UC Santa Barbara

Molecules that self-assemble to form thin coatings on surfaces are of great interest to many industries (e.g., biomedical, marine, and solar) for imparting hydrophobic, conductive, and/or anti-fouling properties. For example, in capillary electrophoresis (CE), inner surfaces of a capillary are coated to control electroosmotic flow (EOF), prevent adsorption, and increase solubility of proteins; this coating reverses the direction of EOF relative to electrophoresis (EP), thereby enhancing separation efficiency and resolution. The rapid degradation of these coatings, however, causes band broadening and variable migration times as the desorbing species changes the EOF velocity and interferes with the detector. Unfortunately, studies to understand desorption in these conditions show different kinetic behaviors and employ low-resolution analysis techniques. Therefore, techniques with greater temporal resolution and control of solution and transport properties are needed to elucidate degradation mechanisms in CE and other applications.

This work presents a high frequency, in situ platform for extracting surface charge (i.e., zeta potential) within a silica microcapillary during surface coating or degradation. A continuous platform was adapted from the current monitoring method, which extracts EOF velocity by measuring the time for fluid to traverse a channel of known length in an electric field; the traversal endpoint is measuring by a linear change in current and subsequent steady state as a solution of different conductivity fills the channel. Zeta potential is calculated using the Helmholtz–Smoluchowski equation and the EOF velocity. Automated zeta potential analysis (AZA) detects the traversal endpoint using the first-derivative, then alternates the electric field polarity for consecutive measurements.

This novel automated approach for monitoring zeta potential was first applied to study aminosilane coating formation and stability for different monomer types and solution conditions. The density of cationic coatings was inferred by an increase in zeta potential relative to the bare surface value; coating stability was monitored by the relative decay in zeta potential after exposure to conditions without the monomer. First, applying AZA to study aminosilanes, we observed higher densities for trimethoxy vs. monomethoxy silanes and faster coating kinetics for trimethoxy vs. triethoxy silanes. Upon exposure, we observed faster decay for higher-pH solutions and shorter-alkyl-length monomers. Coatings deposited in aqueous vs. anhydrous conditions exhibited a lower density and greater stability, indicating that aqueous conditions promote more lateral crosslinking.

We also applied AZA to study adsorption and desorption of CTAB, a cationic surfactant, in conditions relevant to CE (e.g., capillary diameter, EOF/EP, and pressure). In contrast to previous studies, our kinetics were multiphasic with an intermediate "stagnant regime” at distinct zeta potentials in adsorption or desorption. These stagnant regimes occurred at positive zeta potentials where the EOF was equal and opposite to EP, inducing a near-zero net transport of CTA+ (EP+EOF). The kinetics depended strongly on the capillary surface-area-to-volume ratio and the EP mobility of CTA+, which changed with concentration. We confirmed these trends by recasting the zeta potential kinetics in terms of net CTA+ transport volume divided by surface area; this normalization collapsed and linearized the responses for a range of diameters. We showed that varying the voltage algorithm and applying pressure could prolong or eliminate the stagnant regime; new AZA algorithms were developed to further control the transport-limited kinetics by maintaining a fixed net transport of CTA+. With higher resolution and control of transport during coating formation and degradation, this versatile platform enables the discovery of optimal coating chemistries for an array of applications.