© 2004 Robert A. Freitas Jr. and Ralph C. Merkle. All Rights Reserved.

Robert A. Freitas Jr., Ralph C. Merkle, Kinematic Self-Replicating Machines, Landes Bioscience, Georgetown, TX, 2004.

B.4.3.3.5 Physisorption and Desorption of Nonsolvent Molecules

Since there is one acetylene molecule for every ~59 n-octane molecules in the hydrocarbon feedstock solution, the possible physisorption of solvated acetylene molecules from the external solvent bath to the piston cavity passivated diamond walls must also be evaluated. Experiments [3189] on the physisorption of acetylene on the diamond C(111)-H(1x1) surface from gas phase show that the C₂H₂ molecule adsorbs nearly flat on terraces but is slightly tilted on defect sites, covering four surface hydrogens. This in-plane orientation of the acetylene monolayer is not unexpected because C₂H₂ has high p electron density around the C=C triple bond, the most energetically favorable position to be attacked by hydrogen-containing species to form a hydrogen bond, and because electrostatic interactions also favor the horizontal orientation of C₂H₂ on the diamond surface.

Will n-octane or acetylene be preferentially adsorbed on the passivated diamond surface? In principle, the heat of adsorption is of the same order of magnitude as the heat evolved during phase changes such as freezing or condensation [2987], since all these processes are governed by van der Waals interactions. The heat of fusion for n-octane at 1 atm is 34.4 zJ/molecule [2987]; for acetylene, the heat of fusion is only 4.17 zJ/molecule [3255] and 1.34 zJ/molecule for the heat of sublimation [3256], which would appear to strongly favor the persistence of octane.

A molecular dynamics simulation by Xia and Landman [3217] found that in an equal-weight mixture of hexane and hexadecane on the Au(001) surface at 315 K, both species initially adsorb in the nearest-surface monolayer with a slight preference in number of attached segments for the shorter-chain hexane molecules. But the longer chain quickly crowds out the shorter chain by reptation motions, so that by 0.8 x 10^-9 sec the attached segment counts are about equal and by 8 x 10^-9 sec there are 20 times more hexadecanes than hexanes attached. Similarly, a simulation of an octane-butane mixture on a waxy surface at 223 K produced an early preferential adsorption of the butane in the layer nearest the surface, but the simulation was halted after only 0.2 x 10^-9 sec so any subsequent possible substitution effects could not be observed [3232]. In a mixed-hydrocarbon solution of C₁₄H₃₀ in benzene on a graphite surface, alkane chains were found to rapidly penetrate the previously adsorbed benzene layers near the surface and adhere to the graphite [3249]. In one early study, n-alkanes with carbon numbers from 16-32 in n-heptane solution readily displaced the shorter-chain heptane and physisorbed to surfaces of cast iron, graphite, MoS₂ and WS₂ [3251, 3252]. This preferential adsorption of longer-chain hydrocarbons is believed to be driven by lateral intermolecular interactions within the adsorbed layer [3250], rather than by adsorbate-substrate interaction, which suggests that the much larger n-octane molecules will adsorb to diamond in preference to acetylene molecules, leaving a weakly physisorbed close-packed n-octane monolayer at the surface with the carbon skeleton planes of the octane molecules oriented parallel to the surface [3250], and relatively few surface-physisorbed acetylene molecules. The adsorption stability of the alkanes increases with increasing chain length [3257].

Vitamin molecules are present in feedstock at the parts-per-million level and can be designed to minimize their physisorption to diamond walls, hence should not significantly contribute to piston drag forces or drag power losses. The authors would encourage a more comprehensive molecular dynamics analysis of nanoscale piston operations.

Last updated on 13 August 2005