In this paper, we demonstrate that organic thiosulfates (Bunte salts) with the general formula R−SSO3M,
where R is either an aliphatic or aromatic group and M a monovalent cation, constitute a novel class of
surface-active compounds with a sulfur-containing headgroup. Bunte salts form self-assembled monolayers
(SAMs) on gold under anaerobic conditions and chemisorb forming a Au−S bond, in which the chemical
nature of sulfur is indistinguishable by X-ray photoelectron spectroscopy (XPS) from gold thiolate formed
upon chemisorption of thiols and disulfides. The S−SO3 bond in the thiosulfate is cleaved during adsorption
on the gold surface and the sulfite moiety is released. We have prepared one alkyl thiosulfate (sodium
S-dodecylthiosulfate, C12SSO3Na) and two aromatic redox-active thiosulfates (potassium S-(2,5-dihydroxyphenyl)thiosulfate, QSSO3K, and dipotassium S,S
‘-(3,6-dihydroxy-1,2-phenylene)bisthiosulfate,
Q(SSO3K)2) and compared the formation and properties of the SAMs prepared from these Bunte salts and
the corresponding thiols (1-dodecylmercaptan, C12SH, and 1,4-dihydroxy-2-mercaptobenzene, QSH) using
XPS, cyclic voltammetry, and ac impedance spectroscopy. The chemisorption of Bunte salts takes place
1−2 orders of magnitude slower than the adsorption of thiols. The SAMs formed from aromatic Bunte salts
QSSO3K or Q(SSO3K)2 have lower surface coverage than those prepared using QSH. With aliphatic
compounds, the films prepared from Bunte salts are either slightly or relatively well-blocking, although
they do not reach the quality achieved with thiol-based SAMs. The differences in the adsorption time scale
and surface coverage are attributed to the bulky thiosulfate headgroup. A major advantage of using Bunte
salts derives from the general synthetic pathway to organic thiosulfates, generally involving a one-pot
synthesis starting from the corresponding halides and yielding the products as odorless crystalline
compounds. This offers a synthetically feasible way of introducing a sulfur-containing surface-active
headgroup into several redox-active or other functional molecules, allowing their incorporation in SAMs.
This facilitates the preparation of functional monolayers for applications in sensor technology and molecular
electronics.
Galena oxidation was investigated by AFM in acetate buffer under potentiostatic control and by photoelectron spectroscopy on potentiostatically pretreated specimens. At +236 mV (SHE) formation of sulfur protrusions could be observed with AFM. XPS showed the formation of elemental sulfur to start at potentials more anodic than +161 mV (SHE). Elemental sulfur could only be retained on the galena surface if sample cooling was started before the beginning of the evacuation in the spectrometer entry chamber. Sulfur-oxygen species could not be detected on galena samples oxidized in acetate buffer even when investigated with synchrotron-excited X-ray photoelectron spectroscopy. AFM images showed two important features: Oxidation starts with a roughening of the sample surface. At slightly more anodic potentials oxidation products are present on the samples as protrusions of 10-200 nm in height and with mutual distances of several hundred nanometers. Two types of sulfur deposits are formed differing in the emergence potential, size, and mutual distance. The formation of such protrusions can only be understood if the reactants for the depositions reach the growing protrusion by diffusion in the liquid phase. Therefore, it is proposed that the process causing the surface roughening is a dissolution of PbS to lead(II) ions and hydrosulfide ions while the deposition reaction is the electrochemical oxidation of hydrosulfide ions to elemental sulfur. By removal of the hydrosulfide ion from the aqueous solution, further dissolution becomes possible at other sample regions. The sulfur formation occurs at distinct points which are not preferentially located at steps. It is likely that the sulfur formation starts at impurity locations. Different impurities may be responsible for different rates of deposit formation, leading to protrusions of different size which however cannot be distinguished by XPS.
X-ray photoelectron spectroscopy (XPS) was used to study the kinetics and mechanisms of sphalerite
activation in a 10-4 M CuSO4 solution at pH 9.2. The activation was fast during the first 10 min, after
which the rate decreased exponentially. The increase in the amount of copper ion uptake was accompanied
by the displacement of zinc ions from the sphalerite surface, indicating that the mechanism of copper
uptake is one of displacement rather than adsorption. The XPS data show that the kinetics of activation
increased considerably in a deoxygenated CuSO4 solution. Both the conventional XPS and synchrotron
radiation XPS (SR-XPS) analyses show that copper activation involves a mechanism in which Cu2+ ions
are reduced to the Cu(I) state, while the sulfur in ZnS is oxidized. Activation in the absence of oxygen
and at an open circuit results in the formation of a CuS-like product, while in an air-saturated solution
copper polysulfides are formed. The latter mechanisms are supported by the appearance of two distinct
S(2p) doublets representing S- and S0 species at higher binding energies. When copper-activated sphalerite
was conditioned in an air-saturated solution of pH 9.2, the CuS-like activation product is oxidized in
preference to unactivated ZnS. The oxidation resulted in a loss of copper from the sphalerite surface,
which may be detrimental to flotation.
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