Role of Emulsifiers and SPF Booster in Sunscreen Performance: Assessing SPF, Rheological Behavior, Texture, and Stability

Abstract

This study investigates the impact of emulsifier substitution and booster concentration on sunscreen characteristics, including physical properties, the sun protection factor (SPF), and sensory attributes. The impact of substituting Polysorbate^® 80 with Beautyderm^® K10 as an emulsifier in sunscreen formulations, along with the effect of increasing concentrations of the Sunhancer™ Eco SPF Booster, was thoroughly evaluated. Spectrophotometric methods were used to determine SPF, while texture analysis and rheological measurements assessed physical characteristics. Stability was evaluated using a centrifuge stress test, and sensory analysis was conducted on the top-performing formulation. The results indicated that the choice of emulsifier and booster concentration significantly influenced SPF values and stability. The influence of booster concentration on textural properties was most significant in formulations containing Beautyderm^®. Centrifuge testing revealed phase separation in certain formulations. Notably, the formulations that exhibited the greatest stability were those in which Beautyderm^® was combined with either Polysorbate^® or Span^®. Following the stability test results, the cream formulation containing Beautyderm^® and Polysorbate^® as emulsifiers was further evaluated through sensory analysis. Independent assessors determined that the sensory attributes of the cream did not undergo significant changes even when zinc oxide was added at a concentration of 1% (w/w) to the formulation. These findings underscore the importance of carefully selecting emulsifiers and boosters to achieve high sun protection efficacy, stability, and desirable sensory properties in sunscreen formulations.

1. Introduction

UV irradiation has several adverse effects including mutagenicity, immune suppression of the skin, accelerated skin aging, and photodermatoses [1]. The WHO has classified it as a carcinogen. The unprotected exposure to UV irradiation damages the skin cells by launching a cascade of reactions, including the generation of reactive oxygen species (ROS), lipid and protein oxidation, DNA damage, p53 mutation, the release of cytokines and matrix metalloproteinases, and reduced Langerhans cell antigen presentation function [2]. Many studies have confirmed that UVB radiation has a damaging effect. UVA also contributes to DNA damage and photoaging. When sequential pyrimidine bases of the same DNA strand absorb UV photons, this direct DNA damage leads to dimeric photolysis [3]. UV irradiation, especially shorter wavelength UV-B, crosslinks neighboring bases (cytosines or thymidines) in DNA to form “dimers” that interfere with DNA replication. This causes the cytosines in the DNA to mutate into thymidines. C→T and CC→TT mutations are imprints of UV damage to DNA and account for 80% of carcinogenic mutations found in skin cancer [4]. Indirect DNA damage is mainly caused by free radicals, ROS, and reactive nitrogen species. Exposure to UV radiation increases ROS production and depletes antioxidants. These free radicals mainly target guanine in DNA. ROS contribute to gene mutations, DNA strand breaks, aging, mitochondrial damage, organelle membrane damage, and functional damage to proteins (including DNA repair proteins), all of which can promote carcinogenesis [3].

Sunscreens are defined as substances that protect the skin from the harmful effects of solar UV radiation by absorbing, reflecting, scattering, or otherwise deflecting UV photons, thereby preventing their absorption by the skin [5]. They contain one or more UV filters, which may be physical, chemical, or in combination. In addition, they also contain other excipients such as plasticizers, preservatives, emulsifiers, stabilizers, colorants, or fragrances [6].

UV filters, which provide a protective effect, are defined in the Cosmetic Regulation of the European Commission (EC) as “substances which are exclusively or principally intended to protect the skin against certain UV radiation by absorbing, reflecting or scattering UV radiation” [7]. Sunscreen products are subject to different requirements and regulations in different countries. In comparison, while the FDA dates 16 approved filters, the EC currently registers up to 27. This discrepancy is due to different inspection and safety requirements [8].

Photoprotective cosmetics are designed to shield the skin from the damaging effects of ultraviolet (UV) radiation. The active components responsible for this protection are UV filters, which function through either chemical or physical mechanisms [6]. Chemical filters, also known as organic or molecular filters, absorb, transform, or block UV radiation, preventing its penetration into the skin [9]. These are typically aromatic compounds conjugated to a carbonyl group [10]. To ensure broad-spectrum protection, photoprotective cosmetics often incorporate a combination of these filters [11]. Organic filters are categorized into classes such as para-aminobenzoic acid (PABA) derivatives, benzophenones, camphor derivatives, salicylates, cinnamates, and others [10]. Common traditional UVA filters include oxybenzone, meradimate, and avobenzone [12], while newer chemical filters encompass Mexoryl SX, Mexoryl XL, Tinosorb M, and Tinosorb S [13]. The basic physicochemical characteristics of the chemical UV filters used in the formulations are summarized in

Table 1. The characterization of the chemical filters used in sunscreens.

Commercial Name	Tinosorb®S	Tinosorb®M	Eusolex®/ Uvinul® MC 80	Eusolex® OS
Chemical name	Bemotrizinol	Bisoctrizol	Octinoxate	Octisalate
State	Solid	Solid	Liquid	Liquid
Molecular formula	C38H49N3O5	C41H50N6O2	C18H26O3	C15H22O3
CAS *	187393-00-6	103597-45-1	5466-77-3	118-60-5
Molecular weight (g·mol−1)	627.8	658.9	290.4	250.3
Melting point (°C)	>400	195	˂−13	˂25
Log P	80.4	12.7	5.8	5.9
Solubility in water (mg·L−1)	0.339	Insoluble	0.200	0.074
Function	UVA/UVB filter	UVA/UVB filter	UVB filter Anti-aging agent	UVB filter Fragrance

* Chemical Abstracts Service Number.

. Physical filters, on the other hand, create a physical barrier on the skin that scatters or absorbs UV light. Traditional physical filters include zinc oxide and titanium dioxide. These are frequently combined with chemical filters to enhance overall protection [14].

The efficacy of a sunscreen is expressed as the sun protection factor (SPF), which is defined as the ratio of the UV energy required to produce the minimum erythema dose (MED) on protected skin to the UV energy required to produce the MED on unprotected skin [15]. The minimum erythema dose is defined as the lowest time interval or dose of exposure to UV light sufficient to produce minimal, perceptible erythema on unprotected skin [16].

The aim of this experimental study was to evaluate the effect of the substitution of Polysorbate^® 80 with Beautyderm^® as an emulsifier on the physical characteristics of the products and their final SPF. A further experimental objective was to evaluate how increasing concentrations of the booster in sunscreen formulations affect their quality and final SPF and to verify whether the spectrophotometric method for determining the SPF is independent of the sample homogenization time in ethanol. Recognizing the importance of not only formulating sunscreens with high sun protection efficacy but also ensuring their stability and aesthetic appeal, we have also focused on these aspects in this study.

2. Materials and Methods

2.1. Materials

Tinosorb^® M, Tinosorb^® S, and Uvinul^® MC 80 were purchased from BASF (Mannheim, Germany). Eusolex^®, Eusolex^® OS, Homosalate^®, and Span^® 80 were purchased from Merck KGaA (Darmstadt, Germany). Sunhancer^TM Eco SPF Booster was purchased from Lubrizol (Hamburg, Germany). Wool fat (Adeps lanae, Ph. Eur.), stearinum (stearic acid), beeswax (Cera alba, Ph. Eur.), glycerol 85% (Ph. Eur.), olive oil (Olivae oleum raffinatum, Ph. Eur.) and zinc oxide, ZnO, (Zinci oxidum, Ph. Eur.) were purchased from Galvex (Banska Bystrica, Slovakia). Polysorbate^® 80 and ethyl alcohol 96%, Ph. Eur., were from CentralChem (Bratislava, Slovakia). Beautyderm^® K10 was purchased from Handymade (Prievidza, Slovakia). Cetyl alcohol (Alcohol cetylicus, Ph. Eur.) was purchased from Galvex. Tocoferoli alfa acetas (tocopherol) was purchased from Fagron (Olomouc, Czech Republic). Ambiderman^® HBF was from Herbacos Recordati (Pardubice, Czech Republic). The purified water was prepared at the Department of Galenic Pharmacy Comenius University by reverse osmosis using LabAqua Bio from Biosan (Riga, Latvia).

2.2. Sunscreen Formulation

The composition of the examined formulations (F1–F8) is shown in

Table 2. The composition of sunscreens.

The Ingredient	F1 (%)	F2 (%)	F3 (%)	F4 (%)	F5 (%)	F6 (%)	F7 (%)	F8 (%)
Olive oil	16.30	15.80	15.30	15.30	15.30	15.30	15.00	15.00
Polysorbate® 80	5.40	5.40	5.40	-	-	-	-	2.50
Beautyderm® K10	-	-	-	7.00	7.00	7.00	0.50	4.00
Stearin	8.10	7.90	7.70	8.10	7.90	7.70	8.00	8.00
Tinosorb® M	6.00	6.00	6.00	6.00	6.00	6.00	10.00	10.00
Tinosorb® S	3.40	3.40	3.40	3.40	3.40	3.40	10.00	10.00
Eusolex®	7.00	7.00	7.00	7.00	7.00	7.00	10.00	10.00
Eusolex® OS	4.80	4.80	4.80	4.80	4.80	4.80	5.00	5.00
Wool fat	8.10	7.90	7.70	8.10	7.90	7.70	10.00	10.00
Uvinul® MC 80	7.00	7.00	7.00	7.00	7.00	7.00	-	-
Booster	1.00	3.00	5.00	1.00	3.00	5.00	3.00	3.00
Purified water	27.30	26.50	25.8	27.22	25.62	24.02	13.00	14.50
Tocopherol	0.08	0.08	0.08	0.08	0.08	0.08	3.00	3.00
Span® 80	-	-	-	-	-	-	2.50	-
Cetyl alcohol	-	-	-	-	-	-	5.00	-
Glycerol (85%, w/w)	5.32	5.22	5.10	5.00	5.00	5.00	5.00	5.00

. The general procedure of their preparation lies in the melting of all lipophilic components (olive oil, stearin, wool fat, Tinosorb^® S, Eusolex^®, Eusolex^® OS, Beautyderm^®, Uvinul^® MC 80, Span^® 80, cetyl alcohol, and tocopherol) together in a mortar using an infrared lamp. Subsequently, hydrophilic components (purified water, glycerol, Polysorbate^® 80, and Tinosorb^® M) were dissolved in a beaker, heated to 60 °C, and added to the lipophilic phase gently while stirring constantly so that the system was emulsified to form a sunscreen. The resulting cream was stirred continuously until it cooled to the desired temperature. Subsequently, the booster was added in batches to ensure uniform distribution.

2.3. Sectrophotometric Determination of SPF

The assessment of the SPF of the samples was based on the methodology by Mbang et al. [1]; 1.0 g of the sample was transferred into a wide-mouth flask and replenished by ethanol (96%) to 100.0 g. The neck of the flask was covered with cellophane and allowed to sonicate by the SonoPlus UV 3100 (Bandelin electronic, Berlin, Germany) for 5 min, followed by stirring on a magnetic stirrer for 15 min (at 1000 rpm). The content of the flask was filtered through cotton wool. A part of the filtrate (5 mL) was pipetted into a volumetric flask (100 mL). The volume was replenished to 100 mL with ethanol (96%). Again 5 mL of the solution was taken and transferred into a volumetric flask (25 mL), which was topped up with ethanol (96%). The sample thus prepared was subjected to spectrophotometric determination in the wavelength range from 290 to 320 nm while ethanol (96%) was used as a blank. In this way, absorbance at 290–320 nm (each 5 nm) was measured for all samples in five repetitions. The SPF was calculated according to Equation (1) [1]:

$$SPF=CF\sum _{290}^{320}EE\left(\mathsf{\lambda }\right)·I\left(\mathsf{\lambda }\right)·A\left(\mathsf{\lambda }\right)$$

(1)

where EE is the erythemal effect spectrum, I is the solar intensity spectrum, A is the absorbance of the sunscreen, and CF is the correction factor. The multiples of EE and I for each wavelength are the predetermined constants shown in

Table 3. The predetermined multiples of the EE (erythemal effect) and I (solar intensity) at different wavelengths [1].

Wavelength (nm)	EE ∙ I
290	0.0150
295	0.0817
300	0.2874
305	0.3227
310	0.1864
315	0.0837
320	0.0180

.

2.4. Texture Analysis

Texture profiles of the sunscreens were examined in compression mode using a TA.XT Plus texture analyzer (Stable Micro System, Godalming, UK) at room temperature (25 °C). A measuring probe of 12 mm diameter penetrated to 5 mm depth of the sample (10 g) at a speed of 1 mm/s while the cycle was repeated twice. Data collection and data analysis were performed using Texture Exponent software (Version 6.1.11.0, Godalming, UK). Parameters such as hardness, compressibility, cohesiveness, and adhesiveness were determined from the force–time plots. The obtained data are expressed as the mean of three parallel measurements ± SD.

2.5. Rheological Behavior

A rotational viscometer RheolabQC (Anton Paar, Graz, Austria) was used to determine the rheological behavior of the prepared formulations. Approximately 2 mL of the sample was placed in the slit of a stationary cylinder and connected to the measuring system. The sample was allowed to temper to 25 °C for 20 min. The viscometer measured the main rheological parameters: dynamic viscosity and shear stress at increasing shear rates from 6.45 s⁻¹ to 650 s⁻¹ in the first interval and then at decreasing shear rates in the second interval. Data collection and analysis were carried out using the software RheoCompass^TM, Version 1.31.69 (Anton Paar).

The flow behavior index (n) was calculated using linear regression on the log-transformed data of the shear stress (τ) measured in Pascals (Pa) and the shear rate (γ) measured in reciprocal seconds (s⁻¹). The consistency index K, expressed in Pa·sⁿ, is derived from Equation (2):

$$\log \left(\mathsf{\tau }\right)=\log \left(K\right)+n·\log \left(\mathsf{\gamma }\right)$$

(2)

2.6. Microscopic Structure

Microscopic observation was performed with the optical microscope Leica (VWR, Prague, Czech Republic) using imaging software Proview (Optica Microscopes, Ponteranica, Italy) at 40- and 100-fold magnification without prior staining or any other treatment of the samples.

2.7. Stability

The stability of the sunscreens was tested by the accelerated stability test, i.e., the centrifuge stress test. Due to the capacity limitations of the laboratory centrifuge, the test [17] was slightly modified. Approximately 10.0 g of each sunscreen was weighed into the test tube and left to centrifuge for 5 min at 5000 rpm. After this time, the samples were removed and any changes in the structure were observed.

2.8. Sensory Analysis

Sensory analysis of the best formulation was performed according to Plocica et al. [18]. Based on organoleptic tests through smell, sight, and touch, 10 volunteers evaluated the 8 attributes of the top formulation: appearance, fragrance, cushion effect, adhesion, spreadability, stickiness, residual, and absorption. This is a subjective scoring of the given parameters by the individuals from 1 to 5, with 1 representing the least satisfactory attribute. The average value of each attribute was projected onto a graph.

2.9. Effect of Zinc Oxide Addition on the Physical Properties of the F8 Formulation

The texture and rheological behavior of the F8 formulation with added zinc oxide (F8 + ZnO) were evaluated following the procedures outlined in Section 3.3 and Section 3.4.

2.10. Statistical Analysis

The obtained data are expressed as mean values of three parallel measurements (n = 3) ± standard deviation (SD). The data were compared using the Student’s t-test. p values of 0.05 or less were considered statistically significant, indicated in the graphs and tables by the symbol (* p < 0.05; ** p < 0.01; *** p < 0.001). The triplets F1 to F3, F4 to F6, and the pair F7 versus F8 were compared.

3. Results

3.1. Sunscreen Formulation

In sunscreen formulations, excipients such as emollients, emulsifiers, thickeners, humectants, antioxidants, and solvents play critical roles by imparting specific functional benefits. Typically, creams consist of two immiscible phases, aqueous and oily, which are stabilized through the incorporation of emulsifiers. In the formulations studied, the aqueous phase comprises purified water and glycerol, with glycerol functioning as a humectant to enhance skin hydration. Among the chemical UV filters evaluated, only Tinosorb^® M exhibited significant hydrophilicity, warranting its dissolution in the aqueous phase. Conversely, hydrophobic excipients such as stearin and cetyl alcohol serve as thickeners, contributing to the formulation’s viscosity, texture, and overall structural integrity. Emollients and oils, including olive oil and wool fat, provide essential skin conditioning properties by softening and moisturizing the skin. Additionally, these lipophilic components facilitate the solubilization and uniform dispersion of chemical UV filters (Tinosorb^®M, Tinosorb^®S, Eusolex^®, and Eusolex^®OS) as well as the booster within the oil phase, thereby enhancing their photoprotective efficacy. Emulsifiers employed in the formulations—Polysorbate^® 80, Beautyderm^® K10, and Span^® 80—are unavoidable for stabilizing the emulsion. They promote the homogenous mixing of the aqueous and oily phases, ensuring product consistency and preventing phase separation. Beautyderm^® K10 is an emulsifier based on olive oil, primarily composed of a combination of surface active substances, including cetearyl glucoside, sorbitan olivate, and cetearyl alcohol with an emollient effect. Tocopherol was included as an antioxidant to enhance the stability of the formulation and protect against oxidative degradation. The prepared sunscreens exhibited a color range from creamy to yellowish. Higher concentrations of the booster resulted in a more intense yellow discoloration, which could potentially deter consumers from using the product. To balance this issue, only 3% (w/w) of the booster was used in the formulations of F7 and F8. This approach allowed for an improvement in the SPF value while maintaining an acceptable level of discoloration. Additionally, Polysorbate^® 80 was replaced with Beautyderm^® of a neutral fragrance in formulations F4 to F6. While the commonly recommended range for Polysorbate^® 80 is 1–3%, in F1–F3, an increased concentration of 5.4% was necessary to achieve adequate emulsion stability. During preliminary formulation trials, lower concentrations of Polysorbate^® 80 resulted in phase separation and visible instability, likely due to the specific composition, i.e., the presence of multiple chemical UV filters. Increasing the emulsifier concentration to 5.4% effectively stabilized the emulsion system, preventing phase separation. Higher concentrations (around 5–6%) are documented in the literature [19,20]), particularly in complex sunscreen formulations with high oil content or multiple UV filters.

The combinations of emulsifiers in F7, including the addition of cetyl alcohol, were selected based on the calculation of the hydrophilic–lipophilic balance (HLB). This was based on the knowledge that the system is stable when the oil phase has approximately the same HLB as the mixture of emulsifiers, considering their percentage in the composition. The composition of F8 was optimized several times. Originally, we wanted to maintain a 1:1 ratio of Polysorbate^® 80 and Beautyderm^®, thereby reducing the content of the less pleasant-smelling Polysorbate^® 80, but we found that for the phases to emulsify, it was necessary to increase the amount of emulsifier. We chose the one with a more pleasant smell, i.e., Beautyderm^®.

3.2. Spectrophotometric Determination of SPF

To verify the dependence of the measured SPF value of the samples on the time of mixing the samples in ethanol, the F1, F2, and F3 formulations were selected. The sample preparation procedure was like that described in Section 2.3: spectrophotometric determination of the SPF, with the difference that the samples were stirred using a magnetic stirrer for 15, 30, or 45 min after a previous 5 min of sonication. The measured values are provided in Table S1, which is included in the Supplementary Material. They are expressed as the mean of 15 measurements (SPF₁₅ + SPF₃₀ + SPF₄₅) ± SD. At the same time, the coefficient of variation (CV), which is the ratio of the SD to the mean value, was calculated. A lower CV indicates less variability. Generally, in all cases, it was less than 0.02. The obtained data are clearly summarized and presented in Figure 1.

The control sample for F1, i.e., without the booster, exhibited an SPF of 44.15 ± 0.08, which is approximately one unit lower than the sample containing the lowest concentration of the booster (F1), which had an SPF of 45.09 ± 0.82.

3.3. Texture Analysis

The texture parameters, such as hardness, compressibility, adhesiveness, and cohesiveness (

Table 4. The main texture parameters characterized by texture analysis.

	Hardness (g)	Compressibility (g.s)	Adhesiveness (g.s)	Cohesiveness
F1	12.81 ± 0.10	45.24 ± 0.44	−11.25 ± 0.10	0.96 ± 0.00
F2	11.07 ± 0.10	40.05 ± 0.39	−10.62 ± 0.11	1.03 ± 0.01
F3	9.25 ± 0.09	33.01 ± 0.32	−8.54 ± 0.09	1.05 ± 0.01
F4	14.25 ± 0.14	50.79 ± 0.51	−11.30 ± 0.10	0.97 ± 0.10
F5	14.93 ± 0.14	51.83 ± 0.50	−14.38 ± 0.13	1.09 ± 0.01
F6	34.64 ± 0.30	110.07 ± 1.09	−24.24 ± 0.23	0.84 ± 0.00
F7	8.84 ± 0.61	31.25 ± 1.94	−0.05 ± 0.00	0.98 ± 0.01
F8	8.36 ± 0.32	29.82 ± 0.48	−0.05 ± 0.00	1.00 ± 0.07
F8 + ZnO	7.88 ± 0.12	28.17 ± 0.20	−0.05 ± 0.00	1.03 ± 0.00

), help to predict how the product behaves on the skin upon application. The texture profiles of the sunscreens, as illustrated in Figure S1, reveal distinct differences between formulations containing Polysorbate^® (F1, F2, and F3) and those with Beautyderm^® (F4, F5, and F6).

The analysis of the effects of increasing concentrations of boosters in Polysorbate^®-containing sunscreens (F1 to F3) and Beautyderm^®-containing sunscreens (F4 to F6) reveals notable differences in texture profiles, as illustrated in Figure S1a,b. However, when examining the hardness values of the individual formulations, the results present a paradox. In the Polysorbate^®-containing formulations (F1 to F3), an increase in booster concentration leads to a decrease in hardness. Conversely, for the Beautyderm^®-containing formulations (F4 to F6), the hardness increases with higher booster concentrations. Among the samples, the sunscreen F6 exhibited the highest compressibility, while the others averaged lower values. Notably, formulation F6, which contained the highest amount of the booster, also demonstrated superior adhesiveness compared to samples F7 and F8, which recorded the lowest adhesiveness values. Regarding cohesiveness, all formulations achieved values close to one, indicating high deformation resistance of the structure [21]. The texture profiles for formulations F7 and F8 do not differ significantly despite significant compositional differences (see Figure S1c).

3.4. Rheological Behavior

The results obtained from this study dealing with the rheological behavior of the prepared formulations are summarized in

Table 5. Basic characteristics of rheological behavior of the sunscreens—hysteresis loop area (Pa·s⁻¹), consistency index (K), and flow index (n) at ascending (+) and descending (−) torque.

	K+ (Pa·sn)	K− (Pa·sn)	n+	n−	Hysteresis Loop Area (Pa·s−1) 103
F1	74.13 ± 0.31	5.31 ± 0.46	0.22 ± 0.00	0.55 ± 0.00	97.36 ± 0.01
F2	37.67 ± 5.21	12.47 ± 0.85	0.31 ± 0.00	0.45 ± 0.00	35.26 ± 0.10
F3	30.33 ± 0.20	14.22 ± 0.11	0.38 ± 0.00	0.50 ± 0.00	18.27 ± 0.85
F4	8.09 ± 0.19	7.24 ± 0.17	0.62 ± 0.00	0.63 ± 0.00	6.47 ± 0.00
F5	23.49 ± 0.18	3.88 ± 0.11	0.42 ± 0.00	0.71 ± 0.00	11.64 ± 0.32
F6	30.99 ± 0.23	3.67 ± 0.18	0.40 ± 0.00	0.75 ± 0.01	19.11 ± 0.52
F7	60.59 ± 0.39	15.79 ± 0.23	0.34 ± 0.00	0.55 ± 0.00	31.58 ± 1.80
F8	29.04 ± 0.18	5.98 ± 0.90	0.36 ± 0.00	0.60 ± 0.00	19.49 ± 0.80
F8 + ZnO	19.91 ± 0.24	7.57 ± 1.15	0.43 ± 0.00	0.59 ± 00	8.50 ± 0.50

. As can be seen, the formulations F1 and F7 exhibit significantly high viscosity at the beginning of the measurement at a shear rate of 6.45 s⁻¹ (i.e., 15 486.2 mPa·s and 19 555.2 mPa·s). The sunscreens have strong structural characteristics, leading to greater resistance to flow under stress. Other formulations except F4 show moderate viscosity η⁺ (10,150.3 ± 1132.7 mPa·s), indicating a more balanced rheological behavior.

All sunscreens exhibit thixotropic behavior manifested by a reduction in viscosity and the consistency index measured at a shear rate of 6.45 s⁻¹ at ascending and descending torque.

In formulations that contain Polysorbate^®, we observe an interesting paradox: as the amount of the booster increased (F1 < F2 < F3), the viscosity of the sunscreen decreased. This may suggest an incompatibility between the booster and Polysorbate^®. In contrast, for Beautyderm^®-based formulations (F4 to F6), the viscosity increased as the amount of booster was enhanced.

As can be noticed in Figure 2 showing the rheological behavior (the viscosity—the shear rate curve), the formulations exhibit a time-dependent flow, with viscosity decreasing due to shear stress; that is, in all cases the systems are thixotropic, with the hysteresis loop being the largest in the Polysorbate^®-containing sunscreen F1 and Beautyderm^®/Span^®-based sunscreen F7.

3.5. Microscopic Analysis

Microscopic images (Figure 3) show the internal structure of the emulsion system. The electrical conductivity tests confirmed that all the prepared formulations represent the emulsion type o/w, i.e., the dispersed oil phase in the form of droplets of 10–50 µm size can be seen in the images. Following the dispersion of the droplet size of the internal phase, the sunscreens containing Beautyderm^® appear to be more uniform. At the same time, however, insoluble particles can be observed in these formulations, dark formations of irregular shape, probably corresponding to the booster, indicating that the Polysorbate^® provides better solubilization than Beautyderm^®.

3.6. Sensory Analysis

Organoleptic testing was performed to assess the texture, appearance, and aroma of the selected sunscreen formulations. A panel of ten volunteers participated in the evaluation. Eight key sensory attributes of the final formulations were assessed as follows: appearance, fragrance, cushion effect, adhesion, spreadability, stickiness, residue on the skin, and absorption, in accordance with the methodology described by Wojciechowska et al. (2021) [18]. Each volunteer tested two samples—F8 and F8 containing zinc oxide (F8 + ZnO)—following standardized instructions for each attribute (Table S2). Evaluations were conducted using a five-point scale, where one indicated the least satisfactory and five the most satisfactory results [18]. The mean scores for each attribute were calculated and graphically represented in Figure 4.

The cushion effect refers to the perceived amount of cream felt between the thumb and forefinger during rubbing. To assess this, the sunscreen sample was applied to the forefinger, then rubbed against the thumb, and the perceptible amount was evaluated. Adhesion refers to how well the cream adheres to the finger and maintains its shape. This was tested by applying the sunscreen to a finger and observing its consistency and shape retention. Spreadability measures how easily the cream spreads on the skin. The sample was applied to the inner arm, and the resistance encountered during spreading was evaluated. Stickiness indicates whether the sunscreen leaves a sticky residue after application. This was assessed by applying the sample to the inner arm and evaluating stickiness by touch. Residual feeling assesses the presence of a greasy layer remaining on the skin some time after application. Absorption refers to how quickly the cream is absorbed into the skin. Appearance was evaluated visually, considering factors such as texture and uniformity, while fragrance was assessed based on scent intensity and pleasantness. The table provided to the evaluating participants as a guide to perform the sensory analysis in their native language is available in the Supplementary Material (Table S2) [18].

3.7. Effect of Zinc Oxide Addition on the Physical Properties of the F8 Formulation

The rheological behavior of both formulations is compared in Figure 5. Both samples exhibit a reduction in viscosity as the shear rate increases, indicating shear-thinning (pseudoplastic) behavior. The addition of ZnO results in a slight reduction in viscosity, which is most noticeable at low shear rates. At higher shear rates, the behavior of both samples becomes nearly identical.

The measured data are included in Table 4 and Table 5. The texture profiles of the samples with and without zinc oxide show only minor differences (see Supplementary Material—Figure S2).

4. Discussion

Spectrophotometric determination of SPF: We implemented the spectrophotometric method for determining the SPF as outlined by Mbang et al. [1] for the first time in our research. Following a thorough theoretical and practical familiarization with this technique, we proceeded to analyze initial pilot samples. This process raised questions regarding whether varying extraction times in ethanol could influence the resulting SPF values. Consequently, our primary objective was to investigate the differences in SPF values when samples were extracted and mixed in ethanol for durations of 15, 30, or 45 min, following an initial 5 min ultrasonication. To optimize resource utilization, this investigation was conducted exclusively with samples F1–F3. The results indicated minimal differences across all tested conditions, confirmed by statistical analysis to be non-significant (p > 0.05). This finding suggests that the extraction time in ethanol does not significantly affect the spectrophotometric determination of SPF values in the tested formulations.

Additionally, we observed that in both sets of creams (F1–F3 and F4–F6), an increase in the concentration of the booster corresponded with a rise in SPF values. Specifically, increasing the booster concentration by 4% (w/w) resulted in an average SPF increase of approximately 3.3 ± 0.2 for both groups. Given that the booster we used imparted a yellow color to the formulations, we opted for a more moderate approach in the subsequent formulations (F7 and F8), using a concentration of 3% (w/w) in each of them.

Several commonly used organic UV filters have been detected in various aquatic organisms, raising concerns about their potential environmental impacts. A significant challenge has emerged to reduce the amounts of UV filters in sunscreen formulations while still maintaining high sun protection efficacy [22]. Typically, the SPF-enhancing ability of boosters stems from the production of pigments or phenolic compounds, which can block or absorb the sun’s UV radiation [23].

Boosters can generally be categorized into two main types: natural boosters, such as coffee oil, spruce bark stilbenes, and lignin-based substances, and synthetic polymer-based boosters. Among the synthetic options, Sunhancer™—which we employed in our formulations—along with SunSpheres™, is commercially available. A notable drawback of these synthetic boosters is their poor photostability, primarily due to the presence of organic components that absorb UV radiation [22].

The Sunhancer™ Eco SPF Booster consists of micronized particles made from a blend of natural and sustainably sourced waxes, specifically Carnauba wax (Copernicia cerifera wax) and Rice bran wax (Oryza sativa wax) [24]. The Brazilian palm (Copernicia cerifera) has leaves coated with a waxy layer. After extraction, this material serves as the raw source for Carnauba wax production. Carnauba wax is utilized in organic UV filters due to its high content of cinnamic acid derivatives, such as aliphatic diesters of p-methoxycinnamic acid and hydroxycinnamic acid. Studies on isolated cinnamate-rich fractions from Carnauba wax have demonstrated notable UV absorption capacity and antioxidant activity [25]. Rice (Oryza sativa) contains various bioactive compounds, including flavonoids, carotenoids, phytosterols, and phenolic compounds. Among these, flavonoids are known to provide protective effects against ultraviolet radiation. The incorporation of this SPF booster into sunscreen formulations allows for a reduction in the concentration of chemical UV filters while maintaining equivalent levels of sun protection [26]. Additionally, this natural booster improves the texture of the sunscreen, reducing greasiness, and is both biodegradable and environmentally non-persistent [27]. Due to its presence in a cream basis, the risk of aggregation and crystallization of UV filters is reduced, which often limits their efficacy.

Although formulations F2, F5, F7, and F8 were all prepared with the same concentration of the booster, notable differences in their measured SPF values were observed (see Figure 1). These findings highlight that the final SPF is influenced not only by the presence of an SPF booster but also by the overall composition of the formulation, particularly the choice and combination of emulsifiers and UV filters. The higher SPF observed in F7 compared to F2 and F5 can be attributed to several factors. First, F7 contains a combination of emulsifiers (Polysorbate^® 80 + Beautyderm^®), whereas F2 and F5 rely on single emulsifiers. The synergistic effect of emulsifier combinations likely improved the dispersion and film-forming properties of the UV filters and booster particles, resulting in a more uniform dermal product. Additionally, F7 and F8 were formulated with the maximum recommended concentrations of chemical UV filters, contributing to the elevated SPF values, at least in the case of formulation F7. Not only do emulsifiers enhance the distribution of UV filters, but, as noted by Santos et al. [28], the emulsification technique itself also has a significant impact on the photoprotective efficacy of the final product. The comparison of the SPF of the samples with identical booster concentrations revealed statistically significant differences between all tested pairs (F1 vs. F4, F2 vs. F5, and F7 vs. F8), except for the F3 vs. F6 formulation (Figure 1). These results demonstrate that formulation composition substantially impacts the booster’s effectiveness.

Additionally, certain inorganic filters like zinc oxide and titanium dioxide can also function as boosters. However, caution is advised when combining organic and inorganic filters, as titanium dioxide may lead to the photodegradation of organic filters or the generation of harmful free radicals [22].

Texture analysis: Texture analysis of dermal semisolids serves as a valuable tool for understanding, optimizing, and ensuring the quality of products in the pharmaceutical and cosmetic industries. This method, used in both research and product development, is helpful for the evaluation of the basic texture parameters, such as consistency, elasticity, and spreadability, providing information about how the product behaves when applied to the skin, which is after all the key moment influencing the consumer’s choice of a given product [29].

Formulations F1 to F3 exhibited lower hardness values compared to those containing Beautyderm^® (F4 to F6). This suggests that the Polysorbate^®-containing sunscreens are softer and potentially easier to spread on the skin. The compressibility values further support this observation; lower compressibility in Polysorbate^®-containing formulations indicates a more favorable texture for consumer application. In contrast, Beautyderm^®-containing sunscreens showed increased hardness and compressibility, which may affect their perceived comfort during use.

Adhesiveness is a crucial factor influencing how well a sunscreen stays on the skin after application. The area above the negative peak in the first cycle indicates adhesiveness, defined as the energy needed to separate the sample from the probe. This area represents the cohesive forces between the molecules of the sample. A larger area value suggests that greater energy is necessary to pull the sample away from the probe during retraction, which implies that the cohesive forces within the sample are stronger [30].

However, cohesiveness was notably higher in Polysorbate^® formulations, suggesting that they maintain their structure better during application. This characteristic could enhance consumer satisfaction as it implies a smoother application without residue.

The texture attributes directly correlate with consumer preferences in skincare products. Products that feel pleasant and are easy to apply are more likely to be favored by consumers. The findings suggest that formulations with Polysorbate^® may be more appealing due to their softer texture and better spreadability.

Rheological behavior: Based on the data summarized in Table 5, it can be argued that all formulations exhibit shear-thinning (pseudoplastic) behavior (n is less than one) [31] with a relatively high consistency index at the first measurement (K⁺), which decreases in the second measurement cycle at decreasing speeds, which is the so-called manifestation of thixotropy. The most pronounced reorganization of the internal structure quantified as the hysteresis loop area was observed for formulation F1. The data are presented in Table 4; Table 5 shows a direct correlation between viscosity and hardness for nearly all formulations. The only exceptions are formulations F7 and F8. Texture analysis revealed similar hardness values for both formulations (Table 4), but formulation F7 exhibited significantly higher viscosity (Table 5).

Our assumption of the beneficial properties of sunscreens with thixotropy is based on the observation that these creams spread more easily on the skin. This improved application occurs because the cream becomes less viscous when subjected to stress, such as gentle circular movements or skin massage. Once the application ceases, the cream returns to its original, thicker consistency, which enhances stability and ensures the forming of a photoprotective film on the skin surface [32]. However, this perspective contradicts findings from a study by Gaspar and Compos [33], which suggests that sunscreens with lower thixotropic properties tend to offer better SPF performance.

This claim does not align with all our findings. The F4 formulation, which exhibits the smallest hysteresis loop area, does not correspond to the highest SPF value. However, for the trio of formulations F1, F2, and F3, we observed a decrease in the thixotropy of the formulations, respectively, as indicated by the decrease in the hysteresis loop area, while simultaneously noting an increase in the SPF values measured. Therefore, we can conclude that there is no clear relationship between thixotropy and the SPF.

Both texture analysis and rheological measurements provide complementary insights into the physical stability of the formulations [34]. For instance, Maruyama et al. [35] found that the storage modulus G′ was significantly correlated with hardness and viscosity, suggesting a strong relationship between these parameters. Regarding the stability of the products estimated from their rheological properties, high Δη values imply a slowing down of the phase separation, which means an improvement in the shelf-life of the formulation [36].

One notable disadvantage of texture analysis is that the texture parameters are highly dependent on the specific measurement conditions, such as the probe speed, probe diameter, probe material, and depth of penetration [34]. As a result, the reproducibility and consistency of these data can be challenging, making it difficult to obtain reliable measurements upon re-evaluation. A similar issue arises with rheological analysis, where variations in testing conditions can also impact results. Based on these considerations, a simpler and more straightforward method for assessing the physical stability of sunscreens may be to monitor changes in pH over a defined time.

Microscopic analysis: Figure 3 illustrates the variations in the microscopic structure across the different formulations. Notably, F1 and F2 exhibit relatively similar variances in internal phase droplet sizes. However, F3 shows an increased size variance, likely attributable to the higher concentration of the booster. In the Beautyderm^®-containing formulations (F4 to F6), dark regions are visible, most prominently in F4. These are likely agglomerates resulting from the interaction between the booster and Beautyderm^®. Formulations F7 and F8 share a comparable microscopic structure despite the content of various emulsifiers. The addition of ZnO (1%; w/w) to F8 did not induce substantial alterations. While ZnO-containing sunscreens can interact with excipients or form agglomerates, potentially modifying the viscosity of the final product [32], this phenomenon was not observed microscopically in our formulation.

Sensory analysis: While sensory analysis may not provide an entirely objective evaluation of creams based on individual sensations such as touch, smell, and sight, it plays a crucial role in pre-formulation studies for manufacturers. By leveraging sensory analysis, manufacturers can gain valuable insights into consumer preferences, enabling them to predict the potential popularity of a cream among end-users. This approach not only aids in product development but also aligns formulations with consumer expectations, ultimately enhancing market success. Morávková and Stern [37] refer that rheological analysis was found to be more effective than sensory analysis for testing storage stability.

A sample that exhibited the best stability during the stress test in the centrifuge, namely F8, underwent sensory analysis. Additionally, ZnO (1% w/w) was incorporated into the formulation to enhance sun protection. Zinc oxide, along with titanium dioxide, is classified as a physical filter, providing sun protection through the scattering and reflection of UV radiation [38].

We prioritized the use of ZnO due to its suitability for sensitive skin, including conditions such as rosacea, psoriasis, or eczema [39]. Moreover, while titanium dioxide primarily blocks UVB rays, ZnO offers protection against both UVA and UVB rays [40]. However, a significant drawback of using physical filters is the white residue they can leave on the skin after application. To mitigate this issue, these ingredients are often utilized in micronized forms or encapsulated in nanoparticles [41]. The ability of these particles to attenuate UV radiation is influenced by the surrounding medium and size-related particle properties [38].

An important consideration when using titanium dioxide (TiO₂) or ZnO nanoparticles is the risk of their penetration into the systemic circulation, potential accumulation in organs, and the production of ROS [39]. Safety guidelines from the FDA stipulate that ZnO and TiO₂ nanoparticles must demonstrate aggregation in sunscreen formulations to be classified as Generally Recognized As Safe (GRAS) [42]. The use of powdered forms of TiO₂ and ZnO in photoprotective formulations is still considered safe [39].

By adding ZnO to formulation F8, we aimed to enhance sun protection since it was determined that formulation F7 had a lower SPF despite performing better in the stress test. The graph for sensory analysis indicates that the addition of zinc oxide only slightly affected the tested attributes/characteristics, suggesting minimal differences between F8 and F8 + ZnO. Most attributes received an average rating from independent evaluators.

Effect of zinc oxide addition on the physical properties of the F8 formulation:

As summarized in Table 4, the primary textural parameters, including hardness, compressibility, adhesiveness, and cohesiveness, did not exhibit significant changes upon ZnO incorporation. Statistical analysis using the Student’s t-test confirmed that differences in texture were generally non-significant, except for hardness. This observation is further supported by the texture profile curves for each formulation presented in Figure S2. Notably, hardness was slightly decreased due to ZnO addition.

This reduction in hardness aligns with the observed changes in rheological behavior, where the F8 + ZnO formulation demonstrated a decrease in viscosity. Specifically, viscosity measurements at a shear rate of 6.45 s⁻¹ revealed a drop from 9235.1 ± 84.1 mPa·s (F8) to 7369.4 ± 47.3 mPa·s (F8 + ZnO) at ascending torque. The viscosity versus shear rate profiles are compared in Figure 5.

Although ZnO is insoluble in water and alcohol, it may exhibit partial solubility or dispersion within certain lipophilic components of the cream base. A similar decrease in viscosity has been reported in zinc oxide pastes [43], explained by the presence of glycerol. It is also a constituent of the F8 formulation. Generally, the presence of humectants like glycerol or propylene glycol can modulate viscosity changes. This suggests that interactions between zinc oxide and glycerol or other formulation components may contribute to the observed viscosity drop.

Stability testing: The stability of sunscreen formulations is critical to ensuring their efficacy and safety throughout their shelf life. Several factors influence stability, including temperature, light exposure, and interactions between active ingredients and excipients. To guarantee product reliability, comprehensive stability testing is required, either under normal storage conditions or by accelerated tests at elevated temperatures and humidity levels. Physical stability is assessed by evaluating parameters such as appearance, viscosity, and pH over prolonged storage or after exposure to stress conditions. These stress conditions may include elevated temperatures, high humidity, or centrifugal force [44]. Such tests help identify potential issues like phase separation or changes in texture that could compromise the product’s performance.

In our study, the physical stability of the formulations was evaluated using a centrifuge stress test. This method applies centrifugal force to simulate long-term storage conditions in a shorter time frame. For formulations F1 to F6, a small drop of water was observed on the surface of the sunscreens after testing, indicating phase separation and instability. It can be assumed that the elimination of this manifestation would require an increase in the amount of emulsifier. In contrast, formulations F7 and F8 remained stable with no visible signs of instability following the test. These results suggest that F7 and F8 exhibit superior physical stability under stress conditions compared to the other formulations.

To stabilize the lipophilic phase of the sunscreens, tocopherol, i.e., vitamin E was used. Although vitamin E and its derivatives are the most widely used antioxidants in sunscreens, other compounds are also frequently used, including vitamin C and its derivatives, oxothiazolidine, ferulic acid and its derivatives, ectoin, and niacinamide [45]. Some of these substances may even enhance the sun protection efficacy of the formulations. For instance, ferulic acid has been shown to increase the in vivo SPF value of a formulation from 19.7 to 26 [45].

5. Conclusions

This study demonstrates that the concentration of the SPF booster significantly influences the physical characteristics, SPF values, and stability of sunscreen formulations. While increasing Sunhancer™ Eco SPF Booster concentration generally improved the SPF, its impact on the texture profiles of the sunscreens differed depending on the emulsifier used. Centrifuge testing identified stability issues in some formulations, emphasizing the need for careful formulation design to prevent phase separation. Sensory analysis provided valuable insights into the aesthetic appeal of the sunscreens, guiding optimization efforts. Overall, this research underscores the complex interplay between formulation components and their effects on sunscreen performance, emphasizing the importance of a comprehensive approach to sunscreen development that considers efficacy, stability, and sensory attributes. Based on the test results, formulations F7 and F8 appear to be the most promising options for sun protection.