Electrochemical Reactions of Sodium 2-Ethylhexyl Sulfate Salt

Electrocatalysis (2017) 8:270–278 DOI 10.1007/s12678-017-0356-z ORIGINAL RESEARCH Electrochemical Reactions of Sodium 2-Ethylhexyl Sulfate Salt Aleksandra Perek-Dlugosz 1 & Adam Socha 1 & Jacek Rynkowski 1 Published online: 1 March 2017 # The Author(s) 2017. This article is published with open access at Springerlink.com Abstract The electrochemical reactions of sodium 2-ethylhexyl sulfate (EHS) and its effect on the Zn2+ electroreduction have been investigated at a mercury electrode using cyclic voltammetry. It has been shown that the reduction takes place in two steps. The presence of EHS in the solution containing Zn2+ ions moves slightly the potential of zinc reduction towards more negative potentials and causes a slight increase in current density. The differential capacity-potential and differential capacity-time measurements indicate strong adsorption in a wide potential range on the electrode surface. In the potential range −0.46 to −0.86 V vs. saturated calomel electrode and at the concentration lower than the critical micelle concentration (CMC), adsorption for the longer time is hardly reversible. At the concentration higher than the CMC, the formation of hemispherical surface micelles is observed. The theoretical maximum degree of electrode coverage computed with the use of quantum-chemical calculations is equal to 3.53 × 1014 particles cm−2, and it is larger than the value determined experimentally from cyclic voltammograms. In the case of electrochemical reaction, at a current of 0.3 A, during 180 min, the obtained mineralization of EHS is only 3%. Keywords Adsorption . Electroreduction . Mineralization . Sodium 2-ethylhexyl sulfate * Aleksandra Perek-Dlugosz 1 Faculty of Chemistry, Institute of General and Ecological Chemistry, Lodz University of Technology, ul. Zeromskiego 116, 90-924 Lodz, Poland Introduction The electrodeposits of zinc are considered to be one of the main methods enabling anticorrosive protection of steel. Such properties can be achieved by adding certain organic compounds, i.e., arenes and surface-active agents to the zinc bath [1, 2]. Surfactants are commonly used in zinc electrodeposition to control both the shape and size of metallic crystals and, in consequence, to obtain smooth and bright coatings [3, 4]. Solubilization of organic compounds in the presence of their aggregates is also a well-known phenomenon. The most often used surfactants may be classified as ionic (anionic, cationic, zwitterionic) and non-ionic [5]. Anionic surfactants with a negatively charged head group and a positively charged counterion, mostly sodium, have many applications, i.e., in galvanic industry. Their specific activity during electrodeposition depends on their concentration and molecule adsorption on the cathode surface [6]. An example in this group of surfactants is 2-ethylhexyl sulfate (EHS), a commonly used additive in electroplating baths that allows dissolution of sparingly soluble substances in water. At low concentrations, surfactant molecules exist in an aqueous medium in the form of solvated monomers. However, above a particular concentration, known as the critical micelle concentration (CMC), they exhibit an ability to form self-aggregated structures—micelles. Then, their separated water hydrophobic tails aggregate into a hydrophobic interior with a hydrophilic surface [7]. The CMC depends on many factors such as temperature, ionic strength, surfactant chemistry, and the presence of other organic additives in the solution [8, 9]. The use of different surfactants may result in different effects in the process of metal deposition. The accelerating effect of an anionic surfactant on Zn2+ electroreduction was observed in the work of Nieszporek [10] and Gomes and da Silva Pereira [3]. The measurements performed at the dropping mercury electrode showed a current increase without the change in the peak Electrocatalysis (2017) 8:270–278 potential with the addition of 1-decanesulfonate to the zinc solution. Opposite results were obtained in the presence of another anionic alkyl surfactant such as sodium dodecyl sulfate (SDS) [3, 4], for which the authors proved a negative shift with its addition. The process of zinc electroreduction at electrode materials such as Pt, Pd, Au, and steel is characterized by underpotential deposition (UPD) and bulk deposition [11–14]. The UPD process at steel surface starts at the potential of −0.85 V vs. saturated calomel electrode (SCE), and the maximum current is reached at −1.05 V vs. SCE [13]. Simultaneously, hydrogen evolution takes place [13, 14]. The effect of cationic (cetyltrimethylammonium bromide, CTAB), anionic (SDS), and non-ionic (N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, Triton X-100) surfactants on Zn electrodeposition was investigated by Gomes and da Silva Pereira [13]. They found a distinct effect of CTAB and Triton X-100 addition on the reaction of zinc bulk deposition whereas the presence of SDS does not cause significant changes. Zn deposits prepared in the absence of surfactants and in the presence of SDS are more crystalline and have a higher grain size than the ones obtained in the presence of CTAB and Triton X-100. The mass of detergents adsorbed on the surface of various electrode materials grows practically linearly in relation to the c/cCMC ratio equal to 1. Further increase in concentration does not increase the mass of surfactants adsorbed [15]. As it results from the research and literature reports, surfactants are adsorbed on the electrode surface in a wide range of potentials. Excessively high concentration of such substances can obstruct the access to the electrode for diffusing zinc ions. Adsorption of an organic compound on an electrode surface is considered as a replacement not only with water molecules but also with electrolyte ions [6, 16]. Anionic surfactants such as SDS strongly adsorb over a wide range of potentials at the mercury electrode [17]. The formation of the dodecyl sulfate film characterized by multistep adsorption is directly connected with the electrode potential [18, 19]. It was proposed that on hydrophobic surfaces such as gold [20, 21], graphite [22], and mercury electrode [23], surfactants are adsorbed in the form of monolayer, hemicylindrical, or hemispherical structures. Below the CMC, self-organizing aggregates are not formed in solution. In this case, individual molecules adsorb perpendicularly on solid surfaces forming a submonolayer or monolayer. Above the CMC, micelles adsorb and form multilayer on the surface [6, 24–26]. In the longer time, the transformation into hemispherical surface micelles or bilayers of parallel adsorbed molecules occurs [26]. In the case of complete adsorption of films of sodium decyl and dodecyl sulfate, the shift of the point zero charge towards positive potentials has been proved [6]. The phenomenon of surfactant adsorption at different kinds of electrodes has been widely studied. However, the literatu (...truncated)