ORIGINAL PAPER Ion and solvent transfer of polyaniline films electrodeposited from deep eutectic solvents via EQCM Hani K. Ismail 1 & Hasan F. Alesary 2 & Azhar Y. M. Al-Murshedi 3 & Jalil H. Kareem 4 Received: 20 August 2019 /Revised: 27 September 2019 /Accepted: 27 September 2019 # Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract We report herein the electrochemical polymerisation of aniline from a choline chloride (ChCl)-based deep eutectic solvent (DES) at room temperature in both the presence and absence of deionised water in the solvent. The current study used Propeline as the DES electrolyte, which we found to be rich in protons (i.e. similar to protic ionic liquids) and which allowed the polymerisation of aniline in this system. The morphology of films was characterised by scanning electron microscopy (SEM) and atomic force microscopy (AFM), which showed the polymer film to have a compact structure and rough surface. The electrochemical quartz crystal microbalance (EQCM) was used for the first time to investigate the mass changes that occur during the electrochemical deposition of polyaniline (PANI) in situ, and was used to monitor cation, anion, and solvent transport when PANI films were immersed in pure Propeline and 0.5 M H 2 SO 4 solution (monomer free) at 25 °C and 50 °C. In the case of the PANI exposed to pure Propeline at room temperature, the mass change increased continuously in both oxidation and reduction switching due to anion transfer dominating the mass change, while the PANI exposed to pure Propeline at 50 °C was found to form due to a two- step combined mechanism, showing excellent redox switching stability with changing mass. These variations in mass tend to be dominated by cation expulsion in the first oxidative process (the later reductive process being cation insertion) and by anion insertion in the second oxidative stage (the early reductive process being anion expulsion). In aqueous systems, the mechanism of mass change was dominated by anion movement associated with solvent transfer. The movement of neutral species in Propeline and water is in the reverse direction to those of cation or anion transfer. Keywords Conducting polymers (polyaniline) . EQCM . Deep eutectic solvent (DES) . Ion transfer . Solvent transfer . Redox switching Introduction Generally speaking, electrochemical studies of conducting polymers require a recognition of the dynamics of polymer growth in systems such as transport analyte species, solvents, and rate of charge balance, which are affected by selective electrolyte content [1, 2]. In the current study, we investigate these factors by electrodepositing a conducting polymer (i.e. polyaniline) at room temperature from a deep eutectic solvent (DES—a type of ionic liquid) as the background electrolyte, and then expose the polymer to DES and aqueous solutions. Basically, conducting polymers have attracted considerable attention as ‘intelligent’ materials that can be employed in a wide range of interesting applications such as batteries, sen- sors, full cells, supercapacitors, and photovoltaic and electrochromic display devices [3–5]. Among what is a sig- nificant number of conjugated polymers, polyaniline (PANI) is considered one of the more promising due to its particular properties: simplicity of preparation, good reversibility of re- dox switching, good chemical stability, environmental friend- liness, and good capacitive and conductivity features. In addi- tion, it can be formed from the aniline monomer by simple chemical or electrochemical polymerisation [6–8]. The redox behaviour, and the physical and chemical properties of polyaniline, can be influenced by various additives such as * Hani K. Ismail hani.khalil@koyauniversity.org 1 Department of Chemistry, Faculty of Science and Health, Koya University, Koya KOY45, Kurdistan Region F.R., Iraq 2 Department of Chemistry, College of Science, University of Kerbala, Karbala, Iraq 3 Department of Chemistry, Faculty of Education for Girls, University of Kufa, Najaf, Iraq 4 Department of Petroleum Technology, Erbil Technology Institute, Erbil Polytechnic University, Erbil, Iraq Journal of Solid State Electrochemistry https://doi.org/10.1007/s10008-019-04415-1