Non-Invasive FTIR Spectroscopy for Photostability Analysis of Commercial Sunscreens with a Conceptual Proposal for Lignin-Based Enhancements

Abstract

The stability of commercial sunscreen formulations during prolonged ultraviolet (UV) exposure is a crucial factor influencing their protective effectiveness and consumer compliance. This work utilises a rapid, non-invasive analytical method employing Fourier Transform Infrared (FTIR) spectroscopy to examine the chemical properties and photostability of sunscreen lotions with different Sun Protection Factors (SPF 15, 30, and 50). Spectral analyses during regulated UV exposure durations (3–6 hours) demonstrate specific degradation pathways, characterised by diminished transmittance of O–H and N–H groups and heightened signals in the C–O, C–O–C, and C=N stretching regions signifying oxidative degradation, photofragmentation, and molecular reconfigurations of active compounds. These spectral variations are consistent with oxidative degradation, photofragmentation, and molecular reconfiguration of active ingredients. Based on these observed degradation pathways, a conceptual encapsulation strategy employing methylated and syringyl-type lignin derivatives is proposed as a potential means to mitigate UV-induced instability. Owing to their aromatic structure and reported UV-absorbing properties, such biopolymeric materials may offer a promising route for enhancing the photostability and durability of sunscreen actives, although experimental validation was not performed in this work. Overall, the study establishes FTIR spectroscopy as an effective tool for real-time, non-invasive assessment of sunscreen photodegradation and provides a framework for the future development of lignin-based stabilisation strategies in cosmetic and UV-protective materials.