Advisor:Ph.D Diego Camilo Pradilla Raguá

O/W cosmetic emulsions: Influence of UV agent andthickener on rheology, stability and sensory analysisCristian Fabian Rodríguez Rodríguez1,† and Sebastian Camilo CasadiegoFierro2,‡1,2Chemical and Food Engineering Department, Universidad de los Andes | Bogotá, Colombia† cf.rodriguezr@uniandes.edu.co‡ sc.casadiego@uniandes.edu.co

AbstractThe cosmetic industry has implemented different innovative emulsion formulation and processing techniques intending toimprove the cosmetic products that are marketed, where the most important thing is to meet the needs of consumers. Inconsequence, an analysis was developed to determine the formulation variables that directly affect the rheological, stable,and textural properties in direct cosmetic emulsions (O/W). The effect of the incorporation at different concentrations ofthickener (carbopol) and UV filter (TiO2) in the systems was studied. Similarly, the variation in the amount of dispersedphase was taken into account to define the most convenient formulation in those emulsions. For the rheological properties,the elastic module in the different emulsions were analyzed, which made it possible to determine that, with a higherconcentration of dispersed phase, UV filter, and thickener, the elasticity of the emulsion increases, which inhibits someinstability mechanisms. In the stable aspects analysis, it was evident that with the TiO2 incorporation and carbopol, thedroplet size is significantly reduced, which allows greater stability to the emulsion. However, the presence of these twoagents could generate sedimentation phenomenon and the absence of thickener causes creaming. Finally, for the texturalproperties as relevant results, it is evident that the greater the dispersed phase is the lower the spreadability parameterand the hardness is directly influenced by the droplet size.Key words: Direct emulsions, rheological properties, stable properties, textural properties, thickener, UV filter, elasticmodulus, elasticity, sedimentation, creaming, spreadability, hardness.

Introduction

In recent years, different companies have had a greater inter-est in developing products that are market leaders, that meetthe needs of consumers and are economically competitive. Ac-cording to Tanguy and Marchal [1], current trends are focusedon increasing demand for products with improved performanceand accelerating innovation in product development. For this,they have carried out investigation focused on the study of theproperties of the products and how the process and formula-tion variables on the quality and performance of those prod-ucts. The investigation or research process begins by identify-ing needs, followed by prototyping, analysis of results and finalproduct, where the effect of the factors mentioned above on theresponse variable being studied is observed. However, productdevelopment involves problematic scenarios or adverse effectsand the cosmetic field is no exception. In consequence, someof the products (i.e, emulsions) present instability, changes intheir appearance, unwanted texture, among other aspects thatdecrease their quality. For this, investigations have been de-veloped that contribute to the establishment of correlations be-tween the parameters to have a product according to the objec-tives set.

For example, in the emulsions rheology, F. Mendoza [2] re-

ported that the increase in the emulsions elasticity occurs asthe interactions between drops obtained [3]. These behavioroccurs by increasing the oil concentration in O/W emulsions,where drops are closer to each other. The particle size distribu-tion in these cases is no longer monomodal and small dropletsare created, thus allowing the formation of smaller clusters andan increase in molecular interactions. Additionally, C. Gómez[4] complemented the study and establishes that the additionof active ingredients as thickeners increases the values of theelastic moduli by reducing the size of the droplet size, whilethe same relationship happens with the UV agent reported byM. Rossano, et al [5], causing what F. Mendoza raises relatedto the proximity between the drops. On the other hand, separa-tion can occur due to some phenomena such as creaming, floc-culation, sedimentation, phase inversion, among others [6]. M.Grecco and R. Dos Santos [7] reported that small droplet diame-ters favor stability since it prevents the emulsion breaking andphase separation. Regarding the addition of thickeners, S. DeSouza and M. Bruschi [6] established that stability is greater asthe concentration of those thickeners increases. This happensbecause the thickeners increase the system viscosity, which de-creases the droplet size of the emulsion and this is related to amore stable product. The effect of the UV agent on the stabilityof the emulsion is described by M. Rossano, et al [5], where it

Compiled on: May 29, 2021.

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is reported that the addition of TiO2 on the systems increasesstability and this in turn contributes to the decrease in dropletsize.

However, the study of products, in general, goes beyondrheology and stability. Texture studies are also needed to es-tablish how it is perceived on the skin (cosmetics) at the timeof use and what are aspects to improve. Concerning sensoryproperties and analysis, qualitative and quantitative evalua-tion mechanisms have been implemented. However, qualita-tive mechanisms such as Quantitative Descriptive Analysis arenot as viable since they require a select group of panelists andmay be perceptual biased [8]. For this, other types of strategieshave been implemented, such as Spectrum Descriptive Analy-sis, which is based on quantified analysis using instruments[9]. Different studies have modified properties and their for-mulation to analyze the impact on sensory analysis. For exam-ple, dispersed phase concentrations, thickening agent addition,UV filter, surfactant change, and other aspects [10]. With theresults obtained, it is possible to make decisions about the for-mulation of new products that satisfy needs. It should be notedthat some of the properties change over time and it is very im-portant to observe their behavior for decision making.

Literature mentions studies focused on hardness and theaddition of thickeners and UV agent increases this parameter(hardness) because the molecular interactions will be greater[4, 11]. Other parameters such as adhesiveness, compressibil-ity and, spreadability have not been studied in-depth together.However, a proportional correlation between droplet size andtexture parameters is expected according to studies that havebeen carried out individually [4, 11, 12, 13, 14]. Following theabove, there is a need to study the rheology, stability and tex-ture of the diluted O/W emulsions to complement studies thathave been carried out previously. The difference of our studyconcerning those mentioned in the literature is that three for-mulation conditions are being analyzed simultaneously: dis-persed phase, thickener and UV agent, where the last two referto active ingredients popularly used in the cosmetic industryand to that generally studies analyze only one variable. Forthat reason, it is necessary to develop a time-based stabilitystudy and analyze the effect of the amount of thickener and UVagent on rheology, stability and sensory analysis, to determinethe most influential variable in the study.

Materials and methods

Materials:

The emulsions were made with deionized water (Universidadde los Andes, Colombia) and mineral oil USP-grade (QuímicosCampota, Colombia). The surfactants used for the emulsionsare non-ionic from the sorbitan mono-oleate family. The sur-factant was Tween 20 (Croda, England) with an HLB of 16.7 inthe aqueous phase, while in the oil phase the surfactant wasSpan 80 (Croda, England) with an HLB of 4.3. On the otherhand, titanium dioxide (BioQuim.ISM, Colombia) was used asUV agent. In turn, carbopol (BioQuim.ISM, Colombia) was usedas a thickening agent. Finally, to regulate the pH of the solu-tion, it was necessary to use triethanolamine (Universidad delos Andes, Colombia), a product widely used to neutralize car-bopol gels.

Methods:

Process parameters:

They are those parameters that were implemented in all theemulsions developed. The values were kept constant becausethe study is focused on the formulation variables.

• Impeller: Propeller.• Tip velocity:

Vp = π · N · D60 = π · 750 rpm · 0.055 m

60 = 2.15 m/s (1)Where:D: Impeller diameter (0.055 m).N: Rotation speed (750 rpm).

• Homogenization-incorporation speed: 750 rpm.• Homogenization time: 10 minutes.• Peristaltic pump rate: 30 ml/min.Formulation variables:

It refers to the conditions of each of the samples. The levelsselected affect our response variables: elastic module, stability,and texture profile.

• Dispersed phase concentration: Taking into account thatthe oil is the dispersed phase, this indicates the amount ofoil dispersed in the mixture of water and surfactant. Sincethe emulsions were diluted (φ ≤ 40% w/w) , the levels areas follows:– 10% w/w.– 40% w/w.

• Surfactant concentration: It refers to the concentration ofsurfactant in relation to the total amount of the emulsion.The value used was 4% w/w according to the literature andpreviously developed studies [15].

• UV agent concentration: It refers to the concentration ofUV agent in relation to the total amount of the emulsion. Itis our first study variable. For this, the levels selected forsubsequent analysis are the following:– 1% w/w.– 5% w/w.

• Thickener concentration: It refers to the concentration ofthickener in relation to the total amount of the emulsion. Itis our second study variable. For this, the levels selected forsubsequent analysis are the following:– 0.5% w/w.– 2.5% w/w.

All the emulsions formulation are summarized in Table 1. Theformulation is for 500 grams.Emulsification process:

The aqueous phase with deionized water (Universidad de losAndes, Colombia) and Tween 20 (Croda, England) was homoge-nized for 10 minutes at 750 RPM with a propeller. The oil phasewith mineral oil USP-grade (Químicos Campota, Colombia) andSpan 80 (Croda, England) was homogenized at the same con-ditions already mentioned. Once the homogenization processwas completed, the incorporation of the dispersed phase began.To do this, incorporation took place with a peristaltic pump(Fischer Scientific, USA) at a constant flow of 30 ml/min whilethe system was stirred with a propeller and the mixing deviceHei-Torque Value 400 (Heidolph, Germany). When the dis-persed phase was being added, the UV agent and carbopol wereincorporated into the mixture. At the end of the process, thepH of the emulsion was regulated to 5.5 (pH of some facial cos-metic products) with a pH-meter (Metter Toledo, Switzerland)because they were remaining acidic. For this, triethanolamine(Universidad de los Andes, Colombia) was used.

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Table 1. Emulsions formulation.Emulsion Tween 20 (g) Span 80 (g) UV agent (g) Carbopol (g) Mineral oil (g) Water (g)

60% continuous phase - 40% dispersed phase1 12.57 7.12 25 12.5 177.88 264.932 13.11 7.42 5 12.5 185.58 276.393 12.84 7.27 25 2.5 181.73 270.664 13.59 7.69 0 0 192.31 286.41

90% continuous phase - 10% dispersed phase5 12.57 7.12 25 12.5 39.13 403.686 13.11 7.42 5 12.5 40.83 421.147 12.84 7.27 25 2.5 39.98 412.418 13.59 7.69 0 0 42.31 436.41

Characterization:Microscopy:To analyze the droplet size and the impact of the formulationvariables on the diameter, micrographs were taken on a BA310series microscope (Motic, China) with a 100X objective lens andimmersion oil. There, using the microscope software, the di-ameter of the drops were measured. Weighted average diame-ter is calculated using the Sauter’s method (d3,2) [16]:

d3,2 =∑ni nid

3i∑n

i nid2i

(2)

Where:ni: Number of droplets with diameteri.

di: diameteri.On the other hand, the surface area of a drop (assuming it iscompletely spherical) is:

SA = 4π(D3,2

2)2 (3)

Where:d, 3, 2: Sauter’s weighted average diameter.

Rheology:Studies such as the one carried out by Ingrid C. Gómez, empha-size the importance of rheology and texture, its relationship ofmacroscopic properties [4]. Taking into account the influenceof the formulation and composition of the product on thosemacroscopic properties, a flow sweeps test was developed onthe AR-G2 Rheometer (TA Instruments, US) to analyze the rhe-ological behavior of each of the developed emulsions. The pa-rameters are:• Geometry: 20 mm parallel plates.• Shear rate: 1 s–1 to 100 s–1.• Oscilation amplitude: 0.1 rad/s to 100 rad/s.• Controlled effort: 1 Pa.

The rheometer applies a force to the sample and measures thereaction to the force and the force it exerts on the rheometer.In this way, the dynamic modules (G’) of the emulsions areestablished looking for the zone of linear viscoelasticity.Stability:To analyze the instability phenomena in the emulsions, aTurbiscan (Lab Formulaction, France) was implemented usingthis parameters or configuration:• Test: Scheduled analysis.• Temperature: 25 ºC.• Scan time: 10 minutes.• Number of scans: 25.

Each sample was added to the 20 mL Turbiscan vial with aheight of approximately 40000 µm. This height can vary a fewµm according to the experimenter. The Turbiscan works withthe Lamber-Beer law, where the transmission and backscatter-ing of light is measured in an infrared close to 820 nm from thebottom to the neck of the vial. Instability phenomena can beanalyzed with transmittance or backscattering [17]. However,the literature performs more analyzes related to backscatter-ing than transmittance. Therefore, it is chosen to analyze onlywith this method. The backscattering is calculated by the Tur-biscan as follows [17]:

BS =√3φ(1 – g)Qs2d (4)

Where:φ: Dispersed phase volume.

g: Mie theory optical parameter.Qs: Mie theory optical parameter.

d: Average particle size.

Texture analysis:Texture analysis is directly related to the sensory properties ofthe product. In order to compare the emulsions a SpectrumDescriptive Analysis (SDA) has been implemented, an analysisfocused on the comparison of attributes of products and emul-sion sensory characteristics. There are different attributes forthe SDA, however, the ones that we consider most importantfor the study are:• Appearance: Brightness and color.• Pick-up: Hardness, compressibility and adhesiveness.• Rub-out: Spreadability.

The analysis was developed in the texture analyzer TA.HD Plus(Stable Micro Systems, UK) with 5 kg cell, according to previousinvestigations [18]. The mechanism of the texture analyzer isrelated to the analytical probe that measures vertical displace-ments. Further, sensor position and resistance to displacementare recorded in order to calculate each of the texture propertiesof the emulsion [18]. Therefore, a topical with double compres-sion test in the Cosmetics Skincare Product Texture MeasurementAnalysis Category was performed with 100 mL of each emulsion.These were introduced to the cylindrical accessory of the tex-turometer with dimensions of 6 cm diameter and 7.5 cm height.The cylinder to perform the compressions has a diameter of 4cm and 0.5 cm high. According to the texture analyzer settings,the test conditions are as follows:• Pre-test speed: 1 mm/sec.• Test speed 2 mm/sec.• Post-test speed: 2 mm/sec.• Targed mode: Distance.• Distance: 15 mm.• Time: 5 sec.

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• Trigger type: Auto (Force).• Break mode: Off.• Tare mode: Auto.• Advanced options: On.• Control oven: Disabled.• Frame deflection correction: Off (XT2 compatability).

Results and discussion

Microscopy:

Figure 1 shows the droplet size of the emulsions without UVagent and carbopol. The objective of this micrograph is to an-alyze the effect of oil concentration on the droplet size of theemulsion. It is evidenced that the 10% w/w emulsion has alarger average droplet size compared to the 40% w/w emulsion.It should be noted that the droplet size depends on factors in ad-dition to the oil concentration, it also depends on the impellerused, speed and time of stirring and incorporation, type of sur-factant, among others [19]. Under the conditions used there isan weighted average droplet size of 60.86 µm for the 10% w/wemulsion and 26.22 µm for the 40% w/w emulsion. The valuesobtained are consistent with the literature and confirm the re-sults obtained by T. Dapčević et al [20], where it is stated thatwith a higher concentration of oil, there is a gradual decreasein the specific surface area. Assuming that the drops are com-pletely spherical, the surface area is 11637.9 µm2 for the 10%w/w emulsion and 2160.96 µm2 for the 40% w/w emulsion, inaccordance with the Dapčević’s relationship. To analyze whathappens with the addition of UV agent and carbopol, the mi-crograph in Figure 2 was taken. The average diameter of the10% w/w emulsion (5% w/w UV agent + 2.5% w/w carbopol)is 4.76 µm and the 40% w/w emulsion (5% w/w UV agent +2.5% w/w carbopol) average diameter is 4.14 µm. A drop di-

ameter decrease of 99.38% and 97.5%, respectively. This alsoconfirms results that relate carbopol and UV agent as viscositymodifiers and therefore, the droplet size decreases accordingto the addition of these active ingredients [21, 22].Rheology:

For the rheology characterization, the influence of the formu-lation variables on the elastic modulus was analyzed. As canbe seen in Figure 3, emulsions formulation with 40% w/w, theelastic modulus (G’) values are significantly higher than thosethat can be observed in Figure 4. There is an influence of theconcentration of the dispersed phase on the elastic modulus.From the figures mentioned above, it can be determined that,at a higher concentration of dispersed phase, the elasticity ofthe system is greater, a relationship developed by Tadros et alin the analysis of formation, stability, and rheology of emul-sions [23]. Additionally, it can be seen that the rheology ofthe systems also depends on the composition and amount ofthickener and UV filter present.

Also, according to Figure 3 the 40% w/w emulsion (1% w/wUV filter + 2.5% w/w thickener) have an increasing elastic mod-ulus. This means that the system continues to maintain a vis-coelastic behavior with direct dependence on the entire rangeof the angular frequency. Additionally, another similar behav-ior is seen in Figure 4 where, the 10% w/w emulsion (5% w/wUV filter + 0.5% w/w thickener) also have a direct dependencebetween the elastic modulus and the entire range of the angu-lar frequency. For these specific systems, it can be determinedthat the physical interactions are low. Unlike the other sys-tems, which have less steep slopes and have higher physicalinteractions [24].

Figure 1. Droplet size of blank emulsions.

Figure 2. Droplet size of 5% w/w UV agent + 2.5% w/w carbopol emulsions.

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Figure 3. Elastic modulus G’ (Pa) for the 40% w/w emulsions (Week 0).

Figure 4. Elastic modulus G’ (Pa) for the 10% w/w emulsions (Week 0).To summarize the weekly results, Figure 5 has been cre-

ated. In Figure 5 it can be seen that the amount of thickener inthe emulsion is a factor that significantly influences the elasticmodulus (G’) of the system. According to the rheological dataobtained, it can be determined that there is a direct correla-tion, where at a higher concentration of carbopol the elasticityindex increases. This is because the swelling of the thickenerparticles leads to the viscous and elastic modulus values beinglarge in the zone of linear viscoelasticity. Carbopol has flexi-ble structures that allow it to increase its apparent viscosity inaccordance to its low sensitivity to high shear rates [10]. Onthe other hand, it is also important to mention the influencethat the UV filter has on the elasticity index of an emulsion.Observing in the same way Figure 5, for 10% w/w emulsionswith 2.5% w/w of thickener, the elastic modulus could be com-pared at different UV filter concentrations. With higher TiO2composition in the system, the elasticity index also increases.

Figure 5. Elastic modulus average (G’) for the O/W diluted emulsions.

Based on the previous analysis, in the cosmetic industry itis necessary that for the formulations and processes that arecarried out in the development of O/W emulsions, it is takeninto account that the greater the physical interaction betweenthe droplets of the system, the more stable will be the prod-uct. In this way, the rheology of the systems must be analyzedfrom their elastic modulus and the behavior that this has con-cerning the variation of angular frequency. On the other hand,the adequate elasticity of a system depends on the cosmetic tobe designed. For facial and body creams, the elastic modulusranges between 1x101 and 1x103 Pa [25]. In the study carriedout, it can be determined that the systems that satisfy withthis range are the 10% w/w emulsions, since they have lowermagnitudes of elastic modulus compared to those that contain40% w/w. Additionally, it must be taken into account that thiselasticity parameter increases for the other components thatare added in the systems (humectants, antioxidants, antifoams,etc). Therefore, the base formulation must contemplate lowmagnitudes in the elastic modulus and that it is independentof the variation of the angular frequency.Stability:

To establish the influence of the dispersed phase concentra-tion on the stability, the data related to the blanks have beenselected because these emulsions do not have a UV agent or car-bopol, which allows a more direct analysis of the results relatedto the dispersed phase. In addition, to understand the behaviorof each of the emulsions, they have only been plotted with theinitial week (week 0) and the final week of the measurements(week 5) of the emulsions. This is done to analyze results morepractical and simple. However, in Annex 1 are supplementarymaterial for the blanks emulsions. Before the results analy-sis in this section, it is important to mention that accordingto the Turbiscan vials height, the values have been adjusted to40000µm - 45000µm to be more comparative. According toinformation provided by the laboratory worker, the vial shouldnot be completely filled to avoid spillage inside the equipment.For that reason, it is necessary to leave a safe distance beforethe cover. Although attempts were made to approximate thesemeasurements, they were not always exact and therefore a vari-ation arises in the final values of the curve. However, this doesnot affect the results obtained because it only indicates that onevial was a little fuller than the other, since the instability phe-nomena do not depend on the amount of emulsion, it dependson the back-scattering scan carried out by the Turbiscan.• Dispersed phase influence | Emulsion stability:

Figure 6. Week 0 and week 5 | Delta backscattering results of O/W dilutedemulsions (blanks).

In relation to Figure 6, it is evident that the percentage of thedispersed phase on the stability is a factor that influences theresults. Higher instability values are observed in the 10% w/w

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emulsions compared to the 40% w/w emulsions. This compar-ison is made through the delta backscattering values, when thebackscattering is higher, it presents more instability indices.According to the literature, studies carried out by C. Sun and S.Gunasekaran [26] confirm that increasing the concentrationof the oil phase influences the decrease in the stability, that is,more unstable. At higher oil phase concentration, the viscosityof the emulsion increases. In consequence, the droplets have ahigher packing density, where the interactions of the dropletsincrease and a network is formed that helps to decrease theinstability rates [26]. Furthermore, when the oil concentrationis higher, the droplet size is smaller and polydisperse, whichresults in the development of a more stable system [20].It is possible to observe in Figure 6:

Figure 7. Week 0 and week 5 | Zones of instability phenomena of O/W dilutedemulsions (blanks).

Instability phenomena in the zones:• Clarification: Clear bands form in the lower zones or areas

of the emulsion due to the separation that occurs betweenwater and oil. Likewise, movement of the dispersed phasetowards the upper part of the emulsion is evidenced [27].

• Creaming: Creaming in the 10% w/w emulsion. Althoughcreaming is related to clarification, this phenomenon is di-rectly influenced by the amount of oil. The difference indensities between the two phases causes a separation wherethe oil droplets migrate towards the top of the emulsion.For the 10% w/w emulsion it was more evident since thereis less oil, which makes the speed in which the oil drops(creaming velocity) remain at the top, is higher comparedto the 40% w/w emulsion [27].

Key aspects of the dispersed phase influence on stability:

1. The dispersed phase concentration increases the viscosityof the emulsion, which influences the stability (higher).2. When the concentration of dispersed phase is higher, thedroplet size is smaller, a factor that promotes the stability ofthe system due to polydispersity of the droplets.3. In the absence of UV agent and carbopol, a lower amountof oil promotes creaming given the difference in densities be-tween the two phases and the higher creaming velocity in the10% w/w emulsion.To analyze the influence of the thickening agent (carbopol) onthe stability, the UV agent has remained constant. As well asthe influence of the dispersed phase on stability, the data fromthe initial week (week 0) and the final week of the measure-ments (week 5) will be analyzed. Likewise, in Annex 2 are sup-plementary material for emulsions not shown in the resultsanalysis.

• Thickening agent influence | Emulsion stability:

Figure 8. Week 0 and week 5| Delta backscattering results of O/W dilutedemulsions with 5% w/w UV agent + 0.5% w/w and 2.5% w/w carbopol.

According to Figure 8, it is evidenced that they presentlower values in the delta backscattering compared to the blanks(y-axis). Thus, it is possible that the effect of carbopol is toincrease stability. The mentioned above aspect is consistentwith studies results and analysis such as the one carried out byIngrid C. Gómez, where the same relationship is presented [4].The justification for this phenomenon is because the thickenerdecreases the droplet size of the emulsion, making it morestable. In addition, thickeners form networks that prevent theinteraction between the droplets [28]. The other conditionsare plotted in order to analyze if the instability phenomenonchanges according to the dispersed phase concentration:

Figure 9. Week 0 and week 5 | Delta backscattering results of O/W dilutedemulsions with 5% w/w UV agent + 0.5% w/w and 2.5% w/w carbopol.

In Figure 9 the same tendency to show greater instability withless amount of carbopol is confirmed for the reasons alreadyexplained above. Also, compared with the initial week for bothemulsions it is observed that the delta backscattering valuesincrease and this is because the emulsions are thermody-namically unstable systems, therefore liquid/liquid mixturestend to separate naturally and reduce their interfacial area,which is represented in higher values of delta backscattering.In consequence, it is suitable that kinetic stability decreasesover time. Finally, comparing the 10% w/w emulsion and the40% w/w emulsion, it is evident that the delta backscatteringvalues are higher, which agrees with the idea that has beenmentioned throughout the analysis and the greater instability.It is possible to observe in Figure 8 and Figure 9:

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Figure 10. Week 0 and week 5 | Zones of instability phenomena of O/W dilutedemulsions with 5% w/w UV agent + 0.5% w/w and 2.5% w/w carbopol.

Figure 11. Week 0 and week 5 | Zones of instability phenomena of O/W dilutedemulsions with 5% w/w UV agent + 0.5% w/w and 2.5% w/w carbopol.

Instability phenomena in the zones:• Possible sedimentation: Different from blanks emulsions,

the addition of carbopol inhibits clarification and creamingand generates possible sedimentation in low areas. Thereis droplets suspension at the bottom of the sample. It isimportant that this phenomenon is recognized by the max-imum peak presented in the lower areas. Also, it is evidentthat although there is a change in the concentration of theoil phase, sedimentation behavior curve continues to occur.Different from the 40% w/w emulsions and the same con-ditions of UV agent and thickener, the delta backscatteringvalues are in higher ranges, which indicate greater instabil-ity and in accordance with the results obtained in the dis-persed phase influence section.

It is key to mention that according to the behavior of the deltabackscattering, the sedimentation that occurs is very minimalbecause the emulsion is stable. It could be considered as a hy-pothesis given the behavior of the curves. Also, it is observedthat in the final week the phenomenon is the same, the sedi-mentation trend continues through the increase in the valuesof the delta backscattering. J. Gómez, D. Pradilla and O. Ál-varez carried out a study based on O/W emulsions where theinstability phenomena of those samples were flocculation withthe formation of "flocs" that produce droplet agglomeration[29]. However, our results are not far from those obtained bythe aforementioned study, because when flocculation occursthere is the possibility of it becoming sedimentation and thisis confirmed by J. Pérez, M. Santos and N. Zaritzky [30]. Fromthe above, it can be stated that flocculation is the beginning ofsedimentation, which leads us to hypothesize a possible sedi-

mentation over time for the developed emulsions.Oscillations occur throughout the measurements given the

creaminess of the emulsion, where wavelengths can influencedifferently throughout the sample. Different from the blanksthat were liquid and had constant values, the variations in thedelta backscattering is normal in creams and is observed inprojects such as the one carried out by M. Suárez [31].Key aspects of the thickening agent influence on stability:

1. The thickener reduces the droplet size, which influencesan increase in viscosity and therefore greater stability for theemulsion.2. With the variation in the concentration of the dispersedphase, the thickener continues to increase stability, however,it is evident that the effect is not the same in both emulsionsdue to the amount of oil in the sample.3. The possible sedimentation in the emulsion due to the max-imum peaks in the low areas or zones that the delta backscat-tering presents is hypothesized. However, a more in-depth orlonger analysis would be required to test that hypothesis.• UV agent influence | Emulsion stability:

To analyze the influence of the UV agent on stability, only twoemulsions will be chosen since the trend is the same for allthose carried out. However, supplementary material can befound in Annex 3.

Figure 12. Week 0 | Delta backscattering results of O/W diluted emulsionswith 2.5% w/w carbopol + 1% w/w and 5% w/w UV agent.

It is evident that the UV agent is one of the compounds thatinfluences stability. With less amount of UV agent, the emul-sion is more unstable and has a higher delta backscatteringvalues. The above aspect is in accordance with the literaturebecause, for example, there are studies that implement TiO2as an emulsion stabilizer [17]. Also, J. Wang et al [32] reportedthat TiO2 nanoparticles are responsible for increasing Van DerWalls interactions with oil droplets. Other studies confirm themodification of the droplet size by the UV agent (TiO2), andtherefore the stability increases [33]. Hence the reasons whydelta backscattering values indicates greater instability at alower amount of UV agent.It is posible to observe in Figure 12:

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Figure 13. Week 0 | Zones of instability phenomena of O/W diluted emulsionswith 2.5% w/w carbopol and 1% w/w + 5% w/w UV agent.

Instability phenomena in zone:• Possible sedimentation: Literature indicates that emul-

sions with only UV agent generate creaming separation phe-nomena [32]. However, this happens since there is no thick-ening agent within the formulation. In this case, the impactthat the carbopol has on the UV agent is greater and even ifthere is a UV agent, the creaming is inhibited and the combi-nation could generates sedimentation. Therefore, creamingis not noticeable and the particles reach the lower areas forsedimentation.

In the same way, it is important to mention that accordingto the behavior of the delta backscattering, the sedimentationthat occurs is very minimal because the emulsion is stable. Itcould be considered as a hypothesis given the behavior of thecurves. Like the previous stability results, when the UV agentchanges the concentration of the dispersed phase, it continuesto present possible sedimentation behaviors and can be seen inAnnex 3. In addition, it is key to specify or report that to fullyidentify which phenomenon of instability causes each of theformulation variables it is required more extensive analysis andwith a greater number of formulations systems, formulationscombining the components, with the individual components,among others. At the moment with results, it was only possibleto identify possible sedimentation in each of the 6 emulsionsmade different from the blanks.Key aspects of the UV agent influence on stability:

1. TIO2 increases the viscosity of the emulsion, resulting ingreater stability.2. Although in studies they implement TiO2 as a stabilizer, itis not recommended to use it alone since it does not have asmuch influence as carbopol on stability.Stability most influential factor:Between UV agent and carbopol, it is important to note thatthe factor that most influences stability is the addition of car-bopol. Although emulsions stabilized with TiO2 generate a pos-itive effect compared to emulsions that do not have it due tothe increase in intermolecular forces, these emulsions end upseparating more quickly. On the contrary, when carbopol is im-plemented a drastic modification is made in the viscosity andmore when the pH is regulated with triethanolamine since theefficiency of the polymer increases. This is confirmed by thestudies mentioned above [4, 32, 33].Texture and sensory characteristics:

In relation to the Spectrum Descriptive Analysis, the appear-ance of most of the emulsions were bright, white in color and

smooth in texture. No significant changes in appearance. How-ever, the following cases should be highlighted:• Blanks appearance:

Figure 14. Appearance of O/W diluted emulsions (blanks).

According to the Figure 14, regarding color, the continuousphase acquires a cloudy color, while the dispersed phase hasa white color. In addition, for the two emulsions made with-out UV agent and carbopol, the liquid state in which they arefound is evident. However, the fact that they are liquid is di-rectly related to the absence of carbopol, mainly. The UV agentalso influences but it will be the subject of analysis in the nextitem. Carbopol is classified as a water-soluble polymer and inemulsions it functions as a texturizing or thickening agent [21].Consequently, it modifies properties such as viscosity, whichresults in a change in the appearance of the emulsion (specifi-cally in consistency). For this reason, if carbopol is not includedin the formulation, problems associated with the consistencyof the product may arise. Additionally, the absence of carbopolas a thickening agent makes the phase separation evident aswell. Although the separation is not complete, in terms of in-tegrated product design, such phase separation is a negativeeffect of what a consumer would look for when buying a cos-metic product. The fact that it is separated results in the lossof characteristics or properties from the moment it was cre-ated. Visually, there is a wide separation in both cases and anappearance of this type can become counterproductive if the ob-jective is to have a successful product, generating disapprovalon the part of consumers. Therefore, it is suggested that forthe development of emulsions a thickening agent be includedto stabilize the product.• UV agent 5% w/w + 2.5% w/w carbopol appearance:

In the literature there are studies such as the one developed byM. Karsheva, where amounts of UV agent of 0.2% w/w are usedin emulsions [34]. Following the sensory analysis methodol-ogy and to evaluate the influence and effect of the UV agenton the texture, values higher than the literature were chosen.In Figure 15, it is evident that for 2.5% w/w carbopol and 1%w/w UV agent a cream appearance is observed in both dispersedphase concentrations. However, if it is compared with the 5%w/w level of UV agent, the lumps formation is observed, anundesirable aspect in the sensory analysis. This is because theeffect that the UV agent has on the texture is the considerableincrease in consistency. The consistency was not expected toincrease widely to create lumps within the emulsion. There-fore, as a result of this study, it is not advisable to implement5% w/w of UV agent and 2.5% w/w of thickener. Regulatingthe amount of UV agent or carbopol is required to create betterproducts concerning sensory analysis.

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Figure 15. Appearance of O/W diluted emulsions with 1% w/w and 5% w/wUV agent + 2.5% w/w carbopol.

To analyze the pick-up and rub-out of the SDA methodol-ogy, bar chart have been constructed. While property valuesmay change over the weeks, the objectives are not focused onanalyzing the sensory properties change over time (only stabil-ity is analyzed with respect to time). The objectives in textureanalysis are related to analyzing which sensory variable is mostaffected according to the formulation variables. Therefore, anintermediate data measurement was selected to analyze theseinfluences (week 3). The results are the following:• Dispersed phase influence on sensory analysis:

Carrying out the double compression test, the following sen-sory bar chart is obtained:

Figure 16. Sensory bar chart for O/W diluted blanks emulsions.

In first place, it is observed that the factor most affected byoil concentration is compressibility/spreadability. The aboveis directly related to viscosity, where by definition it indicatesthe resistance to flow or deformation of the sample under anal-ysis. When there are less amounts of oil, the viscosity of theemulsion decreases as the droplet size is larger, which resultsin less resistance to deformation, therefore, the compressibil-ity is higher. In the case of spreadability, as the resistance toflow is lower, it is easier to spread the emulsion on the sur-

face. The behavior of compressibility/spreadability accordingto the concentration of the dispersed phase agrees with stud-ies such as those developed by C. Picard [12], where it is alsosaid that the decrease in the amount of oil increases the com-pressibility/spreadability of the sample. Secondly, given thatthe blanks emulsions are in a liquid state due to the absenceof thickener, the texturometer cannot calculate the adhesive-ness and that is why it gives values of zero or very close. Theanalysis of the sensory parameter already mentioned will bemade later with the other emulsions. Likewise, the hardnessis better analyzed with another type of emulsion that allows amore complete bar chart to be visualized. To observe in moredetail the influence of the concentration of the dispersed phaseon the sensory analysis, a percentage of UV agent and carbopolhas been established. The results of the double compressiontest are:

Figure 17. Sensory chart bar for O/W diluted emulsions with 5% w/w UVagent + 0.5% w/w and 2.5% w/w carbopol.

Analyzing Figure 17, it is observed that the correlation men-tioned in Figure 16 is still presented. When there is less dis-persed phase concentration, greater compressibility/spreadingis. Likewise, the amount of oil phase modifies the droplet sizeof the emulsion, where the smaller the droplet size, the greaterthe hardness the emulsion will have due to the changes in vis-cosity level that occur [4]. Additionally, the viscosity not onlyinfluences the hardness of the emulsion but also the adhesive-ness due to the better structure at the molecular level that theseemulsions have. Therefore, in consumer terms, when pres-sure is applied to the skin there will be a noticeable differencein tackiness [13]. Finally, it is important to mention that thecompressibility-spreadability values of Figure 17 vary with re-spect to the blanks (Figure 16) because they are liquid (blanks).For that reason, the texturometer at the time of performing thetwo compression cycles, the liquid does not leave as much as acream.• Thickener influence on sensory analysis:

According to Figure 17 the increase in the hardness of the emul-sion is evidenced with the increase in the amount of carbopol.As mentioned throughout the report, carbopol serves as a thick-ening agent by modifying droplet size and creating molecularnetworks that modify viscosity. Consequently, the hardnessalso changes according to the increase or decrease of the thick-ener. Although the spreadability is mainly modified by the oilconcentration in the O/W emulsion, the thickener also playsa fundamental role. The change in consistency caused by thethickener makes it easier or more difficult to spread our cream.Therefore, it is easier to spread a blank (liquid) because it isless consistent than the 40% w/w emulsion (5% w/w UV agent+ 2.5% w/w carbopol), for example. The customer will prefer aproduct with greater spreadability, however, it must be bornein mind that not adding components such as carbopol on the

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formulation will generate unwanted changes in the first partof the SDA method, the appearance. Likewise, the phase sepa-ration of the sample and although they are thermodynamicallyunstable products, it is important to maintain the oil and wa-ter mixture for as long as possible. Finally, as in the increaseof the oil phase, the adhesiveness also increases with the ad-dition of carbopol due to the modification in the viscosity andthe factors that were explained previously.• UV agent influence on sensory analysis:

Figure 18. Sensory chart bar for O/W diluted emulsions with 2.5% w/wcarbopol + 1% w/w and 5% w/w UV agent.

The addition of active ingredients as UV agent to formulationsin visual terms generate more effects on sensory analysis thanstability. For example, in the case of stability, it does not gen-erate such a significant impact compared to carbopol since theaddition of the UV agent does increase the stability, but notto a great extent. In sensory analysis, the UV agent increasesconsistency and makes a product "rougher" when applied tothe skin (affirmation based on own experimentation). Thischange in consistency influences the hardness of the emul-sion, where the higher the amount of UV agent, the higherthe hardness value as illustrated in Figure 18 (keeping theamount of carbopol constant). Additionally, the compressivity-spreadability decreases to the point where it can be a bad ex-perience to apply the product, perhaps due to the formation oflumps in the case of 5% w/w UV agent. The results in the tex-turometer confirm the sensory perception made by us and inthe literature as in studies carried out by M. Fossa [11]. In addi-tion, with the previous conditions, as the viscosity and consis-tency of the emulsion change, the adhesiveness of the sampleincreases. These results can be seen visually in Figure 15 or theresults in the texturometer in Figure 18.

Finally, from the results obtained in the textural parametersfor the systems, it was determined that the 40% w/w emulsion(1% w/w UV filter + 2.5% w/w carbopol) significantly adjusts tothe conditions and requirements proposed by the industry forthe design of O/W cosmetic emulsions [14]. This specific sys-tem has adequate adhesiveness, which is of great importancein the production of facial and body creams because the prod-ucts must adhere to the skin properly and for a considerablylong time. On the other hand, it was evidenced that the systemalso has a high spreadability, which has a positive effect on themain feeling that the consumer has when using it. The cos-metic emulsion can be spread very easily, it covers the largestsurface in the area of application. Finally, it was evidenced thatthis emulsion mentioned above has a low hardness, which al-lows it to have a high spreadability and adequate stability. Inthis way, it was determined that this system at these condi-tions satisfies the sensory criteria applied by the cosmetic in-dustry for products. The other emulsions are not chosen for aconsumer-industry relationship since the blanks samples have

quite high instability indices, which is unfavorable for a prod-uct. On the other hand, emulsions with 5% w/w UV agent and2.5% w/w carbopol present lump formation as mentioned inthe texture analysis, an unwanted appearance and that gener-ate a strange sensation when applied, which is why it is alsodiscarded.

Conclusions

The behavior of the rheological, stable, and textural proper-ties was analyzed with the variation of the dispersed phase,thickener, and UV filter in the systems. From the analysis car-ried out, it was determined that when the concentration of theaforementioned substances was increased, the elastic modulusalso increased, which leads to several phase separation mech-anisms being significantly inhibited in the system. Likewise,the rheological and stable properties of each emulsion could berelated.

On the other hand, it was observed that the UV filter and thethickener significantly affect the droplet size in the emulsions.From the data, it was established that a higher concentration ofthese agents considerably reduced the droplet diameter. In ad-dition, it was determined that at higher concentrations of thick-ening agent and UV protector, the delta backscattering valuesdecreased, which resulted in a significant increase in stabil-ity. In this way, the analysis determined that the most influ-ential variable for stability in the systems is carbopol. However,the presence of both agents probably promotes the sedimenta-tion mechanism and the absence of thickener in the systempromotes creaming. Additionally, it was established that overtime the stability of the systems was decreasing since the deltabackscattering increased according to the tests that were de-veloped each week.

From the data obtained on the influence on the texturalproperties, it was determined that the greater the dispersedphase, the greater the spreadability in the emulsion, which ac-cording to the sensory criteria is one of the most importantconditions and taken into account when preparing emulsions.high-quality cosmetics. In addition, it is carried out in the tex-ture tests that the lower the hardness the systems are easierto spread to the medium. On the other hand, concerning theadhesiveness parameter, it was evidenced that this has a directrelationship with the viscosity of the emulsion. The more vis-cous the emulsion, the greater the adhesiveness of the system.For compressibility it could be determined that a lower amountof dispersed phase has a lower resistance to deformation and,therefore, the compressible parameter is high. Finally, it issuggested that the UV concentration be less than 5% w/w toavoid the formation of lumps in the sample. In our study itwas possible to identify that an 40% w/w emulsion (1% w/wUV agent + 2.5% w/w thickener) meets the requirements of thetextural properties in the industry, since these systems mustconsist of high spreadability, adhesiveness and a not very highvalue of hardness, although this parameter is proportional tothe increase in stability in a cosmetic emulsion. It is of greatimportance to know all these aspects developed in this study,because they allow the timely analysis of decisions for the re-spective formulation of emulsions in the cosmetic industry.

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Annexes

Annex 1:Delta backscattering curves: Blanks

WEEK 0 WEEK 3

WEEK 4 WEEK 5

Figure 19. Backscattering weekly results for O/W diluted blanks emulsions.

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Annex 2:Delta backscattering curves: Thickening agent influence

WEEK 0 AND 5 WEEK 0 AND 5

WEEK 3 AND 4 WEEK 3 AND 4

WEEK 3 AND 4 WEEK 3 AND 4

Figure 20. Backscattering weekly results for O/W diluted emulsions.

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Annex 3:Delta backscattering curves: UV agent influence

WEEK 3 WEEK 4

WEEK 3 WEEK 4

Figure 21. Backscattering weekly results for O/W diluted emulsions.