Electrochemical study of the anomalous salt extraction from water to a polar organic solvent
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Electrochemical study of the anomalous salt extraction from water to a polar organic solvent Vladimír Mareček 1 & Zdeněk Samec 1 Received: 14 May 2020 / Revised: 15 May 2020 / Accepted: 16 May 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Recently, we have reported an unusual phenomenon occurring at the polarized interface between two immiscible electrolyte solutions (ITIES), which is characterized by the anomalous salt extraction seemingly violating the classical partition law [1]. The preliminary results indicated that this extraction is probably driven by the formation of the hydrated salt clusters in the organic solvent phase [1]. Accumulation of water clusters may finally result in the formation of a water-in-oil emulsion [2]. Spontaneous emulsification at liquid/liquid boundaries can be of the great practical significance for separation, pharmaceutical, chemical, foods, and cosmetic applications [3]. Related studies have dealt with the structure and stability of water clusters [4, 5], and the hydration of ions [6, 7], in the hydrophobic polar solvent like nitrobenzene (NB) or 1,2-dichloroethane (DCE). Initial electrochemical measurements were carried out using the cell described schematically in Fig. 1 [1]. The ITIES was supported at the tip of an L-shaped capillary with the orifice diameter of 1.5 mm. The capillary was filled with the aqueous phase (w) and immersed in the organic solvent phase (o) represented by an electrolyte solution in DCE. A small air bubble was formed in the capillary tip to avoid the contact between the phases prior to the open circuit potential (OCP) measurements. At the start of the OCP measurement, the air bubble was ejected using a syringe connected to the capillary. Prior to the OCP measurements, the organic phase was equilibrated with the pure water. The initial composition of the test cell is described by the Eq. 1, AgjAgClj10 mM TAAClðwÞj10 mM TAATPBðoÞj10
ð1Þ
mM TAAClðw’ÞjAgCljAg’ * Zdeněk Samec [email protected] 1
J. Heyrovský Institute of Physical Chemistry, Dolejškova 3, 182 23 Prague 8, Czech Republic
where the vertical bars represent the individual phase boundaries. The phases (w), (o), and (w’) contain the common ion, i.e., tetraalkylammonium (TAA+) cation, the partition of which was supposed to control the Galvani potential difference at the w/o and w’/o interfaces. Experiments were carried out with tetrabutylammonium (TBA+) and tetrapentylammonium (TPeA+) cations. The scheme above describes a symmetric galvanic cell, the potential of which (i.e., the difference between the Galvani electric potentials of the phases Ag and Ag′) can be assumed to equal to zero. However, the OCP measurements of this cell showed instead a remarkable increase of the OCP with time t upon establishing the contact between the phases (w) and (o), which is demonstrated in Fig. 2A. Such increase of the cell potential can be related to a decrease in the concentration + cw;0 TAAþ of the potential-determining TAA cation on the aqueous side of the w/
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