On the two modes of condensation displayed by thiourea at the mercury-water interface

Abstract

In ihe présence of " inert" elecirolytes (sodium fluoride, sulfate, carbonate, phosphate) there eusts a range of températures and thiourea (TU) concentrations (low and high respectively) where two distinct condenscd TU monolayers. lotally devoid of anions, are observed. The two relevant molecular structures which are suggested rest on the analysis of the following data: (1) £-r-[TU| phase diagrams derived from capacitance measurements, (2) superficial excesses and molecular areas. (3) charge densities, (4) inhibition power, and (S) kinelics displayed by the various phase transitions involved. Anions like CIO^. NOf, CIO," are only coadsorbed within the gas-like film of TU, with the resuit that the pure TU condenscd régions are squeezed detectably. On the other hand, anions known tb interact strongly with Hg (Cl". Br'. l'. SCN") do coadsorb significantly al ail potentials. However, pure*TU films tend lo reappear if the concentration of thèse anions is lowered sufnciently with respect to that of the surfactant.

Electrochemical double layer at the interfaces of Hg/choline chloride based solvents

“…For Reline a pair of sharp peaks is observed around 0 V (vs. Ag wire). The peaks are similar to those obtained for aqueous thiourea electrolyte solutions on Hg [30,31] and these origins will be discussed later.…”

“…At potentials close to zero the capacitance for reline and choline chloride: Thiourea mixtures shows characteristic features associated with formation of a film involving chloride ions as reported earlier for thiourea adsorption for aqueous chloride solutions [30,31].…”

The adsorption of thiourea on the mercury from neutral and acidic solutions of perchlorates

“…It is interesting to compare this with the results of Buess-Herman et al [16], who determined the area occupied by one TU molecule in the three types of monolayers occurring at high TU concentrations: for Ôdip regionÕ 0.166 nm 2 , for Ôpit regionÕ 0.197 nm 2 and for the region of the non-condensed phase 0.286 nm 2 . The latter is almost equal to our value.…”

“…These phase transitions are favoured by lowering the temperature, but also by the presence of specifically adsorbing anions such as ClO À 4 ; NO À 3 and the halides, which are thought to involve coadsorption with the TU [16,17]. The appearance of two condensed TU monolayers which are stable in particular ranges of potentials resulted in capacity troughs in the C-E curve called a ÔpitÕ and a ÔdipÕ, separated by a high pseudocapacitive peak.…”

Structural Aspects of Tetramethylthiourea Adsorption on Au(111) Electrodes Investigated by In Situ Scanning Tunnelling Microscopy

“…There are large amounts of electrochemical data for the adsorption of organic molecules on the dropping mercury electrode [6,7), for instance thiourea [8) or TMTU [9). Such investigations have more recently been extended to solid surfaces [10 and see references therein) with the applications of techniques, such as differential capacitance, chronocoulometry and spectroscopy.…”