Butane monolayer

The inelastic neutron TOF spectrum of a butane monolayer adsorbed on Carbopack B at 80 K (10) is shown at the top of Fig. 6. The background inelastic scattering from the substrate has been subtracted. Typical counting time for such a spectrum is -100 hours. The intramolecular torsional modes of the CH, and CH groups observed in the bulk liquid and solid (23) are also found in the monolayer spectrum. In the two methyl torsionsal modes the CH, groups rotate either in the same or opposite sense about the terminal C-C bonds, and in the CH2-CH torsion the two halves of the molecule rotate in the opposite sense about the internal C-C bond. In addition to the intramolecular torsional modes, new features appear in the monolayer spectrum which are not observed in bulk samples an intense peak at -112 cm"-1 and a broader band centered at 50 cm-1. 

Figure 6. Comparison of observed and calculated vibrational spectra for a butane monolayer (19). (Topi Observed spectrum for monolayer butane adsorbed on a graphitized carbon powder at 80 K. The background inelastic scattering from the substrate has been subtracted, (a) Calculated spectrum for the butane molecule adsorbed with its carbon skeleton parallel to the graphite layers and the bottom layer of four hydrogen atoms bonded to the surface with force constants listed in Table I. (b) Same orientation but only the carbon atoms are bonded to the surface with a force constant of 0.12 mdyn/A. (Bottom) Butane carbon plane perpendicular to the graphite layers and the bottom layer of four hydrogen atoms bonded to the surface with the same force constants as in the parallel orientation.

In the case of the ethane (45) and butane (46) monolayers adsorbed on graphite, it has been possible to analyze the neutron diffraction patterns using all three Euler angles of the molecule as orientational parameters. Here we limit discussion to the butane monolayer which we have taken as a model system and whose vibrational spectrum was discussed in Sec. II. 

It is known that even condensed films must have surface diffusional mobility Rideal and Tadayon [64] found that stearic acid films transferred from one surface to another by a process that seemed to involve surface diffusion to the occasional points of contact between the solids. Such transfer, of course, is observed in actual friction experiments in that an uncoated rider quickly acquires a layer of boundary lubricant from the surface over which it is passed [46]. However, there is little quantitative information available about actual surface diffusion coefficients. One value that may be relevant is that of Ross and Good [65] for butane on Spheron 6, which, for a monolayer, was about 5 x 10 cm /sec. If the average junction is about 10 cm in size, this would also be about the average distance that a film molecule would have to migrate, and the time required would be about 10 sec. This rate of Junctions passing each other corresponds to a sliding speed of 100 cm/sec so that the usual speeds of 0.01 cm/sec should not be too fast for pressurized film formation. See Ref. 62 for a study of another mechanism for surface mobility, that of evaporative hopping. 

The results obtained for the adsorption of butane on a ball-milled caldte" are also of interest. When the solid was outgassed at 150°C to remove physically adsorbed water, the butane isotherm was ofType II with c = 26 (Fig. 5.5, curve (ii)) but outgassing at 25°, which would leave at least a monolayer of molecular water on the surface, resulted in a Type 111 isotherm (Fig. 5.5, curve (i)). Though butane is nonpolar its polarizability is... 

Therefore, monolayers may consist of two different chemisorption modes ordered in different domains, simultaneously coexisting homogeneous clusters, each characterized by a different conformer in their unit cell. This may explain the observation of 2D Hquid in butane- and hexanethiolate monolayers on gold (278), where VDW interactions do not provide enough cohesive energy to allow for small domains to coexist as a 2D soHd. 

SAQ 10.1 The following data refer to the adsorption of butane at 0°C onto tungsten powder (area 16.7 m2g 1). Calculate the number of moles adsorbed in a monolayer, and hence the molecular area for the adsorbed butane (at monolayer coverage) and compare it with the value of 32 x 10 20 m2 estimated from the density of liquid butane. 

The paraffins adsorb with their chain axis parallel to the platinum substrate. Thus their surface unit cell increases smoothly with increasing chain length as shown in Fig. 5.3. The n-butane molecules, unlike the larger molecules, form several monolayer surface structures as the experimental conditions are varied. It appears that the smaller the paraffin the more densely packed it is on the surface. Evidently, as the packing becomes too dense for n-butane in one surface structure it forms a different one. 

Recently, n-butane and isobutane have been studied by RAIRS when adsorbed at low temperature on Ag(110) (138a). They gave undissociated monolayers which exhibited intermolecular attraction. 

C. A model system for neutron vibrational spectroscopy of adsorbed molecules monolayer butane on graphite. In this section we concentrate on a particular system, monolayer n-butane adsorbed on graphite, for which a considerable effort has been made to analyze the inelastic neutron spectra for the orientation of the adsorbed molecule and the forces bonding it to the substrate (10,19,20). By treating one system in greater detail, we can better illustrate the capabilities and limitations of the technique. 

The empirical potential calculations also give the perturbation of the intramolecular torsional modes of the adsorbed butane. A negligible shift is predicted for the CH- torsions while a -25% increase over the free-molecule frequency rs found for the GHp-CH- torsion in the adsorbed molecule. This frequency shift is treasonable agreement with that observed for the ChL-CH torsion between the monolayer and bulk liquid spectrum (231 winch best approximates that of the free molecule. 

The tilted configuration of butane was not considered in the analysis of the monolayer vibrational spectrum in Sec. II.C.2. Normal mode calculations are now being performed (20) with this orientation to see if the fit to the observed spectrum can be improved. The tilting of the molecule may be related to the inconsistency encountered in the plane-parallel model in which different atom-substrate force constants had to be introduced for the co-planar CH- and CH, hydrogens. In the tilted configuration this bottom layer of hydrogens is split into two separate levels of atoms. This difference in height above the surface may provide a physical basis for two different force constants. 

Fig. 1.5. Dependence of the electroenz3maatic response (Ay) of lactate dehydrogenase modified gold electrodes on the time of protein layer growth (a) mixed 3-mercaptopropamol and 3-mercaptopropionic acid SAM derivatized with 1,4-diamino-butane ligand fi-ee- ( ), CB- (O) and Ll-anchored (A) (b) cystamine SAM derivatized with fumaric acid ligand fi-ee- ( ), CB- (O) and Ll-anchored (A). The monolayers were incubated in a 0.36mgml enzyme solution in 50 mM Na-phosphate buffer, pH 7.0. Reproduced from [240] with permission.

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