Structure of Mixed Carboxylic Acid Terminated Self-Assembled Monolayers:  Experimental and Theoretical Investigation

Abstract

The structure and stability of mixed self-assembled monolayers (SAMs) of 3-mercaptopropionic acid (MPA) and 11-mercaptoundecanoic acid (MUA) prepared by immersion in ethanolic solutions were studied by cyclic voltammetry, electrochemical impedance spectroscopy, ellipsometry, and STM as a function of the thiol composition of the forming solution. The presence of a single reductive desorption peak in the voltammograms of the mixed SAMs and the lack of phase segregation observed by STM support the formation of homogeneous SAMs despite the large chain length difference between MPA and MUA. To explain the driving force leading to the formation of a homogeneous mixture, intermolecular interactions within the SAM were investigated using density functional theory. The carboxyl groups of adjacent MPA and MUA molecules in a compact monolayer can form a stable head to head cyclic dimer with a hydrogen bond strength of 16.2 kcal/mol. The flexibility of the alkyl chain of MUA allows the carboxyl groups of adjacent MPA and MUA molecules to be located on the same plane. However, the carboxyls of adjacent MPA-MPA and MUA-MUA pairs form much weaker hydrogen bonds because steric constraints avoid the formation of the stable cyclic dimer. Therefore, the prevalence of MPA-MUA interactions over MPA-MPA and MUA-MUA interactions explains the homogeneous mixing of MPA and MUA. The potential chemical switchability properties of mixed monolayers of mercaptoalcanoic acids of different chain lengths as a function of the solution pH are discussed.

Factors governing the reversible change in ionic permeability of a low-density monolayer

“…For example, in the low-frequency regime, where the impedance spectrum is dominated by the SAM, the magnitude of the imaginary component of the measured impedance (Z 00 )-which is inversely proportionally to the capacitance of the SAM-drops by nearly a factor of two between +0.3 V and 0 V. As the potential is shifted in the positive direction from À0.1 V to +0.3 V, the magnitude of the CPE for both films increases by about a factor of two and the CPE exponent decreases monotonically (from 0.93 and 0.86 for the TEA-MHA ion-pair SAM and from 0.97 to 0.91 for the loosely-packed MHA SAM). Lower values of CPE exponent (greater deviations from 1.0) indicate that the film is a less effective (more leaky) capacitor and have been correlated with structural disorder within SAMs [36,37]. The magnitude of the CPE parameter is only equivalent to the capacitance of the film (C) if the CPE exponent (n) is equal to 1.…”

Subnanometer‐Resolved Patterning of Bicomponent Self‐Assembled Monolayers on Au(111)

“…The unit cell shown in Figure 3 a, b contains two molecules: larger and brighter spots are located at the edges of the unit cell, whereas a dark spot is found at its centre. Similar domains are typically found in monocomponent SAMs formed by alkanethiol species exposing a head group which is capable of forming hydrogen bonds, as in the cases of 1mercaptoundecanoic acid, [20] mercaptopropionic acid, [21] and 1-mercapto-2-propanol [22] (the authors explained these structures in terms of alternating rows of hydrogen-bonded and non-H-bonded molecules).…”

Enhancing the Sensitivity of Biotinylated Surfaces by Tailoring the Design of the Mixed Self-Assembled Monolayer Synthesis

“… 15 , 27 , 28 Moreover, a homogeneous mixed SAM can be prepared from ethanolic solutions. 29 According to the literature, 15 this would lead to a highly packed layer of antibodies. The biofunctionalization process was carried out by activating the carboxylic moieties of 3MPA:11MUA SAMs through 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide/ N -hydroxysulfosuccinimide sodium salt (EDC/NHSS) coupling.…”