Formation of cluster ions A<sup>−</sup>(H<inf>2</inf>O)<inf>n</inf> in nonpolar liquid

Abstract

Study of processes connected with formation of cluster ions with water molecules is of interest due to the fact that such ions can be produced by ionization of nonpolar dielectric liquids in conditions when the liquid has a contact with water or a humid air. In this work, results of investigations of the processes leading to the formation of the negative ion clusters A <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> (H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sub> are presented. Three experimental approaches were utilized in the study on the ion cluster formation. The first one is connected with a change in the ion mobility after the clusterization. The second one is based on a changing the cross section of the photodetachment of the electron from negative ion. The third approach that was utilized for the ions, which are not stable in the nonpolar liquid, is based on the measurement of the life time of the negative ion in respect to the autodetachment of the electron. Tetramethylsilane (TMS), the liquid with a high mobility of the excess electrons, was used as a model medium. The primary ions O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> were produced by the capture of excess electrons by molecules of corresponding substances dissolved in the liquid. The excess electrons were generated near the negative electrode of a conductivity cell by photoionization of an admixture introduced in the liquid in a small amount. The cluster ions were formed in the reactions of the primary ions with molecules of water dissolved in the liquid. It has been established that, in the range of low water concentrations 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> -10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> M, the main part of ions exists in a form of the small clusters A (H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O). Formation of the small clusters is followed by a decreasing the ion mobility as well as by a shift of the threshold of the electron photodetachment. The transition into this form results in a stabilization of ions, which are short- lived in the waterless medium. The rate constants for the processes responsible for the ion cluster formation were determined. It has been shown that the coupling the primary negative ions with the water molecule is controlled by the diffusion of the reagents in the liquid. The big ion clusters are observed when the water concentration is close to the solubility limit. The big cluster ions are formed rather in the coupling the small cluster ions with neutral water clusters being in small concentrations in the nonpolar liquid near the saturation point. So, it was shown that even a trace amount of water in nonpolar liquid leads to hydration of ions. This fact should be taken into account by analysis of the electroconductivity of the dielectric liquids being in a contact with water or the moist atmospheric air.