the G′ and G″ plots obtained after 1 day, 1 month, and
`3 months were very similar except for the G″ modulus at low
`frequencies. These results are in agreement with the oscilla-
`tory analysis results that revealed good stability for the
`emulsion.
`It is interesting to compare the G″ spectra between the
`gel and the emulsion. As can be seen from Figs. 6 and 7, the
`emulsion system in the high frequency range exhibited quite
`similar behavior, with slightly greater G″ values compared to
`the gel. The presence of the oily phase caused greater
`frequency dependence, which can be explained by a more
`organized structure and consequently greater initial energy
`dissipation during the frequency sweep. As mentioned in the
`introduction, the oil in the emulsion–gel formation mainly
`served as an interface able to interact with the gel matrix,
`providing general
`improvement in the system rheological
`characteristics.
`Instead, a different behavior was present in the low
`frequency region, where the curve related to the system
`analyzed after 1 day revealed a trend very similar to that of
`the corresponding Sepineo sample (3% w/w concentration),
`which showed, as previously mentioned, a certain deform-
`ability. After 1 and,
`in particular, after 3 months,
`this
`deformability gradually decreased, indicating a progressive
`interfacial stabilization of the emulsion. On the other hand,
`the G′ modulus can be considered practically identical
`between gel and emulsion (Figs. 6 and 7).
`
`Fig. 3. Frequency sweep test of Sepineo P 600 systems 0.5% (w/w)
`with Doi–Edwards spectra fitting (continuous line)
`
`samples were identical and characterized by a linear depen-
`dence on frequency. On the other hand, this linear trend was
`followed by a sharp modulus increase at lower frequency
`when the Sepineo concentration rose to 3% and 5% (w/w).
`The behavior of the latter two concentrations may seem
`unusual compared to the classical rheological trend, due to
`the fact that during the tests, the polymer chains showed
`significant modification in their deformability. This greater
`deformability at
`low frequency values indicates that the
`polymer/polymer interactions responsible for the gel-like
`behavior were not particularly strong. These interactions
`showed short-lasting elastic properties that probably failed
`when the system was stressed for a longer time. This change
`in the system structural characteristics modified the mecha-
`nisms of interaction between the sound and the samples (see
`“MATERIALS AND METHOD”) and consequently
`changed the resultant attenuation parameter. The structural
`contribution, which is characteristic of gel-like systems, was
`dominant at higher frequencies. At low frequencies, the
`
`Fig. 4. Frequency sweep test of Sepineo P 600 systems 1% (w/w) with
`tube dilation spectra fitting (continuous line)
`
`Fig. 5. Frequency sweep of Sepineo P 600/almond oil emulsion
`
`6
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`

`374
`
`Bonacucina, Cespi and Palmieri
`
`Fig. 6. Mean curves (standard deviation bars are omitted to avoid overlapping) of G″ and G′
`moduli from the acoustic spectroscopy of the different Sepineo P 600 concentration samples
`(0.5–5% w/w)
`
`The results obtained from the analysis of the particle size
`(mean diameter values in micrometer) confirmed the previ-
`ous statements concerning the stability of the emulsion. Mean
`diameter values (micrometer) of the dispersed almond oil
`droplets at the storage time of 1 day, 1 month, and 3 months
`were, respectively, 1.567±0.14, 1.592±0.22, and 1.578±0.25.
`
`CONCLUSION
`
`Oscillatory rheology and acoustic spectroscopy analyses
`were utilized in conjunction in order to characterize the Sepineo
`gel systems as well as to study how the addition of oil affects gel
`characteristics and the final stability of the resultant emulsion.
`Both techniques revealed that Sepineo P 600 thickens and gels
`
`well, a property that depends strongly on polymer concentra-
`tion. Concentration increases from 0.5% (w/w) to 5% (w/w)
`modified the viscoelastic properties of the Sepineo samples,
`changing the typical behavior of a concentrated non-entangled
`solution to that of a “gel-like” sample. On the other hand, the
`microrheological parameters obtained from acoustic spectros-
`copy showed that the physical interactions forming this gel-like
`structure were not particularly strong.
`Concerning the emulsions, the most important result is
`surely the fact that the addition of an oily phase increased
`system consistency only minimally. The viscoelastic character-
`istics depended exclusively on the gel structure.
`In conclusion, Sepineo P 600 is a prime candidate for use
`in the formulation of gels and emulsion gels with rheological
`properties suitable for topical administration.
`
`Fig. 7. Mean curves of G″ and G′ moduli from the acoustic spectroscopy of Sepineo/
`almond oil emulsion
`
`7
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`Characterization and Stability of Emulsion Gels
`
`375
`
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