Molecular flatness

It was shown by surface analysis (AFM and STM) that these films cover large areas (micrometer scale) without the occurrence of pinholes, furnishing a molecular flat amorphous surface with good short-range order but no long-range order, and thus free of domain boundaries (Fig. 6.13). 

For liquids near surfaces the radial distribution functions become asymmetrical and are generally more difficult to handle. The situation is again relatively simple for molecularly flat surfaces when only density variations normal to the surface have to be considered. This is, for instance, the case for spherically symmetrical molecules for rods, or molecules with asymmetrical interaction (water), the situation is again more complicated. In that case one can Introduce a (linear) distribution function g[z) as... 

Since now we have evidence for the molecular flatness of the fumed silica surface, we may indeed use this silica for comparison with other silicas and then with other mineral oxide surfaces. 

Sasou M, Sugiyama S, Yoshino T, Ohtani T. Molecular flat mica surface silanized with methyltrimethoxysilane for fixing and straightening DNA. Langmuir 2(X)3 19 9845-9. 

The argument just made assumes, implicitly, a model in which the substrate is molecularly flat. Appropriate revision to account for roughness effects can be made, which will be relevant when the substrate microtopography is known. The mathematical methods that have been employed to treat the contact angle of a liquid on a rough surface (28) may be employed. For example, we may define as actual area of interface and A as actual area projected on a plane parallel to the envelope of the surface. The roughness ratio, p, is... 

Oscillating forces are not a prerogative of smectics, but have been reported as well for nematics [45,50,51] and even for simple liquids of nearly spherical molecules [52]. In fact, as it was mentioned in the Introduction, the presence of a molecularly flat surface always produces some layering (i.e. positional ordering) of the nearby liquid molecules, because it breaks the translational invariance of the liquid. In liquid crystals this tendency is often enhanced by the presence of a fully layered bulk phase at low temperature (smectic, hexatic, columnar phases, etc.). 

The surface charge is considered homogeneous, and the surface itself is supposed to be molecularly flat. 

The air-water interface has interesting features as a medium for molecular recognition. For example, (1) a molecularly flat environment is formed at the interface, (2) a boundary region is facing the two phases with different dielectric constants, (3) macroscopically dynamic changes can be taken place... 

Yet another theory has emerged recently from the field of nanotribology. Surface force apparatus studies, combined with molecular dynamics simulations, of simplified model systems, such as molecularly flat mica separated by a few molecules thick lubricant layers, have identified a solid-melt transition as the cause for stick-slip motion to occur for such confined liquids (18, 106, 111-113, 144-149). A similarly confined liquid can be found in macroscopic friction systems in the boundary lubrication regime wherein thin lubricant layers are trapped between surface asperities in very close proximity. 

Percec V, Cho W-D, Ungar G, Yeardley DJP (2000) From molecular flat tapers, discs, and cones to supramolecular cylinders and spheres using Frechet-type monodendrons modified on their periphery. Angew Chem hit Ed 39(9) 1597-1602. doi 10.1002/(SlCl)1521-3773 (20000502)39 9<1597 A1D-AN1E1597>3.0.CO 2-1... 

Nada and Furukawa [67, 68] performed an MD simulation study on both ice basal and 1010 prismatic plane surfaces at 5 K temperature intervals in the temperature (T) range from 170 to 250 K using the TIP4P model. The simulation results indicated that structural transitions occur, at least twice, on both surfaces as T approaches from below. The transitions observed in the simulations were a transition from a molecularly flat surface to a surface with admolecules and a transition from a surface with admolecules to a surface at which surface melting occurred. The simulation results also indicated that the thickness of the QLL at T> 235 K is larger for the basal plane surface than that for the prismatic plane surface. This anisotropy in the QLL thickness between the basal and prismatic plane surfaces was qualitatively consistent with that obtained by an experimental study by Furukawa et al. [10]. 

---