Hydrocarbon Self-assembled Monolayer Research Articles

Fluorination to enhance superlubricity performance between self-assembled monolayer and graphite in water

HypothesisAchievement of superlubricity is an effective method to reduce friction and wear, which has a prominent influence on the operational efficiency and lifetime of a device. However, some burning issues still remain to be solved for the practical applications of superlubricity, such as the poor load-bearing capacity, especially in liquid superlubricity. Therefore, exploring an effective method to enhance the superlubricity performance is essential to accelerate the application of superlubricity. ExperimentsThe friction properties between two different self-assembled monolayers (SAMs)—a perfluorocarbon SAM and a hydrocarbon SAM—and graphite in water were explored and compared by atomic force microscopy (AFM). FindingsEnhanced superlubricity performance due to the fluorination was observed. Specifically, we observed an approximately 85% reduction of the friction coefficient after fluorination, and superlubricity was achieved with extremely low friction coefficient of 0.0003. Moreover, 2.4-fold greater load-bearing capacity of the superlubricity was obtained after fluorination. The molecular origin of the superlubricity enhancement by fluorination was revealed by molecular dynamics (MD) simulations, indicating that the greater load-bearing capacity of the perfluorocarbon SAM was ascribed to the enhanced interaction between the water and SAM by fluorination to form a more robust layered water structure confined in the contact zone, which played a pivotal role in the superlubricity.

Performance of a rigid rod statistical mechanical treatment to predict monolayer ordering: a study of chain interactions and comparison with molecular dynamics simulation

A statistical mechanical model that treats hydrocarbon self-assembled monolayer (SAM) chains as rigid rods is examined to interrogate the mechanisms involved in monolayer ordering. The statistical mechanical predictions are compared to fully atomistic molecular dynamics simulations of SAMs with different packing densities. The monolayer chain order is examined as a function of surface coverage, chain-surface interactions, and chain–chain interactions. Reasonable interaction potentials are deduced from ab initio electronic structure calculations of small model systems. It is found that the chain-surface interaction is the most important parameter in formation of flat-lying monolayer phases, while formation of standing phase monolayers is driven most importantly by increased density of molecules at the surface. A brief discussion of the utility and validity of the rigid rod treatment is given in light of the molecular dynamics results.

The Water-Hydrophobic Interface: Neutral and Charged Solute Adsorption at Fluorocarbon and Hydrocarbon Self-Assembled Monolayers (SAMs)

Adsorption of small molecular solutes in an aqueous solution to a soft hydrophobic surface is a topic relevant to many fields. In biological and industrial systems, the interfacial environment is often complex, containing an array of salts and organic compounds in the solution phase. Additionally, the surface itself can have a complex structure that can interact in unpredictable ways with small solutes in its vicinity. In this work, we studied model adsorption processes on hydrocarbon and fluorocarbon self-assembled monolayers by using vibrational sum frequency spectroscopy, with methanol and butylammonium chloride as adsorbates. The results indicate that differences in surface functionality have a significant impact on the organization of adsorbed organic species at hydrophobic surfaces.

Collisions of Polar and Nonpolar Gases with Hydrogen Bonding and Hydrocarbon Self-Assembled Monolayers

Molecular beam scattering experiments are used to explore collisions of 60 kJ/mol Ne, CD4, ND3, and D2O with long-chain CH3-, NH2-, and OH-terminated self-assembled monolayers (SAMs) created via the chemisorption of alkanethiols on gold. Time-of-flight measurements for the scattered gases reveal the extent of energy exchange and the propensity for a gas to thermally accommodate with the surface during a collision. Of the four gases studied, Ne transfers the least amount of translational energy into the monolayers and D2O the most. Neon atoms recoil from the OH-SAM with an average of 14.4 kJ/mol of energy, while D2O retains only 6.4 kJ/mol of its 60 kJ/mol incident energy when scattering from the same surface. Overall, the trend in final translational energies follows the order Ne > CD4 > ND3 > D2O for scattering from all three SAMs. The observed trend in the energy exchange is correlated with the gas−surface attractive forces, as determined by ab initio calculations. The thermal accommodation efficiencies of the four gases follow the opposite trend. Thermalization for the Ne atoms is nearly negligible for all three monolayers, whereas D2O and ND3 approach near complete accommodation on all of the monolayers studied. The overall energy exchange and thermal accommodation efficiencies also depend markedly on the terminal group of the SAM. For Ne scattering, the trend for the overall energy transfer follows: CH3- > NH2- ≈ OH-SAMs. In contrast, the overall D2O energy transfer is greater when colliding with the OH-SAM than the nonpolar CH3-SAM. Together, the results show that the extent of energy transfer depends on a balance between the rigidity of the surface, as affected by intrasurface hydrogen bonding, and the strength of the gas−surface attractive forces, as determined by intermolecular interactions.

Chemical Modifications of Inert Organic Monolayers with Oxygen Plasma for Biosensor Applications

In this paper we present a study of using oxygen plasma for chemically modifying inert hydrocarbon self-assembled monolayers of octadecyltrichlorosilane (OTS-SAMs) and rendering active surfaces for protein immobilization. Detailed surface modification and protein immobilization were characterized by using ellipsometry, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared-attenuated total reflectance spectroscopy, and fluorescence microscopy. Our XPS results showed that the surface reaction between OTS-SAMs and oxygen plasma can generate new surface functional groups such as alcohol (C-O), aldehyde (C=O), and carboxylic acid (O-C=O), and their compositions can be controlled by using different treatment times and powers. A short treatment time ( approximately 1 s) and high power (10 W) can lead to a higher density of aldehyde groups, which can serve as linker groups for protein immobilization through the formation of Schiff bases with the amine groups of proteins. By using the fluorescence immunostaining method, we confirmed that human immunoglobulin (IgG) can be immobilized on a glass slide, only if the surface was decorated with OTS-SAMs and if the OTS-SAMs were pretreated with oxygen plasma. The protein immobilized on the oxygen-plasma-treated surface can only be recognized by using a highly specific antibody, FITC-anti-IgG, but not FITC-anti-biotin.

Theoretical studies of hyperthermal O(3P) collisions with hydrocarbon self-assembled monolayers.

We present a dynamics study of inelastic and reactive scattering processes in collisions of hyperthermal (5 eV) O(3P) atoms with a hydrocarbon self-assembled monolayer (SAM). Molecular-dynamics simulations are carried out using a quantum mechanics/molecular mechanics (QM/MM) interaction potential that uses a high quality semiempirical Hamiltonian for the QM part and the MM3 force field for the MM part. A variety of products coming from reaction are identified, including H abstraction to generate OH, O atom addition to the SAM with subsequent elimination of H atoms, and direct C-C breakage. The C-C breakage mechanism provides a pathway for significant surface mass loss in single reactive events whereas the O addition-H elimination channel leads to surface oxidation. Reaction probabilities, product energy, and angular distributions are examined to gain insight on polymer erosion in low Earth orbit conditions and on fundamentals of inelastic and reactive hyperthermal gas-surface interactions.

Selective Adsorption of DMSO from an Aqueous Solution at the Surface of Self-Assembled Monolayers

The dynamics of wetting a hydrocarbon self-assembled monolayer by water/DMSO (dimethyl sulfoxide) mixtures and the resulting structure are studied by molecular dynamics computer simulations. A mixture of the two liquids in contact with a methyl terminated C18H37 self-assembled monolayer shows preferential wetting of the surface by the DMSO molecules over a wide range of concentrations. The dynamics of the water desorption and DMSO adsorption starting from a nonequilibrium state are followed. The process is shown to involve a continuous replacement of approximately four water molecules for every single DMSO molecule. In the final equilibrium state, the DMSO methyl groups are oriented toward the surface while the SO bonds form hydrogen bonds with the water. This gives rise to an overlayer of water adjunct to the DMSO surface layer.

Chain Length Effects for Cluster Ion Formation during High Energy Ion/Surface Collisions with Self-Assembled Monolayer Surfaces

A high-resolution time-of-flight secondary ionization mass spectrometer (TOF-SIMS) was used to investigate chain length effects for the formation of cluster ions. The clusters were formed from high energy monatomic ion collisions with hydrocarbon self-assembled monolayer (SAM) surfaces that were deposited onto gold substrates. A wide range of n-alkanethiols was used to make the SAM surfaces, which included both odd and even hydrocarbon chain length thiols. The mass range of the TOF-SIMS made it possible to monitor all of the cluster ions that were formed by the high energy ion/surface interactions. The focus of this work was on the formation of high mass gold-sulfur and gold-adsorbate cluster ions. Mechanisms for the formation of these large cluster ions are proposed, which build upon a precursor cluster mechanism for high energy collisions with hydrocarbon SAM surfaces. 1 The formation of high mass cluster ions is a function of the chain length of the alkanethiol chemisorbed to the gold surface. It has been demonstrated that both the attenuation of the ions by steric effects and van der Waals interactions dictate their formation.

Neutralization of methyl cation via chemical reactions in low-energy ion-surface collisions with fluorocarbon and hydrocarbon self-assembled monolayer films

Low-energy ion-surface collisions of methyl cation at hydrocarbon and fluorocarbon self-assembled monolayer (SAM) surfaces produce extensive neutralization of CH 3 +. These experimental observations are reported together with the results obtained for ion-surface collisions with the molecular ions of benzene, styrene, 3-fluorobenzonitrile, 1,3,5-triazine, and ammonia on the same surfaces. For comparison, low-energy gas-phase collisions of CD 3 + and 3-fluorobenzonitrile molecular ions with neutral n-butane reagent gas were conducted in a triple quadrupole (QQQ) instrument. Relevant MP2 6-31G*//MP2 6-31G* ab initio and thermochemical calculations provide further insight in the neutralization mechanisms of methyl cation. The data suggest that neutralization of methyl cation with hydrocarbon and fluorocarbon SAMs occurs by concerted chemical reactions, i.e., that neutralization of the projectile occurs not only by a direct electron transfer from the surface but also by formation of a neutral molecule. The calculations indicate that the following products can be formed by exothermic processes and without appreciable activation energy: CH 4 (formal hydride ion addition) and C 2H 6 (formal methyl anion addition) from a hydrocarbon surface and CH 3F (formal fluoride addition) from a fluorocarbon surface. The results also demonstrate that, in some cases, simple thermochemical calculations cannot be used to predict the energy profiles because relatively large activation energies can be associated with exothermic reactions, as was found for the formation of CH 3CF 3 (formal addition of trifluoromethyl anion).

Low-Energy Collisions of Pyrazine and d6-Benzene Molecular Ions with Self-Assembled Monolayer Surfaces: The Odd−Even Chain Length Effect

The low-energy reactive collisions of two independent probe ions, pyrazine and d6-benzene, illustrate an odd−even hydrocarbon chain length effect for a wide range of hydrocarbon self-assembled monolayer (SAM) surfaces. SAM surfaces prepared from alkanethiols ranging from CH3(CH2)10SH to CH3(CH2)17SH chemisorbed to polycrystalline gold are shown to exhibit an odd−even effect where the orientation of the terminal methyl group determines the reaction behavior of the thin film. X-ray photoelectron spectroscopy shows the surfaces to be homogeneously covered and provides evidence for the presence of a thiolate (Au−SR) on the SAM surface. When 30 eV incident ions were used, the extent of hydrogen addition to the incident probe ion was larger for odd carbon chain length SAM surfaces when compared to the even chain length films. For an odd chain length SAM surface, the terminal methyl group exposes a C−H bond perpendicular to the surface, increasing hydrogen addition reactivity for the probe ions. The low-energy p...

Reactions of BBr(n)(+) (n = 0--2) at fluorinated and hydrocarbon self-assembled monolayer surfaces: observations of chemical selectivity in ion--surface scattering.

Ion-surface reactions involving BBr(n)(+) (n = 0--2) with a fluorinated self-assembled monolayer (F-SAM) surface were investigated using a multi-sector scattering mass spectrometer. Collisions of the B(+) ion yield BF(2)(+) at threshold energy with the simpler product ion BF(+)* appearing at higher collision energies and remaining of lower abundance than BF(2)(+) at all energies examined. In addition, the reactively sputtered ion CF(+) accompanies the formation of BF(2)(+) at low collision energies. These results stand in contrast with previous data on the ion-surface reactions of atomic ions with the F-SAM surface in that the threshold and most abundant reaction products in those cases involved the abstraction of a single fluorine atom. Gas-phase enthalpy data are consistent with BF(2)(+) being the thermodynamically favored product. The fact that the abundance of BF(2)(+) is relatively low and relatively insensitive to changes in collision energy suggests that this reaction proceeds through an entropically demanding intermediate at the vacuum--surface interface, one which involves interaction of the B(+) ion simultaneously with two fluorine atoms. By contrast with the reaction of B(+), the odd-electron species BBr(+)* reacts with the F-SAM surface to yield an abundant single-fluorine abstraction product, BBrF(+). Corresponding gas-phase ion--molecule experiments involving B(+) and BBr(+)* with C(6)F(14) also yield the products BF(+)* and BF(2)(+), but only in extremely low abundances and with no preference for double fluorine abstraction. Ion--surface reactions were also investigated for BBr(n)(+) (n = 0-2) with a hydrocarbon self-assembled monolayer (H-SAM) surface. Reaction of the B(+) ion and dissociative reactions of BBr(+)* result in the formation of BH(2)(+), while the thermodynamically less favorable product BH(+)* is not observed. Collisions of BBr(2)(+) with the H-SAM surface yield the dissociative ion-surface reaction products, BBrH(+) and BBrCH(3)(+). Substitution of bromine atoms on the projectile by hydrogen or alkyl radicals suggests that Br atoms may be transferred to the surface in a Br-for-H or Br-for-CH(3) transfer reaction in an analogous fashion to known transhalogenation reactions at the F-SAM surface. The results for the H-SAM surface stand in contrast to those for the F-SAM surface in that enhanced neutralization of the primary ions gives secondary ion signals one to two orders of magnitude smaller than those obtained when using the F-SAM surface, consistent with the relative ionization energies of the two materials.