Adsorption of thiols

The initial step of the adsorption of thiols on a Mo(100) surface is the formation of adsorbed thiolate groups. Phenyl thiolate is formed upon adsorption of thiophenol at 120 K on a clean Mo(110) surface.381 The thiolate intermediate subsequently undergoes competing C-S bond hydrogenolysis to form benzene, or C-S and C-H bond scission to form surface benzyne. The adsorption of thiophenol was also studied on a sulfur-covered Mo surface382 Phenyl disulfide is formed via S-H bond scission and S-S bond formation. The S-S linkage is oriented perpendicular and the phenyl ring parallel to the surface. 

It is obvious that the change in stability upon UPD of Cu and Ag (Figure 5.15) can be harnessed for manipulation of SAMs, as demonstrated by the scheme of Figure 5.20a where nanopores were created in a sequence of steps involving UPD of both Ag and Cu as well as reductive desorption and adsorption of thiols [219,220]. 

A well researched and popular class of monolayers is based on the strong adsorption of thiols (R - SH), disulfides (R - S - S - R) and sulfides (R - S - R) onto metal surfaces. Although thiols, disulfides, and sulfides strongly align with a number of different metals Hke gold, silver, platinum, or copper, gold is usually the substrate of choice because of its inert properties and the formation of a well-defined crystal structure. 

Adsorption of Thiols Self-assembly of alkanethiols (RSH) on gold has especially been studied intensively. 

All the work discussed in this section involves rather ill-defined substrates and one is on firmer ground when discussing the adsorption of thiols on single crystal gold in Section 6.4. 

Thiols are the sulfur analogs of alcohols. They have high binding energy to a gold surface and form self-assembled monolayers, which are molecular assemblies that are prepared by spontaneous adsorption of thiols from solution onto a gold substrate [i, iii]. 

The mechanism of formation of gold-thiol monolayers has been intensively investigated over the last decade. A detailed record of these studies is given in a recent review. Formation of monolayers from thiols is understood best, although some information on adsorption of disulphides and sulphides is also available. XPS analyses of the sulphur 2p peak in monolayers indicate that adsorption of thiols and disulphides on gold produces thiolate species, Raman spectra of monolayers of disulphides and thiols,... 

The energy of the adsorption process could be estimated on toe basis of toe literature data using bond energies for RS—H, H2, RS—SR and RS—Au at 87, 104, 74 and 40kcalmol , respectively . Values of —5 and —6 kcalmol per gold thiolate unit are obtained for adsorption of thiols and disulphides, respectively. Schlenoff and coworkers estimated values of —5 and —12 kcalmoP for adsorption of thiols and disulphides, from his own and earher published data on toe voltammetric curves corresponding to the thermodynamically controlled (equiUbrium) electrodeposition of alkanethiolates on gold . Recently, accurate thermodynamic parameters were obtained... 

Goh, S.W., Buckley, A.N., Lamb, R.N., Woods, R. (2006) The ability of static secondary ion mass spectrometry to discriminate submonolayer from multilayer adsorption of thiol collectors. Min. Eng., 19,571-581. 

The earliest, and also the most widely studied, SAM systems are those formed by the adsorption of alkyl-siloxanes onto silicon dioxide and by the adsorption of thiols onto noble metal surfaces (Ref. 11 is an extensive review of the subject), but many other SAM systems have been reported. Besides gold, alkanethiols also form ordered, close-packed monolayers on the surfaces of a number of noble metals (including Ag, Cu, Pd, and Pt) and on GaAs, while alkylphosphonic acids and carboxylic acids form SAMs on oxide surfaces. Siloxane monolayers are the most stable, utilizing strong, covalent interactions (Si-O-Si linkages) to tether the molecules to a silanol-terminated oxide surface. However, the chemistry of interaction between the adsorbates and the surface is complex. Silanes (RSiCb) and alkoxysilanes (RSiORj) tend to self-polymerize, which confers additional stability on the... 

This chapter reviews the adsorption of thiols at metal and metal sulfide surfaces. The motivation for studying these systems derives from the interaction of this interesting group of organic compounds with native metal and sulfide mineral surfaces being the key chemical step in the separation and concentration of such minerals from their ores by froth flotation. A number of different species can be formed by the interaction of thiols with metals and sulfides, the identity of the adsorbate depending on the particular thiol and mineral, the adsorption conditions, and the extent of reaction. This review focuses on the chemisorbed species formed in these systems. The concept of chemisorption is one of the formation of chemical bonds between the thiol and atoms in the mineral surface without removal of component atoms from their positions in the metal or metal sulfide lattice. 

Thus, three theories had been developed and each had difficulties formation of metal thiols did not fit thermodynamics, adsorption of ions did not fit charge requirements, and adsorption of thiol acids did not fit stability criteria. 

The literature on SAMs will be considered in this chapter only when there is information that elucidates the adsorption of thiols relevant to flotation, and the results of these publications will be cited in the following... 

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