In situ study of the deposition of (ultra)thin organic phosphonic acid layers on the oxide of aluminum

Two Steady-State Adsorption Modes of Phosphonic Acids on Aluminum Surfaces

The phosphonic acid (PA) surface treatment on various metal substrates is of high industrial relevance, and the PA molecular structure significantly affects its quality. In this work, systematic variation of the PA molecular steric and electron environment helps discern two steady-state adsorption modes on an aluminum surface. The PA molecular structure was varied systematically, which included inorganic phosphorus acid, alkyl phosphonic acids, and phenyl phosphonic acids. To explore their in situ dynamics of adsorption/desorption on the electrochemically unstable aluminum, techniques such as electrochemical impedance spectroscopy and inductively coupled plasma optical emission spectrometry were employed. A range of different types of interfacial layers are formed on the aluminum surface, namely, from the dissolution-limiting physisorbed layer to a quasi-inhibiting chemisorbed layer on the aluminum surface in acidic (pH ≈ 2.2) solution. Presented findings establish the dynamic steady-state nature of this type of interface. They reveal fundamental relationships among adsorbent steric or electronic effects, the steady-state interface morphology, and the steady-state aluminum dissolution rate. The study brings also a more differentiated molecular structure-related description of the aluminum dissolution inhibition of PAs and relates it to molecular density functional theory calculations.

The nature of self-assembled octadecylphosphonic acid (ODPA) layers on copper substrates

HYPOTHESIS

The self-assembly of amphiphilic molecules onto solid substrates can result both in the formation of monolayers and multilayers. However, on oxidized and non-oxidized copper (Cu), only monolayer formation was reported for phosphonic acids possessing one phosphate head group. Here, the adsorption of octadecylphosphonic acid (ODPA) on Cu substrates through a self-assembly process was investigated with the initial hypothesis of monolayer formation.

EXPERIMENTS

The relative amount of ODPA adsorbed on a Cu substrate was determined by infrared reflection/absorption spectroscopy (IRRAS) and by atomic force microscopy (AFM) investigations before and after ODPA deposition. X-ray photoelectron spectroscopy (XPS) with sputtering was used to characterize the nature of the layers.

FINDINGS

The results show that the thickness of the ODPA layer increased with deposition time, and after 1 h a multilayer film with a thickness of some tens of nm was formed. The film was robust and required long-time sonication for removal. The origin of the film robustness was attributed to the release of Cu ions, resulting in the formation of Cu-ODPA complexes with Cu ions in the form of Cu(I). Preadsorbing a monolayer of octadecylthiol (ODT) onto the Cu resulted in no ODPA adsorption, since the release of Cu(I) ions was abolished.

Corrosion Inhibition of Mild Steel by Cetrimonium trans-4-Hydroxy Cinnamate: Entrapment and Delivery of the Anion Inhibitor through Speciation and Micellar Formation

Chemical inhibitors are widely used to protect metallic alloys from corrosion in aqueous environments. This Letter investigates the possible synergistic behavior of a quaternary ammonium carboxylate compound toward the development of a new system taking advantage of the surface activity of a known antimicrobial surfactant molecule, hexadecyl trimethylammonium cation, combined with a known organic corrosion inhibitor, the trans-4-hydroxy-cinnamate anion. Short-term potentiodynamic polarization (PP) studies combined with immersion in aqueous chloride solutions demonstrated the high inhibition efficiency of the combination of ions, and NMR pfg-diffusion measurements revealed micellar formation that was concentration- and pH-dependent. The NMR data suggest that speciation changes occur in the solution that correlate with enhanced corrosion inhibition efficiency at higher pH and at concentrations above the CMC of the compound. This new contribution may provide a rational molecular design toward delivering corrosion inhibitors to a metal surface through controlled speciation in solution.

Molecular dynamics simulations of phosphonic acid–aluminum oxide self-organization and their evolution into ordered monolayers

We outline an unprejudiced molecular dynamics simulation approach to study the mechanisms of self-organization encompassing the evolution of surfactant–surface interactions to the growth of self-assembled monolayers (SAMs).

The interaction of carboxylic acids with aluminium oxides: journeying from a basic understanding of alumina nanoparticles to water treatment for industrial and humanitarian applications

Carboxylic acids are found to react with aluminium oxides via a topotactic reaction such that the carboxylate acts as a bridging ligand. This reaction allows for carboxylate-functionalized alumina nanoparticles to be prepared directly from boehmite (AlOOH). Understanding the structural relationship between molecular and surface species allows for the rationalization/prediction of suitable alternative ligands as well as alternative oxide surfaces. The identity of the carboxylate substituent controls the pH stability of a nanoparticle as well as the porosity and processability of ceramics prepared by thermolysis. Through the choice of functional groups on the carboxylic acid the properties of the alumina surface or alumina nanoparticle can be tailored. For example, the solubility/miscibility of nanoparticles can be tuned to the solvent/matrix, and the wettability to be varied from hydrophobic to super hydrophilic. The choice Zwitter ionic substituents on alumina micro-/ultra-filtration membranes are found to enhance the flux and limit fouling while allowing for the facile separation of organic compounds from water. Examples are presented of purification of frac and flow-back water from oil well production as well as providing drinking water from contaminated sources in underdeveloped regions.

Stability of (bio)functionalized porous aluminum oxide

Porous aluminum oxide (PAO), a nanostructured support for, among others, culturing microorganisms, was chemically modified in order to attach biomolecules that can selectively interact with target bacteria. We present the first comprehensive study of monolayer-modified PAO using conditions that are relevant to microbial growth with a range of functional groups (carboxylic acid, α-hydroxycarboxylic acid, alkyne, alkene, phosphonic acid, and silane). Their stability was initially assessed in phosphate-buffered saline (pH 7.0) at room temperature. The most stable combination (PAO with phosphonic acids) was further studied over a range of physiological pHs (4-8) and temperatures (up to 80 °C). Varying the pH had no significant effect on the stability, but it gradually decreased with increasing temperature. The stability of phosphonic acid-modified PAO surfaces was shown to depend strongly on the other terminal group of the monolayer structure: in general, hydrophilic monolayers were less stable than hydrophobic monolayers. Finally, an alkyne-terminated PAO surface was reacted with an azide-linked mannose derivative. The resulting mannose-presenting PAO surface showed the clearly increased adherence of a mannose-binding bacterium, Lactobacillus plantarum, and also allowed for bacterial outgrowth.