Molecular Dynamic dilution, role

The last group of papers are relative to the computer-simulation study of the rotation-vibration correlations in pure liquids. The main authors are Levesque, Weis and Oxtoby (31,32) who examined liquid and HCl in much detail. A molecular dynamics simulation was carried out using 500 molecules for N2 and 256 for HCl periodic boundary conditions were imposed It was found that, in the isotropic Raman spectrum of liquid N2>intermolecular rotation-vibration interactions are mainly due to short range repulsion and dispersion forces the role of centrifugal forces seems secondary which is an unexpected result. The presence of important interference effects makes any partition illusory. Only intermolecular rotation-vibration correlation effects were examined in the case of infrared and anisotropic Raman spectra of liquid HCl. It results from this calculation that the simple product correlation function is indistinguishable from the total correlation function within the uncertainty of the simulation. This conclusion is similar to that reached by Tarjus and Bratos (12). It should not be forgotten, however, that the latter theory applies to diluted solutions whereas the function determined by Levesque, Weis and Oxtoby is an approximate correlation function of a pure liquid. 

Some possible approximations have been considered by Cates [56], who concentrated attention on macromolecular entanglements, which play an important role in the description of the behaviour of block polymers [86-89]. Cates believes that the fact that the concept of polymer fractal neglects the effects of macromolecular entanglements is the main drawback of this theory. Nevertheless, Cates [56] introduced several simplifications that make it possible to ignore these effects for dilute solutions and relatively low molecular masses. However, in the opinion of Cates, even in the case of predominant influence of entanglements, theoretical interpretation of this phenomenon is impossible without preliminary investigation of the properties of the system in terms of Rouse-Zimm dynamics, which can serve as the basis for a more complex theory. It was assumed [56] that the effects of entanglement can be due to the substantially enhanced local friction of macromolecules. 

Since the time of my first graduation in science, I have been an enthusiastic reader of Boltzmann, whose dynamical vision of physical becoming was for me a model of intuition and penetration. Nonetheless, I could not but notice some unsatisfying aspects. It was clear to me that Boltzmann introduced hypotheses foreign to dynamics under such assumptions, to talk about a dynamical justification of thermodynamics seemed to me an excessive conclusion, to say the least. In my opinion, the identification of entropy with molecular disorder could contain only one part of the truth if, as I persisted in thinking, irreversible processes were endowed with the constructive role that I never cease to attribute to them. For another part, the applications of Boltzmann s methods were restricted to dilute gases, while I was most interested in condensed systems. [Prigogine, 1977/1993, p. 258]... 

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