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Complementary atomic force microscopy (AFM) measurements and molecular dynamics (MD) simulations were conducted to determine the work of adhesion for diamond (C)(111)(1 × 1) and C(001)(2 × 1) surfaces paired with carbon-based materials. While the works of adhesion from experiments and simulations are in reasonable agreement, some differences were identified. Experimentally, the work of adhesion between an amorphous carbon tip and individual C(001)(2 × 1)-H and C(111)(1 × 1)-H surfaces yielded adhesion values that were larger on the C(001)(2 × 1)-H surface. The simulations revealed that the average adhesion between self-mated C(001)(2×1) surfaces was smaller than for self-mated C(111)(1×1) contacts. Adhesion was reduced when amorphous carbon counterfaces were paired with both types of diamond surfaces. Pairing model diamond nanocomposite surfaces with the C(111)(1 × 1)-H sample resulted in even larger reductions in adhesion. These results point to the importance of atomic-scale roughness for adhesion. The simulated adhesion also shows a modest dependence on hydrogen coverage. Density functional theory calculations revealed small, C-H bond dipoles on both diamond samples, with the C(001)(2 × 1)-H surface having the larger dipole, but having a smaller dipole moment per unit area. Thus, charge separation at the surface is another possible source of the difference between the measured and calculated works of adhesion. of hydrogen into DLC films was shown to reduce friction in self-mated DLC-DLC contacts [18].By definition, the work of adhesion (W ) is the energy per unit area required to separate two semi-infinite surfaces from their equilibrium separation to infinity. For two dissimilar materials, this quantity is equivalent to a sum of the surface energies, γ 1 and γ 2 , of the two surfaces (because, effectively, the separation process creates the two surfaces), minus the interfacial energy, γ 12 . Hence, W = γ 1 + γ 2 − γ 12 , where W is also known as the Dupré energy of adhesion. To control adhesion forces between two materials, the work of adhesion can be varied by changing the surface termination. However, this assumes that the two surfaces are perfectly flat. If one or both of the surfaces is rough, then the actual area of contact will be less than the apparent contact area [19], even at the atomic scale [20][21][22]. Taller asperities (or protruding atoms) at the interface will prevent some of the smaller features (or other atoms) from closely approaching each other, increasing the separation between different regions of the surface and, therefore, reducing the energy of interaction. While the extent to which nano-or even atomic-scale roughness affects adhesion measurements is not fully understood [23], recent MD simulations indicate that the effects could be dramatic [20,21].At present, both modeling and experiment have been used to investigate adhesion at the nanometer scale. In an AFM experiment, adhesion is measured by recording pull-off forces (L C ) between an AFM tip and a sample surface in a controlled...

A computational investigation of the conformational preferences of 2-phenethylamine has been carried out with a variety of techniques. To determine the intrinsic (in the absence of a solvent medium) conformational preferences of the 2-phenethylamine system, ab initio calculations at various levels of theory up to the MP2/6-311+G(d,p)//MP2/6-31G(d,p) level were carried out. This is the most sophisticated level of theory that has been applied to this biologically important system to date. In the absence of a solvent medium, phenethylamines prefer a folded gauche conformation for both the charged and neutral amines, indicating a favorable interaction between the amino group and the aromatic ring. To probe the nature of this intramolecular interaction further the effects of ring substituents on the conformational preferences were studied. The results have been compared to those obtained with semiempirical and molecular mechanics force field methods. The molecular mechanics force fields employing default parameters typically performed poorly for this system, but the results were improved significantly if the electrostatic charges were replaced. The effects of aqueous solvation have also been investigated with the GB/SA and the SM2 continuum solvation models. The best agreement with experiment is obtained when the MP2/6-311+G-(d,p)//MP2/6-31G(d,p) results are combined with the SM2-calculated solvent effect. Results of nearly the same quality can be obtained if the solvent effect is calculated with the GB/SA solvation model using AM1-CM1A charges.