Hydrogen-Bond Structure and Dynamics at the Interface between Water and Carboxylic Acid-Functionalized Self-Assembled Monolayers

Abstract

Molecular dynamics computer simulations are used to study hydrogen-bond structure and dynamics at the interface between water and carboxylic acid-functionalized self-assembled monolayers (CAFSAMs). Water-water, water-CAFSAM, and internal CAFSAM hydrogen bonds are examined. Roughly half of all adjacent carboxylic acid-terminated hydrocarbon chains are hydrogen-bonded to one another. This is consistent with experimental results reflecting two pKa values for CAFSAMs. Hydrogen-bond dynamics are expressed in terms of hydrogen-bond population autocorrelation functions and are found to be nonexponential. The water-water hydrogen-bond dynamics are slower at the interface than in the bulk, which is similar to what was found at the interface between water and weakly polar liquids such as nitrobenzene. The water-CAFSAM hydrogen bonds are found to be long-lived, on the order of tens of picoseconds. Internal CAFSAM chain-chain hydrogen bonds show almost no relaxation on the simulation time scale.

Determination of ζ‐potential, charge, and number of organic ligands on the surface of water soluble quantum dots by capillary electrophoresis

“…As mentioned in the introduction, pKa values of organic acids attached to very smooth and well‐defined surfaces are higher than those measured in a free state, due to the formation of self‐assembled monolayers and increased interactions between carboxy groups. Similarly, the pKa values of ligands on the surface decrease with increasing ionic strength due to the blocking of hydrogen bonding .…”

“…This is caused by blocking of strong hydrogen bonding due to formation of ion pairs between carboxylic group and counterions. Thus, a disrupted hydrogen bonding is the reason for easier deprotonation of acids and their dissociation at lower pH . The influence of ionic strength on pKa of TGA bonded on gold disk electrode was observed by Lu et al.…”

Reaction of gas phase OH with unsaturated self-assembled monolayers and relevance to atmospheric organic oxidations

“…This is similar to our previous results 51,68 where oxidation of the film using O 3 or KMnO 4 only slightly enhanced the uptake of water. Possible reasons for this are hydrogen-bonding between polar groups on the surface so they are not available for interaction with water 51,[69][70][71] or the polar groups being buried inside the organic surface film in such a way that they are not available for interaction with the gas phase. 72 …”

The role of subphase chemistry in controlling monolayer behaviour

“…The 1:1 ratio of acid to alcohol would maximise the carbonyl-oxygen to hydroxyl-hydrogen bonding, reported to be the longest lived [21]. This is echoed in the condensation behaviour of the L 2 /L 2 to LS transition pressure data.…”