Adsorption Of HS Research Articles

Early Stages of ZnS Growth Studied by Stopped-Flow UV Absorption Spectroscopy: Effects of Educt Concentrations on the Nanoparticle Formation

The growth of ZnS nanoparticles by precipitation from supersaturated aqueous solution is studied by stopped-flow UV absorption spectroscopy. The average size, size distribution, and concentration of the particles are monitored within the sub-second time regime with a resolution of 1.28 ms. Particle growth at these early stages is governed by pronounced ripening. The UV absorption data strongly suggest that growth occurs by preferential adsorption of HS- anions relative to Zn(2+) or ZnOH(+) cations. Correspondingly, the initial sulfide concentration has a much more pronounced influence on the growth kinetics than the initial zinc concentration. These findings are verified by zeta-potential measurements which confirm that the particle surfaces are negatively charged under near-neutral pH conditions.

Surface Gradient Material: From Superhydrophobicity to Superhydrophilicity

We have developed a simple method to fabricate a gradient from superhydrophobicity to superhydrophilicity. It is based on the controlled self-assembled monolayer consisting of a thiol molecule on a gold surface and the amplifying effect of the wetting property on a rough surface. Using a relatively dilute HS(CH2)11CH3 solution (0.05 mmol/L), we found that the density of molecules on the surface can be controlled by varying the immersion time. Slowly adding the dilute solution to the container holding the rough gold substrate will lead to a density gradient along the surface. After the complementary adsorption of HS(CH2)10CH2OH, the surface exhibits a gradient from superhydrophobicity to superhydrophilicity. The slope of the gradient can be conveniently tuned by varying the speed of addition. Cassie-Baxter and Wenzel equations are employed to explain this special property based on the rough structure and the molecular composition gradient that have been determined by XPS. This kind of material would provide a larger oriented driving force for many important biological and physical processes and might have potential applications in water droplet movement, oriented axonal specification of neurons, protein adhesion, and so on.

The effect of sulphide ions on the corrosion inhibition of copper in acidic chloride solutions

The corrosion behaviour of copper in dilute HCl solution of pH 1.8‐2, in the absence and presence of different additives of Na2S, was studied by open circuit potential measurement, scanning electron microscopy and polarisation techniques. A complete covering of the surface with the strongly chemisorbed HS− is achieved at ≥10 ppm Na2S concentration. The addition of different concentrations of ethylene diamine (EDA) to HCl of pH 1.8‐2 promoted the dissolution rate, due to an increase in autocatalytic dissolution at the Cu metal surface. However, the synergistic inhibition effect between EDA and Na2S, resulting from the adsorption of HS− on the Cu metal, facilitated the adsorption of EDA.

Electrochemical Growth of Ag2S on Ag(111) Electrodes. Coulometric and X-ray Photoelectron Spectroscopic Analysis of the Stepwise Formation of the First and Second Monolayers of Ag2S

Stepwise electrochemical growth of the first and second monolayers of Ag2S at a Ag(111) electrode in aqueous hydrosulfide (HS-) solutions is reported. The deposition of the first and second monolayers occurs at different electrode potentials and can be resolved in voltammetric experiments prior to the formation of a bulk Ag2S layer. Oxidative formation of the first Ag2S monolayer occurs by a kinetically facile two-electron, two-step mechanism, involving the one-electron oxidative adsorption of HS- (Ag + HS- ⇌ Ag−SH + e-) followed by a one-electron oxidative phase transition to yield a complete Ag2S monolayer (Ag−SH + Ag + OH- ⇌ Ag2S + H2O + e-). An irreversible wave corresponding to the formation of a second monolayer of Ag2S in a single two-electron step (2Ag + HS- + OH- ⇌ Ag2S + H2O + 2e-) is voltammetrically resolved at slow scan rates (<25 mV/s). Coulometric analysis of the voltammetric data demonstrates that the quantity of electricity (∼164 μC/cm2) consumed during the formation of the first and seco...

Calorimetric investigation of the formation of ZnS nanoparticles in w/o microemulsions

The enthalpies of precipitation of ZnS nanoparticles within water containing reversed micelles of sodium bis(2-ethylhexyl) solfosuccinate, L-α phosphatidylcholine, tetraethyleneglycol-mono-n-dodecyl ether and didodecyldimethylammonium bromide as a function of the molar concentration ratioR (R=[water]/[surfactant]) were measured by calorimetric technique. The results indicate that the energetic state of ZnS nanoparticles confined in the aqueous core of the reversed micelles is different from that in bulk water. Effects due to nanoparticle size, adsorption of HS− ions on the nanoparticle surface and interactions between nanoparticles and water/surfactant interfaces are discussed.

Oxidation of aqueous sulfide solutions by dioxygen Part II: Catalysis by soluble and immobilized cobalt(II) phthalocyanines

AbstractThe oxidation of sulfide in oxygen‐saturated aqueous solutions is accelerated by dissolved or silica‐bonded cobalt phthalocyanines. On the basis of thermodynamical considerations it is postulated that the catalyst enhances the formation of disulfide as the initial elementary reaction step. The following reaction steps are largely unaffected by the catalyst, as indicated by a product ratio sulfate/thiosulfate=0.86, comparable to that of the uncatalyzed autoxidation. A Langmuir‐Hinshelwood formalism is developed for the catalytic reaction step and is demonstrated to fit best with the kinetics. In addition, from the kinetic data free enthalpies for the adsorption of HS− (i) at the dissolved phthalocyanine (ΔG = −17.6 kJ/mole) and (ii) at the immobilized complex (ΔG = −20.0 kJ/mole) are calculated.

Electrochemical Deposition of Hydrosulfide and Ethanethiolate Adlayers on Silver(111). Voltammetric Measurement of Structural Phase Transitions During Adlayer Formation

AbstractReversible, oxidative adsorption of hydrosulfide (HS−) and ethanethiolate (CH3CH2S−) on highly‐ordered, Ag(111) electrodes in aqueous 0.5 M NaOH solutions is reported. Oxidation of HS− and CH3CH2S− results in adlayer formation, as determined by voltammetry, coulometry, and in‐situ electrochemical quartz‐crystal measurements. The voltammetric response of Ag(111) in HS− solutions displays three characteristic waves. Two waves correspond to the reversible 1‐e− oxidative adsorption of HS”, resulting in the formation of a Ag‐SH adlayer. The third wave corresponds to the kinetically‐slow 1‐e− oxidation of the Ag‐SH adlayer, yielding an underpotential deposited monolayer of Ag2S. The voltammetric response observed in CH3CH2S− solutions is qualitatively similar to that of HS−, displaying two well‐resolved waves corresponding to oxidative adsorption. The observation of two voltammetric waves is suggestive of a mechanism of CH3CH2S− adlayer formation involving at least two distinct structural phases. However, unlike the situation for HS”, the resulting Ag‐SCH2CH3 adlayer is unable to undergo a second oxidative transition. The free energies of adsorption of HS” and CH3CH2S− on Ag(111) are measured to be ca. −23 kcal/mol.

Self-assembled monolayers of alkanethiols on copper provide corrosion resistance in aqueous environments

Alkanethiols adsorb onto copper surfaces and form densely packed monolayer films that inhibit corrosion of the underlying copper substrate. The performance of these self-assembled monolayers are insensitive to the presence of water as they provide equal levels of protection in both dry and humid oxygen-containing atmospheres. This observation suggests that the monolayer films provide a barrier against water penetration, as the corrosion of untreated copper samples is much more rapid in the presence of water. In these applications, longer-chained adsorbates — C 18H 37SH and C 22H 45SH — provided the greatest levels of protection. Monolayers formed from these adsorbates were also effective as barrier layers toward oxidation when the samples were immersed in oxygen-saturated water for over 1 week. These densely packed, chemisorbed layers maintained their level of performance during the weeklong exposure without the need for adding corrosion inhibitor molecules to the contacting water phase. Preliminary experiments show that bilayer assemblies formed from the sequential adsorption of HS(CH 2) 22OH and CH 3(CH 2) 17SiCl 3 provide a stable molecular architecture for forming thicker, robust coatings on the copper surface; however, the use of chlorine-containing reagents in the formation of this assembly results in defects that seriously impede the inhibition properties of the layers.

Electrochemistry of Sulfur Adlayers on the Low-Index Faces of Silver

The formation and reactivity of sulfur adlayers on single-crystal Ag electrodes ((111), (110), and (100) orientations) in aqueous solutions (pH = 13) containing HS- are reported. Oxidative adsorption of HS- (Ag + HS- → AgSH + e-) occurs on all three low-index surfaces at potentials ranging between −0.5 and −0.7 V of the thermodynamic value for bulk Ag2S formation. Voltammetric and electrochemical quartz crystal measurements demonstrate that the resulting AgSH adlayer undergoes a second one-electron oxidation (AgSH + Ag + OH- → Ag2S + H2O + e-) at the (111) and (110) surfaces prior to bulk Ag2S formation, yielding an underpotential deposited Ag2S adlayer (surface coverages (S/Ag): θAg(111) = 0.46 ± 0.02 and θAg(110) = 0.54 ± 0.03). In contrast, the AgSH adlayer on Ag(100) is chemically inert prior to bulk Ag2S formation. Structural models indicate that the formation of a nearly stoichiometric Ag2S adlayer (i.e., θ ∼ 0.5) is feasible on the (111) and (110) surfaces without significant reconstruction of the...

An electrokinetic study of ZnS and CdS surface chemistry

ζ-potential-vs-pH measurements on ZnS and CdS powders were performed in oxygen-free, nonoxidizing indifferent supporting electrolytes, in Zn and Cd salt solutions, respectively, and in (NH 4) 2S solutions of different concentrations. ZnS and CdS ζ-potential-vs-pH curves in Zn and Cd salt solutions, respectively, are S-shaped, showing three charge reversal points: CR1, CR2, and CR3. Up to CR1 points, ζ potentials are positive due to H + adsorption on S Lewis sites; between CR1 and CR2 points, they are negative owing to ionization of S Brønsted sites; between CR2 and CR3 points, they are positive as a result of Zn 2+ and ZnOH + or Cd 2+ and CdOH + chemisorption on S Brønsted sites; they are negative beyond CR3 due to OH − adsorption on Zn or Cd Lewis sites and due to the surface precipitation of hydroxides. ZnS and CdS ζ-potential-vs-pH curves in (NH 4) 2S solutions show minima owing to adsorption of HS − anions on Zn or Cd Lewis sites. ZnS and CdS ζ potentials show a logarithmic dependence(i) on [HS-] in the pH range 6 to 9, (ii) on [Zn 2+] in the pH range 4 to 6, and (iii) on [Cd 2+] in the pH range 4 to 7. H +, OH −, HS −, Zn 2+, ZnOH +, Cd 2+, and CdOH + are potential-determining ions. The ZnS pH iep is 2.3; the CdS pH iep is 1.8.

Formation of monolayers by the coadsorption of thiols on gold: variation in the head group, tail group, and solvent

Long-chain alkanethiols, HS(CH2)nX, adsorb from solution onto gold and form oriented. ordered monolayers. Monolayers exposing more than one functional group at the surface can be generated by coadsorption of t*'o or more thiols from solution. In general, the ratio of the concentration of the t\r'o components in a mixed monolayer is not the same as in solution but reflects the relative solubilities of the components in solution and interactions between the tail groups, X, in the monolayers. Multicomponent monolayers do not phase-segregate into single-component domains large enough to influence the contact angle (a few tens of angstroms across) and also do not act as ideal two.dimensional solutions. In the two-component system HS(CH2)J/HS(CH2)CH3 in ethanol, where X is a polar tail group such as CH2OH or CN, adsorption of the polar component is particularly disfavored at low concentrations of the polar component in the monolayer. These isotherms may arise from poor solvation of the polar tail groups in the quasi-two-dimensional alkane solution provided by the methyl tail groups. From dilute solutions in alkanes, adsorption of HS(CH2)r9CH2OH is strongly preferred over HS(CHz)roCHr, probably due to the stabilization afforded by intramonolayer hydrogen bonds between the hydroxyl tail groups. The wettability of mixed monolayers is not linear in the composition of the surface. (n a surface comprised of a polar and a nonpolar component, the polar component is more hydrophilic when its concentration in the monolayer is low than when the monolayer is composed largely of the polar component.

The potentiodyanmic formation and reduction of a silver sulfide monolayer on a silver electrode in aqueous sulfide solutions

A potentiodynamic study of silver electrodes in aqueous sulfide solutions, carried out to form phase silver sulfide films, revealed that a monolayer of silver sulfice forms as a distinct and separate stage of film growth at an underpotential of about 0.12 V. The monolayer peak (and its cathodic counterpart) was also characterized by a linear relationship between peak current density and potential sweep rate and a constant charge density of about 0.2 mC/cm 2. The potentiodynamic E/ i curves for the silver sulfide monolayer were simulated by computer on the basis of a mechanism of the initial adsorption of HS − on the silver surface in a fast equilibrium step followed by a rate determining electron transfer step to form AgHS as a surface intermediate. The AgHS species then rapidly diffuses on the surface and joins a growing two-dimensional silver sulfide monolayer nucleus. Under the experimental conditons studied here, the formation and reduction of the silver sulfide monolayer was found to be of intermediate kinetic reversibility.