Interfacial activity and emulsifying performance of poly(ethylene glycol)-and hydroxybenzophenone-modified...

Emulsifiers are essential components in pharmaceutical, cosmetic, and industrial formulations, enabling the stabilization of immiscible phases. Among these, silicone-based surfactants offer unique advantages, including thermal stability, biocompatibility, and tunable surface activity. However, current silicone emulsifiers are predominantly linear structures, which limits precise control over interfacial organization and structure-property relationships. Cyclosiloxanes, with their well-defined cyclic architecture and potential for modification, represent an unexplored opportunity to develop emulsifiers with predictable behaviour and enhanced interfacial control. This study investigated cyclotetrasiloxanes functionalized with hydroxybenzophenone and polyethylene glycol (PEG) groups as novel emulsifying agents. Mono- and bifunctionalized derivatives were synthesized via hydrosilylation using Karstedt's catalyst and characterized by NMR, FT-IR, and contact angle measurements. Emulsions with varying water-to-oil ratios and emulsifier concentrations were evaluated for stability using centrifugation, multiple light scattering, optical microscopy, and laser diffraction over a 60 day-period. Bifunctionalized cyclosiloxanes demonstrated markedly superior emulsifying performance compared to monosubstituted analogs. Optimal stability was achieved with a 30:70 water-to-oil composition and a PEG-to-hydroxybenzophenone ratio of 3:1, resulting in emulsions with monomodal droplet distributions (mean diameter 2.2 ± 0.1 μm). The system exhibited a strongly negative zeta potential (-42.2 mV) and remained stable throughout the storage period. These findings establish functionalized cyclosiloxanes as promising emulsifiers for pharmaceutical, cosmetic, and technical applications, offering a new platform for rational formulation design.