Photoprotective Cosmetic Emulsions Based on Brazilian Smectite Clays

Abstract

Photoprotective cosmetic emulsions have gained significant attention in the cosmetic industry due to their ability to protect against harmful ultraviolet radiation (UV). In this work, photoprotective cosmetic emulsions were prepared by adding 5% natural (Branca and Verde Inferior) and commercial (Brasgel and Cloisite) Brazilian clays to different photoprotective emulsions with and without UVA and UVB chemical filters (EB, EB1 and EB2). All clays were benefited (crushed, milled and sieved) and characterized (X ray fluorescence and X ray diffraction). Additionally, a thermal treatment (120 °C by 24 h) was accomplished in the natural clays, aimed at decontamination. The emulsions were characterized for their organoleptic properties, centrifugation test, viscous behavior, pH values and in vitro sun protection factor (SPF). The cosmetic emulsions prepared without any addition of UV chemical filters presented the lowest in vitro and UVB absorption values because the clays used (Cloisite and Branca) did not have the potential to absorb UV radiation. Although some of the cosmetic emulsions prepared from EB1 and EB2 photoprotective emulsions presented phase separation, all of them presented in vitro SPF values according to the Brazilian standard (above 6), indicating that they have the potential to be used in the cosmetic industry.

1. Introduction

Smectite clays have extensive applications in the pharmaceutical industry, serving as excipients, active substances or dispersion agents that fulfill various technological functions [1,2,3]. Indeed, smectite clays are employed in multiple applications, but their employment as active ingredients in pharmaceutical and cosmetic formulations stands out and is well documented in the scientific literature [1,2,4,5,6,7,8,9,10,11]. The numerous advantages offered by clays make them the preferred choice for formulating cosmetic products.

Clays are valuable components in the cosmetics industry because they include a variety of necessary minerals, including Si, Al, Fe, Zn, Mg, Ca, K and Ti, which have a major impact on the health and appearance of the skin. Iron acts as an antiseptic and promotes the renewal of skin cells, while silicon aids in the reconstruction of skin tissues, offering hydration and a soothing effect. Zinc and magnesium possess invigorating properties that rejuvenate and enliven the skin [12]. Calcium and potassium contribute to improved circulation and invigoration of tissues. Titanium, a mineral of particular interest in cosmetology, is primarily utilized in photoprotection formulations due to its remarkable ability to reflect UV radiation [5]. These characteristics, however, rely on the size of the particles. For example, titanium dioxide with an average particle size of 230 nm scatters visible light, but its counterpart with an average particle size of 60 nm scatters ultraviolet radiation and reflects visible light. In addition, such necessary minerals are generally considered nontoxic to humans and animals [13].

There are many advantages to using smectite clays for fabricating photoprotective cosmetic emulsions. Their absorption capacity for substances like grease and toxins is of particular interest, providing effective protection against the harmful effects of the sun. Furthermore, clays exhibit properties that make them suitable for treating inflammatory conditions, offering aesthetic improvements and stimulating collagen production, suggesting that clay may be a good therapeutic option for facial rejuvenation and enhancing the skin’s barrier function [1,14,15]. In addition, smectite clays are scientifically proven to be safe for topical application, and combined with their wide availability and cost-effectiveness, these materials are highly desirable for the formulation of photoprotective cosmetic emulsions [6].

Hoang-Minh et al. [16] demonstrated the effectiveness of incorporating clays into sunscreen creams to block ultraviolet (UV) radiation within the 400 to 250 nm wavelength range. The level of UV transmission was influenced by the iron content in the minerals, with a higher iron content resulting in lower transmission levels. However, the intense coloration of ferrous clays renders them unsuitable for commercial sunscreen production, requiring further research to overcome this limitation. Consequently, exploring alternative clays or clay mixtures as potential UV protection agents is necessary. In this context, using Brazilian smectite clays with a low iron content holds promise for sunscreen formulations. These natural clays possess desirable characteristics, including a relatively high surface area, chemical inertness, low toxicity and cost effectiveness, which could facilitate the development of colorless photoprotective cosmetic emulsions.

Thus, the main objective of this work is to prepare photoprotective cosmetic emulsions containing Brazilian smectite clays which can enhance the cosmetics industry by offering natural, cost-effective solutions. The knowledge generated through this study is expected to enrich the range of cosmetic products available in the market, providing consumers with improved options for effective and safe sun protection.

2. Materials and Methods

2.1. Materials

Cosmetic emulsions were prepared from natural and commercial clays, both of the polycationic type. The natural clays used were the Branca clay (BRA) from Olivedos city in Paraíba State, Brazil, and the Verde Inferior clay (VI) from Cubati city in Paraíba State, Brazil. The commercial clays used were Brasgel (BRG) and Cloisite (CL). Brasgel is a bentonite-type clay supplied by the Bentonit União Nordeste company, located in Campina Grande city in Paraíba State, Brazil. Cloisite is a sodium montmorillonite clay provided by the company Buntech, located in São Paulo city in São Paulo State, Brazil. Sodium hexametaphosphate (Vetec, P.A.) and absolute alcohol (Itajá FR, 99.5% INPM) were also used. The materials used in the preparation of the photoprotective emulsions were purchased from Pharma Face, located in Campina Grande City, Paraíba State, Brazil.

Table 1. Materials and their respective concentrations used to prepare photoprotective and cosmetic emulsions studied in this work.

Materials	INCI # Name	Concentration	Function
Aristoflex® AVC	The neutralized copolymer of sulfonic acid acryloyldimethyltaurate and vinylpyrrolidone	0.5%	Stabilizer
10% NaOH solution	Sodium Hydroxide	2.4%	Stabilizer
Glycerin	Glycerin	0.5%	Emulsifier
Nikkomulese 41	Polyglyceryl-10 Behenate Alcohol, Sodium Stearoyl Lactylate	3.0%	Emulsifier
Xiameter™ PMX—200 5 cSt	Polydimethylsiloxane	8.0%	Film-forming agent
Neopele™PE	Methylisothiazolinone—MIT and Phenoxyethanol	0.6%	Preservative
Univul® A Plus	Diethylamino hydroxy benzoyl hexyl benzoate	3.5%	UVA sunscreen
Tinorsorb® S	Bis—ethylhexyloxyphenol methoxyphenyl triazine	3.5%	Broad spectrum UVB/UVA sunscreen
Univul® T-150	Ethylhexyl Triazone	2.0%	UVB sunscreen
Parsol® MCX	Ethylhexyl methoxycinnamate	8.0%	UVB sunscreen
FloraGLO Lutein®	Tagetes erecta L.	0.1%	Antioxidant and visible light absorbers

^# International Nomenclature Cosmetic Ingredient.

shows the materials and their respective concentrations to prepare the photoprotective emulsions that were used to prepare the cosmetic emulsions studied in this work. For a better understanding throughout the text, emulsions without added clays were called photoprotective emulsions, while emulsions with added clay were nominated cosmetic emulsions.

2.2. Beneficiation and Characterization of the Clays

The natural clays (BRA and VI) were crushed (Pavitest, I4198), milled (CT-242, Servitech, Dodge City, KS, USA) and sieved (0.044 mm). After this, the natural clays were subjected to decontamination, heating them at 120 °C in an oven for 24 h [9]. The commercial clays (BRG and CL) were purchased in powder form (≤0.074 mm) and were sieved (0.044 mm). Such procedures were carried out because the clays used in the cosmetic industry need to present fine granulometry, high specific surface area, be easily moldable and provide a pleasant feeling when applied directly to the skin.

The chemical composition was determined via X-ray fluorescence (Shimadzu, Kyoto, Japan, EDX 720 model). The mineralogical composition was determined via X-ray diffraction (Shimadzu, XRD 6000 model). All XRD experiments were conducted at room temperature using Cu Kα radiation (40 kV/30 mA) with a goniometer speed equal to 2°/min and a step of 0.02°. The International Centre for Diffraction Data Sample Database was used to identify the mineralogical phases.

Fourier transform infrared spectroscopy (FTIR) was performed in a Perkin Elmer spectrophotometer equipment (Spectrum 400 model), coupled to the ATR module, diamond crystal and ZnSe prism. The experiments were carried out at room temperature in the 4000 to 400 cm⁻¹ spectral region, with a resolution of 4cm⁻¹ and 64 scans.

2.3. Preparation of the Cosmetic Emulsion

Three photoprotective emulsions were prepared using the materials shown in Table 1. The first one, EB photoprotective emulsion, was prepared without adding UVA and UVB chemical filters. The second one, EB1 photoprotective emulsion, was prepared using all the materials presented in Table 1. The final photoprotective emulsion, EB2, was prepared without adding the Uvinul T-150 chemical filter. The EB1 emulsion was the first formulation carried out in this study, so after analyzing the appearance of the emulsions and conducting the emulsion stability test using the VI and BRG clays, it was decided not to add the VI and BRG clays to the EB and EB2 emulsions.

The cosmetic emulsions were prepared by adding natural and commercial clays into the photoprotective emulsions. For this, clay additions (5%, w/w) into photoprotective emulsions were carried out in an electric mixer (Fisatom, 713D) at 5000 rpm in a temperature range between 70 and 80 °C until they acquired a viscous consistency. The mixture was cooled in the mixer itself. All cosmetic emulsions prepared in this work and their respective nomenclatures are summarized in

Table 2. Nomenclature of all cosmetic emulsions prepared from the BRA, VI, BRG and CL clays and EB, EB1 and EB2 photoprotective emulsions in this work.

EB photoprotective emulsion
Clay	Cosmetic emulsions
Cloisite (CL)	EBCL
Branca (BRA)	EBBRA
EB1 photoprotective emulsion
Branca (BRA)	EB1BRA
Verde Inferior (VI)	EB1VI
Brasgel (BRG)	EB1BRG
Cloisite (CL)	EB1CL
EB2 photoprotective emulsion
Cloisite (CL)	EB2CL
Branca (BRA)	EB2BRA

.

2.4. Characterization of the Photoprotective and Cosmetic Emulsions

2.4.1. Organoleptic Analysis and Hydrogen Potential (pH)

The organoleptic characteristics (appearance, color and odor) were visually evaluated. Cosmetic emulsions were diluted 10% (w/v) in water to the pH measurements. All measurements were accomplished on an LT2404 digital pH meter, previously calibrated with pH 4.0 and 7.0 buffer solutions.

2.4.2. Colorimetry

Experiments to investigate the colorimetry of photoprotective and cosmetic emulsions were carried out on an iPad device model MW752LL/A using the Hooloovoo software (version 1.1). The experiments were analyzed by Equation (1) where L* = Luminosity, a* = red/green coordinate (+a* indicates red and −a* indicates green) and b* = yellow/blue coordinate (+b* indicates yellow and −b* indicates blue).

$${∆\mathrm{E}}_{\mathrm{a}\mathrm{b}}^{\mathrm{*}}=\sqrt{{\left({\mathrm{L}}_2^{\mathrm{*}}-{\mathrm{L}}_1^{\mathrm{*}}\right)}^2+{({\mathrm{a}}_2^{\mathrm{*}}-{\mathrm{a}}_1^{\mathrm{*}})}^2+{\left({\mathrm{b}}_2^{\mathrm{*}}-{\mathrm{b}}_1^{\mathrm{*}}\right)}^2}$$

(1)

2.4.3. Accelerated Stability Tests

The accelerated stability test was conducted according to the Stability Guide for Cosmetic Products [17] published by the National Health Surveillance Agency (Brazilian regulatory agency, whose acronym is ANVISA). For this, the samples were centrifuged in a Baby^® I Model 206-BL centrifuge for 30 min at 3200 rpm at room temperature and at the end, the occurrence, or not, of phase separation was observed.

2.4.4. Viscosity Measurements

The viscosity experiments were carried out in a Brookfield viscometer, model DV3TLV, with spindle number 64, with rotation speeds of 1.5 rpm, 2.5 rpm and 5 rpm.

2.4.5. Determination of the In Vitro Sun Protection Factor (SPF)

Here, 30 mL of ethanol was added to a volumetric flask (100 mL) containing 0.5 g of cosmetic emulsion. The flask was placed on a magnetic stirring plate for 2 min at room temperature to obtain a homogeneous mixture. After that, 70 mL of ethanol was added to the volumetric flask with subsequent manual homogenization. A 1mL aliquot of the solution was removed from the 100 mL volumetric flask and transferred to a 25 mL volumetric flask. The volume was completed with ethanol to obtain a solution with a final concentration equal to 0.2 mg/mL. Here, 1 mL aliquots were used for spectrophotometric analysis. These experiments were carried out in a UV/Vis spectrophotometer (Perkin Elmer, model LAMBDA 35) with a quartz cuvette and a 1cm optical path. The in vitro determination of SPF was performed according to the Mansur method [18,19]. For this, measurements at various wavelengths were performed (290, 295, 300, 305, 310, 315 and 320 nm) and the data were analyzed according to Equation (2):

$$\mathrm{S}\mathrm{P}\mathrm{F}=\mathrm{F}\mathrm{C}{\sum }_{290}^{320}\mathrm{E}\mathrm{E}\left(\mathsf{\lambda }\right)\times \mathrm{I}\left(\mathsf{\lambda }\right)\times \mathrm{A}\mathrm{b}\mathrm{s}\left(\mathsf{\lambda }\right)$$

(2)

where FC is a correction factor (equal to 10); EE (λ) is the erythematous effect of radiation of wavelength λ; I (λ) is sunlight intensity at wavelength λ; Abs (λ) is the spectrophotometric reading of the absorbance of the cosmetic emulsion solution at wavelength λ. The products EE (λ) and I (λ) were calculated according to Sayre et al. [20], see

Table 3. Relationship between wavelength and EE × I [20].

λ (nm)	EE × I
290	0.0150
295	0.0817
300	0.2874
305	0.3278
310	0.1864
315	0.08390
320	0.0180
Total	1.0000

.

3. Results and Discussion

3.1. Characterization of Clays

The chemical compositions of the Brasgel, Cloisite, Branca and Verde Inferior clays are summarized in

Table 4. Chemical composition (wt%) of Brasgel, Cloisite, Branca and Verde Inferior clays.

	Commercial Clays		Natural Clays
	Brasgel (wt%)	Cloisite (wt%)	Branca (wt%)	Verde Inferior (wt%)
SiO2	65.00	49.89	60.37	57.01
Al2O3	18.54	13.91	26.18	27.61
Fe2O3	9.51	10.42	3.38	8.22
CaO	1.33	0.60	4.20	1.77
MgO	2.80	0.60	4.19	3.17
K2O	0.37	0.15	0.53	0.89
Na2O	1.11	0.36	-	-
TiO2	0.96	0.74	0.80	1.05
ZnO	0.01	0.08	0.008	0.016
Outros óxidos	0.33	1.30	0.33	0.23

. As expected, SiO₂ and Al₂O₃ are the major components of these clays [21,22]. The commercial clays, Brasgel and Cloisite, presented SiO₂ contents equal to 65.00 wt% and 49.89 wt%, respectively. The SiO₂ contents of natural clays, Branca and Verde Inferior, were 60.37 wt% and 57.01 wt%, respectively. It is known that SiO₂ is important in cosmetic emulsions because it contributes to stability and viscosity and increases resistance to phase separation. Also, SiO₂ contributes to the durability of cosmetic emulsions since it can act as an antioxidant agent [23]. The highest Al₂O₃ contents were measured in the natural clays, Branca and Verde Inferior, at 26.18 wt% and 27.61 wt%, respectively. The highest Al₂O₃ percentage in these clays can indicate a greater adsorption capacity for organic compounds. On the other hand, the lowest Al₂O₃ content (13.91 wt%) was detected in Cloisite, which is a commercial clay. Furthermore, the presence of CaO can have a positive effect on the emulsion stability since calcium can act as a stabilizer. However, the presence of Fe₂O₃ in relatively high amounts can affect the color of the cosmetic emulsion since it is known that clays with Fe₂O₃ concentrations above 8 wt% have reddish tones. Small TiO₂ and ZnO amounts were also identified in all clays investigated. ZnO has a broad UVA-UVB absorption curve, while TiO₂ offers better UVB protection [24]. Indeed, several FDA-approved mineral sunscreens contain TiO₂ and/or ZnO particles, which function as physical barriers that reflect and scatter UV rays [25].

The mineralogical phases present in the Brasgel, Cloisite, Branca and Verde Inferior clays were identified by XRD analysis, see Figure 1a–d. The smectite [JCPDS 13-0135] and quartz [JCPDS 46-1045] phases were identified in all investigated clays. The presence of quartz is justified by the fact that it is a common accessory mineral in several clay types [26]. As observed, the decontamination method (thermal treatment at 120 °C for 24 h) did not influence the mineralogical phases. Bentonite particles are very fine and can delaminate when in water, in addition to having the ability to separate layers. The spacing of the layers, whether greater or lesser, depends on factors such as the nature of the sites that generate the charges, the presence of contaminants and, especially, the nature of the cation, as less intense hydration occurs if it is a cation other than sodium [27]. Smectite is a variation of kaolinite and belongs to the montmorillonite group, and it stands out due to its high cation exchange capacity. The kaolinite phase [JCPDS 78-2110] was identified only in Brasgel and Verde Inferior clays. Kaolinite can increase the stability of different emulsions (water/oil, oil/water, oil/oil and anhydrous) since it affects the migration and absorption capacity in hydrophobic and hydrophilic environments [28].

3.2. Organoleptic Properties, Resistance to Phase Separation and Viscous Flow

3.2.1. Organoleptic Properties

All photoprotective and cosmetic emulsions proposed in this work were successfully prepared. Figure 2a shows portions of the EB photoprotective emulsion and their respective cosmetic emulsions prepared by adding Cloisite and Branca clays (EBCL and EBBRA, respectively). EB photoprotective emulsion presented an ice-white color, creamy appearance and an odor of the cream without fragrance. The EBBRA presented color, odor and an appearance very close to EB photoprotective emulsion; however, EBCL presented a light yellow color and an odor characteristic of clay, while the appearance remained creamy. The color change in the EBCL cosmetic emulsion may have occurred due to the Fe₂O₃ concentration in Cloisite clay being three times higher than in Branca clay (10.42 wt% and 3.38 wt%, respectively). In Figure 2b, the EB1BRG and EB1VI cosmetic emulsions presented a color, odor and appearance very different from their respective photoprotective emulsion (EB1). Both showed a color with brown tones and the absence of a creamy appearance. The high levels of Fe₂O₃ measured in Brasgel and Verde Inferior clays (9.51 wt% and 8.22 wt%, respectively) also explain the color with brown tones observed in EB1BRG and EB1VI cosmetic emulsions. In contrast, the color and odor of EB1CL and EB1BRA cosmetic emulsions did not change significantly but showed a decrease in the creamy appearance. Figure 2c shows cosmetic emulsions prepared by adding the Cloisite and Branca clays to the EB2 photoprotective emulsion. In general, such emulsions showed a light orange color, which did not change significantly; however, adding clays decreased the creamy appearance.

Colorimetric measurements were accomplished from the emulsions prepared in this work, see

Table 5. L*, a* and b* values measured from the protective and cosmetic emulsions studied in this work.

	Cosmetic Emulsions Based on EB Photoprotective Emulsion
Formulations	Color	L*	a*	b*	ΔE
EB	ice-white	90.489	−0.512	0.359
EBCL	light yellow	69.412	−2.736	15.745	24.1998
EBBRA	white ice	83.473	0.871	6.435	9.3835
	Cosmetic emulsions based on EB2 photoprotective emulsion
EB1	light yellow	72.144	−2.521	20.768
EB1BRG	brown	47.811	2.783	15.837	25.3876
EB1VI	light brown	57.042	−0.259	17.234	15.6739
EB1CL	light yellow	71.026	−2.804	23.539	3.0015
EB1BRA	light yellow	77.912	−2.941	20.306	5.8018
	Cosmetic emulsions based on EB3 photoprotective emulsion
EB2	light orange	71.779	4.942	61.567
EB2CL	light orange	71.755	3.521	54.731	6.9822
EB2BRA	light orange	73.252	3.425	57.351	4.7165

. As expected, the L*, a* and b* values measured from the EB and EBBRA cosmetic emulsions were relatively close since both showed a similar coloration, see Figure 2a. Comparatively, the color change in the EBCL cosmetic emulsion can also be seen from the L*, a* and b* values, since these values were much lower than those measured for their respective photoprotective emulsion. As for the colorimetric data obtained from the EB1 photoprotective emulsion and their respective cosmetic emulsions (EB1BRG, EB1VI, EB1CL and EB1BRA), it was possible to observe that the L*, a* and b* values measured from EB1, EB1CL and EB1BRA were relatively similar; for EB1BRG and EB1VI, they were significantly smaller than those measured from EB1. Such results are also in agreement with the macroscopic analysis carried out in Figure 2b, since EB1BRG and EB1VI presented a brown and light brown tone, respectively, while EB1, EB1CL, EB1VI and EB1BRA presented a relatively light-yellow tone. The L*, a* and b* values measured from EB2, EB2CL and EB2BRA were also very close and are in accordance with the macroscopic analysis performed in Figure 2c.

As expected, the ΔE values (Table 5) also converge with the discussion based on the L*, a* and b* values. For this analysis, the greater the ΔE value calculated between two materials, the larger the difference between the colors presented by them. The highest ΔE value was calculated from the EB1 photoprotective emulsion and EB1BRG and EB1VI cosmetic emulsions (25.3876 and 15.6739, respectively). These data also align with the macroscopic analysis conducted in Figure 2b as these emulsions exhibited a brown and light brown color that was significantly different from the light-yellow color displayed by the respective photoprotective emulsion. On the other hand, the analysis conducted on the EB2, EB2CL and EB2BRA emulsions resulted in low ΔE values (6.9822 and 4.7165, respectively). Indeed, the EB2, EB2CL and EB2BRA emulsions exhibit a very similar color, as seen in Figure 3c.

3.2.2. Resistance to Phase Separation

Cosmetic emulsions were subjected to centrifugation tests to verify their homogeneity or presence of phase separation. Figure 3a–c shows the appearance of all cosmetic emulsions investigated after the centrifugation test. In general, the cosmetic emulsions obtained from the EB photoprotective emulsion (EBCL and EBBRA) did not show macroscopic evidence of phase separation and were considered stable, see Figure 3a. The EB1, EB1BRA and EB1CL cosmetic emulsions also remained homogeneous after the centrifugation test; however, the EB1BRG and EB1VI cosmetic emulsions were characterized as unstable because a second phase was detected, see Figure 3b. The EB2 and EB2BRA cosmetic emulsions also remained homogeneous after the centrifugation test, see Figure 3c. However, evidence of phase separation (indicated by a red arrow) was observed in the EB2CL cosmetic emulsion. The relatively high SiO₂ levels in the Brasgel and Verde Inferior clays (65.00 wt% by weight and 57.01 wt%, respectively) did not prevent phase separation in the EB1BRG and EB1VI cosmetic emulsions. Furthermore, the relatively low SiO₂ levels in the Cloisite clay also did not play a decisive role in the occurrence of phase separation. This is evident from the fact that the EB2CL cosmetic emulsion exhibited separation, while the EBCL cosmetic emulsion did not show any separation. This is a strong indication that most of the silica detected in the chemical analysis is not in free form and refers to silica that is bound to other minerals in the clay structure and is not readily available. This form of silica is incorporated into the crystalline or amorphous structure of the clay, and its reactivity is limited due to these bonds with other elements in the clay. Unavailable silica is less concerning regarding occupational exposure as its reactivity is reduced.

3.2.3. Viscous Behavior of the Cosmetic Emulsions

The viscosity behavior as a function of different rotation speeds (1.5 rpm, 2.5 rpm and 5.0 rpm) was investigated for all cosmetic emulsions prepared in this work, see Figure 4a–c. In general, the viscous behavior of all investigated cosmetic emulsions decreased with increasing rotational speed. Such behavior characterizes them as a non-Newtonian fluid of the pseudoplastic type, which is suitable for cosmetic formulations for topical use, considering that after shearing or increasing the rotation speed and applying a spreading force under the emulsion, the initial resistance for the cosmetic to flow decreases, reflecting in a better application and spreadability.

Regarding the viscous behavior of the cosmetic emulsions prepared from the photoprotective emulsion EB, it was possible to verify that the viscosity increased with the addition of Cloisite clay, while the addition of Branca clay decreased the viscosity, see Figure 4a. The high viscosity of cosmetic emulsions reduces droplet movement and controls destabilization phenomena such as coalescence, producing stable emulsions without phase separation. In general, adding Branca, Verde Inferior, Brasgel and Cloisite clays to the photoprotective emulsion EB1 decreased viscosity values as a function of rotation speed, see Figure 4b. On the other hand, adding Cloisite and Branca clays to the EB2 photoprotective emulsion had different effects on viscous behavior, see Figure 4c. The EB2BRA cosmetic emulsion showed lower viscosity values than the EB2 photoprotective emulsion, while the EB2CL cosmetic emulsion showed higher viscosity values at low rotations (<2.5 rpm) and relatively equal viscosity values for rotation speeds above 2.5 rpm.

3.3. pH Determination

The pH values measured from the cosmetic emulsions and their respective photoprotective emulsions are shown in

Table 6. pH values measured from EB, EB1 and EB2 photoprotective emulsions and their respective cosmetic emulsions.

Photoprotective/Cosmetic Emulsions	pH Values
EB	7.0
EBBRA	7.2
EBCL	7.0
EB1	7.5
EB1BRG	6.9
EB1CL	7.2
EB1BRA	7.2
EB1VI	6.9
EB2	6.6
EB2CL	7.2
EB2BRA	6.8

. Topical emulsions must have a pH that ensures the stability of the components and biological compatibility; therefore, the pH is directly linked to the safety and efficacy of a given product [19]. The pH values measured from the EB, EB1 and EB2 photoprotective emulsions were equal to 7.0, 7.5 and 6.6, respectively. The EBCL cosmetic emulsion presented the same pH value as its respective photoprotective emulsion (pH = 7.0), while the EBCL presented a slightly higher value (pH = 7.2). The difference of 0.2 in pH values measured from the EB and EBCL emulsions can be related to the slight difference in color presented, see Table 5. The EB1BRG and EB1VI cosmetic emulsions, which showed slightly modified coloring and phase separation (see Figure 2b and Table 5), presented the relatively lowest pH values, at 5.7 and 6.1, respectively, if compared with their respective photoprotective emulsion. On the other hand, the pH values measured from the EB1BRA and EB1CL cosmetic emulsions were the same (pH = 7.2) and very close to that measured from their respective photoprotective emulsion (EB1, pH = 7.5). This small difference in pH values was reflected in the organoleptic properties since these cosmetic emulsions were the ones that presented color, odor and appearance more like that of their respective photoprotective emulsion. The EB2BRA cosmetic emulsion, on the other hand, presented a pH value very similar to the measurements of its respective photoprotective emulsion (EB2), at 6.8 and 6.6, respectively. Although the EB2CL cosmetic emulsion has a color, odor and appearance like its respective cosmetic emulsion, the measured pH value was higher (pH = 7.2). This indicates that the pH value, in this case, is not decisive for the organoleptic properties. It is important to emphasize that slightly acidic pH values favor skin integrity, preventing the installation of pathogenic organisms that often prefer more alkaline media [29].

3.4. Determination of In Vitro Sun Protection Factor

The UV/Vis spectra obtained from photoprotective emulsions (EB, EB1 and EB2) and their respective cosmetic emulsions are shown in Figure 5a–d. In Figure 5a, it is possible to verify that no absorption in the UV region was detected in the EB photoprotective emulsion, while for the EB1 and EB2 photoprotective emulsions, it was possible to identify two bands: one in the UVB region (280–320 nm) and another in the UVA region (320–400nm). The lowest absorption values were observed for EB2. As with its EB photoprotective emulsion, no UV adsorption band was detected in the EBCL and EBBRA cosmetic emulsions, see Figure 5b. This behavior was expected because no chemical filters were added to these emulsions. On the other hand, it is possible to conclude that the Cloisite and Branca clays do not have the potential to absorb UV radiation. The absorption band observed in the UVB region in the EB1 photoprotective emulsion corresponds to the absorption of the Parsol^® MCX, Tinosorb^® S and Univul^® T-150 filters, which present peaks of λ_max equal to 309, 310 and 314 nm, respectively. In the EB2 photoprotective emulsion, this absorption band in the UVB region showed the same behavior, however with lower absorption values due to the absence of the Univul^® T-150 filter. In the UVA region, the absorption band is related to the presence of Tinosorb^® S and Univul^® A Plus filters, which present λ_max values equal to 340 nm and 354 nm, respectively. The UV/Vis spectra measured from the EB1BRG, EB1CL, EB1BRA and EB1VI cosmetic emulsions also showed two absorption bands, shown in Figure 5c. Such cosmetic emulsions showed absorption bands in both the UVB and UVA regions, with the EB1BRG emulsion having the highest absorption and EB1VI having the lowest absorption. The UV/Vis spectra of the EB2CL and EB2BRA cosmetic emulsions showed a sharp absorption band in the UVB region (λ_max = 310 nm). Due to the absence of Uvinul^® A-Plus, the absorption band in the UVA region was smooth.

In vitro sun protection factor (SPF) values were calculated from the experimental data shown in Figure 5a–d, Equation (2) and Table 3. SPF primarily assesses the ability of a sunscreen to absorb UVB rays. It represents the amount of ultraviolet radiation necessary to induce a minimal erythema dose (MED) on protected skin relative to the amount required to produce MED on unprotected skin after applying 2mg/cm² of sunscreen. As expected, in vitro SPF and UVB absorption (%) values measured in EB, EBCL and EBBRA emulsions were relatively slow compared with the other photoprotective and cosmetic emulsions (Table 6). As seen in Figure 5a, this occurred due to two factors: (i) no chemical filter was added to the EB photoprotective emulsion and (ii) Cloisite and Branca clays do not have the potential to absorb UV radiation. The in vitro SPF values of the cosmetic emulsions EB1BRG, EB1CL, EB1BRA and EB1VI and their respective photoprotective emulsion (EB1) are also summarized in Table 6. Among the cosmetic emulsions, only EB1BRG demonstrated in vitro SPF values near the EB1 photoprotective emulsion (23.0639 ± 0.2085 and 24.7254 ± 0.1506, respectively). On the other hand, the SPF values of the EB1CL, EB1BRA and EB1VI cosmetic formulations were lower than the EB1 emulsion (18.8737 ± 0.3552, 17.6305 ± 0.0624, and 15.6413 ± 0.0367, respectively). However, all of the cosmetic emulsions formulated, namely EB1BRA, EB1CL, EB1BRG and EB1VI, exhibited very similar UVB absorption values (

Table 7. Compares in vitro SPF values and UVB Absorption (%) measured from EB, EB1 and EB2 photoprotective emulsions and their respective cosmetic emulsions.

EB photoprotective emulsion and their respective cosmetic emulsions
Photoprotective/cosmetic Emulsions	In vitro SPF	UVB absorption (%)
EB	1.3095 ± 0.0335	24.5
EBBRA	1.3923 ± 0.0389	29.5
EBCL	1.7559 ± 0.1172	41.5
EB1 photoprotective emulsion and their respective cosmetic emulsions
EB1	24.7254 ± 0.1506	95.96
EB1BRA	17.6305 ± 0.0624	94.29
EB1CL	18.8737 ± 0.3552	94.65
EB1BRG	23.0639 ± 0.2085	95.66
EB1VI	15.6413 ± 0.0367	93.59
EB2 photoprotective emulsion and their respective cosmetic emulsions
EB2	25.3994 ± 0.9299	96.0
EB2CL	9.9872 ± 0.0055	89.0
EBBRA	25.5519 ± 0.2060	96.0

), suggesting comparable levels of protection. These findings align with the existing literature, which indicate that an SPF of 15 corresponds to approximately 93.3% UVB absorption, SPF 30 corresponds to 96.7%, SPF 45 to 97.8% and SPF 50 to 98% UVB absorption [30].

In Table 7, the EB2 photoprotective emulsion, which was formulated without the Univul^® T-150 chemical filter, presented in vitro SPF values equal to 25.3994 ± 0.9299, while the emulsion cosmetic obtained from the addition of the Cloisite and Branca clays presented in vitro SPF values equal to 9.9872 ± 0.0055 and 25.5519 ± 0.2060, respectively. Despite the absence of the Uvinul T-150 filter, the cosmetic emulsions obtained from the EB2 photoprotective emulsions showed satisfactory in vitro SPF values since the requirement of the Agência Nacional de Vigilância Sanitária (Anvisa—Brazil) is that the minimum in vitro SPF value is equal to six [17]. The EB2CL cosmetic emulsion showed an in vitro SPF value equal to the EB2 photoprotective formulation, i.e., without adding clay. Such a result is a strong indication that the investigated Brazilian clays have the potential to also be used to replace filters in cosmetic emulsions.

4. Conclusions

Cosmetic emulsions were successfully prepared by adding 5 wt% Brazilian natural and commercial clays to emulsions with and without chemical filters. All cosmetic emulsions prepared exhibited a suitable viscosity behavior and pH from a commercial standpoint. However, the EB1BRA, EB1VI and EB2CL cosmetic emulsions presented phase separation after centrifugation tests. In vitro SPF values measured from the EBBRA and EBCL cosmetic emulsions, which were prepared from emulsions without chemical UV filters, were the lowest, indicating that Branca and Cloisite clays do not have the potential to absorb UV radiation. The in vitro SPF values measured from the cosmetic emulsions prepared based on the EB1 and EB2 photoprotective emulsions were relatively lower; nevertheless, these values still complied with those recommended by the Agência Nacional de Vigilância Sanitária (Anvisa—Brazil), that is, equal to or above six. This study suggests that Brazilian natural (Branca and Verde Inferior) and commercial (Brasgel and Cloisite) clays can be used to prepare cosmetic emulsions from photoprotective emulsions without a significant loss in SPF values. Besides the health benefits, as it is commonly known that clays contribute to skin oil absorption and have effects on wrinkle prevention and skin aging, among others, they also open up possibilities for the clay and cosmetic industries to add new products to their portfolio.