pH-Dependent interfacial complexation mediates astringency-related lubrication loss in a gallic acid-mucin model...

This study employs an in vitro model to investigate the pH-dependent interfacial complexation of gallic acid (GA) with mucin, a key molecular mechanism underlying astringency, through systematic analysis of phase behavior, electrokinetics, adsorption dynamics, and lubrication properties. Results show that GA induces concentration-dependent mucin aggregation, with the hydrodynamic diameter increasing 2.8-fold (152 ± 3 nm to 423 ± 12 nm) at pH 3.5, while remaining largely stable at pH 7.5. A critical aggregation threshold emerged at a 60 : 40 w/w GA : mucin ratio at pH 3.5, coinciding with peak turbidity (OD₅₀₀ = 0.69 ± 0.05) and sedimentation (88 ± 2% mass recovery). At this ratio, electrokinetic measurements revealed near-complete charge neutralization (-0.8 ± 0.3 mV at pH 3.5 vs. -22.4 ± 0.5 mV for mucin alone), indicating that aggregation is driven by reduced electrostatic repulsion in conjunction with other noncovalent interactions. QCM-D analysis displayed that, at pH 3.5, GA formed a highly dissipative (ΔD₅ ≤ 6.76 ± 0.01 × 10^-6), thick (Δt = 14.41 ± 0.02 nm), and water-rich viscoelastic adlayer with substantial mass uptake (Δm = 633.67 ± 0.09 ng cm^-2). Conversely, at pH 7.5, GA adsorption yielded a thinner (Δt = 3.81 ± 0.01 nm), less dissipative (ΔD₅ ≤ 0.87 ± 0.01 × 10^-6), and more rigid film with markedly lower mass accumulation (Δm = 380.37 ± 0.10 ng cm^-2). ΔD-Δf plots confirmed more extensive viscoelastic changes at pH 3.5, indicating a pronounced interfacial restructuring. Tribology directly linked this structural change to lubrication loss; mucin alone exhibited low friction (μ ∼ 0.002-0.003), GA increased friction roughly 7-fold (μ ∼ 0.022) at pH 3.5, while medium-chain triglyceride partially restored lubrication, reducing μ to ∼0.010 (∼54.5% decrease) at pH 3.5 and to ∼0.009 (∼30.8% decrease) at pH 7.5. These results confirm that pH-dependent GA-mucin aggregation and adlayer restructuring coincide with increased friction, linking interfacial changes to astringency-related lubrication loss.