Effect of Anionic Hydration on Counterion Incorporation in Poly(3,4-ethylenedioxythiophene): An X-ray Photoelectron Spectroscopy Study Sarah A. Spanninga, † David C. Martin, †,‡,§ and Zhan Chen †,|ˆ, * Macromolecular Science and Engineering Center, Department of Chemistry, and Department of Materials Science and Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109, and Materials Science and Engineering, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed: May 19, 2010; ReVised Manuscript ReceiVed: July 27, 2010 Poly(3,4-ethylenedioxythiophene) (PEDOT) is widely used in organic electronics and biomedical device coatings because of its outstanding electrical properties and chemical stability. In our previous research, X-ray photoelectron spectroscopy (XPS) was used to investigate the incorporation of various counterions into electrochemically polymerized PEDOT. In this research, XPS was used to further investigate the chemical composition of electrochemically polymerized PEDOT films to determine whether anionic hydration determined counterion affinity in a more systematic way. These counterions probed in this research included sodium citrate tribasic dIhydrate (Na 3 C 6 H 5 O 7 2H 2 O), potassium citrate tribasic monohydrate (K 3 C 6 H 5 O 7 H 2 O), sodium carbonate (Na 2 CO 3 ), calcium carbonate (CaCO 3 ), sodium thiosulfate (Na 2 S 2 O 3 ), sodium acetate trihydrate (NaC 2 H 3 O 2 3H 2 O), sodium phosphate dibasic hepthydrate (Na 2 HPO 4 7H 2 O), sodium phosphate monohydrate (NaH 2 PO 4 H 2 O), sodium chloride (NaCl), sodium bromide (NaBr), lithium bromide (LiBr), sodium perchlorate (NaClO 4 ), lithium perchlorate (LiClO 4 ), and sodium nitrate (NaNO 3 ). Various mixtures containing the above anions were also studied in detail. The thiosulfate was found to act as the dominate counterion for electrochemically polymerized PEDOT within every mixture it was present in regardless of the anionic charge, cation, or anionic hydration. From our systematic study, we deduced the general qualitative trend of PEDOT counterion affinity (from strongest to weakest) as: S 2 O 3 2- > COO - (citrate), Br - , ClO 4 - > Cl - , COO - (acetate), CO 3 2- > NO 3 - > H 2 PO 4 - /HPO 4 2- . These results did not follow the Hofmeister Series rigidly, but some general trends were observed such as the dominance of Br - and ClO 4 - in all mixtures not containing thiosulfate, which matches the conclusions reported in our previous publication. The dominance of the S 2 O 3 2- (hydrated) and citrate COO - (strongly hydrated) as well as the lack of NO 3 - (weakly hydrated) contributions were the major points of deviation from the Hofmeister series, suggesting that anion hydration alone is not the only factor determining PEDOT counterion affinity. 1. Introduction Poly(3,4-ethylenedioxythiophene) (PEDOT) (Figure 1) is a highly conductive, π-conjugated polymer that can be polym- erized by either oxidative chemical polymerization, organic vapor deposition, or electrochemical polymerization. Due to its high conductivity of ∼300 S/cm, PEDOT has been used in antistatic coatings, organic light-emitting devices, 1 solar cells, 2 batteries, 3 and even as biological tissue interfacing agent. 4,5 The incorporation of various counterions into electrochemi- cally polymerized PEDOT focused on how different counterions affected both the polymer surface morphology and electrical properties of the PEDOT polymer film. 6-8 Recently, we investigated the polyanion effect on PEDOT counterion incor- poration. Results show that the polyanions, such as poly(sodium 4-styrenesulfonate) (PSSNa), act as the major PEDOT counte- rion in anionic mixtures. In addition, we examined PEDOT counterion affinity in mixtures of -1 charged anions and a mixture with -2 charged anions. The findings suggest that PEDOT counterion incorporation “loosely” follows the hydra- tion of the anions. The motivation driving this research was to deduce whether anion hydration was indeed the driving force dictating PEDOT counterion incorporation. More specifically, this research consists of three main components: (1) a systematic study on a series of anion mixtures with the gradual addition of more weakly hydrated anions, (2) investigation of mixtures composed of two or three counterions, and (3) the examination of mixtures composed of four or five counterions. Seen previously, X-ray photoelectron spectroscopy (XPS) was used to deduce the respective PEDOT film’s chemical composition. As we extensively introduced before, XPS is an ultrahigh * To whom all correspondence should be addressed. Phone: 734-615- 4189. Fax 734-647-4865. E-mail zhanc@umich.edu. † Macromolecular Science and Engineering Center, University of Michigan. ‡ Department of Materials Science and Engineering, University of Delaware. § Department of Materials Science and Engineering, University of Michigan. |ˆ Department of Chemistry, University of Michigan. Figure 1. Molecular structure of PEDOT. J. Phys. Chem. C 2010, 114, 14998–15004 14998 10.1021/jp104592n  2010 American Chemical Society Published on Web 08/19/2010