Abstract
Properties of solid-liquid interfaces and surface charge characteristics mediate ionic and molecular transport through porous systems, affecting many processes such as separations. Herein, we report experiments designed to probe the electrochemical properties of solid-liquid interfaces using a model system of a single polyethylene terephthalate (PET) pore in contact with aqueous and propylene carbonate solutions of LiClO4. First, the existence and polarity of surface charges were inferred from current-voltage curves recorded when a pore was placed in contact with a LiClO4 concentration gradient. Second, the electro-osmotic transport of uncharged polystyrene particles through the PET pore provided information on the polarity and the magnitude of the pore walls' zeta potential. Our experiments show that the PET pores become effectively positively charged when in contact with LiClO4 solutions in propylene carbonate, even though in aqueous LiClO4, the same pores are negatively charged. Additionally, the electro-osmotic velocity of the particles revealed a significantly higher magnitude of the positive zeta potential of the pores in propylene carbonate compared to the magnitude of the negative zeta potential in water. The presented methods of probing the properties of solid-liquid interfaces are expected to be applicable to a wide variety of solid and liquid systems.
Highlights
Single pores of well-defined geometries and chemistries are often employed as model systems to probe the electrochemical properties of solid–liquid interfaces and how these properties can tune ionic and molecular transport.1–3 In nanoscale pores, excess surface charges can induce ionic selectivity such that negatively charged pores will transport mostly positive ions.4–8 Surface charges in conjunction with the asymmetric geometry of the nanopores have led to ion current rectification and the preparation of ionic diodes.9–12 In meso- and micro-pores, surface charges are responsible for electrokinetic phenomena such as electro-osmosis.1,13 Single pores are often probed by measuring current–voltage curves in symmetric and asymmetric salt solutions
The properties of the solid–liquid interface in aqueous and propylene carbonate solutions of LiClO4 were probed using a model system, which utilizes a single cylindrically shaped pore prepared in 12 μm thick films of polyethylene terephthalate (PET)
Asprepared, track-etched PET pores have surface carboxyl groups at a density of ∼1/nm2 with pKa ≈ 3.5.39 For some experiments, the pore walls and membrane surfaces were rendered positively charged by the electrostatic adsorption of a polyelectrolyte, poly(allylamine hydrochloride) (PAH), as described previously
Summary
Single pores of well-defined geometries and chemistries are often employed as model systems to probe the electrochemical properties of solid–liquid interfaces and how these properties can tune ionic and molecular transport. In nanoscale pores, excess surface charges can induce ionic selectivity such that negatively (positively) charged pores will transport mostly positive (negative) ions. Surface charges in conjunction with the asymmetric geometry of the nanopores have led to ion current rectification and the preparation of ionic diodes. In meso- and micro-pores, surface charges are responsible for electrokinetic phenomena such as electro-osmosis. Single pores are often probed by measuring current–voltage curves in symmetric and asymmetric salt solutions. We illustrate how probing electro-osmotic transport through the recording of current–voltage curves and the electro-osmotic passage of particles in polymer pores can provide information on the polarity and magnitude of the effective surface charge/electric potential in aqueous and organic media. Using largely uncharged polystyrene particles as a probe, we measured their electro-osmotic velocity in a single PET pore in the two solvents as a function of applied voltage.1 These measurements allowed us to determine the polarity of the surface charge of the pore walls and estimate the zeta potential of the polymer wall/solution (solid– liquid) interface. This paper depicts experiments performed in LiClO4 solutions in propylene carbonate and with polymer films, the methods developed to probe the surface properties of the pores using electro-osmotic transport are expected to be applicable to any material/solvent interface
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