UC Irvine

Meso- and nano- pores of well-defined geometries and surface charge characteristics are often employed as model systems to probe the electrochemical properties of solid-liquid interfaces. The high surface area to volume ratio and tunable surface charge of these porous systems gives them fascinating ion transport properties that dominate by controlling the solid-liquid interface. These unique transport properties also affect many processes such as the performance of membrane separation (desalination) systems. In this doctoral dissertation, we report experiments designed to facilitate the systematic studying of ionic transport and solid-liquid interfaces using single polyethylene terephthalate (PET) pores in two separate projects:

The first using cylindrical pores, ~ 1 μm in diameter (and 12 μm in length), and the resistive-pulse technique, to investigate the electroosmotic transport of unfunctionalized polystyrene particles through the PET pore in organic, propylene carbonate solutions of LiClO4. The direction of electroosmotic particle translocation informs us about the surface potential of the pore walls and their duration, a measure of the particles’ velocity and the pore walls’ zeta potential. Our experiments show that the carboxlyated, as prepared surface of PET pore(s) become positively charged in LiClO4 solutions of propylene carbonate, even though in aqueous media, the same pore(s) are negatively charged. These findings were also verified by measuring current-voltage curves in a propylene carbonate, LiClO4 concentration gradient. The electroosmotic velocity of the particles also revealed that the positive zeta potential of the pores in propylene carbonate is significantly higher than the negative zeta potential in water.

Additionally, conically shaped pores, 4-25 nm (tip), 700-1100 nm (base) diameter, KCl and bulky chromium, trivalent (Cr3+) cations, were used to probe the switching of the PET surface potential through charge inversion. Briefly, charge inversion occurs when a (negatively charged) pore is placed in contact with oppositely (positively) charged multivalent ions at a sufficiently high (≥ 2) Z and bulk electrolyte concentration. At these experimental conditions, the cations attract, over screen and invert the sign of surface potential here, from negative to positive. The origin of charge inversion is often described through the concept of a strongly correlated liquid that is formed by the multivalent ions in contact with the charged surface. When the surface potential of the pore is inverted, the observable current-voltage curve changes as well and can be experimentally probed. In this work, it was successfully demonstrated through recorded current-voltage curves and ion current signal measurements in time that when the bulk Cr3+ concentration reached 1 mM, the walls of the conically shaped nanopore became positively charged. Also, pores whose tip diameter is less than or equal to 10 nm exhibited ion current fluctuations in time and negative incremental resistance(s). This finding suggests that the pores become partially sterically occluded with the bulky ions which in sub-5 nm pores has major implications on charge inversion and the pore ionic conductance.