Physicochemical Characterization, Rheological Stability, and in Vitro Photoprotective Evaluation of Topical O/W Emulsions Loaded with <i>Silybum marianum</i> Seed Extract

Abstract

Background: Chronic exposure to solar ultraviolet (UV) radiation is a principal contributor to cutaneous oxidative stress, photoaging, and skin cancer. Plant-derived phytochemicals have attracted interest as biocompatible optical filters and potential complements to synthetic UV absorbers. Objectives: To prepare a hydroalcoholic milk thistle (Silybum marianum) seed extract, quantify its total phenolic content (TPC), formulate topical oil-in-water (O/W) emulsions at four nominal extract loads (5%, 10%, 20%, and 30% w/w; corresponding to actual dry-extract contents of approximately 4.2%, 8.3%, 16.7%, and 24.2% w/w), and characterize their physicochemical properties, rheology, storage behavior, and in vitro sun protection factor (SPF), to identify the extract load that best balances photoprotection with formulation quality. Methods: The extract was obtained by maceration in 70% ethanol (three 24-h cycles), and TPC was measured using the Folin–Ciocalteu assay with tannic acid as the standard. Creams were prepared by hot emulsification. Characterization included organoleptic inspection, six freeze–thaw cycles, 90-day pH monitoring, spreadability, emulsion-type testing (conductivity and dye staining), and rotational rheometry at 5 - 50 rpm. SPF was estimated in vitro using the spectrophotometric Mansur method across 290 - 320 nm with an ethanol solvent blank. No extract-free vehicle (0%) or reference sunscreen was measured in parallel; therefore, all photoprotection values are reported as Mansur-derived in vitro SPF screening estimates rather than validated product-level performance. All determinations used three independent formulation batches (n = 3); data were analyzed using one-way and repeated-measures ANOVA with Tukey HSD (exact P values and partial eta-squared reported). Results: The extraction yield was 2.00 ± 0.08%, and the TPC was 23.67 ± 0.22 mg tannic acid equivalents (TAE)/g dry extract. All creams withstood the freeze–thaw challenge without phase separation; the pH remained between 6.30 ± 0.05 and 7.28 ± 0.04, within the range generally accepted for leave-on topical products, and did not change significantly over 90 days (P > 0.05). In all formulations, the apparent viscosity decreased with increasing rotational speed, consistent with pseudoplastic, shear-thinning behavior (power-law n < 1; R2adj > 0.96); spreadability was acceptable (4.80 ± 0.10 to 5.50 ± 0.10 cm). The in vitro SPF increased with extract load [F(3, 8) = 2318.27, P < 0.0001, ηp2 = 0.9989], from 7.05 ± 0.19 (nominal 5%) to 12.42 ± 0.12 (10%), 15.92 ± 0.20 (20%), and 18.02 ± 0.18 (30%); however, the marginal gain per unit extract decreased markedly beyond the nominal 20% load. Conclusions: Within this series, the nominal 20% (actual 16.7% w/w) cream is proposed as a provisional preferred candidate for further formulation optimization rather than as a proven optimum, combining a Mansur-derived in vitro SPF screening estimate of 15.92 (a theoretical UVB attenuation of ~93.7%, calculated as (1 − 1/SPF) × 100) with good short-term physical stability and favorable rheology. Because no extract-free vehicle or reference sunscreen was tested in parallel, these results represent in vitro screening values only; confirmatory vehicle- and benchmark-controlled testing, chemical (HPLC) standardization of the extract, and in vivo biological and clinical evaluation are required before any photoprotective claim can be made. Milk thistle seed extract warrants further standardized study as a botanical adjunct for cosmeceutical photoprotection.