Structure-flexibility relationships at low dielectric dialkyl ester liquid surfaces of DES, DEO, DEA, and DBS: A vibrational spectroscopy and MD simulations study

Dialkyl esters, such as dibutyl sebacate (DBS), diethyl sebacate (DES), and diethyl adipate (DEA), are attractive, less-toxic alternatives to phthalate plasticizers, yet a mechanistic understanding of their behavior at soft interfaces, where wetting, adhesion, and physicochemical interactions are governed, remains limited. Resolving molecular orientation and organization at liquid-air interfaces is challenging with conventional techniques. Here, we combine sum frequency generation (SFG) vibrational spectroscopy with complementary infrared, Raman measurements, and molecular dynamics (MD) simulations to elucidate the interfacial structure of low-dielectric esters [DBS, DES, DEA, and diethyl oxalate (DEO)]. SFG reveals that terminal methyl group orientations vary substantially across the series, with DES exhibiting a pronounced tilt toward the surface plane relative to DBS, DEA, and DEO. The analysis of CH2/CH3 symmetric stretching intensity ratios indicates a higher population of gauche conformations and increased conformational disorder at the DES interface despite its shared sebacate backbone with DBS. MD simulations support these observations, showing differences in alkyl chain ordering, flexibility, and carbonyl-carbonyl separation, and yielding tilt angles broadly consistent with experiment for DES while highlighting sensitivity in SFG-derived values to optical and hyperpolarizability assumptions. Notably, the CH3 Fermi resonance ratio and surface tension follow similar trends (DES > DEA > DBS ≈ DEO) across the dialkyl ester series, linking enhanced conformational flexibility and intermolecular packing heterogeneity at the molecular scale to macroscopic interfacial properties. These results enhance our understanding of soft low-dielectric liquid interfaces more broadly.