Surface phenomena, computer simulations

However, the assumption of molecule orientation normal to the surface is not convincing enough for this author, and it does not consist well with the results of the molecular d5mamics simulations for the alkane confined between solid walls. An example in Fig. 3 shows that the chain molecules near the wall are found mostly lying parallel, instead of normal, to the wall [6]. This means that the attractions between lubricant molecules and solid wall may readily exceed the inter-molecule forces that are supposed to hold the molecules in the normal direction. Results in Fig. 3 were obtained from simulations for liquid alkane with nonpolar molecules, but similar phenomenon was observed in computer simulations for the functional lubricant PFPE (per-fluoropolyether) adsorbed on a solid substrate [7], confirming that molecules near a solid wall lie parallel to the surface. 

Concerning ices, it has been discussed that they must be amorphous (Smoluchowski 1983) in the interstellar medium and not crystalline. This implies that the adsorbed H atoms are localized in deep traps so that their wavefunctions have a limited spatial extent. This fact reduces significantly their mobility and hence the interaction with another H atom absorbed on another site is slow as compared to the residence time unless the two atoms happens to be localized near each other. This phenomenon reduces the rate of H2 formation by several orders of magnitude when compared to the situation on crystalline surfaces. Computational simulations on soft and hard ice model surfaces have shown that for a cross-section of 4,000 nm2 the reaction probability is 1 (Takahashi et al. 1999). Furthermore, the H2 formed, due to the high amount of energy liberated is rapidly desorbed in an excited state from the ice mantle in timescales of 500 fs (Takahashi et al. 1999). 

A more accurate description of the structure of colloidal plasmas can be obtained by means of MC computer simulations based on the microscopic model of asymmetric two-component plasmas (TCP). As shown above, the nonlinear grain screening obtained within PB theory has the direct analogue in the MC simulations with the microscopic description of plasma background, the phenomenon of plasma condensation near grain surface. This suggests that the above phenomenon should manifest itself in MC simulations of asymmetric two-component plasmas affecting its structural properties as well. 

In this contribution, a brirf review of selected results was presented on the elastic and viscoelastic behavior of amorphous polymer nanocomposites. The ovei-view was done in a simplified manner, in order to support its miderstaudability. Below the matrix Tg, a nanocomposite behaves like a two component system due to the low-entropy/low-mobility state of polymer matrix. Above the mati-ix Tg, the polymer chains near the nano-filler surface become perturbed in respect to their dynamics. These changes occurring on the molecular level cause severe effects observable on micro- and macroscopic levels. Due to the extensive nanofiller surface area, the filler nanoparticles are able to cause these effects even at a very low filler concentration. Interestingly, the immobilization phenomenon in polymers filled with high specific surface area fillers has already been addressed in the 60s by DiBenedetto [40], Lipatov [44,56] and others [39,43,45]. The cited authors interpreted the results properly although very poor computer simulation possibilities were available at that time. 

The unique maximum in the crystallization rate has already been noted. Several explanations have been put forth to explain this observation. One is the so-called poisoning effect proposed by Sadler and collaborators.(286-290) This self-poisoning of the crystal growth surface is postulated to be caused by the deposition of almost stable folded crystalUtes that retard the development of the extended chain crystallites. On this basis this phenomenon would be limited to crystallization temperatures in the vicinity of the junction of Regions II and HI. Rate equation models based on the hypothesis, as weU as computer simulations, have reproduced the qualitative features of the unique rate-crystallization temperature curves.(284,291)... 

Now, theoretical calculation methods of sufficient accuracy may fill the lack of quantitative information concerning so elusive species. On the other hand, the use of a monocoordinated complex as being the simplest molecular model to simulate a chemisorption phenomenon on a metallic surface, for instance the chimisorption of carbon monoxide on iron or nickel [16,17,18] enables to predict the shifts of the CO stretching vibration of the adsorbed species. Similar effects observed with cyanide anions CN on a cathode of platinum, silver or gold, using non-linear optics techniques can be rationalized by computing the CN vibration mode of the corresponding triatomic systems [19,20,21]. 

A modern stage in the development of notions of specific adsorption is masked by a changeover to describing this phenomenon on a microscopical level, using computational approaches to simulation of adsorbed layers. We can expect a rapid progress in these directions, because the information on the microscopic structure of adlayers becomes progressively more available because of studies on singlecrystal electrodes and the results of physical surface science techniques. 

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