Accommodation to the surface

The energy of an incident molecule will not normally be the same as that of the molecule when it is scattered from the surface, i.e., ZsP Ef There will be an accommodation to the surface and an exchange of energy with the surface. Complete accommodation or equilibration with the surface would imply that the scattered molecules have the same temperature as the surface. The energy accommodation coefficient, ac, is defined for each surface involved in the problem by the expression... 

Hurst et al. attributed the second of these spectral peaks (the peak occurring at later times) to Xe atoms which had accommodated to the surface (i.e. come into equilibrium with the surface at a surface temperature of Ts) and subsequently desorbed from the surface. The other peak present in the spectra was attributed to Xe atoms that had directly scattered, without thermal accommodation, from the surface. 

As a first example of low-density heat transfer let us consider the two parallel infinite plates shown in Fig. 12-14. The plates are maintained at different temperatures and separated by a gaseous medium. Let us neglect natural-convection effects. If the gas density is sufficiently high so that A — 0, a linear temperature profile through the gas will be experienced as shown for the case of A. As the gas density is lowered, the larger mean free paths require a greater distance from the heat-transfer surfaces in order for the gas to accommodate to the surface temperatures. The anticipated temperature profiles are shown in... 

We have already mentioned that the temperature-jump effect arises as a result of the failure of the molecules to accommodate to the surface temperature when the mean free path becomes of the order of a characteristic body dimension. The parameter which describes this behavior is called the accommodation coefficient a, defined by... 

Scattering results from the well-ordered surfaces have certain features in common. In all these cases, direct-inelastic scattering of NO(v=l) —> NO(v=l) was observed, as characterized by angular distributions peaked near the specular direction and translational energies much higher than expected from accommodation to the surface temperature. 

Figure 12.5 Schematic potential energy surface for dissociative adsorption-desorption [adapted from ErtI (1982)]. In the drawing, the barrier to dissociation-recombination occurs when the molecule is already at the surface and is along the bond distance. This will lead to vibrationally excited desorbed molecules. Note also the precursor well along the approach to the (physical) surface. This well will slow down the approaching reactant but may not be deep enough to insure that it fully accommodates to the surface.

The sequence space of proteins is extremely dense. The number of possible protein sequences is 20. It is clear that even by the fastest combinatorial procedure only a very small fraction of such sequences could have been synthesized. Of course, not all of these sequences will encode protein stmctures which for functional purjDoses are constrained to have certain characteristics. A natural question that arises is how do viable protein stmctures emerge from the vast sea of sequence space The two physical features of folded stmctures are (l)in general native proteins are compact but not maximally so. (2) The dense interior of proteins is largely made up of hydrophobic residues and the hydrophilic residues are better accommodated on the surface. These characteristics give the folded stmctures a lower free energy in comparison to all other confonnations. 

Theories of the oxidation of tantalum in the presence of suboxide have been developed by Stringer. By means of single-crystal studies he has been able to show that a rate anisotropy stems from the orientation of the suboxide which is precipitated in the form of thin plates. Their influence on the oxidation rate is least when they lie parallel to the metal interface, since the stresses set up by their oxidation to the pentoxide are most easily accommodated. By contrast, when the plates are at 45° to the surface, complex stresses are established which create characteristic chevron markings and cracks in the oxide. The cracks in this case follow lines of pores generated by oxidation of the plates. This behaviour is also found with niobium, but surprisingly, these pores are not formed when Ta-Nb alloys are oxidised, and the rate anisotropy disappears. However, the rate remains linear it seems that this is another case in which molecular oxygen travels by sub-microscopic routes. 

As may be seen from Fig. 2, it seems impossible to accommodate at the surface of the enzyme simultaneously a molecule of sucrose and of L-sorbose in such a way as to permit a switch of the fructosidic linkage... 

Analogous to the slip velocity between gas and particle at Kn above the continuum flow range discussed in Section A above, a temperature discontinuity exists close to the surface at high Kn. Such a discontinuity represents an additional resistance to transfer. Hence, transfer rates are generally lowered by compressibility and noncontinuum effects. The temperature jump occurs over a distance 1.996kl 2 — a )/Fva k + 1) (K2, Sll) where is the thermal accommodation coefficient, interpreted as the extent to which the thermal energy of reflected molecules has adjusted to the surface temperature. 

The physical and chemical processes occurring in a gas-liquid system are often treated in terms of a resistance model described in Box 5.2. As discussed there, the net uptake of gas (yIK.t) can be treated under some conditions in terms of conductances, T, normalized to the rate of gas-surface collisions. Individual conductances are associated with gas-phase diffusion to the surface (Tg), mass accommodation across the interface (a), solubility (rsol), and finally, reaction in the bulk aqueous phase (Tlxn). This leads to Eq. (QQ) ... 

FIGURE 5.16 Schematic of resistance model for diffusion, uptake, and reaction of gases with liquids. Tg represents the transport of gases to the surface of the particle, a the mass accommodation coefficient for transfer across the interface, rso, the solubilization and diffusion in the liquid phase, riM the bulk liquid-phase reaction, and rinlcrl.ll c the reaction of the gas at the interface. 

---