The Role of Cl- Ions During the Adsorption of Polyethylene Glycol (MW 20,000) onto a Polycrystalline Gold Electrode

The adsorption of polyethylene glycol (MW 20,000) (PEG(20000)) onto a polycrystalline gold electrode in the presence of Cl- ions was studied by differential capacitance measurements and quartz crystal microbalance. This study provides new physical insights into the adsorption behavior of this polymer onto a solid electrode in the presence of Cl- ions and KClO4. Three potential-based behaviors were observed: in the potential range from -0.90 to -0.62 V vs. the saturated calomel electrode (SCE), the principal adsorption process was observed to form a condensed PEG(20000) phase. In addition, there was a slight desorption of PEG(20000). In the potential range from -0.62 to -0.10 V vs. SCE, the addition of Cl- ions to the solution reduced the amount of PEG(20000) adsorbed. This behavior was associated with the lower number of available active adsorption sites on the gold surface due to the adsorption of Cl- ions. In addition, we observed the superposition of the two adsorption processes on different active sites. PEG was adsorbed on lower-energy adsorption sites (the most accessible adsorption sites), whereas Cl- ions were adsorbed on higher-energy adsorption sites (the least accessible adsorption sites). The potential range from -0.10 to 0.35 V vs. SCE was dominated by the adsorption of Cl- ions, and a displacement of adsorbed PEG(20000) molecules associated with the adsorption of Cl- anions was observed.