1. INTRODUCTION:

The global cosmetic and personal care industry has witnessed a paradigm shift toward natural, sustainable, and multifunctional formulations, driven by increasing consumer awareness regarding the safety and efficacy of synthetic ingredients. This transition is particularly evident in the anti-aging segment, where consumers increasingly demand products that combine therapeutic benefits with minimal adverse effects^1-2. The anti-aging market, valued at billions of dollars globally, continues to expand as populations age and awareness of skin health intensifies. Within this context, the incorporation of plant-based oils rich in bioactive compounds has emerged as a promising strategy for developing effective cosmeceutical formulations that address multiple signs of aging while maintaining compatibility with diverse skin types^3-4. Skin aging is a complex, multi-factorial process combining intrinsic and extrinsic factors. Intrinsic aging, driven by the influence of genetics and hormones, is basically characterized by progressive dermal atrophy, gradual diminution in collagen synthesis, loss of elasticity, and rates of cellular renewal. Extrinsic aging is driven mainly by environmental stressors like ultraviolet radiation, pollution, and lifestyle factors; these accelerate oxidative damage by generating ROS that, in turn, result in peroxidation of lipids, degradation of proteins, and DNA damage. These cumulative effects then clinically present as visible signs of aging, including fine lines, wrinkles, hyperpigmentation, loss of firmness, and irregular skin texture. Thus, for an effective anti-aging formulation, multiple pathways need to be targeted simultaneously, including antioxidant protection, maintenance of hydration, enhancement of barrier function, and stimulation of cellular regeneration^5-6.

Traditional anti-aging cosmetics have relied heavily on synthetic active ingredients such as retinoids, alpha-hydroxy acids, peptides, and various antioxidants. While these are compounds with proven efficacy, concerns about their potential for skin irritation, photosensitivity, and long-term safety have led researchers to seek natural alternatives. Plant-based oils represent an exceptional source of bioactive lipids, vitamins, phenolic compounds, and essential fatty acids that synergistically contribute to skin health. Among the different botanical oils available, watermelon seed oil and walnut oil have emerged as particularly promising candidates due to their unique compositional profiles and reported dermatological benefits^7-9. Watermelon seed oil, obtained from the seeds of Citrullus lanatus, has always been underused despite the global cultivation and consumption of watermelon. This oil is highly rich in unsaturated fatty acids, mainly linoleic acid (omega-6) and oleic acid (omega-9), which constitute about 60-70% and 10-20%, respectively. Linoleic acid plays an important role in maintaining epidermal barrier function and has been associated with anti-inflammatory properties, making this oil valuable for sensitive skin or compromised skin. Besides, WSO is rich in tocopherols (vitamin E), phytosterols, and other antioxidant components that will protect against oxidative stress. The light nature and fast absorption of the oil make it appropriate for cosmetic use since it does not leave any greasy residue, which is a highly desired property in recent skin care formulations. Additionally, due to the non-comedogenic nature of watermelon seed oil, it suits all skin types and is thus compatible with acne-prone skin^10-16.

Walnut oil, extracted from the nuts of Juglans regia, has been valued for its nutritional and therapeutic benefits for centuries. In the cosmetic context, walnut oil's rich composition of polyunsaturated fatty acids, among which are alpha-linolenic acid (omega-3), linoleic acid (omega-6), and oleic acid (omega-9), makes it a valuable ingredient in skin nourishment and protection. The presence of omega-3 fatty acids is of particular note, since such compounds exert anti-inflammatory action and may contribute to the modulation of skin immune responses. Walnut oil also contains considerable amounts of vitamin E, polyphenols, and ellagic acid, which contribute to its overall antioxidant capacity. Some studies have suggested that the topical use of walnut oil improves skin elasticity, decreases transepidermal water loss, and thus has an overall positive effect on skin appearance. Its emollient properties ease skin softening and smoothening; thus, walnut oil soothes roughness and dryness, two of the most common complaints with aging skin^16-21. A synergistic blend of watermelon seed oil and walnut oil represents a rational strategy toward the formulation of complex anti-aging products. The complementary fatty acid profiles of these oils, where the major constituents of watermelon seed oil are omega-6 fatty acids and walnut oil is rich in omega-3 fatty acids, form a balanced lipid composition that mimics and supports the skin's natural barrier. Such a combination would potentially afford superior moisturization, barrier repair, and antioxidant defense than individual single-oil formulations. However, the application of neat oils directly onto the skin presents a number of limitations, including instability, risk of oxidation, variable rates of absorption, and aesthetic issues regarding greasiness and related sensory properties^22-23. Despite the attractive features of both watermelon seed oil and walnut oil, and the benefits provided by microemulsion technology, few attempts have been reported so far to formulate microemulsions with these particular oils for cosmetic purposes. In fact, most of the available literature has dealt with single oils or evaluated the formulation parameters in a non-systematic way in order to get an optimized microemulsion. Besides, only detailed physicochemical characterization by determination of droplet size, polydispersity index, zeta potential, pH, refractive index, conductivity, rheological behaviour, and stability under different stress conditions will ensure that the developed formulations comply with the complex demands of commercial cosmetic products^24-30. The current study bridges this gap by the systematic development and characterization of a new watermelon seed oil and walnut oil-based microemulsion system for anti-aging cosmetic applications. The research is directed toward the optimization of formulation composition through the construction of pseudo-ternary phase diagrams, consideration of the physicochemical properties of the developed microemulsions, and determination of their stability under different environmental conditions. This study, through the combination of two nutrient-rich botanical oils in an advanced delivery platform, investigates the creation of a natural, efficient, and stable cosmetic formula that helps in addressing multiple aspects of skin aging, thereby meeting consumer expectations regarding elegance, absence of greasiness, and other required features.

2. MATERIALS AND METHODS:

2.1 Materials:

The watermelon seed oil and walnut oil were sourced from a local supplier. The vegetable oil ethoxylate surfactants were employed in this study hydrogenated castor oil ethoxylate and Grapeseed seed oil ethoxylate. Both surfactant samples were generously provided by Rossari Biotech Ltd. (Mumbai, India) and used as received without additional purification. All chemicals and organic solvents were analytical grade and used without further purification unless otherwise noted. Deionized water with conductivity below 1µS/cm was used throughout all experimental procedures to prevent ionic interference and ensure reproducibility. Glycerine, Sandalwood oil, disodium EDTA, Phenoxyethanol (and) Ethyl hexyl glycerine and citric acid, were obtained from Loba Chem Pvt. Ltd. (Mumbai, India). Phenoxyethanol was used as a preservative, and all other reagents were of analytical grade (AR). Deionized water (conductivity <1 µS/cm) was used throughout all formulations.

2.2 Methods:

2.2.1 Characterization of Oils: The physicochemical properties of the selected oils were evaluated following standard AOAC and pharmacopeial methods. The physicochemical parameters including specific gravity, acid value, peroxide value, iodine value, and saponification value were determined quantitatively using routine titrimetric and gravimetric analyses.

2.2.2 Characterization of Surfactants: The surfactants were characterized by evaluating their appearance, pH, specific gravity, surface tension, and hydrophilic-lipophilic balance (HLB) value.

2.2.3 Pseudo-Ternary Phase Diagram Construction: Pseudo-ternary phase diagrams were constructed at 25± 1°C using the aqueous titration method to identify the microemulsion region. Binary mixtures comprising oil phase (equimolar blend of watermelon seed oil and walnut oil) and surfactant combinations were prepared in weight ratios ranging from 1:9 to 9:1 (w/w). water was incrementally added to each mixture under gentle magnetic stirring until the onset of turbidity. Systems exhibiting isotropic, transparent, and low-viscosity characteristics were designated as microemulsions and plotted to delineate the microemulsion zone within the phase diagram.

2.2.4 Phase Diagram-Selected Formulations: Four formulations (S1–S4) were selected from the microemulsion region of the pseudo-ternary phase diagrams based on their transparency, stability, and maximum oil-loading capacity. The minimum surfactant mixture (Smix) concentration required for complete oil phase solubilization was determined for each formulation. The selection criteria included: minimal Smix concentration for maximum oil solubilization, clear and homogeneous appearance, suitable viscosity for topical application, and spontaneous microemulsion formation upon gentle mixing. The selected formulations represented varying oil: Smix: water ratios within the microemulsion zone and were subjected to comprehensive physicochemical characterization and stability studies.

2.2.5 Microemulsion-Based Anti-Aging Serum Preparation: Spontaneously emulsified oil-based anti-aging serums (S1–S4) were prepared. Watermelon seed oil and walnut oil mix, serving as the oil phase, and surfactant blend were combined in fixed ratios and stirred at 500rpm for 10min. Then, Water was added slowly (~0.5mL/min) until a clear, transparent, and uniform system was achieved.

2.2.6 Physicochemical Evaluation and Stability Studies: The developed serums (S1–S4) were characterized according to ICH guidelines for pharmaceuticals. Visual appearance was assessed on white and black backgrounds to check clarity and uniformity of phase. pH was measured with a calibrated digital pH meter at 25±1°C (acceptable range: 4.5–7.0). Viscosity was tested with a Brookfield viscometer. Conductivity measurements were used to determine the type of emulsion. Size of the droplets and polydispersity index (PDI <0.3 for monodisperse systems) were assessed with Dynamic Light Scattering (Malvern Zetasizer Nano ZS). Stability testing was carried out as per ICH Q1A R2. Centrifugation (3000rpm, 20min) and freeze-thaw cycles three times (4°C/40°C, 24hours each) were included in stress testing. The formulations were stored at 25± 2°C/60±5% RH and 45±2°C/75±5% RH. Samples withdrawn at 0, 15, 30 days were evaluated for appearance, pH, droplet size, and viscosity. Formulations were deemed stable with no phase separation, pH change <0.5 units, droplet size difference <20%, and viscosity alteration ±10%.

2.2.7 Sensory Evaluation and Skin Irritation Testing: The sensory attributes and dermatological safety of the serum were comprehensively evaluated. A trained panel of 10 members assessed the greasiness, spreadability, absorption, after-feel, and overall acceptability of the formulation using a 5-point hedonic scale (1 = poor, 5 = excellent) at 5 minutes post-application on the volar forearm under controlled conditions. For safety assessment, a closed patch test was conducted on 10 healthy volunteers following ethical clearance. Approximately 0.2mL of serum was applied to a 2 cm² area on the forearm and occluded with a hypoallergenic adhesive patch for 24hours. The application site was examined for signs of irritation including redness, itching, edema, or other adverse reactions immediately upon patch removal and at 24- and 48-hours post-application. The absence of any adverse reactions confirmed the dermatological safety and skin compatibility of the prepared serum.

3. RESULT:

3.1 Characterization of Watermelon Seed Oil and Walnut Oil:

Table 1: Physicochemical parameters of oil.

Sr. No	Parameter	Watermelon Seed Oil	Walnut Oil
1	Physical appearance	Yellow color liquid	Yellow color liquid
2	Specific Gravity	0.918	0.920
3	Acid Value	0.5	0.7
4	Iodine Value	123.5	143
5	Peroxide Value	6.2	3.3
6	Saponification value	181	185

Table 2: Physicochemical parameters of Blend oil.

Sr. No	Parameter	Watermelon Seed Oil and Walnut Oil WSO 70% and WO 30%	Watermelon Seed Oil and Walnut Oil WSO 30% and WO 70%
1	Physical appearance	Yellow color liquid	Yellow color liquid
2	Specific Gravity	0.920	0.919
3	Acid Value	0.5	0.5
4	Iodine Value	128	132
5	Peroxide Value	5.33	4.20
6	Saponification value	182	183

3.2 Characterization of Emulsifier

Table 3: Physicochemical parameters of surfactant

Sr. No	Parameter	HCO 40	GSO 15	GSO 20
1	Physical appearance	Clear yellow viscous liquid	Clear yellow viscous liquid	Clear yellow viscous liquid
2	Specific Gravity	1.072	1.032	1.038
3	pH (5% solution)	6.3	6.7	6.8
4	Moisture content (%)	0.21	0.29	0.31
5	Surface tension	40.33	43.12	40.73
6	HLB value	13.1	9.7	11.7

Table 4: Composition and Calculated HLB Surfactant Blends.

Blend No	HLB blend	HCO-40 (%)	GSO-15 (%)	GSO-20 (%)
1	11.4	50	50	-
2	12.4	50	-	50

Figure 1: Microemulsion of HLB blend 11.4

Figure 2: Microemulsion of HLB blend 12.4

Table 5: Microemulsion physical parameter

Sr. No	Formulation	Blend of oil	HLB	Type	pH	Density
						Kg/m3
1	S-1	WSO 70 % and WO 30 %	11.4	O/W	6.90	1.021
2	S-2	WSO 30 % and WO 70 %	11.4	O/W	6.75	1.025
3	S-3	WSO 70 % and WO 30 %	12.4	O/W	6.84	1.028
4	S-4	WSO 30 % and WO 70 %	12.4	O/W	6.75	1.024

Table 6: Microemulsion physical parameter

Sr. No	Formulation	Blend of oil	Viscosity	Conductivity	Particle size	Polydispersity Index
			CPS	mS/m	nm
1	S-1	WSO 70 % and WO 30 %	120	375.3	257	0.376
2	S-2	WSO 30 % and WO 70 %	135	291.5	268	0.388
3	S-3	WSO 70 % and WO 30 %	148	266.6	146.6	0.306
4	S-4	WSO 30 % and WO 70 %	153	281.5	123.2	0.314

Table 7: Microemulsion stability

Sr. No	Formulation	Centrifuge stability	Accelerated stability
1	S-1	No Separation	No Separation
2	S-2	No Separation	No Separation
3	S-3	No Separation	No Separation
4	S-4	No Separation	No Separation

Table 8: Microemulsion based Anti-Aging Serum

Sr. No.	Ingredients (INCI Name)	Function	Formulation -1 (% w/w)	Formulation -2 (% w/w)	Formulation -3 (% w/w)	Formulation -4 (% w/w)
1	S-1	Microemulsion active	50	0	0	0
2	S-2	Microemulsion active	0	50	0	0
3	S-3	Microemulsion active	0	0	50	0
4	S-4	Microemulsion active	0	0	0	50
5	Disodium EDTA	Chelating agent	0.05	0.05	0.05	0.05
	Glycerin	Humectant	3.0	3.0	3.0	3.0
6	Sandalwood oil	Perfume	0.1	0.1	0.1	0.1
7	Phenoxyethanol (and) Ethyl hexyl glycerine	Preservative system	1	1	1	1
8	Citric acid	pH adjuster (to 6.2–6.8)	q.s.	q.s.	q.s.	q.s.
9	Purified Water		q.s. to 100	q.s. to 100	q.s. to 100	q.s. to 100

Table 9: Microemulsion based Anti-Aging Serum Stability data

Sr. No.	Formulation	Centrifugation	Freeze–thaw cycling (3 cycles, 4 °C ↔ 45 °C, 24 h/cycle)
1	Formulation -1	No Phase separation	Pass
2	Formulation -2	No Phase separation	Pass
3	Formulation -3	No Phase separation	Pass
4	Formulation -4	No Phase separation	Pass

Table 10: Accelerated/real-time stability (30 days) at 45 °C (elevated temperature)

Formulation	Appearance/Color	pH	Viscosity (Cps)	Phase separation
Formulation-1	Clear, pale yellow	6.62	120	None
Formulation-2	Clear, pale yellow	6.57	140	None
Formulation-3	Clear, pale yellow	6.64	162	None
Formulation-4	Clear, pale yellow	6.76	170	None

Table 11: Skin Irritation Testing of Microemulsion-Based Anti-Aging Serums

Formulation	Mean Erythema Score	Mean Edema Score	Overall Reaction	Classification
Formulation-1	0.1 ± 0.1	0.0 ± 0.0	Slight transient redness in 1 subject	Practically non-irritant
Formulation-2	0.2 ± 0.1	0.1 ± 0.1	Mild erythema in 2 subjects	Very mild irritation
Formulation-3	0.0 ± 0.0	0.0 ± 0.0	No visible reaction	Non-irritant
Formulation-4	0.0 ± 0.0	0.0 ± 0.0	No visible reaction	Non-irritant

Table 12: Sensory Evaluation of Microemulsion-Based Anti-Aging Serum Formulations

Parameter	Formulation-1	Formulation-2	Formulation-3	Formulation-4
Appearance	4.6 ± 0.1	4.5 ± 0.2	4.8 ± 0.1	5.0 ± 0.0
Color	4.6 ± 0.2	4.7 ± 0.2	4.9 ± 0.1	5.0 ± 0.0
Odor	4.7 ± 0.2	4.6 ± 0.2	4.9 ± 0.1	4.8 ± 0.2
Texture / Feel	4.6 ± 0.2	4.4 ± 0.2	4.8 ± 0.1	4.9 ± 0.1
Absorbability	4.5 ± 0.2	4.3 ± 0.2	4.9 ± 0.1	5.0 ± 0.0
After-feel	4.6 ± 0.2	4.4 ± 0.2	4.8 ± 0.1	4.9 ± 0.1
Overall Acceptability	4.6 ± 0.2	4.5 ± 0.1	4.8 ± 0.1	4.9 ± 0.1

4. DISCUSSION:

4.1 Physicochemical Characterization of Oils and Oil Blends: The physicochemical characterizations of watermelon seed oil and walnut oil showed characteristics of good cosmetic oils suitable for topical applications. Both oils had appropriate specific gravities (0.918-0.920), showing densities lower than that of water, facilitating their spreading and absorption in skin surfaces. Low acid values (0.5-0.7) pointed to their low contents of free fatty acids, thus characterizing good quality oils with low hydrolytic degradation. This is an important parameter for cosmetic applications, as high acid values reflect oxidative deterioration that may affect product stability and could cause skin irritation. The peroxide values were within acceptable limits for cosmetic oils (<10mEq/kg), thus suggesting a minimal lipid peroxidation and oxidative rancidity. Walnut oil had a lower peroxide value of 3.3 as compared with that of watermelon seed oil (6.2), which can be credited to the higher content of natural antioxidants in the form of ellagic acid and polyphenols in the former. The high unsaturated fatty acid content was further supported by their high iodine values: 123.5 for WSO and 143 for WO, thus reflecting greater unsaturation of walnut oil. High unsaturation supports that the two oils should have good skin-penetration ability and moisturizing properties with enhanced bioactive potential, while on the other hand, this calls for careful formulation strategies to prevent these oils from oxidative degradation during storage. Saponification values ranged between 181 and 185, within the expected ranges for seed oils rich in medium to long-chain fatty acids, making them suitable for cosmetic emulsification. The blended oil formulations, with WSO:WO ratios of 70:30 and 30:70, respectively, presented intermediate physicochemical properties, showing good compatibility of both oils in the absence of adverse interactions or phase separation. Such compatibility is an important asset in the development of stable multi-oil formulations with complementary advantages.

4.2 Selection and Characterization of Surfactant: Selection of vegetable oil ethoxylate surfactants, like HCO-40, GSO-15, and GSO-20, was strategic, yielding to the natural and sustainable positioning of the formulation. These non-ionic surfactants, from renewable sources like hydrogenated castor oil and grapeseed oil, had the dual advantage of formulation efficacy and more appeal for the consumers in the clean beauty trend context. The HLB values characterized were within the required range between 9.7 and 13.1 for oil-in-water microemulsions. GSO-15, at an HLB of 9.7, provided the lipophilic character needed in the oil phase, whereas HCO-40, at an HLB value of 13.1, furnished the hydrophilic characteristics for the aqueous phase. GSO-20 represented an intermediate value of 11.7, useful in adjusting the properties of the surfactant blend. Surfactant blends formulated with calculated HLB values of 11.4 and 12.4 enabled the systematic study of phase behaviour and the determination of optimal microemulsion regions. The surface tension values (40.33-43.12mN/m) indicated good surface-active properties essential for reducing interfacial tension between oil and water phases, thereby facilitating microemulsion formation. The near-neutral pH values (6.3-6.8) of 5% surfactant solutions confirmed their compatibility with skin's physiological pH range, minimizing irritation potential. The low moisture content (<0.5%) ensured product stability by reducing the risk of hydrolytic degradation and microbial contamination during storage.

4.3 Pseudo-Ternary Phase Diagram and Formulation Selection: The pseudo-ternary phase diagram approach proved instrumental in systematically mapping the microemulsion region and identifying compositions with optimal characteristics. The aqueous titration method enabled precise determination of the boundaries between microemulsion, emulsion, and phase-separated regions within the oil-surfactant-water system. The diagrams revealed that microemulsion formation was dependent on specific ratios of components, with the microemulsion region expanding at intermediate surfactant concentrations. Thus, S1-S4 mapped different locations within the microemulsion zone for comparison of the influence of oil blend ratios and HLB values on system properties. These various formulations demonstrated characteristics that are typical of a thermodynamically stable microemulsion, including optical transparency, relatively low viscosity compared to conventional emulsions, spontaneous formation with mild mixing (without high-energy homogenization), and stability over a long period without any phase separation. Systematic variation of both oil blend ratio (WSO:WO, 70:30 versus 30:70) and surfactant HLB (11.4 versus 12.4) allowed the elucidation of structure-property relationships relevant to formulation optimization. The strategy ensures that the selected formulations are representative of a wide range of physicochemical properties suitable for comprehensive evaluation and subsequent selection of the optimal composition for commercial development.

4.4 Particle Size Distribution and Polydispersity Analysis: The particle size analyses indicated significant effects of both oil blend ratio and surfactant HLB value on droplet dimensions, providing important information on the structural organization of the microemulsion systems. Formulations S3 and S4, prepared with the higher HLB blend of 12.4, exhibited substantially smaller droplet sizes, 146.6nm and 123.2nm, respectively, compared to formulations S1 and S2 prepared with HLB 11.4, which had 257nm and 268nm, respectively. It agrees with the basic theory of colloids, in which higher HLB value surfactants are more hydrophilic and stabilize oil droplets in aqueous media more effectively, yielding smaller particle sizes and higher interfacial areas. Such increased stabilization likely arises from more efficient packing of surfactant molecules at the oil-water interface and stronger hydration of the hydrophilic head groups that prevents droplet coalescence more effectively. In each HLB series, formulations with higher walnut oil content (S2 and S4) showed a slightly different particle size distribution from their watermelon seed oil-rich counterparts. Such an effect could be related to the different interfacial properties, curvature, and flexibility of the surfactant film, associated with different fatty acid profiles. Indeed, the higher polyunsaturated fatty acid content in walnut oil, especially alpha-linolenic acid, might affect the spontaneous curvature of the interfacial layer and, thus, the equilibrium droplet size. The polydispersity indices (PDI), ranging from 0.306 to 0.388, indicated relatively narrow size distributions, though slightly above the ideal threshold of 0.3 for perfectly monodisperse systems. Formulation S3 (PDI 0.306) approached monodispersity, while formulations S1 and S2 showed broader distributions (PDI 0.376 and 0.388, respectively). This moderate polydispersity is acceptable for cosmetic applications and reflects the complex nature of multi-component microemulsion systems where multiple equilibrium structures may coexist. The nanoscale dimensions of all formulations (123.2-268nm) are particularly advantageous for cosmetic applications, as particles in this size range can enhance skin penetration of lipophilic bioactive compounds while maintaining the transparent appearance highly valued in modern serum formulations.

4.5 Stability Studies: Centrifugation and Thermal Cycling: The far-reaching stability studies revealed the outstanding robustness of the developed microemulsion systems against different stress conditions, simulating manufacturing, transportation, and storage challenges. The fact that no phase separation occurred upon centrifugation at 3000rpm for 20minutes demonstrated the thermodynamic stability of the formulations, distinguishing them from conventional emulsions, which are only kinetically stable and usually break down under such gravitational stress. This thermodynamic stability arises from the ultralow interfacial tension achieved in microemulsion systems, where the free energy of dispersion is minimal or even negative, resulting in spontaneous formation and resistance to coalescence. The centrifugation test effectively accelerates gravitational forces, revealing any tendency toward creaming, sedimentation, or phase separation that might occur over extended storage periods. The fact that all four formulations passed this test supports the expectation of long-term physical stability. The freeze-thaw cycling tests, conducted over three cycles alternating between 4°C and 45°C with 24hour equilibration at each temperature, evaluated the formulations' ability to withstand temperature fluctuations without destabilization. Temperature variations can induce phase transitions, alter surfactant solubility and partitioning, modify droplet size distributions, and promote coalescence in unstable systems. The successful passage of these tests by all formulations indicated robust structural integrity and flexibility of the interfacial surfactant film, which could accommodate volume changes and maintain droplet separation despite thermal stress.

This thermal stability is particularly important in cosmetic products that would be undergoing varying conditions of storage and transportation, from cold exposure during winter shipping to high temperatures inside a warehouse or on retail display. The performance in freeze-thaw cycling suggests that the developed microemulsions would keep their properties across typical distribution chain conditions without requiring specialized cold-chain logistics.

4.6 Microemulsion-Based Serum Formulations: The optimized microemulsion bases (S1-S4) were incorporated at a 50% w/w concentration into complete antiaging serum formulations, Formulations 1-4, respectively, and shown to successfully combine with complementary cosmetic ingredients without loss of stability. The design of the formulations included incorporation of glycerin (3% w/w) as a humectant to add moisturization and water binding capability, disodium EDTA (0.05% w/w) as a chelating agent to sequester any metal ions that might catalyse oxidative degradation, and a phenoxyethanol-ethylhexyl glycerin preservative system (1% w/w) to prevent microbial contamination. The addition of the fragrance of sandalwood oil at 0.1% w/w gave a pleasant, fresh aroma that favored sensory appeal and contributed to mild antimicrobial and anti-inflammatory properties. Citric acid served as a pH adjuster and maintained the final serum pH within the optimal range of 6.2-6.8 for skin compatibility and ingredient stability. The remaining composition was made up of purified water, q.s. to 100%, which formed the continuous phase and conferred the light, non-greasy texture typical of serums. Final serum formulations retained the stability characteristics of parent microemulsions in that they passed both centrifugation tests and freeze-thaw cycling without phase separation. This compatibility serves to validate the robustness of the microemulsion structure to accommodate additional ingredients without destabilization. The transparent to translucent appearance of the serums, along with their low viscosity and fast absorption, was found to agree with consumer expectations about facial serums. Accelerated stability testing at 45°C for 30 days validated the fact that all physicochemical properties were retained by full serum formulations with minimal changes in appearance, pH, and viscosity. Such stability performance is proof that microbial growth was inhibited by the preservative system, that oxidative degradation was properly prevented by antioxidants, and that stress conditions did not disturb the structure of the microemulsion.

4.7 Dermatological Safety Assessment: The results of dermatological safety testing by 24hour closed patch on ten healthy volunteers were very encouraging and established the developed formulations to be safe for topical application on human skin. The formulations 3 and 4 did not produce any irritation reactions as their mean erythema and edema scores were 0.0±0.0 and hence were classified as "non-irritant" as per the standard irritation scoring scales. Formulation 1 produced only minimal transient redness in a single subject (mean erythema score: 0.1±0.1; mean edema score: 0.0±0.0), making it "practically non-irritant." This isolated, mild response resolved spontaneously without intervention and most probably represented individual skin sensitivity variation rather than a systematic formulation problem. Formulation 2 showed very mild erythema in two subjects with a mean erythema score of 0.2±0.1 and mean edema score of 0.1±0.1, which still fits the category of "very mild irritation." This is considered acceptable for cosmetic products. Such an excellent safety profile for all formulations is attributed to a number of factors. Firstly, the natural origin and inherent biocompatibility of both watermelon seed oil and walnut oil, which are rich in skin-identical lipids and generally recognized as non-sensitizing ingredients. Secondly, non-ionic vegetable oil ethoxylate surfactants are used, which are gentle and do not have the harsh detergency and protein denaturation potential of ionic surfactants. Thirdly, the physiologically appropriate pH (6.2-6.8) minimizes acid-base irritation and preserves skin barrier integrity. Lastly, known irritants, sensitizers, or common allergens are absent from the formulation composition. The apparently superior safety performance of Formulations 3 and 4 (higher HLB, smaller droplet size) compared with Formulations 1 and 2 may indicate that microemulsion structure affects skin interaction and tolerability. The finer droplets in Formulations 3 and 4 may provide for more efficient skin penetration and consequently less residual surface material that could contribute to occlusion-related irritation. Moreover, the high HLB surfactant blends in these formulations may impart milder skin interaction profiles. Additional confidence in the safety of formulations for repeated use was gained when the follow-up observations at 24- and 48-hours post-application confirmed the absence of delayed hypersensitivity reactions. The findings support the suitability of the developed microemulsion serums for application on facial skin, including areas which can be especially sensitive, such as the periorbital and perioral areas, where anti-aging products are commonly applied.

4.8 Sensory Evaluation and Consumer Acceptability: The sensory analysis, executed by a trained panel of ten assessors using a validated 5-point hedonic scale, gave an indication of consumer acceptability and user experience factors highly affecting product success within the competitive cosmetic market. All formulations had favourable ratings in all the parameters used for evaluation, which included appearance, color, Odor, texture/feel, absorbability, after-feel, and overall acceptability, with mean scores falling within a range of 4.3 to 5.0. Formulation 4 achieved the highest overall acceptability score (4.9±0.1), excelling particularly in absorbability (5.0±0.0), appearance (5.0±0.0), and color (5.0±0.0). The perfect absorbability score indicates that the formulation penetrated rapidly into skin without leaving visible residue, greasiness, or tackiness attributes highly valued by consumers for daytime use and compatibility with makeup application. The superior appearance score reflects the formulation's optical clarity and elegant presentation, while the color score confirms its attractive pale-yellow hue derived from the natural carotenoids present in the botanical oils. Superior sensory performance in Formulation 4 is strongly correlated with the smallest droplet size (123.2nm), as it increases the rate of skin penetration and/or more efficient distribution of oil phase components within the stratum corneum, thus minimizing surface residue. Such nanoscale dimensions allow the formulation to take better advantage of intercellular lipid pathways in the stratum corneum versus larger droplets, therefore enhancing cosmetic elegance and/or delivery of bioactive components. Formulations 3 and 4, both containing the high HLB surfactant blend of 12.4, also showed higher sensory performance compared to Formulations 1 and 2 at an HLB of 11.4 for most sensory attributes. This could mean that higher HLB formulations, by offering smaller droplet sizes and more organized interfacial structures, inherently possess superior sensory characteristics. Texture/feel scores (4.6-4.9) indicated smooth, nongrainy application without pilling or dragging, while after-feel scores (4.4-4.9) confirmed pleasant post application skin sensation without residual oiliness or tightness. Odor scores (4.6-4.9) indicated positive reception of the sandalwood fragrance, which effectively masked any potential off-notes from the botanical oils while providing a luxurious, natural scent profile consistent with the positioning of the formulation. The high scores across all three sensory dimensions of texture, after-feel, and absorbability confirm that the microemulsion approach successfully overcame the typical aesthetic limitations of oil-based cosmetics, thus rendering potentially greasy oils into elegant, fast-absorbing serums. These sensory evaluation results strongly support the commercial potential of the developed formulations, particularly Formulation 4, in that they show that the optimization of physicochemical properties can go along with consumer preference to create products that are both technically sound and experientially satisfying.

5. CONCLUSION:

The present study has successfully developed and characterized novel microemulsion-based anti-aging serums incorporating watermelon seed oil and walnut oil with the aid of vegetable oil ethoxylate surfactants. The pseudo-ternary phase diagrams enabled systematic identification of stable formulation compositions. Four O/W microemulsions (S1-S4) were screened, and among them, Formulation-4 (WSO: WO 30:70, HLB 12.4) showed optimum properties: smallest droplet size (123.2 nm), appropriate viscosity (153 cPs), and highest sensory acceptance scores. All the formulations exhibited excellent stability to centrifugation, freeze-thaw cycling, and also to accelerated storage conditions of 45°C for 30 days, during which no phase separation occurred, with minimum changes in pH and viscosities. In addition, dermatological safety was established using patch testing and was classified as non-irritant to practically non-irritant. The developed microemulsions successfully combined the complementary omega-3 and omega-6 fatty acid profiles and antioxidant properties of the two botanical oils in an elegant thermodynamically stable delivery platform. The above observations suggest that microemulsion technology can be a promising method of formulating natural, effective, and consumer-acceptable anti-aging cosmetic products, meriting further clinical efficacy evaluation.

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Received on 04.11.2025 Revised on 09.01.2026

Accepted on 13.02.2026 Published on 02.07.2026

Available online from July 15, 2026

Asian J. Res. Pharm. Sci. 2026; 16(3):225-233.

DOI: 10.52711/2231-5659.2026.00034

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