Effect of Size Asymmetry and Zwitterionic Character of Solvent Ions on Impedance Spectra of Ultradilute Colloidal Suspensions

Abstract

Buffer chemistry modulates colloidal impedance critically for label-free biosensing and electrokinetic diagnostics, yet classical models neglect buffer-specific interactions. We studied carboxylated polystyrene (PS) microspheres in conductivity-matched zwitterionic 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) and phosphate-buffered saline (PBS). Two concentration-dependent regimes emerged: oscillatory at ultradilute concentrations (<0.02 w/v%; Region 1) and monotonic at higher concentrations (Region 2). In Region 1, HEPES at pH 6.9 (approximately 80% zwitterionic) produced the largest deviation of delta Z from the O'Brien prediction. The conductivity increment was opposite in sign to classical expectation. We attribute this to zwitterion-driven interfacial ordering that enhances local conductivity and promotes self-limiting particle clustering. Dynamic light scattering detected cluster formation at 0.008 w/v%, well below aggregation thresholds. Changing pH from 6.9 to 8.1 strongly altered impedance amplitude; DLS profiles remained invariant. This indicates zwitterions modulate electrochemical polarization, not physical assembly. Molecular dynamics simulations showed pH-dependent HEPES organization at the PS surface. At pH 6.9, interfacial charge- and number-density profiles showed comparable oscillatory periodicity. At pH 8.1, anionic HEPES produced a more diffuse profile with increased Na+ compensation. These results support interfacial ordering as a contributor to enhanced conductivity but do not establish discrete layers or registry. Zeta potentials distinguished stable HEPES interfaces from PBS, where magnitude increased with ionic strength, contrary to classical screening theory. These nonlinear signatures and mesoscale organization underscore the role of buffer selection in high-sensitivity impedance biosensing.