Hidden Metastable States, Hysteresis, and Slow Dynamics of Silica–Aqueous Interface Revealed with Second Harmonic Microscopy

Silica-aqueous interfaces are the foundation of electrokinetics, microfluidics, and countless physics and biophysics experiments. Yet despite their ubiquity, their molecular structure and electrostatics under flow conditions remain poorly understood. Here we use second-harmonic microscopy combined with Raman spectroscopy in glass microcapillaries, to directly track surface potential dynamics during pH cycling. Acidic and basic transitions from neutral pH conditions drive the interface along distinct kinetic pathways: forward pH jumps elicit instantaneous responses, whereas reverse relaxations are markedly slower, producing path- dependent hysteresis. Extended pH cycling experiments reveal that the silica interface can occupy multiple quasi-stable charge states with surface potential variation from −110 mV to −25 mV along distinct acid- and base-mediated pathways. Such history-dependent behavior has broad consequences wherever glass substrates are employed, from electrokinetic devices to single-molecule and condensed-matter studies.