OR IGINAL P A PE R: CHAR ACT ER IZA TION METHODS OF S OL-GEL AND HYBRID MATERIALS
Assessment of kinetic stability of cosmetic emulsions formulated with different emulsi fi ers using rheological and sensory analyses
A. Franzol1,2●T. M. Banin3●T. R. Brazil 1●M. C. Rezende1,2
Received: 12 May 2021 / Accepted: 3 July 2021 / Published online: 12 August 2021
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2021
Abstract
Emulsions are considered thermodynamically unstable systems and the establishment of technical criteria to select suitable components and concentrations to achieve the kinetic stability is a complex task. Theoretical studies have been aimed at obtaining better results and more stable emulsion systems, but relatively little information is available in the literature to support the development of complex emulsion systems. Thus, to address this knowledge gap, the kinetic stability of a cosmetic emulsion containing three different emulsifiers (anionic, cationic and non-ionic) was studied using rheological analysis. The emulsifiers were tested at different concentrations (1.0, 3.0, 5.0 wt%) and the emulsions were aged at different temperatures (5–60 °C) and for various time periods (2 weeks to 6 months). Rheological results based onflow curves, frequency sweep and amplitude sweep show the influence of the concentrations and chemical aspects of the emulsifiers on the emulsion stability. The best behaviors were observed for the samples with 3.0 wt% of cationic and anionic emulsifiers. This result presented good agreement with sensorial tests performed with consumers, who had a preference for the emulsion with 3% of cationic emulsifier. The correlation between the rheological and sensorial results verified the effectiveness of predictive studies based on rheological analysis, which can eliminate or reduce the long time periods required for kinetic studies (aging) and sensory testing in humans.
Graphical Abstract
Keywords Emulsion●Emulsifiers ●Stability● Rheology●Kinetics stability
* T. R. Brazil
1 Instituto de Ciência e Tecnologia, Universidade Federal de São Paulo, São José dos Campos, SP, Brasil
2 Engenharia Aeronáutica e Mecânica, Instituto Tecnológico de Aeronáutica, São José dos Campos, SP, Brasil
3 Centro de Engenharia, Modelagem e Ciências Sociais Aplicadas (CECS), Universidade Federal do ABC, Santo André, SP, Brasil
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Highlights
● This article focuses on recent progresses involving the characterization of emulsions based on rheological and sensory tests.
● Cosmetic emulsions containing three different emulsifiers (anionic, cationic, and non-ionic) in different concentrations were deeply characterized by rheological analyses.
● Sensory and kinetics stability predictions were the main outcomes.
1 Introduction
An emulsion is a polyphase system comprised of a dis- persed or fragmented phase (bubbles, droplets, or particles) within a continuous phase [1], where the size of the frag- mented phase strongly influences the emulsion stability [2, 3]. Emulsions are of great importance in many appli- cations, notably in the food industry (e.g., mayonnaise, butter, ice cream, milk), coatings (paints), agrochemicals, oils and cosmetics [4–6]. Therefore, the stability of emul- sions (time required for the visual initiation of phase separation) needs to be well understood in order to select the appropriate emulsifiers to stabilize the mixing of the discontinuous and continuous phases [7, 8]. Thus, con- sidering that the stability of emulsions can vary from min- utes to years, studies involving this theme attract considerable attention [3,9–12].
In the cosmetics industry, the development of emulsions is a challenge associated with different products. It affects the quality control, stability during shelf life, and benefits and attributes of the product, in addition to the prediction of product behavior in terms of consumer perception. In this context, sensory evaluations need to be conducted under controlled conditions, using appropriate tools and techniques, in order to obtain consistent and reliable results [8,13,14].
Sensorial analysis is a scientific approach used to mea- sure, considering thefive human senses, attributes or global characteristics of a product and how these are perceived by consumers [6, 15–19]. Two types of methods can be applied: analytical or affective. In the analytical methods, a trained professional defines a descriptive and discriminative analysis of the tested samples, so that a sensorial profile can be determined. In the affective methods, potential con- sumers, with no expertize on the subject, evaluate samples and the results are based on their acceptance and preference in relation to the tested samples [20].
The purpose of sensory evaluation is to measure the physiological response to a stimulus, minimizing influences presented by psychological factors, such as culture, age, and environmental aspects. The brain’s ability to receive and process sensory information is defined as the perception of the individual, which plays a fundamental role in the sen- sory evaluation of a product. Thus, it is important to study separately the physiological responses and psychological
influences on sensory evaluation, supported by scientific concepts. In this regard, rheological studies provide an important analytical tool that can support the development and success of a product, based on experimental data [2,21].
Laba (1993) highlighted the application of rheological profiles as a competitive tool in the prediction of the sta- bility and alignment of emulsions, correlating the results with the sensory expectation of consumers [22]. Based on this observation, the use of rheology started to be con- sidered as a tool to quickly reproduce consumer sensation and support safer product developments [23,24]. Brummer (2006) showed that rheological analysis based on frequency sweep could successfully predict the stability of emulsions under severe conditions of transport vibration, up to the point of sale [25]. Mercurio, Calixto and Campos showed that full factorial design of experiments in the development of cosmetic formulations, characterized by rheological and sensory analyses, are promising tools that minimize empirical formulation process [26]. Korać, Krajišnik and Milićshowed that rheological measurements are related to certain sensory attributes and can be used for faster devel- opment of cosmetic emulsions [27]. Morávková and colla- borators estimated sensory attributes of cosmetic emulsions by fast rheological measurement and observed that small change in emulsifier composition influences the emulsion behavior. These authors present that the correlation of rheological and sensory results may save time andfinancial resources in the development of formulations [28, 29].
Thus, the use of concepts of thixotropy, viscosity, yield value and frequency sweep can be very useful for the conceptualization of primary and secondary sensory aspects [30–34]. The primary sensory aspect, for skin creams, is positive when the consumer feels a full-bodied product in contact with the skin before spreading while the secondary sensory aspect involves a decrease in the viscosity (per- ceived as consistency by the consumer) while the product is being spread to facilitate the application.
Rheological studies have proven to be a useful tool to evaluate the effect of stabilizers, used to minimize or pre- vent sediment or aggregate formation in dispersed liquid systems, and assess the consistency of products, which is influenced by several factors, such as adhesion and cohe- sion forces, elasticity, viscosity, thixotropy, and micro- structure [2, 33]. However, although the literature shows
that the application of rheological concepts in stability and sensory studies on cosmetic emulsions [22,25,35] is pro- mising, the review presented in this article confirms that considerably more studies are needed on this theme.
Most of liquids presents a rheological behavior that classifies them between liquids and solids, i.e., they are elastic and viscous and, therefore, they are called viscoe- lastic [25]. The resistance of a fluid to any irreversible change of its volume elements (deformation) is called viscosity. For the conservation of flow in a fluid, energy must be added continuously. Measuring the viscosity of liquids requires defining the parameters that are involved in theflow. Therefore, suitable test conditions must be met so that the measurement offlow properties is reproductive. The basic law of viscometry is described by theflow behavior of an ideal liquid (Eq.1) [22]:
τ = η.
ð1Þwhere:τis the shear stress (N/m2or Pa);ηis the viscosity (N/m2.s or Pa.s), and is the shear rate.
Shear stress is a force F applied tangentially to an area A, where the interface between the upper plate and the liquid below causes a deformation rate (Eq.2). Theflow velocity, which can be maintained with a constant force, is controlled by the internal resistance of the liquid, i.e., its viscosity.
τ¼F=A ð2Þ
The velocity gradient in the sample is called the shear rate and is defined as a differential. The shear rate conducts the liquid to a special flow profile. The maximum flow velocity is in the upper layer and the speed decreases through the specimen until it reaches zero in the stationary layer. The shear stress causes deformation in solids, but in liquids it causes a deformation rate. This means that solids are elastically deformed while liquidsflow [36].
In rheological analyses aflow curve shows a hysteresis graph, obtained with the destroying of the material´s structure under analysis. The hysteresis area shows what is required for the material´s structure to collapse. This information is very important in the cosmetic emulsion area.
For example, for a sunscreen to be effective, its structure must be rebuilt, as soon as the shear ceases [36].
In oscillatory regime tests it is possible to separate the elastic and viscous contributions of the material in relation to time or frequency. This test should be performed within the linear viscoelastic regime, i.e., using sufficiently small stress amplitude (or strain) so that the response (stress or strain) has the same shape as requested [30].
The determination of the linear viscoelastic regime range of a material, at a given temperature, is made in amplitude sweep at a constant frequency. In this range the viscoelastic properties, i.e., storage modulus (Gʹ) and loss modulus (G”),
do not vary with the applied stress. When the material’s responses do not match the applied stresses, it means that the material’s internal structure has been destroyed. Fre- quency sweeping allows determining the viscoelastic behavior of the material at various times [36].
The correlation of the shear rate with the shear stress results in the named flow curve. This curve, obtained in stationary regime, allows determining the yield value, viscosity, and thixotropy of samples. The yield value is important information in the characterization of emulsions, as it is related to the kinetic stability and sensory aspects of this type of samples. The yield value is obtained at the point where the curve profile changes, which characterizes the rupture of the emulsion [30].
The content of this paper is aimed at highlighting the potential for rheological analysis to replace extensive studies involving human subjects, to promote more competitive development processes, and reduce the number of people exposed to chemicals. Specifically, the influence of three different emulsifiers (anionic, cationic and non-ionic) used in a cosmetic emulsion with water as the outer phase (continuous) and oil as the inner phase (fragmented) (O/W emulsions) was evaluated. The results obtained are compared with observations noted in sen- sorial analysis.
2 Materials and methods
2.1 Base emulsion preparationFirstly, a base emulsion (BE) was prepared and used to study the influence of the three different emulsifiers. The BE components were selected considering market research on commercial body moisturizers and based on analysis of the chemical compatibility between them. The BE prepared was similar to those used in commercial body moisturizers, satisfying competitive cost requirements and lacking com- plexity in terms of components or the manufacturing pro- cess. It was formulated with a water phase, an oil phase, emulsifiers and fragrance, as follows.
The oil phase was comprised of the following emollients:
1.0 wt% glyceryl stearate, 2.0 wt% mineral oil, 2.0 wt% C12- 15 alkyl lactate, 1.0 wt% caprylic triglyceride, 1.2 wt% stearyl alcohol, 2.8 wt% cetyl alcohol, 0.5 wt% dimethicone (sensory enhancement), and 0.7 wt% of phenoxyethanol (preservative).
The water phase was comprised of 0.2 wt% disodium ethy- lenediamine tetraacetic acid (chelating agent), 5.0 wt% gly- cerin (humectant), 0.3 wt% hydroxyethylcellulose (thickener), 0.2 wt% triethanolamine (pH adjuster), 0.9% benzyl alcohol (preservative), and deionized water (vehicle) to complete the overall emulsion sample. All wt% of the used components of the water and oil phases are related to the overall emulsion
sample. The compositions also contained 0.3 wt% of fra- grance. The relation of water:oil phases in the overall emul- sion sample was 9:1, in wt%, respectively.
Glycerin was used as the humectant rather than propy- lene glycol (PG) based on the literature [37], considering that PG could solubilize the emulsifier and displace it from the interface at different levels for each emulsifier studied.
This would hinder an assessment of the effect of the emulsifier in the BE. Another point to highlight is the choice of the dimethicone concentration (0.5 wt%). This value is applied in the market and gives a satisfactory result in terms of emulsion spreadability on the skin during con- sumer application, as reported in the literature [37].
The BE was prepared at 85 °C under mechanical agi- tation (100 rpm). Firstly, the aqueous phase and the oil phase were heated separately. At 85 °C, the oil phase was added over the aqueous phase and a viscosity gain was observed. After this step, the emulsifier (anionic cationic or non-ionic) was introduced into the BE. The stirrer speed was adjusted at this time to ensure homogeneous mixing of the components. The mixture was then cooled to 40 °C and benzyl alcohol and the fragrance were added.
When the emulsion reached 25–30 °C, the BE preparation was complete.
2.2 Emulsifiers
Table1 shows the main characteristics of the three emul- sifiers studied (anionic, cationic and non-ionic), aiming to
obtain O/W emulsions. The incorporation of the emulsifiers into the BE was carried out at 85 °C, under mechanical mixing (100 rpm). The three emulsifiers were used in dif- ferent concentrations (1.0, 3.0, and 5.0 wt%, Table 1).
These emulsifiers have different molar masses, structural arrangements and physico-chemical characteristics.
The anionic emulsifier has an alkyl chain with 16 car- bons, which ensures good solubility of the oily portion of the BE, and is similar to the phospholipid structures abun- dantly found in the skin, making it more biocompatible. The potassium counterion ensures good solubility with the aqueous phase [1,10].
The cationic emulsifier (distearyldimonium chloride) is a quaternary ammonium with a chlorine counterion that shows rapid dissociation in water. Considering that the initial pH of the formulas was ~5.5 (same pH as skin) and that in the accelerated stability studies a change in pH might occur, the use of a quaternary ammonium is appropriate, since it is not sensitive to pH changes. This emulsifier has two 18-carbon chains linked to the nitrogen, that provides good solubility in the oily phase and a satisfactory sensory experience for the consumer [37]. At the same time, it poses a challenge in terms of kinetic stability, because it can form large clusters.
The non-ionic emulsifier (ethoxylate) is readily available on the market, does not dissociate into ions in aqueous solution and is compatible with other types of emulsifiers, being widely used in complex formulations as a co- emulsifier or secondary emulsifier [1].
Table 1 Emulsifiers used in this study
Emulsifier Formula Main characteristics
Potassium cetyl phosphate, commercial name Amphysol K®(by DSM Nutritional Products)
Anionic type with polar phosphate group and potassium as counterion.
Distearyldimonium chloride, commercial name Varisoft TA 100®(by Evonik Nutrition &
Care GmbH)
Cationic type, a quaternary ammonium with chlorine as counterion.
Glyceryl Stearate+PEG-100 Stearate (raw materials used in the emulsifier), with commercial name of Arlacel 165®(by Croda Personal Care)
Non-ionic type, an ethoxylate with a hydrophilic group and a chain of polymerized ethylene oxide molecules attached to a nonpolar part.
2.3 Conditioning of samples
To accelerate the aging of the samples studied, conditioning was carried out at different temperatures (5.0, 25.0, 40.0, 50.0 and 60.0 °C) and applying various exposure times (2 weeks to 6 months). The aging was performed according to legal requirements for launching cosmetic products, established by the Brazilian regulatory agency, ANVISA (National Health Surveillance Agency) [38]. The samples kept at 5.0, 25.0 and 40.0 °C were conditioned for 6 months. However, the samples conditioned at 50.0 °C remained at this temperature for 1 month while those kept at 60.0 °C were aged for only 15 days. Conditioning at 5.0 (±0.5) °C was conducted in a refrigerator and aging at 25.0 (±0.5) °C was carried out in a laboratory environment with controlled temperature (reference conditions). In addition, the conditioning was performed at 40.0, 50.0 and 60.0 (±0.5) °C using a conventional hydrothermal chamber, with relative humidity of 75%.
During aging the samples should not show any organo- leptic (color, odor, appearance) and/or physico-chemical (viscosity and pH) changes.
2.4 Characterization
The sample characterization was performed by rheological, sensory and pH tests.
The rheological analysis was performed using a Haake strain control rheometer (CS), model Rheo Stress 6000, with 35 mm diameter and 2° (C35/2° Ti) cone-plate sensors.
The experiments were performed at 25 °C, controlled by a thermostat device (RS6000—Peltier TC81). The rheological tests were conducted to obtain the flow curves (stationary regime), the storage (Gʹ) and loss (G”) modulivs. frequency sweep, and the Gʹ and G” moduli vs. amplitude sweep.
These tests were carried out in an oscillatory regime. The parameters used in this study are given in Table2.
The pH measurements for all samples were obtained using a Micronal pH meter, model B-474, at 25 °C, with a combined glass electrode and Ag/AgCl internal reference electrode.
A questionnaire, considering the characteristics asso- ciated with the acceptance of a body moisturizer, was used for the sensory tests. It presented questions to address the following aspects: preference, absorption, hydration/soft- ness, oiliness/stickiness, sensation, spreadability, con- sistency, texture and whitish skin. Only samples with 3.0 wt
% of the three emulsifiers were evaluated, because this is the concentration most commonly used in this type of product found on the market [37]. In this trial, 19 people (panelists) received the three samples at a room (25 °C), and they were instructed to apply the product on the back of their hands and arms (sensitivity threshold area compatible with the purpose of this study). The samples were color coded. These tests were conducted with the support of Johnson&Johnson Co., São José dos Campos/Brazil.
3 Results and discussion
3.1 Rheological characterizationAll fluids can be characterized by rheological analyses applying shear stress on the material. When material is flowing, there are different forces acting on it, as drag forces (due to contour motion), gravitational forces, forces caused by the pressure gradient, and intermolecular forces (caused by interactions of the molecules that make up the system) [39,40].
The rheological characterization of emulsions is a useful tool for obtaining information on the microstructure and, consequently, the kinetics stability of this type of sample [6, 31]. Rheological experiments do not involve sample destruction and the correlation of results provides infor- mation on the spatial arrangement of droplets and molecules
Table 2 Rheological parameters
used in the analysis Flow curve Frequency sweep Amplitude sweep
Stationary Regime Oscillatory Regime Oscillatory Regime
Determination of yield value, viscosity and thixotropy
Determination of Gʹand G”as a function of frequency variation
Determination of linear
viscoelastic parameters Gʹand G” Shear rate: 0–300 s−1 Frequency: 1–100 Hz Frequency: 0.1 Hz
Equilibration time: 300 s Shear stress constant in linear viscoelastic region
Shear rate: 0–200 s−1 Amplitude: 0.001–1.00 Trimming position at gap:
0.105 mm
Equilibration time: 300 s Equilibration time: 300 s Gap: 6 mm Trimming position at gap: Trimming position at gap:
0.105 mm 0.105 mm
Gap: 6 mm Gap: 6 mm
in the system, which allows the intrinsic characteristics of the system under study to be predicted [41]. It is worth mentioning that the droplet sizes of the prepared emulsions were evaluated in a previous work [11], and these data varied from 31 to 116 µm for the anionic emulsifier based samples, from 54 to 76 µm for the cationic and from 44 to 68 µm for the non-ionic based emulsions. Only the rheo- logical curves (flow curves, frequency sweep, and ampli- tude sweep) performed with the samples conditioned for 6 months are presented and discussed herein, because the curves obtained for the other periods of time are very similar to those shown in Fig.1.
Figure1 shows representative flow curves (shear stress (τ)vs. shear rate
( ))
of the emulsions with non-ionic (Fig.1a–c), cationic (Fig. 1d, e) or anionic (Fig. 1f–h) emulsi- fiers, conditioned at different temperatures (5.0, 25.0, and 40.0 °C) for 6 months. In thisfigure, the green continuous lines represent ideal curves drawn according to the Herschel–Bulkley model [30], which allows to fit the data
when the stress-shear rate curve not show a linear portion at high shear rates. This model predicts the hysteresis area under optimal conditions. The closer the sample data fol- lows the ideal curve the greater the probability of kinetic stability of the system will be [30].
Figure1a, f shows theflow curves for the samples with 1.0 wt% of non-ionic and anionic emulsifier, respectively, at the three conditioning temperatures, where the similarity of the thixotropic curve profile with the ideal curve can be observed for the samples conditioned at 5.0 °C (pink curve) and 25.0 °C (blue curve). This result shows that con- ditioning at 5.0 and 25.0 °C did not affect the rheological behavior of these samples. Similar behaviors can be observed for the samples with 3.0 and 5.0 wt% of the three emulsifiers conditioned at 5.0 °C. Thus, the curves obtained at 5.0 °C for the samples with 3.0 and 5.0 wt% of emulsi- fiers were excluded, and only theflow curves at 25.0 °C and 40.0 °C are shown. The flow curves for the sample with 1.0 wt% of cationic emulsifier are not provided because this Fig. 1 Flow curves (shear stress (τ)vs. shear rate
( ))
of the samples conditioned for 6 months at 5.0 °C (pink curve), 25.0 °C (blue curve) and 40.0 °C (green curve), with 1.0, 3.0 and 5.0 wt% of the non-ionic, cationic and anionic emulsifierssample showed separation of phases. Figure 2a shows a sample with separation of phases prepared with 1.0 wt% of cationic emulsifier and Fig.2b–d (with 3.0 wt% of the three emulsifiers studied) is representative of all samples that did not show separation of phases and presented a homo- geneous aspect.
The rheological curves in Fig.1a–c show that the sam- ples with non-ionic emulsifier present non-Newtonian pseudoplastic behavior, with thixotropy, according to the literature [36]. A comparison of the curve obtained at 25.0 °C (blue curve) with that obtained at 40.0 °C (green curve) shows, for the same sample, the displacement of the thixotropy area and an increase in viscosity. This behavior is attributed to physico-chemical changes induced in the emulsified system by the temperature increase, which con- sequently affected the emulsion microstructure. Similar behavior is described in the literature [30–32]. This phe- nomenon is attributed to the presence of stearyl alcohol that promotes the esterification reaction with the free acids of the oily portion [42]. This behavior is more prominent when the temperature is increased from 24 °C to 37 °C.
In general, the comparative analysis of all curves in Fig.
1 shows similar thixotropic behavior, regardless of the emulsifier, concentration and conditioning parameters used.
This consideration is valid even for the most critical con- ditioning, i.e., 40.0 °C for 6 months. According to Tadros (2004) [30], thixotropy can be considered as a parameter associated with the kinetic stability. A hysteresis curve indicates that the emulsion has sufficient flexibility to
support the accelerated stability study without losing its original characteristics. However, viscosity changes may occur, but this does not imply emulsion breakdown, i.e., destruction of the smallest organized system (emulsified globule).
In general, thixotropic behavior is observed for weakly flocculated systems, because when shear is applied the smallest organized structure is ruptured or broken and when shear is removed the organized system recovers its integrity (hysteresis). Strongly flocculated emulsions usually have little or no thixotropy [30].
Anomalous behavior is observed in the thixotropic curve of the sample with 5.0 wt% of anionic emulsifier (Fig.1h), where a significant change in the low shear rate region, associated with viscosity loss, is observed. According to Tadros (2011), this behavior is indicative of kinetic instability [31], attributed to a fragile condition of the anionic emulsifier at this concentration.
Figure3shows the curves of viscosity (η) as a function of the shear rate for the emulsions with non-ionic (Fig.
3a–c), cationic (Fig. 3d, e) or anionic (Fig. 3f–h) emulsi- fier. These curves show similar behaviors, with a slight change in the viscosity for the samples with 3.0 wt%
cationic emulsifier (Fig. 3d) and 5.0 wt% anionic emulsi- fier (Fig. 3h), both conditioned for 6 months at 40.0 °C.
The observed viscosity behavior suggests a modification of the system microstructure, but without significant changes in the hysteresis area. This behavior is attributed Fig. 2 Visual aspect of the prepared emulsions with 1.0 wt% of the cationic emulsifier and separation of phases (a) and with 3.0 wt% of the three emulsifiers (b–d)
Fig. 3 Curves of viscosityvs. shear rate for the samples conditioned for 6 months at 5.0 °C (pink curve), 25.0 °C (blue curve) and 40.0 °C (green curve), with 1.0, 3.0 and 5.0 wt% of the non-ionic, cationic or anionic emulsifier
Fig. 4 aYield value curves (shear ratevs. shear stress) for the samples with 3.0 wt% of anionic emulsifier, conditioned for 6 months at 25.0 °C (blue curve) and 40.0 °C (red curve); and (b) curves of Gʹ, G”andη*vs. frequency (frequency scan) for the samples with 5.0 wt% of cationic emulsifier, conditioned at 25.0 °C and 40.0 °C for 6 months
to the components used and their concentrations in the sample composition.
Although there is a small change in the viscosity of the samples conditioned at different temperatures, this para- meter does not show a decrease, which is a positive aspect according to Brummer and Tadros [25,32]. These authors predicted that the decrease in viscosity over time is corre- lated with the phenomenon of coalescence, and both authors recommend determining the droplet size in order to gain a better understanding of this behavior. Castro (2001) [33]
noted that a rapid decrease in the emulsion viscosity sug- gests someflocculation while a rapid increase might mean a slight coalescence. The results show that the changes that occur have a tendency to gradually increase over the 6-month period, without, however, compromising stability.
This correlation is considered in this study, with the eva- luation of sensory tests (Section3.3).
Figure4a correlates the shear rate with the shear stress, the so-called yield value curves, for the samples containing 3.0 wt% of anionic emulsifier conditioned for 6 months at 25.0 °C and 40.0 °C.
Table3 shows the yield values (in Pa) for all samples studied, obtained from curves similar to those in Fig. 4a.
This table shows that the yield values obtained with a conditioning temperature of 40.0 °C are lower than those for conditioning at 25.0 °C, for samples with 1.0 wt% of non- ionic emulsifier and 1.0–5.0 wt% of anionic emulsifier. In these cases, there is a more significant influence of the temperature increase on the microstructure of the emulsions, due to the coalescence phenomenon. Tadros (2004) men- tions that the higher the yield value the more robust the organized system will be [30]. Table3also shows that the yield values for the samples conditioned at 40.0 °C are higher than the values for those conditioned at 25.0 °C.
These data show that the system studied is more robust and more viscous over time under the conditions applied. The most stable structures with increasing temperature are those with 3.0 wt% and 5.0 wt% of non-ionic and cationic emulsifiers.
By increasing the viscosity of the continuous phase of an oil/water emulsion (with the addition of cross-linked poly- mers, for example), the mobility of oil droplets is reduced, preventing their coalescence. Increasing the yield value also inhibits the internal phase droplet approach. Therefore, by increasing these two rheological parameters, viscosity and yield value, more stable emulsions are obtained [33].
Oscillatory tests show that when the sample storage modulus (Gʹ) is greater than the loss modulus (G”), this characterizes a viscoelastic solid and the emulsion has a thick cream texture. On the other hand, the emulsion exhibits a viscoelastic liquid behavior when G”> Gʹ, char- acterizing a light cream texture. Considering that the intersection of the Gʹand G”curves defines the disruption of the material structure, the greater the distance between these parameters the more stable the material will be [5,25].
Frequency sweep tests can determine the characteristics of kinetic stability when vibration is involved, by simulating, for example, the conditions present during product transport. This procedure is applied to assess which changes in the sample result from the progressive increase in frequency, and Fig.4b shows a typical graph obtained. The behavior observed in this figure is representative of all samples studied.
The rheology is strongly dependent on the hydrodynamic forces acting on the particle surface (aggregates), regardless of its density [43], and this technique can be successfully applied to provide information on the physical stability of emulsions [30]. Cosmetic emulsions have viscoelastic behavior and, therefore, their characterization is usually focused on the region of linear viscoelasticity, so changes in the microstructure are only observed when oscillatory stu- dies are performed. Ideally, the complex viscosity (η*) should show a linear and decreasing behavior with increasing frequency [25] and this behavior was observed for all samples in this study.
The relationship between viscous (G”) and elastic (Gʹ) moduli gives the tan delta parameter (tanδ), i.e., the tangent of the losses. In general, the tan δvalue allows determining the level of association in microstructures. This parameter provides Table 3 Elastic modulus (Gʹ),
viscous modulus (G”), tanδ, and yield values of the samples conditioned for 6 months at 5.0, 25.0 and 40.0 °C, obtained by amplitude sweep
Emulsifier Concentration Gʹ G” tanδ(mean values) Yield value (Pa)
(wt%) (Pa) (Pa) 25 (°C) 40 (°C) 5 (°C) 25 (°C) 40 (°C)
Non-ionic 1.0 1131 724 1.57 1.86 52 52 43
3.0 783 408 0.97 1.43 53 53 143
5.0 434 265 1.13 1.34 54 54 116
Cationic* 3.0 908 508 1.64 2.77 120 120 196
5.0 1846 945 3.93 2.39 101 101 101
Anionic 1.0 296 115 0.66 0.69 43 43 34
3.0 171 70 0.67 0.69 73 73 64
5.0 1440 709 7.17 2.39 243 243 195
*Sample with 1.0 wt% of cationic emulsifier showed separation of phases and was not characterized
information on the viscoelastic behavior of the material when subjected to increasing frequency. The lower the tanδvalue the greater the elasticity of the emulsion will be [43].
Figure 5 shows the tan δ variation with increasing fre- quency for the samples with non-ionic emulsifier, conditioned at 25.0 °C and 40.0 °C, for 6 months. Thisfigure shows that the samples analyzed presented good results in terms of sta- bility, i.e., tan δ< 1, even in the higher frequency region (10–100 Hz). Ideally, tanδmust be <1 in higher frequencies.
This behavior shows the domination of the elastic component over the viscous component and represents a well-constructed (stable) microstructural arrangement. Table 3 shows this behavior with the domination of Gʹover the G”component.
Similar behavior is discussed in the literature [25]. Samples with cationic and anionic emulsifiers exhibit similar behaviors to that observed in Fig.5, particularly with the predominance of tanδ< 1 in the frequency range of 0.01 and 100 GHz.
Table3shows tanδvalues, which represent the average over the frequency range evaluated. It is observed that, in a general way, they are higher for the conditioned (aged) samples. This behavior is expected, since the microstructure more likely to be modified when an emulsion is subjected to relatively higher temperatures for a longer time, due to the greater degree of system disorganization. An exception is observed for the samples with 5.0 wt% of cationic or anionic emulsifier. According to the literature, this behavior is attributed to the higher concentration of emulsifier being associated with greater packaging of the microstructure [44]. The samples with the anionic emulsifier show lower tanδvalues, which indicates better kinetic stability.
In the amplitude sweep, the microstructure is analyzed without being destroyed and the elastic and viscous com- ponents are revealed so that, comparatively, viscosity and stability behavior characteristics are attributed, considering that with a larger elastic component the microstructure will be more resistant. However, it should be considered that the relationship between G”/Gʹ(tan δ) is more important than the magnitude of the elastic component.
Riscardo (2005) [45] describes that the application of a certain strain or deformation (γ) in the low viscoelasticity
region (LVR), below the critical deformation
( ) ( < )
allows the characterization of the rheo- logical behavior of the emulsion microstructure. Conse- quently, the evaluation of the emulsion in the LVR can provide information without interference from external factors, while high strain can promote structural changes in the emulsion. The critical strain is the maximum strain amplitude, regardless of the strain applied and the higher this value the greater the shear force required breaking the structure will be. The storage modulus (Gʹ) is the elastic component of the material in the LVR. A higher Gʹvalue indicates a more pronounced solid characteristic of the material [30].Figure6 shows the Gʹcurves as a function of the shear rate within the LVR of the emulsions with non-ionic (Fig.
6a–c), cationic (Fig. 6d, e) or anionic (Fig. 6f–h) emulsi- fiers. The profiles of these curves allow a qualitative assessment of the possible change in the amplitude sweep, which indicates whether or not the emulsion is highly flocculated [46]. In this study, these curves were used to determine the parameters Gʹ and G” (this parameter was also obtained from G” vs. γ, strain amplitude). Table 3 shows the medium values of Gʹ and G” for the samples conditioned at 25.0 °C.
Based on the rheological results, it appears that samples with 1.0 and 3.0 wt% of anionic emulsifier best meet the requirements for kinetic stability. These results are obtained prior to the lengthy kinetic studies of shelf aging, normally used to predict the behavior of emulsions before their commercialization.
3.2 Stability assessment by visual inspection and pH measurements
The visual inspection of the samples conditioned at 5.0, 25.0, 40.0, 50.0 and 60.0 °C, for periods of 2 weeks and 1, 2, 3 and 6 months, showed phase separation and droplets of the oily phase in some samples. The samples conditioned at 5.0 and 25.0 °C did not show alterations. The emulsions Fig. 5Curves of tanδvs. frequency of samples conditioned for 6 months at 25.0 °C (red), 40.0 °C (blue) and 5.0 °C (green) with (a) 1.0 (b) 3.0 and (c) 5.0 wt% of non-ionic emulsifier
with 5.0 wt% of one of the three emulsifiers, conditioned at 40.0 °C for 3 and 6 months, presented phase separation and the others conditioned for 1 and 2 months showed small oil droplets on the sample surface. Samples with 5.0 wt% of one of the three emulsifiers, conditioned at 50.0 °C, pre- sented oil droplets after 1 month of conditioning and the same samples at 60.0 °C showed phase separation after 2 weeks. Samples with 1.0 wt% of anionic or non-ionic
emulsifier conditioned at 60.0 °C also showed oil droplets on the surface. These observations corroborate the rheolo- gical results, where changes in stability were observed in the samples with 5.0 wt% of emulsifier. Good agreement was also observed for the samples conditioned at 5.0 °C and 25.0 °C, which showed no differences in terms of phase separation or in theflow curves (Fig.1).
The pH is an important measurement in the evaluation of the chemical stability of emulsions, because when the pH is adjusted to a value that allows charges in the interfacial region between the two emulsion phases to be nullified, the van der Waals attraction forces can cause aggregation of particles [33,43, 47]. In this study, the main objective of measuring the pH was to adjust this value for the samples that did not present phase separation, in order to enable sensory analysis with the panelists. For this, the pH values of these samples were adjusted around 5, considering that the skin has a pH close to this value [4,17].
Fig. 7 Results of sensory evaluation
Fig. 6 Curves of G’vs. shear rate for the samples conditioned for 6 months at 5.0 °C (pink curve), 25.0 °C (blue curve) and 40.0 °C (green curve), with 1.0, 3.0 and 5.0 wt% of the non-ionic, cationic and anionic emulsifiers
3.3 Sensory evaluation
Samples containing 3.0 wt% of emulsifier, which provided the best rheological results, were tested by consumers. In addition, this concentration of emulsifier is used in products currently available on the market [48]. Figure7 shows the results obtained for each sample evaluated, considering the aspects included in the questionnaire, and the best results are observed for the sample containing the cationic emul- sifier (3.0 wt%). This result is supported by experimental rheological analysis and also by Brummer’s theory [25].
According to Brummer [25], the sensation of applying a cosmetic on the skin can be divided in primary and sec- ondary sensory aspects. As previously mentioned, the pri- mary sensory aspect is positive when the consumer feels a full-bodied product in contact with the skin before spreading.
Correlating this concept with the rheological analysis performed, a high yield value gives this characteristic to the product and the highest yield value was determined for the sample with 3.0 wt% of cationic emulsifier (Table3). With regard to the secondary sensory aspect, when the product is almost completely spread over the skin it should show a decrease in viscosity, to facilitate its application. At the same time, the product must withstand higher shear rates and exhibit thixotropy.
In a previous study [11], the authors show that, for the same samples used in this study, the three emulsifiers tested changed the droplet size distribution, and that the emulsions with 3.0 wt% of cationic and anionic emulsifiers are more stable. This observation was based on an increase in the size of the emulsified globule with an increase in the emulsifier concentration to 5.0 wt%. This size increase characterizes a more kinetically unstable system. For the non-ionic emul- sifier, the most suitable concentration was around 3.0–5.0 wt
%, since the smallest size of the system’s organized unit continuously decreases with increasing concentration. The sample with 3.0 wt% of anionic emulsifier favored the for- mation of the emulsified system with a smaller droplet size, that is, better packing of the smallest organized structures in the emulsified system and thus a more stable system. In this case, the anionic emulsifier favored the best physico- chemical interaction between the oily and aqueous por- tions in the BE. Unexpectedly, the results show an increase in the size of emulsified globules when the anionic and cationic emulsifier concentration increased. This observation demonstrates the influence of the chemical structure of the emulsifier molecule, where the presence of large clusters in the chains promoted steric hindrance, hindering the particle size decrease of the emulsified globules.
The correlation of the rheological results, pH and droplet size distribution [11] shows good agreement and evidences that the two most promising (more stable) samples, were obtained with 3.0 wt% of anionic or cationic emulsifier.
4 Conclusions
Rheological analysis based on flow curves, frequency sweep and amplitude sweep show that the chemical aspects of emulsifiers, their concentrations in the emulsion, and the parameters used in the aging tests contribute differently to the emulsion stability. The samples containing 3.0 wt% of the cationic (distearyldiammonium chloride) or anionic (potassium cetyl phosphate) emulsifier showed the best stability results. In the sensory tests performed with customers, the formulation with 3.0 wt% of cationic emulsifier gained the best acceptance.
This result is in agreement with the rheological analysis and shows the reliability of the use of rheological analysis in the prediction of emulsion stability, which contributes to reducing not only costs but also the time required for the development of products. It could also eliminate or reduce the need for kinetic studies (aging) and sensory testing with consumers.
Acknowledgements The authors acknowledge the financial support received from FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo) (2018/09531-2), CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) (305123/2018-1), CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) (001), and Johnson&Johnson Co. in São José dos Campos/SP, Brazil.
Compliance with ethical standards
Conflict of interest The authors declare no competing interests.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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