Reorientation of Thiols during 2D Self-Assembly: Interplay between Steric and Energetic Factors

Abstract

Reorientation of thiols during their 2D self-assembly is well established; however, little is known about its energetics and the factors that control its onset. We have developed a new strategy to determine the critical reorientational surface concentration (crsc) of thiols at the substrate/solution interface, which makes use of a cathodic stripping protocol. Its application to distinct homologous series of alkylthiols shows that the magnitude of the crsc and its variation with the molecular size is strongly dependent on the nature of the terminal group. Methyl-terminated alkylthiols reorient close to the saturation coverage of the lying-down phase, thus following their molecular size trend; whereas reorientation of alkylthiols bearing a negatively charged end group starts well below the monolayer coverage of the lying-down phase, with its onset being almost independent of the molecular size. Hydroxy-terminated alkylthiols show an intermediate behavior. A theoretical approach is developed to determine the reorientation equilibrium constant from the crsc value. The standard free energy of reorientation has been found to vary linearly with the alkyl chain length, and to increase upon replacing the terminal methyl group by a negatively charged one. A quantitative correlation between the reorientation equilibrium constant and the hydrophobicity of the molecule has been established. Overall, these findings have allowed us to disentangle the role of steric and energetic factors in the onset of the reorientation process of alkylthiols, demonstrating that their interplay can be finely tuned by varying either the alkyl chain length or the nature of the terminal group.

Aliphatic Alcohols Facilitate Interfacial Reorientation of Thiols: Correlation with Alcohol Adsorptivity

“…The surface area occupied by the thiol molecule in the lying-down orientation (A I ), with all the methylene groups in contact with the electrode surface, was calculated as described elsewhere. 37 Results and Discussion a. Effect of Ethanol on the Interfacial Reorientation of Alkanethiols.…”

“…However, we have recently shown that alkylthiols bearing a negatively charged terminal group reorient well below the saturation coverage of the lying-down state, its onset being almost independent of the molecular size. 37 Hydroxy-terminated alkylthiols were shown to have an intermediate behavior. Since the reorientational transition is determined by a detailed balance between the interactions of the hydrophobic and hydrophilic portions of the molecule with the solution components, its onset is expected to be sensitive to the nature of the solvent.…”

“…In the present article, we report on the effect of a series of aliphatic alcohols on the onset of the interfacial reorientation of nalkanethiols at the mercury|solution interface by using the cathodic stripping voltammetric protocol described in a previous work. 37 The presence of an alcohol in the depositing solution is shown to facilitate the interfacial reorientation of the thiol, whose onset can be conveniently tuned by varying the length or branching of the alcohol hydrocarbon chain. An extension of our previous model is developed to account for the effect of a second adsorbate in the critical reorientational surface concentration (crsc) of alkylthiols, by means of a quantitative description of the competition between the alcohol and the thiol hydrocarbon chain for the surface adsorption sites.…”

Nanostructured Platforms for the Sustained and Local Delivery of Antibiotics in the Treatment of Osteomyelitis

“…129,130 Despite the presence of calcium, phosphate, and hydroxyl ionic groups on the particle surface, which, in theory, would allow the binding of an array of functional groups, the intense ionic exchange between the particle surface and its ionic milieu renders this approach inoperative for dispersed particles. Compared with calcium phosphate nanoparticles, silanol groups on the surface of silica nanoparticles offer greater stability and more facile functionalization with organic molecules, having the same role as monolayers of thiol groups chemisorbed on the surface of silver, copper, or gold 132 and carboxylic or phosphonic acid moieties on the surface of metal oxides or quantum dots. Their downside, however, is an uncertain fate in the body and an array of inflammatory and oxidative stresses possibly induced in it, ranging from mitochondrial dysfunction to genotoxicity to pulmonary congestion to hepatocyte necrosis.…”

Covalently functionalized gold nanoparticles

“…al. 187 and others 188,189 describe SAM formation in a two-step process: a rapid (24 hour) chemisorption of thiols to gold, followed by reorganization of the molecular backbone to eliminate gauche and pinhole defects. Whereas Georgiadis et.…”